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Geotechnical engineering is concerned with the engineering properties of earth materials. Geotechnical engineers investigate the soil and bedrock below a site to determine their engineering properties and how they will interact with the proposed construction. The geotechnical engineer determines and designs the type of foundations, earthworks, and pavements required for the intended man-made structures to be built.
Geotechnical engineers design foundations for such structures as high-rise buildings, bridges, and medium to large commercial buildings but also work on smaller structures where the soil conditions do not allow code-based design. The foundations built for above-ground structures include shallow foundations (footings), deep foundations (driven piles and drilled piers), and retaining walls. Geotechnical engineers also design structures built in or of soil or rock, including tunnels, embankments, levees, earth dams, channels, reservoirs, and hazardous waste and sanitary landfills.
Geotechnical engineers also assess the risk to humans, property and the environment from natural hazards such as earthquakes, landslides, sinkholes, soil liquefaction, debris flows, and rock falls (all involving natural materials). Geotechnical engineering is also applicable to coastal and ocean engineering applications, such as construction of wharves, marinas, jetties, as well as foundation/anchor systems for offshore structures such as oil platforms.
Karl Terzaghi is widely considered the father of soil mechanics and geotechnical engineering.
Engineering properties of soils
- Main article: Soil Mechanics
Soils have three main components: rock or mineral particles, water, and air. The rock and mineral particles range in size from boulders (defined as solid rock particles larger than 300 mm in diameter) to clay minerals less than 2 µm across. The properties of soils are affected by the predominant size of the soil particles, the amount of fines, defined as particles less than 2 µm in diameter, and the amount of clay minerals present, as well as by the amount of water and air present in the soil matrix.
Soils where no air is present are saturated, and are usually found below the water table.
Soil properties relevant to engineering include the soil's bulk density, the porosity of the soil (usually expressed as the void ratio, the ratio of the volume of air and water to the volume of solid particles), the permeability of the soil, the consolidation state of the soil, the shear strength of the soil, the plasticity of clay and silt soils, and expansion properties of the soil when abosorbing water.
Geotechnical investigation
- Main article: Geotechnical investigation
Geotechnical engineers perform geotechnical investigations to obtain information on the physical properties of soil and rock underlying (and sometimes adjacent to) 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 testing of the soil samples retrieved.
Surface Exploration
Surface exploration can include Geologic mapping, geophysical methods, and Photogrammetry, or it can be as simple as an engineer walking around on the site to observe the physical conditions at the site.
Subsurface Exploration
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 rock include test pits, trenching (particularly for locating faults and slide planes), borings, and cone penetration tests.
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.
CPT - Cone Penetration Test. A Cone Penetration Test 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.
Geophysical Methods
- Seismic waves (pressure, shear, and Rayleigh waves)
- Crosshole method
- Downhole method (Seismic CPT)
- Surface wave methods
- Seismic reflection
- Seismic refraction
- Electromagnetic (radar, resistivity)
- Optical/Acoustic Televiewer Survey
Soil Sampling
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.
- SPT - Standard Penetration Test. This test returns a sample as well as providing in-situ soil data. SPT samples are disturbed samples.
- 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.
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 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 being indicated.
- In-situ density. This test requires an undisturbed sample, and measures the bulk density of the soil.
- Moisture content. This test provides the water content of the soil, normally expressed as a percentage of the dry weight of the soil.
- Grain Size Analysis using sieves and Hydrometer Tests. These tests are performed on dried soils and do not require undisturbed samples, and determine the distribution of grain sizes within the soil sample.
- Atterberg Limits (ASTM D4318). These tests determine the moisture contents at which the portion of the soil smaller than 2 mm grain size transitions from a brittle solid to a plastic solid, and from a plastic solid to a viscous liquid. The results are called the Plastic Limit and the Liquid Limit, respectively. The Plasticity Index is the difference between the Plastic Limit and Liquid Limit, and is the range of moisture contents over which the soil acts as a plastic solid. Atterberg Limits tests are used to determine whether the soil will act primarily as a silt or a clay, and whether it is considered "highly plastic".
- Expansion Index Test. This test uses a remolded sample of the soil to estimate the amount of expansion which can be expected in expansive soils due to changes in moisture content.
- Direct Shear Test (ASTM D3080)
- Unconfined Compression (UC) (ASTM D2166)
- Triaxial Shear Tests
- CD - Consolidated drained
- CU - Consolidated undrained (ASTM D4647)
- UU - Unconsolidated undrained (ASTM D2850)
- Oedometer Test - including consolidation (ASTM D2435) and swell tests (ASTM D4546)
- Soil Suction Tests (ASTM D5298)
- Compaction Tests - Standard Proctor (ASTM D698), Modified Proctor (ASTM D1557), and California Test 216. These tests are used to determine the maximum bulk density to which a soil can be compacted given a specified compaction energy. The soil sample is divided into parts, where each part is brought to a different moisture content through adding water or drying, and compacted into a mold using a specified number of blows of a hammer of standard size and weight falling through a specified distance. The density obtained varies with different moisture contents; the "maximum density" is the highest obtained at any moisture content, while the "optimum moisture" is the moisture content at which the maximum density is obtained. This test is used primarily for providing field control for earthwork, where typical specifications will require that soil be compacted to at least a certain percentage of the maximum density obtained in a compaction test.
- California Bearing Ratio (ASTM D1883) Test. This test measures the response of a compacted sample of soil or aggregate to a bearing pressure, and is used primarily for the design of pavement sections. This test was developed by CalTrans, but is no longer used in the CalTrans pavement design method. It is still used by other agencies as a cheap method to estimate resilient modulus.
- 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.
Foundations
- Shallow Foundations
- Footings
- Deep Foundations
- Drilled Piers
- Driven Piles
Lateral Earth Support Structures
- Retaining Walls
- Conventional Retaining Walls
- Basements
- Gabion Walls
- Keystone Walls
- Reinforced Slopes
- Excavation Shoring
- Sheet Piles
- Soldier Beams and Lagging
- Tiebacks
Earth Structures
- Pavements
- Embankments
- Reserviors
- Engineered Slopes
Geosynthetics
Geosynthetics is the umbrella term used to describe a range of synthethic products used to solve geotechnical problems. The term is generally regarded to encompass four main products; geotextiles, geogrids, geomembranes, and geocomposites. The synthetic nature of the products make them suitable for use in the ground where high levels of durability are required, this is not to say that they are indestructible. Geosynthetics are available in a wide range of forms and materials, each to suit a slightly different end use. These products have a wide range of applications and are currently used in many civil and geotechnical engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering
Recommended Reading
- Holtz, R. and Kovacs, W. (1981), An Introduction to Geotechnical Engineering, Prentice-Hall, Inc. ISBN 0-13-484394-0
- Bowles, J. (1988), Foundation Analysis and Design, McGraw-Hill Publishing Company. ISBN 0-07-006776-7
- Cedergren, Harry R. (1977), Seepage, Drainage, and Flow Nets, Wiley. ISBN 0471141798
- Kramer, Steven L. (1996), Geotechnical Earthquake Engineering, Prentice-Hall, Inc. ISBN 0-13-374943-6
- Freeze, R.A. & Cherry, J.A., (1979), Groundwater, Prentice-Hall. ISBN 0133653129
- NAVFAC (Naval Facilities Engineering Command) (1986) Design Manual 7.01, Soil Mechanics, US Government Printing Office
- NAVFAC (Naval Facilities Engineering Command) (1986) Design Manual 7.02, Foundations and Earth Structures, US Government Printing Office
- NAVFAC (Naval Facilities Engineering Command) (1983) Design Manual 7.03, Soil Dynamics, Deep Stabilization and Special Geotechnical Construction, US Government Printing Office
See also
Geotechnical Engineering Directory
"Geotechnical engineering"