Research On Backfill Disposal And Settlement Control Technology For High-speed Rail Crossing Giant Karst Caves

With the rapid development of China’s economy,the high-speed rail network continues to expand to mountainous areas in southwestern China,Most of these areas are karst geology with extremely complex ambient conditions,the construction of long tunnels will inevitably pass through karst caves of all sizes.The "cavern ballast backfill + upper grouting" disposal scheme was adopted for the first time in the huge karst cave of the Guizhou Chang Chang railway tunnel.The super thick backfill body was formed after the disposal,and the settlement problem was remarkable.Based on this project,this paper makes a systematic study on the scheme comparison and selection before the disposal of the karst cave,the settlement of the super thick backfill and its control technology after the disposal.This study provides reliable experience and scientific guidance for the future Karst Tunnel Engineering in similar areas and conditions,and has important technical value and economic significance for solving the major engineering problems of the tunnel crossing the giant karst cave.The research contents are as follows:1.The risk of giant karst cave in Gaoshan tunnel is evaluated.According to the evaluation results and the characteristics of the cave,11 kinds of cave disposal schemes are put forward.The analytic hierarchy process(AHP)is used to establish the comparison and selection model of cave backfill disposal schemes,Considering many factors,the disposal scheme of "processing tunnel ballast backfill + upper grouting reinforcement" is the best one.The safety protection measures during the implementation of the scheme and the optimization measures of the tunnel lining structure in the karst cave affected area are put forward;2.Analysis of settlement monitoring datas and numerical calculation results show that:(1)The settlement development law of super thick filling body is greatly affected by construction load,and has a large correlation with the construction load location.The settlement during the construction period accounts for more than 90% of the total settlement;(2)The settlement of the filling body is mainly caused by the compression deformation of tunnel ballast layer without grouting and the later accumulation body,which accounts for 65% of the total deformation;(3)The longitudinal differential settlement of the filling body increases with the increase of the construction load,which shows that the settlement in the center of the line is the largest,the settlement at both ends of the line is the smallest,and because of the different filling thickness of the tunnel ballast on the longitudinal section,the "settlement tank" is slightly inclined to the small mileage side;(4)Due to the constraint of the side wall of the karst cave,the settlement of the filling body near the side wall of the karst cave is large.3.Based on the measured data,a variety of prediction methods are used to predict the settlement of the filling body.The exponential curve model is the best fit for the settlement development situation,and the predicted the post-construction residual settlement meets the railway standard’s requirement.A double exponential settlement prediction model considering grouting reinforcement is put forward,which can well predict the development law of backfill settlement in grouting.4.Based on the practice of cave backfill disposal project,a variety of settlement control technologies are put forward.During the construction period,technologies such as layered compaction,grouting reinforcement and preloading reinforcement can be used to control the settlement of the filling body;after the construction,technologies such as tunnel reserved clearance,grouting at the bottom of the subgrade board or structural adjustment of the subgrade board can be used to control the problem of slab bottom void,settlement over limit or uneven settlement.5.The grouting reinforcement design and defect analysis of the upper tunnel ballast backfill are described in detail,and It is found that the main reason for the defect is the poor forming of the surrounding grout-stop wall;The corresponding treatment measures are put forward to make the grouting effect meet the standard’s requirement;...