Numerical Analysis of the Influence of a River on Tunnelling-Induced Ground Deformation in Soft Soil

When tunnels are constructed in coastal cities, they will inevitably undercross a river. Exploring the influence of rivers on tunnelling-induced deformation in costal soft soil is of great significance for controlling excessive settlement and protecting surrounding buildings. This paper presents a case study of twin tunnels undercrossing a river in soft soil in Hangzhou, China. The soft soil of Hangzhou refers to cohesive soil in a soft plastic or fluid plastic state with high natural water content, high compressibility, low bearing capacity, and low shear strength. Considering the influence of the river, the research region was divided into two parts, inside and outside the river-affected area, based on monitoring data of the Zizhi Tunnel. The development law of surface settlement is divided into three stages. In the first and second stages, the surface settlement within and outside the river-affected area showed a similar trend: the settlement increased and the growth rate of settlement in the second stage was smaller within the river-affected area. In the third stage, the surface settlement continued to increase within the river-affected area, while it converged outside the river-affected area. Within the river-affected area, there was an asynchronization of the sinking rate and stability of vault settlements and surface settlements. A numerical model was established by simulating different reinforcements of the tunnel. The numerical model revealed that the ground movement is influenced by the distribution and amount of the excess pore water pressure. The excess pore pressure was concentrated mostly in the range of 1.0Ht–3.0Ht (Ht is the tunnel height) before the tunnel face, especially within the river-affected area. Inside the river-affected area, the dissipation of excess pore water pressure needs more time, leading to slow stabilization of surface settlement. When undercrossing a river, a cofferdam is necessary to reduce excessive ground deformation by dispersing the distribution of excess pore water pressure.