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completion of concreting, digging within the now concrete wall-enclosed area can
proceed. To prevent the concrete wall from collapsing into the newly open area,
temporary supports such as tiebacks or thrusts are installed. Special care is required
for the interconnections between successive panels and local soil intrusions. They
can be traced by using monitoring techniques. Recent tests on 20-metre deep walls
have shown that acoustics during construction is relatively cheap and reliable. A
transmitter tube (50 kHz) and several receiver tubes near joints detect density
changes which indicate improper inclusions in the concrete.
Slurry walls or cut-off trenches filled with cement-bentonite, soil-bentonite or
plastic concrete may form a continuous barrier, and besides building pit walls they
can also be used to limit the migration of liquid or gaseous contaminants, to allow a
localised lowering of the water table or to stabilise existing and future slopes.
D
REINFORCED WALLS
Earth slopes can be stabilised by the application of strips, mesh reinforcement
(e.g. gabions), or soil nailing, and sometimes facing-plates are included (Fig
11.9a). It is used in retaining walls, quays, abutments, earthen dams, dikes and in
temporary works. The construction is usually relatively easy and fast. The soil
involved must be of suitable quality, i.e. moderate stiffness, low creep and allowing
a proper connection with the reinforcement. The reinforcement should possess
adequate strength, moderate (geotextile) to high stiffness (nails), low creep, proper
durability and a good bond with soil. The facing plates or skin provide
containment, prevent erosion, and may contribute to anchoring of the
reinforcement. Grouting at anchors or facing may be added.
q
A
C
D
facing
tie
active
v
P
2
resisting
H
H
H
W
P
1
T
H/2
H/3
E
B
R
(a) external mechanical system (b) internal mechanical system (c) failure zone
Figure 11.9 Reinforced earth-retaining structures
Stability is improved by enhancing shear strength of a failure zone through
compressive and tensile strains in the reinforcement. In some cases bending
stiffness of the reinforcement contributes as well. In the design analysis the
required forces in the reinforcement have to match allowable forces by using
proper spacing and both internal and external stability must be checked. It should
be noted that the development of the full shear resistance of the ties may require
relatively large deformations. These residual deformations and subsequent creep
should be acceptable.
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