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Abstract Weak rock brings about a series of difficult issues to tunnel support and maintenance in mining engineering. Further study of the mechanism and law of weak rock deformation and breakage has great significance to the safety and stability of the tunnel wall rock. Based on the conventional triaxial compression test of weak mudstone，complete stress-strain curves under different confining pressures are obtained. Then based on the assumption that the stress state at a point satisfies Mohr-Coulomb critical failure criterion，building the weak rock subsequent yielding plane model characterized by generalized cohesion and generalized internal friction angle using the test data，several Mohr¢s stress circles of post-peak rock at different softening conditions are drawn；then using tangent method to build the fitting equation of Mohr¢s strength envelope curve， and under different confining pressures could be determined. And by means of Matlab software，the relationship between generalized cohesion ，generalized internal friction angle ，equivalent plastic shear strain and confining pressure are fitted by using least square surface. Through the relationship，the post-peak mechanical parameters softening law of weak rock can be obtained. The results show that with the increase of confining pressure，generalized cohesion shows a rapid increase trend；but generalized internal friction angle decreases significantly；and the influence that the softening condition brings to the generalized cohesion is evident；the generalized cohesion has a rapid decrease when the stress state changes from the peak value state to the residual state；the average of the decrease is 53.88%，while generalized internal friction angle almost maintains stable during the change of the softening condition. At last，the fitting equations of generalized cohesion and generalized internal friction angle are embedded into the strain softening constitutive relation of the fast Lagrangian analysis of continua in 3 dimensions(FLAC3D)；and using FLAC3D software，numerical verification is made to the model. The results show that the numerical simulation curves agree well with the test curves comparatively.
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Received: 17 June 2009
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