Lessons Learned From Foundation And Slab Failures On Expansive Soils PdfBy Searlas P. In and pdf 29.03.2021 at 04:35 4 min read
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- Expansive Clay
- Expansive Soils: Problems and Practice in Foundation and Pavement Engineering
- Geotechnical engineering
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Geotechnical engineering , also known as geotechnics , is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles and methods of soil mechanics and rock mechanics for the solution of engineering problems and the design of engineering works. It also relies on knowledge of geology , hydrology , geophysics , and other related sciences.
Geotechnical engineering is important in civil engineering, but also has applications in military , mining , petroleum , coastal , ocean , and other engineering disciplines that are concerned with construction occurring on the surface or within the ground, both onshore and offshore. The fields of geotechnical engineering and engineering geology are closely related, and have large areas of overlap. However, while geotechnical engineering is a specialty of civil engineering , engineering geology is a specialty of geology : they share the same principles of soil mechanics and rock mechanics, but may differ in terms of objects, scale of application, and approaches.
The tasks of a geotechnical engineer comprise the investigation of subsurface conditions and materials; the determination of the relevant physical, mechanical, and chemical properties of these materials; the design of earthworks and retaining structures including dams , embankments , sanitary landfills, deposits of hazardous waste , tunnels , and structure foundations ; the monitoring of site conditions, earthwork, and foundation construction; the evaluation of the stability of natural slopes and man-made soil deposits; the assessment of the risks posed by site conditions; and the prediction, prevention, and mitigation of damage caused by natural hazards such as avalanches , mud flows , landslides , rockslides , sinkholes , and volcanic eruptions.
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 BCE that were found in ancient Egypt , ancient Mesopotamia and the Fertile Crescent , as well as around the early settlements of Mohenjo Daro and Harappa 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 , 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. The application of the principles of mechanics to soils was documented as early as when Charles Coulomb a physicist, engineer, and army Captain developed improved methods to determine the earth pressures against military ramparts.
In the 19th century Henry Darcy developed what is now known as Darcy's Law describing the flow of fluids in porous media.
Joseph Boussinesq a mathematician and physicist developed theories of stress distribution in elastic solids that proved useful for estimating stresses at depth in the ground; William Rankine , an engineer and physicist, developed an alternative to Coulomb's earth pressure theory. Albert Atterberg developed the clay consistency indices that are still used today for soil classification. Modern geotechnical engineering is said to have begun in with the publication of Erdbaumechanik by Karl Terzaghi a mechanical engineer and geologist.
Considered by many to be the father of modern soil mechanics and geotechnical engineering, Terzaghi developed the principle of effective stress, and demonstrated that the shear strength of soil is controlled by effective stress.
Terzaghi also developed the framework for theories of bearing capacity of foundations, and the theory for prediction of the rate of settlement of clay layers due to consolidation.
The interrelationships between volume change behavior dilation, contraction, and consolidation and shearing behavior were all connected via the theory of plasticity using critical state soil mechanics by Roscoe, Schofield, and Wroth with the publication of "On the Yielding of Soils" in Critical state soil mechanics is the basis for many contemporary advanced constitutive models describing the behavior of soil.
Geotechnical centrifuge modeling is a method of testing physical scale models of geotechnical problems. The use of a centrifuge enhances the similarity of the scale model tests involving soil because the strength and stiffness of soil is very sensitive to the confining pressure.
The centrifugal acceleration allows a researcher to obtain large prototype-scale stresses in small physical models. Geotechnical engineers are typically graduates of a four-year civil engineering program and some hold a masters degree.
In the US, geotechnical engineers are typically licensed and regulated as Professional Engineers PEs in most states; currently only California and Oregon have licensed geotechnical engineering specialties. GE certification in State governments will typically license engineers who have graduated from an ABET accredited school, passed the Fundamentals of Engineering examination, completed several years of work experience under the supervision of a licensed Professional Engineer, and passed the Professional Engineering examination.
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. Some of the important properties of soils that are used by geotechnical engineers to analyze site conditions and design earthworks, retaining structures, and foundations are: .
Geotechnical engineers and engineering geologists 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 the 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 in-situ testing two common examples of in-situ tests are the standard penetration test and cone penetration test.
In addition site investigation will often include subsurface sampling and laboratory testing of the soil samples retrieved.
The digging of test pits and trenching particularly for locating faults and slide planes may also be used to learn about soil conditions at depth.
Large diameter borings are rarely used due to safety concerns and expense but are sometimes used to allow a geologist or engineer to be lowered into the borehole for direct visual and manual examination of the soil and rock stratigraphy. A variety of soil samplers exists to meet the needs of different engineering projects. 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 less disturbed samples. More advanced methods, such as the Sherbrooke block sampler, are superior, but even more expensive. Coring frozen ground provides high-quality undisturbed samples from any ground conditions, such as fill, sand, moraine and rock fracture zones. Atterberg limits tests, water content measurements, and grain size analysis, for example, may be performed on disturbed samples obtained from thick-walled soil samplers.
Properties such as shear strength, stiffness hydraulic conductivity, and coefficient of consolidation may be significantly altered by sample disturbance. To measure these properties in the laboratory, high-quality sampling is required. Common tests to measure the strength and stiffness include the triaxial shear and unconfined compression test.
Surface exploration can include geologic mapping , geophysical methods , and photogrammetry ; or it can be as simple as an engineer walking around to observe the physical conditions at the site.
Geologic mapping and interpretation of geomorphology are typically completed in consultation with a geologist or engineering geologist. Geophysical exploration is also sometimes used.
A building's foundation transmits loads from buildings and other structures to the earth. In general, geotechnical engineers:. 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 a 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 climatic changes, frost expansion of soil, melting of permafrost, slope instability, or other causes. 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.
Shallow foundations are a type of foundation that transfers the building load to the very near the surface, rather than to a subsurface layer. Shallow foundations typically have a depth to width ratio of less than 1. 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 another long line loads. 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. 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. Deep foundations are used for structures or heavy loads when shallow foundations cannot provide adequate capacity, due to size and structural limitations.
They may also be used to transfer building loads past weak or compressible soil layers. While shallow foundations rely solely on the bearing capacity of the soil beneath them, deep foundations can rely on end bearing resistance, frictional resistance along their length, or both in developing the required capacity.
Geotechnical engineers use specialized tools, such as the cone penetration test , to estimate the amount of skin and end bearing resistance available in the subsurface. There are many types of deep foundations including piles , drilled shafts, caissons , piers, and earth stabilized columns. Large buildings such as skyscrapers typically require deep foundations.
For example, the Jin Mao Tower in China uses tubular steel piles about 1m 3. In buildings that are constructed and found to undergo settlement, underpinning piles can be used to stabilize the existing building. There are three ways to place piles for a deep foundation.
They can be driven, drilled, or installed by the use of an auger. Driven piles are extended to their necessary depths with the application of external energy in the same way a nail is hammered. There are four typical hammers used to drive such piles: drop hammers, diesel hammers, hydraulic hammers, and air hammers. Drop hammers simply drop a heavy weight onto the pile to drive it, while diesel hammers use a single-cylinder diesel engine to force piles through the Earth.
Similarly, hydraulic and air hammers supply energy to piles through hydraulic and air forces. The energy imparted from a hammerhead varies with the type of hammer chosen and can be as high as a million-foot pounds for large scale diesel hammers, a very common hammerhead used in practice.
Piles are made of a variety of material including steel, timber, and concrete. Drilled piles are created by first drilling a hole to the appropriate depth, and filling it with concrete. Drilled piles can typically carry more load than driven piles, simply due to a larger diameter pile. The auger method of pile installation is similar to drilled pile installation, but concrete is pumped into the hole as the auger is being removed. 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 weight of the retained material is creates lateral earth pressure behind the wall, which can cause the wall to deform or fail.
The lateral earth pressure depends on the height of the wall, the density of the soil, the strength of the soil , 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 away from the backfill.
Expansive Soils: Problems and Practice in Foundation and Pavement Engineering
To a lesser degree, the same could be said of agronomists and foresters. Simple engineering rules and guidelines have been developed based on investigations of damaged houses and other buildings, and the experience gained from implementation of these guidelines. The intent of the geotechnical engineer and footing designer is to reduce the risk of damage to property to an acceptable level. However our understanding of trees in the urban environment is quite poor; for example, there is still little evidence to differentiate between species, and not much to go on regarding the influence of soil types and different regional weather patterns. Further development of engineering guidelines can only follow with increased research effort and improved models for prediction of soil moisture re-distribution near trees.
Geotechnical engineering , also known as geotechnics , is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles and methods of soil mechanics and rock mechanics for the solution of engineering problems and the design of engineering works. It also relies on knowledge of geology , hydrology , geophysics , and other related sciences. Geotechnical engineering is important in civil engineering, but also has applications in military , mining , petroleum , coastal , ocean , and other engineering disciplines that are concerned with construction occurring on the surface or within the ground, both onshore and offshore. The fields of geotechnical engineering and engineering geology are closely related, and have large areas of overlap.
Also, it discusses problems associated with swelling soil, classification of structural damages caused to buildings, and various foundation designs to combat the.
Soils and Foundations for Architects and Engineers pp Cite as. Expansive clay is a generic term used by architects, engineers, and contractors to indicate any soil that exhibits, by observation or by tests, the characteristic of volumetric expansion or contraction when subjected to an increase or decrease in moisture content. This phenomenon occurs only in clays, but does not occur equally in all clays.
The swelling properties of expansive soils can be reduced by the addition of modifiers. Nevertheless, the performance deterioration after modification occurs when weathering for a long term. Therefore, in this study, the effect of drying-wetting cycles on swelling behaviour and compressibility of modified expansive soils with the iron tailing sand and calcium carbide slag has been investigated. The swelling potential initially increases and subsequently decreases with the increasing number of cycles, reaches the peak at the seventh cycle, and tends to equilibrium after the tenth cycle. These results show that drying-wetting cycles will destroy the soil structure.
Differential settlement is the term used in structural engineering for a condition in which a building's support foundation settles in an uneven fashion, often leading to structural damage. All buildings settle somewhat in the years following construction, and this natural phenomenon generally causes no problems if the settling is uniform across the building's foundation or all of its pier supports. But when one section of the foundation settles at a faster rate than the others, it can lead to major structural damage to the building itself. Differential settlement is not usually a sign of carpentry construction flaws, although some people view it that way.
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