OpenGTi - Open Geotechnical Intelligence

Retaining Walls

Retaining Walls

Behaviour, earth pressures and design of gravity, cantilever and L-shaped retaining walls, from classical Rankine/Coulomb theory to modern partial-factor design for external, internal and overall stability.

Where they are used

  • Basements, cut-and-fill platforms
  • Road and rail embankments
  • Bridge abutments and ramp retaining systems

Key questions

  • What lateral earth and water pressures act on the wall?
  • Is the wall stable against sliding, overturning and overall failure?
  • Are internal stresses, crack widths and deformations acceptable?

Main tools

  • Rankine & Coulomb earth pressure theory
  • Limit equilibrium and partial-factor design
  • Finite element / finite difference modelling

Overview

A retaining wall supports soil at different levels by resisting lateral earth and water pressures and other actions (surcharge, traffic, seismic). Design combines soil mechanics (earth pressures, bearing capacity, global stability) with structural design of the wall stem, base slab and connections, informed by appropriate ground investigation and groundwater assessment.

Design focus: ensure adequate external stability (sliding, overturning, bearing, overall stability) and internal stability (bending, shear, cracking, reinforcement) for all relevant limit states, and verify serviceability performance (deflections, rotations, crack widths) for the intended use and tolerance criteria.

Wall Types

Gravity & Semi-Gravity Walls

Massive walls (concrete, masonry, large blocks, gabions) that resist earth pressures primarily through their own weight, sometimes with modest structural reinforcement. Behaviour is essentially rigid-body: stability relies on a favourable self-weight and lever arm relative to overturning and sliding actions.

  • Simple detailing, but heavy and bulky.
  • Suited to low to medium heights where space is available and bearing capacity is adequate.

Cantilever & Counterfort Walls

Reinforced concrete walls with a slender stem and base slab (heel and toe). The soil over the heel acts as a counterweight, and the stem and base behave as cantilevers fixed at their junction; counterforts may be added for higher walls to reduce stem bending.

  • Efficient up to moderate heights for simple cantilevers; higher with counterforts.
  • Common for basements, abutments and cuttings where a structural solution is appropriate.

L-Shaped Walls

Variant of a cantilever wall with most or all of the base on the low side, forming an “L” shape. Used where a heel cannot project into the retained soil (e.g. boundaries, services, existing structures).

  • Relies more on toe length and front soil for stability.
  • Useful for property line or tight urban sites but can increase base pressure.

Other Systems

Mechanically stabilised / reinforced soil (MSE) walls, anchored walls, sheet pile and secant pile walls introduce reinforcement or deep structural elements. Earth pressure concepts remain relevant, but internal stability (reinforcement pull-out/rupture, connections) and overall stability require dedicated design models.

Active, Passive & At-Rest Pressures

Lateral earth pressure depends on how much the wall and soil are allowed to move. Three idealised states are commonly considered: active, at-rest and passive.

Earth Pressure States

  • Active (Ka): wall moves or rotates sufficiently away from the soil to reduce horizontal stress to a minimum compatible with equilibrium. Requires measurable outward movement.
  • Passive (Kp): wall moves or rotates into the soil, increasing horizontal stress to a maximum before failure of the front wedge. Requires substantial displacement and careful justification before full Kp is assumed in design.
  • At-rest (K0): restrained wall with negligible lateral strain; K0 lies between Ka and Kp and is appropriate for stiff, braced or propped walls with limited movement.

Rankine, Coulomb & Code-Based Design

  • Rankine: stress-based solution assuming a vertical wall, horizontal backfill and no wall friction. Provides simple closed-form Ka, Kp values; mainly applicable when geometry closely matches assumptions.
  • Coulomb: wedge equilibrium including wall friction, wall inclination and sloping backfill. More general and widely used for conventional retaining walls.
  • Code-based design: uses Rankine/Coulomb or numerical models to obtain characteristic pressures, then applies partial factors to soil strength and actions to derive design earth and water pressure diagrams.
The choice between Ka, K0 and Kp depends on anticipated movements and stiffness of the wall–soil system. Flexible cantilevered walls often mobilise conditions close to active; stiff braced or propped walls may stay close to at-rest. Passive resistance is typically treated conservatively and reduced to allow for limited mobilisation, disturbance and uncertainty.
Water pressures are not included in K-values. Hydrostatic and seepage pressures must be evaluated separately and superimposed on soil pressures. Blocked drainage can rapidly increase lateral loads beyond drained design values.

Lump Factor of Safety vs Partial Factor Methods

Retaining walls were historically designed using global (lump) factors of safety. Modern standards generally adopt limit state and partial-factor methods for more transparent and consistent reliability, with explicit ultimate and serviceability limit states.

Lump Factor of Safety Partial Factor (Limit State)
Concept Compute ultimate resistance and divide by a single FS. Apply factors to actions, soil properties and resistance separately.
Loads Service/unfactored. Factored (e.g. permanent, variable and special actions with separate factors).
Soil strength Characteristic values used directly. Reduced design values (φd, cd) using appropriate partial factors.
Resistance Rallow = Rult/FS. Rd = R(φd, cd)/γR.
Use Legacy design, simple checks. Required or preferred under modern limit-state geotechnical and structural codes.

In everyday language, designers still refer to “factor of safety against sliding/overturning”, but calculations are usually carried out within a partial-factor framework for ultimate and serviceability limit states using cautious characteristic soil parameters.

External Stability

Sliding

Sliding resistance is provided mainly by base friction, and in some cases by cohesion or passive resistance at the toe. The design check compares factored lateral forces with available factored resistance:

Fd,h ≤ (N′d tan φ′d,base + c′d,base A) / γR,sliding

where N′d is the factored effective normal force on the base and A is base area. In many designs cohesion at the base is neglected or heavily reduced, and passive resistance is only included where adequate confinement and long-term availability can be demonstrated. In working stress format this reduces to a factor of safety against sliding (typically of order 1.3–1.5).

Overturning

Overturning is assessed by summing moments about the toe. Restoring moments from self-weight and soil above the heel must exceed overturning moments from earth and water pressures by the required margin:

ΣMresisting ≥ ΣMoverturning × (required margin)

In partial-factor design, actions and resistances are factored rather than using a single global factor. Eccentricity is checked to ensure the resultant lies within an acceptable portion of the base; in working stress design this is often the “middle third” for no-tension conditions. Some limit-state approaches allow local tension provided the compressed area remains adequate and structural design accounts for uplift and cracking.

Bearing & Global Stability
  • Bearing: check maximum and minimum contact pressures under the base, accounting for eccentricity, uplift and pore pressures, against bearing resistance based on design soil strength. Non-uniform contact (loss of contact at heel or toe) should be considered explicitly.
  • Global stability: consider potential deep-seated slip surfaces passing beneath or behind the wall, particularly for high walls or walls near slopes or weak strata. Typically assessed using slope stability software (limit equilibrium) or numerical analysis with soil parameters and factors consistent with geotechnical design requirements.

Structural Design

Wall Stem

  • Modelled as a cantilever (for free-standing walls) or spanning between supports (for propped/braced walls).
  • Design for bending and shear from factored earth and water pressures derived from the geotechnical model.
  • Check crack control, deflection and durability (cover, concrete quality, exposure classification) to ensure long-term performance.

Base Slab, Keys & Connections

  • Heel and toe slabs behave as cantilevers from the stem; design for soil and surcharge loads and any concentrated reactions from superstructure.
  • Check punching shear around the stem and any columns or concentrated loads; provide shear keys where required for sliding resistance and detail them structurally.
  • Detail reinforcement for continuity and constructability, including development lengths, anchorage, and compatibility with joints, waterstops and drainage details.

For propped or braced walls, earth pressure distributions depend strongly on stiffness and staging; pressures should be derived from at-rest or numerically determined distributions rather than assuming simple active conditions.

Drainage & Backfill

  • Backfill: granular, well-compacted material reduces earth pressures and improves drainage. Avoid expansive or highly plastic clays directly behind the wall where possible.
  • Drainage layer: free-draining gravel or geocomposite behind the wall to intercept seepage and route water to collection drains or weepholes.
  • Weepholes / collector drains: relieve water pressure; ensure outlets remain free-draining and protected from clogging, freezing and scour at the toe.
  • Filters & geotextiles: gradation and filter design should prevent migration of fines while allowing adequate water flow, in accordance with filter criteria.
Poor drainage or blocked weepholes can lead to water pressures that significantly exceed the intended design state, greatly increasing lateral loads and reducing stability. Construction control is required to keep drains and outlets free of laitance, debris and later obstruction.

Construction & Monitoring

  • Founding level QA: check bearing strata, remove soft spots and document levels, soil conditions and groundwater. Confirm that ground conditions match design assumptions.
  • Backfill placement: compact in thin layers; limit the use of heavy compaction plant near the wall (typically within a specified distance) to avoid excessive compaction-induced pressures and damage.
  • Sequence: consider staged backfilling, curing and any temporary supports; avoid surcharge or traffic loads on the backfill before the wall is structurally capable.
  • Records & QA: retain as-built records of founding levels, backfill materials, compaction results and drain locations for future reference.
  • Instrumentation (for critical/high walls): monitor deflection and pore pressure using inclinometers and piezometers; establish trigger levels and response plans, and back-analyse soil parameters where appropriate.

Worked Example (Sketch)

Given: 4.0 m high cantilever retaining wall supporting level granular backfill (φ = 32°, γ = 19 kN/m³). Backfill is drained; wall stem is vertical and smooth. Ignore surcharge and groundwater for this illustrative example. Approximate active pressure using Rankine and sketch sliding and overturning checks in a working stress format.

  1. Active earth pressure coefficient:
    Ka = (1 − sin φ) / (1 + sin φ)
    For φ = 32°, Ka ≈ 0.31
  2. Lateral pressure at base:
    σh,base = Ka γ H ≈ 0.31 × 19 × 4.0 ≈ 23.6 kPa
    Resultant active force (per metre run):
    Pa = ½ Ka γ H² ≈ 0.5 × 0.31 × 19 × 4² ≈ 47 kN/m
    Acting at H/3 above the base.
  3. Overturning and restoring moments:
    Compute overturning moment about the toe from Pa. Evaluate restoring moments from the self-weight of stem, base slab and soil over the heel. Check that the factor of safety against overturning is acceptable or, in limit-state terms, that factored restoring moments exceed factored overturning moments and that base eccentricity is within allowable limits.
  4. Sliding check:
    Determine the vertical effective normal force N′ at the base (self-weight minus any uplift) and evaluate base friction resistance R = N′ tan φbase (plus any justified cohesion or passive contribution). Compare with Pa to obtain a sliding factor of safety or verify the relevant limit-state inequality with partial factors.
  5. Next steps: refine the analysis using the actual wall geometry, surcharges, groundwater and partial factors required by the governing standard; then complete structural design of the stem, base and connections under the resulting design pressure diagrams.

This example is illustrative only. Project-specific wall geometry, soil parameters, groundwater conditions, water pressures and applicable standards must be used for actual design. Overall stability of the wall and adjacent slopes must also be verified where relevant.

Further Reading

  • Standard geotechnical texts on earth retaining structures and soil mechanics.
  • National or regional retaining wall design guidelines and geotechnical design manuals issued by relevant authorities.
  • Applicable structural and geotechnical design standards for earth-retaining structures in your jurisdiction.
  • Manufacturer design manuals and approvals where proprietary retaining systems are used.
This page provides general technical guidance on retaining wall behaviour and design. Always confirm project-specific requirements, governing standards, site conditions and investigation data before design or construction.