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H
H
Q
-
?
?
Q
H/3
H/3
H
Q
H
-
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H/3
H/3
Q
(a) passive states (b) active states
Figure 11.5 Curved slip lines for a rough wall
?
: the soil rises, -
?
: the soil settles
Theoretical and experimental studies support the conclusion that values of active
earth pressures, the lowest horizontal thrust, using Coulomb's theory, are on the
safe side and only slightly deviate from results obtained by more refined methods,
but values of passive earth pressure, the highest horizontal thrust, can be much
higher due to wall friction. In practical guidelines one may find tables with earth
pressure coefficients
K
i
, which incorporate cohesion, wall friction, soil weight and
surface loads, and inclined surfaces.
B
EMBEDDED WALLS
Embedded walls are usually installed in undisturbed soils and afterwards
excavation at one side takes place. Such walls may be composed of hammered or
vibrated driven sheet-piles, grouted or installed contiguous piles or slurry or
diaphragm walls, acting as cantilever or supported by anchors or props (see Fig
11.1). In the stability analysis not their weight but their flexibility is essential, as
embedded walls obtain support at the toe from passive resistance mobilised in the
soil. Failure modes that need to be considered are rotation of the wall, slip of the
anchor, buckling of the strut, excessive bending of the wall, excessive settlement
behind the wall, bottom heave and overall slip failure (Fig 11.6). One may
distinguish temporary and permanent structures. The latter should be designed
more rigorously. Particular attention should be paid to groundwater pressures and
leakage.
A freestanding (cantilever) sheet-pile wall obtains support by the depth of its
embedment
D
. The assumption that the wall will rotate around a point
B
at depth
d
and that active and passive earth pressures will develop, as shown in Fig 11.7a,
allows to define a stability factor
F
, according to
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