Study On Structural Mechanical Properties During Tunnel Constrction Considering Short-Term Creep Effect Of Argillaceous Soft Rock

With the continuous extension of China’s transportation network to the western region,a large number of tunnels passing through soft rock strata will be built in the western mountainous area.This kind of rock mass has low strength,broken rock mass and remarkable creep characteristics.In a short time scale,the creep of soft rock also affects the safety of tunnel construction,especially in argillaceous soft rock.However,in the tunnel structure design and actual construction,the adverse impact of soft rock creep characteristics on tunnel construction is often rarely considered.Therefore,it is urgent to carry out relevant research considering the short-term creep characteristics of argillaceous soft rock.Relying on Dongmachang No.1 Tunnel of Huali expressway,through literature research,rock creep test,field monitoring,model test and numerical simulation,this paper studied the evolution law of tunnel deformation and mechanical properties of supporting structure with time and space during construction considering the short-term creep characteristics of argillaceous soft rock.The influence of different construction and support parameters on tunnel excavation was also analyzed.On this basis,the parameters of deformation control technologies were optimized through the mathematical models.The main research work and conclusions are as follows:(1)The in-situ stress test was carried out by hydraulic fracturing method.The results showed that the in-situ stress in the tunnel site is mainly horizontal tectonic stress,the maximum initial stress is 12.66 MPa.According to the results of on-site tunnel deformation,contact pressure,steel arch stress and secondary lining internal force,the deformation of surrounding rock is obviously affected by rock creep effect from tunnel excavation to completion of secondary lining construction.After adopting the double primary support scheme,the tunnel excavation is stable as a whole.(2)According to the results of uniaxial compression creep test and field monitoring displacement inversion,cvisc model can better describe the creep characteristics of argillaceous siltstone at the inlet of Dongmachang No.1 Tunnel.Through the results of creep test and displacement back analysis,the parameters that can reflect the short-term creep effect of argillaceous soft rock were determined.(3)Considering the short-term creep characteristics of rock mass,the middle step length in tunnel excavation and the construction time of the second primary support were discussed by means of model test and numerical simulation.The results show that reducing the step length and advancing the construction time of the second layer of primary support are conducive to controlling the displacement around the tunnel and reducing the surrounding rock pressure,concrete stress and steel arch internal force of the first layer of primary support,but the initial support stress of the second layer increases.Therefore,the length of the middle step can be appropriately extended and the construction time of the second primary support can be delayed to release the surrounding rock pressure and ensure the safety of the second primary support.(4)By means of numerical simulation,the effects of double primary support thickness ratio and primary support closure time on the deformation law of tunnel and the mechanical properties of support structure were studied.The results showed that properly increasing the thickness of the first primary support is beneficial to reduce the displacement at the top of the tunnel,the volume of surrounding rock plastic zone and the primary support stress.There is a positive correlation between the closure time of the primary support and the displacement around the tunnel and the volume of the plastic zone.After delaying the closure time of the primary support,the stress of the first primary support and the second primary support except the inverted arch gradually increases,and the stress at the inverted arch gradually decreases.Through the analysis of variance,it was concluded that the length of the middle step has the greatest impact on the support effect,followed by the construction time of the second primary support,and the closure time of the primary support has the least impact.(5)Taking the primary support stress as the constraint condition and the vault settlement as the optimization objective,the parameters of deformation control technologies are optimized by genetic algorithm.The results showed that in the actual construction,the steps should pass quickly,and the construction time of the second primary support should be delayed to release the surrounding rock pressure,and the primary support should be closed in time.However,since the primary support closure time has little impact on the support effect,which can be delayed in the construction process to improve the construction convenience.