A typical geotechnical engineering project begins with a review of project needs to define the required material properties. Then follows a site investigation of soil, rock, fault distribution and bedrock properties 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 building, 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, dikes, 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.
The fields of geotechnical engineering and engineering geology are closely related, and have large areas of overlap. However, the field of geotechnical engineering is a specialty of engineering, where the field of engineering geology is a specialty of geology.
History
Humans have historically used soil as a material for flood control, irrigation purposes, burial sites, building foundations, and as construction material for buildings. First activities were linked to irrigation and flood control, as demonstrated by traces of dykes, dams, and canals dating back to at least 2000 BCE that were found in ancient Egypt, ancient Mesopotamia and the Fertile Crescent, as well as around the early settlements of Mohenjo Daro and Arappa in the Indus valley. As the cities expanded, structures were erected supported by formalized foundations; Ancient Greeks notably constructed pad footings and strip-and-raft foundations. Until the 18th century, however, no theoretical basis for soil design had been developed and the discipline was more of an art than a science, relying on past experience.
Several foundation-related engineering problems, such as the Leaning Tower of Pisa, prompted scientists to begin taking a more scientific-based approach to examining the subsurface. The earliest advances occurred in the development of earth pressure theories for the construction of retaining walls. Henri Gautier, a French Royal Engineer, recognized the "natural slope" of different soils in 1717, an idea later known as the soil's angle of repose. A rudimentary soil classification system was also developed based on a material's unit weight, which is no longer considered a good indication of soil type.
Classical geotechnical mechanics began in 1773 with Charles Coulomb's (a physics) introduction of mechanics to soil problems. Using the laws of friction and cohesion to determine the true sliding surface behind a retaining wall, Coulomb inadvertently defined a failure criteria for soil. By combining Coulomb's theory with Christian Otto Mohr's theory of a 2D stress state, the Mohr-Coulomb theory was developed, a graphical construction still used today.
Other relevant developments during this period include: Henry Darcy's defining of hydraulic conductivity; Joseph Boussinesq's (a mathematician and physicist) theory of stress distribution; William Rankine's simplification of Coulomb's earth pressure theory; and Albert Atterberg's examination of clay consistency.
Modern geotechnical engineering began in 1925 with the publication of Erdbaumechanik by Karl Terzaghi (a civil engineer and geologist). Generally recognized as the father of modern soil mechanics and geotechnical engineering, Terzaghi's research on the settlement of clays and failure due to piping beneath dams was considered groundbreaking.
Practicing engineers
Geotechnical engineers are typically graduates of a four-year civil engineering program and often hold a masters degree. Geotechnical engineers are typically licensed and regulated as Professional Engineers (PEs) in most states; currently only California and Oregon have licensed geotechnical engineering specialties. 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
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 grain size distribution, 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.
Soil properties
The following properties of soils are used by geotechnical engineers in analysis of site conditions and design of earthworks, retaining structures, and foundations.
Unit Weight
Total unit weight: Cumulative 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.
void ratio is the ratio of the volume of voids to the volume of solid particles in a soil. Void ratio is mathematically related to the porosity and is more commonly used in geotechnical formulae than porosity.
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.
Shear strength
Amount of shear stress a soil can resist without failing.
Atterberg Limits
Liquid limit, plastic limit, and shrinkage limit, related to the plasticity of a soil. Used in estimating other engineering properties of a soil and in soil classification.
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. Geologic mapping and interpretation of geomorphology is typically completed in consultation with a geologist or engineering geologist.
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 (CPT) or standard penetration test (SPT). CPT allows continuous recording of soil changes with depth, whereas SPT only records major changes at discrete steps of 150 mm (6 in); however, SPT allows soil sampling for laboratory testing.
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 to 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 is typically performed using an instrumented probe with a conical tip, pushed into the soil hydraulically. A basic CPT instrument reports tip resistance and frictional resistance along the friction sleeve, which is located just above the tip. 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).
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. In general, methods intended to collect undisturbed samples cost more than disturbed samples. It should be noted that the discussion of soil disturbance is almost entirely in reference to clay and silty-clay soils. Sands can only be collected undisturbed using very expensive ground freezing techniques, and are rarely practiced.
A variety of soil samplers exist to meet the needs of different engineering projects and budgets. Small projects, those predicting minor stress changes and straightforward topography, can be suitably designed with disturbed samples. Projects involving large stress changes or complicated topography will require undisturbed samples for stress-strain-strength testing in a laboratory. Speaking terrestrially, the standard penetration test (SPT), which uses a thick-walled split spoon sampler, is the most common way to collect disturbed samples. Piston samplers, employing a thin-walled tube, are most commonly used for the collection of undisturbed samples. More advanced methods, such as the Sherbrooke block sampler, can be employed for research quality soil sample collection, but it has a limited depth of recovery.
Offshore soil collection introduces many difficult variables. In shallow water, work can be done off a barge. In deeper water a ship will be required. Deepwater soil samplers are normally variants of Kullenberg-type samplers, a modification on a basic gravity corer using a piston (Lunne and Long, 2006). Seabed samplers are also available, which push the collection tube slowly into the soil.
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. Some of the more commonly performed laboratory tests include: Atterberg limits, California bearing ratio, hydraulic conductivity, consolidation, particle-size analysis, soil compaction, triaxial shear, unconfined compression, density index (called relative density in USA) and water content tests.
BOOKS ON GEOTECHNICAL ENGINEERING
No comments:
Post a Comment