Civil Engineering Reference
In-Depth Information
Possible failure zone
φ
(a)
(b)
(c)
Fig. 8.16
Anchorage systems for sheet pile walls.
The anchor or prop force required can be obtained by equating horizontal forces: T
=
P
a
−
P
p
, from
which a value is obtained per metre run of wall. The resulting value of T is increased by 25% to allow for
flexibility in the piling and arching in the soil. Anchors are usually spaced at 2-3 m intervals and secured
to stiffening wales.
Anchorage can be obtained by the use of additional piling or by anchor blocks (large concrete blocks
in which the tie is embedded). Any anchorage block must be outside the possible failure plane (Fig.
8.16a
),
and when space is limited piling becomes necessary (Fig.
8.16b
). If bending is to be avoided in the anchor-
age pile, then a pair of raking piles can be used (Fig.
8.16c
).
8.6.4 Depth of embedment for anchored walls
As the depth of embedment is not as great as for the cantilever wall, the toe of the wall is not rigidly
fixed into the ground and is free to move slightly. The analysis of this condition is thus referred to as the
free earth support method
. With this method it is assumed that rotation occurs about the anchor point
and that sufficient yielding occurs for the development of active and passive pressures. The pressure
distribution assumed in design is shown in Fig.
8.15b
, and the wall is considered free to move at its base.
By taking moments about the anchor at A an expression for the embedment depth, d, can be obtained.
The traditional methods of assessing the ratio of restoring moments to overturning moments described
for cantilever walls are also used for anchored walls. The design of anchored walls to Eurocode 7 involves
the use of the GEO limit state to assess the rotational stability, as illustrated in Example
8.4.
