Geology Reference
In-Depth Information
and replaced with concrete or other suitable material. Karstic condi-
tions with voids at depth that may be particularly dif
cult to investi-
culties for foundation
design and construction, as illustrated by a case example in
Chapter 7
and discussed by Houghten & Wong (1990). Conversely, if there are
particularly strong areas
gate comprehensively can pose particular dif
for example, an igneous dyke through
otherwise weak rock in a pad foundation, then this must be accounted
for, otherwise the foundation may fail structurally. In all cases, it is
essential to check any assumptions from preliminary design as the
foundation excavation is exposed. If the ground is worse than antici-
pated then redesign may be required. In severe cases where, for exam-
ple, a major fault is exposed unexpectedly, the required change in
design may be drastic, but that is the price paid for an inadequate site
investigation. Time must be allowed for checking during construction
and taking any actions that prove necessary.
For soils, compressibility and settlement is often the main concern
and much more so than for rock. The presumed values given in
Table 6.1
should restrict settlement to less than 50mm in the long-
term, but estimates may be widely in error and even supposedly
sophisticated methods of prediction are often inaccurate. For founda-
tions on granular soils, empirical methods relying on SPT or CPT data
tend to be used for predicting settlement. Burland & Burbridge (1985)
compiled data for sand and gravel and showed that predictions of
settlement are often in error by factors of two or more. Das &
Sivakugan (2007) provide an updated review.
For cohesive soil, where relatively undisturbed samples can be taken to
the laboratory, oedometer tests are used to determine settlement potential
and to predict rate of consolidation. Estimates of settlement can be made,
given the thicknesses of the various strata in the ground pro
-
le, their
compressibility and the stress changes. Details are given in many refer-
ences, including Tomlinson (2001) and Bowles (1996). For major struc-
tures, engineers will often carry out numerical modelling using software
such as Plaxis or FLAC, which can be used for sensitivity studies. Such
software is also used to predict deformations during different stages of
excavation and construction and to determine support requirements.
If the weight of the soil removed from an excavation is the same as the
building constructed within the excavation, then no settlement should
occur, as illustrated schematically in
Figure 6.4.
This design concept
has been used for many major structures incorporating deep base-
ments which can be utilised for parking spaces. There may be a need
to include holding-down piles or anchors in the design to combat any
uplift forces. Construction of deep foundation boxes often involves the
construction of diaphragm walls using the same techniques as for
