HOME

TheInfoList



OR:

Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical or vertical slope). They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a
seawall A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservatio ...
or a bulkhead.


Definition

A wall for holding in place a mass of earth or the like, as at the edge of a terrace or excavation. A retaining wall is a structure designed and constructed to resist the lateral pressure of soil, when there is a desired change in ground elevation that exceeds the
angle of repose The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope fac ...
of the soil. A basement wall is thus one kind of retaining wall. But the term usually refers to a cantilever retaining wall, which is a freestanding structure without lateral support at its top. These are cantilevered from a footing and rise above the grade on one side to retain a higher level grade on the opposite side. The walls must resist the lateral pressures generated by loose soils or, in some cases, water pressures. Every retaining wall supports a "wedge" of
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
. The wedge is defined as the soil which extends beyond the failure plane of the soil type present at the wall site, and can be calculated once the soil friction angle is known. As the setback of the wall increases, the size of the sliding wedge is reduced. This reduction lowers the pressure on the retaining wall. The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained material to move downslope due to gravity. This creates lateral earth pressure behind the wall which depends on the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...
of internal friction (phi) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes. Lateral earth pressures are zero at the top of the wall and – in homogeneous ground – increase proportionally to a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a
drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
system causes hydrostatic pressure on the wall. The total pressure or thrust may be assumed to act at one-third from the lowest depth for lengthwise stretches of uniform height. It is important to have proper drainage behind the wall in order to limit the pressure to the wall's design value. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. Drystone retaining walls are normally self-draining. As an example, the
International Building Code The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted for use as a base code standard by most jurisdictions in the United States. The IBC addresses both health and saf ...
requires retaining walls to be designed to ensure stability against overturning, sliding, excessive
foundation Foundation may refer to: * Foundation (nonprofit), a type of charitable organization ** Foundation (United States law), a type of charitable organization in the U.S. ** Private foundation, a charitable organization that, while serving a good cause ...
pressure and water uplift; and that they be designed for a
safety factor In engineering, a factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), expresses how much stronger a system is than it needs to be for an intended load. Safety factors are often calculated using detailed analy ...
of 1.5 against lateral sliding and overturning.


Types


Gravity

Gravity walls depend on their mass(stone, concrete or other heavy material) to resist pressure from behind and may have a 'batter' setback to improve stability by leaning back toward the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units). Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing. 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: geosynthetics such as geocell cellular confinement earth retention or 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 granular material).


Cantilevered

Cantilevered retaining walls are made from an internal 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 strength resisting high loads. Buttresses are short
wing wall A wing wall (also "wingwall" or "wing-wall") is a smaller wall attached or next to a larger wall or structure. Bridges In a bridge, the wing walls are adjacent to the abutments and act as retaining walls. They are generally constructed of the sam ...
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.


Diaphragm wall

Diaphragm walls are a type of retaining walls that are very stiff and generally watertight. Diaphragm walls are expensive walls, but they save time and space, and hence are used in urban constructions.


Sheet piling

Sheet pile retaining walls are usually used in soft soil and tight spaces. Sheet pile walls are driven into the ground and are composed of a variety of material including steel, vinyl, aluminum, fiberglass or wood planks. For a quick estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back
anchor An anchor is a device, normally made of metal , used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ''ancora'', which itself comes from the Greek ἄγ ...
, or "dead-man" placed in the soil a distance behind the face of the wall, that is tied to the wall, usually by a cable or a rod. Anchors are then placed behind the potential failure plane in the soil.


Bored pile

Bored pile retaining walls are built by assembling a sequence of
bored pile In conventional usage, boredom, ennui, or tedium is an emotional and occasionally psychological state experienced when an individual is left without anything in particular to do, is listlessness and dissatisfaction arising from a lack of occup ...
s, followed by excavating away the excess soil. Depending on the project, the bored pile retaining wall may include a series of
earth anchor An earth anchor is a device designed to support structures, most commonly used in geotechnical and construction applications. Also known as a ground anchor, percussion driven earth anchor or mechanical anchor, it may be impact driven into the gr ...
s, reinforcing beams, soil improvement operations and
shotcrete Shotcrete, gunite (), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique, first used in 1907 invented by Carl Akeley. It is typically ...
reinforcement layer. This construction technique tends to be employed in scenarios where sheet piling is a valid construction solution, but where the vibration or noise levels generated by a pile driver are not acceptable.


Anchored

An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Usually driven into the material with boring, anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a bulb in the soil. Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.


Alternative retaining techniques


Soil nailing

Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements – normally steel reinforcing bars. The bars are usually installed into a pre-drilled hole and then
grout Grout is a dense fluid which hardens to fill gaps or used as reinforcement in existing structures. Grout is generally a mixture of water, cement and sand, and is employed in pressure grouting, embedding rebar in masonry walls, connecting sect ...
ed into place or drilled and grouted simultaneously. They are usually installed untensioned at a slight downward inclination. A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface.


Soil-strengthened

A number of systems exist that do not consist of just the wall, but reduce the earth pressure acting directly on the wall. These are usually used in combination with one of the other wall types, though some may only use it as facing, ''i.e.'', for visual purposes.


Gabion meshes

This type of soil strengthening, often also used without an outside wall, consists of
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is co ...
mesh "boxes", which are filled with roughly cut stone or other material. The mesh cages reduce some internal movement and forces, and also reduce erosive forces. Gabion walls are free-draining retaining structures and as such are often built in locations where ground water is present. However, management and control of the ground water in and around all retaining walls is important.


Mechanical stabilization

Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing via layered horizontal mats ( geosynthetics) fixed at their ends. These mats provide added internal shear resistance beyond that of simple gravity wall structures. Other options include steel straps, also layered. This type of soil strengthening usually needs outer facing walls (S.R.W.'s – Segmental Retaining Walls) to affix the layers to and vice versa. The wall face is often of precast concrete units that can tolerate some differential movement. The reinforced soil's mass, along with the facing, then acts as an improved gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep or thick as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall. Cellular confinement systems (geocells) are also used for steep earth stabilization in gravity and reinforced retaining walls with geogrids. Geocell retaining walls are structurally stable under self- weight and externally imposed loads, while the flexibility of the structure offers very high seismic resistance. The outer fascia cells of the wall can be planted with vegetation to create a green wall.


See also

* Civil engineering *
Direct shear test A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses. The U.S. and U.K. standards defining how the test ...
* Earthquake engineering * Flying arch * Foundation (engineering) * Geotechnical engineering * Landslide mitigation * Lateral earth pressure *
Revetment A revetment in stream restoration, river engineering or coastal engineering is a facing of impact-resistant material (such as stone, concrete, sandbags, or wooden piles) applied to a bank or wall in order to absorb the energy of incoming water an ...
*
Seawall A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservatio ...
*
Slope stability analysis Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of earth and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. Slope stability refers to the condition of i ...
*
Structural engineering Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and muscles' that create the form and shape of man-made structures. Structural engineers also must understand and cal ...
* Terraced wall *
Trench shield Trench shields (also called trench boxes or trench sheets) are steel or aluminum structures used for protecting utility workers while performing their duties within a trench and avoid cave-ins. They are customarily constructed with sidewalls ...
*
Trench shoring Trench shoring is the process of bracing the walls of a trench to prevent collapse and cave-ins. The phrase can also be used as a noun to refer to the materials used in the process. Several methods can be used to shore up a trench. Hydrauli ...


References


Further reading

*Bowles, J.,(1968). Foundation Analysis and Design, McGraw-Hill Book Company, New York *Ching, F. D., Faia., R., S., & Winkel, P. (2006). Building Codes Illustrated: A Guide to Understanding the 2006 International *Crosbie, M. & Watson, D. (Eds.). (2005). Time-Saver Standards for Architectural Design. New York, NY: McGraw-Hill. {{DEFAULTSORT:Retaining Wall Geotechnical shoring structures Types of wall