Study On Mechanical Properties Of Soil-Rock Mixture And Stability Of Pile-Supported Tunnel In Soil-Rock Mixture Backfill Area

In recent years,more and more geological conditions such as soil-rock mixture have been encountered in engineering construction.However,the understanding of soil-rock mixture is not deep enough.Moreover,due to the complexity of its mechanical properties,such geological conditions have a great influence on the safety of tunnel construction and long-term stability.Based on the actual engineering,this paper conducts research through the method of combining laboratory test,theoretical analysis and numerical simulation.The main research contents are as follows:(1)Through field sampling and laboratory screening and matching,in order to get the density of soil samples and soil-rock mixture samples under different pressures,the large compression experiment was carried out.The obtained densities were used to conduct triaxial tests to obtain the stress-strain curves and failure characteristics of pure soil samples and soil-rock mixture samples under different confining pressures,and the mechanical parameters of soil-rock mixture were calculated.(2)Samples are drilled on site and processed into standard cylindrical specimens for wave velocity test,uniaxial test,and triaxial test.The wave velocity,uniaxial and triaxial test stress-strain curves and failure characteristics of the soil-rock grouting specimens were obtained.The physical and mechanical parameters of the soil-rock grouting specimens were obtained after considering strength reduction.(3)According to the stress-strain curve obtained from the indoor test of pure soil sample,soil-rock mixture sample,and grouting sample,the PFC-FLAC coupling analysis method is used to establish the numerical analysis model of triaxial test and uniaxial test,and the meso-contact parameters among soil-soil,soil-stone,stone-stone and grouting particles were calibrated.Combined with the numerical simulation results and laboratory test results,the deformation and failure law of the soil-rock mixture sample and the evolution characteristics of deformation and contact force chain were obtained,and the failure characteristics of the soil-rock mixture grouting sample were obtained.(4)Based on PFC-FLAC coupling analysis method,a multi-scale analysis model of tunnel excavation in soil-rock mixture stratum was established to analyze the excavation stability of pre-grouting tunnel.Based on the strength reduction method,the PFC-FLAC coupling analysis method and the FLAC finite difference method are used to analyze the deformation and failure characteristics of pile-supported tunnels in the soil-rock mixed backfill area after grouting and excavation.The results show that the pre-grouting reinforced tunnel is relatively stable after excavation,and the pre-grouting reinforced tunnel can be divided into three stages: the rapid deformation stage,the slow deformation stage and the deformation stability stage.The safety factor of the tunnel reinforced by grouting is about 1.6,the Angle between the crack surface and the horizontal direction(large principal stress surface)caused by top collapse is about 65°.Cracks first appeared in the inner side of the arch waist in the pre-grouting reinforcement area,and then cracks began to appear in the inner side of the vault after the cracks were connected.If the tunnel has the tendency of collapse,the arch waist in grouting area will be damaged first.The continuous increase of the horizontal convergence value or the failure of the arch waist is regarded as a sign before the collapse.The tunnel completely collapsed suddenly,the collapse happened instantaneously.(5)The PFC-FLAC coupling analysis method and FLAC finite difference method are used to analyze the deformation and stress characteristics of the surrounding rock and supporting structure of the pile-supported tunnel,and the influence of the construction time of the secondary lining on the stress and deformation is discussed.The results show that the secondary lining is a whole settlement,and the local deformation features are as follows: the secondary lining vault is subsided,the middle of the invert weakly heave relative to the arch foot,the waist of the arch has horizontal displacement to the outside of the tunnel,and the upper part of the pile foundation weakly bends to the inside.The maximum tensile and compressive stresses of concrete are far less than the designed values of compressive strength of C40 concrete.The displacement of the key points of the secondary lining decreases linearly with the delay of the application time of the secondary lining,and the stress reduction of the key cross-sections of the secondary lining decreases with the delay of the application time of the secondary lining.When the secondary lining is applied after the convergence occurs 80%,the stress changes of the secondary lining and pile foundation are small.