Chongqing — In mountainous Chongqing, hazardous rock formations pose persistent risks to communities and transport routes along the Yangtze River and its tributaries. In 2023, the municipality launched a three-year campaign to mitigate risks from 431 identified hazardous rock formations in the Three Gorges Reservoir Area.
Reinforcement: “lock the top, cinch the middle, brace the base”
Chongqing engineers tailor their approach to each hazardous rock formation, using a method described as “locking the top, cinching the middle and bracing the base,” said Zhang Zhihua, deputy head and chief engineer of the Chongqing Bureau of Geology and Minerals’ 208 Geological Team.
“Locking the top” involves driving anchor cables into loose rock high on the cliff and securing it to the mountain. “Cinching the middle” uses anchor rods and grout to form a stabilizing belt around the rock mass, while “bracing the base” reinforces weakened rock at the bottom with grout.
The near-vertical cliffs above the river present another challenge: creating a safe platform for construction crews.
Diaozui is a hazardous rock formation in Qutang Gorge, Fengjie County, rising nearly 200 meters above the Yangtze River. Engineers adopted cantilevered scaffolding modeled on Chongqing’s traditional diaojiaolou, or stilt houses, said Wang Xu, head of the Diaozui rockfall-prevention project and deputy general manager of Chongqing Nanjiang Survey and Design Co.
Crews drove parallel rows of anchors into the cliff to form a platform extending outward from the rock face. Planks, scaffolding and safety nets were then installed above it.
The cantilevered structure at Diaozui in Fengjie exceeded 50 meters. At a separate site in Longmenzhai, Wushan County, 60 meters of cantilevered scaffolding was combined with an 80-meter ground-supported structure, bringing the total height to 140 meters, a scale rarely seen in China. A special technical review was conducted to ensure safety.
“Steel jacket” defense against rockfalls
Completed in 2021, an earlier project beside the Wujiang River illustrates how engineers reinforced a similarly unstable cliff to protect the Yuhuai Railway, also known as the Chongqing–Huaihua Railway, from rockfalls.
Opened in 2005, the Yuhuai Railway is a typical mountainous rail line winding through rugged peaks and deep valleys. Along a 360-meter stretch of cliff, unstable rock formations pose a significant hazard. The most dangerous section involves a 25,000-tonne rock mass separated by cracks up to 1.4 meters wide. Without stabilization, further disturbance could trigger a collapse.
To stabilize the cliff, workers drilled deep anchor holes and secured the unstable rock mass with steel cables before installing active rockfall protection nets. The project also includes reinforced concrete frames and curtain nets to provide additional support and prevent loose rocks from threatening train operations.
Depending on the condition of each unstable rock formation, the project employs a range of stabilization measures, including crack grouting, prestressed anchor systems, and rock mass monitoring and early warning systems.
Incorporating multiple advanced engineering technologies, it is regarded as one of the most challenging rockfall prevention projects of its kind in China.
(Li Jiayan, as an intern, also contributed to this report.)