Research On Support Technique Of Large Deformation Tunnel With Soft Rock Based On Loose Circle Theory

With the vigorous development of national highway and railway construction,it is necessary to construct tunnels that pass through various complex geological conditions.During the construction of soft rock tunnels,engineering problems such as large deformation and limit violations are prone to occur.Therefore,it is necessary to conduct deep research of surrounding rock stability and support technology among soft rock tunnels.Based on a newly built tunnel of Chengdu-Lanzhou Railway,this paper analyzes the mechanical characteristics of soft rocks,and then analyzes the stability of surrounding rocks based on the theory of surrounding rock loose circle.Then,theoretical calculations are performed and the support parameters are optimized based on the results.The numerical simulation is used to compare and analyze the stability of surrounding rock before and after optimizing the parameters.Finally,through comparison and analysis of the results obtained from field measurements after optimizing the parameters,the results of theoretical calculations and simulation of count values,the main conclusions are as follows:?1?From the physical and hydraulic tests of the phyllite,it is known that the phyllite of the tunnel site has a low water content,strong joint development,and poor integrity.The acoustic emission characteristic curve during the uniaxial compression process shows that the specimen has less acoustic emission activity during the compaction phase and the elastic phase,and the acoustic emission activity spikes to the peak while the specimen is near destruction.Based on the Kaiser effect,the original rock is under high geostress.The peak stress under the uniaxial compression test is small,showing the properties of soft rocks.Meanwhile,the failure modes of the specimens are mainly tensile-shear composite failure mode.?2?The conventional triaxial tests show that the peak strength,residual strength value of the specimen is proportional to the confining pressure.The post-peak deformation characteristics show strain softening.The peak strain reaches the order of 10^-2.The failure mode of the rock also transitions from tensile-shear failure with low confining pressure to shear failure with high confining pressure,which meansthat the specimen has a ductile failure characteristic..?3?Taking the relationship between the strength parameters and strain softening parameters as intermediate variables,a strain softening model based on unified strength theory was established.The post-peak stress-strain relationship of the phyllite under different confining pressures was fitted by the proposed model and the comparisons between the conventional triaxial tests and theoretical calculation results verified the feasibility of the model.Based on this model,the calculation formula of the surrounding rock displacement and surrounding rock loose circle radius of the tunnel is derived.?4?Relying on the actual engineering,the bolt parameters were optimized based on the theoretical calculation of the loose circle range.Based on this,FLAC 3D numerical simulation software is used to compare and analyze loose circle range,surrounding rock displacement,surrounding rock stress and bolt stress before and after the optimized parameters.The thickness of the loosening circle after the optimized parameters is reduced by 0.2?0.3m.At the same time,the dome settlement and peripheral convergence has been reduced by 48.5%and 45.4%,the maximum principal stress of the surrounding rock has been reduced by 43.5%,and the maximum stress of the bolt has been reduced by 28.3%,which indicates that the optimization of the support parameters based on the theory of the loosening circle can provide support resistance in a timely manner and effectively.Then deformation of the surrounding rock is controlled to increase the stability of the surrounding rock.?5?The thickness of the loose circle and the displacement of the surrounding rock are obtained through field tests,and the results are compared with the calculation results of theoretical calculations and numerical simulations.The variation laws of the thickness of the loosening circle and the displacement of the surrounding rock are further obtained.The results of theoretical calculation and numerical simulation are close to those of field measurement.The results indicated that strain-softening model based on unified strength theory has good applicability to the elastoplastic analysis of surrounding rock in soft rock large deformation tunnels.