Study On Settlement Mechanism Of Backfill Subgrade Of Railway Tunnel Crossing Giant Cavity

The huge karst cave of the alpine tunnel is composed of three parts: main karst fissure passage,hall corridor and branch 1.The tunnel passes through hall corridor at a high level with a length of 124 m,a width of 32~63 m and a height of 46~65 m.It is the first karst cave crossed by a 100-metre tunnel in China.Its karst treatment project is one of the key projects of Qian-Zhang-Chang Railway.The karst cave is filled with ballast,which is the first time used in China.The thickness of the filling body is large,and the total thickness of the original accumulation at the bottom of the karst cave is more than 100 m.The settlement of the backfill roadbed is particularly prominent.In this paper,by means of on-line monitoring,numerical simulation and theoretical calculation and analysis,the settlement of backfill subgrade under giant karst conditions is studied in the following aspects:(1)The schemes of avoidance,bridge span and backfilling of karst cave treatment are compared and analyzed.Finally,the treatment scheme of ballast backfilling +upper grouting in backfilling scheme is adopted.The scheme can provide a stable foundation for tunnel structure,and reduce the impact of karst cave collapse on tunnel during operation.It has the advantages of simple construction technology,low construction cost and less construction before comprehensive protection.By strengthening the structure of the upper open tunnel,the safety of high-speed railway operation can be ensured.(2)The analysis of surface monitoring data shows that the development of backfill settlement can be divided into two stages when the upper load increases:(1)The rapid development of backfill settlement,from the establishment of monitoring system to the completion of side wall construction(0-125 days),belongs to this stage.In this stage,settlement accounts for more than 60% of the total settlement.(2)Settlement develops slowly to the stage of gradual stabilization,and it belongs to this stage after the completion of side wall construction(126-382d).In this stage,the settlement of backfill continues to increase,but the settlement rate gradually decreases.(3)Pearl curve method,exponential curve method and hyperbola method are used to predict the settlement of backfill.The predicted values of the three methods are highly correlated with the measured values.The mean square errors of the two-point Pearl curve model are 0.56,2.77,the exponential curve model is 5.62,9.99,and the hyperbolic model is 0.72,44.41.The Pearl curve model has the highest prediction accuracy and the best stability.The final settlement predicted by three methods is BU1: 231.67 mm,259.76 mm,218.86 mm;BM5: 225.18 mm,313.74 mm,211.36 mm.(4)The analysis of stratified monitoring data shows that the compression of graded gravel layer(0-5m)and ballast grouting layer(5-25m)under the upper load is small;the compression of backfilling ballast below the ballast grouting layer and the deposit at the bottom of karst cave(below 25m)is relatively large,which is the main source of settlement.The numerical simulation analysis of settlement also shows that the settlement mainly comes from the untreated soil below 25 m.After the completion of construction,the compression of this part of the soil is 44.10 mm,accounting for76% of the total.(5)By changing the elastic modulus of the soil layer,the influence of the change of the stiffness of each soil layer on the settlement is analyzed.The stiffness change of the bottom soil layer(61-85m)has little effect on the top settlement,and the closer the soil layer is to the top,the more obvious the effect of the stiffness change on the settlement.When the elastic modulus of the bottom layer increases by 80 Mpa,the settlement at the top of graded gravel layer decreases by 3.21 mm;when the elastic modulus of the middle layer(25-61 m)increases by 80 Mpa,the settlement decreases by 13.71 mm;and when the elastic modulus of the upper layer(0-25 m)increases by80 Mpa,the settlement decreases by 13.90 mm.This shows that for high fill subgrade,the key to control settlement is to ensure the backfill quality of the upper soil layer.(6)The dynamic characteristics of backfill subgrade under train dynamic load are analyzed by finite element software.Because of the existence of reinforced concrete slabs with a thickness of 4.8m(3.0m thick subgrade + 1.8m thick inverted arch)in the lower part of ballast layer,the dynamic stress in the ballast layer with a thickness of0.8m has attenuated nearly 84%,and when it reaches the top surface of graded gravel layer,the dynamic stress is close to zero;the dynamic displacement and acceleration attenuate faster,so the train can be considered.Dynamic load has no effect on backfill,and the deformation of backfill subgrade under dynamic load mainly comes from the accumulative deformation of ballast.