In Situ Tests Used in Soil Exploration | Soil Engineering

In situ tests give a more realistic picture of soil properties basically because of two reasons:

i. The in situ tests cover a larger area and volume of soil mass than the laboratory tests which are conducted on small-size samples of soil.

ii. Several site factors, such as non-homogeneity of soil, groundwater table, moisture content, cracks, fissures exist­ing in the site are included in the in situ tests subject to the zone of influence of the stresses applied in the in situ tests and the number of in situ tests conducted. Hence the properties of soil obtained from in situ tests are more representative than those obtained from laboratory tests.

Following are the various types of in situ tests that are used in soil exploration:

1. Standard penetration test.

2. Static cone penetration test.

3. Dynamic cone penetration test.

4. Pressure-meter test.

5. Geophysical exploration.

1. Standard Penetration Test:

This test determines the in situ penetration resistance of the soil in a borehole. The test is the most popular, simple, and economical. This test is also widely used in India.

Test Procedure:

The test consists of driving a standard split-spoon sampler into the soil under the blows of a hammer and counting the number of blows for each 15-cm penetration of the sampler. The test is conducted in course of soil exploration at specified depths in a borehole.

Test Preparations:

Where a casing is used, it should not be driven below the level at which the test is made. For cohesionless deposits, which cannot stand without a casing, the bottom of the casing should not be deeper than 15 cm below the level at which SPT is conducted, and where bentonite solution is used as the drilling fluid, this depth can be up to 30 cm.

Driving the Sampler:

The standard split-spoon sampler is driven into the soil by the blows of a 63.5-kgf weight, falling freely from a height of 75 cm, for a distance of 45 cm. The number of blows required for successive 15-cm penetrations of the sampler is noted.

The first 15-cm penetration of the sampler is taken as the seating drive. The total blows required for the second and third 15-cm penetration of the sampler is termed penetration resistance (N). If the sampler cannot be driven for the 45-cm penetration, the number of blows for the last 30-cm penetration of the sampler is taken as the penetration resistance (N).

The penetration resistance is reported as refusal and the test is stopped if:

i. Fifty blows are required for any 15-cm penetration.

ii. Total 100 blows are obtained.

iii. Ten successive blows produce no advance of the sampler.

Corrections to SPT N Value:

Following two corrections are generally done to the observed standard penetration resistance:

i. Correction for overburden pressure.

ii. Correction for dilatancy.

These are explained below:

i. Correction for Overburden Pressure:

If two soils, having the same relative density, but with different confining pressures, are subjected to SPT, the soil with the higher confining pressure gives a higher penetration resistance. Since the confining pressure in cohesionless soils increases with depth, SPT gives a lower N value at shallow depth and a higher N value at large depth.

For uniformity and standardization, the SPT N value is always reported at a standard confining pressure of about 100 kPa (10 t/m²), by applying a correction for overburden pressure. As per IS – 2131-1981, the correction for overburden pressure is applied using Fig. 14.18.

It may be noted from Fig. 14.18 that the overburden correction is positive if σ₀‘ < 100 kPa and negative if σ₀‘ > 100 kPa.

Murthy (1995) proposed Eqs. (14.6) and (14.7) for the computation of correction for overburden pressure –

However, deviations are observed in the values computed from Eq. (14.7) from those obtained from Fig. 14.18. Equations (14.8)-(14.10) are proposed for the computation of correction for overburden pressure.

where N’ is the observed N value, N_co the SPT N value corrected for the overburden pressure, and σ₀‘ the effective overburden pressure at the depth, where SPT is done.

ii. Correction for Dilatancy:

Saturated fine sand or silt below the water table gives an apparently higher SPT N value, since such soils dilate during shear and develop negative pore pressure. IS – 2131-1981 recommends the following correction for the observed N value for fine sand and silt below the water table –

The above correction is applied only if N_co is more than 15. Thus, the correction for dilatancy is always negative.

Correlation of SPT N with Engineering Properties:

Based on the experience in the study of various types of soil, correlations were developed between standard penetration resistance (N) and the engineering properties of soil. These correlations for sandy soils and cohesive soils are given in Tables 14.5 and 14.6, respectively.

2. Static Cone Penetration Test:

Principle:

This test involves forcing a cone into the ground and measuring the pressure needed for each increment of pene­tration. The result is expressed in the form of cone resistance (q_c) in kgf/cm². Since the cone is pushed rather than driven, the test is also known as static cone penetration test (SCPT). The most commonly used cone test is the Dutch cone test. The Dutch cone has an apex angle of 60° and a base area of 10 cm².

Test Procedure:

For obtaining the cone resistance, the cone is pushed downward at a steady rate of 1.5-2 cm/s, which is too fast for drainage to occur in many fine-grained soils. The resistance offered by the soil to the penetration of the cone is directly measured in kgf/cm² and recorded as cone resistance (q_c).

The resistance to penetration is the sum of point resistance at the base and frictional resistance on the sides of the shaft. The more sophisticated systems can differentiate between the point and the frictional components of cone resistance.

Types of Penetrometers:

The following two types of cone penetrometers are available for cone testing:

i. Mechanical penetrometer.

ii. Electrical penetrometer.

The electrical penetrometer gives a continuous record of the penetration resistance and hence reflects better the nature of the soil layers penetrated. The repeatability of the cone test is also good with the electrical penetrometer. It is indispensable for offshore soil investigation but requires skilled operators and better maintenance.

Correlation with Engineering Properties:

IS – 6403-1981 recommends a method for determination of ultimate bearing capacity of soil using static cone pene­tration resistance (q_c). A separate procedure is given for estimation for bearing capacity for cohesionless soils and cohesive soils.

The cone penetration results are correlated to the SPT N value, and indirect correlations are obtained between the cone test results and the engineering properties of soil. Table 14.7 gives the correlation between the cone penetration resistance and the SPT N value for various soils.

Correction:

The observed static cone resistance (q_c) is to be corrected for the dead weight of the sounding rods.

Advantages:

Some of the advantages of the cone penetration testing are the following:

i. It is very rapid; the speed of operation allows considerable data to be obtained in a short period of time.

ii. The use of electrical penetrometers allows precise measurements, and improvements in the equipment allow deeper penetrations.

iii. It is very useful in very soft soils, where recovery of undisturbed samples would be very difficult.

iv. It allows the use of a number of correlations between cone resistance and a desired engineering property.

Disadvantages:

Some of the major drawbacks of CPT are as follows:

i. It is not possible to recover samples for identification.

ii. It is difficult to advance cone in dense or hard deposits.

iii. A stable and fairly strong working surface to support the rig is needed for conducting the cone test.

iv. The method is applicable only in fine-grained deposits (clay, silt, and fine sands), where the material does not have massive resistance to cone penetration.

Suitability:

SCPT is most successful in soft or loose soils, such as silty sands, loose sands, and layered deposits of sands, silts, and clays, as well as in clayey deposits.

3. Dynamic Cone Penetration Test:

In this test, the cone is driven into the ground by application of blows with a 65-kgf hammer, falling through a height of 75 cm, instead of pushing. Cone resistance (N_cd) is measured in terms of the number of blows per 30-cm penetration.

Dynamic cone penetration test (DCPT) is of the following two types:

i. Dry DCPT.

ii. Wet DCPT.

Dry DCPT is performed using a 5-cm-diameter cone without bentonite slurry [IS – 4968 (Part I)]. Wet DCPT is per­formed using a 6.5-cm-diameter cone with bentonite slurry [IS – 4968 (Part II)]. Correlations have been developed between dynamic cone resistance (N_cd) and standard penetration resistance (N) by the Central Building Research Institute (CBRI), Roorkee, India, as shown in Table 14.8.

4. Pressure-Meter Test:

Pressure-meter test (PMT) is an in situ stress-strain test performed on the walls of a borehole using a cylindrical probe that can be inflated radially. PMT was developed in 1954 by a young French engineering student Louis Menard. PMT is widely used in France and Germany. It is used only occasionally in other parts of the world. However, the test is now becoming more and more popular in India.

Pressure-Meter:

Pressure-meter consists of three independent chambers, made of inflatable rubber membranes, stacked one above the other, and held together by steel discs at top and bottom and a rigid hollow tube at the center, as shown in Fig. 14.19.

The top and bottom cells, known as guard cells, are connected to a gas cylinder that contains air, CO₂, or N₂ under pressure. The middle chamber, known as the measuring cell, is connected to a water reservoir. The measuring cell is also called the probe, the real part of the pressure-meter. The purpose of the guard cells is to protect the measuring cell from the end effects, caused by the finite length of the apparatus.

Test Procedure:

IS – 1892-1979 describes PMT as follows:

i. The test is conducted in pre-bored, uncased boreholes at 1-m intervals.

ii. The test is started by opening the valves in the control unit for admitting water into the measuring cell and gas into the guard cells.

iii. The pressure in the guard cells is normally kept equal to the pressure in the measuring cell.

iv. As the pressure is increased, the probe presses against the unlined walls of the borehole.

v. Volumetric deformation of the borehole is obtained by noting the fall in the water level in the water reservoir.

vi. The pressure of water in the measuring cell is increased in stages, and the pressure at each stage is held constant for a fixed time, usually 1 min.

vii. Volumetric deformation is measured at each stage after the elapsed time.

viii. Normally, 10 equal increments of pressure are applied for the soil to reach the ultimate pressure.

Pressure-Volumetric Strain Relation:

Figure 14.20 shows a typical pressure-volumetric strain curve. The soil is initially in an elastic state at low pressure and it enters the plastic phase at high pressure. After that stage, there will be no change in the volume with further increase in pressure.

Uses of PMT:

The pressure deformation data obtained may be used to determine the modulus of deformation, un-drained shear strength, angle of shearing resistance, and other engineering properties of the soil. The test gives valuable informa­tion for the design of foundation.

Merits of PMT:

The merits of pressure-meter test are:

i. The test sets up a stress field in the ground, similar to the one induced by the actual foundation.

ii. PMT makes more direct measurement of soil compressibility and lateral pressures than do SPT and CPT.

iii. In addition, the stress applied from the measuring cell is spread over a larger area of soil than in SPT or CPT, and thus, is likely to be not adversely affected by gravel in the soil.

iv. It forms a better theoretical basis for settlement analyses and for pile capacity analyses.

Demerits:

Following are the demerits of pressure-meter test:

i. PMT is a difficult test to perform.

ii. It is limited by the availability of the equipment, which is very expensive.

iii. It requires skilled personnel, trained to perform the test.

5. Geophysical Exploration:

Geophysical surveys explore large areas rapidly and economically, in contrast to borings, which are conducted at isolated points to large depth. They indicate average soil and ground conditions along an alignment or in an area, rather than along the restricted vertical line at a single location, as in boring. This helps to detect irregularities of the bedrock surface and the interface between different strata.

Geophysical methods are generally suitable for prospecting sites for dams, reservoirs, tunnels, highways, and large groups of structures either onshore or offshore. They are also used to locate gravel deposits and sources of other construction materials.

Geophysical methods can be broadly divided into the following methods:

i. Seismic-refraction method.

ii. Electrical resistivity method.

iii. Magnetic surveys.

iv. Gravity surveys.