Underground Singapore 2011 The overlying geology comprises of backfill, Kallang Formation, residual soil of Bukit Timah Granite and bedrock of varied depth due to different degrees of weathering. The tunnels were mainly driven through residual soil using Earth Pressure Balance Machines (EPBMs). The foundations were bored piles with diameter 600mm, 800mm and 1000mm, ranging from 27 to 34 m long. In this study, 5 piles were installed with strain gauges. Figure 5 shows the layout of the five instrumented piles relative to the alignment of the twin tunnels. The levels at which the strain gauges were installed below ground were 2.5m, 5.5m, 19.3m, 21.8m, 25.3m and 28.3m. They were mostly located between tunnel crown and tunnel invert level. The results of the five working piles instrumented with strain gauges were evaluated to check the response of piles during tunnelling. The study concluded that the maximum induced axial forces can be as high as 48% and 72% of the pile structural capacity after the advancement of single and twin tunnels. Maximum induced bending moments are measured near to tunnel level and their magnitude is small compared to their ultimate capacity. In addition, induced forces are found to be negligible for horizontal offset beyond 3 times the tunnel diameter. Figure 6 shows the measured and predicted induced axial forces and bending moments. It is observed that the shapes of the profiles are identical, however the predictions over-estimate the induced axial force and the bending moment by about 5% to 10%. In conclusion, although the proposed approach is relatively simple, it is capable to perform a reasonable good analysis of pile response caused by tunnelling.
Figure 6 Measured and predicted induced forces for case study 2
4 PARAMETRIC STUDIES OF PILE RESPONSES DUE TO TUNNELLING The valuable field monitoring data of the case studies described in the paper was only limited to a specific range of tunnel-pile configurations, tunnelling parameters and soil properties. Further understanding of the tunnel-pile interaction problem outside the range is therefore required. To bridge
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