Retaining walls are relatively rigid walls used for supporting the soil mass laterally so that the soil 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). Earth retaining structures, such as retaining walls, bridge abutments, quay walls, anchored bulk-heads, braced excavations, and mechanically stabilized walls, are used throughout seismically active areas. They frequently represent key elements of ports and harbors, transportation systems, lifelines, and other constructed facilities.
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 homogenous 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.
As an example, the International Building Code requires retaining walls to be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift; and that they be designed for a safety factor of 1.5 against lateral sliding and overturning.
The problem of retaining soil is one of the oldest in geotechnical engineering; some of the earliest and most fundamental principles of soil mechanics were developed to allow rational design of retaining walls. Many different approaches to soil retention have been developed and used successfully. In recent years, the development of metallic, polymer, and geotextile reinforcement has led to the development of many innovative types of mechanically stabilized earth retention systems.
Retaining walls are often classified in terms of their relative mass, flexibility, and anchorage conditions. Gravity walls are the oldest and simplest type of retaining wall.
To design retaining walls, it is necessary to define "failure" and to know how walls can fail. Under static conditions, retaining walls are acted upon by body forces related to the mass of the wall, by soil pressures, and by external forces such as those transmitted by braces. A properly designed retaining wall will achieve equilibrium of these forces without inducing shear stresses that approach the shear strength of the soil. During an earthquake, however, inertial forces and changes in soil strength may violate equilibrium and cause permanent deformation of the wall. Failure, whether by sliding, tilting, bending, or some other mechanism, occurs when these permanent deformations become excessive. The question of what level of deformation is excessive depends on many factors and is best addressed on a site-specific basis.
Gravity walls usually fail by rigid-body mechanisms such as sliding and/or overturning or by gross instability. Sliding occurs when horizontal force equilibrium is not maintained (i.e., when the lateral pressures on the back of the wall produce a thrust that exceeds the available sliding resistance on the base of the wall). Overturning failures occur when moment equilibrium is not satisfied; bearing failures at the base of the wall are often involved. Gravity walls may also be damaged by gross instability of the soils behind and beneath them. Such failures may be treated as slope stability failures that encompass the wall. Composite wall systems, such as crib walls, bin walls, and mechanically stabilized walls, can fail in the same ways or by a number of internal mechanisms that may involve shearing, pullout, or tensile failure of various wall elements.
Cantilever walls are subject to the same failure mechanisms as gravity walls, and also to flexural failure mechanisms. Soil pressures and bending moments in cantilever walls depend on the geometry, stiffness, and strength of the wall—soil system. If the bending moments required for equilibrium exceed the flexural strength of the wall, flexural failure may occur. The structural ductility of the wall itself may influence the level of deformation produced by flexural failure.
Braced walls usually fail by gross instability, tilting, flexural failure, and/or failure of bracing elements. Tilting of braced walls typically involves rotation about the point at which the brace acts on the wall, often the top of the wall as in the cases of basement and bridge abutment walls. Anchored walls with inadequate penetration may tilt by "kicking out" at their toes. As in the case of cantilever walls, anchored walls may fail in flexure, although the point of failure (maximum bending moment) is likely to be different. Failure of bracing elements can include anchor pullout, tierod failure, or bridge buckling. Backfill settlements can also impose additional axial and transverse loading on bracing elements such as tierods and tiebacks.