1 Geotechnical Engineering - · PDF file · 2012-10-04The mechanical properties of the soil, such as its compressibility, strength, permeability, ... (fluids). The relative amount

Geotechnical

Engineering

TED D. BUSHELL, P.E., AECOM (GROUND IMPROVEMENT); TONY A. KIEFER, P.E., AECOM (FOUNDATIONS); SARA E. KNIGHT, P.E., STRATA EARTH SERVICES, LLC (SUBSURFACE EXPLORATION AND TESTING); STEVEN W. NICOSON, P.E., GEI CONSULTANTS, INC. (EXCAVATION SUPPORT SYSTEM); RONALD P. PALMIERI, P.E., GEI CONSULTANTS, INC. (ROADWAYS/PAVEMENTS); FERNANDO SARABIA, PH.D., AECOM (SOIL PROPERTIES AND LABORATORY TESTING); AND DAVID ZENG, PH.D., FRANK H. NEFF PROFESSOR AND CHAIRMAN, DEPT. OF CIVIL ENGINEERING, CASE WESTERN RESERVE UNIVERSITY (TECHNICAL EDITOR)

§ 1.1 Soil Properties and Laboratory Testing§ 1.1.1 Introduction§ 1.1.2 Soil Classification

Gradation and PlasticitySoil Volume and Density Relationships

§ 1.1.3 Flow of Water§ 1.1.4 Concept of Stresses§ 1.1.5 Soil Compressibility§ 1.1.6 Strength

§ 1.2 Subsurface Exploration and Testing§ 1.2.1 Soil Borings

SamplingBoring AdvancementCone Penetrometer Testing

§ 1.2.2 In-Situ Testing§ 1.2.3 Test Pits and Trenches

§ 1.3 Ground Improvement§ 1.3.1 Dynamic Compaction

IntroductionSoil TypesDepth and Degree of ImprovementDesign RequirementsMonitoring ImprovementSite ConstraintsUtilities and Buried Structures

§ 1.3.2 Lime StabilizationIntroductionEngineering PropertiesDesign Lime ContentInstallation MethodsCaution

1

kub87633_01_c01_p001-054.indd 1kub87633_01_c01_p001-054.indd 1 10/2/12 1:50 PM10/2/12 1:50 PM

§ 1.4 Foundations§ 1.4.1 Bearing Capacity

Design Charts§ 1.4.2 Settlement

Allowable SettlementTypes of Settlement

§ 1.4.3 Foundation TypesShallow FoundationsDeep Foundations

§ 1.5 Excavation Support System Design and Construction§ 1.5.1 Design

Design ResponsibilitySite InvestigationsSelection of the Excavation Support SystemsDesign Engineering

§ 1.5.2 ConstructionGeneral Construction MonitoringPre- and Post-Construction SurveysInstrumentation

§ 1.6 Roadways/Pavements§ 1.6.1 Subgrade Preparation§ 1.6.2 Pavement Design

TrafficDesign PeriodSubgrade PropertiesBase CoursePavement Surface

§ 1.6.3 Designs

2 Infrastructure from the Ground Up


§ 1.1 SOIL PROPERTIES AND LABORATORY TESTING

§ 1.1.1 Introduction

The materials that form the earth’s surface can be arbitrarily divided into rock and soil. The term “rock” typically defines large-sized solid particles formed by minerals strongly bonded to one another. “Soils” are a system of small-sized particles that are weakly bonded together. This particulate system is formed by minerals and, in some cases, organic materials. These particles or grains are derived from rocks subjected to mechanical or chemical processes.

§ 1.1.2 Soil Classification

Gradation and PlasticityThe mechanical properties of the soil, such as its compressibility, strength, permeability, and so forth, depend on the properties of the different particles that comprise it. One of the most important properties of these particles is their size. Soil particle sizes can vary from a fraction of a micron (1 micron = 10-6 m) to a few centimeters.

Relatively simple laboratory tests can be performed on soil samples to obtain the particle size distribution of the soil mass. The particle size distribu-tion, or percentile distribution, in weight, of particles with similar diameters can be determined following standard procedures recommended by ASTM International (ASTM), listed below:

• “Standard Test Method for Particle-Size Analysis of Soils,” ASTM D422-63

• “Standard Test Methods for Amount of Material in Soils Finer than No. 200 Sieve,” ASTM D1140-00

The gradation test is used for soils containing primarily coarser grains (i.e., particle sizes larger than 0.075 mm). This test, often referred to as Sieve Analy-sis, uses a series of sieves to separate the soil into groups of particles of similar sizes. A graduated scale then weighs the amount of soil with similar sizes.

Sedimentation analysis estimates the particle size distribution when the particle size is too small for using sieves. This test method mixes fine soils with water and uses a hydrometer to estimate the particle size distribution of the solids in suspension.

Although fine-grained soils can be classified in terms of their grain size, their mechanical properties are best defined by their plasticity. Plasticity measures the tendency of soil to act like a fluid or solid when it is mixed with water. The Plasticity Index can be correlated to the actual mineralogy of the soil and its mechanical properties. The most common way to measure plas-ticity is through Atterberg Limit tests:

• “Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils,” ASTM D4318-10

Geotechnical Engineering 3


In geotechnical practice, gradation and plasticity tests are typically per-formed in representative soil samples. When test results are not available, experienced engineers can estimate their classification by visual inspection of the soil samples.

Soil particle distribution, in conjunction with the soil’s plasticity, is used to classify the soil into groups that share similar properties and uses in civil engineering practice. Different classification systems exist in the industry. The Unified Soil Classification System is widely utilized in most geotechnical work and has been adopted by the United States Bureau of Reclamation and Corps of Engineers. The AASHTO classification, proposed by the Highway Research Board’s Committee on Classification of Materials for Subgrades and Granular Type Roads, is widely utilized to relate the different groups of soils with their suitability as roadway subgrade materials. Even though significant differences exist among the classification systems, they all classify soil into four groups: clay, silt, sand, and gravel.

Soil Volume and Density RelationshipsSoil is a three-phase material, typically formed by a system of grains (solids), air (gases), and water (fluids). The relative amount of solid, gas, and fluid per unit volume define, among other things, the density of the soil. The soil density is an indirect measurement of the tightness of the solid structure: the tighter the solid particle arrangement, the less compressible is the soil and the higher strength the soil exhibits.

Different tests conducted in the laboratory environment determine the volume-weight relationship between the different phases of soil. These tests include:

• “Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass,” ASTM D2216-10

• “Standard Test Methods for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density,” ASTM D4254-00

§ 1.1.3 Flow of Water

The voids encountered between the soil particles form pathways or channels for water to flow through the soil mass. The size of these channels determines the “permeability” of the soil—the ability of fluids to flow through the soil. “Permeability” may be measured in the laboratory environment via the fol-lowing tests:

• “Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter,” ASTM D5084-10



§ 1.1.4 Concept of Stresses

The stresses applied to soils are carried in part by the solid particles and in part by the fluids (water) in the soil skeleton void space. “Effective stresses” are the stresses carried by the soil skeleton and transmitted across the soil particles through the points of contact of the soil grains. The “pore water pressure” is the pressure within the fluid. The magnitude of the effective stress influences the compressibility and strength of the soil.

§ 1.1.5 Soil Compressibility

The deformation of soils is time-dependent and can occur almost instanta-neously or over long periods of time after the application of a load. When the soil is compressed, the applied pressure is initially transferred to the water trapped in the void space. Because water is practically incompressible, no ini-tial volume change is apparent. Over time, as the water is squeezed out of the soil, the stresses applied to the fluids are transferred to the compressible soil skeleton. The rate of settlement depends on the amount of time it takes for the fluids to be squeezed out of the soils (i.e., the permeability). For soils with high permeability, such as sands or gravels, where the fluids flow at high rates, settlements and soil deformations occur almost immediately. For soils with low permeability, such as clays and silts, where the fluids flow at slow rates, settle-ment occurs very slowly over long periods of time.

After the water has been squeezed out of the soil mass and the excess pressure in the water has dissipated, the total applied pressure is carried by the soil skeleton, increasing the effective stress in the soil.

Water is almost incompressible. For this reason, the amount of settle-ments or soil deformations depends largely on the compressibility of the soil skeleton. Field and lab tests can assess the compressibility of the soil skeleton.

Since the compressibility of the soil depends on its density, it is neces-sary to obtain undisturbed samples to test in the laboratory. Because it is very difficult to obtain undisturbed samples of granular soils (such as sands and gravels), the compressibility of these type of soils is usually determined using tests performed in the field. Laboratory tests are typically performed in fine-grained soils. The most common laboratory test utilized to measure soil compressibility is:

• “Standard Test Methods for One-Dimensional Consolidation Proper-ties of Soils Using Incremental Loading,” ASTM D2435-11

§ 1.1.6 Strength

The maximum pressure that a soil can sustain is related to its so-called soil strength. The frictional resistance of the soil grains and cohesive forces



provide the strength of a soil. In other words, the soil strength is provided in part by the grain-to-grain frictional resistance (at the points of contact), which depends on the grains’ type, shape, and size, and by cohesive resistance, which depends primarily on cementation and capillarity stresses.

Cohesive forces tend not to be significant in coarse-grained soils, such as sands and gravels. Thus, they are typically referred to as “cohesion-less soils.” Therefore, the inter-particle friction primarily provides the strength of cohesion-less soils.

Cohesive forces are significant in fine-grained soils. Thus, plastic silts and clays are typically referred to as “cohesive soils.” Cohesive forces tend to vanish with time as capillarity forces decrease. When the water is allowed to flow, pore water pressure dissipates. For this reason, strength of the soil is also time-dependent. Lab tests can determine soil strength via the following standard test procedures:

• “Standard Test Method for Unconfined Compressive Strength of Cohesive Soil,” ASTM D2166-06

• “Standard Test Method for Direct Shear Test of Soils under Consoli-dated Drained Conditions,” ASTM D3080-11

• “Standard Test Method for Unconsolidated-Undrained Triaxial Com-pression Test on Cohesive Soils,” ASTM D2850-03a

• “Standard Test Method for Consolidated Undrained Triaxial Com-pression Test for Cohesive Soils,” ASTM D4767-11

§ 1.2 SUBSURFACE EXPLORATION AND TESTING

The geotechnical engineer uses various exploration methods and techniques to obtain necessary soil information, including soil borings, cone penetrom-eter testing, in-situ testing, and test pits or trenches. Typically, the geotechni-cal engineer dictates the exploration plan, including sampling, testing, and the number of test locations, based on the proposed construction and the anticipated soil conditions.

§ 1.2.1 Soil Borings

Soil borings are commonly used for subsurface exploration. Soil borings are small diameter holes drilled vertically into the ground. Borehole diameters may vary but are typically three to six inches in diameter. The geotechni-cal engineer determines the sampling interval for the borings. Borings are accomplished using a machine mounted to a truck, skid, all-terrain-vehicle, or barge, depending on the site conditions. Where site conditions present challenging conditions, such as low overhead or steep slope, the geotechni-cal engineer may resort to performing borings by manual auger.



SamplingThe two most common ways to obtain samples are by driven split-barrel (also known as split-spoon) and pushed Shelby tube methods.

• ASTM D1586-11: “Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils”

• ASTM D1587-08: “Standard Practice for Thin-Walled Tube Sampling of Soils for Geotechnical Purposes”

The split-spoon sampling method may be used on any type of soil. In general, the split-spoon sample is driven into the soil 18 inches using a 140-pound hammer dropping 30 inches in free fall. The energy required to drive the sampler is measured by how many hammer blows per each six-inch increment. The last foot of blows are added together to provide a “blow count,” or “N-value,” which is used to determine the in-situ relative den-sity of the soil. The blow count is not typically used to classify strength in cohesive soils. The samples obtained from the split-spoon sampler should be considered disturbed soils and are typically used for visual classification and laboratory classification testing, such as moisture content and Atterberg limit testing.

Shelby tube sampling is used for obtaining cohesive soil samples with minimal disruption of the sample. The Shelby tube consists of a steel tube, typically two to three inches in diameter, which is pushed into the cohe-sive soil, retracted after a period of time, and returned to the soil laboratory, where the samples are extruded for further testing. The Shelby tube sampler provides a less-disturbed sample than the split-spoon method. If the goal is the least-disturbed samples, then an Osterberg piston sampler is the preferred equipment as it uses fluid pressure.

Boring AdvancementThe industry has developed many methods for advancing the boreholes. The method selected should take into account the anticipated soil conditions, the depth of the borings, and local practice. Borings performed for geotechnical purposes are most commonly advanced using solid stem, hollow stem, or rotary drilling methods.

The solid stem auger method advances no casing; it typically limits the boring depth to areas of relatively stiff cohesive soils and granular soils above the water table.

The hollow stem auger method consists of an outer auger with a plug in the base. To sample the soil, the plug is removed and replaced by the sam-pler. For this method, the auger serves as casing against squeezing or slough-ing soils. If this method is used in granular soils below the water table, the soils at the base of the auger may loosen and create a blow-in, which in turn produces a lower N-value (an artificially lower strength).

Rotary drilling methods are commonly used for deep soil borings. This method uses smooth-sided casing and drilling mud to maintain an open



borehole. Casing is either used as surface casing or continuously, depending on the soil conditions. The drilling mud is used to help maintain an open borehole.

Rock coring is necessary for sites where the rock properties will be used for design or excavation purposes. Core barrels are typically 5 to 10 feet in length and range from 1 7/8 inch to 3 7/8 inches in diameter. The barrel is connected to either a diamond- or carbide-tipped drill bit.

Groundwater readings should be recorded when encountered during drill-ing and after boring completion. After completion of the drilling, sampling, and groundwater readings, the boreholes are typically backfilled using soil cut-tings from the boreholes, bentonite chips, and a cementitious grout. It is impor-tant to backfill borings properly as improper backfilling may provide a path for groundwater flow or may leave surface voids as the backfill material settles.

Cone Penetrometer TestingCone Penetrometer Testing (CPT) has become a popular testing method over the last two decades. In the cone penetrometer test, a drill rig pushes a cone tip with a friction sleeve into the ground. The CPT measures the tip resistance and friction resistance, respectively, as well as pore water pressures. One of the primary advantages of the CPT is a near-constant readout that provides a continuous soil profile. Soil sampling is not performed in CPT boreholes.

§ 1.2.2 In-Situ Testing

In-situ tests are performed in the soil boring, typically by lowering apparatus down into the borehole. One of the main benefits of in-situ testing is the ability to test soil in a minimally disturbed state. In-situ testing is often combined with Vane Shear Testing (VST), which consists of a four-bladed vane that is pushed into soft to medium-stiff cohesive soils. As the vane rotates in the cohesive soils, it measures the resistance and provides the unconfined compressive strength.

• ASTM D2573-08: Standard Test Method for Field Vane Shear Test in Cohesive Soil

Pressuremeter testing (PMT) consists of an expandable cylindrical probe that is inserted into the borehole and expanded radially against the walls of the borehole. The stress-strain curves are measured to provide soil stress-strain and strength characteristics of the soil tested.

• ASTM D4719-07: Standard Test Methods for Prebored Pressuremeter Testing in Soils

§ 1.2.3 Test Pits and Trenches

Test pits or trenches are very useful tools for observing bedding planes, subsurface obstructions, and existing structures, such as adjacent building



footings. Test pits are also very helpful for observing fill conditions at previ-ously developed sites. Common obstructions in fill soils in urban environ-ments include construction debris and abandoned foundations from previous structures at the site. The depth and lateral limits of the obstructions should be determined by the test pits. Test pits are typically used in combination with soil borings and not as the only source of geotechnical information. In-place density testing can be performed in the base of the excavation, and bag or bulk sampling can also be obtained from the excavated material.

§ 1.3 GROUND IMPROVEMENT

Buildings, roads, and other constructed facilities are often proposed for sites that have poor ground conditions. Development on such sites is increasing due to the unavailability of sites with good ground support conditions. Previ-ous solutions included the use of deep foundations or conventional removal and replacement techniques. Ground improvement techniques are now avail-able to allow these new structures to be built directly on the improved ground.

§ 1.3.1 Dynamic Compaction

IntroductionDynamic compaction is an improvement technique that can treat the ground to depths up to 30 feet. Dynamic compaction is defined as the densifica-tion of the ground by repeatedly dropping a heavy weight onto the surface. Other terms such as heavy tamping, pounding, and dynamic consolidation have been used for dynamic compaction. Dynamic compaction is typically performed with weights ranging from 6 to 30 tons. The weights are lifted and dropped at free-fall from heights ranging from 30 to 75 feet. Figure 1.1 illustrates the dynamic compaction process. This process temporarily leaves craters across the surface at the weight impact points as shown in figure 1.2.

Dynamic compaction restructures the soil grains into a denser state at lower moisture content and/or collapses voids within the ground mass. But dynamic compaction is distinct from conventional fill compaction in the fol-lowing ways:

• During conventional compaction, soil is placed in multiple thin loose lifts of 12 inches or less and then compacted layer by layer from the bottom up. In dynamic compaction, the deposits are compacted in-situ in one lift thickness from the ground surface down to some predetermined depth.

• Conventional compaction equipment can generally densify materi-als to a depth of about 2 to 4 feet, whereas dynamic compaction can improve the ground to depths of 15 to 30 feet.



• The conventional method restricts compaction to particles under six inches, whereas dynamic compaction can be used on larger parti-cles, such as broken concrete and boulders.

• Dynamic compaction is effective both above and below the ground-water table. However, the working surface needs to be maintained about five to six feet above the water table. Conventional compac-tion must be performed completely above the water table.

(A) Advantages of Dynamic Compaction

• Simple equipment• Serves as both an exploration and improvement technique• Degree of compaction can be observed in the field as the work

progresses• Applies to a wide variety of material types• Minimizes differential settlement by providing more uniform bearing

conditions• Materials below the water table can be densified• Can be performed in wet weather and with limited frost penetration• Generally less expensive than other ground improvement techniques

w

D=Depth of improvement

H

Figure 1.1 Dynamic CompactionFigure 1.2 Dynamic Compaction

Craters



(B) Disadvantages of Dynamic Compaction

• Generates off-site vibrations• Causes several inches of lateral ground movement, which may

impact buried utility lines• Ground surface must be five to six feet above the water table for this

method to be effective• Very loose deposits, such as recent landfills, require addition of

imported granular material to provide a working platform for crane, limit penetration of weight, and provide confinement of weak deposits

Soil TypesDynamic compaction can be used on a wide variety of materials ranging from naturally occurring soils to heterogeneous fill deposits. This partial list identifies materials where dynamic compaction has been utilized:

• Natural, loose gravel, sand, and silt• Landfill material• Building rubble and construction debris• Mine spoil• Partially saturated clay soils above the water table• Collapsible soils such as loess• Near surface karst or sinkhole formation• Loose liquefiable sand and silt• Special waste

Dynamic compaction is performed on materials at their prevailing moisture content. During dynamic compaction, the energy imparted into the ground increases the pore water pressure between the soil particles. The material must be permeable enough for the pore pressure to dissipate quickly, thereby permitting the soil particles to move into a denser state of packing and increase the density of the deposit—meaning materials with a higher permeability and better drainage features are the most favorable can-didates for dynamic compaction. Consequently, low permeability materials are not favorable for dynamic compaction. Following this reasoning, soils are typically grouped into the following three major categories as shown in figure 1.3.

(A) Most Favorable Soil Deposits—Zone 1Dynamic compaction is most effective on materials with a low degree of saturation and high permeability—pervious granular deposits such as natu-ral sand and gravel, building rubble, slag, and decomposed refuse deposits. Above the water table, these materials move into a denser state of packing during densification. Below the water table, the permeability of these deposits



is high enough that excess pore water pressures generated during the impact of the weight dissipate quickly and densification is immediate.

(B) Unfavorable Soil Deposits—Zone 3Saturated, clayey soils are not appropriate for dynamic compaction. Clayey soils have low permeability such that dissipation of excess pore water pres-sures generated during dynamic compaction cannot dissipate quickly as shown in figure 1.4.

(C) Intermediate Soil Deposits—Zone 2Silt, clayey silt, and sandy silt fall into an intermediate category (Zone 2) between the most favorable (Zone 1) and unfavorable soils (Zone 3) for dynamic compaction. These deposits have an intermediate permeability fall-ing between the pervious granular soils from Zone 1 and the impervious clay soils from Zone 3. Due to the lower permeability, dynamic compaction must be performed in multiple passes to allow excess pore pressure to dissipate. This process may take days to weeks.

Perc

en

t fi

ne

r b

y w

eig

ht

Perc

en

t c

oa

rse

r b

y w

eig

ht

100

90

80

70

60

50

40

30

20

10

0 100

90

80

70

60

50

40

30

20

10

0

5 mm 0.5 mm 0.75 mm 0.005 mm 0.001 mm

4 40 200U.S. standard sieve numbers

Grain size in millimeters

Silt or clayFineMediumCoarse

Sand

Zone 3

Impervious soils P.I.>8permeability less than

1 x 10-6 cm/sec

Zone 1

Pervious soilsplasticity index (P.I.) = 0

permeability greaterthan 1 x 10-3 cm sec

Zone 2Semi-PerviousSoils O< P.I.<8

Permeability in theRange of

1 x10-3 to 1x 10-6 cm/sec

Figure 1.3 Grouping of Soils for Dynamic Compaction

Source: Robert G. Lukas, U.S. Dep’t of Transp., Fed. Highway Admin., Dynamic Compaction, 1 GEOTECHNICAL ENG’G CIRCULAR (Pub. No. FHWA-SA-95-037, Oct. 1995).



Depth and Degree of ImprovementDynamic compaction is performed to improve the geotechnical properties of the deposit to a certain depth below grade, dependent on the desired perfor-mance of the structure. For example, a building can tolerate less settlement than an earth embankment, and, therefore, the depth of improvement typically needs to be greater for a building. Dynamic compaction creates a more uni-form condition across the site, which reduces the risk of differential settlement.

Several elements impact the depth and degree of the ground improve-ment. The depth of improvement typically increases with the amount of energy applied. Crane capacity places a limitation on the depth of improve-ment. The largest cranes normally available have 150- to 175-ton ratings. These cranes can lift 20- to 22-ton weights with a maximum drop height of 66 to 98 feet. The depth of improvement is also influenced by the con-tact pressure of the weight. The contact pressure is the weight of the tamper divided by the contact area. Contact pressures typically range from 800 to 1,600 pounds per square foot (psf). If the contact pressure is too low, the depth of improvement would be reduced. A weight with a low contact pres-sure, as shown in figure 1.5, is used for final “ironing” of the site, where it is desired to limit the depth of improvement to the depth of the craters.

0 2 4 6 8 10 12 14 16 18 20

Legend

Piezometer at 10ft. depthPiezometer at 15ft. depth1 inch of mercury ≈ 1 ft. of water 1 psi = 2.3 ft. of water

+6

+4

+2

0

-2

-4

-6

-8

Days after dynamic compaction

Pie

zo

me

ter

rea

din

g-

= in

ch

es

of

me

rcu

ry+

= p

ou

nd

s p

er

sq

ua

re i

nch

(P

SI)

BeforeDynamicCompaction

Before DynamicCompaction

Figure 1.4 Pore Pressure Dissipation



The average improvement is less than the maximum improvement. The maximum improvement typically occurs at a depth range of one-third to one-half of the maximum depth of improvement. Lesser improvement occurs at shallower depths due to the penetration of the weight. The improvement also diminishes at greater depths. Figures 1.6 and 1.7 illus-trate dynamic compaction improvement based on standard penetration and pres-suremeter testing.

Figure 1.5 Ironing Weight

50/3"

50/3"

0.0

2.0

5.0

7.5

12.0

10.0

15.0

17.0

20.0

0 10 20 30 40 50 60

Standard Penetration Test -"N" (BPF)

Dep

th (

FT

)

Before Dynamic compaction

After Dynamic compaction with 15 ton weight

Fill: Fine coarse slag;cinders gravel, brick, and broken concrete

Weight of hammer

Weight of hammer

Figure 1.6 Dynamic Compaction Improvement-Standard Penetration Testing Results



Design RequirementsA geotechnical engineer must address the following items when developing a dynamic compaction design plan:

• Weight and drop height• Applied energy• Area to be densified• Grid spacing and number of drops• Number of passes• Need for a surface-stabilizing layer

The weight and drop height will be selected to compact the loose depos-its to the required depth as defined from a subsurface exploration program.

0.0

2.0

5.0

7.5

12.0

10.0

15.0

17.0

20.0

0 10 20 30 40 50 60

Limit Pressure (TSF)

Dep

th (

FT

)

Before DC

After DC with 15 ton weight

Fill: Fine coarse slag;cinders gravel, brick, and broken concrete

Figure 1.7 Dynamic Compaction Improvement-Pressuremeter Testing Results



A sufficient amount of energy must be applied during dynamic compaction to cause ground compression to yield the improvements required in the design. The applied energy is a function of the num-ber of drops, the mass of the weight, the drop height, the number of passes, and the impact grid spacing. The initial den-sity of the deposit will also influence the selected energy.

Dynamic compaction is generally performed over an area larger than the footprint of the structure to compact the soils within the zone of influence or pres-sure distribution surrounding the struc-

ture. On most projects, dynamic compaction is performed a distance beyond the edges of the structure equal to the depth of the compacted deposit.

Dynamic compaction is typically performed on a grid spanning the area to be compacted as shown in figure 1.8. Continuous application of the weight to the area is not necessary since the energy distributes laterally away from the weight into the ground mass. A drop spacing of 1.5 to 2.5 times the diameter of the weight or 7 to 10 feet is typical. The number of drops at each grid point depends on the applied energy, the weight mass and drop height, grid spacing, and the number of passes. Normally, 7 to 15 drops per grid point are used. The grid spacing should be adjusted to maintain the number of drops in this range. Figure 1.9 shows a typical dynamic compaction plan.

The surface soils are generally loosened during dynamic compaction to a depth equal to the crater depth. This loosened zone is compacted by a low-energy application called an ironing pass. After leveling the craters with a bulldozer, a square tamper with a low-contact pressure is generally used to iron out the area. The ironing occurs continuously across the surface. If the crater depth is less than 18 to 24 inches, the ironing can be performed using a conventional compactor after the surface is leveled.

The number of drops that can be applied at a grid location is limited by the crater depth. Crater depths that are greater than the height of the weight can cause problems such as cable breakage, crater collapse, and excess dis-turbance of the surface materials. The crater depths should be limited to the height of the weight plus one foot. If the required amount of energy cannot be applied without creating deep craters, dynamic compaction should be per-formed in multiple passes. As previously discussed, multiple passes may also be required in semi-pervious (Zone 2, in figure 1.3) soils to allow for dissipa-tion of high pore pressure caused by the initial pass of dynamic compaction.

At sites where the surface is extremely loose, such as landfills, it may be necessary to add granular material to stabilize the surface. The granular mate-rial creates a working mat that provides a stable platform for the dynamic

Figure 1.8 Dynamic Compaction Grid



compaction crane. In addition, the granular stabilizing fill will help to limit the crater depths. Working mats are typically one to four feet thick and consist of coarse-grained granular mate-rial such as gravel, crushed rock, or building rubble.

Working mats should be avoided where possible since the dense surface material can limit the penetration of energy to the weak deposit. Heavier weights are typically required where working mats are neces-sary so as not to limit the depth of improvement.

Monitoring ImprovementSoil boring data allows the engi-neer to monitor the dynamic compaction process as the work is being performed and after completion. During the work, production control testing con-sists of monitoring crater depths and ground settlement. After dynamic compaction, conven-tional soil boring sampling and testing assesses the compaction.

Crater depths should be measured and plotted as the work progresses. A pattern of excessive crater depths may indicate the presence of a weak layer and the need for another pass of dynamic compaction.

Ground heave is also moni-tored and measured if it occurs. The ground heave occurs between the craters if plastic

deformation takes place. Plastic deformation is associated with high excess pore water pressures. The continuation of dynamic compaction must be delayed until the excess pore pressures have dissipated. This condition may require the use of multiple passes to allow the pore pressures to dissipate.

Figure 1.9 Dynamic Compaction Plan

Buliding exterior wall

Impact Point(Crater)

Grid Pattern9 ft. x 9 ft.



Once the area has been compacted and leveled, and craters have been filled, the surface should be treated with an ironing pass or compaction. The area will then be surveyed on a grid and compared with the elevations before dynamic compaction. The elevation change represents the average ground settlement that occurred during the dynamic compaction. Typically, the average ground settlement is on the order of 5 percent to 10 percent of the thickness of the densified zone.

Following dynamic compaction, verification testing confirms the level of compaction. These tests are performed through soil borings near the location of similar borings prior to dynamic compaction—to provide a basis of com-parison. A minimum of one boring with testing should be performed for each 10,000 square feet of densified area. The borings should extend through the deposit to be densified and should not be performed until excess pore pres-sures have dissipated.

Site ConstraintsA heavy weight striking the ground during dynamic compaction causes vibra-tions to be transmitted away from the point of impact and may travel off-site. Also, lateral ground displacements occur that may cause utility lines or other structures to shift during dynamic compaction. Airborne debris may cause injury or damage to occur.

(A) Ground VibrationsThe dynamic compaction weight striking the ground causes vibrations on and around the site. These vibrations travel varying distances away from the point of impact. On a large site, where dynamic compaction occurs at the interior of the site, the off-site effects are usually negligible. Dynamic compaction performed near the boundaries of the property, however, may cause vibrations that travel off-site and cause human annoyance or damage to property. The U.S. Bureau of Mines has established threshold particle velocities related to cracking of walls in residential structures as shown in figure 1.10. Precondition surveys of the adjacent facilities should also be performed to document preexisting conditions prior to dynamic compaction. Critical structures may be protected by digging isolation trenches between the dynamic compaction and the structures as shown in figure 1.11.

(B) Lateral Ground DisplacementLateral displacements will occur in the ground due to dynamic compaction. These displacements are greatest near the point of impact. For low energy applications with 6-ton weights and 30- to 40-foot drop height, displacements of about 0.1 inches are anticipated a distance of 25 feet from impact. For higher energy applications (16- to 30-ton weights and 65- to 100-foot drop heights), displacements on the order of 1 to 5 inches for clay soil or 10 to 14 inches for



sandy soil are expected at a distance of 10 feet from impact. At a point 20 feet from the impact, a displace-ment of 0.5 to 3 inches is anticipated for these higher energy applications. Close to the point of impact, the maxi-mum displacement occurs about 5 to 15 feet below the ground surface. However, at a distance of 20 feet or more, the maximum displacement is closer to the ground surface.

Based on these parameters, high-energy dynamic compaction should not be performed within 25 feet of a structure situated within the upper 30 feet below the ground surface. This may include buried utilities as well as foundations.

(C) Airborne DebrisDebris may become airborne when the weight impacts the ground. This debris may range from large particles or pieces of debris to mud or dust. Site personnel should wear protective safety equipment, such as hard hats and eye protection, and should main-tain a safe distance from the work. Protective shields such as traveling plywood boards can be placed near the drop point to deflect flying debris.

Utilities and Buried StructuresThe effect of permanent lateral dis-placement should be considered to protect buried utility lines or other belowground structures close to the

dynamic compaction work. Field measurements of lateral displacement and vibrations should be utilized to evaluate potential damage to these structures.

Studies indicate that buried pipes can withstand a particle velocity of three inches per second. Measurements on high-pressure pipelines indicate the particle velocities of 10 to 20 inches per second have not caused dam-age, undoubtedly because of the confinement on the buried pipe. Other stud-ies have shown that reinforced concrete foundations can withstand particle velocities of 10 inches per second or higher.

Figure 1.11 Isolation Trench

10.0

1.0

.11 10 100

Frequency, Hz

Pa

rtic

le V

elo

cit

y, in

/se

c

0.75 IN/SECDrywall

0.50 IN/SECPlaster

SISKIND (1980)

2 IN/SEC

Figure 1.10 U.S. Bureau of Mines Vibration Criteria

Source: D.E. SISKIND ET AL., BUREAU OF MINES, DEP’T OF INVESTIGATION, STRUCTURE RESPONSE AND DAMAGE PRODUCED BY GROUND VIBRATIONS FROM SURFACE MINE BLASTING, RI 8507 (1980).



§ 1.3.2 Lime Stabilization

IntroductionUnsuitable ground may also be improved using soil stabilization tech-niques. Soil stabilization involves improvement of a soil for use as a base for pavements and floor slabs. Soils that are unsatisfactory in their natural state can be altered using admixtures. One such admixture is lime. When lime is added to a fine-grained soil, several chemical reactions occur. Cation exchange is a reaction where the calcium in the lime replaces the cations in the soil particles. Flocculation and agglomeration takes place, with the soil particles attracting and bonding to one another. Cementitious bonds also form via reaction with the alumina and silica in the soil. This reaction is similar to the hydration of Portland cement and is called a poz-zolanic reaction.

Engineering PropertiesThe various chemical reactions associated with lime stabilization modify the engineering properties of the soil.

(A) Plasticity IndexThe liquid limit is normally reduced and the plastic limit will increase by lime treatment. The result will be a substantial reduction in the plasticity index. In many cases, the soil becomes nonplastic. The soil becomes more workable since it becomes more silty and friable.

(B) SwellingSwelling is an undesirable characteristic due to the effect on overlying pave-ments and floor slabs. Lime treatment substantially reduces the swell poten-tial of fine-grained soils. The lime-soil mixture has a decreased affinity for water due to the calcium content, and the cemented nature of the mixture also helps to resist volume change.

(C) California Bearing RatioThe lime-soil mixture exhibits an increased California Bearing Ratio (CBR) due to flocculation and agglomeration reactions. Following lime treatment, it is not uncommon to see five- to ten-fold increases in the CBR value. The CBR is a commonly run test to predict the support capability of a subgrade and to design bituminous concrete pavements.

(D) StabilityThe modulus and shear strength of a lime-treated soil will also increase. The increased stability occurs immediately due to flocculation and agglom-eration. Lime treatment can yield increases in compressive strengths of fine-grained soils up to 500 pounds per square inch (psi) as shown in figure 1.12. A flexural strength of 75 to 100 psi or greater may be achieved in lime-treated soil. This change tightens up the plastic subgrade and creates a working table for expediting construction operations.



(E) Moisture ResistanceLime-treated soils display increased resistance to moisture, which is consis-tent with the treated soil’s decrease in swell potential. The permeability of the lime-soil mixture is also reduced. This increased resistance to moisture makes it possible to proceed with construction operations on lime-treated soil during wet construction seasons.

Design Lime ContentLime contents for typical fine-grained soils normally range from three to eight percent by weight of dry soil. Soils with higher plasticity and clay content require a higher lime percent-age. Treatments less than two to three percent are generally not used due to practical construction considerations related to distributing and mixing these small quantities. If the strength of the lime-treated soil does not increase at least 50 psi, the soil is considered non-reactive and will not produce a pozzo-

lanic reaction. Lime will improve the workability of these soils by reducing the PI. Therefore, the mix design of these soils is based on PI reduction. For reactive soils, the design lime content is based on increases in unconfined compressive strength. The authors recommend that a mix design be performed using the site soils and proposed lime to determine the lime content and effect on the soil.

Installation MethodsRelatively routine equipment is necessary for soil-lime construction. Lime can normally be spread by bulk tankers. If the soil is very wet and soft, special equipment such as bulldozer-towed trailers may be required. The authors recommend pulverizing equipment be used to mix the lime with the soil. This results in a more uniform lime soil mixture. This equipment may be utilized to stabilize soil to a depth of 12 inches. Conventional compaction equipment also may be used for soil-lime construction.

CautionLime is often used at the end of the construction season in an effort to “dry out” the soil. Before applying lime, a mix design should be developed with consultation by the geotechnical engineer. The authors have been called into

500

400

300

200

100

00 1 2 3 4

% Strain

Str

es

s (

PS

I)

5% LimeModulus = 54,000 PSI

Natural SoilModulus = 5,700 PSI

Figure 1.12 Typical Stress-Strain Curves for Natural and Lime-Treated Soil

Source: M.R. THOMPSON, LIME STABILIZATION FOR PAVEMENT CONSTRUCTIOn (Dep’t of Civil Eng’g, Univ. of Ill. 1971).



a number of projects where lime was not applied properly or the site condi-tions (perched water table) were not favorable for lime stabilization and pre-mature slab and pavement distress has occurred.

§ 1.4 FOUNDATIONS

The purpose of a structure’s foundation is to spread out and transmit the con-centrated structure loads into the ground at an intensity that the ground can withstand without causing unacceptable total or differential settlement of the structure. Foundations can lessen the intensity of a structure’s load by spread-ing out concentrated loads over a large area of ground or by extending the foundation deep into the ground until it reaches a soil or rock layer capable of supporting the structure.

Figure 1.13 shows the Second and Great Pyramids of Giza, Egypt, con-structed around 2600 BC. Until the twen-tieth century, the Great Pyramid was the heaviest structure ever constructed by humans, weighing about 16 billion pounds. And yet, the Great Pyramid was constructed without any real foundation, supported directly on bedrock, trans-mitting an average bearing pressure of around 28,000 psf. Despite this high pres-sure over an immense area, the pyramid likely settled less than two inches.

Approximately 4,500 years later, in 1899, humans designed the Auditorium Building in Chicago (figure 1.14), support-ing it on shallow spread footings designed for a constant bearing pressure of 4,000 psf. The footings were supported by a crust of very stiff clay over deep soft clay. Despite weighing perhaps 1/400th of the Great Pyramid and imparting 1/7th the pressure on its foundations, the Audito-rium settled about 20 inches differentially. As a result of this poor performance, mod-ern Chicago structures over four stories in height are now typically supported on deep foundations.

Two of the world’s tallest build-ings, the Petronas Towers (figure 1.15), are supported on perhaps the deepest

Figure 1.13 The Pyramids of GizaThe Pyramids of Giza, Egypt, constructed circa 2600 BC are supported directly on bedrock.

Figure 1.14 The Auditorium BuildingThe Auditorium Building in Chicago, constructed in 1889, is on a shallow footing foundation.



abeck

Text Box

note: we added a reference to figure 1.15

foundations ever constructed. The aver-age pressure below the two towers is approximately equal to the pressure exerted below the Great Pyramid. To achieve an allowable settlement of less than 1 inch, the Petronas Towers were supported on barrettes that extended as deep as 420 feet (approximately the height of the Great Pyramid) below the ground surface.

The geotechnical engineer’s role is to determine the depth to a suitable bearing layer for a structure’s foundation, estimate the bearing capacity of the soil or rock, and estimate the likely total and differen-tial settlement of the foundation.

§ 1.4.1 Bearing Capacity

The bearing capacity of soil or rock is the ultimate pressure that the material can support. Generally, large, uncontrolla-ble settlement will result if a foundation exerts a pressure that reaches the bear-ing capacity of the soil or rock. Since this

would be highly undesirable for most any structure, geotechnical engineers first estimate the likely bearing capacity and then apply a factor of safety of typically 3.0 to this value to arrive at an allowable bearing pressure. The safety factor has two purposes. First, it allows for the uncertainty in estimat-ing the bearing capacity and allows for the natural variability of typical geologic strata. Second, it reduces the applied pressure significantly so that the settlement of the foundation will be within an acceptable range.

Figure 1.16 illustrates the simple concept of how a shallow footing dis-perses the applied pressure of a structural column. If the concentrated col-umn load of a multistory building is 75,000 pounds, the pressure (or stress) in the column assuming it is 1 foot square would be 75,000 psf. This pres-sure would be much too high to be supported directly on soil. However, if the column is supported on a spread footing that is 5 feet by 5 feet square, the average applied soil pressure reduces to 75,000 pounds per 25 square feet or 3,000 psf.

Three thousand psf is considered to be a common or desirable allow-able bearing pressure for shallow footings. If the allowable bearing pressure for a soil equals or exceeds this value, then typical-size footings would result for low-rise structures and the foundations are likely to be economical. If,

Alluvium

Kennyhillformation

N = 20–300

Fissuresand

cavaties

Slump zoneN = 0–20

Fissuredlimestone bedrock

RQD = 0–100

0 100M

Scale

Figure 1.15 The Petronas TowersThe Petronas Towers in Kuala Lumpur, Malaysia (1995), may be supported on the deepest barrette foundations ever constructed.



however, the allowable bearing pressure were determined to be less than 3,000 psf, excessively large footings might result and a mat foundation or deep foundation might be more economical.

Figure 1.17 depicts the same column of a multistory building supported on a pile foundation. The pile does not spread the load out laterally, but carries the column load deeper into the ground. This pile could be extended through a very soft compress-ible clay or organic layer to bear on stiffer soil or rock below, with the end bearing on the base of the pile supporting the load. Because the area at the base of the pile is only about 7 square feet, the end bearing pressure would be 10,700 psf, which a very dense soil or rock could support.

If no hard layer exists at the base of the pile, the column load could be transferred to the soil surrounding the pile shaft via side friction. In this scenario, the area of the pile shaft (236 square feet) is much larger than

the pile end bearing area or the area of the spread footing. Consequently, the 75,000-pound column load would be supported by an average side friction of only 320 psf. This low value of side friction could be supported by even a soft clay. This type of pile design is known as a friction pile.

Design ChartsThe various methods for estimating the bearing capacity of soil are depen-dent on the type of soil and the method used for estimating soil strength. For sand deposits, which cannot be sampled without disturbing the sand, bearing capacity for foundations is most usually based on the Standard Penetration Test (SPT).

Two design charts for allowable soil-bearing pressure of footings on sand are shown in figure 1.18. These charts correlate experience with actual foot-ings to the SPT blow count or “N” value. The charts show that the allowable soil pressure is influenced not only by the soil strength (as represented by the N value), but also by the width, length, and depth of the footing in addition to the location of the water table.

These charts demonstrate an important point related to footings on sand: the allowable soil pressure increases for footings up to a width of two to four feet, but then remains constant or even decreases for increased footing sizes. This leveling off, or apparent reduction in allowable bearing

Column Load75,000 lbs

5ft 5ft

Area of footing:5ft x 5ft = 25 sq.ft.

Average soil bearing pressure =

75,000 lbs/25 sq ft = 3,000 lbs/sq. ft (PSF)

Figure 1.16 A Shallow Footing Spreads Out the Concentrated Column Load to a Soil-Bearing Pressure of 3,000 psf



Column Load

75,000 lbs

Pile cap

Pile 3ft diameter x 25ft long

Shaft area = π DL = 3.143 X 3 X 25 = 236 ft2

Tip area = π D 2

4

If no friction, end bearing =75,000lbs / 7 sq.ft =10,700 lbs / sq.ft

If no end bearing,shaft friction =75,000 lbs/236 sq.ft =≈ 320 psf

= 3.14⋅3ft2

4= 7 sq.ft

Figure 1.17 A Deep, Pile Foundation Supports the Column Load with Some Combination of End Bearing and Side Friction

pressure, does not result from a reduction in bearing capacity, but is neces-sary to reduce the settlement of the footing to an acceptable level, typically to less than one inch. Thus, the allowable settlement criterion controls the allowable bearing pressure for the most common sizes of shallow footings on sand.



Figure 1.18 Design Charts for Allowable Bearing Pressure of Shallow Foundations on Sand Based on SPT Blow Counts

From Peck, Hanson, and Thornburn (1974) and Terzaghi and Peck (1967).

Width of footing, B, ft

0 5 10 15 20

6

5

4

3

2

1

0 1 2 3 4 0 1 2 3 40

(a) Df/B=l (b) Df/B=0.5 (c) Df/B=0.25

N=50 N=50

N=5 N=5

N=10 N=10

N=15 N=15

N=20 N=20

N=30 N=30

N=40 N=40

0 1 2 3 4 5 6

N=50

N=5

N=10N=15

N=20

N=30

N=40

So

il p

ressu

re, to

ns/s

q f

t

Very Dense

Dense

Medium

Loose

Width B of Footing in Feet

N=50

N=30

N=10

7

6

5

4

3

2

1

0

Allo

wa

ble

So

il P

ressu

re i

n T

on

s p

er

sq

ft

(Wate

r Tab

le B

elo

w D

ep

th 2

B)



For design of foundations supported on clay, rather than rely on the SPT, bearing capacity is typically based on the unconfined strength of the clay, which can be determined from laboratory tests performed on relatively undisturbed samples. In-situ tests using the Vane Shear or Cone Penetration Test (CPT) can also be used to measure the strength of clay.

Figure 1.19 shows a popular design chart and procedure to determine the bearing capac-ity of a foundation supported on a clay soil. The chart provides a bearing capacity factor Nc for dif-ferent foundation geometries at different depth-to-width ratios. The allowable bearing pressure of a square (or circular) footing supported on the surface of a clay deposit would be approximately equal to the unconfined strength of the clay. For deep circular foundations bearing on clay, the allowable bearing pressure for a deep foundation increases by 50 percent, approximately equal to 1.5 times the unconfined strength of the clay.

§ 1.4.2 Settlement

Allowable SettlementBearing capacity of the soil defines an ultimate limit associated with a plung-ing failure or large settlement of a foundation. Because significant settlement is usually unacceptable, foundations cannot be designed only to a bearing capacity reduced by a factor of safety. Rather, the engineer must select allow-able bearing pressures that allow total and differential settlement across the structure without resulting in detrimental structural damage.

The amount of total or differential settlement that a structure can undergo without resulting in unsightly building cracking, structural damage, or loss of intended use is dependent on the type of structure, structural system type, and planned use. The structural engineer provides the total and differential settlement tolerances for the structure. But these values should not be “zero,” because every structure that imparts a load on the ground must cause some settlement, even if it is very small.

Table 1.1 on the next page provides a guideline to the maximum accept-able total and differential settlements. Tolerable total settlements could be as large as 6 to 24 inches, provided that the settlement is uniform. Total settle-ments of most structures, however, are limited to one to two inches to limit the possibility of differential settlement exceeding tolerable levels.

Be

ari

ng

ca

pa

cit

y f

ac

tor,

Nc

Df /BRatio of depth of surcharge, Df,

to width of floating, B

0 1 2 3 4 5

10

9

8

7

6

5

4

Continuous B/L=0

Square and circular, B/L=1

Figure 1.19 Estimation of Allowable Bearing Pressure for Foundations Sup-ported on Clay from Work by Skempton

Adapted from Peck, Hanson, and Thornburn (1974).



Differential settlement within a structure is more critical than the total settlement. If different portions of a structure settle in different amounts, unintended stresses will result in the structure. If these stresses become large enough, elements of the structure will crack. For larger differential settle-ments, windows and doors would no longer open or close properly because

TABLE 1.1 Allowable Settlement

Type of Movement Limiting Factor Maximum Settlement

Total Settlement Drainage 6–12 inch

Access 12–24 inch

Utility connections 6–12 inch

Probability of non-uniform settlement:

Masonry walled structure 1–2 inch

Framed structures 2–4 inch

Smokestacks, silos, mats 3–12 inch

Tilting Stability against overturning Depends on H and W

Tilting of smokestacks, towers 0.004L

Rolling of trucks, etc. 0.01L

Stacking of goods 0.01L

Machine operation 0.002–0.003L

Crane rails 0.003L

Drainage of floors 0.01–0.02L

Differential movement High continuous brick walls 0.0005–0.001L

One-story brick building, wall cracking 0.001–0.002L

Plaster cracking (gypsum) 0.001L

Reinforced-concrete-building frame 0.0025–0.004L

Reinforced-concrete-building curtain walls 0.003L

Steel frame, continuous 0.002L

Simple steel frame 0.005L

* L = distance between adjacent columns that settle different amounts, or between any two points that settle differently. Higher values are for regular settlements and more tolerant structures. Lower values are for irregular settlement and critical structures. H = height and W = width of structure.

Source: Adapted from Sowers (1962)



of loss of alignment. Machines or mechanical equipment within the structure (such as crane rails) might cease to operate properly. Consequently, 3/4-inch allowable differential settlement between columns is a general rule of thumb.

Types of SettlementSettlement of a structure results from the compression or consolidation of the soil beneath a foundation in response to the applied load. Settlement can also occur due to external factors not related to the structure, such as adja-cent excavation for tunnels, basements, utilities, and deep foundations, that can undermine the existing foundation. Other external causes could include changes in the water table level—either due to off-site pumping or inunda-tion from burst water mains; loss of ground below a structure due to sewer or tunnel failures; and densification or vibration of the soil from pile driving adjacent to a structure.

Settlement of a foundation supported on sand or gravel can be described variably as compression, elastic, immediate, or short-term settlement. As the names imply, settlement on sand occurs relatively quickly, as the load is applied, and is largely completed by the end of construction. For rela-tively inexpensive structures, the estimation of settlement on sand is typically limited to using empirical charts based on SPT blow counts. For expensive or major structures, more sophisticated in-situ testing, such as pressuremeter testing, could be done to measure the soil stiffness.

Settlement of a foundation supported on clay is termed consolidation, which is a time-dependent phenomenon. A clay soil has low permeability due to much smaller pore spaces than sand, which means the water typically occu-pying the pore space of a clay soil cannot easily escape. When a foundation load is applied to the clay, the load is largely supported by the incompressible water in the pore space. Over time, as the water is slowly squeezed out of the pores, the foundation load shifts from the water to the soil skeleton, causing the foundation to settle. The rate and magnitude of the settlement is most typically estimated by performing laboratory consolidation tests on undisturbed sam-ples. A typical structure founded on clay might see 10 percent to 20 percent of its total settlement occur during construction, with the remaining settlement occurring over the next 10 to 20 years. Selection of the foundation type is criti-cal, as long-term settlement on clay will occur long after sensitive finishes have been installed and the building has been occupied.

§ 1.4.3 Foundation Types

The foundation type selected for a project should be the most economical available to support project loads while meeting the settlement requirements for the structure. Deep piles that result in negligible settlement are not practi-cal for every project. Selecting the most economical foundation type is based on many factors, including the building load, geologic setting, settlement



tolerance, and local availability of materials, equipment, and contractors to install a particular foundation type.

Foundations types can be roughly divided into two categories: shallow and deep.

Isolated or spread footing undera single column

Strip or continuous footingunder a wall

Combined footings undertwo or more columns

Mat or raft foundation supportingmany columns under a largeportion of a building, or the entirebuilding

Figure 1.20 Types of Shallow Foundations



Shallow FoundationsWhere light building loads permit, shallow foundations are preferred, because of ease of installation by relatively unspecialized methods. Con-structing shallow foundations can be as simple as excavating a hole and filling it with concrete. As shown in figure 1.20, shallow foundation types include isolated column footings, strip footings under walls, combined foot-ings under multiple columns, or mat foundations.

Geotechnical engineers and/or building codes specify minimum sizes for isolated column and strip footings. Shallow footings are also designed, per applicable building codes, with a minimum depth of embedment for frost heave protection and to provide adequate bearing capacity when on sand.

When isolated strip or column footings become too large due to high loads or poor soils, the footings can be combined so that multiple columns are supported on a single footing. Where possible, footings are designed so that the column or wall load is concentric with the footing, which reduces the possibility of foundation tilting. Space limitations at a property line make it common to use combined footings to lessen tilt and differential settlement that could result due to load eccentricities.

If the combined footings become too large, it is possible to combine all footings for a structure into a single thick concrete mat or raft that supports all of the columns. Mats will typically be 2 to 10 feet thick and use a consid-erable amount of concrete and reinforcing steel. Thus, while a mat may be good at limiting differential settlement, its high material cost might typically be greater than a deep foundation system that would provide similar or better settlement performance.

Mats can be more cost-effective when combined with a deep basement for a tall structure. Such a foundation is called a compensated mat or floating foundation. Every foot of soil removed in a basement reduces the pressure on the underlying bearing soil by around 125 psf, which is approximately equal to the weight of an entire floor of a building. Consequently, excavating two basement levels (or 20 feet) would be equivalent to removing the entire weight of a 20-story building. A mat constructed below this basement would be expected to undergo negligible settlement because the weight added by the 20-story building would be less than or equal to the weight removed by excavating the soil.

Deep FoundationsWhen shallow foundations are not possible because of heavy building loads or poor soils at normal excavation levels, it may be necessary to extend the foundations to a deeper bearing stratum by using pile foundations. Piles can be divided into three categories: driven piles, cast-in-place piles, and special piles.

Driven piles can be made from timber, steel, or concrete. Steel shapes include H-piles, pipe piles, and tapered, fluted piles. Concrete piles are pre-cast and pre-stressed in square, octagonal, and round shapes, which can either be solid or cylindrical with hollow cores (see figure 1.21).



H-piles are considered nondisplacement-type piles; they can be inserted into the ground without displacing and disturbing much existing soil volume. This can be advantageous adjacent to existing structures. H-piles are typi-cally limited to being driven to end bearing because they would not generate a high side resistance in comparison to other driven piles.

Solid piles, pipe piles driven with closed ends, or any type of concrete piles are considered displacement piles, because they compress or displace a significant volume of soil laterally to make room for the installed pile. These piles are ideal for loose sand deposits since the action of installing the pile can significantly densify the sand and increase its resistance.

As their name implies, driven piles are pounded into the ground with special pile-driving equipment supported from a crane. The pile-driving hammer includes a heavy weight that is repeatedly lifted and dropped onto the top of the pile to drive it into the ground. The number of blows needed

Depth

(ft)

0

30

60

90

1

23

4

5

6

Figure 1.21 Common Types of Driven Piles

1. Timber

2. Steel H-pile

3. Steel pipe pile (open or concrete-filled), direct or mandrel-driven

4. Tapered steel piles (open or concrete-filled), direct or mandrel-driven

5. Pre-stressed, pre-cast solid concrete, square, circular, or octagonal

6. Pre-stressed, pre-cast hollow concrete pile (open or concrete-filled), circular, square, or octagonal



to advance the pile is related to the pile resistance. Piles can also be installed by hydraulic equipment that presses the pile into the ground, or by hammers that vibrate the pile into the ground.

Use of driven piles has its disadvantages. The process of driving or vibrating the piles into the ground causes significant noise and vibration that may be objectionable in an urban environment. Generally, multiple piles are needed to support a single column, meaning the piles are driven in a group and attached to a formed and cast concrete “pile cap” that supports the col-umn. The cost of the pile caps must be considered in the overall cost of the foundation when comparing to other systems.

Cast-in-place piles come in various forms (see figure 1.22). Bored piles are also known as drilled piers, drilled shafts, or caissons. These piles are constructed by drilling a hole into the ground, installing temporary or perma-nent steel casing, or drilling mud to keep the hole open. When the excava-tion reaches the desired bearing level, the pile is formed by filling the hole with concrete. The steel casing can be removed, left in place, or replaced by

Depth

(ft)

0

30

60

90

1 2

3

4

5

Figure 1.22 Most Common Types of Cast-in-Place Concrete Piles

1. Cast-in-place concrete, bored pile, drilled pier, drilled shaft or caisson

2. Cast-in-place under-reamed bored pile or belled caisson

3. Augered, cast-in-place concrete or grout pile

4. Enlarged base, expanded base-compacted, or Franki concrete pile

5. Cast-in-place concrete barrette



a thin corrugated steel liner. Bored piles have the advantage of relatively low noise or vibration, but can cause substantial ground subsidence of neigh-boring sites if improperly excavated. Bored piles also have the advantage of being very large and can thus support enormous loads. Bored piles can be designed for end bearing, friction, or a combination of both.

Auger-cast piles are constructed with a continuous hollow-flight auger that is drilled to the bearing level. High-strength sand-cement grout is then pumped under pressure from the bottom of the hollow stem as the auger is slowly with-drawn. Careful construction and observation are needed for this type of pile to ensure that the pile is fully formed. Auger-cast piles are typically used in sand deposits where an open shaft could not easily be drilled. Where auger-cast piles have been used in loose sands below the water table, however, ground subsid-ence has often occurred at neighboring sites, which has resulted in lawsuits.

Less common specialty piles are used regionally, such as micropiles, heli-cal piles, rammed aggregate piers, and stone columns as shown in figure 1.23.

Depth

(ft)

0

30

60

90

1

2

3

4

Figure 1.23 Special Types of Foundation Piles Typically Used for Under-pinning or Ground Improvement

1. Grout-filled or open steel pipe micropiles, minipiles, or pinpiles typically pushed or drilled into place. Three to nine inches in diameter.

2. Steel helical or screw piles, square or tubular sections, 1 1/2 to 3 1/2 inches, screwed into the ground

3. Rammed aggregate pier

4. Stone columns



Typical capacity and size of the piles are summarized in table 1.2.

TABLE 1.2 Typical Capacities and Sizes of Piles

Pile Type Size Length* (feet) Allowable Load† (tons)

Timber 8-inch tip 50 10–30

Steel H-pile (50 ksi) HP 10 × 42 60+ 50–120

HP 14 × 102 100+ 150–315

Steel pipe 10 × 1/4 wall 80+ 50–100

16 × 3/8 wall 100+ 150–300

Precast, pre-stressed concrete

12 × 12 square 80+ 100–175

24 × 24 square 100+ 420–705

36 to 66 inches 100+ 500–1,250

Steel tapered 10-inch diameter 60 120

14-inch diameter 80 180

Bored pile 2.5 feet 80 250–1,500

5 feet 150 1,800–4,000

10 feet 150+ 8,000–22,000

Belled pile‡ 9-foot bell 120 650–1,250

27-foot bell 120 11,500

Augercast pile 14 inch 60–80 80

22 inch 100 220

Franki pile 16 to 24 inches 50 35–150

Barrette 3 feet × 9 feet 400+ 5,000

Micro pile 3 to 9 inches 50 to 100 10–200

Helical pile 10- to 14-inch plates 40 to 60 10–20

Rammed aggregate pier 2 to 3 feet 10 to 15 (4)

Stone column 2 to 3 feet 20 to 40 (4)

* Typical maximum length

† Structural capacity indicated

‡ End bearing on Chicago hardpan

(4) Ground Improvement



abeck

Callout

AU/PUB: could not read correction here. Please send revision

§ 1.5 EXCAVATION SUPPORT SYSTEM DESIGN AND CONSTRUCTION

Many projects require open excavations to allow construction of below-grade portions of structures, such as basements for buildings, foundations for buildings and bridges, underground utilities, and underground transportation facilities, including roadways and commuter rails. The sides of excavations are typically sloped to provide excavation stability when existing structures are not nearby, when the excavations are relatively shallow, and where other constraints, such as property lines and poor soil conditions, do not prohibit the use of slopes.

Where slopes are not possible or practical, the vertical sides of exca-vations must be supported. The selection of the type of excavation support system, and the method of design and construction of that system, depends upon many factors. This section will present some basic considerations for excavation support systems utilized in construction of buildings or other civil engineering structures. Note that this section does not cover the special cases of temporary support of tunnels, underground chambers, mines, or coffer-dams placed in open water.

§ 1.5.1 Design

Design ResponsibilityA professional engineer with training and experience in geotechnical and structural engineering will design the excavation support system. The engi-neer may be retained by the contractor constructing the support system or may be an agent of the owner as the structural engineer of record. When the contractor is responsible for design, the structural engineer of record will typically review the contractor’s engineer’s shop drawings. Depending on local laws and local engineering practice, the design may also have to be reviewed and approved by local or state regulating agencies.

Site InvestigationsA thorough investigation of the site’s subsurface conditions is an important first step toward the successful design and construction of an excavation sup-port system. The site investigation includes a survey of existing site condi-tions, evaluation of structural integrity of any existing structures above- or below-grade, and analysis of the local geology and groundwater.

The scope of the investigation depends on the scale of excavation, the need to protect adjacent structures or utilities, and the nature of subsurface materials. One of the main tools in the subsurface investigation is soil borings. The number and depth of borings may be governed by local code or regula-tions. For major and complex work, the investigation may be performed in



phases. The preliminary phase consists of limited widely spaced borings to obtain overall geological conditions of the site, identify potential design and construction issues, and evaluate feasibility of viable support system options. The next phase is a final investigation consisting of a more detailed soil bor-ing and design program to obtain the necessary information for final design and construction.

Before the site investigation occurs, the engineer should conduct a desk-top study, which includes review of all available design and construction documents for existing buildings and critical structures at or adjacent to the site, USGS maps, historic aerial photos, and previous soil boring records from around the project site.

(A) Subsurface Explorations and TestingSoil borings are required to collect data for the design of an excavation sup-port system. Often the spacing of the soil borings performed for the design of the structure (building, utility, roadway, etc.) will be sufficient for the design of the excavation support system. A reasonable preliminary spacing of bor-ings for a long wall is approximately every 100 feet along the support wall. For excavations with four sides, where each side is under 100 feet, a boring in each corner should be performed. If subsurface conditions appear to vary between the preliminary borings, additional borings should be added along the wall alignment. Additional borings should also be performed along the wall at locations where the proposed structure is deeper.

In addition to borings along the support wall alignment, borings should be performed behind the anticipated alignment of the wall (outside the future excavation) if tiebacks will be used to support the wall. The borings should be located within the approximate bond zone of the tiebacks. Tieback bond zones in soil are typically between 20 and 40 feet in length. To locate these borings, it is reasonable to assume that the bond zones will begin either out-side a line extending up from the bottom of the final excavation at an angle of between 30 and 45 degrees to the support wall, or a minimum of 15 feet from the wall, whichever is greater.

Boring depths along the alignment of a continuous cantilever wall or a wall supported with one level of bracing should extend to a depth below the existing ground surface of approximately two times the maximum excava-tion depth. This boring depth is appropriate in stable soils (e.g., cohesion-less soils with a relative density of “dense”). For deeper excavations in stable soils above the groundwater level, where the excavation support wall will be sup-ported with more than one level of bracing, borings should extend to a depth of approximately 15 feet below the maximum excavation depth.

Borings in tieback bond zones should extend to a depth determined by the geometry conditions described above, assuming the lowest tieback is approximately 10 feet above the bottom of the final excavation.

Groundwater observation wells may be installed to determine the loca-tion of the groundwater table. Where artesian water pressure is present



within soil deposits at or near the base of the excavation, piezometers must be installed to determine the artesian pressure level. For large-scale excava-tion work, field permeability and pumping tests may be required to obtain the necessary information to select and design an appropriate dewatering system to allow excavation to be performed safely and in a relatively dry condition.

Buried obstructions can cause significant construction difficulties and delays to the installation of the excavation support walls. Test pits should be excavated where man-made fill soils contain potential obstructions.

In-situ sampling and testing in the borings should include, at a mini-mum, Standard Penetration Tests in cohesion-less soils and the collection of undisturbed samples using thin-walled Shelby tubes in cohesive soils. Basic laboratory index and strength tests should be performed on the undisturbed cohesive samples. The strength tests should consist of unconfined and tor-vane tests. In-situ Vane Shear Testing should be performed in the borings in cohesive soils to estimate the undrained shear strength of the soil. Consolida-tion tests may be required if the settlement of adjacent ground and structures due to construction drawdown is a concern. Depending upon the complexity of the project in terms of the soil conditions and the surrounding structures, more elaborate laboratory testing may be needed for use in finite element computer-modeling of the excavation support system performance.

(B) Investigation and Conditions Survey of Existing StructuresNext, the engineer must investigate all existing structures on or near the exca-vation site. The following items are necessary for the evaluation: the plan location, elevation, and original design drawings of all existing structures near the excavation, including below-grade structures, such as major utilities and transportation tunnels and basements and foundations for above-grade structures; the structural integrity of all those structures and their tolerance to total and differential settlements; and field measurements. All existing cracks and structural defects of the surveyed structures should be properly docu-mented and continuously monitored throughout the excavation. In addition, the review should include all reasonably available documents to determine if remnant underground structures may exist. If the presence of these structures goes undetected during design, encountering them during construction can result in significant additional costs and schedule delays.

For property with adjacent building foundations, test pits work best to determine or confirm the depths and sizes of those foundations. Nondestruc-tive testing involving the use of the parallel seismic technique may be used if the adjacent building is supported on deep foundations. This information is crucial in the selection of the appropriate excavation support system and the building underpinning design, if required.

Selection of the Excavation Support SystemsMany types of excavation support walls are available. More common wall types include soldier pile and lagging walls, steel sheet piling walls,



cast-in-place (CIP) slurry (diaphragm) walls, CIP soldier pile and tremie con-crete (SPTC) walls, and CIP secant and tangent pile walls. Less common wall types include soil nail walls, deep soil-mix walls, jet grout walls, and varia-tions of any of these walls. The following discussions are intended to address the more common wall types.

(A) Wall Types(1) Soldier Pile and LaggingSoldier pile and lagging walls consist of discrete vertical members (the soldier piles) with “lagging” spanning between the piles. The soldier piles are typically structural steel HP or wide flange steel sections; CIP piles may be used. Steel soldier piles are either driven into the ground or placed in pre-augered holes. The pre-augered holes are often filled with a low-strength cement/sand mixture or some other similar low-strength concrete mix that can later be excavated by hand or light machinery. Typical pile spacings are between 6 and 10 feet. The lagging is typically timber boards, although precast concrete panels can be used, especially if the wall is to serve as a permanent structural wall. Typical timber lagging thicknesses are three to four inches. As the excavation proceeds downward and the soldier piles are exposed, the lagging is placed behind the front flanges of the HP sections. The exposed excavation face is typically lim-ited to two to four feet in height, depending on the soil conditions. Voids that occur behind the lagging, resulting from overexcavation, should be filled with sand, small gravel, or cementitious grout.

(2) Sheet PilingSheet pile walls generally consist of a continuous line of steel sheet piling, although timber sheeting is sometimes used in relatively shallow excavation not exceeding 20 feet in depth. The sheet piling is typically driven into the ground with impact or vibratory hammers.

(3) CIP Diaphragm and SPTCThe construction of a CIP diaphragm is similar to that of SPTC; both walls are installed in slurry-filled, excavated panels, or trenches. The panels are con-structed in two alternating stages—a primary panel and secondary panel. The panel lengths will typically range between 6 and 25 feet. Wall thicknesses can range between 18 and 60 inches, but are commonly between 30 to 40 inches. Each primary panel gets concrete before the intermediate secondary panels are excavated and cast. A structural connection is provided between the panels by a “key.” The final product is a continuous wall of alternating, interlocked primary and secondary panels.

The panels are maintained full of slurry during excavation to provide sta-bility. When the panel excavation is complete, steel reinforcement is installed. The difference between diaphragm and SPTC walls rests in the type of rein-forcement they use. Reinforcement for diaphragm walls consists of conven-tional deformed steel reinforcing bars, assembled as a “cage,” with dimensions approximately equal to the full height and width of each primary and second-ary panel. Reinforcement for SPTC walls consists of structural steel W sections



abeck

Text Box

note: updated heads to correctly match other chapters.

(soldier piles) placed at predetermined spacings in the primary and second-ary panels. Depending upon the spacing of the soldier piles, steel reinforcing cages may be placed between the piles to allow the wall to span between the piles. After the placement of the reinforcement, tremie concrete is poured in to displace/replace the slurry, leaving each panel filled with concrete.

(4) CIP Secant and Tangent PileThese walls are similar to diaphragm and SPTC walls in that they are a con-tinuous cast-in-place, steel-reinforced wall; they are also constructed in two alternating stages using primary and secondary wall elements. The primary and secondary wall elements of secant and tangent pile walls, however, are drilled piles. The drilled piles are overlapped (typically three to four inches) to form the “secant” pile wall. For a tangent pile wall, the piles do not over-lap, but instead run contiguous/adjacent to one another.

Depending upon the soil conditions and the presence of groundwater, the piles are either drilled under slurry or dry. The reinforcement can consist of either circular steel reinforcing bar cages, or structural steel W sections. Both the primary and secondary piles can be reinforced, or alternating piles can be reinforced. The concrete is poured in to displace/replace the slurry, leaving each panel filled with concrete, in a similar manner to diaphragm and SPTC walls.

(B) Soil and Groundwater Conditions(1) Cohesion-less SoilsEach soil type has a limited “stand-up” time; that is, a vertical excavated face will remain stable for only a limited amount of time, or not at all. In the inter-est of the safety of the site and crew as well as financial concerns, a stable vertical excavated face is necessary to allow sufficient time for excavation between soldier piles and placement of the lagging. The soil and ground-water conditions directly impact the type and success of excavation support system.

Because of the disruption of soils that accompanies use of the separate components, the use of soldier pile and lagging walls is generally not well suited for projects with the following conditions: subsurface has dry, cohesion-less soils with a very loose to loose relative density; subsurface has a mini-mal percentage of clay- or silt-sized fines; subsurface has a uniform gradation (grain sizes are within a relatively narrow size range); and granular soils exist below the groundwater level. One possible exception is that soldier pile and lagging may work where the subsurface conditions are well-graded, dense to very dense soils with a relatively low or high permeability, or where the water table is lowered prior to excavation to a depth of at least several feet below the cut level.

By contrast, sheet pile walls are well suited for these dynamic soil and groundwater conditions. Diaphragm, SPTC, secant, and tangent pile walls can be constructed in these soils above and below the groundwater level with the careful use of slurry and good workmanship; the length of their panels will typically need to be reduced to help maintain panel excavation



stability. Secant and tangent piles can use casing, in combination with or in lieu of slurry, to maintain pile excavation stability. Tangent pile walls con-structed below the groundwater table may experience problems with exces-sive seepage through the wall, however, which can lead to significant ground loss and adjacent ground and structure movements. For these reasons, the best practice is to lower the water table outside the excavation prior to instal-lation of the tangent pile walls.

Well-graded, dense to very dense soils below the groundwater level, as noted above, will typically have a low permeability. As a result, the excavated face will remain stable long enough to allow lagging placement. Because of limited groundwater seepage from the excavated face, the volume of ground-water that must be pumped from the excavation may be controlled with low-cost sumps in the bottom of the excavation. By contrast, if the excavated soils have a relatively high permeability, dewatering wells or wellpoints along the wall may be able to lower the groundwater level, both at the wall and for a sufficient distance outside the wall, to stabilize the excavated face.

When the excavation site has cohesion-less soils below the water table, “bottom failure” may occur if the hydraulic head at the base of excavation is significantly large enough to cause a “quicksand condition.” One preven-tive measure under these conditions is to drive the sheeting deeper or lower than the groundwater level outside the excavation. A bottom “blow-in” may occur if an artesian sand layer is present below the base of excavation and the artesian water pressure head exceeds the soil’s weight above it. This con-dition may be alleviated by pumping water from within the excavation site to relieve the underground artesian pressures.

Another risk to the excavated site is settlement behind the wall due to significant ground loss at the excavation face. If the groundwater is allowed to enter the excavation from permeable soils, the in-situ soils will be lost through seepage out the excavation face. One preventive measure is to place a geotextile “filter” fabric against the excavated face behind the lagging to limit ground loss. Or, if dewatering wells or wellpoints are used, they should include properly sized well screens, and the soils surrounding the well should be properly graded.

In very dense to extremely dense cohesion-less soils, it will be difficult to drive soldier piles for a distance of more than several feet. If soldier piles must penetrate these soils, the pile locations can be pre-augered. Similarly, sheet piling will have limited success penetrating these soils. Diaphragm, SPTC, secant piles, and tangent piles, however, can be advanced through these soils.

If the excavation will extend below the groundwater level in cohesion-less soils, the engineer must consider the impact of lowering the groundwater level outside the excavation. A depressed groundwater level will increase the effective stress in the soil and could cause settlement in compressible soils, such as organic soils or loose cohesion-less soils. Settlement, in turn, could cause distress to adjacent buildings. In addition, depressed groundwater lev-els could expose timber pile foundations that were previously continuously submerged in groundwater. If exposed long enough, the timber will start to



deteriorate, thereby decreasing the structural capacity of the piles and pre-cipitating settlement of the building.

If groundwater levels cannot be lowered outside the excavation, sheet piling or CIP walls can be used to cut off water from the excavated area. CIP walls are essentially impermeable. Sheet piling is relatively impermeable, but does allow some seepage through its interlocks. For surrounding soils that have a relatively low to moderate permeability, seepage through interlocks can lower groundwater levels outside the excavation.

(2) Cohesive SoilsEach of the excavation support walls discussed above is generally suitable for excavations in cohesive soils, but it may not be practical to drive sheet pil-ing into hard to very hard cohesive soils. Similarly, the risks of bottom “blow in” also exist in cohesive soil and could lead to significant ground move-ments outside the excavation and failure of the excavation support system. The primary concern in cohesive soils is basal instability. Similar to the bear-ing failure of a spread footing, basal instability occurs when the soil outside the excavation support wall starts to settle; that movement stresses the soils beneath the site, causing the bottom of the excavation to rise. Basal instabil-ity can lead to significant ground movements outside the excavation as well as distress to existing structures.

(C) Ground Movements and Adjacent StructuresAs discussed earlier in this chapter, an important component in the selection and design of an excavation support system is a survey of the locations, struc-tural integrity, and tolerance of existing structures relative to the excavation.

Ground movements are a natural side effect of an excavation. Typically, installation of the bracing and the excavation itself are the primary sources when ground movement affects adjacent structures.

Although ground movements from excavation and bracing comprise the greatest source of total movements experienced during a project, many other factors can contribute to the total ground movements, and some can be sig-nificant, including:

• Excavation basal instability• External groundwater lowering due to active dewatering or due to

seepage into the excavation through the excavation support wall• Open panel excavations during installation of CIP diaphragm and

SPTC walls, or open holes during installation of CIP secant and tan-gent pile walls

• Vibrations from installation of the excavation support wall elements• Removal of existing underground structures or foundation elements

and installation of new deep foundations• Excessive over-excavation below bracing levels to install braces at

the design levels• Poor lagging installation practices for soldier pile and lagging walls

These activities are discussed further in subsequent sections.



(D) Other ConsiderationsEach of the common excavation support walls discussed here is suitable for temporary construction; that is, the temporary wall must remain in service until (1) a permanent cast-in-place foundation wall is constructed inside the temporary wall, and (2) earth and water pressure loads are transferred to the permanent wall. In addition, however, CIP excavation support walls may also serve as the permanent, load-bearing wall, which is commonly seen in underground parking garages or similar structures where the interior finish of the wall is not of high architectural importance. Of the CIP walls discussed herein, the diaphragm and SPTC walls are the most common wall types that are incorporated into the final structure. Further, although sheet pile, soldier pile, and lagging walls are not generally suitable as the permanent wall, they can serve as concrete “forms” for the permanent concrete foundation wall. That is, the foundation wall is cast directly against the sheet pile or soldier pile wall.

Design Engineering

(A) Earth and Water PressuresLateral earth and water pressures exist on both sides of the excavation sup-port wall. “Driving” pressures are exerted on the wall from the outside. These driving pressures arise from the weight of the soil itself, from “surcharge pres-sures,” and from construction surcharge pressures. Surcharge pressures arise from existing structures and vehicular/rail traffic. Construction surcharge pressure arises from material stockpiles, equipment loads, vehicular traffic, and isolated equipment loads (such as cranes). “Resisting” pressures occur on the inside of the wall below the bottom of the excavation and contribute to the stability of the wall.

For the integrity of the excavation wall, the estimation and balance of all pressures—inside the wall and out—are of critical importance.

(1) Water PressureAs a general rule, water pressure outside the wall is 50 percent greater than the earth pressure (not including surcharges). Water pressure, there-fore, represents approximately two-thirds of the total pressure from earth and water. If water pressures on the wall can be eliminated, the excavation support system would likely be much less expensive. Further, if a support system is not designed for water pressure, it is critical that the water pres-sure be maintained and controlled throughout the design life of the wall, lest significant wall movements, and possible overstressing, could lead to wall failure.

(2) Earth PressurePrior to excavation, the stresses in the ground are referred to as “at-rest” earth pressures. The engineer estimates the exterior lateral earth pressures on the temporary excavation support system from the weight of the soil and surface surcharges, based on anticipated and tolerable wall movement during con-struction. If the control of wall movements is not critical for the protection of



existing structures and facilities, the design of the wall can assume that the wall will deflect a minimum amount. This minimum movement will allow the retained soil to move, thereby mobilizing the internal strength of the soil. This mobilization of soil strength will reduce the lateral earth pressure imposed on the wall from the at-rest pressure to an “active” earth pressure. Active pressures are the minimum earth pressures that may act on the wall. Depending upon the type of soils and its relative density and consistency, the required wall movement varies between 0.05 and 2 percent of the wall height. Typically, some wall movement will occur so that the earth pressures are reduced below the at-rest pressures.

The lateral earth pressure on the inside of the wall below the bottom of the excavation is referred to as the “passive” pressure. As the wall deflects inward below the bottom of the excavation, the internal strength of the soil is mobi-lized and the lateral earth pressure increases above the at-rest pressure. The amount of wall movement (or rotation) required to mobilize the full passive earth pressure can be significant and depends on the type of soil and its den-sity/consistency. A factor of safety of 1.5 is typically applied to the passive soil strength during design to limit lateral wall movements to acceptable levels.

It is important to note that the wall and soil movement do not affect the magnitude of the water pressure. For the purpose of excavation support sys-tem design, the water pressure will not change as a result of a change in the state of stress in the soil. Therefore, the water is usually the major component of the total lateral earth and water pressures.

If, as noted in the prior section, the temporary excavation support system will also serve as a permanent wall for the final structure, the design must also consider long-term earth pressures. Regardless of the earth pressures that exist at the end of construction, these pressures will change with time and eventually correspond to the at-rest pressures. Long-term water pressures will not change from the temporary condition, unless the long-term design water level differs from the design level adopted for construction.

(B) General Wall and Bracing DesignThe design of all excavation support systems, other than cantilevered sys-tems, must include staged analyses that duplicate the field sequence of the excavation process, bracing installation, and bracing removal. During exca-vation and installation of a support system with one or more levels of brac-ing, each bracing level is installed in stages. During the first stage, the crew excavates an initial, relatively shallow area to two to three feet below the design centerline of the first level of bracing and installs the bracing. Follow-ing the installation of this bracing level, they excavate a second stage to the design centerline of the second level of bracing and install this bracing. The crew repeats this staged sequence until they reach the final bottom of exca-vation. The earth and water pressures, tributary load areas of the braces, and the length of wall spans will change with each excavation stage, resulting in different stresses in the excavation support system. Consequently, the engi-neer’s staged analysis of the support system is critical.



The staged analyses must also evaluate the impact of the sequence of installation of the permanent foundation walls and elevated slabs and the related relocation or removal of the temporary support system bracing. Dif-ferent bracing tributary load areas and wall spans may result in stresses that control the overall design of the support system. Once the final bottom of excavation is reached, the earth and water pressures corresponding to the final stage are temporarily “locked into” the support system. That is, these maximum pressures will not change as the permanent foundation wall and slabs are installed and the bracing is relocated or removed. Therefore, these pressures must be used in the latter stages of analyses.

The design of the various elements of the excavation support system should follow generally accepted design guidelines and/or the applicable code provisions. In the United States, reinforced concrete design should be in general accordance with the American Concrete Institute. Design of steel wall elements and internal steel bracing should be in general accordance with the American Institute of Steel Construction. Design of timber structures should be in general accordance with the National Design Specification for Wood Construction. Design of tiebacks should be in accordance with the Post-Tensioning Institute.

(C) Designing to Minimize Ground MovementEarlier, this chapter identified ancillary activities associated with under-ground construction that can contribute to ground movement. This section discusses some of these activities as well as others that can reduce ground movements.

(1) External Groundwater LoweringIn the event that active dewatering is not performed outside the excavation limits, a reduction in the external groundwater level may still occur. The type of excavation support system can lower the exterior groundwater level. This reduction may be the result of active dewatering inside the excavation in combination with a discontinuous excavation support wall (soldier pile and lagging), a leaking continuous support wall (typically sheet piling), or a con-tinuous support wall that does not provide a groundwater cutoff (sheet piling, or CIP walls). A change in the type or design of the support wall can address each of these problems.

(2) Open Excavations for Support Wall InstallationThe excavation of long panels for the installation of CIP diaphragm and SPTC walls will inherently make it more difficult to maintain stability of the soils than shorter panel excavations and allow a greater risk for ground movement. The use of CIP secant or tangent pile walls instead would reduce this risk.

(3) Vibrations During Support Wall InstallationVibrations can cause ground settlement. The source can be the installation of driven soldier piles or sheet piles, the removal of shallow bedrock, or the chiseling in excavations for CIP walls. In addition to vibrations, settlement requires a particular type of soil and soil density. Settlement is most likely to



occur from compression of cohesion-less soils, with very loose to loose rela-tive densities, above the groundwater table. Settlement can also occur when these same soils below the groundwater table liquefy.

With advance planning, several methods exist to avoid vibrations. For instance, soldier pile locations can be pre-augered; CIP secant and tangent pile walls can core into the bedrock rather than chiseling it; and the align-ment of sheet piling can be pre-excavated. Typically, pre-excavation is most cost-effective at depths of approximately 15 feet or less. If vibrations are still a concern below the pre-excavation, the engineer should consider an alter-nate wall type.

(4) Removal of Existing Underground Elements and Installation of Deep FoundationsIf possible, the presence of existing structures and foundations should be iden-tified early in the design phase. The removal of buried structures and aban-doned deep foundations inside the excavation may decrease the passive soil resistance that supports the excavation wall and thereby allow increased lateral wall movements. The wall design and/or the construction sequence should be modified to reduce the impact on the excavation support wall from removal of these features. Further, as discussed earlier, the installation of new deep foundations inside the excavation can impact existing structures outside the excavation, causing additional lateral movement of the excavation support wall. This movement can especially be a problem if more than one drilled-shaft excavation is open at the same time. To reduce the impact of drilled-shaft installation, the shafts should be installed prior to the general excavation.

(5) Excessive Over-Excavation below Bracing LevelsIf excavation proceeds below the specified excavation elevation for installa-tion of a bracing level, lateral movements of the support wall can occur. To reduce this risk, the maximum depth of excavation below the centerline of the bracing level assumed in the support wall design should be clearly noted on the construction drawings.

(6) Poor Lagging Installation PracticesExcessive excavation below the last level of in-place lagging prior to instal-lation of the next lagging boards can allow ground loss to occur between the soldier piles, as well as associated ground settlement outside the excavation. Similarly, poor workmanship or delay in backpacking the over-excavated areas behind the lagging can allow ground loss and settlement. Further, in the time between excavation and placement of the lagging and backpacking, the stability of the excavated soil face between the soldier piles depends on soil pressures arching to the soldier piles. If excavation occurs behind the soldier piles to allow for lagging installation, this soil removal eliminates the means by which the pressures on the excavated face can arch to the soldier piles, thus reducing the stability of the excavated soil face.

Each of these issues should be considered during design and/or clearly addressed on the construction drawings, as appropriate.



(7) Bracing Design and InstallationReduction of the vertical and horizontal spacing of the support wall brac-ing (internal supports or tiebacks) will increase the stiffness of the excavation support system and thereby assist in reducing ground movements. In addi-tion, limiting the height of the first stage of the excavation to the first bracing level will help limit ground movement behind it.

During the installation of internal steel bracing, each bracing level should be preloaded before excavation resumes to the next bracing level below. Preloading is accomplished by installing a hydraulic jack between one end of the brace and the support wall. The jack is used to impose a com-pression load in the brace. After the load in the brace is increased to a speci-fied level, the load is held constant while the brace is shimmed and welded to the support wall. After the welded connection of the brace to the support wall is completed, the jack is removed. The magnitude of the preload typi-cally ranges between approximately one half and three quarters of the design load for the brace. Preloading braces removes the “slack” in the connections of the support system. The preload also removes a portion of the elastic com-pression for the brace. Both items help tighten the support wall and reduce lateral deflection and ground movement during subsequent excavation.

Following installation and testing of tiebacks, they are loaded to between 75 percent and 100 percent of the design load of the tieback and “locked off” at that load. This pre-stressing of the tiebacks provides a result similar to that of preloading the braces—it reduces ground movements. When ground movement is a concern, both brace preloads and tieback lock-off loads should be at the upper end of the applicable ranges.

§ 1.5.2 Construction

General Construction MonitoringIn excavation support systems, good design practices alone are not sufficient to assure good performance. Monitoring the installation and performance of the system throughout the construction process is a key component to a suc-cessful project. Many elements can contribute to problems during construc-tion, including use of improper materials, improper installation methods, poor workmanship, and a lack of monitoring work performance as well as the site’s response to the excavation. Indeed, the engineer must be attentive to address any subsurface conditions encountered during construction that differ from those anticipated during design.

The late Dr. Ralph Peck, a prominent American geotechnical engineer and internationally recognized expert in that field, emphasized the use of the “observational method” in geotechnical engineering. In essence, the obser-vational method is an acknowledgement that the materials geotechnical engineers must design and use for construction (i.e., soil and groundwater) are inherently highly variable. Standard practices of the profession can be



followed during the design phase of a project to quantify the characteristics of the soil and groundwater at a site and to incorporate those properties into the design, using appropriate safety factors.

But those steps cannot prevent unanticipated conditions from arising dur-ing construction due simply to the inherent variability of the soil and ground-water. Thus, it is critical that the geotechnical engineer observe closely the construction process so as to recognize those conditions, evaluate their impact on the performance of the excavation support system, choose how to respond, and implement field adjustments to accommodate those conditions in a timely manner.

All budgetary discussions aside, the project benefits when the geotech-nical engineer of record (EOR) for the design of the excavation support sys-tem, foundations, and earthwork is retained to monitor construction progress. In fact, this involvement is imperative. The EOR is intimately familiar with the project conditions and is best suited to provide that continuity between design and construction.

The EOR should be present on-site at times appropriate to the size and complexity of the project. For small projects, part-time observation may be adequate. On larger projects, full-time observation is usually necessary, pos-sibly with more than one field representative of the geotechnical engineer.

The following sections present some of the more important activities that should be considered for implementation during construction.

Pre- and Post-Construction SurveysOn projects where the excavation may distress adjacent structures, a pre-construction survey can document the condition of the structures. This docu-mentation will assist in evaluating future apparent movements or distress to the structure and can be an important tool in resolving claims from adjacent property owners as a result of the project.

Pre-construction surveys typically consist of photographic and/or video-graphic documentation of the interior and exterior of the structures, accom-panied by notes of observed distress. These surveys should be performed by qualified personnel experienced with these types of evaluations. The photo-graphic or videographic records should be of such quality that the observed distress is clearly visible. In buildings, the walls, ceilings, and floors of each room on each floor should be documented. If the primary source of poten-tial distress is potential ground movement from the excavation and bracing process, then the engineer should select which buildings (or portions thereof) to include in the survey, based in part on the estimated lateral extent of the ground movement.

These limits will likely not be sufficient where vibrations are expected during construction. Vibrations can travel to considerable distances beyond the excavation, especially if bedrock is shallow. Vibrations can be the cause of structural distress or only a source of perceived distress by, or a nuisance



to, adjacent property owners. Consequently, when determining the limits of the pre-construction survey, the engineer should consider the “sensitivity” of the public to vibrations and the potential impact from construction.

At the end of construction, post-construction surveys can be performed to document the conditions at that time. As with the earlier counterparts, post-construction surveys provide a snapshot in time that could be important in evaluating future claims of distress filed by property owners.

InstrumentationInstrumentation is necessary to monitor the performance of the excavation support system, ground responses to the excavation, and responses of adja-cent structures to the excavation and/or construction. Many types of instru-mentation are in use, each type with a particular purpose. Depending upon the complexity of the project, instrumentation monitoring may include any of the following:

• Vibrations at adjacent structures• Horizontal deflections of the excavation support system, vertical

movement of the base of the excavation, horizontal and vertical movement of the ground outside the excavation (both at the surface and at depth), and horizontal and vertical movement of adjacent structures

• Stresses and loads in the steel bracing for the excavation support system

• Groundwater pressures inside and outside the excavation• Volume of groundwater pumped during dewatering activities

The instrumentation should be installed, monitored, and reported by qualified and experienced personnel to ensure the most accurate data pos-sible. Data that are not representative of the site can result in unnecessary and possibly costly construction delays as well as complicated factual and legal issues.

The instrumentation program for complex projects should establish dif-ferent magnitudes, or levels, of the instrument readings that can be used as “flags” to trigger further action. For example, the engineer can establish threshold and limiting values for horizontal deflection of the excavation sup-port wall. The limiting value would be slightly less than or equal to the maxi-mum tolerable wall deflection. The threshold value would be less than the limiting value. The threshold value would serve as a warning that the wall deflection is approaching the maximum value. Prior to the start of construc-tion activities, baseline readings should be obtained on all instruments. The baseline readings will typically include several readings, for instance at dif-ferent times of the day. These readings serve as the basis against which future instrument readings during construction are compared.



During construction, the project team should develop and execute a schedule for reading the instrumentation periodically. The frequency of the readings may vary, depending upon the schedule of the construction activi-ties. The project team would be prudent to reduce, report, and review the data from the readings on a timely basis.

§ 1.6 ROADWAYS/PAVEMENTS

A common pavement cross section consists of a subgrade, base course, and surface, as illustrated in figure 1.24. Common pavement surfaces are Port-land cement concrete (concrete) and bituminous concrete (asphalt). Base course material can be either a stabilized layer or an untreated aggregate base. Subgrades are either cohesive (silts and clays) or noncohesive (sand and silty sands).

§ 1.6.1 Subgrade Preparation

This section assumes that the subgrade soils are suitable and, if fill is neces-sary to raise the site, that the fill has been placed and compacted. Once the subgrade is either cut or filled to design grade and all unsuitable soils (organic, frost susceptible soils, expansive clump and uncontrolled fill soils) have been removed, the prepared subgrade should be proof-rolled using a rubber-tired

dump truck or semi-tractor trailer having a minimum gross load of 25 tons—two passes in perpendicular directions over the subgrade. The geotechnical engineer of record or his representative should be pres-ent during the proof-roll. Any areas of pumping and/or rutting should be corrected prior to base course instal-lation. Pumping occurs when the subgrade deflects under the weight of the proof-roller and then rebounds to near its original elevation. Rutting is the permanent deformation of the subgrade under the load of the proof-roller. Passing or failing of the subgrade is often a subjective deci-sion. The areas and recommended depth of improvement should be documented on a site diagram.

Tack coat

Prime coat

Bitminousconcrete surfacecourse

Bitminousconcrete bindercourse

Aggregate base

Prepared Subgrade

Figure 1.24 A Typical Pavement Section



Some municipalities have their own criteria (e.g., ruts greater than one inch fail). One criterion does not fit all. When determining if the subgrade needs improvement prior to placement of aggregate base course, the engineer con-siders the pavement’s intended use (e.g., heavy-duty trucks versus light-duty cars only), the design pavement section, surface and subdrainage design, and subgrade soil type. The geotechnical engineer of record or representative typi-cally provides recommendations for improving the subgrade. Typical subgrade improvement procedures include discing and aerating the soil to near its opti-mum moisture content to a specified depth, recompacting the lift(s) to the specified compaction criteria, adding a stabilizer (lime, lime fly ash, cement, asphalt), or overexcavating the unstable material and replacing it with moisture-conditioned on-site soils, imported soils, or imported granular soils.

Geotextiles can also reinforce marginal soils and act as a separator between the subgrade and the aggregate base course. Prior to performing the proof-roll, the engineer should confirm that the subgrade has been cut or filled to the design grade, either by surveying elevations or string lining between two finished points. This test will confirm that the subgrade has been prepared to an elevation or grade that will allow for the appropriate total pavement thickness. Lack of total pavement thickness is a frequent cause of premature pavement distress.

§ 1.6.2 Pavement Design

The initial key inquiry for pavement design is: Where will the pavement be placed and how will it be used? As one can imagine, significant differences exist between roadways and parking lots, and distribution and warehouse pavements. The differences relate to traffic volume, vehicle weights, vehicle speed, and distribution over the pavement.

The industry has developed numerous design procedures for bitumi-nous concrete (Asphalt Institute, AASHTO, DOT, etc.) as well as for Port-land cement concrete pavements (Portland Cement Association, AASHTO, DOT, etc.). But these design procedures were developed for roadways. Some design procedures have been modified for parking lots. The point here is that not all of these design processes will be appropriate for every application.

Regardless of the design procedure, pavement design procedure requires at least the following considerations: traffic (volumes, weight, distribution, and anticipated growth); design period; subgrade; and pavement materials (base course, surface [PCC, AC]).

TrafficTraffic, particularly for commercial pavements, is often not well defined at the design phase of the project. Conversely, traffic for streets and highways is often well defined by traffic counts that provide the Average Daily Traffic, the percentage of light passenger vehicles and trucks, the vehicle weights, and



the distribution of traffic by lane. When traffic information is not available at the design phase, the engineer should make reasonable assumptions based on the intended use of the pavement and discuss them with the project stake-holders to obtain agreement.

Traffic data are typically converted from vehicle type (number and type of axles), axle weights, and number of repetitions to Equivalent Singe Axle Loads. The AASHTO road test conducted in Ottawa, Illinois, in the late 1950s and early 1960s provided these axle load equivalency factors.

Design Period“Design period” refers to the number of years to which the traffic data relate. The design period is commonly selected to be 20 years but does vary depend-ing on the owner or agency requirements. A 10- to 20-year design period is not uncommon for commercial property pavement. In setting the design period, the project team (and especially the owner) must understand the importance of routine maintenance, repairs (e.g., patching and crack seal-ing), and rehabilitation (e.g., overlay). Rehabilitation should be anticipated throughout the design period.

Subgrade PropertiesFor bituminous concrete pavements, the California Bearing Ratio (CBR) is commonly the measurement used to calculate subgrade load-bearing strength. The harder the surface, the higher the CBR rating. The CBR is a test that mea-sures the pressure required to penetrate a soil sample to determine its strength.

For the design of Portland cement concrete pavements, the modulus of subgrade reaction (K) is frequently used as the subgrade design parameter. There is no laboratory test to determine K value. Although a “field plate load test” is used to determine the K value of the subgrade, this test is not typically feasible, except for large projects or airfield pavement design. Consequently, over the years, correlations between the CBR value and the K value have been made for estimating the K value.

If provided the approximate design subgrade elevations, the geotechni-cal engineer typically provides subgrade parameters for design.

Base CourseThe primary function of base course under a bituminous course pavement is to provide load distribution to protect the subgrade. The primary function of a base course under a concrete pavement is to provide for a working mat dur-ing construction and to control pumping of the subgrade. Whether under a bituminous concrete or Portland cement concrete pavement, the base course should allow water to pass through to a subdrainage system.



The base course of a pavement structure can be either a stabilized base, such as cement, cement fly ash, asphalt, or an untreated material, such as an aggregate base course. An untreated aggregate base is commonly used for the base course for local roads, collectors, and parking lots. Crushed aggre-gate should be used where it is locally available. A crushed aggregate pro-vides for a stronger base course. If the aggregate base course has too many fine particles, the base will hold water that enters through pavement surface cracks. A wet or saturated base course loses its strength and results in higher pavement deflections under traffic loads. Higher deflections lead to prema-ture pavement distress. In areas where freezing conditions occur, a saturated base course will freeze, resulting in pavement heave and premature failure.

Pavement SurfaceThe pavement surface is a permanent surface upon which vehicles drive. This surface may be a treated or untreated base course, topped with bituminous concrete or Portland cement concrete. Bituminous concrete and Portland cement concrete surfaces are the most commonly used for roads and park-ing lots. In selecting the surface type, the designer should take into account the traffic (type, loads, frequency, distribution, speed), the facility type that the pavement services, and the economics (life cycle cost). A bituminous concrete surface is commonly used for lower-volume roads and commercial parking lots, whereas concrete pavement surface is more commonly used on interstates and warehouse/distribution center pavements. In the case of ware-house/distribution centers, where there are slow-moving, turning, and parked tractor trailers, a Portland cement concrete surface typically performs better than bituminous concrete.

§ 1.6.3 Designs

Proper surface and subdrainage are important considerations that affect the performance of the pavement over the design period. To ensure proper drainage from the surface, a minimum three percent grade is desired for bituminous concrete pavements, and a minimum one percent surface grade is desired for a Portland concrete pavement. Surface drainage is critical because standing water (“ponding”) on pavement creates unsafe conditions for the motorist and also allows water to penetrate into the base course and subgrade, leading to the deterioration of the pavement.

Many commercial pavements are built without subdrainage because of economics. But the cost-benefit analysis calls that decision into question. Proper subdrainage removes water that has penetrated the pavement surface cracks and is therefore beneficial to the long-term performance of the pavement.



References

G. Wayne Clough & Thomas D. O’Rourke, Construction Induced Movements of Insitu Walls, in Am. Soc’y of Civil Eng’rs Geotechnical Special Publ’n No. 25, DESIGN & PERFORMANCE OF EARTH RETAINING STRUCTURES (proceedings from conference at Cornell University, 1990).

R.W. DAY, GEOTECHNICAL AND FOUNDATION ENGINEERING-DESIGN AND CONSTRUCTION (1999).

DEEP FOUNDATIONS INST., INDUSTRY PRACTICE STANDARDS AND DFI PRACTICE GUIDELINES FOR STRUCTURAL SLURRY WALLS (2005).

D.T. GOLDBERG, W.E. JAWORSKI & M.D. GORDON, LATERAL SUPPORT SYSTEMS AND UNDERPINNING, vols. 2 & 3 (Apr. 1976).

Robert G. Lukas, U.S. Dep’t of Transp., Fed. Highway Admin., Dynamic Compaction, 1 GEOTECHNICAL ENG’G CIRCULAR (Pub. No. FHWA-SA-95-037, Oct. 1995), available at http://ww2.media-teca.com/mediateca/index.php/it/guidelines-a-specifications/doc_download/446-02-fhwa-geotechnical -engineering-circular-dynamic-compaction.html.

Naval Facilities Engineering Command Design Manual 7.02, Foundations & Earth Structures (Sept. 1986).

R.B. PECK, W.E. HANSON & T.H. THORNBURN, FOUNDATION ENGINEERING (2nd ed. 1974).

Pile Buck® Steel Sheet Piling Design Manual (1987).

POST-TENSIONING INST., RECOMMENDATIONS FOR PRESTRESSED ROCK AND SOIL ANCHORS (4th ed. 2004).

P.J. Sabatini, D.G. Pass & R.C. Bachus, U.S. Dep’t of Transp., Fed. Highway Admin., Ground Anchors and Anchored Systems, 4 GEOTECHNICAL ENG’G CIR-CULAR (Pub. No. FHWA-IF-99-015, June 1999), available at http://www.fhwa .dot.gov/engineering/geotech/pubs/if99015.pdf.

D.E. SISKIND ET AL., BUREAU OF MINES, DEP’T OF INVESTIGATION, STRUCTURE RESPONSE AND DAMAGE PRODUCED BY GROUND VIBRATION FROM SURFACE MINE BLASTING, RI 8507 (1980).

G.F. SOWERS, INTRODUCTORY SOIL MECHANICS AND FOUNDATIONS (3d ed. 1962).

K. TERZAGHI & R.B. PECK, SOIL MECHANICS IN ENGINEERING PRACTICE (2d ed. 1967).

M.R. THOMPSON, DEEP-PLOW LIME STABILIZATION FOR PAVEMENT CONSTRUCTION (Dep’t of Civil Eng’g, Univ. of Ill. 1971).



1 Geotechnical Engineering - · PDF file · 2012-10-04The mechanical properties of the soil, such as its compressibility, strength, permeability, ... (fluids). The relative amount

Documents