Geology Reference
In-Depth Information
STONECUTTERS
SIDE (East Tower)
Tower for
bridge
Piers
LIKELY GROUND
INVESTIGATION
METHODS
Planned
drillholes
Existing drillholes
sea level
Rockhead profile
seismic traverses
Superficial deposits
(fill, alluvium)
and weak saprolite
resisitivity
magnetic surveys
vertical drilling
Possible
rockhead
profile
Fault zones and
dykes
inclined, intersecting
drilling
downhole geophysics
seismic tomography
between boreholes
Low angle clay-filled thrusts?
Deep weathering?
Suspected fault zone
(from geomagnetic
survey)
Mapped fault zone
Figure 4.3 Preliminary assessment of ground conditions by Halcrow for East Tower of
structures.
on excavated rock (
Chapter 6)
and with plan dimensions of up to 40×
20m. For each of these foundations, there were usually about six
boreholes, typically one put down at the conceptual stage, three for
the basic design and two for detailed design. For most of the other
viaduct piers with plan caisson dimensions of 17 × 17m, there were
from one to
five boreholes
-
less where the geology was better known,
close to shore.
Obviously, where the site reconnaissance, together with desk study
or
findings from preliminary boreholes, indicate potentially complex
and hazardous conditions, it may prove necessary to put down far
more boreholes. For the design of the new South West Transport
Corridor near Brisbane, Australia, the preliminary investigation over
a critical section of more than 500 m comprised
five boreholes and a
few trial pits, mostly along the centre line of the road. As the earth
works were approaching completion, minor landslides occurred
at road level, together with some indications of deeper-seated
movements. Over the next few months, an additional 70+ deep
boreholes were put down, 56 trial pits and 54 inclinometers installed,
despite almost 100% rock exposure in the cuttings (which was care-
fully examined and mapped). This intensive investigation allowed the
landslide mechanisms to be identi
ed in this very complex site and
