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| Study on backfill treatment of huge karst caves and settlement mechanism of super thick backfill |
| WANG Jun1，2，GUO Bijun1，FENG Guosen3，LIU Tongjiang2，SUN Yafei3，DUO Shengjun4，YU Mingyang3 |
| (1. School of Civil Engineering，Shandong Jianzhu University，Jinan，Shandong 250101，China；2. China Railway 14th Bureau Group Co.，Ltd.，Jinan，Shandong 250101，China；3. China Railway 14th Bureau Group Construction Engineering Co.，Ltd.，Jinan，Shandong 250101，China； 4. China Railway First Survey and Design Institute Group Co.，Ltd.，Xian，Shaanxi 710043，China) |
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Abstract Gaoshan tunnel of Qianjiang-Zhangjiajie-Changde railway goes through a huge karst cave with a long crossing distance (the affected area of the cave up to 124 m) and a large suspended height(the clearance under the tunnel floor up to 30–55 m)，and the cave has unfavorable conditions including large volume，complex structure，poor stability and high risk of falling rock，which makes the construction and treatment of the tunnel to be very difficult. In order to overcome the above-mentioned issues，a treatment scheme of “backfill with tunnel ballast + upper grouting” was proposed through comprehensive comparisons，and the settlement pattern of the backfill under static and dynamic loads was comprehensively studied by using field monitoring，numerical simulation and theoretical analysis. The monitoring results during the construction period show that the surface settlement of the super thick backfill-body is mainly affected by the gravity and the load of the upper structure and the convergence rate of the settlement is fast. The settlement is mainly resulted from the lower un-grouted backfill and the surface part of the deposit at the bottom of the karst cave. A 3D numerical simulation model was established based on finite element method to simulate the influence of the dynamical load of trains on the super thick backfill-body. The simulation results show that，due to that the transfer of the dynamical stress of the train to the super thick backfill is largely reduced because of the exist of the ballast track and the 3m-thick reinforced concrete subgrade and further blocked by the upper grouting layer of the backfill body，the un-grouted layer of the backfill body is nearly not affected by the dynamic load，that is，the dynamic load of the train will not cause an additional settlement value of the backfill. The engineering practice shows that the backfill treatment of the huge karst cave in Gaoshan tunnel of Qianjiang-Zhangjiajie-Changde railway is reliable，that the settlement after the construction is under control and meets the design requirements. The proposed technology can be adopted as a reference for other similar kind of projects.
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2020, Vol. 39 
