Prevention of the ground subsidence by using the foot reinforcement side pile during the shallow overburden tunnel excavation in unconsolidated ground

Abstract Large settlements of the ground surface, crown and foot of a tunnel have been measured at a construction field that an NATM tunnel has been excavated under shallow overburden and unconsolidated ground conditions. For these settlements, it was assumed that if the settlement at the foot of the tunnel had been prevented, the ground subsidence can be controlled effectively. Based on this idea, Foot Reinforcement Side Pile (FRSP) has been utilized to prevent the ground settlement. It was reported that the FRSP could effectively prevent the settlements of ground surface and tunnel during the construction period. However, the mechanism of how the FRSP prevents the settlements of tunnel and surrounding ground has not been clearly understood. In practice, several parameters of the FRSP, such as the length, the spacing and the diameter of the pile, have been determined through the practical construction works. In this paper, three dimensional trapdoor model experiments and the corresponding numerical analyses, and numerical analyses for the actual tunnel excavation are carried out to examine the effect of FRSP on preventing settlements of ground surface and crown and foot of tunnel, and to discuss its mechanism. As a result, it was observed that the FRSP can prevent settlements of the ground surface and tunnel effectively, and these effects of the FRSP are connected with the distance from the tunnel lining to the slip line. When the FRSP is long enough to cross the shear line, it can exert the effect of the shear reinforcement, load redistribution and internal pressure to prevent the settlements of the tunnel and surrounding ground effectively.