Study On Dynamic Response Of Super Thick Backfill Subgrade Of Giant Karst Cave Under Train Loadismic Reaction

In recent years,the rapid development of high-speed rail technology in China has gradually adapted to the construction needs under complex conditions.At the same time,the problem of foundation vibration caused by high-speed rail train has received increasing attention.In the process of railway tunnel construction,sometimes it will encounter giant karst caves.At present,the disposal method of backfilling is often used in construction.The super thick backfill subgrade formed after backfilling is the weak link in the railway subgrade,which may produce large settlement in the operation period,and even threaten the safety of the train.At present,the research on the vibration and long-term dynamic response of the super thick backfill Subgrade of the giant karst cave is not enough.Whether the super thick backfill subgrade can meet the requirements of the long-term operation of the train needs further demonstration.In view of the above problems,this paper studies the dynamic response law of the super thick backfill subgrade under the train load by using the similarity model test and numerical simulation method,which is based on the disposal project of the giant karst cave in the Gaoshan Tunnel of Qian-Zhang-Chang Railway.The main research work and achievements are as follows:(1)Based on the principle of similarity simulation,the similar constants with the geometric similarity ratio of 1:5 as the main control factor is derived.Combined with the engineering practice and laboratory conditions,a scale-down test model of dynamic response of high-speed railway subgrade is established.The test monitoring scheme is designed with vibration speed,vertical displacement,subgrade slab reinforcement strain,earth pressure and acceleration as the monitoring items.The half sine wave test loading scheme based on the CRH380AL train is developed.(2)The vibration response of the train through the super thick backfill subgrade of the giant karst cave section in a single pass was obtained through analysis.The vibration speed of the super thick backfilled subgrade surface increases with the increase of the train speed,and the variation amplitude of the speed response time history curve in a single cycle is approximately linearly related to the train speed.There is no obvious settlement before and after the load effect of the super thick backfill subgrade.The difference between the strains of the transverse and longitudinal reinforcement at the same position in the subgrade slab is small,and the rebar strain is more likely to make a difference in the direction perpendicular to the line than in the direction of the line.The dynamic earth pressure in backfill has obvious attenuation trend along the depth direction.The smaller the thickness of the subgrade slab,the more sensitive the vibration speed and the vertical acceleration of the surface layer of the backfill to the change of the speed,the more likely to cause the instability of the displacement response amplitude of the subgrade surface,the dynamic earth pressure and vertical acceleration peak of the backfill surface increase.(3)Through the analysis,the dynamic response law and dynamic settlement of super thick backfill subgrade with giant karst cave under long-term train load are obtained.The change amplitude of the speed response time history curve in a single cycle hardly changes with the number of load actions.according to the prediction and analysis,the final settlement of the super thick backfill subgrade under the train load is about 2.5mm,and the settlement after the train passes through the karst cave section for two thousand times will reach more than 90%of the predicted value.The settlement rate of each layer of the backfill will gradually decrease with the increase of the number of load actions.Grouting in the upper part of backfill can reduce the settlement caused by train load to a certain extent.The strain value of reinforcement in subgrade slab will fluctuate up and down within a reasonable range.The dynamic earth pressure inside the backfill will increase slightly with the increase of loading times.The peak value of vertical acceleration will increase in parabola shape in all layers of the backfill,and it will approach the convergence value faster in the upper backfill.The greater the thickness of subgrade slab is the smaller the action range of train load in backfill.(4)The distribution and variation of dynamic displacement,dynamic stress and acceleration in super thick backfill Subgrade of giant karst cave are obtained by numerical simulation.The most obvious response area of dynamic displacement,dynamic stress and acceleration is the position under the track of ballast surface layer,they attenuate rapidly in the ballast layer,with the maximum attenuation rate of 97%,80%and over 99%respectively under the design speed.Within the speed range of 150 km/h to 300 km/h,the maximum vertical displacement change of each position below the tunnel floor is not more than one thousandth of a millimeter.The dynamic displacement of the surface layer of the subgrade slab near the middle line of the tunnel and the place with large backfill thickness has a large response amplitude.The area of subgrade dynamic displacement response exceeding-0.02mm is all above the bottom surface of the grouting layer,which shows the rationality of upper grouting to some extent.The maximum response amplitude of the dynamic displacement at the design speed is about 0.66mm,and the speed of its attenuation along the depth is related to the backfill material.The maximum response amplitudes of dynamic stress and acceleration at design speed are 21.3kPa and877.61mm/s~2 respectively.At the same depth,the dynamic stress and acceleration increase with the increase of train speed.