Study On The Effect Of The Principal Stress On The Failure Behavior Of Surrounding Rocks With Three-Dimensional Numerical Simulation

In recent years, with the economy growing and technology progressing, more and more engineering projects are constructed for the underground space utilization in China. Meanwhile, the security and stability of underground projects is always the attention focus. Especially, because the stress conditions of the underground engineering are often very complex and rock masses are usually in three-dimensional stress states, the surrounding rocks show strong nonlinear deformation and failure characteristics. The plane strain hypothesis generally ignores the effects of the axial stress and intermediate principal stress and has no enough advantages to gain insight into the rock failure problem as well. Hence, it is necessary to study the failure mechanism of the surrounding rocks of underground openings under multiaxial stress conditions considering the three-dimensional stress effect in order to provide reliable bases for disaster monitoring and prediction, underground excavation supporting design and stability evaluation, etc.Statistical damage theory and numerical simulation method are employed in this paper to research the fracture mechanism of underground openings under triaxial stress states in complex stress conditions. The three-dimensional numerical model is established considering the heterogeneity of rock masses that is described by Weibull distribution. Strength reduction method is used, which can not only achieve progressive failure and failure modes of surround rock masses but also assess the stability of underground openings in different stress fields quantitatively. Three-dimensional effect is embodied in the process of calculation. Failure patterns under different axial stresses and lateral pressure coefficients are simulated. The influence of intermediate principal stress on the opening failure patterns and stability is analyzed. The production condition and failure law of zonal disintegration are discussed. The main contents and achievements of this paper are as follows:(1) The opening failure law when the axial stress is the minimum principal stress, intermediate principal stress, maximum principal stress respectively and the safety factor under different axial stresses are studied. The results show that when the axial stress is small, local failure zones are promoted and the surrounding rock stability is threated by the shear failure and tensile failure; when the axial stress is large, the distribution of failure zones are more uniform and surrounding rocks tend to overall instability. Simultaneously, the effect of the axial stress on the safety factor of the opening is different under various lateral pressure coefficients. (2) The opening failure mode and stability affected by different lateral pressure coefficients are studied. The results show that the lateral pressure coefficient has significant effect on the initial rupture location and development trend. When the lateral pressure coefficient is small, fracture firstly appears near the two cavern wall, and then the local rupture zones develop into the deeper surrounding rocks, which threats the opening security; when the lateral pressure coefficient is large and the bias compression level is low, the distribution of the fracture zones is more uniform, but the failure pattern is complex and the overall failure mode is exhibited generally. Moreover, with the lateral pressure coefficient increasing, the security and stability of openings is basically improved. However, the improvement speed of the safety factor has obvious differences because of the various axial stresses.(3) The surrounding rock failure mode and stability affected by different intermediate principal stresses are analyzed. The results show the intermediate principal stress has significant effect on the opening failure law, the safety and stability of underground openings has intermediate principal stress effect and the influence of intermediate principal stress on the surrounding rock stability is different in different directions. When the axial stress is intermediate principal stress, safety factor shows two different stages; when the horizontal stress is intermediate principal stress, safety factor increases firstly and then decreases with the intermediate principal stress increasing.(4) The production condition and mechanical mechanism of zonal disintegration is discussed. When the direction of tunnel axis is parallel with that of the maximum principal stress, zonal disintegration of underground openings will appear because of large axial stress and the rupture zone exists tensile strain concentration. Simultaneously, the formation of the zonal disintegration is conditional and the surrounding rocks may produce local failure with the zonal disintegration trend instead of a typical zonal disintegration phenomenon.