Observed ground and pile group responses due to tunneling in Bangkok stiff clay

Abstract

A 5.15 m diameter water diversion tunnel was driven into Bangkok stiff clay using an earth pressure balance shield. The tunnel was driven within a clear distance of 2 m from the closest pile of a 3 × 4 pile group supporting an expressway. During construction, tunnel driving parameters as well as induced ground and pile group responses were recorded. To avoid cutting the piles supporting the expressway, the alignment of the tunnel was adjusted and curved. As a result of this change in tunnel alignment, the tunnel advancing rate was reduced from an average 17 m/day for a straight drive to an average of only 6 m/day for the curved alignment, and the ratio between the tunnel face pressure and overburden pressure was changed from 0.5 to 0.4, accordingly. Due to the reduction of the tunnel face pressure, up to a 280% larger inward ground movement towards the tunnel was observed. As the shield penetration rate decreased, the torque required for tunnel driving was reduced by 33%, while the ratio between shield penetration rate and soil extraction was almost constant throughout the tunnel route. A transverse influence zone due to tunnel driving was identified to extend up to a distance that was twice the tunnel diameter radially from the longitudinal tunnel axis. The maximum tilting of the expressway pier and deduced differential settlement of the pile located within the influence zone were up to 1:2600 and 2.0 mm, respectively. Tilting of all the piers was mainly caused by long-term subsurface settlement having the tilting direction towards the tunnel. This long-term subsurface settlement was up to about 80% of the total.