Structure and support · Below Ground & Retaining
Basement Structure System
A reference for owners and designers who need to see a basement as a closed structural box, so that questions about floors, openings and alterations are recognised as questions about the retaining perimeter.
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 basement structure is
A basement structure is the whole below-ground enclosure treated as one object: the perimeter walls that hold back ground and groundwater, the base slab they stand on, and the floor plates that span between them. Drawn on separate sheets these look like ordinary walls and slabs. Built, they behave as a closed box whose faces restrain each other, and none of them can be assessed on its own.
The boundary against the embedded-retaining-wall-system entry is testable by looking at the building. An embedded retaining wall has ground on one side and is complete in itself. A basement box has ground on every side and floor plates crossing it. Take the ground floor away from a basement and the perimeter walls change how they carry load; take a slab away from a lone retaining wall and the wall carries on unchanged.
Because a basement wall is normally held near its head by a floor plate and near its foot by the base slab, those plates are structural components of the wall, not merely surfaces to walk on. That is why cutting an opening in a basement floor, moving a stair, or replacing a ground floor over an old cellar is a structural question about the perimeter rather than a flooring decision. The box is also where the building sits in groundwater, so its geometry is shared with a water-resisting strategy drawn by other people, and the quality of a basement is usually decided in that overlap.
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.
- below-ground structural box
- basement shell
- sub-structure enclosure
- retained box structure
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 back surrounding ground and any water pressure acting on the buried faces of the building.
- To carry superstructure loads down to founding level through walls and base slab working together.
- To use the floor plates as restraint at the head of the perimeter walls, so no wall is asked to act alone.
- To create enclosed usable volume below ground that the rest of the building can be planned around.
- To give the water-resisting and ground gas strategies a continuous and geometrically simple surface to follow.
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 supportPerimeter retaining walls
The buried faces of the box. They collect ground pressure and any water pressure acting on the outside and pass it into the base slab at their feet and into the floor plates at their heads, while also carrying vertical load from the structure above.
- Primary supportBase slab
The floor of the box. It spreads building load onto the founding stratum, resists any upward water pressure trying to lift it, and holds the feet of the perimeter walls in position so they cannot slide inwards.
- Primary supportPropping floor plates
The ground floor and any intermediate basement floors act as horizontal props. They carry the inward thrust from one wall across the plan and deliver it into the opposite wall, which is being pushed the other way, so those opposing thrusts balance through the plate.
- Jointing and sealingWall-to-slab connection and construction joints
The kicker and the joints where casting stopped and restarted. Reinforcement changes direction here, the water-resisting strategy has to cross here, and the wall and slab exchange force here, so the same short line answers to several designers.
- Jointing and sealingMovement joints
Deliberate discontinuities that give shrinkage and thermal movement somewhere chosen to occur. They cut through the very continuity that the box relies on, which is why their position is a joint decision between the structural and the water-resisting design.
- Edge and terminationOpenings and trimming
Stair voids, lift shafts, plant openings and light wells interrupt the floor plates. Each has to be trimmed so that thrust still travels around the hole, and each is a place where the propping action of the plate is locally absent.
- Edge and terminationSuperstructure bearing interface
The line where the box hands over to the building above. Column positions, wall lines and stability elements rarely coincide exactly with the basement perimeter, so load is redistributed here and the transfer has to be deliberate rather than assumed.
- Service zonePenetrations through the buried envelope
Drainage, incoming utilities and pumped discharges have to cross a face that is both structural and water-resisting. Each crossing opens the structure and the water-resisting layer at the same moment, and is designed as a detail rather than formed wherever convenience suggests.
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.
The relationship that defines the box runs from ground to slab and back into the walls. Ground pushes on a perimeter wall; the wall leans on the base slab at its foot and on the floor plate near its head; the plate carries that push across the building and delivers it into the opposite wall, which is being pushed the other way. Remove the plate and the wall has to find its restraint elsewhere, usually by bending far more or by being propped.
This is why a hole in a basement floor is not a floor question. A stair moved, a lift shaft cut through, or a void opened for a double-height room interrupts the route by which one wall's thrust reaches the wall opposite. The remaining plate has to be trimmed so the force still travels around the opening, and whether that is achievable depends on the wall being propped, not on the span of the floor.
The base slab and the walls meet at the hardest-working line in the structure. It is a construction joint, a change of direction in reinforcement, a place the water-resisting strategy must cross, and the point where uplift under the slab and inward thrust from the wall arrive together. A detail that satisfies any of those and ignores the rest is where basements crack and where water finds its way in.
Movement joints and construction joints cut the continuity everything else depends on. Positions that give the structural engineer relief from shrinkage may be exactly where the water-resisting designer least wants an interruption, and joints chosen independently produce a box that is structurally reasonable and hydraulically awkward. Reconciling them is a design task, not a decision to be made on site.
The box is not a box until it is closed, so its behaviour changes as it is built. Before the propping floors exist the perimeter is held by temporary works, and each stage of digging, casting and striking alters what is holding what. The permanent arrangement described on the drawings is only the last of a series of arrangements the wall passes through.
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
These families are commonly encountered in below-ground walls and base slabs. Whether any of them suits a particular basement depends on ground and groundwater conditions, the loads involved, the water-resisting strategy chosen and the engineer's design, and on the manufacturer's documentation for the product itself.
Control layer
Materials commonly met where a barrier is applied to or against the buried faces of the structure. No layer resists water on its own; behaviour belongs to the completed and correctly detailed assembly, and product selection is a matter for the specialist designer and the manufacturer's literature.
Jointing and sealing
Commonly encountered at construction joints, movement joints and around penetrations in below-ground structures. Compatibility with the surrounding concrete, with the barrier system and with each other governs whether any of them can be used together, and that is a question for the designer.
Drainage plane
Studded membranes of this kind are commonly encountered on the inner face of basement walls and under floors where a drained approach is taken. Whether a drained approach is appropriate at all depends on the water regime, the intended use of the space and specialist advice.
Protection
Encountered where ground gas or aggressive ground chemistry is a consideration on the buried faces. Whether protection is needed, and of what kind, follows from ground investigation and from the assessment made by qualified professionals rather than from the structure alone.
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.
Embedded wall retained as the permanent basement perimeter
Where piles, panels or sheets formed before excavation become the finished perimeter, the box inherits a wall whose tolerances and face were set by a boring or driving process. The connection between that wall and the basement floor plates is the moment a temporary support arrangement becomes permanent structure.
Read about Embedded Retaining Wall →Continuous barrier across walls, slab and joints
A barrier approach asks the structure for a surface it can follow without interruption, including around re-entrant corners, at the wall-to-slab junction and at every penetration. Structural joints placed for shrinkage reasons become the barrier's most demanding details, so both designs are coordinated rather than sequenced.
Read about Barrier Waterproofing →Internal drained cavity, channels and sumps
A drained approach accepts that water may reach the inner face and manages it, which changes what the structure has to provide: a route at the base of the wall, a fall in the floor construction, and access for maintaining the collection system. It also introduces pumped discharge, which is a service the box has to accommodate.
Read about Drained Cavity Protection →Gas-resistant layer and its continuity with the structure
Where ground gas is a consideration, the protective layer follows the same buried surface as the water-resisting layer and is defeated by the same discontinuities. Ventilated voids below the slab, if used, alter the slab construction, so this is settled while the structure is still being designed.
Read about Ground Gas Protection →Drainage crossing the buried envelope
Foul and surface water pipework has to pass through walls or under the slab, and each crossing is simultaneously a structural opening and an interruption in the water-resisting layer. Changing a drainage route late is therefore rarely a drainage-only change.
Read about Buried Drainage Network →Handover of stability to the superstructure
Wind and other horizontal forces from the building above have to reach the ground through the box, and the elements that provide stability above rarely sit precisely over the basement walls. The ground floor plate is often doing this job at the same time as propping the perimeter.
Read about Lateral Stability →External drainage relieving the buried faces
Drainage outside the wall reduces the water that ever arrives, which changes the demands on both the structure and the barrier. It is a separate system with its own maintenance needs, and it is normally inaccessible once the ground is reinstated.
Read about Wall Base Drainage →
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 reaches a basement as groundwater under pressure, as water perched in the backfill and as vapour from the ground and from use of the space. These are different problems and a measure aimed at one may not address another; the assessment belongs with a specialist rather than with the structural design.
- Movement
- Concrete shrinks as it matures and the box responds to temperature and to ground behaviour after the structure is complete. Where that movement is accommodated, and whether it is expected to occur at planned joints or as distributed cracking, is a design position taken early and recorded.
- Buildability
- How the excavation is supported while the box is being built determines what the finished structure looks like and where its joints fall. Temporary works are therefore part of the design conversation, not a separate matter left to the contractor after the drawings are issued.
- Interfaces
- The junction of wall, slab and barrier, and every service crossing, involve several designers with different priorities. Establishing who owns each junction, and who reviews it before it is covered up, matters more than the specification of any individual layer.
- Maintenance and access
- Some parts of a basement can never be inspected again once the ground is reinstated, while others, such as collection channels and pumps in a drained approach, need permanent access. Which parts fall into which category is worth establishing while the layout is still adjustable.
- Thermal
- Ground-facing construction behaves differently from an above-ground wall, and where insulation sits relative to the structure and the barrier changes the assembly type. Whether any arrangement is appropriate depends on the build-up, the use of the space and the climate, and is assessed by a qualified professional.
- Documentation
- Later owners inherit a structure whose critical features are invisible. Records of joint positions, penetrations, the approach taken to water resistance and any monitoring carried out are what make future alterations possible without exploratory opening up.
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.
- Which floor plates are acting as props to the perimeter walls, and at what stage of construction do they take up that duty?
- Where are the construction and movement joints proposed, and have the structural and water-resisting designers agreed those positions together?
- Is the approach to water resistance a barrier, a drained cavity, the concrete itself, or a combination, and who is responsible for the whole of it?
- What openings are proposed in the basement floor plates, and how does thrust travel around each of them?
- How will every service crossing the buried envelope be detailed, and are those routes fixed before the structure is built?
- What temporary support does the excavation need, and does any of it become permanent structure?
- How does stability from the superstructure arrive at the box, and which elements carry it into the ground?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- That a basement floor is simply a floor, when in most basements it is also the prop holding the head of the perimeter wall.
- That the water-resisting layer is a product choice, when its performance depends on continuity across joints, corners and penetrations in the structure it follows.
- That an existing cellar and a designed basement are the same kind of construction, when many older cellars were never designed to resist water pressure at all.
- That deepening or extending a basement affects only the room being altered, when it changes ground support to neighbouring foundations as well.
- That once the ground is backfilled the design is finished, when the drained and monitored parts of the system continue to need attention.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What has the ground investigation established about groundwater, and what water regime has the structure been designed against?
- Which elements of the box are propping which, and is that made explicit on the drawings for future reference?
- Who is coordinating the structural joints with the water-resisting strategy, and when does that coordination happen?
- What grade of internal environment is the basement being designed to achieve, and who decides whether it has been achieved?
- If a drained approach is used, what happens if the pumped discharge fails, and how is that risk being managed?
- How will neighbouring structures and buried services be protected and monitored while the excavation proceeds?
- What records will be handed over that a future owner could use before altering the floors or the perimeter?
What this page does not do
- Below-ground structures involve ground, groundwater and temporary works, and their design belongs with qualified geotechnical and structural engineers.
- Nothing here describes how a basement is built, propped, excavated or made water-resisting, and no dimension, reinforcement arrangement or sequence is implied.
- Water resistance is a property of a completed, correctly detailed and correctly installed assembly, never of a single layer or product named in isolation.
- Works affecting ground beside a neighbouring building may carry legal obligations that are settled with the relevant authority and the neighbour, not on site.
- This entry states no fire, acoustic, thermal or structural performance for any arrangement described, and the tested or designed assembly governs.
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.
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 →