## 2 Background of the Yujingshan Tunnel  
The Yujingshan tunnel is a crucial component of the Chengdu-Guiyang high-speed railway in the Weixin county, Zhaotong city, Yunnan province, China. Figure 1 shows the longitudinal section of Yujingshan tunnel and design profile of tunnel infrastructure. The tunnel is designed as 30% single-side uphill and one tube with two lanes tunnel, and the train speed limit is 250 km/h. As the Yujingshan tunnel is located in the transition region between Sichuan Basin and Yunnan-Guizhou Plateau, the terrain varies greatly in elevation, with a huge relative height difference of 50-650 m. The valleys of mountains are mostly southwest-northeast trending, which is consistent with the direction of the structure line. The tunnel is located at the downstream of Nanguang River, which is one of tributary of Yangtze River. The tunnel is mainly composed of east-west geological structures, crossing the structure line at different angles, the whole tunnel section passes through four faults [8]. Most sections of Yujingshan tunnel is buried at a depth of 200-300 m. The tunnel mainly passes through limestone, sandstone, mudstone, tuff and dolomite. For the section from mileage number of D3K279 + 865-D3K279 + 956, the tunnel passes through the giant karst cave hall. According to the geological survey report, the tunnel infrastructure with dimensions of 17.22 m \(\times\) 25.21 m. The study region can be divided into two layers, the first layer is dominated by the fragment stone, and the second layer is composed of limestone. The length of the tunnel is 6306.28 m from the entrance mileage D3K277 + 860 to exit mileage D3K284 + 164, and the thickest overburden is 350 m. The auxiliary excavation of parallel pilot tunnel is adopted, and construction organization is divided into three parts according to the entrance work region, parallel pilot tunnel work region and exit work region in Yujingshan tunnel. The entrance region and exit region employed open excavation method, and the New Austrian Tunneling Method (NATM) is adopted in the underground excavation section of the parallel pilot tunnel region. The geological survey shows that the Yujingshan tunnel mainly cross the coal measure strata and soluble rock strata, where the mileage number from D3K278 + 990 to D3K279 + 061 contains high gas concentrations, the mileage number from D3K279 + 061 to D3K279 + 500 has coal and gas outburst layers, and the Lower Permian Qixia Maokou formation is founded in the range of mileage number from D3K279 + 500 to D3K280 + 290. In July 2016, when the parallel pilot tunnel region was excavated along the small mileage direction of the tunnel to D3K279 + 872, an unprecedented huge karst cave in the history of tunnel construction of China was found. Geological survey shows that the karst cave hall is almost 100 m along the lane, and the width is almost 230 m. The roof of karst cave is present as the dome shape,  
Figure 1: Design profile of tunnel infrastructure. and the bottom of the karst cave developed on the Yujingshan underground river. Due to the extremely complicated construction condition of the karst cave, the excavation risk is intensely high. Figure 2 shows the geological profile of karst cave, the location of the Yujingshan tunnel developed Niuer slope No. 1 fault, Niuer slope No. 2 fault and Weixin fault, and the Yujingshan tunnel is mainly astride the roof of the karst cave. The geology condition of the karst cave region is dominated by soluble limestone, and the bottom of the karst cave hall filled with talus body which shows strongly layered characteristic. The height of the talus body is almost 78 m and 100 m on the left and right side, respectively, and the width of the talus body is nearly 134 m. According to the engineering construction scheme, the karst cave is backfilled with engineering spoil. The hydrological conditions around the tunnel are shown in Figure 3. The location of the Yujingshan tunnel is mainly distributed on Chishui-river, Nanguang-river and Zhavi reservoir.