Research On The Effects Of Stiffness Reduction For Surrounding Rocks Stability Of Road Neighborhood Tunnel

Rocks stability analysis of tunnel surrounding is one of the hot issues in underground engineering studies.Due to the narrowness of pillar in neighborhood tunnel,its stability is much more complicated than ordinary single-tunnel,and its stability should be more concerned about.The same strength reduction factor to c and? is adopted in mostly traditional finite element strength reduction method,But when the surrounding rock of tunnel sliding the cohesive force(c)and internal friction angle(?)have different function,different degradation order and speed in the failure process.In the same time,the traditional finite element strength reduction method merely considers the impact of strength parameters on surrounding rocks stability,but neglects that the stiffness parameters will be affected by the weakening of the strength parameters.Based on the finite element strength reduction method,this thesis proposes a new way to analyze the surrounding rocks stability of neighborhood tunnel when adopted double reduction factor,and also probe into the effects of stiffness parameters for the stability of surrounding rock,with main contents outlined as follows:(1)Literature at home and abroad are consulted,and the current situation of underground engineering stability research is summarized.A detailed introduction of the stability analysis method,instability failure types,factors leading to rock instability failure,characteristics of instability process and failure criterion of tunnel surrounding rocks.(2)By introducing the effects of yield criterion selection on results analyzing,and elaborating on the Mohr-Coulomb(M-C)criterion and Drucker-Prager(D-P)criterion which are widely used in geotechnical engineering.(3)Based on the basic principle of double strength reduction factors,different strength reduction factors are used into shear strength parameters c and?,with its impact surrounding rocks stability of neighborhood tunnel.Results reveal that by applying such strength reduction method in which internal friction angle(?)degradated as bigger than cohesive force(c),its dome settlement shows great capability in withstanding displacement and its cavern can withstand greater displacement of inner arch spring as well as greater first principal stress.In the shear instability failure of surrounding rocks,its shear capacity of rocks is soundly adjusted,which is more in line with the actual process of surrounding rocks failure.(4)Based on the basic principle and method of stiffness degradation,the Poisson's ratio(v)and elastic modulus(E)were adjusted,and a variable rigidity strength reduction method of tunnel surrounding rock was proposed.The new method was used to analyze the stability of neighborhood tunnel surrounding rocks.The results showed that the Poisson's ratio hasstronger influence in the displacement of arch settlement,dome settlement,and the first principal stress than elastic modulus.The displacement of the tunnel arch spring under the condition of stiffness reduction is greater than that without considering the stiffness decrease.The stiffness adjustment leads to the decrease of the equivalent plastic strain zone.Considering the stiffness degradation,the transfixion of interval pillar plastic zone is delayed.When the variable rigidity strength reduction method of tunnel surrounding rock is used to surrounding rock stability evaluation of neighborhood tunnel,the rationality of the design can be judged according to the size of the safety factor,so as to provide references for the selection of support parameters and construction technology.