Geology Reference
In-Depth Information
ed resistance, they had to be re-driven contrac-
tually. Invariably, it was found that on re-driving, the piles could not
be advanced at all. This was not through design or choice and was a
costly problem because piles had to be manufactured to cater for
longer lengths of penetration to avoid re-drives that were expensive
and caused delays to the programme. An investigation was carried out
using driven sample tubes into the sand, as well as additional cone
penetrometer soundings in groups of piles where they came to an early
set and where the depth of penetration was much greater, respectively.
In addition, some pneumatic piezometers were installed within the
sand horizon and readings of groundwater pressure taken during pile
driving as the piling front approached the instrument positions. Data
are presented i
n Figure 7.20.
It can be seen that for piles more than 4m
away from the piezometers, there was no in
achieving the speci
uence from the pile
driving. However, as the piles approached the piezometer positions,
water pressures increased markedly and for the closest piles were off
scale at more than twice overburden pressure. Clearly, we were dealing
with an effective stress problem. Examination of samples established
that an explanation for the different behaviour was in the soil grading.
In areas where the piles were driven to refusal at unexpectedly high
levels, the sand was uniform and clean. Where they penetrated to
deeper levels, the sand had higher contents of silt and clay and clearly
had lower permeability. Evidently, the lower permeability allowed
higher water pressures to develop, which reduced effective stress and
enabled greater penetration. If the driving was stopped, the water
pressure dissipated and the piles could not be advanced, however
Figure 7.20
Piezometric
pressure vs.
horizontal distance
between pile and
instrument.
550
Off Scale
Piezometer 1
Piezometer 2
450
350
250
150
50
0
2
4 6 8
Distance between pile being driven and piezometer, m
10
12
14
16
18
