Structure and support · Below Ground & Retaining
Cantilever Retaining Wall System
A reference explaining why this form of wall recruits the ground it retains as part of its own structure, and why anything done to the soil behind it, above it or at its toe is a structural change rather than a landscaping one.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What cantilever retaining wall is
A cantilever retaining wall is a bent structural member rather than a heavy object. The stem rises from a base slab and stem and base are made continuous, so pressure collected over the height of the stem arrives at the base as a turning effect. The base resists that turning through soil sitting on the heel behind the wall and through bearing under its underside, while the toe projecting in front helps resist the tendency to slide forward.
The boundary against the gravity-retaining-wall-system entry is whether the wall needs ground standing on its own base in order to work. Dig out behind a cantilever wall, down towards the level of its heel, and it loses a large part of what was holding it. Do the same behind a gravity wall and the mass is untouched, because it never recruited that weight. The other clue visible from the front is slenderness: a cantilever stem is thin for the height it holds, precisely because it is working in bending.
The form varies in ways that address particular constraints. An L-shaped wall puts the base entirely behind or entirely in front of the stem where a boundary prevents a symmetrical base; a T-shaped wall has both heel and toe; counterforts or buttresses stiffen a tall stem so it spans between them rather than bending over its full height. Precast and cast in place versions of each exist, and the difference shows chiefly in where the joints fall.
The consequence an owner has to absorb is that the retained ground is part of the structure. Levels raised behind the wall, a driveway brought close to its top, a heavy planter, an unexpected vehicle, or a service trench dug along the line of the heel all change the demands on a member proportioned for one particular profile of ground and loading.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- reinforced concrete retaining wall
- stem and base retaining wall
- L-wall and T-wall
- reinforced stem wall
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- To hold a change of ground level using bending strength and recruited soil weight rather than sheer mass.
- To retain ground where the width available for a heavy mass wall does not exist.
- To convert horizontal pressure from the retained ground into bearing and friction under a base slab.
- To keep the visible face slender while the working part of the structure sits buried behind and below it.
- To provide a stable edge that surfacing, guarding and drainage above and behind can be planned around.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportReinforced stem
The upstand that collects pressure from the retained ground. It works in bending, with its reinforcement concentrated towards the retained face, and it is at its most heavily worked where it meets the base rather than anywhere up its height.
- Primary supportBase slab with heel and toe
The horizontal member the stem grows from. The heel projects back under the retained soil and the toe projects forward, and their relative lengths are the main variable the designer has for balancing overturning, sliding and bearing pressure.
- Primary supportRecruited soil on the heel
The block of retained ground bearing on the heel is a working part of the structure, not backfill. Its weight holds the wall down against the turning effect of the pressure on the stem, which is why it must not be removed or lightened later.
- SubstrateFounding surface beneath the base
The ground under the base carries a pressure that varies across the width because the wall is leaning, and supplies the friction resisting sliding. Any softening of it, by water or by disturbance at the toe, undermines both mechanisms at once.
- Drainage planeDrainage against the retained face of the stem
A permeable route down the back of the stem to a collector and an outlet. Its function is to stop water standing against the wall, because saturated ground applies pressure the stem was not proportioned for.
- Jointing and sealingConstruction and movement joints
The joint between base and stem, and the vertical joints breaking a long wall into lengths. The stem-to-base joint falls at the point of greatest force, which is why its detailing matters more than its convenience during construction.
- Edge and terminationCoping and top edge
The capping over the stem and the strip where surfacing behind meets the wall. It sheds water off the top of the stem and sets the relationship between finished level behind and the drainage zone below it.
- ProtectionCover and protective treatment to the buried face
The retained face of the stem and the whole base are in permanently damp ground whose chemistry was established by investigation. Concrete cover and any applied protection are what stand between that environment and the reinforcement doing the work.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Stem and base are the same member. The reinforcement resisting bending in the top of the base is the reinforcement running up the retained face of the stem, and the corner where they meet carries the greatest force in the wall. That corner is also, awkwardly, where casting usually stops and restarts. What is convenient as a construction break is the least convenient place structurally, which is why the kicker detail matters disproportionately on a wall that otherwise looks simple.
Heel and toe do different jobs and trade against each other. Lengthening the heel recruits more soil weight and improves resistance to overturning; lengthening the toe spreads bearing pressure and helps against sliding. Boundaries, existing services and neighbouring foundations regularly make one of them impossible, and the wall then has to be reconfigured through a shear key, a buttress or a different form rather than simply made heavier. Geometry here is set by the site, not selected from a picture.
Drainage behind the stem changes the loading, not merely the durability. Water standing against the retained face adds pressure the stem was never proportioned for, and in freezing conditions the ground behind adds another action again. The permeable route, the filter that keeps it from silting and the outlet all have to survive being backfilled over; an outlet crushed during backfilling leaves a wall that looks complete and is quietly overloaded every time the ground behind saturates.
Everything above and behind the wall reaches the stem. Surface water shed towards the wall arrives in the drainage zone, surfacing laid tight to the top of the stem prevents it from getting there, and loads placed near the top edge reach the base magnified because they act high above it. Coping, surface falls behind the wall, guarding and the drainage outlet are usually drawn by different people and all meet within a narrow strip along the top of the wall.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Commonly encountered in the stem and base of walls of this form. Reinforcement arrangement, concrete grade and cover follow from the engineer's design and from the ground chemistry established by investigation, and cannot be inferred from the form of the wall.
Protection
Encountered on retained faces where the intention is to keep the buried concrete from wetting and staining through, or to protect it from aggressive ground. Whether protection is needed at all is a matter for the designer following ground investigation.
Jointing and sealing
Commonly met at the stem-to-base joint and at joints dividing a long wall into lengths. Movement capability and compatibility with concrete and with any protective layer determine whether a product can be used, which is a design and manufacturer question.
Drainage plane
Studded membranes are commonly encountered against the retained face as part of a drainage arrangement. They form a route rather than a complete solution, since the collector and the outlet decide whether water actually leaves the wall.
Edge and termination
Encountered as capping across the top of the stem. How the capping oversails, drips and relates to the surfacing behind is detailing rather than product selection, and exposure governs what any manufacturer will support.
Finish surface
Applied faces of this kind are commonly met on the exposed side of concrete retaining walls. A facing here sits on concrete that may be damp from behind and moves with it, so compatibility and movement accommodation are questions for the designer.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Drainage relieving pressure on the stem
The drainage behind the stem is a load-reducing measure as much as a moisture measure, and its outlet is usually buried. Because the wall was proportioned assuming drained ground, a silted or crushed outlet changes the structural condition without anything visible happening.
Read about Wall Base Drainage →Guarding above the retained edge
Guarding applies horizontal force at the top of the stem, the point furthest from the base and therefore the most demanding place to apply it. Retrofitting posts into the top of an existing wall introduces both that force and penetrations into the cover protecting its reinforcement.
Read about Guarding System →Surfacing and traffic on the retained side
A pavement built behind the wall adds load over the heel, changes where surface water goes and may seal the drainage zone at the top. The construction depth of the pavement also determines how close its excavation comes to the recruited soil the wall relies on.
Read about Pavement Foundation →Where the wall meets a building below ground
A cantilever wall running up to a basement is a free-standing member meeting a propped box, and each responds to ground pressure differently. The junction has to accommodate that difference rather than assume the elements move together.
Read about Basement Structure →Collecting water before it reaches the retained ground
A channel or gully at the top of the slope behind the wall reduces how much water ever enters the ground the wall is holding. It is a separate system whose blockage shows up as pressure on a structure rather than as a puddle.
Read about Surface Water Collection →Treatment of the ground under and behind the wall
Where founding conditions cannot carry the pressure under the toe, or the retained ground pushes harder than the form allows, treating the ground changes the problem the wall has to solve. Either measure alters assumptions the wall was designed against.
Read about Ground Improvement →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Movement
- This is a monolithic reinforced member, so differential settlement along its length appears as cracking at a located section rather than as distortion taken up between units the way it is in dry-laid, gabion and segmental forms. That is why joint positions are a design decision, and why ground varying along the line of the wall is a reason to expect the design to change along it too.
- Moisture
- The retained face and base sit permanently in damp ground while the exposed face wets and dries. Water leaving through the exposed face, seen as staining or salts, generally reports that drainage behind is not taking water away rather than that the concrete is at fault.
- Durability
- The working element is steel embedded in concrete in a wet environment, so cover and concrete quality carry the whole durability argument. Nothing about that can be judged from the finished appearance of the wall once it is faced or rendered.
- Buildability
- The base is cast before the stem in most walls, which puts a construction joint where forces are greatest, and the recruited soil has to be placed and compacted behind the wall without damaging the drainage or overstressing the stem before it is ready.
- Interfaces
- The top of the wall is a narrow strip where the coping, surfacing behind, guarding fixings and drainage all arrive. These are frequently detailed by different parties on different drawings, and the strip is where their assumptions have to reconcile.
- Documentation
- The heel extent, the joint positions and the drainage outlets are all invisible once the wall is complete. A record of them is what allows a later trench, planter or fence to be positioned without cutting into what holds the wall up.
- Maintenance and access
- The only routinely maintainable part is the drainage outlet, and it is usually the part that disappears under planting. Knowing where it is, and that it should discharge during heavy rain, is the practical check available to an owner.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What ground profile, surcharge and loading behind the wall has the design been based on?
- How long is the heel, and does anything planned behind the wall come within that zone?
- Where does the construction joint between base and stem fall, and how is it being detailed?
- How is water intercepted before it reaches the retained ground, and how does it leave the wall?
- Is guarding required at the top, and has its force been included in the design of the stem?
- Does the ground along the line of the wall vary, and does the wall change with it?
- Will the exposed face be finished, and has that finish been considered against a face that may be damp from behind?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- That the wall is the stem, when the working structure includes the base slab and the block of retained soil sitting on the heel.
- That a slender concrete wall is weaker than a thick masonry one, when each resists ground in an entirely different way.
- That backfill behind the wall is simply fill, when its weight and its drainage behaviour are both structural assumptions.
- That a drainage outlet at the base of a wall is a cosmetic detail, when a blocked one increases the pressure the stem carries.
- That fixing a fence or guarding to the top of an existing wall is a fixing question rather than a structural one.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What loading behind the wall has been assumed, including any vehicle access or future raised levels?
- How far back does the heel extend, and can that be marked so nobody excavates within it?
- What has ground investigation established about bearing and about the chemistry the concrete sits in?
- How is the drainage behind the wall arranged, and where can its discharge be observed?
- What movement or cracking should be regarded as routine for this wall, and what should prompt inspection?
- If guarding is to be added later, what would need to be verified before fixing into the stem?
What this page does not do
- The proportioning of a cantilever retaining wall is a structural design task for a qualified engineer working from ground investigation.
- No dimension, reinforcement arrangement, heel or toe length, or drainage arrangement is stated or implied anywhere in this entry.
- Excavating behind an existing wall of this form, or at its toe, can remove part of what is holding it and is not a landscaping decision.
- Raising ground or parking vehicles behind a retaining wall changes its loading and should be discussed before it happens, not after.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Below-Ground Structures, Earth Retention and Water Control
Assemblies built against retained ground, where lateral earth pressure and the water held in that ground decide the design together rather than one at a time.
Browse all below ground & retaining entries →