Soil-Structure Interaction of Tunnels in Soft Clay Soil

Abstract

The construction and use of tunnels can be considered one of the most important features of civilization in developed nations. However, it is found that tunnels effectively helped in the consistency and fluency of traffic as means to carry passengers from one place to another, particularly in over-populated cities. But, it is very difficult to construct tunnels in soft clay soil. So, the objectives of the present study are: i) identify the most suitable shape [Circular, Elliptical and Rectangular with Arch Circular (like horseshoe)] tunnels in soft clay soil. ii) the effect of change shape of tunnels [circular, elliptical and rectangular with arch circular (like horseshoe)] on the internal forces in the tunnel and the internal stresses as well as settlement in soft clay soil. The design of tunnel cross section depends on the values of internal forces and deformations induced in the lining. These values are influenced by many factors such as type of soil and dimensions of the tunnel cross section. In the present study, a numerical analysis using finite element software ADINA version 8.5 (a finite element computer program) was used. A numerical model was presented as a plane strain problem. Two dimensions triangle element of 3-node was used to present the soft clay soil and 2 -node beam element to simulate the lining of tunnels. The material of the lining was chosen as a reinforcement concrete. The Mohr-Coulomb model was used to simulate the non linear soft clay soil with different modulus of elasticity (Es) 1000, 1500 and 1900 kN/m 2 and different Poisson’s ratio () 0.4, 0.45and 0.49. The effect of changing the thickness–radius ratio (t/r) of (0.1, 0.15, 0.2) on the internal forces and the displacement as well as the internal stresses, under the ground surface at a depth 6 times radius of the tunnel were analyzed. The values of internal forces and the displacement at the upper crown (Cr) and the spring (SL) points at the tunnel as well as internal stresses in the soft clay soil were investigated. The analysis showed that the circular shape for tunnels is the most suitable shape in soft clay soil. As the internal forces and the vertical displacement as well as the horizontal stresses for the lining of circular tunnels is less than for the two adjacent elliptical as well as rectangular with arch circular (like horseshoe) tunnels. The numerical results showed that in soft clay soil, the internal forces and stresses are slightly affected by the stiffness of circular tunnel while it is more affected as (Es) of soil increases. As the internal forces decreased up to 75%, the horizontal stresses increased to 35% and the shear stresses decreased to 70%, as well as the displacement decreased to 70% at some cases