Geotechnical investigations (soil Investigation Survey) are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions.
This type of investigation is called a site investigation.
Additionally, geotechnical investigations are also used to measure the thermal resistivity of soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities.
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.
These can also be used to identify contamination in soils prior to development in order to avoid negative environmental impacts.
- Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils’ (ASTM D 1586)
- Standard Practice for Thick Wall, Ring-Lined, Split Barrel, Drive Sampling ofSoils1′ (ASTM D 3550)
- The Standard Practice for Thin-Walled Tube Sampling of Soils for Geotechnical Purposes’ (ASTM D 1587)
- A standard penetration test is an in-situ dynamic penetration test designed to provide information on the properties of soil Investigation survey, while also collecting a disturbed soil sample for grain-size analysis and soil classification.
- A dynamic cone penetrometer test is an insitu test in which a weight is manually lifted and dropped on a cone which penetrates the ground. the number of mm per hit are recorded and this is used to estimate certain soil properties (soil Investigation survey). This is a simple test method and usually needs backing up with lab data to get a good correlation.
- A cone penetration test is performed using an instrumented probe with a conical tip, pushed into the soil Investigation survey 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:
- A piezocone penetrometer 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.
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- seismic piezocone penetrometer 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.
Full flow penetrometers (T-bar, ball, and plate) probes are used in extremely soft clay soils (such as sea-floor deposits / Field soil Investigation survey) 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 and pore water pressure 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. Ultimately the geotechnical professional can use the measured penetration resistance to estimate undrained and remolded shear strengths.
- Helical probe test soil Investigation surveye xploration and compaction testing by the helical probe test (HPT) has become popular for providing a quick and accurate method of determining soil properties at relatively shallow depths. The HPT test is attractive for in-situ footing inspections because it is lightweight and can be conducted quickly by one person. During testing, the probe is driven to the desired depth and the torque required to turn the probe is used as a measure to determine the soil’s characteristics. Preliminary ASTM testing has determined that the HPT method correlates well to standard penetration testing (SPT) and cone penetration testing (CPT) with empirical calibration.
- 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
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.
- 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.
- Expansion Index test
This test uses a remolded soil sample to determine the Expansion Index (EI), an empirical value required by building design codes, at a water content of 50%[clarification needed] 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 any type include remolded, undisturbed, and compacted samples.
- Oedometer test
This can be used to determine consolidation (ASTM D2435) and swelling (ASTM D4546) parameters.
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- 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.
- 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 (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
- 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.
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