Effects of Cut-and-Cover Tunnel Construction on Adjacent Piles of an Overpass in Bangkok Soft Clay

To mitigate heavy traffic in Bangkok, a cut-and-cover tunnel is often constructed at a congested intersection beneath an existing overpass. The effects of stress relief due to long and narrow excavation can cause adverse impact on deep foundation and super structure of the overpass. However, this adverse impact is still not fully understood. To evaluate the tunnel-pile interaction, measured and back-analyzed data collected from a cut-and-cover tunnel construction across beneath an adjacent flyover is interpreted. The tunnel was supported by diaphragm wall, designed for 7 m deep excavation in thick soft clay. Bored piles supported the overpass are located about 8 m away from the diaphragm wall. Numbers of instruments were installed to observe ground and structural responses due to the excavation. Interaction between the cut-and-cover tunnel and deep foundation of the overpass was back-analyzed using finite element method. For numerical simulation, an advanced hardening soil model with small strain stiffness was adopted. Effects of construction time on movement of diaphragm wall was modeled and compared with the measured results. Difficulties and practical construction method adopted under the existing flyover are discussed. Back-analyzed responses of force in piles, movement and stress in soil due to excavation and construction time are reported.