Effect of soil saturation changes on pressure on tunnel linings

Abstract

It is well known that changes in soil degree of saturation have an important influence on mechanical soil properties. Decreasing suction in soil leads to an increase of specific weight and to a decrease of stiffness and strength. These mechanical changes may have an important influence on underground works, particularly on subsurface tunnels. In fact, decreasing soil cohesion due to saturation reduces arching effects above tunnels. Recent developments in unsaturated soil behaviour include numerical models coupling hydromechanical phenomena and considering elastoplastic constitutive laws. However, these techniques are seldom used in practical applications. This paper presents a methodology to compute stress and strain changes in a tunnel lining due to changes in soil saturation, using relatively conventional tools. The motivation of this work comes from a real case in Barcelona, where a Metro tunnel received an increment of loading due to loss of cohesion of upper layers after irrigation. The Finite Element programme “PLAXIS” was used. The paper concludes that the effect of soil saturation changes on underground structures can be assessed by using conventional Finite Element codes, if the variation of stiffness and strength with suction is previously known. The analyses presented in the paper show that changes in density and stiffness due to saturation have little influence on the stresses in the sidewalls of a subsurface tunnel. However, reduction of cohesion due to suction changes may reduce arching effects above the tunnel and would generate high stresses in the tunnel lining.