World's largest double-shield hard-rock TBM completed for 18.4 km Sichuan expressway tunnel

China has completed what is described as the world's largest-diameter double-shield hard-rock tunnel boring machine, developed for excavation of the 18.4 km Yuexi Tunnel on the Jinkouhe-Xichang Expressway in Sichuan Province.

The Yuexi TBM has an excavation diameter of 13.01 m, is approximately 220 m long and weighs around 3,500 tonnes. It was completed at a manufacturing facility in Xinjin, Chengdu, before deployment to the mountainous alignment in southwest Sichuan.

The machine has been specifically designed for difficult geological conditions expected along the tunnel, including high ground stresses, fractured rock and variable rock quality. Its double-shield configuration allows excavation and tunnel support activities to proceed in parallel where ground conditions permit, improving advance rates compared with more conventional excavation methods.

The cutterhead is equipped with 83 19-inch disc cutters that break the rock under high contact forces as the head rotates. Hydraulic thrust systems then drive the machine forward, while excavated material is removed from the face and permanent precast concrete segments can be installed behind the shield.

One of the main advantages of the double-shield arrangement is its ability to adapt between more competent and weaker ground. In stable rock, the machine can brace against the tunnel walls and maintain continuous excavation, while alternative support and thrust arrangements can be used where fractured formations reduce the ability of the surrounding rock to provide reaction.

The TBM also incorporates systems for geological probing ahead of the cutterhead, allowing construction teams to assess changing rock conditions before the machine reaches them. This capability is particularly important on a long mountain tunnel where faulted or fractured zones may create sudden changes in excavation behaviour.

Guidance is controlled through an automatic laser-based system designed to limit measurement errors to approximately 3 mm, supporting millimetre-level alignment accuracy over excavation distances greater than 10 km.

The machine is expected to achieve monthly advances of more than 380 m under suitable conditions. The planned mechanised excavation method is reported to be between three and five times more efficient than relying solely on conventional drilling and blasting for the main tunnel drive.

The Yuexi Tunnel forms part of the Jinkouhe-Xichang Expressway, which crosses mountainous terrain between Leshan's Jinkouhe District and Xichang in Liangshan Yi Autonomous Prefecture. The project represents the first reported use in China of a large-diameter double-shield TBM for the main bore of an expressway tunnel, a machine type more commonly associated with railway and metro projects.

Its record dimensions are significant, but the greater engineering challenge lies in combining a very large excavation diameter with long-distance hard-rock tunnelling under high-stress and fractured geological conditions. Successful performance will depend on cutter wear, face stability, ground support, advance probing and the ability of the machine to maintain production as geological conditions change along the alignment.