Geology Reference
In-Depth Information
first bored tunnels under the sea in
Hong Kong, and the general thinking amongst the design engineers
was that if the tunnels were deep enough, in rock that was grade III or
better, then water in
These tunnels were to be the
low would be low and ground conditions suitable
for excavating with open-face rock TBMs. In the event, most boreholes
put down for the project and available at time of tender did not prove
ground conditions to the depths of the tunnel alignment that was
eventually adopted. Buckingham (2003) summarises as follows:
'
When the SSDS project was conceived, site investigation was under-
taken along the planned alignment to levels determined by this align-
ment. This revealed ground conditions at the planned depth to be worse
than expected, the tunnels were lowered by several tens of metres below
the depth of these boreholes with the idea that they would be below the
poor ground. The tender was based upon this assumption that the
tunnels would be in better rock with minimal water ingress.
'
There was a tender competition and the successful contractors elected
to use mostly refurbished open-face rock tunnel boring machines with
diameters of between 3.2 and 4.3m. These TBMs had very limited
ability to grout ahead of the tunnels and a low level of shielding for
electrical and mechanical devices, which re
ected the general assump-
tion by all parties
-
the HK Government, the design engineers and the
bidding contractors
low. would be low. The contractor
was required to accept all geotechnical risks such that there was no
mechanism for additional payment
-
that water in
in the event of unexpected
conditions.
In the event, once tunnelling commenced, severe water in
ow con-
ditions were encountered in Tunnels F and C especially. In Tunnel A/B
there was almost zero water in
ow, due to the massive nature of the
rock. Fairly onerous conditions had been set for the original contract,
including a limit for groundwater in
ow of 200 litres/minute/1,000m
length of tunnel. This compares to a
ow rate of 5,300 l/min
for the completed Tunnel F, which equates to 1,400 l/min/1,000m
(McLearie
et al
., 2001), despite extensive ground treatment and
re-letting of the contract, as discussed below.
The original contractor had great dif
final in
culty in grouting ahead of the
tunnels because the machines and ancillary drilling rigs were not
designed for such poor conditions. The original speci
cation was for
120 degree drilling capability rather than 360 degrees, which is more
appropriate for stability improvement than preventing water ingress.
In Tunnel F, measured water pressures
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was up to 14 bars. Post-excavation grouting behind the TBM proved
ineffective in meeting the requirements of the contract. When water
was stopped at one location, it reappeared somewhere else.
Figure 7.23
shows water conditions in Tunnel F at one stage.
-
to be overcome by grouting
