Underground Singapore 2011 not high. This may be attributed to the stiff ground at this location. The normalized maximum ground settlements (Sv/H) were less than 0.45% at the final excavation stage, increasing slightly to 0.68% after the strut removal and backfilling stage. The ratio of settlements to wall deflections (Sv/w) ranged from 1 to 2. There is some influence of consolidation on ground settlements although the Kallang Formation is considerably thinner at this location – this would be due to the higher wall permeability in the ERSS compared to other locations.
Kallang Formation Jurong Formation (soils) Jurong Formation (rocks) Soldier pile walls
CBP walls
CBP walls Soldier pile walls
Figure 9. Geological conditions & layout of ERSS at TLB station.
w / H = 1%
Strut removal & backfill completed
150
w / H = 0.5% 100
50
w / H = 0.25%
0 0
5 10 15 20 Depth of excavation, H (m)
Max settlement, Sv (mm)
Max wall deflection, w (mm)
Excavate to FEL
Max settlement vs excavation depth 200
Excavate to FEL
Normalised Settlement vs Deflection 1.0%
Sv / H = 1%
Strut removal & backfill completed
150
Sv / H = 0.5% 100
50
Sv / H = 0.25% 0 25
0
5 10 15 20 Depth of excavation, H (m)
25
Normal. max settlement Sv / H
Max wall deflection vs excavation depth 200
Sv / w = 2
Excavate to FEL
0.8% Strut removal & backfill completed 0.6% 0.4%
Sv / w = 1 Sv / w = 0.5
0.2% 0.0% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% Normalised max wall deflection w / H
Figure 10. Maximum wall deflection and ground settlements at TLB station.
2.6 Discussion Generally, the monitored wall deflections at all the five locations were less than 0.5% times of the excavation depth – this is consistent with the study by Clough & O’Rourke (1990) on excavation case histories where they showed that the maximum wall deflection for excavations in stiff clays were less than 0.5% times of the excavation depth. Table 1 compiles the average maximum wall deflection and maximum ground settlement at the various sites. It is interesting to note that the wall deflections for the excavations at WCT were higher than those at TLB, even though a more rigid contiguous bored pile wall was used compared to the soldier-pile wall. This was similar to the observation made by Clough & O’Rourke for excavations in stiff clays, where they found no significant influence between trends of maximum wall movements for various types of walls and suggested that soil stiffness could have a stronger influence on wall behaviour than wall stiffness. Nevertheless as the excavation at PPJ station has illustrated, it is possible to install a very rigid
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