Structure and support · Below Ground & Retaining
Reinforced Soil Retaining System
A reference for the reversal that defines this family: the retained material is the structure and the wall you can see is a facing, which changes what may be built, dug or planted in the ground behind it.
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 reinforced soil wall is
In a reinforced soil structure the retaining is done by the ground. Sheets, grids or strips of reinforcement are laid between compacted layers as the fill is built up, and friction and interlock between reinforcement and soil stop the mass spreading sideways. The result behaves as a coherent block that resists the ground behind it much as a very wide mass wall would, while the facing at the front keeps surface material in place and gives the structure an appearance.
The boundary against the gravity-retaining-wall-system entry is what is doing the retaining, and it can be settled by asking where the structure ends. A mass wall stops at the back of its own section, with undisturbed or ordinary backfill beginning there. A reinforced soil structure continues back into the slope well beyond its face, because the reinforced block is the wall. That is why a face light enough to handle by hand can hold ground that a masonry wall of the same appearance could not.
Facings vary widely: interlocking concrete units, filled cages, large panels, and wrapped faces that are planted and become green. They are chosen for appearance, for how much differential movement they tolerate and for how they connect back to the reinforcement. Structures with identical faces can be entirely different things, and nothing visible from the front says which is which.
The practical consequence is a zone behind the wall that must not be disturbed. A trench for a service, a soakaway, a pond liner, a footing for a shed or a deep planting pit within the reinforced block cuts the layers holding it together. Records of how far the reinforcement extends are therefore part of what an owner inherits, and are worth having before anything is dug behind such a wall.
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.
- mechanically stabilised earth
- MSE wall
- geogrid reinforced retaining wall
- segmental reinforced block wall
- reinforced fill structure
- reinforced slope construction
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 turn compacted fill into a coherent mass capable of retaining the ground behind it.
- To hold a change of level where a rigid wall would have to be very heavy or very deeply founded.
- To tolerate differential settlement by deforming rather than by cracking as a rigid member would.
- To allow the visible face to be selected for appearance largely independently of the structural action behind it.
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 meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 8. Order is meaningful.Primary supportReinforcement layers within the fill
Horizontal sheets, grids or strips laid between layers of fill as it rises. They carry tension that the soil cannot, and they work only through contact with the material packed around and through them, so they are inseparable from how the fill is placed.
- Layer 2 of 8. Order is meaningful.Primary supportCompacted engineered fill
The body of the structure. Its grading, moisture condition and compaction decide whether force can pass into the reinforcement at all, which makes the fill a designed component rather than material of convenience from the site.
- Layer 3 of 8. Order is meaningful.AttachmentFacing-to-reinforcement connection
The mechanical link between facing units and the reinforcement layers, whether by friction, pins, lips or embedment. It has to hold as the fill settles relative to the face, so it is designed for movement as much as for force.
- Layer 4 of 8. Order is meaningful.Finish surfaceFacing units or wrapped face
The visible front, retaining only the material immediately behind it. Its tolerance of differential movement and the way it lets water out without letting fines out are what distinguish one facing choice from another.
- Layer 5 of 8. Order is meaningful.SubstrateLevelling pad and founding surface
The prepared strip the facing is built up from and the ground beneath the whole block. Because the face is built in courses, an irregularity at the base is carried upward and magnified over the height of the structure.
- Layer 6 of 8. Order is meaningful.Drainage planeDrainage within and behind the block
Routes built into the mass as it rises, and a zone at the back where the reinforced block meets the retained ground. Water inside the block reduces the friction the reinforcement depends on, so this is not simply about the face staying clean.
- Layer 7 of 8. Order is meaningful.Edge and terminationCorners, terminations and coping
Where the structure turns, ends or steps, reinforcement layers from different directions overlap or stop, and the facing has to close. These are the locations where the geometry of a layered structure is most difficult and least standard.
- Layer 8 of 8. Order is meaningful.ProtectionSurface treatment above the block
The surfacing, planting or sealing over the top of the reinforced mass. It controls how much water enters the block from above, and it is also the layer through which any later excavation would have to pass.
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.
Reinforcement does nothing on its own; it works by gripping the soil packed around it. Compaction is therefore a component of the structure rather than a quality of the workmanship. Fill placed in layers too thick to compact throughout, or too fine or too wet to interlock, leaves reinforcement lying in material that cannot transfer force into it, and the block never becomes a block even though every layer is present and correctly spaced.
The facing is connected, and that connection is the relationship most often misread. Facing units carry only the material immediately behind them, but they must stay attached to the reinforcement while the fill settles relative to the face. A connection stiffer than the designer assumed, or slacker, concentrates force at the front. A bulging face is usually reporting a problem in this relationship, or in compaction close to the face, rather than weakness in the units themselves.
Water inside the block matters more than water behind it. Because the mass is the structure, saturating it lowers the friction the reinforcement relies on while adding weight and pressure at the same time. Drainage is therefore built in as the block rises rather than applied afterwards, and it has to reach the outside through or around the facing without carrying fines with it. Fines washing out show as voids behind units and as settlement of whatever sits on top.
Everything above and behind the block is an input to it. The reinforced mass resists the retained ground, so the slope behind, anything placed on it and the surface water arriving on it all set the demands. And because reinforcement lies in horizontal planes, anything that cuts it removes tension from a whole plane rather than damaging a patch: a service trench, a post hole, a tree root plate or a later footing behind the face can all do that, which is why the extent of the reinforcement is information worth keeping.
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.
Finish surface
Commonly encountered as facings to reinforced soil structures. A facing unit identical to one used in a mass wall performs an entirely different duty here, and whether a particular unit can be connected to a given reinforcement is a matter for the designer and the manufacturer.
Substrate
Encountered in the levelling pad the facing is built up from. Its purpose is to give a true starting line rather than to found the structure, since the reinforced mass bears on the ground beneath it as a whole.
Attachment
Encountered where connectors, pins or strip reinforcement are metallic. Ground chemistry established by investigation and the intended design life govern what can be used, and that assessment sits with the designer rather than with the material.
Drainage plane
Studded membranes are commonly met at the back of the reinforced block where it meets retained ground. They form part of a drainage route only, and the collector and outlet decide whether water actually leaves the structure.
Edge and termination
Encountered as capping over the top course and as surfacing across the top of the block. How the top is closed determines how much surface water enters the mass, which is a structural question as well as a finishing one.
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 at the back of the reinforced block
Where the reinforced mass meets undisturbed retained ground, water arriving from the slope has to be intercepted before it saturates the block. This zone is buried deep behind the visible face, so its condition can never be judged by looking at the wall.
Read about Wall Base Drainage →Paving and traffic over the reinforced mass
A pavement built on top of the block imposes load through the reinforced layers and, more significantly, requires excavation into them. Its construction depth and any edge restraint have to be reconciled with where the uppermost reinforcement lies.
Read about Pavement Foundation →Guarding at the top of the face
Posts set into the top of a layered structure both cut reinforcement locally and apply horizontal force to a face designed to carry very little. Guarding at such a wall is usually supported independently behind it, which is a decision taken during design.
Read about Guarding System →Planting within or behind the reinforced block
Trees planted on or behind the structure send roots into the same zone the reinforcement occupies, and a root plate developing between layers acts where the structure is designed to be continuous. Species, position and rooting volume are settled with both the designer and the landscape professional.
Read about Tree Pit System →Land drainage in the slope above
Reducing how much water reaches the retained slope reduces both the pressure on the block and the risk of saturating it. This is a separate system serving an area much larger than the wall, and its outfall is usually nowhere near the face.
Read about Land Drainage →Unbound surfacing over the structure
Surfacing that deforms with settlement suits a mass that continues to compress slightly, whereas rigid surfacing tends to record that movement as cracking. The choice interacts with how much movement the facing itself is expected to accommodate.
Read about Flexible Paving →
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.
- Buildability
- The structure is created by the act of building it, layer by layer, so placing and compacting fill is design being executed rather than groundwork. Supervision and testing during construction carry weight here that they do not carry for a wall cast in one operation.
- Moisture
- Water inside the reinforced mass is the condition that matters most, because it reduces the friction the reinforcement relies on. Drainage is built into the block as it rises, and cannot be added afterwards without excavating the very layers it would protect.
- Movement
- These structures accept differential settlement by deforming, which is a strength in variable ground and a difficulty where anything rigid is attached to them. Steps, copings and adjoining rigid construction need to be able to take up that movement.
- Durability
- Reinforcement is buried and cannot be inspected, so its long-term condition rests on the ground chemistry established before construction and on the material selected against it. The facing weathers visibly while the working part of the structure does not.
- Documentation
- The extent of the reinforced zone is the most valuable record a later owner can hold, because it defines where nothing may be dug. Without it, any excavation behind the wall becomes an exploratory operation requiring professional supervision.
- Maintenance and access
- There is little to maintain in the mass itself and a great deal to observe at the face. Bulging, opening joints between units, voids behind them and settlement of the surface above are the signs that report on parts nobody can reach.
- Interfaces
- Corners, steps and the meeting with rigid construction such as a building, a stair or a bridge abutment are where layered geometry becomes awkward. These are the locations to look at closely on a drawing, because standard details rarely cover them.
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.
- How far back does the reinforced zone extend, and how will that be recorded for whoever owns the structure later?
- What fill is the design based on, and is that material available or being imported?
- How does the facing connect to the reinforcement, and how much movement is that connection expected to take?
- How is water kept out of the reinforced mass, both from the slope behind and from the surface above?
- Is anything to be built, planted or buried behind the face, and has that been checked against the reinforced zone?
- How do corners, steps and terminations work, and are they covered by a drawn detail rather than a standard arrangement?
- What supervision and testing of compaction is proposed while the structure is being built?
- What will be done at the top of the face if guarding or a fence is required?
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 visible wall is the retaining structure, when it is a facing and the reinforced fill behind it is doing the work.
- That a face which looks like a stacked block wall is one, when identical units are used in structures that behave completely differently.
- That backfill is simply what goes behind a wall, when here its grading, moisture condition and compaction are the structure itself.
- That digging behind the wall affects only the ground being dug, when a trench cuts reinforcement across a whole plane.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is the extent of the reinforced zone, and can it be marked on a drawing that stays with the property?
- What fill specification and compaction regime is the design based on, and who verifies it during construction?
- How is drainage arranged inside the block, and where does it discharge?
- What ground chemistry information supports the choice of reinforcement and connectors?
- How much movement of the face should be regarded as normal, and what would indicate a problem?
- What could safely be planted or built behind this structure once it is complete?
- How are corners, steps and any junction with rigid construction being detailed?
What this page does not do
- Reinforced soil structures are engineered works whose design and construction supervision belong with qualified geotechnical engineers.
- This entry states no layer thickness, reinforcement spacing, zone length, fill specification or compaction requirement, and none can be inferred.
- Excavating behind or into the face of such a structure can sever reinforcement and is not a task to undertake without professional advice.
- Similar-looking proprietary technologies exist and their manufacturers set their own requirements, which govern over any general description here.
- Nothing here indicates that any facing, fill or reinforcement suits a given structure; that determination rests with the designer.
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 →