Monitoring of reinforced concrete piles under horizontal and vertical loads due to tunnelling

As part of the North East Line Mass Rapid Transit Project under construction in Singapore, a 1.9km long 3 lane 2 way vehicle viaduct is being constructed on foundations consisting of reinforced concrete piers founded on between 4 and 6 bored and cast in-situ reinforced concrete piles. The alignment of the viaduct foundations follows closely the alignment of the twin bored tunnels being constructed as part of the same contract. The piles for the viaduct were constructed before the tunnels and a number of the piers were instrumented with vibrating wire strain gauges cast into the piles, to monitor the horizontal and vertical forces exerted on the piles due to the construction of the adjacent tunnels, and the viaduct. This paper presents the results of the strain gauges used to monitor the piles during the tunnel construction. Axial loads and bending moments are calculated from the strain gauge results and these are compared with the design working loads and bending moments for the viaduct piles. The results presented in this paper are highly relevant in understanding the effects of tunnel construction on adjacent piled foundations, a potential problem which is being encountered increasingly in urban construction pi-ejects. It is concluded that tunnelling close to piled foundations can increase both the vertical load and horizontal bending of the piles and this may have consequences for adjacent structures.