Parametric Study of Steel-Lined Pressure Shafts in Anisotropic Rock

Steel-lined pressure tunnels and shafts are used to convey water from the reservoir to the turbines in high-head hydroelectric power plants. An axisymmetrical multilayer model is often considered by engineers for the design of the steel liner. Stresses and deformations can then be computed with a closed-form analytical solution in isotropic rock. When the rock is anisotropic, the lowest elastic modulus of the rock measured in situ is often considered in the analytical solution, which is regarded as conservative. In this work, the behaviour of steel liners in anisotropic rock was studied systematically by means of the Finite Element Method (FEM). The materials, namely steel, backfill concrete, near-field rock and far-field rock were modelled as linear and elastic, and a tied contact was assumed between the layers. The influence of geometrical and material parameters under a quasi-static internal water pressure was studied through an extensive parametric study. It was observed that, compared to the corresponding results in isotropic rock, maximum stresses in the steel-liner can be reduced up to 25% when anisotropy is considered. On the contrary, the maximum stresses in the far-field rock can be largely underestimated, namely up to 65%.