Structure and support · Below Ground & Retaining
Soil-Nailed Slope Retention System
A reference account of why reinforcing existing ground where it stands is a different kind of retention from building a wall or from building a reinforced fill, and what an owner should raise before a face is treated this way.
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 soil nailing is
Soil nailing retains a slope by putting reinforcement into ground that is already in position. Nothing is dug out and rebuilt: the existing soil between the reinforcing elements remains the retaining body, and the nails are there to make that body act as a block rather than as a mass free to slide along a surface within itself. What the ground is, therefore, is discovered rather than specified, and it is the material the whole arrangement depends on.
The facing is the part most readily taken for the structure, and it is the part doing least. Mesh, a sprayed shell, discrete panels or a vegetated wrap each hold the material immediately at the surface between the nail heads, while the reinforcement and the ground behind it carry the slope. That is why the same retention can appear as a hard grey face in one place and as a green one in another without anything structural having changed.
The arrangement is formed downwards rather than upwards, in lifts, because each exposed face has to stand unaided between being cut and being reinforced. That reverses the logic of its neighbours: an embedded retaining wall is formed before the excavation, and a reinforced soil structure is built up in layers with the fill. How much face may be open at any moment is a design determination made for the particular ground by a qualified engineer, and it cannot be read off another slope that looks similar.
Drainage is not an accessory here. The nails rely on the ground gripping them and on that ground having the strength the design assumed. Water arriving behind a face that gives it no route out turns the retaining body into the saturated version of itself, which behaves as a different material from the one the ground investigation described.
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.
- soil nailing
- soil nail wall
- nailed slope stabilisation
- in-situ soil reinforcement
- soil nail retention
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.
- Make an existing slope or cut face act as a reinforced block rather than as a mass able to slide along a surface within it.
- Add retention without first removing and replacing the ground that is being retained.
- Hold the material at the surface between reinforcement points so the face does not ravel while the block behaves as one.
- Give water within or behind the slope a route out through the face rather than a place to accumulate.
- Allow a face to be treated progressively from the top where working upward would require the slope to stand unsupported to its full height.
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 supportSoil nails
The reinforcing elements installed into ground that is already in place, commonly grouted into a drilled hole or driven directly. They act with the soil around them rather than independently of it, so what any of them can do is a property of the ground as much as of the element.
- Primary supportThe in-situ ground between the nails
The retaining body itself. It is not placed, compacted or selected, so its strength, its layering and how much water it carries are found on site rather than chosen, and they are the largest uncertainty the arrangement holds.
- AttachmentHead plates and connections
The parts joining each nail to the facing. They decide whether facing and reinforcement act together or separately, and they are the point at which any movement of the face is passed back into the reinforcement.
- ProtectionFacing over the slope surface
Mesh, a sprayed shell, discrete panels or a vegetated wrap covering the ground between the heads. Its work is confinement of the surface rather than retention of the slope, and how stiff it is decides how much the face may move before the facing takes load.
- Drainage planeDrainage taken out through the face
The route by which water within the slope reaches the surface and leaves it. Behind a closed facing this may be the only route available, which is why the facing type and the drainage arrangement are settled together rather than one after the other.
- ProtectionCorrosion protection of the reinforcement
Coating, sacrificial section, sheathing or grout cover around an element that is buried and will not be seen again. How aggressive the ground is towards it is a ground-investigation question rather than something to be assumed from the slope's appearance.
- Edge and terminationCrest, toe and the ends of the treated length
The places where the reinforced block stops. Ground above the uppermost nail is not reinforced, ground beyond the last nail of the run is untreated slope, and both are transitions into something else rather than edges of a structure.
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.
Change the facing and the connections change with it. A flexible mesh permits the face to move and picks up load only once it has moved; a sprayed shell begins taking load as soon as anything moves at all. The head connections, the arrangement of the reinforcement and how much deformation the face is permitted are one decision, so substituting a facing on site alters two other roles without anyone touching them.
The reinforcement and the ground are a composite, so neither can be judged on its own. A nail in ground that grips it is a structural element; the same nail in weaker or wetter ground is an object buried in a slope. This is why an arrangement designed for one cutting says very little about another that looks the same from the road.
Block the route water takes out through the face and the retaining body becomes the saturated version of itself. Nothing visible changes at that moment: the nails are still there, the facing is intact, and the material the whole scheme relies on has quietly become a different one.
Because the face is treated in lifts from the top, every level depends on the level above it already being reinforced and faced. The crest is therefore the most sensitive part of the arrangement rather than the least, and anything standing near it - a fence line, a road edge, a building - is bearing on ground that is the last to be reinforced and the first to feel a surcharge nobody allowed for.
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
Bar and section families of these kinds are commonly encountered as the reinforcing element. Which of them belongs in particular ground, and what protection it carries, is a determination for the geotechnical designer informed by the ground investigation.
Attachment
Cementitious grouts are commonly encountered in the annulus between a nail and the ground it is installed into. How such a grout behaves in a given soil is a matter for the designer and the product documentation rather than a property of the family.
Protection
Facings are commonly met in these families, from a projected concrete shell to discrete panels to steel mesh held between heads. The choice changes how much the face may deform, which makes it a structural question rather than an appearance one.
Protection
Applied protective coatings of these kinds are commonly encountered on exposed steelwork at the face. Protection of the buried length is a separate question and is settled by the designer against the measured ground conditions.
Drainage plane
Studded drainage sheet is commonly encountered where water is collected behind a closed facing and taken to an outlet. Whether a drained arrangement is being relied on at all is a design decision for the particular slope.
Edge and termination
Crest beams, toe kerbs and the terminations at each end of a treated run are commonly formed in these families. What any of them has to do there depends on what stands above the crest and what collects at the toe.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Water within the retained ground
The drainage behind and through a retaining face governs what the retained material actually is. Here that material is the structure, so where the water is intercepted, what filter separation it passes, and where it is released are questions about the retention itself rather than about a drainage add-on.
Read about Wall Base Drainage →Run-off arriving at the crest
Water crossing the ground above a slope reaches the treated face either as run-off over the crest or as infiltration behind it. Whether it is caught before the crest decides whether the face drainage is dealing with the slope's own water or with the catchment above it as well.
Read about Surface Water Collection →A surfaced area at the crest or the toe
A road, path or hardstanding near the crest is a surcharge on ground that is being reinforced, and one at the toe is built over the point where the face is terminated. Both are commonly designed by a different discipline from the one designing the retention.
Read about Pavement Foundation →A wall standing on treated ground
A boundary wall along the crest founds within the same block the nails are reinforcing. Its foundation and the reinforcement occupy one volume of ground, and unless someone reconciles them the wall is designed as though the slope below it were undisturbed.
Read about Freestanding Wall →Structures founded near a treated face
Piles installed close to a nailed slope pass through ground the nails are reinforcing, and each arrangement changes what the other is holding. Which was designed first, and whether the second knew about it, is worth establishing before either is installed.
Read about Piled Foundation →Land, services and ownership beyond the face
The reinforcement extends horizontally into ground behind the visible face, which may be in different ownership or may contain buried services. How far it reaches is a question asked before the face is designed rather than discovered while it is being installed, and what is permitted varies by jurisdiction.
The existing slope and what is growing on it
Vegetation cleared for access changes the water regime of a slope before any reinforcement is installed, and a vegetated facing then has to establish on a face that is partly covered by hardware. What the slope was doing before the work is part of the brief for what replaces it.
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.
- Moisture
- Water is the variable that changes the strength of the retaining body, and the retaining body is the ground itself. How water reaches the slope, how it moves within it and how it leaves through the face are one discussion, and the assessment belongs with a geotechnical engineer rather than with general reasoning about drainage.
- Movement
- These arrangements commonly accept some movement of the face as part of how they work, because the reinforcement has to be engaged before it can resist. How much movement is anticipated, and what nearby structures and services could tolerate, is set by the designer for the particular slope.
- Durability
- The working parts are buried in ground whose chemistry may be aggressive and cannot be inspected once installed. What protection they carry, and what condition the facing and the heads are expected to be in over time, are questions to settle while the design is still open.
- Buildability
- The construction depends on ground standing unaided between exposure and reinforcement, and on plant being able to reach the face. Access, working room at the crest and toe, and what happens if the exposed ground behaves differently from the investigation are practical questions with structural consequences.
- Maintenance and access
- A treated face is not finished work that can be forgotten. Face drainage can block, vegetated facings need managing, and heads and plates are exposed. Who inspects the face, how they reach it, and what they are looking for belong in the discussion at design stage.
- Documentation
- The reinforcement is invisible once installed, so what was built is recoverable only from records: where nails went, how far they reach, what the ground turned out to be, and where drainage was taken out. Anyone later digging near the crest or toe needs that record to exist.
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 investigation supports treating this slope as a reinforced block, and what did it find?
- Is the facing being chosen for how much the face may move, or for how it looks from the property below?
- Where does water within this slope currently go, and where is it intended to go once the face is closed?
- What stands above the crest now, and what might stand there later, that would surcharge reinforced ground?
- How far behind the face does the reinforcement reach, and whose land or services are in that zone?
- Who is designing the face, and is the same party designing the drainage taken out through it?
- What condition is the face expected to be inspected in, and who has access to reach it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The facing is not the retaining structure. It confines the material at the surface between the heads, while the reinforced ground behind it does the retaining, so a face that looks robust says nothing about what is holding the slope.
- This is not a reinforced soil retaining structure. There the reinforcement is laid between compacted layers of fill as a new mass is built up; here nothing is placed at all and existing ground is reinforced where it stands.
- It is also not an embedded retaining wall. That is a wall formed before the excavation and held by the length still buried below formation; there is no wall here and no embedment.
- Soil nails and ground anchors are not the same idea. An anchor is tensioned to hold a structure back against the ground, while these elements are working with the ground to make it act as one body.
- A green face is not an untreated slope. A vegetated wrap is one kind of facing over a fully reinforced block, and what is behind it is as engineered as a sprayed shell.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground model is this design based on, and how was it established?
- What movement of the face has been anticipated, and what nearby structures or services were checked against it?
- How is water expected to reach the face drainage, and what happens to the design if it does not?
- What corrosion protection has been specified for the buried length, and on what evidence about the ground?
- How far does the reinforcement extend behind the face, and has the land or services in that zone been confirmed?
- Who is responsible for the temporary condition of the ground while each lift is open?
- What inspection and monitoring is expected once the face is complete, and who carries it out?
What this page does not do
- This entry describes how the arrangement is organised. It is not a design, a method, a specification, or a substitute for geotechnical design by a qualified engineer.
- No spacing, length, inclination, capacity or sequence is stated here, and none can be inferred from this page; all of them are project-specific and determined by ground conditions.
- Slope stabilisation commonly involves temporary works beneath or beside things that are already standing, and temporary works design is its own discipline.
- Whether reinforcement may extend beyond a site boundary, and what notice or consent that needs, varies by jurisdiction and is a legal question as much as a technical one.
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.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Boundary Structure · External works
- Substructure · Substructure
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.
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 →