In January 2020, contractor Guangdong No. 2 Hydropower Bureau Co., Ltd successfully broke through its Robbins 5.97m diameter Main Beam TBM in Guangxi Province, China, following the excavation of the first of two tunnels, an 11.9km long conduit for Lot 1 of the North Line Water Irrigation Project, Letan Water Reservoir, Drought-Relief.
The tunnel was marked by a gauntlet of challenges, from karst cavities to fault zones and water inflows. The contractor and the owner, Construction Management Bureau for the Letan Water Reservoir, had much to celebrate after completion of what is widely regarded as the most complex and longest tunnel on the North Line project.
Boring with the TBM and continuous conveyor system began in summer 2015. “There was no precedent in this province for using a Main Beam TBM to excavate a tunnel longer than 10km. We didn’t have relevant local experience to use for reference,” explained Yongjiu Jin, Deputy Manager of the Project for Guangdong No. 2 Hydropower Bureau Co., Ltd.The machine did encounter a number of difficult geological obstacles as it bored through limestone rock, but was still able to achieve advance rates up to 40m per day in good ground.
Much of the geology consisted of lightly weathered limestone in rock class II to III, with some sections in class IV to V rock that required the heaviest amount of ground support, ranging from rock bolts to ring beams and mesh. “Our team encountered a coal seam, gasses in the tunnel, two large water inrushes, three fault zones up to 103m long, 11 karst cavities, and more. In order to solve the ground problems, there were more than 160 special technical research meetings held,” said Yongjiu.
While the completion of the first tunnel—the longest single-heading construction on record for water tunnels in Guangxi—is a milestone, there is more to do. The Robbins machine will be inspected and relaunched to bore a second tunnel 4.2km (in length. The ground conditions are predicted to be equally challenging, but the tunnelling operation has some help from ground prediction methodology. Tunnel Reflection Tomography (TRT)—consisting of ground prediction using seismic waves—is being used to detect changing conditions ahead of the TBM. The method can predict the distribution and scale of joints and fissures, allowing the crew to plan ahead.
Located near Laibin City, the North Line project provides much needed drought relief using a network of tunnels totaling 29.4kk. “This tunnel will realize the dream of drought control that people in Central Guangxi have had for many years. The breakthrough is the most important milestone event in this first phase of the North Line project,” said Yongjiu.