Geotechnical engineering
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Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering includes investigating existing subsurface conditions and materials; assessing risks posed by site conditions; designing earthworks and structure foundations; and monitoring site conditions, earthwork and foundation construction.
A typical geotechnical engineering project begins with a site investigation of soil and bedrock on and below an area of interest to determine their engineering properties including how they will interact with, on or in a proposed construction. Site investigations are needed to gain an understanding of the area in or on which the engineering will take place. Investigations can include the assessment of the risk to humans, property and the environment from natural hazards such as earthquakes, landslides, sinkholes, soil liquefaction, debris flows and rock falls.
A geotechnical engineer then determines and designs the type of foundations, earthworks, and/or pavement subgrades required for the intended man-made structures to be built. Foundations are designed and constructed for structures of various sizes such as high-rise buildings, bridges, medium to large commercial buildings, and smaller structures where the soil conditions do not allow code-based design.
Foundations built for above-ground structures include shallow and deep foundations. Retaining structures include earth-filled dams and retaining walls. Earthworks include embankments, tunnels, levees, channels, reservoirs, deposition of hazardous waste and sanitary landfills.
Geotechnical engineering is also related to coastal and ocean engineering. Coastal engineering can involve the design and construction of wharves, marinas, and jetties. Ocean engineering can involve foundation and anchor systems for offshore structures such as oil platforms.
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[edit] History
Karl Terzaghi (1883 - 1963) is generally recognized as the father of modern soil mechanics and geotechnical engineering. [1]
[edit] Practicing engineers
Geotechnical engineers are typically graduates of a four-year civil engineering program and often hold a masters degree. Governments usually license and regulate practicing geotechnical engineers. In the United States, state governments will typically license engineers who have graduated from an ABET accredited school, completed several years of work experience, and passed the professional engineering examination.[2] California has an additional licensing program for geotechnical engineers who have already obtained licensure as civil engineers.
[edit] Soil mechanics
In geotechnical engineering, soils are considered a three-phase material composed of: rock or mineral particles, water and air. The voids of a soil, the spaces in between mineral particles, contain the water and air.
The engineering properties of soils are affected by four main factors: the predominant size of the mineral particles, the type of mineral particles, the amount of fine particles, and the relative quantities of mineral, water and air present in the soil matrix. Fine particles (fines) are defined as particles less than 0.075 mm in diameter.
[edit] Basic soil properties
- Bulk density
- Total unit weight: Cummulative weight of the solid particles, water and air in the material per unit volume. Note that the air phase is often assumed to be weightless.
- Dry unit weight: Weight of the solid particles of the soil per unit volume.
- Saturated unit weight: Weight of the soil when all voids are filled with water such that no air is present per unit volume. Note that this is typically assumed to occur below the water table.
- Porosity
- Ratio of the volume of voids (containing air and/or water) in a soil to the total volume of the soil expressed as a percentage. A porosity of 0% implies that there is neither air nor water in the soil.
- Permeability
- A measure of the ability of water to flow through the soil, expressed in units of velocity.
- Consolidation
- As a noun, the state of the soil with regards to prior loading conditions; soils can be underconsolidated, normally consolidated or over-consolidated.
- As a verb, the process by which water is forced out of a soil matrix due to loading, causing the soil to deform, or decrease in volume, with time.
- Soil Settlement
- A decrease in total soil volume concurrent with a decrease in voids.
- Shear strength
- Amount of shear force which a soil can resist without failing.
- Atterberg Limits
- Liquid limit, plastic limit, and shrinkage limit used in defining other engineering properties of a soil and in soil classification.
- Plasticity
- A defining characteristic of soils, most notably clays and silts.
[edit] 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 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.
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. 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.
Geophysical exploration is also sometimes used; geophysical techniques used for subsurface exploration include measurement of seismic waves (pressure, shear, and Rayleigh waves), using surface-wave methods and/or downhole methods, and electromagnetic surveys (magnetometer, resistivity, and ground-penetrating radar).
[edit] 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 samples are taken using a variety of samplers; some provide only disturbed samples, while others can provide relatively undisturbed samples. Samples can be obtained by methods as simple as digging out soil from the site using a shovel. Samples taken this way are disturbed samples. More sophisticated sampling methods include split-spoon samplers, piston samplers, and pushed samplers. The Standard Penetration Test sampler is a split-spoon sampler, and there are similar samplers with larger sample-barrels. The SPT test returns a sample as well as providing in-situ soil data. SPT samples are disturbed samples, but samples from larger split-spoon samplers can be considered relatively undisturbed. Piston 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 Pitcher Barrel sampler is a direct-push sampler 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.
[edit] 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.
- 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 weight of water to 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 Liquid Limit and Plastic 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.
[edit] Foundations
A building's foundation transmits loads from buildings and other structures to the earth. Geotechnical engineers design foundations based on the load characteristics of the structure and the properties of the soils and/or bedrock at the site.
The primary considerations for foundation support are bearing capacity, settlement, and ground movement beneath the foundations. Bearing capacity is the ability of the site soils to support the loads imposed by buildings or structures. Settlement occurs under all foundations in all soil conditions, though lightly loaded structures or rock sites may experience negligible settlements. For heavier structures or softer sites, both overall settlement relative to unbuilt areas or neighboring buildings, and differential settlement under a single structure, can be concerns. Of particular concern is settlement which occurs over time, as immediate settlement can usually be compensated for during construction. Ground movement beneath a structure's foundations can occur due to shrinkage or swell of expansive soils due to climactic changes, frost expansion of soil, melting of permafrost, slope instability, or other causes. All these factors must be considered during design of foundations.
Many building codes specify basic foundation design parameters for simple conditions, frequently varying by jurisdiction, but such design techniques are normally limited to certain types of construction and certain types of sites, and are frequently very conservative.
In areas of shallow bedrock, most foundations may bear directly on bedrock; in other areas, the soil may provide sufficient strength for the support of structures. In areas of deeper bedrock with soft overlying soils, deep foundations are used to support structures directly on the bedrock; in areas where bedrock is not economically available, stiff "bearing layers" are used to support deep foundations instead.
[edit] Shallow foundations
[edit] Footings
Footings (often called "spread footings" because they spread the load) are structural elements which transfer structure loads to the ground by direct areal contact. Footings can be isolated footings for point or column loads, or strip footings for wall or other long (line) loads. Footings are normally constructed from reinforced concrete cast directly onto the soil, and are typically embedded into the ground to penetrate through the zone of frost movement and/or to obtain additional bearing capacity.
[edit] Slab foundations
A variant on spread footings is to have the entire structure bear on a single slab of concrete underlying the entire area of the structure. Slabs must be thick enough to provide sufficient rigidity to spread the bearing loads somewhat uniformly, and to minimize differential settlement across the foundation. In some cases, flexure is allowed and the building is constructed to tolerate small movements of the foundation instead. For small structures, like single-family houses, the slab may be less than 30cm thick; for larger structures, the foundation slab may be several meters thick.
Slab foundations can be either slab-on-grade foundations or embedded foundations, typically in buildings with basements. Slab-on-grade foundations must be designed to allow for potential ground movement due to changing soil conditions.
[edit] Deep foundations
Deep foundations are foundations for structures or heavy loads that circumvent weak or compressible soil layers to provide adequate foundation support. There are many types of deep foundations including piles, drilled shafts, caissons, piers, and earth stabilized columns.
[edit] Lateral earth support structures
A retaining wall is a structure that holds back earth. Retaining walls stabilize soil and rock from downslope movement or erosion and provide support for vertical or near-vertical grade changes. Cofferdams and bulkheads, structures to hold back water, are sometimes also considered retaining walls.
The primary geotechnical concern in design and installation of retaining walls is that the retained material is attempting to move forward and downslope due to gravity. This creates soil pressure behind the wall, which can be analysed based on the angle of internal friction (φ) and the cohesive strength (c) of the material and the amount of allowable movement of the wall. This pressure is smallest at the top and increases toward the bottom in a manner similar to hydraulic pressure, and tends to push the wall forward and overturn it. Groundwater behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.
[edit] Gravity Walls
Gravity walls depend on the size and weight of the wall mass to resist pressures from behind. Gravity walls will often have a slight setback, or batter, to improve wall stability. For short, landscaping walls, gravity walls made from dry-stacked (mortarless) stone or segmental concrete units (masonry units) are commonly used.
Earlier in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or steel-reinforced backfill soil with precast facing; gabions (stacked steel wire baskets filled with rocks), crib walls (cells built up log cabin style from precast concrete or timber and filled with soil) or soil-nailed walls (soil reinforced in place with steel and concrete rods).
For reinforced-soil gravity walls, the soil reinforcement is placed in horizontal layers throughout the height of the wall. Commonly, the soil reinforcement is geogrid, a high-strength polymer mesh, that provide tensile strength to hold soil together. The wall face is often of precast, segmental concrete units that can tolerate some differential movement. The reinforced soil's mass, along with the facing, becomes the gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep (thick) as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall.
[edit] Cantilever walls
Prior to the introduction of modern reinforced-soil gravity walls, cantilevered walls were the most common type of taller retaining wall. Cantilevered walls are made from a relatively thin stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing; converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their stability against high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.
Cantilever walls resist lateral pressures by friction at the base of the wall and/or passive earth pressure, the tendency of the soil to resist lateral movement.
Basements are a form of cantilever walls, but the forces on the basement walls are greater than on conventional walls because the basement wall is not free to move.
[edit] Excavation shoring
Shoring of temporary excavations frequently requires a wall design which does not extend laterally beyond the wall, so shoring extends below the planned base of the excavation. Common methods of shoring are the use of sheet piles or soldier beams and lagging. Sheet piles are a form of driven piling using thin interlocking sheets of steel to obtain a continuous barrier in the ground, and are driven prior to excavation. Soldier beams are constructed of wide flange steel H sections spaced about 2-3 m apart, driven prior to excavation. As the excavation proceeds, horizontal timber or steel sheeting (lagging) is inserted behind the H pile flanges.
In some cases, the lateral support which can be provided by the shoring wall alone is insufficient to resist the planned lateral loads; in this case additional support is provided by walers or tie-backs. Walers are strutural elements which connect across the excavation so that the loads from the soil on either side of the excavation are used to resist each other, or which transfer horizontal loads from the shoring wall to the base of the excavation. Tie-backs are steel tendons drilled into the face of the wall which extend beyond the soil which is applying pressure to the wall, to provide additional lateral resistance to the wall.
[edit] Earth structures
- Pavements
- Embankments
- Reservoirs
- Engineered Slopes
[edit] Slope stability
Slope stability is the analysis of soil covered slopes and its potential to undergo movement. Stability is determined by the balance of shear stress and shear strength. A previously stable slope may be initially affected by preparatory factors, making the slope conditionally unstable. Triggering factors of a slope failure can be climatic events can then make a slope actively unstable, leading to mass movements. Mass movements can be caused by increases in shear stress, such as loading, lateral pressure, and transient forces. Alternatively, shear strength may be decreased by weathering, changes in pore water pressure, and organic material.
[edit] Geosynthetics
Geosynthetics is the umbrella term used to describe a range of synthethic products used to aid in solving some 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, landfills, bank protection and coastal engineering
[edit] See also
- Soil mechanics
- Civil engineering
- Earthworks
- Effective stress
- Geology
- Geotechnical investigation
- Landfill
- Land reclamation
- Laboratory Information Management Systems
- Soil physics
- Soil science
- Pile
- Retaining wall
[edit] Notes
- ^ Soil Mechanics, Lambe,T.William and Whitman,Robert V., Massachusetts Institute of Technology, John Wiley & Sons., 1969.
- ^ Licensure for Engineers. Retrieved on 2006-12-11.
[edit] References
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