Civil Engineering Reference
In-Depth Information
7.8.3 Compaction effects
During the construction of gravity retaining walls, layers of fill are compacted behind the wall, and this
compaction process induces lateral stresses within the fill which can act against the back of the wall. If
the stresses are high enough they can lead to movement or deformation of the wall, and so the effect of
the compaction is taken into account during the design of the wall. Guidance on the effects of the com-
7.9 Choice of method for determination of active pressure
The main criticism of the Rankine theory is that it assumes conditions that are unrealistic in soils. There
will invariably be friction and/or adhesion developed between the soil and the wall as it will have some
degree of roughness and will never be perfectly smooth. Hence, in many cases, the Rankine assumption
that no shear forces develop on the back of the wall is simply not true and it may be appropriate to use
the Coulomb theory.
As noted earlier it is not easy to obtain measured values of the value of wall friction,
δ
, and the value
of the wall adhesion, c
w
, which are usually estimated.
δ
is obviously a function of the angle of shearing
resistance,
φ
′
, of the retained soil immediately adjacent to the wall and can have any value from virtually
zero up to some maximum value, which cannot be greater than
φ
′
. Similarly the operative value of c
w
is
related to the value of cohesion of the soil immediately adjacent to the wall.
Just what will be the actual operating value of
δ
depends upon the amount of relative movement
between the soil and the wall. A significant downward movement of the soil relative to the wall will result
in the development of the maximum
δ
value.
Cases of significant relative downward movement of the soil are not necessarily all that common. Often
there are cases in which there is some accompanying downward movement of the wall resulting in
the smaller relative displacement. Examples of such cases can be gravity and sheet piled walls and a value
of
δ
less than the maximum should obviously be used (descriptions of different wall types are given in
Chapter
8.
).
When the retained soil is supported on a foundation slab, as with a reinforced concrete cantilever or
counterfort wall, there will be virtually no movement of the soil relative to the back of the wall. In this
case the adoption of a 'virtual plane' in the design procedure, as illustrated in Example
8.2,
justifies the
use of the Rankine approach.
The use of the Rankine method affords a quick means for determining a conservative value of active
pressure, which can be useful in preliminary design work. Full explanations of the procedures used in
retaining wall design are given in Chapter
8.
7.10 Backfill material
The examples used to illustrate lateral earth pressure in this chapter have all been based on gravity walls,
i.e. walls which are constructed using a 'bottom-up' process and backfilled with soil after construction.
The ideal backfill material for such walls is granular, such as suitably graded stone, gravel, or clean sand
with a small percentage of fines. Such a soil is free draining and of good durability and strength but,
unfortunately, it can be expensive, even when obtained locally.
Economies can sometimes be achieved by using granular material in retaining wall construction in the
form of a wedge as shown in Fig.
7.25.
The wedge separates the finer material making up the bulk of the
backfill from the back of the wall. With such a wedge, lateral pressures exerted on to the back of the wall
can be evaluated with the assumption that the backfill is made up entirely of the granular material.
Slag, clinker, burnt colliery shale and other manufactured materials that approximate to a granular soil
will generally prove satisfactory as backfill material provided that they do not contain harmful chemicals.
Inorganic silts and clays can be used as backfills but require special drainage arrangements and can give
