The successful breakthrough of a variable diameter inclined shaft tunnel boring machine (TBM) on China’s Pingjiang pumped storage hydropower project marks a significant milestone not only for the scheme itself, but also for the wider underground construction and hydropower sectors. Delivered by China Railway Construction Heavy Industry Co (CRCHI), the achievement demonstrates how adaptable TBM technology is expanding the boundaries of what is possible in steep, complex underground environments .
The breakthrough was achievexd on the project’s No.1 diversion inclined shaft, a critical component of the underground water conveyance system at the 1.4GW Pingjiang station in Hunan province. Bored at an inclination of 50°, the shaft extends almost 1338m and plays a key role in managing water flow between conveyance tunnels and the powerhouse.
At the heart of the operation was a large, variable diameter hard-rock TBM manufactured by CRCHI and named Tianyue. During the drive, the machine’s excavation diameter was adjusted from 6.5m to 8m, allowing it to meet changing hydraulic and structural requirements along the length of the inclined shaft. According to CRCHI, this makes the machine a first-of-its-kind example designed specifically for steeply inclined shafts in hard rock conditions .
The engineering challenge was considerable. Unlike conventional horizontal tunnels, steep inclined shafts introduce significant risks related to machine stability, thrust control and slippage under gravity. To overcome this, Tianyue was equipped with a triple-gripping hydraulic interlock system. Three sets of grippers anchored into the shaft walls formed what CRCHI describes as an “anti-slip triangular matrix”, providing the stability needed to safely advance uphill.
During excavation, the mid and rear grippers alternated every 1.5m, providing continuous counter-thrust and maintaining precise control of the TBM’s position. The 87m-long, 900-tonne machine was thus able to maintain steady progress despite the steep gradient and changing excavation diameter.
The inclined shaft itself reflects careful hydraulic optimisation within the overall design of the Pingjiang project. On one end of the shaft, its geometry was designed to reduce water flow velocities during plant operation, minimising the risk of cavitation bubbles forming within the system. Cavitation can cause severe long-term damage to hydraulic structures, so designing it out at an early stage significantly improves operational reliability. At the opposite end, the shaft design allowed engineers to reduce steel usage by nearly one-third while still accommodating high hydraulic pressures over the lifetime of the plant. This not only delivered material savings but also reduced construction complexity and associated risks.
Addressing the challenges at Pingjiang
CRCHI says that the adjustable diameter TBM concept was developed to address exactly these kinds of challenges. By enabling a single machine to excavate two large-span openings of different diameters, the approach avoids the need for multiple TBMs or extensive redesigns. The manufacturer likens the solution to equipping the TBM with an “extendable skeletal structure”, combining flexibility with structural strength while helping to control overall project costs.
While the inclined shaft breakthrough is a standout technical achievement, it forms just one part of the much larger Pingjiang pumped storage project.
The project is being developed by State Grid Xinyuan Company, a wholly owned subsidiary of the State Grid Corporation of China, with an estimated investment of around $1.4bn. It is the second pumped storage project in Hunan province, following the 1.2GW Heimifeng plant, which has been operational since 2010
Construction began in August 2019 after the project received approval from China’s National Development and Reform Commission in 2014. The power station will be equipped with four 350MW reversible pump-turbine units housed in an underground powerhouse and designed to operate under a water head of 685m. The first unit is scheduled to come online by July 2026, with the remaining three units expected to follow in 2027.
The scheme is located on Fushou Mountain in Pingjiang County, about 135km from Changsha, the provincial capital. Its upper reservoir will be situated within the Fushoushan National Forest Park, while the lower reservoir lies 2.3km away in Jixing township. Both reservoirs are formed by large dams, creating active storage volumes of more than 6Mcm each.
Against this backdrop, the successful completion of the inclined diversion shaft using an innovative variable diameter TBM highlights the growing sophistication of underground construction technologies supporting China’s energy transition.
For CRCHI, the breakthrough provides a high-profile demonstration of how adaptable TBM design can reduce risk, improve efficiency and open new possibilities for complex underground hydropower infrastructure.
