Numerical analysis of the influence of a river on tunnelling-induced ground deformation in soft soil

Abstract

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 1.0H_t–3.0H_t (H_t 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.

概要

目的

在沿海城市修建隧道时,不可避免地要穿过河流。本文旨在探究河流对沿海软土隧道掘进变形的影响,地下水渗流引起的孔隙水压力对地表变形的影响,以及围堰施工对控制沉降的作用,为控制穿越河流的隧道开挖引起的过度沉降和保护周边建筑物提供理论参考。

创新点

1. 基于紫之隧道现场监测数据系统分析了地铁穿越河流段隧道施工引起的地表沉降在河流影响范围内与外的发展过程和沉降特征;2. 建立了数值模型对现场情况进行模拟,并与实测数据进行对比,验证了数值模型的可行性;3. 研究了河流影响范围外、河流影响范围内无围堰和河流影响范围内有围堰三种工况对地表沉降及超静孔隙水压的影响。

方法

1. 结合隧道施工方案及地表沉降监测数据,分析河流对地表沉降的影响,并对比河流影响范围内外地表及拱顶沉降的发展规律(图5∼7);2. 通过Plaxis 3D数值模型,研究河流影响范围内外的沉降及超静孔隙水压发展情况,并与实测数据进行对比(图11,12,16和17);3. 通过模拟围堰施工与否,研究围堰施工对于地表沉降的控制效果,以及对开挖面超静孔隙水压的影响(图14∼17)。

结论

1. 河流影响范围内外的地表沉降发展规律不同,但都可以分为三个阶段;在第一阶段和第二阶段,河流影响范围内外地表沉降趋势相似,沉降增加,但河流影响范围内第二阶段沉降增长速度较小;在第三阶段,河流影响范围内的沉降继续增加,但在河流影响范围外趋于稳定;河流影响范围内的地表沉降与拱顶沉降不同步增长。2. 数值模型揭示了地表沉降受到超静孔隙水压分布与大小的影响,超静孔隙水压主要集中在掌子面钱1.0H_t∼3.0H_t的范围内(Ht为隧道高度);在河流影响范围内,超孔隙水压力的消散需要更多时间,使地表沉降缓慢稳定。3. 当隧道施工穿越河流时,需要围堰以减少过度的地面变形,因为围堰的施工可使超孔隙水压力的分布更加分散。