Evolution of tunnel uplift induced by adjacent long and collinear excavation and an effective protective measure

Abstract

Tunnel uplift control is an important issue for excavation construction near an existing tunnel, especially for long and collinear excavations. In this study, an effective anti-floating measure combining shaft excavation with the frictional pile slab (FPS) was proposed based on a long and collinear foundation pit above existing metro tunnels. The convergence of the shield segments was carefully measured using automatic total stations over 800 days. Numerical simulation was applied to analyze the effect of the width of the shaft excavation and geometric dimension of the frictional pile on the tunnel uplift. Furthermore, a multiobjective optimization procedure was proposed to optimize anti-floating design considering both safety and economy. A comparison between the FPS method and the high-pressure jet-grouting pile (HJP) method shows that the FPS method is a more effective way to control the heave of the underlying tunnel induced by long and colinear excavation due to its small-scale graded unloading and integral portal anti-floating structure. Combining the FPS method with sectional excavation, all the heaves of the underlying tunnel can be controlled to be less than 9 mm. For granite residual strata, the width of shaft excavation is advised to be lower than 8 m, and the length of the frictional pile is suggested to be longer than the distance between the pit bottom and tunnel invert. The heave of the tunnel crown is suggested as the key index to evaluate the excavation-induced disturbance on the underlying tunnel. The unloading ratio and construction time are the key factors that influence the magnitude of tunnel heave. Compared with the actual design, an optimal design with a lower cost and better anti-floating effect is obtained with the proposed multiobjective optimization procedure. The presented tunnel deformation data, advised anti-floating method and construction sequence can provide useful guidance for similar scenarios.