Failure of soft soils such as sandy media after underground opening usually involves three dimensional stress conditions in the materials. Furthermore, the behavior and stability of complex underground openings commonly involve a number of behavioral components such as: elasticity, plasticity, shear hardening, strength deterioration and high potential of sliding as shear band etc. However, to determine the stress conditions govern fracture of intact materials, a general three dimensional failure criterion is required. In this paper, an integrated planes elastic-plastic model capable of predicting the failure specifications of granular soil on the basis of sliding mechanisms and elastic behavior of particles has been introduced. This model is able to present numerical distribution of plastic strain through even highly anisotropic medium. Active orientations regarding the plastic shear strain, thereafter, is clarified. The orientation takes priority in plastic strain will fail first. However, the induced anisotropy, pre-failure behavior and plastic strain history of soil as represented at all gauss points can be visualized and carried out. Constitutive formulation of the introduced model has been implemented in appropriate finite element formulation and a computer program is developed. The generalization of integrating plasticity of planes as plane integrating framework for all gauss points through the medium leads the solution to clarify rational failure mechanism which may occur first. To show the capability of model for predicting failure specifications, an underground cavity boundary value problem with different linings has been solved.


Many different analytical solution have already been proposed for stability of underground cavities in soft ground [1],[2]. Lack of the conformity of predictions and analytical solutions led the solution to more development of modeling. However, to obtain a better solution, representing the most probable mode of failure as well as deformations and stress analysis, a mathematical model will do the job.

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