Diaphragm walling refers to the in-situ construction of vertical walls by means of deep trench excavations. Stability of the excavation is maintained by the use of a drilling fluid, usually a bentonite suspension. The walls are constructed in discrete panel lengths ranging typically between 2.5m and 7.0m using purpose built grabs or, in appropriate circumstances, milling machines (hydromills). Excavation is typically carried out using either rope-suspended mechanical or hydraulically operated grabs.

Standard grabs range in weight from 8-20 tonnes. The grabs are mounted on 80- 120 tonne hydraulic base crane units providing stability and suitable line pull. Specific applications and ground conditions demand the use of hydromills – hydraulically operated reverse circulation trench cutters where the excavation technique is by ‘cutting’ as opposed to ‘digging’.

This technique is appropriate for deeper diaphragm walls and walls located in granular materials and soft rock. Where panels are constructed in a line, abutting one another to form a retaining wall, the term diaphragm walling applies.