Civil Engineering Reference
In-Depth Information
Figure 6.10
Impermeable slab subject to uplift water pressure.
is reduced and because of downward seepage the active soil pressure is increased. The
question that arises therefore is one of how much difference these assumptions make
in any particular design case.
It has also to be remembered, in the case of deep basements for example, that where
an impermeable slab is cast in the bottom, this slab may be subject to uplift pressures
in the long term unless provision is made for drainage. Thus seepage may cease and
full water pressure on both sides of the wall may return to its original active side level
as indicated in Figure 6.10.
Examination of cases involving cantilever walls in clay soils reveals that, where the
difference in water level is say 3 or 4 m between active and passive sides, the effective
difference in the finally determined wall dimensions and bending moments between
methods (1) and (2) is negligible. Where the difference is say 6m, the case (2) analysis
leads to slightly deeper walls and slightly greater bending moments.
The difference in approach is probably of most concern in relation to walls that
are propped or tied back by ground anchors, which are generally of greater retained
height than normal cantilever walls, and which are often subject to greater water dif-
ferential pressures than cantilever walls. In this case toe penetration below the dredge
or excavation level may be relatively small depending on the selected position of the
tie or anchor, and although the uplift seepage effect on the passive side may be small,
it is prudent to recognize the sensitivity of penetration to this effect and to ensure that
toe penetration is adequate to deal with any uplift seepage pressure that may exist.
In general terms therefore method (1) above is widely used and for most walls proves
to be a satisfactory way of dealing with water pressures, though the assumptions made
are far from rigorous.
In the case of contiguous bored pile walls, where the wall does not provide an
impermeable barrier, and where high water pressures cannot normally exist on the
active side of the wall, it is usual only to take into account a small nominal differ-
ence of water head between active and passive sides in the temporary state. It should
be recognized that in such cases, if the excavation is subsequently lined and water
rises to a higher level in the permanent state, then bending in the wall may be more
severe. In all cases both the temporary and permanent states of the wall need to be
considered.
