Midas GTS NX: Buoyancy effect on shallow tunnels

Osvaldo P.M. Vitalia, Tarcisio B. Celestinob, Antonio Bobeta

"Abstract

When a shallow tunnel is excavated, an overall upward movement appears as a result of the weight removal from the excavation. This movement is analogous to buoyancy. When numerical simulation of shallow tunnels is used, the magnitude of the upward movement increases with the depth of the lower boundary, defined as the distance from the tunnel center to the bottom of the mesh. This seems counterintuitive, and yet it is mathematically correct. This paper investigates the influence of the assumptions made to model the buoyancy phenomenon numerically, and specifically the (typical) plane strain and elasticity assumptions. Three-dimensional and two dimensional plane strain numerical models are carried out using a linear elastic model, where the influence of the ground stiffness increasing with depth is assessed. The results show that the buoyancy effect, i.e. increasing upwards movements with depth of the lower boundary, decreases when the ground stiffness increases with depth."