Editing Retaining wall (section)

==Definition==
A retaining wall is designed to hold in place a mass of earth or the like, such as the edge of a terrace or excavation. The structure is constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the [[angle of repose]] of the soil.<ref>{{Cite book|last1=Ching |first1=Francis D.K. |last2=Winkel |first2=Steven R. |date=2006 |title=Building Codes Illustrated: A Guide to Understanding the 2006 International Building Code |edition=2 |location=Hoboken, New Jersey |publisher=John Wiley & Sons |isbn=978-0-471-74189-3 |url=https://books.google.com/books?id=PYFUAAAAMAAJ}}</ref>

A basement wall is thus one kind of retaining wall; however, the term usually refers to a cantilever retaining wall, which is a freestanding structure without lateral support at its top.<ref>{{Cite book |last=Ambrose |first=James |date=1991 |title=Simplified Design of Masonry Structures |location=New York |publisher=John Wiley and Sons |pages=70–75 |isbn=0-471-17988-4 |url=https://books.google.com/books?id=hZWl-XgKcS8C}}</ref> 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, [[hydrostatic pressure|water pressures]].<ref>{{Cite book |last1=Crosbie |first1=Michael J. |last2=Watson |first2=Donald |date=2005 |title=Time-Saver Standards for Architectural Design |edition=8 |location=New York |publisher=McGraw-Hill |isbn=9780071777339 |url=https://books.google.com/books?id=RTGdn4IiaGIC&q=Time-Saver%20Standards%20for%20Architectural%20Design%20:%20Technical%20Data%20for%20Professional%20Practice}}</ref>

[[Image:Retaining wall terminology.jpg|thumb]]

Every retaining wall supports a "wedge" of [[soil]]. 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 [[angle of repose|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.<ref>{{Cite book |publisher=Allan Block Corporation |date=2011 |title=Commercial Installation Manual for Allan Block Retaining Walls |location=Bloomington |page=13 |url=https://www.allanblock.com/literature/pdf/abcommmanual.pdf}}</ref>

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]] 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]] 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.<ref name="retaining-wall-tests">
{{cite book |last=Terzaghi |first=Karl |author-link=Karl von Terzaghi |title=Large Retaining Wall Tests |publisher=Engineering News Record Feb. 1, March 8, April 19 |year=1934}}</ref>

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]] requires retaining walls to be designed to ensure stability against overturning, sliding, excessive [[Foundation (engineering)|foundation]] pressure and water uplift; and that they be designed for a [[safety factor]] of 1.5 against lateral sliding and overturning.<ref>2006 International Building Code Section 1806.1.</ref>