Pile Design and Construction Practice, Fifth edition

500 Ground investigations, contracts and pile testing

(or other strong relatively incompressible material) and the overburden. Also there must be
adequate correlation with the control boreholes.
Geophysical methods are not usually economical for small site areas, but where the
overburden is soft or loose, either uncased wash probings or continuous dynamic cone
penetration tests (see Section 11.1.4) are cheap and reliable methods of interpolating ground
conditions between widely spaced cable percussion boreholes.
Information on groundwater conditions is vital to the successful installation of driven and
cast-in-place and bored and cast-in-place piles. The problems of installing these pile types in
water-bearing soils and rocks are discussed in Sections 3.4.8 and 3.4.9. Standpipes or piezo-
meters should be installed in selected boreholes for long-term observations of the fluctuation in
groundwater levels. Simple forms of in-situ permeability tests are described in Section 11.1.4.

Levels conjectured

from bore holes

A & B

Hard rock

Pile base level

as installed

1234 1234

1234 1234

A

A

B

B

A B

AB

Weak

rock

Decomposed rock

Planned base

level of piles

Level of hard rock

conjectured from

bore holes A and B

Hard rock Actual level of

hard rock

(a) (b)

(c) (d)

Hard rock

Weak rock

Figure 11.2Misinterpretation of borehole information (a) Horizontal stratification interpreted by
interpolation between boreholes A and B. Piles 1 to 4 planned to have uniform base level
(b) Actual stratification revealed by drilling boreholes for piles 1, 3, and 4, showing base
level required by dipping strata (c) Uniform level of interface between decomposed rock
and hard rock interpreted by interpolation between boreholes A and B. Piles 1 to 4 planned
to have uniform base level (d) Actual profile of hard rock surface.