Geotechnical Investigation

Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil and rocks around a site to design earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved.

Surface exploration can include geologic mapping, geophysical methods and photogrammetry, or it can be as simple as a geotechnical professional walking around on the site to observe the physical conditions at the site. To obtain information about the soil conditions below the surface, some form of subsurface exploration is required. Methods of observing the soils below the surface, obtaining samples and determining physical properties of the soils and rocks include test pits, trenching (particularly for locating faults and slide planes), boring and in-situ tests.

Soil sampling

Borings come in two main varieties, large-diameter and small-diameter borings. Large-diameter borings are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to visually and manually examine the soil and rock stratigraphy in situ. Small-diameter borings are frequently used to allow a geologist or engineer examine soil or rock cuttings from the drilling operation, to retrieve soil samples at depth and to perform in-place soil tests.

Soil samples are obtained in either ‘disturbed’ or ‘undisturbed’ condition; however, ‘undisturbed’ samples are not truly undisturbed. A disturbed sample is one in which the structure of the soil has been changed sufficiently such that tests of structural properties of the soil will not be representative of in-situ conditions, and only properties of the soil grains can be accurately determined. An undisturbed sample is one where the condition of the soil in the sample is close enough to the conditions of the soil in situ to allow tests of structural properties of the soil to be used to approximate the properties of the soil in situ.

Soil samplers

Soil samples are taken using a variety of samplers; some provide only disturbed samples, while others can provide relatively undisturbed samples.

1. Shovel: Samples can be obtained by digging out soil from the site. Samples taken this way are disturbed samples.
2. Hand/Machine-driven auger: This sampler typically consists of a short cylinder with a cutting edge attached to a rod and handle. The sampler is advanced by a combination of rotation and downward force. Samples taken this way are disturbed samples.
3. Continuous flight auger: A method of sampling using an auger as a corkscrew. The auger is screwed into the ground and then lifted out. Soil is retained on the blades of the auger and kept for testing. Soil sampled this way is considered disturbed.
4. Split-spoon/SPT sampler: Utilized in the ‘standard test method for standard penetration test (SPT) and split-barrel sampling of soils’ (ASTM D 1586), this sampler is typically a 18″–30″ long, 2.0″ outside diameter (OD) hollow tube split in half lengthwise. A hardened metal drive shoe with a 1.375″ opening is attached to the bottom end, and a one-way valve and drill rod adapter at the sampler head. It is driven into the ground with a 140-pound hammer falling 30″. The blow counts (hammer strikes) required to advance the sampler a total of 18″ are counted and reported. It is generally used for non-cohesive soils, and samples taken this way are considered disturbed.
5. Modified California sampler: Similar in concept to the SPT sampler, the sampler barrel has a larger diameter and is usually lined with metal tubes to contain samples. Samples from the modified California sampler can be considered undisturbed if the soil is not excessively soft, does not contain gravel or is not a very dense sand.
6. Shelby tube sampler: Utilized in the ‘standard practice for thin-walled tube sampling of soils for geotechnical purposes’ (ASTM D 1587), this sampler consists of a thin-walled tube with a cutting edge at the toe. A sampler head attaches the tube to the drill rod, and contains a check valve and pressure vents. Generally used in cohesive soils, this sampler is advanced into the soil layer, generally 6″ less than the length of the tube. The vacuum created by the check valve and cohesion of the sample in the tube cause the sample to be retained when the tube is withdrawn. Standard ASTM dimensions are 2″ OD, 36″ long, 18 gauge thickness; 3″ OD, 36″ long, 16 gauge thickness and 5″ OD, 54″ long, 11 gauge thickness. It should be noted that ASTM allows other diameters as long as they are proportional to the standardized tube designs, and the tube length is to be suited for field conditions. Soil sampled in this manner is considered undisturbed.
7. Piston samplers: These samplers are thin-walled metal tubes that contain a piston at the tip. The samplers are pushed into the bottom of a borehole, with the piston remaining at the surface of the soil while the tube slides past it. These samplers will return undisturbed samples in soft soils, but are difficult to advance in sands and stiff clays, and can be damaged (compromising the sample) if gravel is encountered. The Livingstone corer, developed by D. A. Livingstone, is a commonly used piston sampler. A modification of the Livingstone corer with a serrated coring head allows it to be rotated to cut through subsurface vegetable matter such as small roots or buried twigs.
8. Pitcher barrel sampler: This sampler is similar to piston samplers, except that there is no piston. There are pressure-relief holes near the top of the sampler to prevent pressure build up of water or air above the soil sample.

In-situ tests

A standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the properties of soil, while also collecting a disturbed soil sample for grain size analysis and soil classification.

A cone penetration test (CPT) is performed using an instrumented probe with a conical tip, pushed into the soil hydraulically at a constant rate. A basic CPT instrument reports tip resistance and shear resistance along the cylindrical barrel. CPT data have been correlated to soil properties. Sometimes, instruments other than the basic CPT probe are used, including:

1. CPTu – piezocone penetrometer: This probe is advanced using the same equipment as a regular CPT probe, but the probe has an additional instrument that measures the groundwater pressure as the probe is advanced.
2. SCPTu – seismic piezocone penetrometer: This probe is advanced using the same equipment as a CPT or CPTu probe, but the probe is also equipped with either geophones or accelerometers to detect shear waves and/or pressure waves produced by a source at the surface.
3. Full flow penetrometers – T-bar, ball and plate: These probes are used in extremely soft clay soils (such as seafloor deposits) and are advanced in the same manner as the CPT. As their names imply, the T-bar is a cylindrical bar attached at right angles to the drill string forming what look likes a T, the ball is a large sphere and the plate is a flat circular plate. In soft clays, soil flows around the probe similar to a viscous fluid. The pressure due to overburdened stress and pore water pressure is equal on all sides of the probes (unlike with CPTs), so no correction is necessary, reducing the source of error and increasing accuracy. It is especially desired in soft soils due to the very low loads on the measuring sensors. Full flow probes can also be cycled up and down to measure remoulded soil resistance. Ultimately, the geotechnical professional can use the measured penetration resistance to estimate undrained and remoulded shear strengths.

Flat plate dilatometer test (DMT) is a flat plate probe often advanced using CPT rigs, but can also be advanced from conventional drill rigs. A diaphragm on the plate applies a lateral force to the soil materials and measures the strain induced for various levels of applied stress at the desired depth interval.

Laboratory tests

A wide variety of laboratory tests can be performed on soils to measure a wide variety of soil properties. Some soil properties are intrinsic to the composition of the soil matrix and are not affected by sample disturbance, while other properties depend on the structure of the soil as well as its composition, and can be effectively tested only on relatively undisturbed samples. Some soil tests measure direct properties of the soil, while others measure index properties that provide useful information about the soil without directly measuring the property desired.

Atterberg limits: The Atterberg limits define the boundaries of several states of consistency for plastic soils. The boundaries are defined by the amount of water a soil needs to be at one of those boundaries. The boundaries are called the plastic limit and the liquid limit, and the difference between them is called the plasticity index. The shrinkage limit is also a part of the Atterberg limits. The results of this test can be used to help predict other engineering properties.

California bearing ratio: This test is used to determine the aptitude of a soil or aggregate sample as a road subgrade. A plunger is pushed into a compacted sample and its resistance is measured. This test was developed by Caltrans, but it is no longer used in the Caltrans pavement design method. It is still used as a cheap method to estimate the resilient modulus.

Direct shear test: The direct shear test determines the consolidated, drained strength properties of a sample. A constant strain rate is applied to a single shear plane under a normal load and the load response is measured. If this test is performed with different normal loads, the common shear strength parameters can be determined.

Expansion index test: This test uses a remoulded soil sample to determine the expansion index (EI), an empirical value required by building design codes, at a water content of 50 per cent for expansive soils, like expansive clays.

Hydraulic conductivity tests: There are several tests available to determine a soil’s hydraulic conductivity. They include the constant head, falling head and constant flow methods. The soil samples tested can be of any type, including remoulded, undisturbed and compacted samples.

Oedometer test: This test is used to determine consolidation and swelling parameters.

Particle size analysis: This is done to determine the soil gradation. Coarser particles are separated in the sieve analysis portion and the finer particles are analysed with a hydrometer. The distinction between coarse and fine particles is usually made at 75 μm. The sieve analysis shakes the sample through progressively smaller meshes to determine its gradation. The hydrometer analysis uses the rate of sedimentation to determine particle gradation.

R-value test (California test 301): This test measures the lateral response of a compacted sample of soil or aggregate to a vertically applied pressure under specific conditions. This test is used by Caltrans for pavement design, replacing the California bearing ratio test.

Soil compaction tests: (Standard Proctor test (ASTM D698), Modified Proctor test (ASTM D1557) and California Test 216.) These tests are used to determine the maximum unit weight and optimal water content a soil can achieve for a given compaction effort.

Triaxial shear tests: This is a type of test that is used to determine the shear strength properties of a soil. It can simulate the confining pressure a soil would see deep in the ground. It can also simulate drained and undrained conditions.

Unconfined compression test: This test compresses a soil sample to measure its strength. The modifier ‘unconfined’ contrasts this test to the triaxial shear test.

Water content: This test provides the water content of the soil, normally expressed as a percentage of the weight of water to the dry weight of the soil.