Diaphragm walls and retaining structures: the key steps of design

Geotechnical data, construction sequence, calculation methods and checks to Eurocode 7 and NF P 94-282: the steps in designing an embedded retaining wall.

Embedded retaining walls make deep excavations possible on constrained sites: metro stations, underground car parks, hydraulic structures, basements in dense urban areas. Among them, the diaphragm wall is often chosen when the wall must retain the ground, limit water inflow and form part of the permanent structure.

The main types of embedded walls

• Diaphragm walls: reinforced concrete panels cast in place in a trench supported by slurry (bentonite or polymer).
• Sheet pile walls: steel sections driven or vibrated into the ground, often temporary.
• King post (Berlin) walls: vertical steel posts with lagging between them, for ground above the water table.
• Secant or contiguous pile walls: an alternative to diaphragm walls depending on the ground and available equipment.

1. Gather the input data

Design starts with knowledge of the site. In France, geotechnical assignments are defined by standard NF P 94-500 (stages G1 to G5). For a retaining wall, the key inputs are:

• the geotechnical profile and the parameters of each layer (unit weights, cohesion, friction angle, moduli);
• groundwater levels, their variations and the permeability of the ground;
• surcharges: neighbouring buildings, roads, site plant;
• how sensitive neighbouring structures are to movement.

2. Define the construction sequence

A wall is analysed stage by stage, because forces change as excavation proceeds. The sequence describes the successive excavation levels, the installation of supports (struts, ground anchors, slabs), any dewatering and, finally, the permanent configuration. Temporary stages are often the governing ones.

3. Choose the calculation method

Subgrade reaction method

This is the most common method for embedded walls: the ground is represented by elasto-plastic springs, bounded by active and passive pressures. It gives the forces in the wall and the supports, and the wall displacements, with short computation times.

Finite element method

A finite element model (for example in Plaxis 2D) represents the soil mass as a continuum. It is particularly useful to assess ground movements behind the wall and their effect on neighbouring structures, or for complex geometries and sequences.

4. Carry out the checks

In France, embedded retaining walls are designed to Eurocode 7 (EN 1997-1) and its national application standard NF P 94-282. The checks cover in particular:

• mobilisation of passive resistance at the toe (sufficient embedment);
• stability of the excavation base: hydraulic heave, piping, heave of clay soils;
• overall stability of the soil mass;
• structural resistance of the wall (bending moments, shear forces) and reinforcement of the cages;
• resistance of the supports: struts, anchors, walings;
• displacements, compared with the acceptable limits for neighbouring structures.

5. Think about construction

The design must remain compatible with construction: panel layout, joints between panels, length and lifting of reinforcement cages, verticality tolerances, block-outs for connections with slabs. These points are dealt with during detailed design to avoid last-minute changes.

In short

A well-designed retaining wall is the result of good knowledge of the ground, a realistic construction sequence and comprehensive checks. ROKENG carries out design studies for foundations, diaphragm walls, retaining structures and buried structures, including soil–structure interaction studies.