Robbins main beam completes extremely challenging drive

The machine excavated 17.5km of tunnel in what Robbins describes as “some of the most difficult geology ever encountered”. The water diversion tunnel traverses the Qinling Mountains of Shaanxi Province, with up to 2,000m of cover.

“Every day was full of challenges,” said a representative for tunnel contractor China Railway Tunnel Group (CRTG).

The ground, consisting of mainly quartzite and granite, was estimated to have a rock hardness of between 107 and 309MPa (15,500 to 45,000psi) UCS, with high abrasivity and a maximum quartz content of 92.6%.

“This was, in my opinion, the most challenging project ever completed by TBMs, and it proves TBMs are up to overcoming even the most difficult conditions. I have great respect for the CRTG crews and management, and I thank them for moving TBM technology to a new level,” said Robbins president Lok Home.

During tunnelling, crews encountered more than 14,000 rock bursts, some with energy as high as 4,080kJ.

“Robbins’ overall equipment performance was excellent from the beginning to the end of breakthrough, and during seven years of excavation. This is despite the super hard rock with high quartz content, strong rock bursts, and substantial water inrushes,” said the CRTG representative.

Water ingress occurred 69 times during the drive, with some inflows extremely high – exceeding 20,000m³ of water in one day from a single point. In-tunnel ambient temperatures peaked at 40 deg C 90% humidity.

Rock bursting was controlled using steel slats in conjunction with the McNally crown support system, while zones of stress were predicted using a micro-seismic monitoring system. The micro-seismic system records rock stresses in a borehole 20m ahead of the face and predicts the potential for rock bursting following comparative analysis with similar rock burst data from other projects, as well as from nearby sections of tunnel in the Qinling Mountains.

Water ingress was controlled by dramatically increasing pumping capacity in the tunnel to 41,000m³ per day. Systematic probing ahead of the TBM was also used to detect water, as well as rock bursting. When ingress exceeded 70 per cent of the in-tunnel pumping capacity, crews then carried out grout injections.

The abrasive, hard rock was another challenge addressed by Robbins through the use of Extra Heavy Duty (XHD) 20in disc cutters that provided long cutter life and lower wear compared to standard 20in discs. The crew also optimised TBM operation with lower production rates when needed.

“Especially with such a huge challenge, a strong cutterhead is required to ensure production. The quality of Robbins’ cutterhead has been proven. The cutterhead can still work properly after the tunnel breakthrough,” said the CRTG representative.

The tunnel is part of two other sections of an 82km-long tunnel linking the Hanjiang and Weihe Rivers in Shaanxi province. The completed tunnel, for owner Hanjiang-to-Weihe River Valley Water Diversion Project Construction Company, will secure a water supply for towns and agricultural areas in central China and also generate hydroelectricity.