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Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil and rock 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 Map ping, 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. 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.
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 has been correlated to soil properties. Sometimes instruments other than the basic CPT probe are used, including:
Flat Plate Dilatometer Test (DMT) is a flate 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 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 ; , 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.23 ; 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 remolded soil sample to determine the Expansion Index (EI), an empirical value required by Building Design Code s, at a water content of 50% for expansive soils, like Expansive Clays .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 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. ; (ASTM D698), Modified Proctor (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. ; Soil suction tests : ASTM D5298. ; 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 a percentage of the weight of water to the dry weight of the soil. GEOPHYSICAL EXPLORATION See Also: 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.8 There are a number of methods used to determine a site's shear wave velocity:
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