Lateral Earth Pressure - Elements of Soil Mechanics - page 211

Fig. 7.25 Use of granular material in retaining wall construction.
rise to swelling and shrinkage problems that are not encountered in granular material. Peat, organic soil,
chalk, unburnt colliery shale, pulverised fuel ash and other unsuitable material should not be used as
backfill if at all possible.
7.10.1 Drainage systems
No matter what material is used as a backfill, its drainage is of great importance. A retaining wall is
designed generally to withstand only lateral pressures exerted by the soil that it is supporting. In any
design the possibility of a groundwater level occurring in the material behind a retaining wall must be
examined and an appropriate drainage system decided upon.
For a granular backfill the only drainage often necessary is the provision of weep holes that go through
the wall and are spaced at some 3 m centres, both horizontally and vertically. The holes can vary in diam-
eter from 75 to about 150 mm and are protected against clogging by the provision of gravel pockets
placed in the backfill immediately behind each weep hole (Fig. 7.26a ).
Generally weep holes can only be provided in outside walls and an alternative arrangement for granular
backfill is illustrated in Fig. 7.26b . It consists of a continuous longitudinal back drain, placed at the foot
of the wall and consisting of open jointed pipes packed around with gravel or some other suitable filter
material. The design of filters is discussed in Chapter 2. Provision for rodding out should be provided.
If the backfill material is granular but has more than 5% fine sand, silt or clay particles mixed within it
then it is only semi-pervious. For such a material the provision of weep holes on their own will provide
inefficient drainage, with the further complication of there being a much greater tendency for clogging
to occur. The answer is to provide additional drainage, in the form of vertical strips of filter material (about
0.33 × 0.33 m 2 in cross-section) placed midway between the weep holes and led down to a continuous
longitudinal strip of the same filter material of the same cross-section as shown in Fig. 7.26c .
For clayey materials blanket drains of suitable filter material are necessary. These blankets should be
about 0.33 m thick and typical arrangements are shown in Figs 7.26d and 7.26 e. Generally the vertical
drainage blanket of Fig. 7.26d will prove satisfactory, especially if the surface of the retained soil can be
protected with some form of impervious covering. If this protection cannot be given then there is the
chance of high seepage pressure being created during heavy rain (see Example 7.11) . In such a situation
the alternative arrangement of the inclined filter blanket of Fig. 7.26e can substantially reduce such
seepage pressures.
The reason for the different effects of the two drainage systems can be seen when we consider the
respective seepage flow nets that are generated during flooded conditions.
The flow net for the vertical drain is shown in Fig. 7.27a . It must be appreciated that the drain is neither
an equipotential nor a flow line. It is a drained surface and therefore the only head of water that can exist
along it is that due to elevation. Hence, if a square flow net has been drawn, the vertical distances between