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An elasto-plastic model for underwater tunnels considering seepage body forces and strain-softening behaviour

This paper proposes an analytical–numerical model for elasto-plastic analysis of underwater tunnels that considers seepage body forces and strain-softening behaviour. Using the finite difference method, a theoretical solution is presented for calculation of the pore-water pressure, stress and strain distributions around the circular tunnels in axisymmetric and plain strain conditions. In this method, strain-softening behaviour and non-linear Hoek–Brown failure criterion are used. The proposed model also takes into account the effects of elastic strain increments and dilatancy angle () of the rock mass in the plastic zone. Through developing previous seepage models, a more accurate model is proposed for calculation of pore-water pressure distribution in all directions around underwater tunnels. Taking into account strain-softening behaviour of the rock mass, the stepwise method is applied to the plastic zone where parameters of strength and dilatancy, stresses, strains and deformation vary from their elasto-plastic boundary values to the tunnel boundary values. On the other hand, the analytical equations are derived for the elastic zone. Accuracy and the practical performance of the proposed model are evaluated by a number of examples. The results present the effects of seepage body forces, elastic strain increment and variations of the dilatancy angle in the plastic zone appropriately.