Geology Reference
In-Depth Information
Safety is a major issue and an engineering geologist working in a
situation where blasting is being conducted may well be involved in
blast monitoring, checking fragmentation and reviewing the overall
suitability of the blast design, given the changing geological situation
as the rock is excavated.
The other major source of potentially damaging vibrations in civil
engineering is from driven piles. Damaging levels are generally
limited to about 10m distance, although this depends on the sensi-
tivity and state of repair of the structure. Predictions can be made
using empirical formulae into which the main inputs are hammer
energy and distance (Head & Jardine, 1992), but these are rarely
very accurate.
There are two main groups of programs commonly used:
finite element
(FE) and
finite difference (FD), time-stepping type software. PLAXIS is a
general purpose FE package that allows geotechnical situations
-
foun-
dations, slopes or tunnels
to be modelled. The model is set up and run
to give a quick solution to complex equations
-
perhaps of deformation
or calculation of Factor of Safety of a model that is split into elements
-
-
mostly triangular. It can also be used to model
flow. As with all
sophisticated software, it should only be used by those knowledgeable
of the underlying mechanics and the way these are dealt with within the
computer program. Following the Nicoll Highway collapse discussed in
Chapter 7,
it was established that there had been a mistake made in the
manner in which the design of the diaphragm walls was carried out
using an inappropriate soil model. The same problemwould have arisen
for any
uid
it is not
unique to PLAXIS. The mistake resulted in excessive deformation of the
walls and an under-design of their moment capacity, although these two
effects did not have any in
finite element package used in this incorrect manner
-
final failure. The mistake was in
adopting effective stress strength parameters in aMohr-Coulombmodel
under undrained conditions and expecting theMohr-Coulombmodel to
predict an appropriate undrained strength. For clay, such as the Kallang
Formation at the Nicoll Highway site, undrained strength is a function
of stress history, in particular overconsolidation ratio, because this
determines whether a soil will attempt to contract or dilate as it is
sheared in an undrained manner and thus generate positive or negative
pore pressures during shearing, which in turn decreases or increases the
strength. The Mohr-Coulomb model does not consider dilation or
uence on the
