Erb Cpd Seminar 11th December 2009

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ERB CPD SEMINAR

11TH DEC EMBER 2009

DEEP EXCAVATIONSPlanning aspects of deep excavations, requisite

investigations feeding into Design andConstruction

Eng. Pande Michael ME-mail: [email protected]


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1.0 General

As all present here should be aware, when a society undergoes socio-economic

development and growth, land/space sooner than later becomes a constraint since it

remains inelastic while demand for it rises.

This is so especially in urban areas (particularly central business centres CBCs) where

demand for office and business space is highest.

Because of the above comes a great push by developers/investors to want to maximize

use of the land/space available to them.

This (maximization of land use) can be achieved basically in three ways:

i) by constructing on 100% of the owned land/space or;ii) by constructing high rise buildings/structures or;iii) by combining the two - (i) & (ii) above.

Naturally, option (iii) is the most preferred by developers as it guarantees maximum

possible space use (and hence more return-on-investment). Consequently, (almost) alldevelopers (will) opt for high rise structures that also occupy 100% of available land.

1.1 So what is the problem with that?

In a context of an urban setup such as Kampala City (say along Kampala road),

construction of a high rise building that occupies 100% plot area presents several criticalchallenges as listed below, yet this phenomenon is fast becoming commonplace:

i) the land on which such construction is to take place (most likely) shall besandwiched between existing buildings;

ii) because the building shall cover 100% of plot area, and shall host hundreds oftenants, provision must be made for adequate car packing;

iii) the Engineer should achieve the above within a tight project budget;

The above therefore necessitates that:

a) in order to provide for parking, basement (below ground) floors MUST becreated;

b) in order to construct high rise building adequate footings (founded on firm base atrequired depths) must be used;

These two cannot be achieved without excavating to (required) depths which so far atseveral sites in Kampala, have been up to 12m and one with excavation reaching a depth

of 23m below ground surface level.

It is therefore a reality that deep excavations have fast become a very important aspect in

the construction industry in Uganda particularly Kampala City. This is not to end here

in Kampala.


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Other towns like Mbale, Mbarara and Jinja where most streets are lined up with the pre-

independence 2-4 storey buildings that have no provision for parking shall soon alsorequire basement parking (deep excavations) as newer and taller buildings begin to be

built therein

1.2 Challenges presented by 1.1 above to the engineer

The necessity (or rather demand) for deep excavations requires very judicious planning

since in most cases a host of activities must (shall) be undertaken simultaneouslyespecially at implementation (excavation) stage.

The sequencing and grouping of activities during: excavation; construction of temporaryand permanent supporting of cuts (embankment); movement of material & temporary

storage of materials; etc within the constrained site space is very vital.

Secondly, the following must be considered in detail and analytically (with no guess-

work or rule-of-thumb) in order to achieve a balance between: space use maximization as demanded by the developer;

safety to the project works, construction personnel, adjacent properties and generalpublic (both during execution & after construction); and

budget.

[The rule-of-thumb approach to construction that (has) worked since 1990 when

construction of bungalows was embarked upon in Namuwongo, through the generation ofdouble-storey structures in Muyenga and Naguru, to construction of the 4 8 level flats

in open spaces cannot apply any in the above circumstance and shall soon cease to

apply.

In dealing with deep excavation construction, is where a clear characterization shall bemade between Engineers and Quasi or Dummy engineers.]

To achieve the above balance, the Engineer thus is challenged to ensure that:

a) The developer MUST maximally utilize his/her land/space.b) Car parking MUST be provided for without reducing on the lettable space (in

order for the rentable space to be marketable). The only way to achieved this is by

creating basement floors.c) Construction MUST proceed without causing failure or distress to existing

(surrounding or adjacent) structures through exposure of their footings or due tosettlement induced by consolidation of underlying soil strata by the new structure,or damage to services such as power lines and sewer lines;

d) plant/ equipment movement at implementation/ excavation stage is planned for more so that there are so many one-way roads in Kampala, not to forget traffic

jams;e) there is a site office as mobilized equipment must be station at some place within

site;


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f) there is proper equipment movement management within the hoarded site withoutcompromising safety of the construction personnel;

g) the drainage system is in place to manage water in the (most likely) event thatunderground water exists;

h) the embankments are well protected against storm water from the neighbourhood;

i) waterproofing can be done safely and effectively; etc.

It can be seen therefore that adequate planning is very imperative before any works are

embarked upon as several activities are (must be) carried out simultaneously after startwith minimal time lapse.

1.3 Requisite Investigations for Deep excavations

The need for a thorough and comprehensive soil/ geotechnical investigations in

anticipation/ preparation for deep excavations cannot be overemphasized.

Soil investigation results are useful in preliminary analysis of the various options/possibilities of walls and support systems which also assists in assessment of the costsand time of construction, assessment of technical requirements on safety and determine

influence of projected cut on the adjacent (infra)structures before selection of the final

option to produce the most safe and economical design.

It is however disheartening that, there has not been a culture of geotechnical

investigations in Uganda since the engineers have been handling bungalows and or

simple 4-8 storey box flats located on open spaces and on the gifted by nature soils ofKampala hills whose average bearing capacity is 200kN/m

2.

Similarly not many developers (have) appreciate(d) the need for geotechnical studies not even Kampala City Council, as it has not been a major requirement in the application

for approval.

Genuine geotechnical investigations not only lower costs incurred by the developer since

design of footings is based on actual (rather than assumed) parameters but also

guarantee safe design for cuts and structures especially where deep excavations are to bemade.

BS 5930: 1981 summarizes the objectives of soil investigations as:

iv) to assess the general suitability of the site and environment for the proposedworksv) to enable an adequate and economic design to be prepared including design of

temporary works

vi) to determine the changes that may arise in the ground and on the environmentalconditions either naturally or as a result of the works, and effects of such changes

on the works, on adjacent works and on the environment in general


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vii) to plan the best method of construction: to foresee and provide against difficultiesand delays that may arise during construction due to ground and other localconsiderations.

viii) Where alternatives exist to advise on the relative suitability of the different sitesor different parts of the same site.

Therefore in order for a designer to come up with a safe and economical design (for deep

basement construction in this case), (s)he must be armed with data on soil properties

which are obtained via both field and laboratory tests.

1.4 Tests

A number of tests are vital where deep excavations are envisaged. As stated in

BS8002:1994. Sufficient information should be obtained on thegroundandgroundwater conditions together with strength and deformation properties of the soil whichwill be retained and the soils that will support the earth retaining structures. Geological

maps/memoirs and handbooks (where available) should be consulted together with anyother source of local knowledge.

Thus proper planning and supervision of subsurface investigations and laboratory testingis utmost important for the designer to obtain accurate and vital info for use in design.

1.4.1 Field tests

These can be (have been) conducted in accordance with BS5930:1981 whichdescribes the general considerations to be taken into account and details the methods

of investigation available.

Generally a number of boreholes should be adequate to establish the groundconditions along the length of the wall and to ascertain the variability in thoseconditions.

Piezocone test would compliment the boreholes particularly in sedimentary loose andsoft deposits. For a large site, geophysical survey would be an advantage to optimize

the subsurface investigations.

Prior to planning for the subsurface investigations it is important to acquire thegeology information of the site. (may be hard to find in Uganda).

According to BS8002:1994, the centres between boreholes should be at interval of

10m to 50m along the length of the wall depending on site conditions thecomplexity of the site geology, subsurface profile and adjacent structures.

Sometimes, subsurface investigations outside the site should be carried out especiallyaround the sensitive adjacent structures.

In addition, dilapidation survey of the adjacent structures should be conducted toassess the effect of a deep excavation on them.


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Common field tests to be carried out for the design of a retention structure comprisesof rotary wash boring (borehole) (Do we have one in Uganda at present?) andincludes Standard Penetration Tests (SPT), Pressuremeter tests, collection of

disturbed and undisturbed soil samples for testing in laboratory.

If the site is underlain by soft clayey material, field tests such as piezocone tests, in-situ penetrating vane and vane shear tests in the borehole are commonly required.

In deep excavation design, an adequate knowledge of the ground water levels,seepage pressures and information on the existence of hydrostatic uplift pressures are

essential.

Preliminary ground water conditions may be predictable from knowledge of the localgeology.

Standpipes of piezometers should be installed in the drilled boreholes to determineand confirm the ground water conditions at the site. It should be noted that water

levels encountered during boring operation s where water is used as a flushing

medium are unreliable and seldom represent equilibrium conditions.

Sealing of the borehole after completion is also important to prevent collapse of thesoil causing loosening of the subsoil. Holes often left open also pose a safety risk to

humans and animals. Boreholes in areas with potential blow-out of ground water

when carrying out excavation also need to be carefully sealed. Usually grout is usedto seal the hole.

1.4.2 Laboratory tests

Laboratory test that are usually (should be) carried out on disturbed and undisturbed soil

samples collected from boreholes include:

Particle size distribution like sieve analysis including clay and silt separation usinghydrometer.

Atterberg limits tests to determine liquid limit, plastic limit and plasticity index.

Tests to determine moisture content, porosity, unit weight and specific gravity.

Chemical tests on pH, chloride, sulphate and organic content.

Shear strength tests like Unconsolidated Undrained Triaxial Tests (UU),

Consolidation tests Tests on rock cores using Unconfined Compression Test (UCT) for strength and

strain gauges can be attached to the rock sample to measure modulus of the rock from

the test.


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1.5 Evaluation of Foundation of Adjacent properties and their Tolerances

Major concern during planning and execution of deep excavation is the impact of

construction related to ground movements on the adjacent properties and utilities.

During excavation, the state of stresses in the ground mass around the excavationchanges.

The most common changes in stresses in the retained are the stress relief on the cutface resulting in horizontal movement and followed by vertical movement for

equilibrium and, increase in vertical stress due to lowering of water table resulting in

both immediate and consolidation settlement of the ground.

The ground movement that vary away from the excavation can cause buildingsespecially those on shallow footings to translate, rotate, deform, distort and finally

sustain damage if the magnitude exceeds tolerable limits.

It is important to carry out analysis to estimate the magnitude and distribution of theground movements due to the proposed excavation.

Damage to adjacent property may include cracks, distortions, settlement, etc

The results from the above investigations and tests form very vital input to both the

design and construction process of deep basements.

The selection of retention structure type and system thus very much depend on the

findings of the investigations

o Foundation of adjacent properties and serviceso Design limits on wall and retained ground movemento Subsoil conditions and ground water level

o Working space requirements and site constraintso Flexibility of the layout of the permanent workso Local experience and available construction planto Maintenance of the wall and support system in permanent condition

The most commonly used retaining structures types areo Sheet pile wallso Soldier pile wall (soldier pile and horizontal laggings)o Contiguous bored piles wallo Secant pile walls

o Diaphragm walls.


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APPENDICES


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Appendix 1

1.6Example of Deep Excavation retention systems

The term "Retaining System" for a deep excavation refers to the structural system that retains soil and water

and prevents it from collapsing into the open cut. Many types of retaining systems exist. The selection of theproper retaining system depends on a wide range of factory such as: Economical, soil conditions, protectionof adjacent structures, ease of construction, environmental issues and more. Typical retaining systemsinclude soldier pile and lagging, sheet piling, secant pile or tangent pile walls, soil mix walls, and diaphragmwalls (also known as slurry walls in the U.S.).

a) SOLDIER PILE WALLS AS RETAINING SYSTEMS FOR DEEP EXCAVATIONS:Soldier Pile and Lagging Walls

Soldier pile and lagging walls are some of the oldest forms of retaining systems used in deep excavations.These walls have successfully being used since the late 18th century in metropolitan cities like New York,Berlin, and London. The method is also commonly known as the "Berlin Wall" when steel piles and timberlagging is used. Alternatively, caissons, circular pipes, or concrete piles can also be used as soldier piles butat an increased cost. Timber lagging is typically used although reinforced concrete panels can be alsoutilized for permanent conditions. Soldier pile walls are formed by:

1. Constructing soldier piles at regular intervals (6 ft to 12 ft, typical)2. Excavating in small stages and installing lagging.3. Backfilling and compacting the void space behind the lagging.

Moment resistance in soldier pile and lagging walls is provided solely by the soldier piles. Passive soilresistance is obtained by embedding the soldier piles beneath the excavation grade. The lagging bridgesand retains soil across piles and transfers the lateral load to the soldier pile system.

Soldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They arealso very easy and fast to construct. The major disadvantages of soldier pile and lagging systems are:1. They are primarily limited to temporary construction.2. Cannot be used in high water table conditions without extensive dewatering.3. Poor backfilling and associated ground losses can result in significant surface settlements.4. They are not as stiff as other retaining systems.5. Because only the flange of a soldier pile is embedded beneath subgrade, it is very difficult to control basalsoil movements.


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b) SHEET PILE WALLS - RETAINING SYSTEMS FOR DEEP EXCAVATIONSSheet Pile Walls

Sheet pile walls are constructed by driving prefabricated sections into the ground. Soil conditions may allow

for the sections to be vibrated into ground instead of it being hammer driven. The full wall is formed byconnecting the joints of adjacent sheet pile sections in sequential installation. Sheet pile walls providestructural resistance by utilizing the full section. Steel sheet piles are most commonly used in deepexcavations, although reinforced concrete sheet piles have also being used successfully.

Steel sheet piling is the most common because of several advantages over other materials:1. Provides high resistance to driving stresses.2. Light weight3. Can be reused on several projects.4. Long service life above or below water with modest protection.5. Easy to adapt the pile length by either welding or bolting6. Joints are less apt to deform during driving.

Sheet pile walls are constructed by:

1. Laying out a sequence of sheet pile sections, and ensuring that sheet piles will interlock.2. Driving (or vibrating) the individual sheet piles to the desired depth.3. Driving the second sheet pile with the interlocks between the first sheet pile and second "locked"4. Repeating steps 2 & 3 until the wall perimeter is completed5. Use connector elements when more complex shapes are used.

Sheet pile wall disadvantages are:1. Sections can rarely be used as part of the permanent structure.2. Installation of sheet piles is difficult in soils with boulders or cobbles. In such cases, the desired walldepths may not be reached.3. Excavation shapes are dictated by the sheet pile section and interlocking elements.4. Sheet pile driving may cause neighborhood distrurbace5. Settlements in adjacent properties may take place due to installation vibrations

SHEET PILE WALL DESIGN WITH DeepXcav 2010

Sheet Pile Wall Section


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Sheet Pile Wall Excavation

c) SECANT PILE WALLS - SECANT PILE WALL DESIGN FOR DEEP EXCAVATIONS:Secant Pile Walls

Secant pile walls are formed by constructing intersecting reinforced concrete piles. The piles are reinforcedwith either steel rebar or with steel beams and are constructed by either drilling under mud or augering.Primary piles are installed first with secondary piles constructed in between primary piles once the latter gainsufficient strength. Pile overlap is typically in the order of 3 inches (8 cm). In a tangent pile wall, there is nopile overlap as the piles are constructed flush to each other. The main advantages of secant or tangent pilewalls are:

1. Increased construction alignment flexibility.2. Increased wall stiffness compared to sheet piles.3. Can be installed in difficult ground (cobbles/boulders).4. Less noisy construction.

The main disadvantages of secant pile walls are:1. Verticality tolerances may be hard to achieve for deep piles.2. Total waterproofing is very difficult to obtain in joints.3. Increased cost compared to sheet pile walls.

Secant pile wall design when steel beams are used involves the use of weaker than normal concrete. Thepile that is lagging the wall between two main beams has to be examined for shear and compressionarching.

Tangent Pile WallTangent pile walls are a variation of secant pile walls and soldier pile walls. However, tangent pile walls areconstructed with no overlap and ideally one pile touches the other. Compared to secant pile walls, tangentpile walls offer the following advantages:


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1. Increased construction alignment flexibility.2. Easier and quicker construction.

The main disadvantages of tangent pile walls are:1. They are can not be used in high groundwater tables without dewatering.2. Each pile is independent from adjacent piles

Soldier Pile and Lagging Wall (Berliner Wall)

Soldier pile and lagging walls are some of the oldest forms of retaining systems used in deep excavations.These walls have successfully being used since the late 18th century in metropolitan cities like New York,Berlin, and London. The method is also commonly known as "Berliner Wall" when steel piles and timberlagging is used. Alternatively, caissons, circular pipes, or concrete piles can also be used as soldier piles butat an increased cost. Timber lagging is typically used although reinforced concrete panels can be alsoutilized for permanent conditions. Soldier pile and lagging walls are formed by:

1. Constructing soldier piles at regular intervals (2 to 4m, typical)2. Excavating in small stages and installing lagging.3. Backfilling and compacting the void space behind the lagging.

Moment resistance in soldier pile and lagging walls is provided solely by the soldier piles. Passive soilresistance is obtained by embedding the soldier piles beneath the excavation grade. The lagging bridgesand retains soil across piles and transfers the lateral load to the soldier pile system.

Soldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They arealso very easy and fast to construct. The major disadvantages of soldier pile and lagging systems are:

1. They are primarily limited to temporary construction.2. Cannot be used in high water table conditions without extensive dewatering.3. Poor backfilling and associated ground losses can result in significant surface settlements.


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4. They are not as stiff as other retaining systems.5. Because only the flange of a soldier pile is embedded beneath subgrade, it is very difficult to control basalsoil movements.

d) Soil Mix Walls

Various methods of soil mixing, mechanical, hydraulic, with and without air, and combinations of both typeshave been used widely in Japan for about 20 years. Soil mixing has been used for many temporary andpermanent deep excavation projects including the Central Artery project in Boston. Known methods includeas Jet Grouting, Soil Mixing, Cement Deep Mixing (CDM), Soil Mixed Wall (SMW), Geo-Jet, Deep SoilMixing, (DSM), Hydra-Mech, Dry Jet Mixing (DJM), and Lime Columns. Each of these methods aims atfinding the most efficient and economical method to mix cement (or in some cases fly ash or lime) with soiland transform soil to become more like a soft rock.

Mechanical soil mixing is performed using single or multiple shafts of augers and mixing paddles. The augeris slowly rotated into the ground, typically at 10-20 rpm, and advanced at 2 to 5 ft (0.5 to 1.5 m) per minute.

Cement slurry is pumped through the hollow stem of the shaft(s) feeding out at the tip of the auger as the

auger advances. Mixing paddles are arrayed along the shaft above the auger to provide mixing and blendingof the slurry and soil. Slurry lubricates the tool and assists in the breaking up of the soil into smaller pieces.Spoils come to the surface since fluid volume is being introduced into the ground. These spoils comprisecement slurry and soil particles with similar cement content as what remains in the ground. After final depthis reached, the tools remain on the bottom of the hole, rotating for about 0.5 to 2 minutes for completemixing. At this point, the tools are raised while continuing to pump slurry at a reduced rate. Withdrawal istypically at twice the speed of penetration, 4 ft to 10 ft (1 m to 3m) per minute.

Steel beams are typically inserted in the fresh mix to provide reinforcement for structural reasons. Acontinuous soil mix wall is constructed by overlapping adjacent soil mix elements. Soil mix sections areconstructed in an alternating sequence with primary elements are formed first and secondary elementsfollowing once the first have gained sufficient strength.

The soil mix method can be very effective at providing very stiff and waterproof retaining systems. However,

it is rather limited to medium and large-scale projects because of high mobilization costs. Insufficient mixstrength may result when mixing organic soils unless a high replacement ratio is maintained. Other issuesinclude difficulties in maintaining consistent compressive strengths throughout the section of a soil mix wall.

Soil mix method


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e) SLURRY WALL RETAINING SYSTEMS FOR DEEP EXCAVATIONS

Diaphragm Walls (U.S. Slurry Walls)

The continuous diaphragm wall (also referred to as slurry wall in the US) is a structure formed and cast in aslurry trench (Xanthakos, 1994). The trench excavation is initially supported by either bentonite or polymer

based slurries that prevents soil incursions into the excavated trench. The term "diaphragm walls" refers tothe final condition when the slurry is replaced by tremied concrete that acts as a structural system either fortemporary excavation support or as part of the permanent structure. This construction sequence is illustratedin Figure 1.The term slurry wall is also applied to walls that are used as flow barriers (mainly in wastecontainment), by providing a low permeability barrier to contaminant transport.

Slurry Wall Equipment

Slurry wall technology hinges on specialized equipment for excavating slurry trenches. The simplest type oftrenching equipment is the mechanical clamshell attached on a kelly bar. Individual contractors havedeveloped their own specialized trenching equipment like hydraulic clamshells, fraise or hydromills (samplemanufacturers: Icos, Bauer, Casagrande, Case Foundation, Rodio etc). Figure 2 shows selected picturesfrom construction of a new subway in Boston (MBTA South Boston Transit way) including two slurry wallconstruction machines

Slurry Wall HistoryThe first diaphragm walls were tested in 1948 and the first full scale slurry wall was built by Icos in Italy in1950 (Puller, 1996) with bentonite slurry support as a cut-off wall. Icos constructed the first structural slurrywall in the late 1950s for the Milan Metro (Puller, 1996). Slurry walls were introduced in the US in the mid1960s by European contractors. The first application in the US was in New York City [1962] for a 7mdiameter by 24m deep shaft (Tamaro, 1990), that was followed by the Bank of California in San Francisco(Clough and Buchignani, 1980), the CNA building in Chicago (Cunningham and Fernandez, 1972), and theWorld Trade Center in New York (Kapp, 1969, Saxena, 1974). The majority of diaphragm wall projects in theUS are located in six cities Boston, Chicago, Washington DC, San Francisco and New York.

Diaphragm walls are extensively used in the Central Artery/Tunnel project (CA/T) in Boston, Massachusetts(Fig. 3). Work in the CA/T involves many cut and cover tunnels constructed under the existing artery. Someof the deepest T- slurry walls, extending 120' below the surface have been constructed for the Central Artery(Lambrechts et al., 1998).

Diaphragm (Structural) Wall ApplicationsEarth retention walls for deep excavations, basements, and tunnels.High capacity vertical foundation elements.Retaining wall-foundationsRetaining wall-water controlUsed in top-down construction method as permanent basement walls

Slurry Wall for Cut-Off Wall ApplicationsWater and seepage control for deep excavationsCut-off curtainsContaminated groundwater / seepage controlGas barriers for landfills

LIMITATIONS OF SLURRY WALLSSlurry wall construction requires the use of heavy construction equipment that requires reasonableheadroom, site area, and considerable mobilization costs. In limited headroom conditions smaller cranescan be used and the technique can be altered to remote backfill mixing, where the excavated soil istransported and mixed to a remote location, and then is returned as backfill.

Cement-bentonite slurry walls also provide another alternative. In this method, the trenches are excavatedunder a slurry that later solidifies and create the permanent barrier/backfill.


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Also, one should check that used bentonite slurry and soil-bentonite slurries are able to withstand chemicalattacks from the insitu soils. In such a case, alternate slurry materials such as attapulgite and treatedbentonites can be used. Other backfill compositions may be used when deemed appropriate (soil-attapulgiteand soil-bentonite with geomembrane inserts). When required, cement-bentonite and soil-cement-bentonitecan provide greater strengths.

SLURRY WALL COSTS

Slurry wall construction cost for cut-off barries is considerably cheeper than diaphragm wall construction fordeep excavations. The differences arise mainly from construction method differences. In cut-off wallsconstruction is much quicker as a continuous trench is excavated and backfilled and reinforcement cagesare seldomly used. In contrast, in diaphragm walls the wall perimeter is constructed panel by panel andreinforcement cages are almost always used.

Figure 1: Typical construction sequence of slurry walls : (A) Trenching under slurry, (B) End stop inserted(steel tube or other), (C) Reinforcement cage lowered into the slurry-filled trench, (D) Concreting by tremiepipes.


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Figure 2: Diaphragm Wall Trenching equipment, (A) Mechanical clamshell in front and hydraulic clamshell in

the back, (B) Smaller size mechanical clamshell

Figure 4: Central Artery Tunnel Project, Boston MA (Ladd et al., 1999) where slurry walls have beenextensively used


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F) Retaining wall design

Retaining wall design can be a tedious simple task to carry out. A retaining wall design has to account for anumber of factors, foremost being the stability of the wall itself. Last, the design has to account for thespecific retaining wall type that is used. In simple terms, different retaining wall types might require someadditional design checks. Typically a retaining wall design has to consider the following:

a) Earth - Water pressures in retaining wall design

Before all, a designer has to appropriately select the type of lateral earth pressures that are expected to acton the wall. For most retaining walls active or at-rest earth pressures are appropriate. Passive soilresistance should be used with caution. The possibility of including water pressures has to be considered ifsufficient drainage is not provided. In the USA, depending on the design approach, some design codes(LRFD) apply safety factors that multiply each pressure by a safety factor. In Europe, a strength designapproach is applied where soil strength is divided by safety factors and loads are multiplied according totheir nature (temporary and permanent). Each method has its benefits and its shortcomings.

b) External Stability checks in retaining wall design

External stability checks refer to calculations that represent the overall stability of the retaining wall as if the

retaining wall structure acts as a whole single body. Two calculations are typically performed:

b1) Sliding stability of retaining walls: This calculation considers the retaining wall stability in thehorizontal direction. The horizontal components of forces are calculated and separated into driving andresisting forces. Soil and retaining wall weights are calculated and then the horizontal shear resistance atthe base can be computed as Base Shear Resistance= Sum of Vertical forces x tan (soil friction angle) +Base Length x Soil-Wall Adhesion. Then the overall sliding stability if given by:

Factor of safety sliding = Resisting horizontal forces / driving horizontal forces

Under normal conditions a safety factor of atleast 1.5 is required.

b2) Overturning stability of retaining walls: This type of calculations considers the stability of the wallagaingst toppling (i.e. turning over). This calculation is performed by calculating the moment each forcecomponent is generating about a given point in the wall. The toe of the wall is usually taken as the point ofrotation. Moments are then subdivided into resisting and driving moments and the overturning safety factoris calculated as:

Factor of safety overturning = Resisting moments / driving moments


c) Bearing Stability in retaining wall design: In all cases a retaining wall has to be founded in some kindof base material (be that rock or soil). When a retaining wall is based on soil the bearing stability tends to bemore critical. The first task in this check is to properly compute bearing stresses on the toe and heel of thewall. The reason why bearining stresses have to be computed on both sides is because the overturningcauses increased stresses in the toe and reduced stresses on the heel base. The bearing stresses have to

be examined againgst the permissible bearing stresses and a minimum safety factor of 3.0 is typicallyspecified. Using such a high safety factor typically ensures that wall settlements are kept within acceptablelevels. Otherwise detailed settlement alculations are required if settlement control is critical.

d) Global stability in retaining wall design:Another item of concern is the overall global stability of aretaining wall. In some cases, while the overturning and sliding resistance as well as the bearing checksyield acceptable factors the wall might be succeptible to an overall rotational type failure that extends wellbelow the retaining wall itself. Such a failure mode is most commonly accounted in hillsides where weakersoil zones exist or when a soft geomaterial is found below the wall base.


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e) Structural checks in retaining wall design: Once a stability checks are satisfactory then one candesign the actual retaining wall structure itself. For concrete retaining walls this involves the proper sizing oflongitudinal and shear reinforcement if required. Limited wall bending is generated in most gravity walls thatsolely rely on their own weight for stability. Hence, in many cases the provided reinforcement is the minimumrequired for thermal and shrinkage effects.

Support Systems for Deep Excavations

The role of support systems is to provide lateral bracing for retaining walls. Support systems can besubdivided into external and internal depending on the load transfer mechanism. External supports (namelytiebacks) work by transferring lateral excavation loads beyond the active zone of soil movements (retainedside). On the other hand, internal supports such as struts, rakers, or floor slabs, transfer lateral loads withinthe excavation (across opposing walls or to other internal structures). In all cases, support and retainingsystems have to work closely together in order to guarantee a high level of performance.

g) SUPPORT SYSTEMS FOR DEEP EXCAVATIONS:

Cross-lot/Internal Bracing - Braced Excavations

Cross-lot or internal bracing transfers the lateral earth (and water pressures) between opposing wallsthrough compressive struts. Rakers resting on a foundation mat or rock offer another internal bracingalternative. Typically the struts are either pipe or I- beam sections and are usually preloaded to provide avery stiff system. Installation of the bracing struts is done by excavating soil locally around the strut and onlycontinuing the excavation once preloading is complete. A typical sequence of excavation in cross-lot bracedexcavations is shown in Figure 1. The struts rest on a series of wale beams that distribute the strut load tothe diaphragm wall.

Pre-loading ensures a rigid contact between interacting members and is accomplished by inserting ahydraulic jack as each side of an individual pipe strut between the wale beam and a special jacking pblatewelded to the strut (Fig. 2, Xanthakos, 1994). The strut load can either be measured with strain gages or canbe estimated using equations of elasticity by measuring the increased separation between the wale and the

strut. Figure 3 shows the basic arrangement for the wedging, and the telescoping preloading methods.

In some earlier projects the struts were not preloaded, and as a result when the excavation progresseddeeper the soil and the wall movements were large (C1). Thus it has become standard practice to preloadstruts in order to minimize wall movements.

Cross-lot bracing makes sense in narrow excavations (60ft to 120ft) when tieback installation is not feasible.The struts can bend excessively under their own weight if the excavation spacing is too large. In addition,special provisions have to taken to account for thermal expansion and contraction of the struts.

The typical strut spacing is in the range of 15ft, both in the vertical and the horizontal direction. This is largerthan the typical spacing when tiebacks are used, because the pre-loading levels are much higher. A clearbenefit of using struts is that there are no tieback openings in the slurry wall, thus eliminating one source of

leakage.


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Figure 1: Typical excavation sequence in cross-lot excavations: (A) V-cut initial cantilever excavation, (B)Strut installation and pre-loading in small trenches in soil berms, (C) V-cut excavation to next level and strut

installation, (B) Final grade.

Figure 2: View of cross-lot strut supported excavation with 3DEEP


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Figure 3: (a) preloading arrangement, and (b) measured brace stiffness (Xanthakos, 1994)

Figure 4: Methods of preloading struts; Wedging (top), Telescoping pipe (bottom)


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Figure 5 Cross-lot supported excavation NYU Medical Center, New York City


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h) Top/Down Construction

Top/down or up/down construction methods are another method for constructing deep excavations. In thiscase the basement floors are constructed as the excavation progresses. The top/down method has beenused for deep excavation projects where tieback installation was not feasible and soil movements had to beminimized. Figures 1 through 2 show construction photographs from two top/down excavations in Boston.

The general top/down construction sequence is shown in Figure 3. The Post Office Square Garage inBoston (7-levels deep) is one of the best-instrumented and documented top/down projects in the US(Whittle, et al., Whitman et al., 1991).

The sequence construction begins with retaining wall installation and then load-bearing elements that willcarry the future super-structure. The basement columns (typically steel beams) are constructed before anyexcavation takes place and rest on the load bearing elements. These load bearing elements are typicallyconcrete barrettes constructed under slurry (or caissons). The top few feet of a barrette with a steel beamcan be seen in Figure 2. Then the top floor slab is constructed with at least on construction (glory) hole leftopen to allow removal of spoil material (Figs. 2, 3).

The excavation starting at the glory hole begins once the top floor has gained sufficient strength. Soil under

the top basement floor is excavated around the basement columns to slightly lower than the first basementfloor elevation in order to allow for the installation of the forms for the first level basement slab. Glory holesare left open within each newly formed basement floor slab and the procedure is repeated. Each floor restson the basement columns that were constructed earlier (Fig. 2).

Figure 1:Top/Down Excavation (Beth Israel Deaconess Hospital, Boston)


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Figure 2: Millenium Place excavation. Left: Looking up at a glory hole, Top right: Lowest most level note LBEon the left and the barrette, Bottom right: close up view the same barrette (LBE) and steel beam.


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Figure 3: Top/down basic construction.a) Slurry wall and basement column construction

b) Ground floor construction and pouringc) Excavation and floor construction under and above the ground floord) Excavation & lowest basement floor completed


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At-Rest Lateral Earth Pressures in Retaining Walls

Soils in nature have an in-situ state of stress. This "in-situ" state of stress is commonly refered to as "At-rest"conditions. If a natural surface is level and all stratigraphy is also level, then the "At-rest" state of stress canbe described by two main stresses:

a) The vertical stress (effective and total)

b) The horizontal stress

All effective horizontal stresses are typically defined as a ratio of the effective vertical stress times acoefficient of lateral pressure. For "At-rest" conditions, this coefficient is typically defined as:

Ko1 = [1-Sin(friction angle)]

However, as many researchers (Ladd et. al) have reported, the initial lateral state of stress is linked to thesoil stress history. For example, soils that have experienced a greater vertical state of stress in the past tendto hold memory of their overloaded history. As a result, these types of soils tend to "lock-in" greater lateralstresses in "At-rest" conditions. These types of soils are typically referred to as overconsolidated. In suchcases, the coeffiecient of at-rest lateral earth pressures can be defined from an equation relating to the

Overconsolidation Ratio (OCR) such as:

Ko = Ko1 x (OCR )^n

Where OCR is the ratio of maximum past to current effective vertical stress. The exponent n can be definedby running a series of laboratory or insitu experiments.

When a sloped ground is included Eurocode 7 recommends multiplying the above coefficients by (1+ sin(Beta)) where Beta is the surface inclination angle.

Do you have to include At-Rest Pressures for Retaining Wall Design?

An important aspect of "At-Rest" lateral earth pressures is that they typically take place at zero lateral wall

displacement. This means that a wall will experience full "At-Rest" lateral pressures only if it does not yield.Such a case could take place if a stiff gravity wall fully bears on bedrock, in such a condition a retaining wallwill essentially feel the full "At-rest" driving soil pressures.

Given that "At-rest" pressures are considerably greater than active earth pressures, one might conclude thatall braced excavations should be designed with at-rest pressures. Doing so, might actually do greaterdamage than good. While having a greater capacity might be beneficial, if a series of supports areprestressed to the "full" theoretical "at-rest" load then "in-practice" the wall might actually move back into theretained soil causing a series of unpredicted problems. The author is aware of a diaphragm wall designedthis way that moved as much as 12inches (30 cm) back into the retained soil causing severe wall andpavement cracking in the process. Part of the reason for such observations is that engineers tend to be onthe safe side when providing "at-rest" pressure coeffiecients and other geotechnical strength parameters.Thus, the actual "at-rest" coefficient might be smaller than originally predicted.

Once you start getting wall movement you are moving into active pressure territory.

Active Lateral Earth Pressures in Retaining Walls

Active pressures what is it and why?

When we excavate on one side of a retaining wall an unbalanced load condition is created. As it is veryobvious, the retained side wants to move into the recently excavated zone. However, we engineers boldly


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introduce our retaining walls in the middle to take care of the unbalance. At a closer glance what ishappening is this:

a) The excavated zone gets unloaded, the zone above subgrade is removed and therefore there is zerolateral pressure on the recentrly excavated depth.

b) As a result the retained soil, which is initially at an "At-rest" state starts moving towards the excavation.

c) The result of this movement on the retained side is a reduction in lateral earth pressures from the initialstate.

d) If this lateral movement keeps increasing, the driving lateral earth pressure reaches a minimum value,beyond which there is no further drop in lateral pressure. Essentially, this condition is a limit state or in otherwords a failure condition. This means that the soil has fully mobilized its strength.

While at a first glance this might seem unsafe, active earth pressures take place at very smalldisplacements, typically less than 3% of the total excavation depth.

As all effective horizontal stresses, active earth pressures are defined as a ratio of the effective verticalstress times a coefficient of lateral pressure. For "active" conditions, this coefficient is typically defined as:

Ka = [1-Sin(friction angle)]/[1+sin(friction angle)]

Lateral active earth pressures can be modified to include wall friction, seismic effects, and surfaceinclination.

Do you have to include Active Pressures for Retaining Wall Design?

Engineers typically design gravity walls for active earth pressures and then apply a safety factor in theoverall wall design. Over many years this practice has proven safe given that the retaining wall is allowed toexperience small lateral displacements.

At-Rest Lateral Earth Pressures in Retaining Walls

Soils in nature have an in-situ state of stress. This "in-situ" state of stress is commonly refered to as "At-rest"conditions. If a natural surface is level and all stratigraphy is also level, then the "At-rest" state of stress canbe described by two main stresses:

a) The vertical stress (effective and total)

b) The horizontal stress

All effective horizontal stresses are typically defined as a ratio of the effective vertical stress times acoefficient of lateral pressure. For "At-rest" conditions, this coefficient is typically defined as:

Ko1 = [1-Sin(friction angle)]


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However, as many researchers (Ladd et. al) have reported, the initial lateral state of stress is linked to thesoil stress history. For example, soils that have experienced a greater vertical state of stress in the past tendto hold memory of their overloaded history. As a result, these types of soils tend to "lock-in" greater lateralstresses in "At-rest" conditions. These types of soils are typically referred to as overconsolidated. In suchcases, the coeffiecient of at-rest lateral earth pressures can be defined from an equation relating to theOverconsolidation Ratio (OCR) such as:

Ko = Ko1 x (OCR )^n

Where OCR is the ratio of maximum past to current effective vertical stress. The exponent n can be definedby running a series of laboratory or insitu experiments.

When a sloped ground is included Eurocode 7 recommends multiplying the above coefficients by (1+ sin(Beta)) where Beta is the surface inclination angle.

Do you have to include At-Rest Pressures for Retaining Wall Design?

An important aspect of "At-Rest" lateral earth pressures is that they typically take place at zero lateral walldisplacement. This means that a wall will experience full "At-Rest" lateral pressures only if it does not yield.Such a case could take place if a stiff gravity wall fully bears on bedrock, in such a condition a retaining wall

will essentially feel the full "At-rest" driving soil pressures.

Given that "At-rest" pressures are considerably greater than active earth pressures, one might conclude thatall braced excavations should be designed with at-rest pressures. Doing so, might actually do greaterdamage than good. While having a greater capacity might be beneficial, if a series of supports areprestressed to the "full" theoretical "at-rest" load then "in-practice" the wall might actually move back into theretained soil causing a series of unpredicted problems. The author is aware of a diaphragm wall designedthis way that moved as much as 12inches (30 cm) back into the retained soil causing severe wall andpavement cracking in the process. Part of the reason for such observations is that engineers tend to be onthe safe side when providing "at-rest" pressure coeffiecients and other geotechnical strength parameters.Thus, the actual "at-rest" coefficient might be smaller than originally predicted.

Active Lateral Earth Pressures in Retaining Walls

Active pressures what is it and why?

When we excavate on one side of a retaining wall an unbalanced load condition is created. As it is veryobvious, the retained side wants to move into the recently excavated zone. However, we engineers boldlyintroduce our retaining walls in the middle to take care of the unbalance. At a closer glance what ishappening is this:


b) As a result the retained soil, which is initially at an "At-rest" state starts moving towards the excavation.

c) The result of this movement on the retained side is a reduction in lateral earth pressures from the initialstate.

d) If this lateral movement keeps increasing, the driving lateral earth pressure reaches a minimum value,beyond which there is no further drop in lateral pressure. Essentially, this condition is a limit state or in otherwords a failure condition. This means that the soil has fully mobilized its strength.


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While at a first glance this might seem unsafe, active earth pressures take place at very smalldisplacements, typically less than 3% of the total excavation depth.


Ka = [1-Sin(friction angle)]/[1+sin(friction angle)]

Lateral active earth pressures can be modified to include wall friction, seismic effects, and surfaceinclination.

Do you have to include Active Pressures for Retaining Wall Design?

Engineers typically design gravity walls for active earth pressures and then apply a safety factor in theoverall wall design. Over many years this practice has proven safe given that the retaining wall is allowed toexperience small lateral displacements.

Passive Lateral Earth Pressures in Retaining WallsPassive pressures what is it and why?

When we excavate on one side of a retaining wall an unbalanced load condition is created. As it is veryobvious, the retained side wants to move into the recently excavated zone. However, we engineers boldlyintroduce our retaining walls in the middle to take care of the unbalance. At a closer glance what ishappening is this:


b) As a result the retained soil, which is initially at an "At-rest" state starts moving towards the excavation,

while the soil on the excavated side gets further compressed into the excavation.

c) The result of this movement on the excavation side is an increase in the resisting lateral earth pressuresfrom the theoretical initial state for the current excavation level.

d) If this lateral movement keeps increasing, the lateral lateral earth pressure reaches a maximum value,beyond which there is no further increase in lateral pressure. Essentially, this condition is a limit state or inother words a failure condition. This means that the soil has fully mobilized its strength.

This condition, if left uncontrolled can be unsafe, partly because passive earth pressures take place at largedisplacements.


Ka = [1+Sin(friction angle)]/[1-sin(friction angle)]

Lateral passive earth pressures can be modified to include wall friction, seismic effects, and surfaceinclination.


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Do you have to include Passive Pressures for Retaining Wall Design?

Engineers typically design gravity walls for passive earth pressures and then apply a safety factor in theoverall wall design. Over many years this practice has proven safe given that the retaining wall is allowed toexperience small lateral displacements and that the passive earth pressure is used to determine a safe wallembedment that is later multiplied by a safety factor. In Eurocode 7 the toe embedment safety factor isincorporated in soil strength reduction factors. Another approach is to divide passive pressures by a safety

factor when designing a retaining wall.

Apparent Lateral Earth Pressures in Retaining WallsApparent Lateral Earth Pressures what is it and why?

While active and passive earth theory are applicable in simple cases, multileve braced excavations tend toexperience more complex earth pressures. Since cantilever walls can typically reach only down to 15ftdepth, deep excavations require bracing if an economical design is to be achieved.

Peck (1969) in a monumental for geotechnical engineering publication compiled a series of case studieswhere bracing loads where measured in struted excavations for the Chicago metro. Peck noticed thatmeasured maximum bracing reactions did not behave as active or at-rest earth pressure theory wouldpredict. From back calculations, Peck noticed that upper struts were more heavily loaded than active theorywould predict, and lower level struts where less loaded compared to active pressures. Peck summarized hisfindings in "Apparent" earth pressures that were purposed as means to determine maximum strut reactions.

In a mishap, Peck noted the maximum pressures as factors times gamma x h where gamma = soil density,and h = excavation depth.

Many engineers take this gamma as the total soil unit weight and do not apply any water pressures. In apersonal communication, Mr. Peck confirmed that the effective unit weight should be taken and waterpressures should be added separetely.

The Federal Highway Administration has also fallen in the same trap. In its design manual, it recommendsusing a total unit weight. Again, as Mr. Peck has quoted, "the apparent earth pressures are essentiallyeffective active pressures mutliplied by a factor and redestributed as a rectangle or a trapezoid".

Unfortunately many engineers have misused the original concept and they typycally apply zero driving soilpressures below subgrade. This practice can lead to unsafe results and is strongly not recommended.

So if one thing is left in your mind from this webpage, it must be to use apparent earth pressures with greatcaution and judgement.


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Shoring design - Excavation Support Design

Shoring is a term used to describe a system that functions to retain earth, water, and adjacent structures,when an excavation has to be performed. Shoring design can be a very complicated matter. The designer

has to content with many unknowns and factors that influence the behavior of the excavation. Typically,there are two systems in excavations that must be designed: A) the Earth Retention System that containsthe earth i.e. the support wall (sheet pile, diaphragm wall, etc.), and b) the Support System (i.e. the internalor external bracing such as rakers, struts, or tiebacks) that supports the earth retention system.

Performing detailed calculations for both systems can be a very time consuming process, especially whenparameters have to be changed. In addition, many current software programs do not offer an integratedplatform of structural and geotechnical analyses required to design shoring excavations. As a result, thedesigner is forced to use numerous software programs to analyze the excavation and the structural systemseperately. With the exception of finite element analyses, there are very few theoretical solutions forcalculating lateral soil pressures from complex surface profiles. Furthermore, the designer has to save underdifferent filenames different stages for the same excavation. As a result, the whole process can becomeunescessarily complicated and time consuming. DeepXcav addresses most of these issues and provides anintegrated structural and geotechnical platform for designing deep excavations.

Shoring can be designed with both traditional and non-linear methods of analyses. While it is realized thattraditional methods of analysis have obvious limitations in predicting real behavior accurately, they areimportant for framing the problem and providing a back-check for more rigorous finite element methods.


Before all, a designer has to appropriately select the type of lateral earth pressures that are expected to acton the wall. For most retaining walls active orat-rest earth pressures are appropriate. Passive soilresistance should be used with caution. The possibility of including water pressures has to be considered ifsufficient drainage is not provided. In the USA, depending on the design approach, some design codes(LRFD) apply safety factors that multiply each pressure by a safety factor. In Europe, a strength designapproach is applied where soil strength is divided by safety factors and loads are multiplied according totheir nature (temporary and permanent). Each method has its benefits and its shortcomings.


External stability checks refer to calculations that represent the overall stability of the shoring system. Twocalculations are typically performed:

b1) Passive resistance of shoring systems: This calculation considers the available earth resistance inthe horizontal direction below the excavation.


For temporary conditions a safety factor of atleast 1.2 is required.

b2) Moment - rotational stability: This type of calculation considers the stability of the shoring for rotationalfailure of the wall.



c) Bearing Stability in retaining wall design: In all cases a retaining wall has to be founded in some kindof base material (be that rock or soil). When a retaining wall is based on soil the bearing stability tends to be


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more critical. The first task in this check is to properly compute bearing stresses on the toe and heel of thewall. The reason why bearining stresses have to be computed on both sides is because the overturningcauses increased stresses in the toe and reduced stresses on the heel base. The bearing stresses have tobe examined againgst the permissible bearing stresses and a minimum safety factor of 3.0 is typicallyspecified. Using such a high safety factor typically ensures that wall settlements are kept within acceptablelevels. Otherwise detailed settlement alculations are required if settlement control is critical.

d) Global stability in for shoring designAnother item of concern is the overall global stability of theexcavation. In some cases, while the other checks yield acceptable factors the wall might be succeptible toan overall rotational type failure that extends well below the retaining wall itself. Such a failure mode is mostcommonly accounted in hillsides where weaker soil zones exist or when a soft geomaterial is found belowthe wall base.

e) Structural checks in a shoring system: Once a stability checks are satisfactory then one can designthe actual individual shoring components. For concrete retaining walls this involves the proper sizing oflongitudinal and shear reinforcement if required.

DeepXcav 2010- Shoring Design


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Retaining wall design

Retaining wall design can be a tedious simple task to carry out. A retaining wall design has to account for a

number of factors, foremost being the stability of the wall itself. Last, the design has to account for thespecific retaining wall type that is used. In simple terms, different retaining wall types might require someadditional design checks. Typically a retaining wall design has to consider the following:


Before all, a designer has to appropriately select the type of lateral earth pressures that are expected to acton the wall. For most retaining walls active orat-rest earth pressures are appropriate. Passive soilresistance should be used with caution. The possibility of including water pressures has to be considered ifsufficient drainage is not provided. In the USA, depending on the design approach, some design codes(LRFD) apply safety factors that multiply each pressure by a safety factor. In Europe, a strength designapproach is applied where soil strength is divided by safety factors and loads are multiplied according totheir nature (temporary and permanent). Each method has its benefits and its shortcomings.


External stability checks refer to calculations that represent the overall stability of the retaining wall as if theretaining wall structure acts as a whole single body. Two calculations are typically performed:

b1) Sliding stability of retaining walls: This calculation considers the retaining wall stability in thehorizontal direction. The horizontal components of forces are calculated and separated into driving andresisting forces. Soil and retaining wall weights are calculated and then the horizontal shear resistance atthe base can be computed as Base Shear Resistance= Sum of Vertical forces x tan (soil friction angle) +Base Length x Soil-Wall Adhesion. Then the overall sliding stability if given by:



b2) Overturning stability of retaining walls: This type of calculations considers the stability of the wallagaingst toppling (i.e. turning over). This calculation is performed by calculating the moment each forcecomponent is generating about a given point in the wall. The toe of the wall is usually taken as the point ofrotation. Moments are then subdivided into resisting and driving moments and the overturning safety factoris calculated as:



c) Bearing Stability in retaining wall design: In all cases a retaining wall has to be founded in some kind

of base material (be that rock or soil). When a retaining wall is based on soil the bearing stability tends to bemore critical. The first task in this check is to properly compute bearing stresses on the toe and heel of thewall. The reason why bearining stresses have to be computed on both sides is because the overturningcauses increased stresses in the toe and reduced stresses on the heel base. The bearing stresses have tobe examined againgst the permissible bearing stresses and a minimum safety factor of 3.0 is typicallyspecified. Using such a high safety factor typically ensures that wall settlements are kept within acceptablelevels. Otherwise detailed settlement alculations are required if settlement control is critical.

d) Global stability in retaining wall design:Another item of concern is the overall global stability of aretaining wall. In some cases, while the overturning and sliding resistance as well as the bearing checksyield acceptable factors the wall might be succeptible to an overall rotational type failure that extends well


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below the retaining wall itself. Such a failure mode is most commonly accounted in hillsides where weakersoil zones exist or when a soft geomaterial is found below the wall base.

e) Structural checks in retaining wall design: Once a stability checks are satisfactory then one candesign the actual retaining wall structure itself. For concrete retaining walls this involves the proper sizing oflongitudinal and shear reinforcement if required. Limited wall bending is generated in most gravity walls thatsolely rely on their own weight for stability. Hence, in many cases the provided reinforcement is the minimum

required for thermal and shrinkage effects.

DEEP2008 - Gravity module performs retaining wall design computations efficiently, in an easy touse manner.


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Appendix-2

PIEZOCONE 3.0

The piezocone test (CPTU) is a cone penetration test (CPT) with

additional measurement of the porewater pressure at one or more

locations (U1, U2and U3) on the penetrometer surface (Figure 8).

Cone penetration testing, with porewater pressure measurements,

gives a more reliable determination of stratification and soil type than a

standard CPT. In addition, CPTU provides a better basis for interpreting

the results in terms of mechanical soil properties.

Mechanical properties to be evaluated are:

shear strength parameters

deformation and consolidation characteristics.

The results from a CPTU can be used, directly, for the design of piled

foundations in clay.

Erb Cpd Seminar 11th December 2009

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