Module 2 - NPTELnptel.ac.in/courses/105104137/module2/Lecture7.pdf · · 2017-08-04nptel - advanced foundation engineering-1 module 2 (lecture 7) natural soil deposits and subsoil

NPTEL - ADVANCED FOUNDATION ENGINEERING-1

Module 2

(Lecture 7)

NATURAL SOIL DEPOSITS AND SUBSOIL EXPLORATION

Topics

1.17 CONE PENETRATION TEST

1.18 PRESSUREMETER TEST (PMT)

1.19 DILATOMETER TEST

1.20 CORING OF ROCKS

1.21 PREPARATION OF BORIN LOGS

1.22 DETERMINATION OF HYDRAULIC CONDUCTIVITY IN THE FIELD

1.22.1Open End Test


CONE PENETRATION TEST

The cone penetration test (CPT), originally known as the Dutch cone penetration test, is a versatile sounding method that can be used to determine the materials in a soil profile and estimate their engineering properties. This test is also called the static penetration test, and no boreholes are necessary to perform it. In the original version, a 60° cone with a base area of 10 cm2 was pushed into the ground at a steady rate of about 20 mm/sec, and the resistance to penetration (called the point resistance) was measured.

The cone penetrometers in use at present measure (a) the cone resistance (𝑞𝑞𝑐𝑐) to penetration developed by the cone, which is equal to the vertical force applied to the cone divided by its horizontally projected area, and (b) the frictional resistance (𝑓𝑓𝑐𝑐) which is the resistance measured by a sleeve located above the cone with the local soil surrounding it. The frictional resistance is equal to the vertical force applied to the sleeve divided by its surface area-actually, the sum of friction and adhesion.

Generally, two types of penetrometers are used to measure 𝑞𝑞𝑐𝑐 and 𝑓𝑓𝑐𝑐 :

a. Mechanical friction-cone penetrometer (figure 2.25). In this case the penetrometer tip is connected to an inner set of rods. The tip is first advanced about 40 mm giving the cone resistance. With further thrusting, the top engages the friction sleeve. As the inner rod advances, the rod force is equal to the sum of the vertical force on the cone and sleeve. Subtracting the force on the cone gives the side resistance.


Figure 2.25 Mechanical friction-cone penetrometer (after ASTM 1992)

b. Electric friction-cone penetrometer (figure 2.26). In this case the tip is attached to a

string of steel rods. The tip is pushed into the ground at the rate of 20 mm/sec. wires from the transducers are threaded through the center of the rods and continuously give the cone and side resistance.


Figure 2.26 Electric friction-cone penetrometer (after ASTM 1992)

Figure 2.27 shows the results of penetrometer tests in a soil profile with friction measurement by a mechanical friction-cone penetrometer and an electric friction cone penetrometer.

Figure 2.27 Penetrometer tests with riction measurement (after Ruiter, 1971)

Several correlations that are useful in estimating the properties of soils encountered during an exploration program have been developed for the point resistance (𝑞𝑞𝑐𝑐) and the friction ratio (𝐹𝐹𝑟𝑟) obtained from the cone penetration tests. The friction ratio, 𝐹𝐹𝑟𝑟 , is defined as

𝐹𝐹𝑟𝑟 = friction resistancecone resistance

= 𝑓𝑓𝑐𝑐𝑞𝑞𝑐𝑐

[2.24]

Lancellotta (1983) and Jamiolkowski et al. (1985) showed that the relative density of normally consolidated sand, 𝐷𝐷𝑟𝑟 , and 𝑞𝑞𝑐𝑐 can be correlated as

𝐷𝐷𝑟𝑟(%) = 𝐴𝐴 + 𝐵𝐵 log10 �𝑞𝑞𝑐𝑐�𝜎𝜎′𝑣𝑣

� [2.25]

Where

𝐴𝐴,𝐵𝐵 = constants

𝜎𝜎′𝑣𝑣 = vertical effective stress

The values of 𝐴𝐴 and 𝐵𝐵 are


A B Unit of 𝑞𝑞𝑐𝑐 and 𝜎𝜎′𝑣𝑣

-98 66 metric ton/m2

Figure 2.28 shows the correlations obtained for several sands. Baldi et al. (1982), and Robertson and Campanella (1983) also recommended an empirical relationship between vertical effective stress (𝜎𝜎′𝑣𝑣), relative density (𝐷𝐷𝑟𝑟 ) and 𝑞𝑞𝑐𝑐 for normally consolidated sand. This is shown in figure 2.29.

Figure 2.28 Relationship between 𝐷𝐷𝑟𝑟 and 𝑞𝑞𝑐𝑐 (based on Lancellotta, 1983, and Jamiokowski et al., 1985)


Figure 2.29 Variation of 𝑞𝑞𝑐𝑐𝜎𝜎′𝑣𝑣 and 𝐷𝐷𝑟𝑟 for normally consolidated quartz sand (based on Baldi et al., 1982, and Robertson ad Campanella, 1983)


Figure 2.30 Variation of 𝑞𝑞𝑐𝑐𝜎𝜎′𝑣𝑣 and 𝜙𝜙 for normally consolidated quartz sand (after Robertson ad Campanella, 1983)

Figure 2.30 shows a correlation between 𝜎𝜎′𝑣𝑣 ,𝑞𝑞𝑐𝑐 , and the peak friction angle 𝜙𝜙 for normally consolidated quartz sand. This correlation can be expressed as (Kulhawy and Mayne, 1990).

𝜙𝜙tan−1 �0.1 + 0.38 log � 𝑞𝑞𝑐𝑐𝜎𝜎′𝑣𝑣�� [2.26]

Robertson and Campanella (1983) also provided a general correlation between 𝑞𝑞𝑐𝑐 , friction and 𝐹𝐹𝑟𝑟 , and the type of sol encountered in the field (figure 2.31). Figure 2.32 shows the general range of 𝑞𝑞𝑐𝑐/𝑁𝑁𝐹𝐹 for various types of soil.


Figure 2.31 Robertson and Campanellas’ correlation (1983) between 𝑞𝑞𝑐𝑐 ,𝐹𝐹𝑟𝑟 , and the soil type

Figure 2.32 General range of variation of 𝑞𝑞𝑐𝑐/𝑁𝑁𝐹𝐹 for various types of soil (after Robertson and Campanella, 1983)


The correlations for 𝑞𝑞𝑐𝑐 as proposed in figures 2.30, 2.31, and 2.32, equations 25 and 26 are for normally consolidated sands. In fact, for a general condition, 𝑞𝑞𝑐𝑐 is a function of 𝜎𝜎′𝑣𝑣, relative density, and vertical and lateral initial effective stress. A more rational theory for that correlation has been provided by Salgado, Mitchell, and Jamiolkowski (1997), and readers may refer to that paper for further information.

According to Mayne and Kemper, (1988), in clayey soil the undrained cohesion 𝑐𝑐𝑢𝑢 , preconsolidation pressure 𝑝𝑝𝑐𝑐 and the overconsolidation ratio can be correlated as

� 𝑐𝑐𝑢𝑢𝜎𝜎′𝑣𝑣� = �𝑞𝑞𝑐𝑐−𝜎𝜎𝑣𝑣

𝜎𝜎′𝑣𝑣� 1𝑁𝑁𝐾𝐾

[2.27]

Or

𝑐𝑐𝑢𝑢 = 𝑞𝑞𝑐𝑐−𝜎𝜎𝑣𝑣𝑁𝑁𝐾𝐾

[2.27a]

Where

𝑁𝑁𝐾𝐾 = bearing capacity factor (NK = 15 for electric cone and NK = 20 for mechanical cone)

𝜎𝜎𝑣𝑣 = 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 vertical stress

𝜎𝜎′𝑣𝑣 = effective vertical stress

Consistent units of 𝑐𝑐𝑢𝑢 ,𝜎𝜎𝑣𝑣 ,𝜎𝜎′𝑣𝑣 , and 𝑞𝑞𝑐𝑐 should be used with equation (27):

𝑝𝑝𝑐𝑐

= 0.243(𝑞𝑞𝑐𝑐)0.96

↑

MN/m2 ↑

MN/m2

[2.28]

And

𝑂𝑂𝑂𝑂𝑂𝑂 = 0.37 �𝑞𝑞𝑐𝑐−𝜎𝜎𝑣𝑣𝜎𝜎′𝑣𝑣

�1.01

[2.29]

Where 𝜎𝜎𝑣𝑣 and 𝜎𝜎′𝑣𝑣 = total and effective stress, respectively

PRESSUREMETER TEST (PMT)

The pressuremeter test is an in situ test conducted in a borehole. It was originally developed by Menard (1956) to measure the strength and deformability of soil. It has also been adopted by ASTM as Test Designation 4719. The Menard-type PMT essentially consists of a probe with three cells. The top and bottom ones are guard cells and the middle one is the measuring cell, as shown schematically in figure 2.33a. The test is conducted in a re-bored hole. The pre-bored hole should have a diameter that is between 1.03 to 1.2 times the nominal diameters of the probe.


The probe that is most commonly used has a diameter of 58 mm and a length of 420 mm. the probe cells can be expanded either by liquid or gas. The guard cells are expanded to reduce the end-condition effect on the measuring cell. The measuring cell has a volume (𝑉𝑉𝑡𝑡) of 535 cm3. Following are the dimensions for the probe diameter and the diameter of the borehole as recommended by ASTM:

Figure 2.33 (a) Pressuremeter; (b) plot of pressure versus total cavity volume

Probe diameter (mm)

Borehole diameter

Nominal (mm) Maximum (mm)

44 45 53

58 60 70

74 76 89

In order to conduct a test, the measuring cell volume, 𝑉𝑉𝑡𝑡 , is measured and the probe is inserted into the borehole. Pressure is applied in increments and the volumetric expansion of the cell is measured. This is continued until the soil fails or until the pressure limit of the device is reached. The soil is considered to have failed when the total volume of the expanded cavity (V) is about twice the volume of the original cavity. After the completion of the test, the probe is deflated and advanced for test at another depth.

The results of the pressuremeter test is expressed in a graphical form of pressure versus volume as shown in figure 2.33b. in this figure, Zone I represents the reloading portion during which the soil around the borehole is pushed back into the initial state (that is, the state it was in before


drilling). The pressure, 𝑝𝑝𝑡𝑡 , represents the in situ total horizontal stress. Zone II represents a pseudo-elastic zone in which the cell volume versus cell pressure is practically linear. The pressure, 𝑝𝑝𝑓𝑓 , represents the creep, or yield, pressure. The zone marked III is the plastic zone. The pressure, 𝑝𝑝𝑡𝑡 , represents the limit pressure.

The pressuremeter modulus, 𝐸𝐸𝑝𝑝 , of the soil is determined using the theory of expansion of an infinitely thick cylinder. Thus

𝐸𝐸𝑝𝑝 = 2(1 + 𝜇𝜇)(𝑉𝑉𝑡𝑡 + 𝑣𝑣𝑚𝑚 ) �Δ𝑝𝑝Δ𝑣𝑣� [2.30]

Where

𝑣𝑣𝑚𝑚 = 𝑣𝑣𝑡𝑡+𝑣𝑣𝑓𝑓2

Δ𝑝𝑝 = 𝑝𝑝𝑓𝑓 − 𝑝𝑝𝑡𝑡

Δ𝑣𝑣 = 𝑣𝑣𝑓𝑓 − 𝑣𝑣𝑡𝑡

𝜇𝜇 = Poisson′sratio (which may be assumed to be 0.33)

The limit pressure, 𝑝𝑝𝑡𝑡 , is usually obtained by extrapolation and not by direct measurement.

In order to overcome the difficulty of preparing the borehole to the proper size, self-boring pressuremeters (SBPMT) have also been developed. The details concerning SBPMTs can be found in the work of Baguelin et al. (1978).

Correlations between various soil parameters and the results obtained from the pressuremeter tests have been developed by various investigators. Kuljawy and Mayne (1990) proposed that

𝑝𝑝𝑐𝑐 = 0.45𝑝𝑝𝑡𝑡 [2.31]

Where

𝑝𝑝𝑐𝑐 = preconsolidation pressure

Based on the cavity expansion theory, Baguelin et al. (1978) proposed that

𝑐𝑐𝑢𝑢 = (𝑝𝑝𝑡𝑡−𝑝𝑝𝑡𝑡 )𝑁𝑁𝑝𝑝

[2.32]

Where

𝑐𝑐𝑢𝑢 = undrained shear strength of a clay

𝑁𝑁𝑝𝑝 = 1 + in � 𝐸𝐸𝑝𝑝3𝑐𝑐𝑢𝑢�


Typical values of 𝑁𝑁𝑝𝑝 vary between 5 to 12, with an average of about 8.5. Ohya et al. (1982) (see also Kulhawy and Mayne, 1990) correlated 𝐸𝐸𝑝𝑝 with field standard penetration numbers (𝑁𝑁𝐹𝐹) for sand and clay as follows:

Clay: 𝐸𝐸𝑝𝑝(kN/m2) = 1930𝑁𝑁F0.63 [2.33]

Sand: 𝐸𝐸𝑝𝑝(kN/m2) = 908𝑁𝑁F0.66 [2.34]

DILATOMETER TEST

The use of the flat-plate dilatometer test (DMT) is relatively recent (Marchetti, 1980; Schmertmann, 1986). The equipment essentially consists of a flat plate measuring220 mm (length) × 95mm (width) × 14mm (thickness)(8.66 in.× 3.74in.×0.55 in. ). a thin flat circular expandable steel membrade having a diameter of 60 mm (2.36 in.) is located flush at the center on one side of the plate (figure 2.34a). The dilatometer probe is inserted into the ground using a cone penetrometer testing rig (figure 2.34b). Gas and electric lines extend from the surface control box through the penetrometer rod into the blade. At the required depth, high-pressure nitrogen gas is used to inflate the membrane. Two pressure readings are taken. They are

Figure 2.34 (a) Schematic diagram of a flat-plate dilatometer; (b) dilatometer probe inserted into ground


1. The pressure A to “lift off” the membrane, and 2. The pressure B at which the membrane expands 1.1 mm (0.4 in.) into the surrounding soil

The A and B readings are corrected as follows (Schmertmann, 1986)

Contact stress,𝑝𝑝𝑡𝑡 = 1.05(𝐴𝐴 + ∆𝐴𝐴 − 𝑍𝑍𝑚𝑚 ) − 0.05(𝐵𝐵 − ∆𝐵𝐵 − 𝑍𝑍𝑚𝑚 ) [2.35]

Expansion stress,𝑝𝑝1 = 𝐵𝐵 − 𝑍𝑍𝑚𝑚 − ∆𝐵𝐵 [2.36]

Where

∆𝐴𝐴 = vacuum pressure required to keep the membrane in contact with its seating

∆𝐵𝐵 =air pressure required inside the membrane to deflect in outward to a center expansion of 1.1mm

𝑍𝑍𝑚𝑚 = gauge pressure deviation from zero when vented to atmospheric pressure

The test is normally conducted at depths 200 mm to 300 mm apart. The result of a given test is used to determine three parameters:

1. Material index, 𝐼𝐼𝐷𝐷 = 𝑝𝑝1−𝑝𝑝𝑡𝑡𝑝𝑝𝑡𝑡−𝑢𝑢𝑡𝑡

2. Horizontal stress index, 𝐾𝐾𝐷𝐷 = 𝑝𝑝𝑡𝑡−𝑢𝑢𝑡𝑡𝜎𝜎′𝑣𝑣

3. Dilatometer modulus, 𝐸𝐸𝐷𝐷(kN/m2) = 34.7(𝑝𝑝1kN/m2 − 𝑝𝑝𝑡𝑡kN/m2)

Where

𝑢𝑢𝑡𝑡 = pore water pressure

𝜎𝜎′𝑣𝑣 = 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑡𝑡𝑢𝑢 vertical effective stress

Figure 2.35 shows the results of a dilatometer test conducted in Porto Tolle, Italy (Marchetti, 1980). The subsoil consisted of recent, normally consolidated delta deposits of the Po River. A thick layer of silty clay was found below a depth of 10 ft (𝑐𝑐 = 0; 𝜙𝜙 ≈ 28°). The result obtained from the dilatometer tests have been correlated to several soil properties (Marchetti, 1980). Some of these correlations are given below.


Figure 2.35 A dilatometer test result conducted at Porto Tolle, Italy (after Marchetti, 1980)

a. 𝐾𝐾𝑡𝑡 = �𝐾𝐾𝐷𝐷1.5�

0.47− 0.6 [2.37]

b. 𝑂𝑂𝑂𝑂𝑂𝑂 = (0.5𝐾𝐾𝐷𝐷)1.6 [2.38] c. 𝑐𝑐𝑢𝑢

𝜎𝜎′𝑣𝑣= 0.22 (for normaly consolidated clay) [2.39]

d. � 𝑐𝑐𝑢𝑢𝜎𝜎′𝑣𝑣�𝑂𝑂𝑂𝑂

= � 𝑐𝑐𝑢𝑢𝜎𝜎′𝑣𝑣�𝑁𝑁𝑂𝑂

(0.5𝐾𝐾𝐷𝐷)1.25 [2.40]

e. 𝐸𝐸 = (1 − 𝜇𝜇2)𝐸𝐸𝐷𝐷 [2.41]

Where

𝐾𝐾𝑡𝑡 = coefficient of at − rest earth pressure (chapter 5)

𝑂𝑂𝑂𝑂𝑂𝑂 = overconsolidation ratio

𝑂𝑂𝑂𝑂 = overconsolidated soil

𝑁𝑁𝑂𝑂 = normally consolidated soil

𝐸𝐸 = modulus of elasticity

Schmertmann (1986) also provided a correlation between the material index (𝐼𝐼𝐷𝐷) andthe dilatometer modulus (𝐸𝐸𝐷𝐷) for determination of soil description and unit weight (𝛾𝛾). This is


shown in figure 2.36.

Figure 2.36 Chart for determination of soil description and unit weight (after Schmertmann, 1986); Note: 1 𝑡𝑡/m3 = 9.81 kN/m3

CORING OF ROCKS

When a rock layer is encountered during a drilling operation, rock coring may be necessary. For coring of rocks, a core barrel is attached to a drilling rod. A coring bit is attached to the bottom of the core bared (figure 2.37). The cutting elements may be diamond, tungsten, carbide, and so on. Table 8 summarizes the various types of core barrel and their sizes, as well as the compatible drill rods commonly used for foundation exploration. The coring is advanced by rotary drilling. Water is circulated through the drilling rod during coring, and the cutting is washed out.


Figure 2.37 Rock coring: (a) single-tube core barrel; (b) double-tube core barel

Two types of core barrel are available: the single-tube core barrel (figure 2.37a) and the double-tube core barrel (figure 2.37b). Rock cores obtained by single-tube core barrels can be highly disturbed and fractured because of torsion. Rock cores smaller than the BX size tend to fracture during the coring process.

When the core samples are recovered, the depth of recovery should be properly recorded for further evaluation in the laboratory. Based on the length of the rock core recovered from each run, the following quantities may be calculated for a general evaluation of the rock quality encountered.

Recovery ratio = length of core recoveredtheoretical length of rock cored

[2.42]


Table 8

Standard Size and Designation of Casing, Core Barrel, and Composite Drill Rod

Outside diameter of core barrel bit

Outside diameter of drill rod

Diameter of borehole

Diameter of core sample

Casing and core barrel designation (mm) (in.)

Drill rod designation

(mm) (in.) (mm) (in.) (mm) (in.)

EX 36.51 1 716 E 33.34 1 5

16 38.1 112 22.23 7

8

AX 47.63 178 A 41.28 15

8 50.8 2 28.58 178

BX 58.74 2 516 B 47.63 17

8 63.5 212 41.28 15

8

NX 74.61 21516 N 60.33 23

8 76.2 3 53.98 218

Table 9 Relation between in situ Rock Quality and RQD

RQD Rock quality

0-0.25 Very poor

0.25-0.5 Poor

0.5-0.75 Fair

0.75-0.9 Good

0.9-1 excellent

Rock quality designation (RQD)= Σ length of recovered pieces equal to or larger than 101.6 mm (4 in .)theoretical length of rock cored

[2.43]

A recovery ratio of 1 will indicate the presence of intact rock; for highly fractures rocks, the recovery ratio may be 0.5 or smaller. Table 9 presents the general relationship (Deere, 1963) between the RQD and the in situ rock quality.


PREPARATION OF BORIN LOGS

The detailed information gathered from each borehole is presented in a graphical form called the boring log. As a borehole is advanced downward, the driller generally should record the following information in a standard log:

1. Name and address of the drilling company 2. Driller’s name 3. Job description and number 4. Number and type of boring and boring location 5. Date of boring 6. Subsurface stratification, which can be obtained by visual observation of the soil brought

out by auger, split-spoon sampler, and thin wall Shelby tube sampler 7. Elevation of water table and date observed, use of casing and mud losses, and so on 8. Standard penetration resistance and the depth of SPT 9. Number, type, and depth of soil sample collected 10. In case of rock coring, type of core barrel used, and for each run, the actual length of

coring, length of core recovery, and the RQD

This information should never be left to memory, because that often results in erroneous boring logs.

After completion of the necessary laboratory tests, the geotechnical engineer prepares a finished log that includes notes from the driller’s field log and the results of tests conducted in the laboratory. Figure 2.38 shows a typical boring log. These logs have to be attached to the final soil-exploration report submitted to the client. Note that figure 2.41 also lists the classifications of the soils in the left-hand column, along with the description of each soil (based on the United Soil Classification System).


Figure 2.38 A typical boring log

DETERMINATION OF HYDRAULIC CONDUCTIVITY IN THE FIELD

Several types of field test are now available to determine the hydraulic conductivity of soil. Two fairly easy test procedures described by the U. S. Bureau of Reclamation (1974) are the open end test and the packer test.

Open End Test

The first step in the open end test (figure 2.39) is to advance a borehole to the desired depth. A casing is then driven to extend to the bottom of the borehole. Water is supplied at a constant rate from the top of the casing, and it escapes at the bottom of the borehole. The water level in the


casing must remain constant. Once the steady state of water supply is established, the hydraulic conductivity can be determined as

Figure 2.39 Hydraulic conductivity-open end test (redrawn after U. S. Bureau of Reclamation, 1974)

𝑘𝑘 = 𝑄𝑄5.5𝑟𝑟𝑟𝑟

[2.44]

Where

𝑘𝑘 = hydraulic conductivity

𝑄𝑄 = constant rate of supply of water to the borehole


𝑟𝑟 = inside radius of the casing

𝑟𝑟 = differential head of water

Any system of consistent units may be used in equation (44).

The head, H, has been defined in figure 2.42. Note that for pressure tests (figure 2.42c and 2.42d) the value of H is given as

𝑟𝑟 = 𝑟𝑟(gravity ) + 𝑟𝑟(pressure ) [2.45]

The pressure head, 𝑟𝑟(pressure ), given in equation (45) is expressed in meters (or feet) of water (1 kN/m2 = 0.102m; 1 lb/in.2 = 2.308 ft).

Packer Test

The packer test (figure 2.40) can be conducted in a portion of the borehole during drilling or after drilling has been completed. Water is supplied to the portion of the borehole under test under constant pressure. The hydraulic conductivity can be determined.

Figure 2.40 Hydraulic conductivity determination-packer test (redrawn after U. S. Bureau of Reclamation, 1974)


𝑘𝑘 = 𝑄𝑄2𝜋𝜋𝜋𝜋𝑟𝑟

log𝑒𝑒 �𝜋𝜋𝑟𝑟� (for 𝜋𝜋 ≥ 10𝑟𝑟) [2.46]

𝑘𝑘 = 𝑄𝑄2𝜋𝜋𝜋𝜋𝑟𝑟

sinh−1 𝜋𝜋

2𝑟𝑟 (for 10𝑟𝑟 > 𝜋𝜋 ≥ 𝑟𝑟) [2.47]

Where

𝑘𝑘 = hydraulic conductivity

𝑄𝑄 = constant rate of flow into the hole

𝜋𝜋 = length of portion of the hole under test

𝑟𝑟 = radius of the hole

𝑟𝑟 = diferential pressure head

Note that the differential pressure head is the sum of the gravity head [𝑟𝑟(gravity )] and the pressure head [𝑟𝑟(pressure )]. The packer test is used primarily to determine the hydraulic conductivity of rock. However, as mentioned previously, it and also be used for soils.

Module 2 - NPTELnptel.ac.in/courses/105104137/module2/Lecture7.pdf · · 2017-08-04nptel - advanced foundation engineering-1 module 2 (lecture 7) natural soil deposits and subsoil

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