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