Diaphragm Wall: Construction, Design and Singapore Practice

A diaphragm wall is a reinforced concrete wall cast in a deep trench in the ground before any excavation takes place. Once complete, the soil on one side can be dug out to form a basement, station box or underpass, with the wall holding back the ground and groundwater on the other side. It is the default retaining system for the deepest and most sensitive urban excavations, and it has shaped much of Singapore’s underground infrastructure.

This article explains how a diaphragm wall is constructed, how it is supported and designed, where it is preferred over alternatives such as piled or sheet walls, and what Singapore’s regulations require of deep excavations. It is written from the viewpoint of an engineer who has to think about both the wall and the buildings next to it.

What a diaphragm wall is

The wall is built in a series of panels, each a rectangular slot in the ground typically several metres long, excavated to the full design depth and then filled with reinforced concrete. Adjacent panels are joined to form a continuous wall. Contractor literature commonly quotes thicknesses from about 0.6 m to 1.5 m, with depths that can exceed 60 m on large projects, though the actual dimensions are set by design.

Because the wall is continuous, stiff and relatively watertight, it serves as both the temporary earth retaining structure during excavation and, very often, the permanent basement wall. That dual role is a large part of its economy on deep projects.

How a diaphragm wall is constructed

Guide walls

Construction starts with two shallow, parallel reinforced concrete guide walls along the line of the diaphragm wall. They set out the position and thickness precisely, guide the excavating tool, support the top of the trench and provide a working platform for handling reinforcement cages.

Excavation under support fluid

Each panel is excavated with a hydraulic or cable-operated grab, or for harder ground and greater depths, a cutter (often called a hydrofraise or trench cutter) that grinds the soil and pumps it out. As the trench is dug, it is kept full of a support fluid, traditionally bentonite slurry and increasingly polymer fluids. The fluid’s pressure against the trench sides, together with the thin filter cake it forms on the soil face, keeps the unsupported trench from collapsing. The fluid level is kept above the groundwater level throughout.

Panel sequence and joints

Panels are usually built in a primary and secondary sequence: alternate primary panels first, then the secondary panels between them once the primaries have hardened. The joints between panels are formed with stop-end elements, or by cutting slightly into the hardened concrete of the adjacent panel with a trench cutter. Joints are the weak points for watertightness, and waterstops are often incorporated.

Reinforcement and concreting

Once the panel reaches depth and the fluid has been cleaned of excess soil, a prefabricated reinforcement cage is lowered in, sometimes in spliced sections for deep panels. Concrete is placed from the bottom up through tremie pipes, displacing the lighter support fluid, which is pumped away for treatment and reuse. The top layer of contaminated concrete is later trimmed off, and a capping beam ties the panels together.

Supporting the wall during excavation

A diaphragm wall on its own can act as a cantilever for shallow excavations, but deep excavations need support as the soil is removed in front of it. The main options are:

Temporary struts and walers: steel struts spanning across the excavation between walers fixed to the wall faces, installed level by level as digging proceeds.
Ground anchors: tensioned tendons drilled through the wall into the ground behind, leaving the excavation clear. Their use under neighbouring land requires permission and is often impractical in dense cities.
Top-down construction: the permanent basement slabs are cast progressively downward, each slab propping the wall as excavation continues beneath it.

The support system and the wall are designed together. Strut levels, spacing and stiffness control how much the wall bends and how much the ground behind it moves. Connections between struts and walers deserve as much attention as the wall itself. Our overview of deep excavation shoring covers the wider family of support systems.

Designing a diaphragm wall

Design begins with ground investigation: soil layers, strengths and stiffnesses, groundwater levels and any variation across the site. The engineer then models each construction stage, since the critical case is often a temporary one, such as excavation to a level just before the next strut is installed. For each stage the design checks bending and shear in the wall, forces in the supports, stability of the base against heave or hydraulic failure, and the depth of toe embedment needed for stability and groundwater cut-off.

Equally important is predicting movement. As the excavation deepens, the wall deflects inwards and the ground behind it settles. Neighbouring buildings, utilities and tunnels respond to that settlement. The engineer estimates wall deflection and ground movement at each stage and compares them with the limits that adjacent structures can tolerate. The geotechnical design of retaining structures in Singapore is carried out to SS EN 1997-1 (Eurocode 7) and the associated National Annex, as listed in the Approved Document. For the principles common to all retaining structures, see our article on retaining walls.

Singapore requirements for deep excavations

The Building and Construction Authority’s Approved Document sets specific requirements for earth retaining or stabilising structures (ERSS). Among them, the structural elements of the ERSS must be designed with a safety factor not less than that of the permanent works in the same project, and the maximum wall deflection is limited according to how close existing structures are:

0.5 per cent of the excavation depth H where existing structures lie within a distance H of the excavation edge (Zone 1);
0.7 per cent of H where they lie between H and 2H (Zone 2);
beyond 2H (Zone 3), 0.7 per cent of H in stiffer ground (Ground Type A) and 1.0 per cent of H in soft clays, silts, organic soils and loose fills such as the Kallang Formation (Ground Type B).

The Qualified Person must also check that ground movements will not damage neighbouring buildings, adopt the more stringent of the two criteria, and specify the allowable deflection as a work suspension level for inclinometer monitoring, with an alert level at 70 per cent of it. If the work suspension level is reached, work must stop and measures be taken to remove the danger. These figures are from Approved Document version 7.08 (October 2025); the current edition should always be checked.

The emphasis on monitoring has a history. The 2004 Nicoll Highway collapse, in a strutted diaphragm-walled excavation for the Circle Line, was found by the Committee of Inquiry to have been initiated by failure of the strut-to-waler connections at the lowest level, with under-design of the wall and warning signs in the instrumentation also identified. It reshaped how deep excavations are designed, checked and monitored in Singapore.

Advantages and limitations

The diaphragm wall’s strengths are stiffness, depth and watertightness. It limits ground movement better than most alternatives, can be taken very deep, cuts off groundwater effectively in permeable ground and can form the permanent structure. Construction is relatively quiet and vibration-free compared with driven piles, which matters next to occupied buildings.

Its limitations are cost, plant and space. It needs heavy rigs, a slurry plant and room for cage handling, which can be difficult on tight sites. The panel joints need care to remain watertight. For shallower basements or where some movement is acceptable, a contiguous pile wall, secant pile wall or sheet piling may be more economical. The choice depends on depth, ground and groundwater, neighbouring structures, programme and whether the wall will form part of the permanent works.

Frequently asked questions

What is a diaphragm wall used for?

A diaphragm wall is used to retain soil and groundwater around deep excavations such as basements, underground stations, shafts and underpasses, and it often becomes the permanent external wall of the structure once excavation is complete.

Why is bentonite used in diaphragm wall construction?

Bentonite slurry fills the trench during excavation and exerts pressure on the trench faces while forming a thin filter cake on the soil, which together keep the deep, narrow trench stable until it is filled with concrete.

How deep can a diaphragm wall go?

Depth depends on the ground, the equipment and the design, and specialist contractors report walls exceeding 60 metres on large projects, although most building basements need considerably less.

What is the difference between a diaphragm wall and a contiguous pile wall?

A diaphragm wall is a continuous concrete wall built in panels and is stiff and relatively watertight, while a contiguous pile wall is a line of separate bored piles with small gaps between them that is cheaper but lets groundwater through unless additional measures are taken.

What deflection is allowed for a retaining wall in Singapore?

BCA’s Approved Document limits wall deflection to between 0.5 and 1.0 per cent of the excavation depth, depending on how close existing structures are and the type of ground, with a stricter limit applying if needed to protect neighbouring buildings.

Planning a basement or concerned about an excavation next to your property? Talk to our consultancy or get in touch.