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Beyond engineering

Chengdu-Guiyang High-speed Railway—Yujingshan Mountain Tunnel Crossing Giant Karst Cave and Underground River

The project takes place in a section of Yujingshan Mountain Tunnel that encounters a giant 1.08 million m karst cave hall and 18km long underground river (on the Chengdu-Guiyang High-speed Railway Line). The major part of the project was the construction of a tunnel crossing a giant karst cave and underground river.

With a designed speed of 250km/h and a length of 515km, the Chengdu-Guiyang High-speed Rail Project (CG HSR) is a key part of the “Lanzhou (Xining)-Guangzhou Corridor”, and one of the 16main high-speed railways (8 horizontal lines & 8 vertical lines) in China. With a height of 50m ~ 120m, the domed cave hall is located 60m below the ground. It is 95m inlength (in the rail line direction) and 230m in width. In terms of area, it is equivalent to three standard football pitches.

The project faces four major challenges as it crosses the giant cave and underground river system rarely seen anywhere else in the world :
- With the risk of cave wall collapse, the first challenge is to ensure the safety of construction personnel and equipment in the cave.
- With the overhanging rail line and the unstable deposition at the bottom of the cave, the second challenge is to determine what kind of engineering structure should be adopted to cross the cave hall safely and economically.
- With a high and giant karst cave and limited working faces for tunnel construction, the third challenge is to complete the project on time.
- With the need of protecting environment and drinking water resources from polluting, the fourth challenge is to maintain the balance of the existing underground water network and the original ecosystem.

Several solutions were set up : design and construction methods to avoid project interruption/pause, large-volume karst cave backfill technology and the "tunnel + bridge" integrated structure, Construction technology innovation to ensure construction safety and meet the deadline and layered groundwater discharge system is designed: underground river is discharged through diversion channels; fissure water seeps from rock layer around the cave, and rainy-season floods are discharged from the spillway.