Civil Engineering Reference
In-Depth Information
Figure 7.6
Bulbous projection on pile shaft caused by overbreak. (Photograph courtesy of CIRIA,
London.)
7.2.1.1 Overbreak
The formation of cavities or overbreak outside the nominal diameter of the pile is
a significant problem in cohesionless soils (Figure 7.6), particularly below the water
table. Under such conditions it is essential that a temporary steel casing is used and
that the cutting edge of the casing is driven below the base of the advancing bore. If
the casing is not at the base of the hole, slumping or washing of soil into the bore can
occur, with subsequent problems on concreting the pile. The cavities may or may not
be filled with water. Even with well-controlled drilling it is possible for the inflow of
water through the base of the advancing bore to seriously loosen the surrounding soil,
with a consequent loss of skin friction.
In very clean sands or rounded gravels, a soil archmay not formaround the borehole,
even in dry conditions. Under these circumstances collapse of soil into the bore will
form large irregular cavities. However, the problem is easily overcome by the use of a
temporary casing.
The problem may be alleviated by drilling under water and maintaining an excess
water head in the casing or by resorting to the use of a bentonite drilling mud or
in appropriate circumstances, a polymer fluid. Alternatively the boreholes may be
permanently cased.
Severe boring problems may occur, with resultant overbreak in pile shafts, in boulder
clay. In such heterogeneous materials it is very difficult to assess the maximum size
of any rock fragments from site investigation data. It is frequently advantageous to
adopt large-diameter piles, drilled with heavy equipment, to enable the largest boulder
to be removed from the bore. The alternatives of chiselling to break up boulders or to
redrill the pile at another position, can be expensive and serious delays to the piling
programme may result.
