A study on the effects of overlying soil strata on the stresses developing in a tunnel lining

Shield tunnel lining in soft ground is usually designed based on empirical and analytical methods. These methods are convenient and easy to use; however, they are based on simplifying assumptions related to soil homogeneity around and above the excavated tunnel. The effects of overlying stiffer layers on the tunnelling induced settlement has been investigated by several researchers and was found to significantly influence ground movements. This study evaluates the effects of overlying stiff layers above a tunnel excavated in soft ground on the stresses developing in the tunnel lining. Laboratory investigation is conducted to study how the presence of these layers influence bending stresses in a model tunnel constructed in soft clay and overlain by a coarse sand layer located at different heights above the tunnel. Validation using the experimental results, elasto-plastic finite element analyses are then performed to explain the role of the relative stiffness between the overlying layer and the soft soil deposit hosting the tunnel. Depending on the thickness and location of the overlying stratum, the presence of a stiff layer above the tunnel can have a significant impact on the stresses developing in the tunnel lining.