Giant karst cave encounter in China Feb 2021

Passage through the Yujingshan Mountain in the middle of the Chengdu-Guiyang high-speed railway in Weixin County in Yunnan Province, required a 6.3km tunnel heading into one of the most intensive karst development regions in China. The region is also the home of the Miao and Yi ethnic minorities and is a sensitive ecological environment in which heavy construction work is difficult.

Fig 1. Giant karst cave and underground river system

With a design speed of 250km/hr, the railway will connect the city of Chengdu, the capital of Sichuan Province, and Guiyang, the capital of Guizhou Province and cut current rail travel time from 14 to three hours.

In July 2016, at about 2km from the portal and 60m below ground, a giant karst cave of 95m long x 230m wide x 50m-120m high was discovered (Fig 1). The deposits at the bottom of the cave are 30m-90m thick, with a scree slope of 30-40 degrees. At the lower side of the slope emerges a large underground river of 5m-15m wide that runs for 18km and has a flow of 70m3/sec in the rainy season.

The solution to continue though the route through such a giant and complicated karst cave was to suspend a structure near the roof of the cave, with a distance of 40m from the cave bottom and running 110m perpendicular to the water surface of the underground river. The conditions presented four major and difficult construction challenges.

Scale of the giant karst cave

The first was for construction safety. Tensile joints and fissures in the rock mass provided for poor stability around the tunnel, with frequent rock falls from the cave roof, some with a maximum diameter of 15m. These directly threatened the safety of the construction workers and equipment. How to ensure construction safety of the structure across the giant cave was challenge one.

The second challenge was for structure and rail operation safety. The tunnel is 40m above the bottom of the karst cave and the overhanging length is 90m. What kind of reliable structure should be selected to meet the strict track settlement control requirements of a high-speed railway was challenge two.

Thirdly, for such a giant karst cave and no matter the adopted solution, the engineering is extremely difficult and would take time. With just three years remaining before the programmed opening of the railway, how to work out a solution as soon as possible and execute it efficiently to ensure the timely opening of the line was challenge three.

Underground river at the bottom of the karst cave

Environment protection was the fourth challenge. Water resources in the project location are scarce. The underground river in the cave is the main water source for downstream villages and the only water source for a downstream hydropower station (Fig 2). How to keep the river flowing, maintain the balance of the existing groundwater network, and protect the original ecosystem was challenge four.

To address the challenges, all the stakeholders formed a team to overcome difficulties, and solved the construction problems through scientific and technological research and repeated many studies and comparisons of different solutions.

With a clear height of the cave of 120m, and the unstable deposits at the bottom of the cave, adopting a traditional solution of a scaffolding platform to protect the cave roof, and to build the railway structure inside, would be difficult, the duration would be long, and the number of construction workers needed high, with their safety not guaranteed. For this reason, the solution was to completely backfill the cave with spoil. The solution adapted to local conditions and recycled the drill+blast excavation spoil. The construction is the simplest, the number of construction workers is fewest, and there would be no casualties or damage to equipment during the construction.

Rock falls from the cave roof

In response to challenge two, the solution to the structural problem of creating a high-speed railway crossing in a giant karst cave was to construct a tunnel through the backfill body and then erect a large-span bridge inside the tunnel (Fig 3). High-pressure grouting was used to stabilize the backfill and surrounding rock and support excavation of the 432m² super-large tunnel section in five top-down benches. To compensate for the potential of uneven settlement and deformation in the foundation, the design imitated the spine of an animal, dividing the tunnel structure into 5m long segments set wide with deformation joints between adjacent segments.

The large-span bridge inside the tunnel was built in a 38m+108m+38m three-span continuous beam as the bearing structure to meet the limits of high-speed rail track deformation of not greater than 15mm.

Fig 2. Villages and a power station downstream depend of the underground river

An automatic monitoring system was installed and a compensation grouting system was established to control any differential settlement of the foundation.

In response to challenge 3, a series of fast construction measures were developed.

First, five adits were excavated on the vertical and at 20m between them, to complete the huge 1.08 million m³ backfill quantity of the 120m high void (Fig 4). The backfill was divided into five layers and completed layer by layer from the bottom-up, taking eight months to complete the operation.

To speed up the progress, a special side-wall formwork was created to cast side wall concrete, and a Bailey beam truss trolley was developed to cast the arch lining and bridge structure in the tunnel. This parallel construction of the tunnel lining and the cast-in-place bridge saved three months on the construction period.

To protect the environment and address the issues of challenge 4, a three-dimensional groundwater drainage system was built to maintain the original groundwater system. Before backfilling, a 450m length of the underground river was redirected to the outside edges of the cave. A layer of large rocks was placed in a 3m area around the outer walls of the cave at 20m above the original underground river level to create a three-dimensional seepage layer to channel fissure water around and back into the underground river system.

Fig 3. A bridge structure within a tunnel in complete backfill of the giant void carries the railway across the karst cave

Through the above measures, the construction and structural safety issues of an overhanging tunnel in a huge unstable cave were successfully solved and this guaranteed the smooth opening and operation safety of the Chengdu-Guiyang high-speed railway. This solved the technical challenges of creating an overhang karst tunnel, formed the construction technology of crossing a giant karst cave, and improved the ability of selecting a high-speed railway route in karst areas.

The spoil of the earlier heading excavation was recycled into the cave, thus saving arable land, and the three-dimensional groundwater drainage system, to maintain the original groundwater system and protect the ecological environment of the Miao and Yi communities, realized a harmony between infrastructure engineering and nature. The new line will greatly facilitate travel for local people and will promote sustainable economic development along the line.

Fig 4. Five adits were created to complete the bottom-up backfilling of the cave

Since the opening of the new line in late 2019 and the start of high-speed train operations, the structure across the giant karst care has performed successfully. The measured maximum settlement of the bridging tunnel at its invert is 7mm, which meets the high-speed rail structural safety requirements, and the water volume of the downstream underground river is stable, meeting the production and domestic water demands of the local villages and the hydropower plant.

In the 2020 series of the international ITA Tunnelling and Underground Space Awards, the Yujingshan Mountain crossing of the giant karst cave and the management treatment of the underground river as designed by the China Railway Eryuan Engineering Group and built by the No 5 Engineering Group Co, was the winner of the Overcoming the Challenge category. The China Railway Eryuan Engineering Group Co and the No 5 Engineering Group Co belong to the Chia Railway Engineering Company which was separated from the China Ministry of Railways about 20 years ago. In further national structural reforms, the former Ministry of Railways of China became the China State Railway Group Co in June 2019.