Embedded wall

Embedded walls rely on the passive resistance of the soil in front of the lower part of the wall to provide stability. Anchors or props, where incorporated, provide additional support.

7.3.1 Sheet pile walls

These walls are made up from a series of interlocking piles individually driven into the foundation soil. Most modern sheet pile walls are made of steel but earlier walls were also made from timber or precast concrete sections and may still be encountered. There are two main types of sheet pile walls: cantilever and anchored.

Cantilever wall

This wall is held in the ground by the active and passive pressures that act on its lower part (Fig. 7.8).

Anchored wall

This wall is fixed at its base as is the cantilever wall but it is also supported by a row, or two rows, of ties or struts placed near its top (Fig. 7.11).

7.3.2 Diaphragm walls

A diaphragm wall could be classed as either a reinforced concrete wall or as a sheet pile wall but it really merits its own classification. It consists of a vertical reinforced concrete slab fixed in position in the same manner as a sheet pile in that the lower section is held in place by the active and passive soil pressures that act upon it.

A diaphragm wall is constructed by a machine digging a trench in panels of limited length, filled with bentonite slurry as the digging proceeds to the required depth. This slurry has thixotropic properties, i.e. it forms into a gel when left undisturbed but becomes a liquid when disturbed. There is no penetration of the slurry into clays and in sands and silts water from the bentonite slurry initially penetrates into the soil and creates a virtually impervious skin of bentonite particles, only a few millimetres thick, on the sides of the trench. The reason for the slurry is that it creates lateral pressures which act on the sides of the short trench panel and thus prevent collapse. When excavation is complete the required steel reinforcement is lowered into position. The trench is then filled with concrete by means of a tremie pipe, the displaced slurry being collected for cleaning and further use.

The wall is constructed in alternating short panel lengths. When the concrete has developed sufficient strength, the remaining intermediate panels are excavated and constructed to complete the wall. The length of each panel is limited to the amount that the soil will arch, in a horizontal direction, to support the ground until the concrete has been placed.

The various construction stages are shown in a simplified form in Fig. 7.4.

Bentonite slurry

(a) Trench dug

(b) Cage of fabric reinforcement inserted

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