Mechanical Characteristics Of Expansive Soft Rock Tunnel Support Structure And Slow-Release Method

The phenomenon of tunnel supporting structure cracking due to argillization and swelling of swelling soft rock is common.The reason is that the surrounding rock of swelling soft rock tunnel swells when encountering water,and the swelling force acts on the support structure in the way of stress concentration.Therefore,it is necessary to study the mechanical characteristics and sustained release method of the supporting structure of the swelling soft rock tunnel in view of the engineering problems such as the deformation and instability of tunnel surrounding rock,the invasion and cracking of the supporting structure,so as to provide theoretical support for the tunnel construction in soft rock areas.Based on the swelling problem of mudstone,this thesis studies the mechanical characteristics and sustained release method of the supporting structure of swelling soft rock tunnel.The swelling characteristics of mudstone and remolded granular bentonite were analyzed through constant volume swelling test of mudstone and remolded granular bentonite using aggregate bentonite and quartz sand as materials of buffer layer,which provided support for physical model test and numerical simulation of surrounding rock swelling force and buffer layer swelling force source.Indoor tunnel scale physical model test and numerical simulation are carried out to compare and analyze the mechanical characteristics of supporting structure of swelling soft rock tunnel,based on the theory of tunnel mechanics and elastic-plastic mechanics,combined with the analysis of experimental data,the expression of sustained release coefficient of supporting structure in swelling soft rock tunnel is deduced.The main results are as follows:(1)Through the swelling force test of mudstone and remolded granular bentonite under constant volume,the following conclusions are drawn.(1)The swelling stress time-history curve of mudstone under constant volume generally shows a rapid increase in the early stage,a deceleration increase in the middle stage,and finally a stable value;the swelling force of remolded granular bentonite increased rapidly to the peak at the early stage and then decreased slightly,and then increased to a stable value.(2)The ultimate swelling stress of mudstone is negatively correlated with the initial water content,and positively correlated with the saturated water absorption.The ultimate swelling stress of remolded granular bentonite was positively correlated with saturated water absorption.(2)Based on the similarity theory,the physical model test of the supporting structure of swelling soft rock tunnel is designed.The comparative test of the physical model without and with buffer layer is carried out,and the mechanical characteristics of the supporting structure under two working conditions are compared and analyzed.The results show that:(1)The trend of swelling stress time-history curve of surrounding rock of tunnel before and after adding buffer layer is basically the same,and the difference of ultimate swelling stress is small,which provides the same stress environment for the deformation comparison test of supporting structure,and makes the subsequent comparative analysis more reliable.(2)The supporting structure before and after adding buffer layer is shown as vault sinking,the bottom of the arch uplifting,and both sides of the arch waist deformation inward,and the maximum deformation is located at the vault.(3)Through the comparative analysis of two physical model test data,it can be seen that the displacement of vault and arch bottom after adding buffer layer is smaller than that before adding buffer layer,and the sustained release coefficien of tunnel supporting structure strain is 30%-50% by calculation.(3)The 3-D numerical model was established by MIDAS GTS NX software and the swelling time-history load of mudstone and remolded granular bentonite was introduced to analyze the stress and deformation characteristics of the supporting structure of swelling soft rock tunnel under swelling load.The numerical simulation results show that:(1)The trend of strain time-history curve of supporting structure before and after adding buffer layer is consistent,which shows rapid increase in the early stage,lasting about 1 hour,and then slow increase until the swelling is stable,reaching the limit swelling strain.(2)The swelling strain and displacement of the supporting structure after adding buffer layer are lower than those before adding buffer layer.The sustained release coefficient of the strain of the supporting structure vault reaches 78.36 %,and the displacement sustained release coefficient of the supporting structure vault reaches 43.64 %.(3)The displacement vector diagram of numerical simulation shows that the vault sinks,the bottom of arch uplifts upward,and the arch waists on both sides of the middle deform inward.(4)Through the comparison and analysis of physical model test and numerical simulation,it can be seen that the variation law of support structure strain under the two research methods is generally consistent,and the sustained release effect of support structure strain by numerical simulation is more obvious than that by physical model test.Based on the analysis of physical model test data and the theory of tunnel mechanics and elastic-plastic mechanics,the expression of sustained release coefficient for supporting structure of swelling soft rock tunnel is deduced.