Civil Engineering Reference
In-Depth Information
8.3 Embedded walls
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.
8.3.1 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.
8.12)
.
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.
8.15
).
8.3.2 Diaphragm walls
A diaphragm wall could be classed either as 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 prevents collapse. When excavation is complete the required steel reinforce-
ment 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.
8.4.
bentonite
slurry
(a)
(b)
(c)
(d)
(a) Trench dug
(b) Reinforcement cage inserted
(c) Bentonite displaced by concrete
(d) Soil excavated in front of wall
Fig. 8.4
The construction stages of a diaphragm wall.
