Basement retaining walls
A basement wall retains soil and, usually, groundwater on every side, while the floors above and below prop it. We design propped and cantilever basement walls with the waterproofing strategy that keeps them dry, for clients across the UK.
How is a basement wall different from a normal retaining wall?
A basement wall is propped at the top by the ground floor and at the bottom by the basement slab, so it spans between them rather than cantilevering from a footing. That makes it efficient, but it also carries full groundwater pressure and forms part of a watertight box, so it is designed for both structure and waterproofing together.
Loads on a basement wall
A basement wall is a retaining wall that is propped top and bottom, held at the head by the ground floor and at the foot by the basement slab, so it spans vertically between them rather than cantilevering. That propping usually makes it a stiffer, more efficient wall than a free-standing one, but the loads are severe. The wall carries the at-rest earth pressure of the full retained height, any surcharge from the building above, a road or a neighbouring foundation, and, critically, the water pressure of the groundwater outside.
Water often governs the design. Below the water table the wall and slab feel full hydrostatic pressure, which increases with depth and can exceed the soil pressure, and the same water tries to float the whole box, so the basement is checked for uplift against its own weight. We design the wall to span between its props under the combined soil and water pressure, size the reinforcement to Eurocode 2, and check the slab and walls together as a watertight box to Eurocode 7.
New build and retrofit basements
A new basement in an open dig can be built as a reinforced concrete box, walls and slab cast together and propped by the permanent floors, with the temporary excavation supported until the structure is complete. A basement formed under an existing house is very different: the walls are often built by underpinning the existing foundations in short, carefully sequenced bays so the house is never left unsupported, then lowering the floor and forming the new box within. The sequence itself is a structural design, not just a method.
Underpinning and lowering a floor changes the load path of the whole house, so we check the existing foundations, the temporary condition at every stage of the dig, and the heave and movement the excavation can cause to the property and its neighbours. Where the site is tight or the ground is poor, embedded walls or piled solutions replace open underpinning. We advise which route suits the property, whether it is a townhouse basement in the UK or a project elsewhere in Europe, since calculations and reports are delivered wherever you are.
Waterproofing and detailing
A structurally sound basement that leaks has failed the client, so waterproofing is designed alongside the structure to BS 8102. We select the grade of protection the finished use needs and combine the methods that suit the ground: an integral watertight concrete box designed for crack control, an external tanking membrane, and an internal drained cavity system that collects any water and pumps it away. Most basements use more than one, so that a single defect does not let water in.
The detailing is where basements are won or lost. We detail the construction joints and water bars, the crack widths the reinforcement must control, the junctions at the wall to slab and wall to floor, and the service penetrations, so the box stays watertight at every interface. The design is coordinated with the drainage, sump and pump arrangement and prepared to Eurocode 2 and 7, BS 8102 and Approved Document A, ready for Building Control and the contractor.
What we check.
The points our calculations resolve for a project like this.
- At-rest earth pressure over the full retained height plus surcharge
- Groundwater hydrostatic pressure and uplift on the box
- Wall spanning between propping floor and basement slab
- Sequenced underpinning where a basement is formed under a house
- Heave and movement affecting the property and neighbours
- BS 8102 waterproofing combining watertight concrete, tanking and drained cavity
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
How is a basement wall different from a normal retaining wall?
A basement wall is propped at the top by the ground floor and at the bottom by the basement slab, so it spans between them rather than cantilevering from a footing. That makes it efficient, but it also carries full groundwater pressure and forms part of a watertight box, so it is designed for both structure and waterproofing together.
Can I build a basement under my existing house?
Often yes, but the existing foundations have to be underpinned in short, carefully sequenced bays so the house is never unsupported, then the floor is lowered and the new box formed. The sequence is a structural design in its own right, and we check the existing structure, every temporary stage and the heave the dig can cause.
How do you stop a basement from leaking?
We design waterproofing to BS 8102, usually combining methods so no single defect lets water in: a watertight concrete box detailed for crack control, an external membrane and an internal drained cavity with a sump and pump. The grade of protection is matched to how the basement will be used.
Does a basement need to resist floating?
Yes. Below the water table the groundwater tries to lift the whole basement like a boat, so we check uplift against the weight of the structure and the ground bearing on it. If the box is too light we increase the slab thickness or add other measures so it cannot float.
Related in structural design & elements.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project