Geotechnical investigation

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Geotechnical investigations are performed by geotechnical engineers to obtain information on the physical properties of the soil and rock at a site either to design earthworks and foundations for proposed structures or to obtain data 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 engineer 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. 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 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.

  • Shovel. Samples can be obtained by digging out soil from the site. Samples taken this way are disturbed samples.
  • The standard penetration test (SPT) sampler is a split-barrel sampler of specified dimensions - 1.4 inches (35.6mm) inside diameter, 2.0 inches (50.8mm) outside diameter, and a minimum of 18 inches (45.7 cm) long. Soil samples obtained from the standard penetration test sampler are considered disturbed.
  • 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.
  • Piston samplers. These samplers are thin-walled metal tubes which 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 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.
  • 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 buildup 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. It uses a standard sampler, and is driven in to the soil at the bottom of a borehole using a standardised driving method - a 140 pound (63.4 kg) hammer falling through 30 inches (76cm) using a rotating cathead to lift the hammer. The number of blows required to advance the sampler 12 inches (30.5cm) is recorded. The number obtained is called the "blow count" or SPT "N-Value", and is often used in empirical formulae in geotechnical engineering.

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

  • CPTu - Piezocone Penetrometer. This probe is advanced using the same equipment as a regular CPT probe, but the probe has an additional instrument which measures the groundwater pressure as the probe is advanced.
  • SCPTu - Seismic Piezocone Penetrometer. This probe is advanced using the same equipment as a CPT or CPTu probe, but the probe also has a transducer for detecting shear waves and/or pressure waves produced by a source at the surface.
  • DMT - Flat Plate Dilatometer Test. This probe is advanced using similar equipment to CPT probes, but can apply a lateral force to the soil in which it is embedded and measure the strain induced by various levels of stress applied.
  • Full Flow Penetrometers - T-bar, Ball, and Plate: These probes are used in soft clay soils 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 flat circular plate. In soft clays, soil flows around the probe similar to a viscous fluid. The pressure due to overburden stress is equal on all sides of the probes (unlike with CPT's), so no correction is necessary, reducing a source of error and increasing accuracy, especially in soft soils. The full flow probes can also be cycled up and down to measure the remolded soil resistance. Ultimately the measured penetration resistance is used to estimate the undrained and remolded shear strengths.

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 only be effectively tested on relatively undisturbed samples. Some soil tests measure direct properties of the soil, while others measure "index properties" which 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.[1]
California bearing ratio
ASTM D 1883. A test 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.[2][3]
Direct shear test
ASTM D3080. 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.[4]
Expansion index test
This test uses a compacted sample to estimate the amount of expansion expected in expansive soils, like expansive clays, due to changes in moisture content.[5]
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 any type include remolded, undisturbed, and compacted samples.[6]
Oedometer test
This can be used to determine consolidation (ASTM D2435) and swelling (ASTM D4546) 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 analyzed 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.[7]
R-Value test
California Test 301[8] 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 (ASTM D698), Modified Proctor (ASTM D1557), and California Test 216[9]. These tests are used to determine the maximum unit weight and optimal water content a soil can achieve for a given compaction effort.
Soil suction tests
ASTM D5298. Used in determining the moisture variation at a side, used in design of post-tensioned slabs.
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 into the ground. It can also simulate drained and undrained conditions.
Unconfined compression test
ASTM D2166. 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 the weight of water divided by the dry weight of the soil, expressed as a percentage.

Geophysical exploration

For more information, see: exploration geophysics.

Geophysical methods are used in geotechnical investigations to evaluate a site's behavior in a seismic event. By measuring a soil's shear wave velocity, the dynamic response of that soil can be estimated.[10] There are a number of methods used to determine a site's shear wave velocity:

  • Crosshole method
  • Downhole method (with a seismic CPT or a substitute device)
  • Surface wave reflection or refraction
  • Suspension logging (also known as P-S logging or Oyo logging)
  • Spectral analysis of surface waves (SASW)
  • Reflection microtremor (ReMi)

Other geophysical methods for examining a site include electromagnetic methods, including soil resistivity surveys, and ground-penetrating radar, which can locate buried metallic objects and subsurface voids.

Geotechnical analysis

After the field exploration and laboratory work has been performed, the engineer performs various geotechnical analyses to provide recommendations for construction at the site. Geotechnical investigations are both site-specific and project-specific; an investigation prepared for a proposed housing development may not be useful for a future owner desiring to build commercial buildings, for example.

Depending on the seismicity of the site, and the specific characteristics of the site, various seismic hazards may be evaluated; frequently, local regulations set forth requirements for which hazards must be examined in different areas. Other geologic hazards are often evaluated as well, such as the risk of landslides and sink-holes.

After the seismic and geologic hazards of a site are evaluated, the geotechnical engineer must prepare recommendations for construction of earthwork, grading, and building foundations. Frequently, the earthworks and foundations are already partially specified before the geotechnical investigation; the geotechnical engineer provides recommendations to make possible the desired construction so that geotechnical distress does not occur, though sometimes the engineer must inform the client that the desired construction methods are not suitable, and recommend alternates.

See also

Notes and references

External links

  • UC Davis Video on typical drilling and sampling methods in geotechnical engineering.