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[[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 ==
=== Foundations ===
{{Unreferenced section|date=September 2010}}
{{Main|Foundation (engineering)}}
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== Earthworks ==
{{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 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 feet) driven to a depth of 83.5m (274 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]]
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.]]
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