Diaphragm walls

In densely built-up inner city areas the use of diaphragm walls as sheeting for deep construction pits in groundwater-bearing strata is frequently advantageous. The construction of a diaphragm wall has no adverse effects on the groundwater and causes only minimal noise and vibration. Prior to excavation of the trench, support walls, usually of in-situ concrete, are made in the upper part of the trench: they stabilize the ground in that area and at the same time serve as a locating and guiding walls for the grab.

The soil is excavated in primary and secondary sections using special grabs, while bentonite slurry supports the trench walls. First the primary sections are excavated with spacing between the slots. Stop-end tubes are then placed at both ends and the slot is reinforced and concreted. The stop-end tubes are removed as soon as the concrete begins to set and harden. The half-round vertical joints which are created this way serve as guides for the grab during excavation of the secondary sections between the primary ones and leave a close watertight contact face for the concrete of the adjacent section. The bentonite slurry is reclaimed during concreting and prepared for re-use.

If required, the prefabricated reinforcement cages can be provided with circumferential reinforcement for anchors or bracing, or with recesses and starter bars for base and floor slabs.

For the construction of precast walls, precast reinforced concrete wall elements are immersed into a hardening supporting liquid with a cement constituent. The faces are completely smooth when exposed.

Diaphragm walls can also be constructed for sealing purposes, to keep excavations dry, to render dams impermeable or to protect the groundwater near industrial plants and refuse deposits from pollution by industrial waste. In this case the stabilizing slurry, by the addition of cement and filler, becomes the sealing compound. After hardening, the compound remains sufficiently plastic to enable the wall to accept minor movements of the soil without cracking.

Both for economic reasons and in order to reduce the risk of defects in the wall (which increases with increasing depth) sealing walls are generally constructed to depths ranging from 30 to 40 m, with thicknesses varying from 0,4 m to 1,2 m.

Thin diaphragm cut-off walls

Thin diaphragm cut-off walls (also with incorporated synthetic sealing membranes) and bored pile walls serve as enclosures for construction pits and waste dumps in order to protect the ground water from pollution. In addition, they are used to render earth dams impermeable and to seal impounded areas of hydroelectric power plants.

With this method a steal beam is vibrated to the required depth using a tandem vibratory hammer. The hollow space which is created after withdrawal of the beam is grouted with a sealing compound consisting of bentonite, cement, filler (e.g. stone dust) and water. Depending on the thickness of the beam tip the wall has a thickness of approx. 10 cm which, in highly permeable soils, due to the penetration of the sealing compound into the surrounding soil, is increased further. In this way, section by section, a structurally sound wall is constructed, with defects being avoided by overlapping the individual sections.

With this method a great number of square meters can be constructed per shift Thin diaphragm cut-off walls may also be constructed as double walls connected by transverse bulkheads at certain intervals to form the so-called cut-off chambers which, combined with hydraulic measures, can be used to good effect for waste dump enclosures.