Civil Engineering Reference
In-Depth Information
Figure 7.21
Defects associated with displacement of reinforcement. (Photograph courtesy of CIRIA,
London.)
Slurries with a concentration of bentonite clay in the range 4 to 6%are normally used
to support the borehole. The fresh slurry can become contaminated with soil arising
from the excavation, resulting in an increase in density and viscosity of the drilling
fluid. The borehole is basically stabilized by the pressure exerted by the drilling mud.
If this pressure is less than the active soil pressure, failure of the sides of the bore will
result. Such failures are most likely to occur near the head of the pile, especially if the
water table is high. Short guide casings should be used and the level of the drilling
mud should be maintained at least 1 m, and normally 1.5 m above the groundwater
level to avoid problems of instability.
Excessive viscosity of the drilling fluid may occur with heavy contamination of soil.
In this situation rapid movement of the drilling bucket may lead to a 'piston effect'.
On withdrawal of the bucket, significant reductions in pressure can occur and local
collapse of the borehole may result. Clearly control of the viscosity of the drilling fluid
should be enforced to avoid this problem and digging buckets should be provided with
a fluid by-pass.
Cleaning the pile base should be carried out with extra care as it is not possible to
inspect the base. Cleaning may be effected by means of a special bucket or with an air
lift, and loosening of the base should be avoided. Fine sand and silt suspended in the
bentonite slurry will gradually settle to the base of the pile, hence final cleaning of the
hole should be carried out shortly before concreting.
An interesting example of bored piles constructed under bentonite slurry in Italy
was quoted by Fleming and Sliwinski (1977). In this case several methods of con-
struction were adopted and the piles were load tested. Details of the tests and the
soil strata penetrated are shown in Figure 7.22. It is clear from the test results that
