Research On Self-stability Of Surrounding Rock Based On Strength Reduction Method

The evaluation of surrounding rock self-stability is traditionally based on rockmass classification. And then the self-stable time and possible collapse degree arepredicted in tennels with different span intervals. The enumeration method is used innearly all of these methods. However, the results of such evaluation methods do notprovide the exact quantitative indicators, but only qualitative description. Also,surrounding rock bearing capacity is not calculated and no quantitative indicator forthe stability is provided. Therefore, it is too empirica l, and can not reflect theself-stability of a tunnel accurately. In response to these shortcomings, and based onstrength reduction method, a quantitative evaluation method of self-stability wasprovided in this paper.(1) Firstly, the basic principles and advantages of strength reduction methodwere introduced in detail. Then, strength reduction method was combined with finitedifference method. The shear strength of the rock and soil, which was simulated withMorh-Coulomb model, was reducted till the limit state. In this way the safty factorcould be obtained, and it could be used to to evaluate the stability of rock mass. Thestrength method can not only be used in geotechnology engineering, but also obtain aquantitative indicator for the stability of geotechnology engineering.(2) According to the mechanism of instability of the tunnel, combined with theoperational characteristics of FLAC, instability criterions of of tunnel surroundingrock in numerical calculation were discussed. FLAC, as finite difference software,was better at numerical analysis in large strain mode than FEM. So it is better to usecrown settlement mutation as the instability criterion. The run-through of plastic zoneis only a necessary condition for instability of tunnel surround ing rock, so it can onlybe used as a complement of other instability criterion.(3) Based on GSI scoring system and the relationship between Hoek-Browncriterion and Mohr-Coulomb criterion, the shear strength parameters of the rock massis determined. Then the finite difference method and strength reduction method areintroduced. And the safety factor of tunnel with no support could be worked out,which is named as self-stability factor. In addition, according to code for design ofroad tunnel (JTG D70-2004), the most unfavorable rock mechanics parameters for alllevels of surrounding rock and the minimum support standards could be chosen. Thesafety factor in this situation was named as low standard safety factor, which could be used as the reference indicators of self-stability of rock mass. In this way, thequantitative evaluation method of self-stability was provided.(4) Finally, the theory of orthogonal design was introduced, and sensitivityanalysis for the basic factors that influence self-stability of surrounding rock wasdone. Range analysis showed that the geological strength index plays a key role,followed by excavation disturbance factor, the uniaxial compressive strength of intactrock, tunnel depth and span.