What Is a Retaining Wall and How Do Different Types Work? An Engineering Guide

A retaining wall is a structure designed to hold back a mass of soil or other material and maintain a difference in ground level on either side of it. Retaining walls are ubiquitous in civil engineering — they support highway cuttings and embankments, stabilise slopes, enable level changes in urban landscapes, form the walls of basement structures, protect riverbanks and harbour edges, and allow construction on sites where space constraints prevent the use of natural slopes. Despite this diversity of application, all retaining walls face the same fundamental engineering challenge: resisting the lateral earth pressure exerted by the retained soil mass and transferring it safely to the foundation or to the retained soil itself. This article explains how retaining walls work, what types exist and what role geosynthetic materials play in their design and performance.

The Fundamental Challenge: Lateral Earth Pressure

Soil exerts pressure on any structure that prevents it from spreading laterally under its own weight. This lateral earth pressure — described by classical earth pressure theories developed by Coulomb (1776) and Rankine (1857) — is the fundamental load that all retaining structures must resist. Its magnitude depends on the unit weight and shear strength of the retained soil, the height of the retained mass, the angle of the retained slope surface, and the movement of the wall relative to the soil.

Three states of lateral earth pressure are recognised: active earth pressure, which acts when the wall moves away from the soil allowing the soil to expand; passive earth pressure, which acts when the wall is pushed into the soil compressing it; and at-rest earth pressure, which acts when the wall does not move. Active earth pressure is the smallest of the three and governs the design of most free-standing retaining walls. Passive earth pressure — the largest — provides resistance at the toe of embedded walls and in front of foundation elements.

In addition to the horizontal earth pressure from the retained soil, retaining walls must resist any surcharge loading on the retained surface (vehicles, buildings, stored materials), hydrostatic pressure from groundwater behind the wall, seismic inertial forces in earthquake-prone regions, and the self-weight of the wall structure itself.

Gravity Retaining Walls

The oldest and simplest type of retaining wall, a gravity wall relies entirely on its own mass — the weight of the wall material — to resist the overturning and sliding forces imposed by the retained earth. The wall must be massive enough that its weight creates sufficient friction on the foundation to prevent sliding, and its geometry must be such that the resultant of all forces passes within the base width to prevent overturning and excessive bearing pressure on the foundation soil.

Gravity walls are constructed from mass concrete, stone masonry, plain concrete or — in the geosynthetics world — from gabion baskets filled with rock. Their advantages are simplicity of construction and robustness, requiring no reinforcement and tolerating some differential settlement. Their disadvantage is that the volume of material required increases rapidly with wall height — gravity walls become uneconomical for heights above approximately 3–4 metres compared with more efficient structural types.

Gabion gravity walls are particularly suited to rural and landscape applications, riverbank protection and situations where locally available stone can be used as fill. The permeable nature of gabion walls eliminates the need for separate drainage design, as water moves freely through the rock-filled wire mesh structure — a significant advantage over impermeable concrete or masonry gravity walls.

Cantilever Retaining Walls

A cantilever retaining wall is a reinforced concrete structure consisting of a vertical stem and a base slab — forming an inverted T or L shape in cross-section. Unlike a gravity wall, a cantilever wall does not rely on the mass of the wall itself to provide stability; instead, the soil resting on the heel of the base slab provides most of the stabilising weight. The vertical stem acts as a cantilever beam, transferring the lateral earth pressure from the retained soil to the base slab, which distributes it to the foundation.

Cantilever walls are more efficient than gravity walls for heights of 3–8 metres, requiring significantly less concrete for equivalent height. However, they require careful structural design of the reinforced concrete elements for bending and shear, and the cantilever action means that horizontal deflection at the top of the wall increases with height — which must be within acceptable limits for the specific application.

The critical role of drainage behind cantilever retaining walls cannot be overstated. Groundwater accumulating behind the wall creates hydrostatic pressure that can double or triple the total lateral force on the wall compared with drained conditions. Geocomposite drainage sheets placed against the back face of cantilever walls intercept groundwater and convey it to weephole outlets at the wall base, eliminating hydrostatic pressure and ensuring that the wall is designed only for earth pressure loads.

Mechanically Stabilised Earth (MSE) Walls

Mechanically Stabilised Earth walls — also called reinforced soil walls or geosynthetic reinforced soil (GRS) walls — represent a fundamentally different approach to retaining wall design. Instead of a single structural wall element resisting earth pressure from the outside, an MSE wall is a mass of reinforced fill in which multiple horizontal layers of geosynthetic reinforcement — geogrids or geotextiles — are embedded at regular vertical intervals within the compacted fill mass.

The reinforcement layers restrain the tendency of the fill to spread laterally under its own weight and any surcharge. The composite reinforced fill mass behaves as a coherent gravity structure, resisting overturning and sliding through its total weight and the tensile resistance mobilised in the reinforcement layers. A facing system — which may be segmental concrete blocks, precast concrete panels, timber, vegetated geotextile wraps or simply a geotextile-wrapped face — provides the visible wall face and protects the fill from erosion.

MSE walls with geosynthetic reinforcement offer several compelling advantages over concrete retaining walls for many applications: they are more economical at heights above 3–4 metres; they can be constructed more rapidly from locally available fill; they tolerate differential settlement better than rigid concrete structures; they can be constructed on relatively weak foundations that would be inadequate for concrete walls; and vegetated-face MSE walls provide significant aesthetic and ecological benefits.

The design of geosynthetic MSE walls involves both internal stability checks — verifying that each reinforcement layer has adequate tensile capacity and embedment length to prevent pull-out or rupture — and external stability checks — treating the reinforced mass as a gravity structure and verifying its resistance to overall sliding, overturning and bearing failure.

Embedded Retaining Walls

Embedded retaining walls — sheet pile walls, soldier pile walls, secant pile walls and diaphragm walls — are installed by driving or casting structural elements into the ground to a depth below the excavation base. The embedded section provides passive earth resistance that balances the active earth pressure on the retained side. Embedded walls are used primarily for deep excavations in urban environments where space constraints preclude the use of gravity or cantilever walls with conventional spread foundations.

Geosynthetics play a more limited role in embedded wall design than in gravity or MSE walls, but are used for drainage and filtration behind permanent embedded walls and for ground improvement of the retained or excavated soil.

The Critical Role of Drainage in Retaining Wall Performance

The single most important factor in the long-term performance of any retaining wall — regardless of type — is drainage. Water accumulating behind a retaining wall creates hydrostatic pressure that can rapidly overwhelm the wall’s designed resistance, leading to overturning, sliding or structural failure. A disproportionately high percentage of retaining wall failures are caused by inadequate drainage rather than by structural deficiency.

Modern retaining wall design invariably incorporates one or more of the following drainage measures: geocomposite drainage sheets against the back face of the wall, conveying groundwater to outlet pipes or weepholes; granular drainage layers behind the wall face, wrapped in geotextile filter fabric to prevent migration of fines into the drainage aggregate; perforated drainage pipes at the wall base collecting and conveying water away from the retained zone; and low-permeability surface drainage to minimise rainwater infiltration into the retained soil mass.

The geotextile filter wrapping the drainage aggregate is particularly important — without filtration, fine particles migrate from the retained soil into the drainage aggregate, progressively clogging the drainage system and rebuilding hydrostatic pressure over time.

Summary

Retaining walls resist lateral earth pressure through mass (gravity walls), structural cantilever action (cantilever walls) or internal reinforcement (MSE walls). Each type has distinct advantages, limitations and appropriate height ranges. Geosynthetics contribute to retaining wall performance at multiple levels: as the primary tensile reinforcement in MSE walls; as drainage geocomposites and geotextile filters that eliminate hydrostatic pressure behind all wall types; and as protection and separation layers in composite drainage systems. Adequate drainage is the single most critical factor in retaining wall long-term performance and must be incorporated as a primary design consideration, not an afterthought.