Research On Loose Circle Of Tunnel With Weak Surrounding Rock Based On Softening Effect

With the extension and expansion of the western transportation network,tunnel projects inevitably need to cross mountainous and hilly areas.This condition makes it a big challenge to design and construct tunnels,especially when crossing weak surrounding rock stratum.Due to the low strength of the weak surrounding rock itself and the broken internal structure,it is very easy to produce safety hazards and serious engineering accidents.From the results of numerous rock mechanics tests,it is clear that most of the weak surrounding rock bodies with poor integrity or low bearing capacity have strain softening characteristics after reaching peak strength.Now the simulations in theoretical and numerical sides of rock strain softening models are relatively mature though,Few studies have considered the effect of rock strain softening after tunneling on the extent of the distribution of the surrounding rock loose circle,and the distribution range of the loose circle can,to some extent,reflect the stability of the surrounding rock of the tunnel which is a guiding value for tunnel construction design.Therefore,based on the knowledge of elasto-plastic mechanics and the loose circle theory,this paper selects the unified strength criterion for the elastic-plastic analysis of the tunnel surrounding rocks,using the Zhonghe Tunnel of Anlan Expressway as the basis of the project.The calculated expressions for the thickness of the conventional surrounding rock loose circle and the displacement of surrounding rock are determined,and then considered the strain softening characteristics of rock after peak stress.An constitutive model considering the strain softening effect was established,and the numerical model was further simulated by using Flac3 D software.Finally,the theoretical under the effect of conventional,the simulated results under the effect of conventional under the effect of conventional and the strain softening are compared with the field measured results respectively.In this way,the reasonableness and applicability of two models are verifed in the mechanical analysis of the soft and weak tunnel surrounding rocks.The details of the study and its findings are as follows:（1）Based on the loose circle theory,the appropriate strength criterion is selected and the elastic-plastic analysis of the tunnel envelope is carried out,based on which the post-peak strain softening characteristics of the rock are considered.The decay weakening relationship of the strength parameters c and φ are introduced into the mechanical analysis of the surrounding rock,and a principal structure model based on the unified strength theory considering the strain softening is established.Further equations for the thickness of the loose circle of the surrounding rock and the displacement of the surrounding rock are obtained for conventional and considering strain softening.（2）According to the actual engineering,the calculation in theory of the thickness of the tunnel loose circle and the surrounding rock displacement without considering and considering strain softening respectively.Results show that for the same rock level,the thickness and displacement of the loose circle of the tunnel without considering strain softening are smaller than that with considering strain softening and the loose circle as well as the surrounding rock displacement of the grade V section are larger than that of the grade IV section.（3）On the basis of theoretical calculations,based on the example of the Zhonghe Tunnel Project,the conventional Mohr-Coulomb elastoplastic model and the strain softening model are built into Flac3 D which is a numerical simulation software used to simulate the excavation of the actual tunneling project respectively.Simulation results show that during the whole simulated excavation process,when the surrounding rock level is the same,the strain softening model produces a larger vertical displacement,horizontal displacement,and loose circle thickness than the Mohr-Coulomb elastic-plastic model.When the model is fixed,the vertical displacement,horizontal displacement,and thickness of loose circle generated during excavation of grade V surrounding rock is larger than that of grade IV surrounding rock.（4）Determine the size of the surrounding rock displacement and the distribution range of the loose circle of the selected grade Ⅳ surrounding rock section and grade V surrounding rock section through field tests.the simulation as well as the theoretical calculation results derived under the conventional and consideration of strain softening numerical model were compared with field measurement results to obtain the soft surrounding rock tunnel surrounding rock deformation law and loose circle distribution characteristics.Further,combining the field test results with the calculated results under the strain softening model,it is verified that the soft and weak rock bodies similar to phyllite,which have the similar characteristics of strain softening after the peak.It can be seen that the strain softening characteristics of the surrounding rock mass should be considered during the pre-design and post-construction of soft and weak surrounding rock type tunnels to ensure the safety and stability of the project.