1701
Technical Committee 204 /
Comité technique 204
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 5. Holes drilled for water-leak test.
and (4) drainage sump. For every 0.5 m of top heading mining,
the top part was supported by the assembled steel
lining-segments. The void between the improved ground the
segment was backfilled to prevent any segment movement.
For every 0.5 m of downward digging for the drainage sump,
the shaft was supported with an assembled steel segment ring,
and the void behind the ring was backfilled with grouting
materials. Even with the JSG grouting, additional chemical
grouting, and water-leak test, small amount of groundwater
seep-in was observed in the unlined drainage sump. With the
help of compressed-air, the amount of groundwater seep-in was
minimized and pumped out of the sump before lining. Lot
CD266 of Taipei MRT was completed and started to operate on
May 31 of 2006.
It should be noted that the soil and groundwater conditions
encountered below the depth of 30 m could be quite
complicated. The geotechnical engineer should never
overestimate the effects of jet grouting and chemical grouting.
Under the threat of the tremendous groundwater pressure, to
keep the construction on the safe side, it is suggested to
maintain a cautious attitude and take conservative measures.
4MULTIPLE LINES OF DEFENSE
For Lot CD266 of Taipei MRT, the groundwater table was
located at about 7.0 m below ground level. The bottom of the
drainage sump was located at the depth of 33.7 m, at which the
excavation zone must be able to resist 26.7 m of pressure head.
Under the challenge of this immense groundwater pressure, any
micro-crack in the improved ground might cause water and soil
to stream into the excavation zone; carry away ground materials
near the cross passage, damage the completed main tunnels, and
Figure 6. Water-leak test before cross passage excavation.
result in exceedingly serious consequences. When facing such
potential hazards, to reduce the risk of construction, the
designer is suggested to adopt the following “Multiple Lines of
ile to deviate laterally. The
resulting discontinuity of the improved ground could create a
r this project, the water-leak test and the following
additional chemical grouting served as the second line of
mpressed air was an effective measure to repress the
seep-in of groundwater in the excavation face of the drainage
e confined in a small and restricted zone.
The contractor should never be overconfident as to give up this
In this paper, the ground improvement and construction of
Figure 7. Additional chemical grouting in tunnel before cross passage
excavation.
Defense” strategy.
4.1
First line of defense - jet grouting
For this project, the first line of defense for the excavation of
cross passage is the JSG ground improvement. The diameter of
the jet-grouted pile was assumed to be 1.4 m. However, the
actual JSG pile diameter at the depth of 35 m was hard to
justify. Besides, the grouting rod was assumed to be totally
vertical. Any slight tilting of the grout rod from verticality may
cause the bottom of the soilcrete p
path for the invading groundwater.
4.2
Second line of defense – water-leak test and chemical
grouting
The disturbance of improved ground by the boring of the shield
machine might create new cracks and fissures in the soilcrete
body. Fo
defense.
4.3
Third line of defense – compressed air
If the first and second defense lines were insufficient to resist
the groundwater inflow driven by the tremendous pressure head,
the compressed air method was used as the third line of defense.
The co
sump.
4.4
Fourth line of defense – safety gates
In case the jet grouting, leak-test and addition grouting, and
compressed air methods all failed to resist the invasion of the
groundwater, the fourth defense line could be activated. This
simple and effective method is to fabricate three steel gates at
the top of the drainage sump, and at both ends of the cross
passage (see Figure 10). If the break in of groundwater appears
irresistible, in the worst case scenario, the tunneling crew could
simply shut the emergency gates. The flow of groundwater and
loss of ground will b
last line of defense.
5 CONCLUSIONS
