Study On Deformation Characteristics Of Soft Rock Tunnel Excavation And Optimization Support Design

The soft rock has low strength,large and rapid deformation after tunnel excavation,and the construction disturbance has a significant effect on its deformation rate,which is easy to produce plastic zone.When a tunnel built under the geological condition of soft rock strata,and after the tunnel excavated,the in-situ stress is redistributed,which is easy to produce large deformation.Tunnel deformation will encroach on clearance,crush tunnel support structure,cause surface collapse,affect the safety of surrounding buildings.How to predict the displacement and stress variation of soft rock tunnel after excavation is very important to optimize tunnel design and ensure safe construction.This paper focuses on the disturbance range and influence degree of soft rock tunnel after excavation and the variation of surrounding rock pressure and displacement with time（space-time effect）.The theoretical parameters and the basis of comparative analysis are obtained by field soil pressure monitoring experiment,monitoring and measurement experiment,drawing experiment and geotechnical experiment;On the basis of the strength criterion of nonlinear Hoke-Brown and linear Mohr-Coulomb,combined with the finite element analysis software FLAC^3D5.0,the relationship between the spatial influence radius of surrounding rock excavation,the development of plastic zone and the relationship between the stress release rate of surrounding rock and the convergence rate of wall displacement is studied.The applicability of the two strength criteria in predicting the deformation of surrounding rock in medium weathered shale is verified;The convergence constraint method is used to determine the best supporting time and virtual supporting force of the second lining,and the original supporting scheme is optimized.The main work and conclusions are as follows:（1）Within the range of values given in the Code for Design of Highway Tunnels（JTG3370.1-2018）,based on the modified rock mass quality index[BQ]value,the relationship between the surrounding rock classification and rock mass parameters and[BQ]value is given,and the surrounding rock grade and rock mass parameter index corresponding to[BQ]are obtained;（2）According to the monitoring results of earth pressure and wall displacement,the pressure distribution trend of tunnel circumferential surrounding rock is obtained after the convergence of surrounding rock is stable;Combined with the convergence of the displacement of the wall of the five monitoring sections and the longitudinal deformation curve of the surrounding rock,the influence range of the tunnel excavation on the longitudinal deformation of the surrounding rock and the amplitude of the displacement convergence are obtained;The results show that the wall rock pressure of the arch roof is about 2.64 times of the arch foot pressure,and the actual influence range of surrounding rock excavation is about4 times of the excavation radius after excavation;（3）Based on the small hole expansion model and the non-correlation flow rule,considering the shear expansion characteristics of rock and soil,and taking Mohr-Coulomb、Hoek-Brown two strength criteria as constitutive models of rock and soil,the deformation characteristics of soft rock after excavation are studied;The results show that the Hoek-Brown strength yield criterion based on residual rock mass parameters is more suitable for the deformation prediction of fracture development or loose surrounding rock than the Mohr-Coulomb strength yield criterion based on initial rock and soil parameters;（4）Based on the component combination rheological model,a constitutive model can be established to describe the rheological properties of soft rock in Pingtian tunnel,and the rheological formula of soft rock is given,which is combined with in-situ experimental data to verify the scientific nature of the formula;The results show that the model has good applicability,and the convergence solution of surrounding rock displacement considering time effect can well reflect the influence of construction on surrounding rock deformation during tunneling;（5）Based on the influence degree of tunnel excavation disturbance on soft rock deformation,80%of the final amount of displacement convergence of surrounding rock is taken as the control point of second liner timbering opportunity,based on the analysis of convergence constraint method,the optimum supporting time of two-liner is obtained;Combined with the field displacement monitoring data,optimizing the original support scheme.The research shows that under the premise of engineering safety,the support design can be optimized,and a part of the surrounding rock displacement can be released or restrained to meet the actual engineering requirements.