Experimental Study On Dynamic Characteristics Of Soil-rock Mixture Backfill Under Subway Train Load

The main urban area of the mountain city Chongqing is divided by four mountains(Jinyun Mountain,Zhongliang Mountain,Tongluo Mountain and Mingyue Mountain)and two rivers(the Yangtze river and the Jialing river),so the available land resources of the city are limited.The population of the main urban area is dense,and in recent years there has been a continuous influx of large numbers of people into the city,resulting in a very crowded urban traffic.In order to alleviate the increasingly severe traffic pressure in urban areas,metro rail transit has been constructed on a large scale as a modern public transport vehicle with faster speed,timeliness,convenience and efficiency,large passenger carrying capacity,saving ground space,low noise and no cross-interference with other transportation systems.However,in the early urbanization process of mountain city chongqing,a lot of backfill areas of soil-rock mixture with loose structure,high porosity,low strength and poor engineering performance often are formed.Dense metro projects will inevitably pass through these deep backfill areas of soil-rock mixture.The backfilling time of soil-rock mixture backfill in Chongqing is not very long,its consolidation has not yet been completed,and the degree of consolidation varies,which will produce large settlement during the operation of the subway.If the settlement caused during operation is neglected,the long-term operation safety of metro will be affected.Aiming at the settlement problem of metro tunnel crossing backfill section during long-term operation,in this paper,the influence of different consolidation degree,rock content and effective consolidation confining pressure on the dynamic pore pressure,stiffness and cumulative deformation of backfill soil with soil-rock mixture under cyclic load of metro train is studied by adopting the combination of laboratory test and theoretical analysis method,and the corresponding empirical models are established.The main research contents and results of this paper are as follows:? A series of physical and mechanical properties tests of backfill under metro track were carried out to obtain the dry density and natural moisture content of soil,the dry density and saturation density of block stone,proportion of soil particles,stone content and limit moisture content of soil,which provided reference for subsequent backfill sample preparation.And the fine grained soil in backfill was verified to be silty clay.? The paper analyzes and determines the control conditions of the dynamic triaxial test of backfilled soil,such as consolidation stress,consolidation stress ratio,frequency,amplitude,cycle number and waveform of dynamic stress,and verifies that the biased sine wave can well simulate the metro train load.? Through the pore pressure test of soil-rock mixture backfill under metro train load,the relationship between excess pore pressure ratio ru and cyclic stress ratio rc under different cyclic times are obtained under different consolidation degree U,different effective consolidation confining pressure ?31 and different rock content P,based on which the corresponding threshold cyclic stress ratio rt is determined,and the influence of each factor on threshold cyclic stress ratio is discussed.At the same time,the relationship between the excess pore pressure Au and the number of cycles N under the respective factors was obtained.And the influence of various factors on the variation of excess pore pressure Au of backfill under cyclic loading of metro trains is analyzed and discussed.Based on the pore pressure test results,a pore pressure model related to the number of cycles N was established.?Through the stiffness softening test of soil-rock mixture backfill under metro train load,the effects of consolidation degree U,effective consolidation confining pressure ?31,rock content P and the number of cycles N on the variation of backfill soil stiffness under cyclic loading of metro train are analyzed.According to the test results,a stiffness softening-hardening model with the softening index 8 and the number of cycles N is established.? Through the strain test of soil-rock mixture backfill under metro train load,the effects of consolidation degree U,effective consolidation confining pressure ?31,stone content P and the number of cycles N on the accumulated strain of backfill soil under cyclic loading of metro train are analyzed..According to the test results of backfill samples with stable strain development model,when the degree of consolidation U=0.98,an accumulated strain model related to the number of cycles N,rock content P and effective consolidation confining pressure ?31 is established;when the rock content P=20%,an accumulated strain model related to the number of cycles N,consolidation degree U and effective consolidation confining pressure ?31 is established.