Geology Reference
In-Depth Information
another major fault to the north of the landslide area. The derived
model is of a mechanism of sliding on the short, impersistent joints
that combine with offset sections of schistose fabric to form a shear
surface. The shearing forces are largely balanced by sliding friction
on joints along the transverse faults and schistosity in one part of the
toe where the failure is kicking out. Resistance is also provided by the
dilating mass towards the right side toe of the slope (facing). One
possible option for remediation that can be derived from this model,
therefore involves strengthening the toe area by anchoring or otherwise
buttressing.
It is to be noted that this model is not numerical but could certainly
be used as the basis for a numerical model that would indeed work for
some realistic set of parameters. Without this understanding of geolo-
gical mechanism, it would be impossible even to begin to design
successful remedial measures.
There is some evidence that groundwater is playing a part in the
failure (some seepage) and therefore it was recommended that long
trial raking drains be installed at points of seepage, in such a way that
they also allow water pressures to be monitored within the slope (a
cost-effective combination of ground investigation and remedial
measure).
One of Terzhagi
'
s principles, as reported by Goodman (2002), was to:
'
guration of properties and boundary condi-
tions consistent with the data from site investigations
assume the worst con
'
, i.e. within the
con
nes of an appropriate ground model. The following two examples
illustrate the consequences of failing to do so.
The failure of a tunnel at Kingston on Hull is reported by Grose &
Benton (2005). The tunnel was constructed with a TBM through a
sequence of saturated Quaternary sediments. During construction,
water and then soil migrated through one of the already constructed
segmental liner joints and the tunnel had to be abandoned temporarily
as the situation deteriorated. A subsequent investigation failed to come
up with a de
nitive answer but it was clearly a matter of soil-structure
interaction and possibly construction defects. The discussions by
Hartwell (2006) and Shirlaw (2006) are instructive and they come
up with various ideas to do with problems with grouting of the liner
to explain the failure, which seem feasible. The bottom line seems to be
that the tunnel design and construction methodology was not robust
enough for the ground conditions. In this case, whatever the geological
variability that ultimately brought about the failure, variability was to