Sheet pile and embedded retaining walls
Embedded walls, sheet piles, contiguous and secant bored piles, retain ground by being driven or bored deep below the excavation rather than resting on a wide footing. We design them cantilevered or propped, for clients across the UK.
What is an embedded retaining wall?
It is a wall that retains ground by being driven or bored deep below the excavation rather than sitting on a wide footing. Steel sheet piles, contiguous bored piles and secant piles are all embedded walls. The buried length in front of the wall provides the passive resistance that holds it, which is why depth of embedment is the key design output.
How embedded walls retain ground
An embedded wall has no wide base. It is a line of steel sheet piles, or a row of bored concrete piles, driven or drilled deep enough that the soil in front of the toe pushes back hard enough to hold the wall. The retained soil pushes actively on the back over the full height, and the soil in front resists passively over the buried length, so the design is a balance of these pressures about the toe. That makes embedded walls ideal where there is no room for a spread footing, against a boundary, in a basement, along a watercourse or in a cofferdam.
The key output is the depth of embedment. For a cantilever wall the buried length has to be enough that passive resistance in front, plus the fixity it gives, balances the active push behind with a proper margin to Eurocode 7. Get it too short and the wall rotates about a point near its toe and fails; the design finds the depth that holds it, then adds an allowance for over-excavation and any softening of the soil in front.
Cantilever and propped walls
A cantilever embedded wall is held only by the ground and suits moderate retained heights, but the deflection and bending grow quickly with height, so beyond a few metres it becomes uneconomic. Adding a prop or an anchor near the top changes everything: it holds the head of the wall, cuts the bending moment and deflection, and lets the same section retain much more ground. In basements the permanent floor slabs prop the wall, while in temporary works steel walings and struts or ground anchors do the job during excavation.
The propping sequence has to be designed alongside the wall, because the loads change at every stage of the dig. We check the wall for each stage, the initial cantilever before the prop goes in, the propped condition as the excavation deepens, and the permanent case, so the section and the prop forces are sized for the worst each experiences. Where anchors are used we design their inclination and length to reach beyond the failure wedge into competent ground.
Wall types, water and movement
The wall type follows the ground and what it has to do. Steel sheet piles interlock into a continuous watertight wall and are fast to drive, ideal for cofferdams and waterfront work. Contiguous bored piles, spaced slightly apart, suit firmer ground where a little seepage between piles is acceptable. Secant piles, where interlocking piles overlap, give a stiff and effectively watertight wall for basements and deep excavations near buildings. We choose on the retained height, the water table, the ground and the vibration and noise the site can accept.
Water and ground movement often govern more than strength. We check seepage and the risk of piping or base heave where the wall retains water or is dug below the water table, and we assess the ground movement the wall and excavation will cause so nearby buildings and services are not damaged. The design is prepared to Eurocode 7 and the relevant execution standards and delivered to clients across the UK, with any temporary works staged to suit the site.
What we check.
The points our calculations resolve for a project like this.
- Active and passive pressures balanced about the toe
- Embedment depth with allowance for over-dig and soil softening
- Cantilever versus propped or anchored arrangement
- Propping sequence and stage-by-stage checks during excavation
- Wall type chosen for water tightness, ground and vibration
- Seepage, base heave and ground movement affecting neighbours
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
What is an embedded retaining wall?
It is a wall that retains ground by being driven or bored deep below the excavation rather than sitting on a wide footing. Steel sheet piles, contiguous bored piles and secant piles are all embedded walls. The buried length in front of the wall provides the passive resistance that holds it, which is why depth of embedment is the key design output.
When is a wall propped rather than cantilevered?
A cantilever wall is held only by the ground and works for moderate heights, but bending and deflection grow fast with height. Beyond a few metres a prop or anchor near the top is added, which greatly cuts the bending and lets a lighter section retain much more ground. Basement floor slabs often act as the permanent props.
What is the difference between contiguous and secant pile walls?
Contiguous piles are spaced slightly apart and suit firm ground where minor seepage between them is acceptable. Secant piles interlock and overlap to form a stiff, effectively watertight wall, which suits basements and deep digs near buildings or below the water table. We choose between them on ground and water conditions.
Will an embedded wall affect neighbouring buildings?
Any deep excavation moves the ground around it, so we assess the movement the wall and dig will cause and design the wall stiffness and propping to keep it within limits that protect nearby buildings and services. Watertight walls and careful propping sequences are often used specifically to control that movement.
Related in structural design & elements.
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