Geotechnical engineering: Difference between revisions

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{{short description|Scientific study of earth materials in engineering problems}}[[Image:Boston CAT Project-construction view from air.jpeg|thumb|upright=1.15|[[Boston]]'s [[Big Dig]] presented geotechnical challenges in an urban environment.]]

[[File:Precastconcreteretainingwall.tif|thumb|Precast concrete retaining wall]]

[[File:slope 2d plain.svg|thumb|A typical cross-section of a slope used in two-dimensional analysesanalyzes.]]

'''Geotechnical engineering''', also known as '''geotechnics''', is the branch of [[civil engineering]] concerned with the engineering behavior of [[earth materials]]. It uses the principles of [[soil mechanics]] and [[rock mechanics]] forto the solution ofsolve its respective [[engineering]] problems. It also relies on knowledge of [[geology]], [[hydrology]], [[geophysics]], and other related sciences. Geotechnical (rock) engineering is a subdiscipline of [[civil engineering]].

In addition to [[civilGeotechnical engineering]], geotechnical engineering also has applications in [[military engineer|militaryengineering]], [[mining engineering|mining]], [[petroleum engineering|petroleum]], [[coastal engineering]], and [[offshore construction]]. The fields of geotechnical engineering and [[engineering geology]] have overlapping knowledge areas that 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 differ in the application.

==History==

Humans have historically used soil as a material for flood control, irrigation purposes, burial sites, building foundations, and construction materials for buildings. Early activities were linked to [[irrigation]] and [[flood control]], as demonstrated by traces of dykesDykes, [[dam]]s, and [[canal]]s dating back to at least 2000 BCEBCE—found thatin wereparts found inof ancient [[Egypt]], ancient [[Mesopotamia]], and the [[Fertile Crescent]], as well as aroundand the early settlements of [[Mohenjo-daro|Mohenjo Daro]] and Harappa in the [[Indus valley]]—provide evidence for early activities linked to [[irrigation]] and [[flood control]]. As cities expanded, structures were erected and supported by formalized foundations. The [[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 experience.<ref name=das>{{cite book | last = Das | first = Braja | title = Principles of Geotechnical Engineering | publisher = Thomson Learning | year = 2006}}</ref>

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 [[lateral earth pressure|earth pressure]] theories for the construction of [[retaining walls]]. Henri Gautier, a French Royalroyal Engineerengineer, recognized the "natural slope" of different soils in 1717, an idea later known as the soil's [[angle of repose]]. AAround the same time, 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.<ref name=das/><ref name=budhu>{{cite book | last = Budhu | first = Muni | title = Soil Mechanics and Foundations | publisher = John Wiley & Sons, Inc | year = 2007 | isbn = 978-0-471-43117-6}}</ref>

The application of the principles of [[mechanics]] to soils was documented as early as 1773 when [[Charles-Augustin de Coulomb|Charles Coulomb]], a physicist, engineer, and army captainengineer, developed improved methods to determine the earth pressures against military ramparts. Coulomb observed that, at failure, a distinct slip plane would form behind a sliding retaining wall and he suggested that the maximum shear stress on the slip plane, for design purposes, was the sum of the soil cohesion, <math>c</math>, and friction <math>\sigma\,\!</math> <math> \tan(\phi\,\!)</math>, where <math>\sigma\,\!</math> is the normal stress on the slip plane and <math>\phi\,\!</math> is the friction angle of the soil. By combining Coulomb's theory with [[Christian Otto Mohr]]'s [[Mohr's circle|2D stress state]], the theory became known as [[Mohr-Coulomb theory]]. Although it is now recognized that precise determination of cohesion is impossible because <math>c</math> is not a fundamental soil property, the Mohr-Coulomb theory is still used in practice today.<ref name="schofield">Disturbed soil properties and geotechnical design, Schofield, Andrew N., Thomas Telford, 2006. {{ISBN|0-7277-2982-9}}</ref> the Mohr-Coulomb theory is still used in practice today.

In the 19th century, [[Henry Darcy]] developed what is now known as [[Darcy's Law]], describing the flow of fluids in a [[Porous medium|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 [[Atterberg limits|clay consistency]] indices that are still used today for soil classification.<ref name=das/><ref name=budhu/> In 1885, [[Osborne Reynolds]] recognized that shearing causes volumetric dilation of dense materials and contraction of loose [[granular material]]s.

Modern geotechnical engineering is said to have begun in 1925 with the publication of ''Erdbaumechanik'' by [[Karl Terzaghi|Karl von Terzaghi]], a mechanical engineer and geologist. Considered by many to be the father of modern soil mechanics and geotechnical engineering, [[Karl von Terzaghi|Terzaghi]] developed the principle of effective [[Stress (mechanics)|stress]], and demonstrated that the [[Shear strength (soil)|shear strength]] of soil is controlled by effective stress.<ref>{{cite journal |last1=Guerriero V. |first1=Mazzoli S. |title=Theory of Effective Stress in Soil and Rock and Implications for Fracturing Processes: A Review |journal=Geosciences |date=2021 |volume=11 |issue=3 |page=119 |doi=10.3390/geosciences11030119|bibcode=2021Geosc..11..119G |doi-access=free }}</ref> 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 (soil)|consolidation]].<ref name=das/><ref name=schofield/><ref name="Lambe and Whitman">Soil Mechanics, Lambe, T.William and Whitman, Robert V., Massachusetts Institute of Technology, John Wiley & Sons., 1969. {{ISBN|0-471-51192-7}}</ref> Afterwards, [[Maurice Biot]] fully developed the three-dimensional soil consolidation theory, extending the one-dimensional model previously developed by Terzaghi to more general hypotheses and introducing the set of basic equations of [[Poroelasticity]].

In his 1948 book, Donald Taylor recognized that the interlocking and dilation of densely packed particles contributed to the peak strength of the soil. Roscoe, Schofield, and Wroth, with the publication of ''On the Yielding of Soils'' in 1958, established the interrelationships between the volume change behavior (dilation, contraction, and consolidation) and shearing behavior with the theory of [[plasticity (physics)|plasticity]] using critical state soil mechanics. [[Critical state soil mechanics]] is the basis for many contemporary advanced [[constitutive model]]s describing the behavior of soil.<ref name="Wood">Soil Behavior and Critical State Soil Mechanics, Wood, David Muir, Cambridge University Press, 1990. {{ISBN|0-521-33782-8}}</ref>

In 1960, [[Alec Skempton]] carried out an extensive review of the available formulations and experimental data in the literature about the effective stress validity in soil, concrete, and rock in order to reject some of these expressions, as well as clarify what expression was appropriate according to several working hypotheses, such as stress-strain or strength behavior, saturated or non-saturated media, and rock, concrete or soil behavior.

In 1960, [[Alec Skempton]] carried out an extensive review of the available formulations and experimental data in the literature about the effective stress validity in soil, concrete, and rock in order to reject some of these expressions, as well as clarify what expressions were appropriate according to several working hypotheses, such as stress-strain or strength behavior, saturated or non-saturated media, and rock, concrete or soil behavior.

In his 1948 book, Donald Taylor recognized that the interlocking and dilation of densely packed particles contributed to the peak strength of the soil.

== Roles ==

Roscoe, Schofield, and Wroth, with the publication of ''On the Yielding of Soils'' in 1958, established the interrelationships between the volume change behavior (dilation, contraction, and consolidation) and shearing behavior with the theory of [[plasticity (physics)|plasticity]] using critical state soil mechanics. [[Critical state soil mechanics]] is the basis for many contemporary advanced [[constitutive model]]s describing the behavior of soil.<ref name="Wood">Soil Behavior and Critical State Soil Mechanics, Wood, David Muir, Cambridge University Press, 1990. {{ISBN|0-521-33782-8}}</ref>

=== Geotechnical investigation ===

[[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 are very sensitive to the confining [[pressure]]. The [[Centrifugal force|centrifugal acceleration]] allows a researcher to obtain large (prototype-scale) stresses in small physical models.

== Geotechnical investigation ==

{{Main|Geotechnical investigation}}

TheGeotechnical tasksengineers ofinvestigate aand geotechnical engineer includedetermine the following: the investigationproperties of subsurface conditions and materials;. theThey determination of the relevant physical, mechanical, and chemical properties of these materials; thealso design ofcorresponding [[Earthworks (engineering)|earthworks]] and retaining structures (including [[dam]]s,Retaining [[Embankment (transportation)wall|embankments]],retaining sanitary landfills, deposits of [[hazardous wastestructures]]), [[tunnel]]s, and structure [[foundation (engineering)|foundations]]; the monitoring of site conditions, earthwork, and foundationmay construction; the evaluation of the [[Slope stability|stability of natural slopes]]supervise and man-madeevaluate soilsites, deposits;which themay assessmentfurther ofinvolve thesite risksmonitoring posedas bywell site conditions; andas the prediction,risk prevention,assessment and mitigation of damage caused by [[natural hazard]]s (such as [[avalanche]]s, [[mud flow]]s, [[landslide]]s, [[rockslide]]s, [[sinkhole]]s, and [[volcanic eruptions]]).<ref name="TerzaghiPeckMesri">Terzaghi, K., Peck, R.B. and Mesri, G. (1996), ''Soil Mechanics in Engineering Practice'' 3rd Ed., John Wiley & Sons, Inc. {{ISBN|0-471-08658-4}}</ref><ref name="HoltzKovacs">Holtz, R. and Kovacs, W. (1981), ''An Introduction to Geotechnical Engineering'', Prentice-Hall, Inc. {{ISBN|0-13-484394-0}}</ref>

Geotechnical engineers and engineering geologists perform geotechnical investigations to obtain information on the [[Physical property|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. AGeotechnical geotechnicalinvestigations investigation will includeinvolve surface exploration and subsurface exploration of a site, often including subsurface sampling and laboratory testing of retrieved soil samples. Sometimes, [[Exploration geophysics|geophysical methods]] are also used to obtain data, aboutwhich sites. Subsurface exploration usually involves in-situ testing (two commoninclude examplesmeasurement of in-situ tests are the [[standardseismic penetration testwaves]] (pressure, shear, and [[coneRayleigh penetration testwaves]]). In addition, sitesurface-wave investigationmethods willand oftendownhole include subsurface samplingmethods, and laboratory[[Prospecting|electromagnetic testingsurveys]] of(magnetometer, the[[Electrical soilresistivity samplesand retrieved. The digging of test pitsconductivity|resistivity]], and trenching (particularly for locating [[Faultground-penetrating (geology)|faultsradar]] and). [[landslideElectrical resistivity tomography|slideElectrical planestomography]]) may alsocan be used to learn aboutsurvey soil conditionsand atrock depth.properties Large-diameterand boringsexisting areunderground rarelyinfrastructure usedin dueconstruction toprojects.<ref>Deep safetyScan concernsTech and(2023): expense[https://www.deepscantech.com/news/deep-scan-tech-uncovers-hidden-structures-at-the-site-of-denmarks-tallest-building.html butDeep areScan sometimesTech useduncovers tohidden allowstructures a geologist or engineer to be lowered intoat the borehole for direct visual and manual examinationsite of theDenmark's soiltallest and rock [[stratigraphy]building].</ref>

Surface [[exploration]] can include on-foot surveys, [[geological map]]ping, [[Exploration geophysics|geophysical methods]], and [[photogrammetry]]. Geological mapping and interpretation of [[geomorphology]] are typically completed in consultation with a [[geologist]] or [[engineering geologist]]. Subsurface exploration usually involves in-situ testing (for example, the [[standard penetration test]] and [[cone penetration test]]). The digging of test pits and trenching (particularly for locating [[Fault (geology)|faults]] and [[landslide|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. Still, they 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 [[Geotechnical investigation#Soil sampling|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.<ref name="Coring frozen ground">{{cite web | url=https://www.geofrost.no/en/ground-investigations/#Undisturbed%20samples | title=Geofrost Coring | publisher=GEOFROST | access-date=20 November 2020}}</ref>

Various [[Geotechnical investigation#Soil sampling|soil samplers]] exist to meet the needs of different engineering projects. The [[standard penetration test]], 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 to collect less disturbed samples. More advanced methods, such as the Sherbrooke block sampler, are superior but expensive. Coring frozen ground provides high-quality undisturbed samples from ground conditions, such as fill, sand, [[moraine]], and rock fracture zones.<ref name="Coring frozen ground">{{cite web | url=https://www.geofrost.no/en/ground-investigations/#Undisturbed%20samples | title=Geofrost Coring | publisher=GEOFROST | access-date=20 November 2020}}</ref>

[[Atterberg limits]] tests, [[water content]] measurements, and [[Grain (unit)|grain]] size analysis, for example, may be performed on disturbed samples obtained from thick-walled [[Geotechnical investigation#Soil sampling|soil samplers]]. Properties such as shear strength, stiffness, [[hydraulic conductivity]], and coefficient of [[Consolidation (soil)|consolidation]] may be significantly altered by sample disturbance. To measure these properties in the laboratory, high-quality sampling is required. Common tests to measure strength and stiffness include the [[Triaxial shear tests|triaxial shear]] and unconfined compression test.

[[Geotechnical centrifuge modeling]] is another 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 soil's strength and [[stiffness]] are susceptible to the confining [[pressure]]. The [[Centrifugal force|centrifugal acceleration]] allows a researcher to obtain large (prototype-scale) stresses in small physical models.

Surface [[exploration]] can include [[geologic map]]ping, [[Exploration geophysics|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]].

=== Foundation design ===

[[Exploration geophysics|Geophysical exploration]] is also sometimes used. Geophysical techniques used for subsurface exploration include measurement of [[seismic waves]] (pressure, shear, and [[Rayleigh waves]]), surface-wave methods and/or downhole methods, and [[Prospecting|electromagnetic surveys]] (magnetometer, [[Electrical resistivity and conductivity|resistivity]], and [[ground-penetrating radar]]).

==Structures==

=== Foundations ===

{{Unreferenced section|date=September 2010}}

{{Main|Foundation (engineering)}}

The foundation of a buildingstructure's or transportation infrastructure transmits loads from the structuresstructure to the earth. Geotechnical [[engineer]]s design foundations based on the load characteristics of the structure and the properties of the soils and/or [[bedrock]] at the site. In generalGenerally, geotechnical engineers: first estimate the magnitude and ___location of loads to be supported before developing an investigation plan to explore the subsurface and determine the necessary soil parameters through field and lab testing. Following this, they may begin the design of an engineering foundation. The primary considerations for a geotechnical engineer in foundation design are [[bearing capacity]], settlement, and ground movement beneath the foundations.<ref name="Han 2015">{{Cite book |last=Han |first=Jie |title=Principles and Practice of Ground Improvement |publisher=Wiley |year=2015 |isbn=9781118421307}}</ref>

# Estimate the magnitude and ___location of the loads to be supported.

# Develop an investigation plan to [[geotechnical investigation|explore the subsurface]].

# Determine the necessary soil parameters through field and lab testing (e.g., [[consolidation (soil)|consolidation test]], [[triaxial shear test]], vane shear test, [[standard penetration test]]).

# Design the foundation in the safest and most economical manner.

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 and/or softer soils, both overall settlement relative to unbuilt areas or neighboring buildings, and differential settlement under a single structure can be of 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 low bearing capacity soils (soft clay, [[silt]], organic, loose sand), volumetric change of expansive soils due to moisture or [[Weathering|freeze-thaw cycles]] or melting of [[permafrost]], or due to unsuitable fill material with low strength, highly compressible and high water content.<ref name="Han 2015">{{Cite book |last=Han |first=Jie |title=Principles and Practice of Ground Improvement |publisher=Wiley |year=2015 |isbn=9781118421307}}</ref> All these factors must be considered during the design of foundations.

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====

{{Unreferenced section|date=September 2010}}

{{Main|Shallow foundation}}

[[Image:Slab on grade.JPG|thumb|right|Example of a slab-on-grade foundation.]]

Shallow foundations are a type of foundation that transfers the building load to very near the surface, rather than to a subsurface layer. Shallow foundations typically have a depth-to-width ratio of less than 1.

=====Footings=====

Footings (often called "spread footings" because they spread the load) are structural elements that 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.

=====Slab=====

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 [[Shear modulus|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 300&nbsp; mm thick; for larger structures, the foundation slab may be several meters thick.

Slab foundations can be either [[slab-on-grade foundation]]s or embedded foundations, typically in buildings with [[basement]]s. Slab-on-grade foundations must be designed to allow for potential ground movement due to changing soil conditions.

====Deep====

{{Unreferenced section|date=September 2010}}

[[Image:PileDriving.jpg|thumb|right|150px|[[Pile driver|Pile-driving]] for a bridge in [[Napa, California]].]]

{{Main|Deep foundations}}

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 contact mechanics|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 [[Deep foundation|piles]], drilled shafts, [[caisson (engineering)|caissons]], piers, and earth-stabilized columns. Large buildings such as [[skyscraper]]s typically require deep foundations. For example, the [[Jin Mao Tower]] in [[China]] uses tubular steel piles about 1m (3.3&nbsp; feet) driven to a depth of 83.5m (274&nbsp; feet) to support its [[weight]].

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 [[Hydraulics|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 materials 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.<ref name="Coduto, et al 2011">{{cite book|last=Coduto|first=Donald|title=Geotechnical Engineering Principles and Practices|year=2011|publisher=Pearson Higher Education|___location=New Jersey|isbn=9780132368681|display-authors=etal}}</ref>

=== Lateral earth support structures ===

{{Unreferenced section|date=September 2010}}

{{Main|Retaining wall}}

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. [[Cofferdam]]s and [[Bulkhead (barrier)|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 creating [[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 mechanics|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. [[Groundwater]] behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.

====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, landscaped walls, gravity walls made from [[Cellular confinement|geocells]], 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 geocell [[retaining wall]]s, 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 free-draining gravel) 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 provides tensile strength to hold the soil together. The wall face is often of a geocell or 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 30 to 40 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.

====Cantilever walls====

Prior to the introduction of modern reinforced-soil gravity walls, cantilevered walls were the most common type of taller retaining wall. [[Cantilever]]<nowiki/>ed 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 buttressed on the front, or include a counterfort on the back, to improve their stability against high loads. Buttresses are short [[wing wall]]s 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.

====Excavation shoring====

{{Unreferenced section|date=September 2010}}

[[Shoring]] of temporary excavations frequently requires a wall design that 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 [[flange]]s.

The use of underground space requires excavation, which may cause large and dangerous displacement of [[soil]] mass around the excavation. Since the space for slope excavation is limited in urban areas, cutting is done vertically. [[Retaining walls]] are made to prevent unsafe soil displacements around excavations. [[Diaphragm wall]]s are a type of retaining wall that are very stiff and generally watertight. The horizontal movements of diaphragm walls are usually prevented by lateral supports. Diaphragm walls are expensive walls, but they save time and space and are also safe, so are widely used in urban deep excavations.<ref name="Bahrami">{{cite journal |last1=Bahrami |first1=M. |last2=Khodakarami |first2=M.I. |last3=Haddad |first3=A.|title=3D numerical investigation of the effect of wall penetration depth on excavations behavior in sand |journal=Computers and Geotechnics |date=June 2018 |volume=98 |pages=82–92 |doi=10.1016/j.compgeo.2018.02.009 |s2cid=125625145 }}</ref>

In some cases, the lateral support which can be provided by the shoring wall alone is insufficient to resist the planned [[Lateral earth pressure|lateral]] loads; in this case, additional support is provided by walers or tie-backs. Walers are structural elements that 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 extends beyond the soil which is applying pressure to the wall, to provide additional lateral resistance to the wall.

=== Earthworks ===

[[Image:Seabees compactor roller.jpg|thumb|A [[compactor]]/[[road roller|roller]] operated by U.S. Navy Seabees]]

{{See also|Earthworks (engineering)}}Geotechnical engineers are also involved in the planning and execution of [[Earthworks (engineering)|earthworks]], which include ground improvement,<ref name="Han 2015" /> slope stabilization, and slope stability analysis.

{{See also|Earthworks (engineering)}}

*Excavation is the process of training earth according to requirement by removing soil from the site, either to level the land or to replace inferior subgrade with a soil with higher bearing capacity.

*Filling is the process of training earth according to requirement by placing the soil on the site and leveling or to add natural or processed geomaterials (e.g., crushed stone aggregate) to increase the soil strength and structure support layers.

*[[Soil compaction|Compaction]] is the process by which the density of soil is increased and permeability of soil is decreased. Fill placement work often has specifications requiring a specific degree of compaction, or alternatively, specific properties of the compacted soil. In-situ soils can be compacted by rolling, deep [[dynamic compaction]], vibration, blasting, [[Gyration|gyrating]], kneading, compaction [[grout]]ing, etc.

====Ground improvement====

GroundVarious improvementgeotechnical or modification is defined as the alteration of site foundation soils or project earth structures to provide better performance under design and/or operational loading conditions.<ref>(Schaefer et al., 2012) in Alderton, D. and Elias, S.A. (2021) Encyclopedia of Geology., 2nd Edition. Elsevier. ISBN 978-0-08-102909-1.</ref> Usually, the properties modified are shear strength, stiffness, and permeability. Ground improvement has developed sophisticatedengineering methods tocan supportbe foundationsused for a wide variety of [[building]]s and [[Transport|transportation infrastructure]], as urbanization and infrastructure spread to areas with challenging geotechnical conditions. [[Mechanically stabilized earth|Soil reinforcement]] is one of the most popular ground improvement techniques used to improve soil stiffness and strength. This can be achieved through different materials and techniques, e.g.,including reinforcement [[geosynthetics]] such as geocells and geogrids, which disperse loads over a larger area, thus increasing the soil's load-bearing capacity of the soil.<ref name="HanThrough 2015"/>these Properly appliedmethods, i.e.geotechnical afterengineers giving due consideration to the nature of the ground being improved and the type and loading of the structures being built, hydraulic, mechanical, chemical, and/or biological ground improvement methodscan reduce direct and long-term costs.<ref>{{cite book | title=Ground Improvement Technologies and Case Histories | publisher=Research Publishing Services | author=RAJU, V. R. | id=Ground Improvement – Principles And Applications In Asia | year=2010 | ___location=Singapore | pages=809 | isbn=978-981-08-3124-0}}</ref>

====Slope stabilization====

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{{Main|Slope stability}}

SlopeGeotechnical stabilityengineers iscan theanalyze potentialand ofimprove soil-coveredslope slopesstability tousing withstandengineering andmethods. undergoSlope [[mass wasting|movement]]. Stabilitystability is determined by the balance of [[shear stress]] and [[shear strength (soil)|shear strength]]. A previously stable slope may be initially affected by preparatoryvarious factors, making the slope conditionallyit unstable. Triggering factors of a [[slope failure]] can be climatic events that can then make a slope actively unstableNonetheless, leadinggeotechnical to mass movements. Mass movementsengineers can be caused by increases in shear stress, such as loading, lateral pressure,design and transientimplement forces.engineered Alternatively,slopes shearto strengthincrease may be decreased by weathering, changes in [[pore water pressure]], and organic materialstability.

Several modes of failure for earth slopes include falls, topples, slides, and flows. In slopes with coarse-grained soil or rocks, falls typically occur as the rapid descent of rocks and other loose slope material. A slope topples when a large column of soil tilts over its vertical axis at failure. Typical slope stability analysis considers sliding failures, categorized mainly as rotational slides or translational slides. As implied by the name, rotational slides fail along a generally curved surface, while translational slides fail along a more planar surface. A slope failing as flow would resemble a fluid flowing downhill.

=====Slope stability analysis=====

{{Main|Slope stability analysis}}

Stability analysis is needed for theto design of engineered slopes and for estimatingestimate the risk of slope failure in natural or designed slopes. Aby commondetermining assumptionthe isconditions thatunder awhich slopethe consiststopmost mass of asoil layerwill ofslip soilrelative sittingto onthe topbase of asoil rigidand base.lead Theto massslope andfailure.<ref>{{cite thebook|last=Pariseau|first=William baseG.|title=Design areanalysis assumedin torock interactmechanics|year=2011|publisher=CRC viaPress}}</ref> friction.If Thethe interface between the mass and the base canof bea planar,slope curvedhas a complex geometry, orslope havestability someanalysis otheris difficult and [[complexNumerical geometryanalysis|numerical solution]] methods are required. The goalTypically, ofthe interface's exact geometry is unknown, and a slopesimplified stabilityinterface analysisgeometry is toassumed. determine theFinite conditionsslopes underrequire whichthree-dimensional themodels massto willbe slipanalyzed, relativeso tomost theslopes baseare andanalyzed leadassuming tothat slopethey failure.<ref>{{citeare book|last=Pariseau|first=Williaminfinitely G.|title=Designwide analysisand incan rockbe mechanics|year=2011|publisher=CRCrepresented by Press}}</ref>two-dimensional models.

If the interface between the mass and the base of a slope has a complex geometry, slope stability analysis is difficult and [[Numerical analysis|numerical solution]] methods are required. Typically, the exact geometry of the interface is not known and a simplified interface geometry is assumed. Finite slopes require three-dimensional models to be analyzed. To keep the problem simple, most slopes are analyzed assuming that the slopes are infinitely wide, and can therefore be represented by two-dimensional models. A slope can be drained or undrained. The undrained condition is used in the calculations to produce conservative estimates of risk.

== Sub-disciplines ==

A popular stability analysis approach is based on principles pertaining to the limit equilibrium concept. This method analyzes a finite or infinite slope as if it were about to fail along its sliding failure surface. Equilibrium stresses are calculated along the failure plane and compared to the soils shear strength as determined by [[Shear strength (soil)#Drained shear strength|Terzaghi's shear strength equation]]. Stability is ultimately decided by a factor of safety equal to the ratio of shear strength to the equilibrium stresses along the failure surface. A factor of safety greater than one generally implies a stable slope, failure of which should not occur assuming the slope is undisturbed. A factor of safety of 1.5 for static conditions is commonly used in practice.

=== Geosynthetics ===

{{Main|Geosynthetics}}

[[Image:Geocollage.JPG|thumb|upright=1.15|A collage of geosynthetic products.]]

[[Geosynthetics]] are a type of plastic [[polymer]] products used in geotechnical engineering that improve engineering performance while reducing costs. This includes [[geotextiles]], [[geogrids]], [[geomembranes]], [[geocells]], and [[geocomposites]]. The synthetic nature of the products makesmake them suitable for use in the ground where high levels of durability are required;. theirTheir main functions include [[drainage]], [[filtration]], reinforcement, separation, and containment. [[Geosynthetics]] are available in a wide range of forms and materials, each to suit a slightly different end-use, although they are frequently used together. Some reinforcement geosynthetics, such as geogrids and more recently, [[cellular confinement]] systems, have shown to improve bearing capacity, modulus factors and soil stiffness and strength.<ref>Hegde, A.M. and Palsule P.S. (2020), Performance of Geosynthetics Reinforced Subgrade Subjected to Repeated Vehicle Loads: Experimental and Numerical Studies. Front. Built Environ. 6:15. https://www.frontiersin.org/articles/10.3389/fbuil.2020.00015/full.</ref> These products have a wide range of applications and are currently used in many civil and geotechnical engineering applications including roads, airfields, railroads, [[Embankment (earthworks)|embankments]], piled embankments, retaining structures, [[reservoir]]s, canals, dams, [[landfill]]s, bank protection and coastal engineering.<ref>{{Cite book |last=Koerner |first=Robert, M. |title=Designing with Geosynthetics |publisher=Xlibris |year=2012 |isbn=9781462882892 |edition=6th Edition, Vol. 1}}</ref>

Geosynthetics are available in a wide range of forms and materials, each to suit a slightly different end-use, although they are frequently used together. Some reinforcement geosynthetics, such as geogrids and more recently, [[cellular confinement]] systems, have shown to improve bearing capacity, modulus factors and soil stiffness and strength.<ref>Hegde, A.M. and Palsule P.S. (2020), Performance of Geosynthetics Reinforced Subgrade Subjected to Repeated Vehicle Loads: Experimental and Numerical Studies. Front. Built Environ. 6:15. https://www.frontiersin.org/articles/10.3389/fbuil.2020.00015/full.</ref> These products have a wide range of applications and are currently used in many civil and geotechnical engineering applications including roads, airfields, railroads, [[Embankment (earthworks)|embankments]], piled embankments, retaining structures, [[reservoir]]s, canals, dams, [[landfill]]s, bank protection and coastal engineering.<ref>{{Cite book |last=Koerner |first=Robert M. |title=Designing with Geosynthetics |publisher=Xlibris |year=2012 |isbn=9781462882892 |edition=6th Edition, Vol. 1}}</ref>

== Offshore ==

=== Offshore ===

{{Main|Offshore geotechnical engineering}}

[[File: Offshore platforms.jpg|thumb|Platforms offshore Mexico.]]

''Offshore'' (or ''marine'') ''geotechnical engineering'' is concerned with foundation design for human-made structures in the [[sea]], away from the [[coast]]line (in opposition to ''onshore'' or ''nearshore'' engineering). [[Oil platform]]s, [[artificial island]]s and [[submarine pipeline]]s are examples of such structures.<ref name="Dean">Dean, E.T.R. (2010). Offshore Geotechnical Engineering – Principles and Practice. Thomas Telford, Reston, VA, 520 p.</ref> [[Oil platform]]s, [[artificial island]]s and [[submarine pipeline]]s are examples of such structures.

There are a number of significant differences between onshore and offshore geotechnical engineering.<ref name="Dean" /><ref name="Randolph&Gourvenec">Randolph, M. and [[Susan Gourvenec|Gourvenec, S.]], 2011. Offshore geotechnical engineering. Spon Press, N.Y., 550 p.</ref> Notably, site investigation and ground improvement (on the seabed) and site investigation are more expensive,; the offshore structures are exposed to a wider range of [[geohazard]]s,; and the environmental and financial consequences are higher in case of failure. Offshore structures are exposed to various environmental loads, notably [[wind]], [[wind wave|wave]]s and [[Ocean current|currents]]. These phenomena may affect the integrity or the serviceability of the structure and its foundation during its operational lifespan – theyand need to be taken into account in offshore design.

In [[subsea]] geotechnical engineering, seabed materials are considered a two-phase material composed of 1) rock or [[mineral]] particles and 2) water.<ref name="Das">Das, B.M., 2010. Principles of geotechnical engineering. Cengage Learning, Stamford, 666 p.</ref><ref name="Atkinson">Atkinson, J., 2007. The mechanics of soils and foundations. Taylor & Francis, N.Y., 442 p.</ref> Structures may be fixed in place in the seabed—as is the case for [[pier]]s, [[jettyjetties]]s and fixed-bottom wind turbines—or maybemay comprise a floating structure that remains roughly fixed relative to its geotechnical anchor point. Undersea mooring of human-engineered floating structures include a large number of [[Offshore drilling rig|offshore oil and gas platforms]] and, since 2008, a few [[floating wind turbine]]s. Two common types of engineered design for anchoring floating structures include [[Tension-leg platform|tension-leg]] and [[catenary]] [[Mooring (watercraft)|loose mooring]] systems.<ref name="Tensionmit200710">

leg mooring systems have vertical tethers under tension providing large restoring [[Moment of inertia|moments]] in pitch and roll. [[Catenary]] mooring systems provide station keeping for an offshore structure, yet provide little stiffness at low tensions."<ref name=mit200710>

[http://web.mit.edu/flowlab/pdf/Floating_Offshore_Wind_Turbines.pdf Floating Offshore Wind Turbines: Responses in a Sea state – Pareto Optimal Designs and Economic Assessment], P. Sclavounos et al., October 2007.</ref>

==Observational method==

InFirst geotechnicalproposed engineering,by during[[Karl theTerzaghi]] constructionand oflater earthdiscussed structuresin (damsa andpaper tunnelsby for[[Ralph example)Brazelton Peck|Ralph B. Peck]], the '''observational method''' is a continuous, managed, and integrated process of design, construction control, monitoring, and review, enablingwhich appropriate, previously-definedenables modifications to be incorporated during (orand after) construction. All these aspects must be demonstrably robust. The objectivemethod isaims to achieve a greater overall economy, without compromising [[safety]] by creating designs based on the most probable conditions rather than the most unfavorable.<ref>Nicholson, D, Tse, C and Penny, C. (1999). The Observational Method in ground engineering – principles and applications. Report 185, CIRIA, London.</ref> Using the observational method, gaps in available information are filled by measurements and investigation, which aid in assessing the behavior of the structure during [[construction]], which in turn can be modified per the findings. The method was described by Peck as "learn-as-you-go".<ref name="peck">Peck, R.B (1969). Advantages and limitations of the observational method in applied soil mechanics, Geotechnique, 19, No. 1, pp. 171–187.</ref>

The observational method may be described as follows:<ref name="peck" />

The observational method was proposed by [[Karl Terzaghi]] and discussed in a paper by [[Ralph Brazelton Peck|Ralph B. Peck]] (1969). This was in an effort to reduce the costs during construction incurred by designing earth structures based on the most-unfavorable assumptions (in other words, geological conditions, soil engineering properties, and so on). Instead, the [[design]] is based on the most-probable conditions rather than the most unfavorable. Gaps in the available information are filled by observations: geotechnical-instrumentation measurements (for example, [[inclinometer]]s and [[piezometer]]s) and geotechnical site investigation (for example, [[borehole]] drilling and a [[Cone penetration test|CPT]]). These observations aid in assessing the behavior of the structure during [[construction]], which can then be modified in accordance with the findings. The method may be described as "learn-as-you-go".<ref name=peck>Peck, R.B (1969). Advantages and limitations of the observational method in applied soil mechanics, Geotechnique, 19, No. 1, pp. 171-187.</ref>

# General exploration sufficient to establish the rough nature, pattern, and properties of [[Deposition (geology)|deposits]].

The observational method may be described as follows:

#Assessment of the most probable conditions and the most unfavorable conceivable deviations.

*Exploration sufficient to establish the general nature, pattern, and properties of the [[Deposition (geology)|deposits]] (not necessarily in detail).

#Creating the design based on a working hypothesis of behavior anticipated under the most probable conditions.

*Assessment of the most probable conditions, and the most unfavorable conceivable deviations from these conditions. Geology plays a major role.

*Creating#Selection theof design,quantities basedto onbe aobserved workingas hypothesisconstruction ofproceeds behaviorand calculating their anticipated values based on the working hypothesis under the most-probable unfavorable conditions.

#Selection, in advance, of a course of action or design modification for every foreseeable significant deviation of the observational findings from those predicted.

*Selection of quantities to be observed as construction proceeds, and calculation of their anticipated values based on the working hypothesis.

#Measurement of quantities and evaluation of actual conditions.

*Calculation of values of the same quantities under the most unfavorable conditions compatible with the available data concerning subsurface conditions.

#Design modification per actual conditions

*Selection (in advance) of a course of action or design modification for every foreseeable significant deviation of the observational findings from those predicted based on the working [[hypothesis]].

*Measurement of quantities to be observed and evaluation of actual conditions.

*Design modification in accordance with actual conditions

The observational method is suitable for construction that has already begun when an unexpected development occurs, or when a failure or [[accident]] threatenslooms or has already occurredhappened.<ref name=peck/> The methodIt is not suitableunsuitable for projects whose design cannot be altered during construction.<ref name="peck" />

The most serious blunder in applying the observational method is failing to select (in advance) an appropriate course of action for all foreseeable deviations (disclosed by observation) from those assumed in the design. The engineer must devise solutions to all problems which could arise under the least favorable conditions. If he or she cannot solve these hypothetical problems (even if the probability of their occurrence is very low), he or she must revert to a design based on the least-favorable conditions.<ref name=peck/>

== See also ==

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* [[Civil engineering]]

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* [[Land reclamation]]

* [[Landfill]]

* [[List of publications in geology#Geotechnical engineering|List of publications in geotechnical engineering]]

* [[Mechanically stabilized earth]]

* [[Offshore geotechnical engineering]]

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* [[Soil science]]

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==Notes==

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* Bates and Jackson, 1980, Glossary of Geology: American Geological Institute.

* Krynine and Judd, 1957, Principles of Engineering Geology and Geotechnics: McGraw-Hill, New York.

* Pierfranco Ventura, Fondazioni, Modellazioni: Verifiche Statiche e Sismiche Strutture-Terreni, vol. I, Milano Hoepli, 2019, pp.770, ISBN 978-88203-8644-3

* Pierfranco Ventura, Fondazioni, Applicazioni: Verifiche Statiche e Sismiche Strutture-Terreni, vol. II, , Milano, Hoepli, 2019, pp.749,ISBN 978-88-203-8645-0 https://www.hoeplieditore.it/hoepli-catalogo/articolo/fondazioni-modellazioni-pierfrancventura/9788820386443/1451

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*[http://www.swedgeo.se/templates/SGIStandardPage____184.aspx?epslanguage=EN Worldwide Geotechnical Literature Database]

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