Civil Engineering Reference
In-Depth Information
Figure 7.7
Blocks of clay fallen from underream of pile. (Photographs courtesy of CIRIA, London.)
7.2.1.2 Base of boreholes
To develop the available end-bearing capacity it is necessary to ensure the base of the
bore is clean and undisturbed. In clay soil the boreholes can be cleaned satisfactorily
with special tools, but, in stiff fissured clays with sand partings and perhaps with
water seepage, blocks of material may fall into the borehole. Underreamed piles are
particularly liable to this problem (Figure 7.7).
The base of boreholes should be inspected before concreting and any debris removed.
High-intensity spot lamps now available may be used for this purpose. The importance
of cleaning the base of boreholes cannot be overstressed: cases of pile failure have been
attributed to neglecting this step.
In granular soils, loosening of the base is almost impossible to avoid, even when
drilling under water or bentonite. For piles founded in such deposits it may be wise to
reduce the contribution of end bearing when computing the allowable capacity of the
pile. Special measures may be adopted to consolidate the loosened soil. These usually
involve placing a stone fill pack at the base of the borehole, followed by high pressure
grouting after the pile is concreted. The main effect of the grouted packing is to preload
the pile and reverse the direction of the shaft friction. The overall load/settlement
performance of the pile is enhanced, but the technique has little effect on the ultimate
bearing capacity of the pile. Sand or silt may accumulate at the base of a pile before
concreting, leading to the effect shown in Figure 7.8. The fines may arise from local
seepages into the borehole, or from sediment falling from the drilling water or mud.
For piles deriving bearing capacity from a rock socket, cleaning the base in a
water-filled hole is obviously crucial to the satisfactory behaviour of the pile. In such
circumstances final cleaning may be effected by the use of an air-lift or by adopting
the reversed-circulation drilling technique.
