Retaining walls - Piling Engineering - page 235

6.4.1 Tension cracks
On many sites, there is overburden of fill or granular material on top of clay on the
active side of a retaining wall, or alternatively the area behind the wall has been built
on or paved over. These features should prevent the formation of shrinkage or tension
cracks in the clay behind the wall in periods of dry weather. However, in some instances
the surface of clay is exposed, and where a value of cohesion c is used in the design it
is implied that a negative active earth pressure can exist in the vicinity of the wall head.
This feature in these circumstances could lead to surface cracking in the clay and the
cracks could fill with water, which would in turn cause local pressure corresponding
to the head of water retained in the crack. The possibility of such cracks would seem
mainly to be confined to cantilever walls rather than to propped or anchored walls
where the position of the anchoring force is usually located at some distance below
the wall head. In this case the deflected form of the wall is such as to produce a closure
of any cracks so formed.
For reasons associated with the requirements of the old British Code of Practice for
Retaining Walls (CP2), it has been common to stipulate in designs a minimum equiv-
alent fluid pressure on the active side of retaining walls of 4.7 kPa/m depth, measured
below the wall head. While this feature is not of much other effect in wall design
when using effective stress parameters, it does serve the useful purpose of providing a
reasonable positive-pressure condition where tension cracks might otherwise occur in
the immediate vicinity of the wall head.
A new British Code of Practice is now available (BS 8002) which does not specify
a minimum equivalent pressure but on the other hand specifies over excavation and
minimum surcharge on the retained soil. The effects may therefore be better directed
to lateral increase of active soil pressure though in general they do not differ very
materially.
6.5 Earth pressures due to ground surcharges
6.5.1 Uniform surcharges
A uniform surcharge acting on the active side of a wall produces a uniform vertical
stress throughout the retained soil, and this is assumed to give rise to a lateral earth
pressure equal to the surcharge multiplied by the active earth pressure coefficient or
coefficients on the whole of the active face of the wall.
Occasions arise in which, for example, a road extends across the width of the active
soil wedge, and in this case it is normally assumed that standard road loadings apply
and these are taken as a uniform surcharge. In clay soils where the design parameters
are chosen on a long-term basis, this approach may be rather too conservative and
some modified lesser surcharge may on occasions be justified.
6.5.2 Point loads and line loads
The presence of a point, line or strip load resting on or in the soil being retained
by a wall gives rise to additional lateral pressures on the wall. The usual meth-
ods for assessing the lateral stresses due to these conditions are those of Boussinesq