Geology Reference
In-Depth Information
Figure 7.12
3D
geological model,
Kong (courtesy of
Dr Chris Fletcher,
Fletcher (2004)).
collapsed material
full platform
superficial deposits
sf
sea
c-m
sf
c-m
c-f
c-f
m-s
volcanic rock
m-s
c-f
granite
m-s
c-f
meta-sediments
cavity fill
reclamation. Prior to the planning of Tung Chung and the formation of
the reclamation, only very limited ground investigations and geophysical
surveys were undertaken. As a consequence, the geology of the sub-
strate below the seabed was essentially unknown, and extrapolation of
the geology from the onshore rock outcrops proved to be misleading.
Initial ground investigations on the newly formed reclamation indi-
cated that the sub-surface bedrock geology comprised rhyolite dykes,
marble, metasedimentary rock and skarn. The thickness of weathered
bedrock varied greatly across the reclaimed area, from less than 50m
to over 150m. At the site of a proposed 50-storey residential tower
block, a very steep gradient in the rockhead surface was identi
ed. In
addition, cavities up to 12m in thickness and of unknown lateral
extent were recorded in the drill logs, deduced from zones of no
sample recovery and sudden drops of the drill string. Using this infor-
mation, a foundation design consisting of 2.5m diameter bored piles
socketed into rock at depths in excess of 120m was proposed. Ground
conditions are illustrated in
Figure 7.12.
However, the costs and risks
involved with this design were considered to be too high, and further
ground investigations were undertaken to investigate the geology more
fully and to determine whether the cavities were actually present or
were
fluids
and improved sampling techniques, followed by down-hole electrical
cylinder resistivity, gamma density and sonar surveys. No open cav-
ities were identi
filled with soil. The new boreholes used polymer drilling
ed in the second phase of drilling and a new geological
model was proposed. The foundation design was then reassessed, but
concern over the mention of open cavities in the original borehole logs
still remained and in the event the tower block was never built.
The key points to be learnt from this project are:
-
The town planning of Tung Chung should have taken into account
the sub-surface geology, so that areas with problematic foundation
conditions could have been avoided, if at all possible.
