عنوان مقاله [English]
In this paper, an elasto-plastic analytical-numerical method for the analysis of underwater tunnels is proposed. Seepage flow and secondary permeability of the rock mass due to the mechanical-hydraulic coupling are taken into accounts for plastic zone. Meanwhile, a modified accurate seepage model is used for elastic zone. As the analytical equations do not have closed form solutions, a computer program has been presented to obtain the solutions.
When a tunnel is excavated below groundwater table, groundwater flows into the tunnel and seepage forces act on the tunnel walls. Any element of rock mass is loaded on all sides by the seepage forces as body forces. Fractures and pores cause the permeability of the rock mass to be deformed by these forces. Therefore, the permeability of the rock mass around the pressure tunnels will be modified by the excavation and installation of lining. This change in permeability in turn affects the seepage flow and forces. This mechanical-hydraulic coupling is rarely considered in the analysis of underwater tunnels.
Methodology and Approaches
A new numerical procedure, using the finite difference method, is proposed for calculating the distribution of stresses, radial displacements and pore pressures around a circular tunnel excavated in a rock mass with strain-softening behavior. For estimation of rock mass strength, the Hoek-Brown model is applied. The stepwise procedure proposed by Brown & Bray (1982) is modified by including the effects of elastic strain increments. For the strain-softening behavior, it is assumed that all the strength parameters are a linear function of deviatoric plastic strain, which is different from the function used by Brown-Bray. The accuracy and practical application of proposed procedure have been shown through some examples.
Results and Conclusions
The results and findings have indicated the effects of dilatancy angle, deviatoric plastic strain as the factor of strain-softening, elastic strain increments and condition of groundwater. It can be observed that both elasto-plastic radius and convergence of the tunnel before installation of lining have been increased by raising the groundwater level. Meanwhile, variations of dilatancy angle have also a significant effect on ground response curve and elasto-plastic radius.