TBM TUNNELLING | TECHNICAL
Above: Use of McNally support systems on Lot T5 of Liaoning NOW project, China
The McNally system was important to help deal with
the challenges, including preventing rocks falls on to the machine and increasing worker safety.
McNally System The McNally system was originally developed, designed and patented by C&M McNally almost 20 years ago and early on used wooden slats, being employed on dozens of projects in Canada (especially Ontario), says Robbins. The TBM manufacturer obtained exclusive license of use on its machines and has evolved the system to steel slats, and the significance of its value to TBM tunneling was highlighted by the Olmos experience. At Olmos, where squeezing ground and numerous
rock bursting events troubled the tunneling works, modification to the machines included in-tunnel welding of wear plates plus removing of roof shield fingers behind the cutterhead support in favor of, instead, a system of pockets for continuous installation of steel slats, then still a relatively novel ground support system for TBM tunneling. A major benefit is the provision of customizable
continuous support running in parallel and close together along the roof area of the tunnels, mainly, but can be extended along the sidewalls. Broken rock stays in location, which helps also to keep the natural rock arch. The strength and size of slats can be adjusted to suit varied tunneling, overburden and rock stress conditions. They can also be become part of the final lining of the tunnel, as in Olmos.
Diversion Tunnel challenges in China The McNally slats made a difference to progress of tunneling at Yin Han Ji Wei diversion tunnel, along with many other important systems. But even for them there were limits in the environment where there were some excessive stress deformation
and especially high energy rock bursts, requiring much extra excavation support – shotcrete, rockbolts, mesh, and steel ribs. Stress relief holes were also drilled in a 120° arc
across the crown. The rock bursts caused frequent damage to the
machine. Following passage of the TBM, in some sections of tunnel insitu concrete lining was constructed for long-term durability. Groundwater inflows only added to the difficulties
and there was a major flood in 2016. Probing ahead of the face helped to assess risk of rock burst (though not so much timing or severity) and of inflows, and when grouting would be necessary. High temperatures underground and severe humidity were further extreme challenges. Improved cooling and ventilation systems were added. A major review by experts from China and Robbins,
in 2016, concluded that a reasonable monthly progress rate should be only up to 240m, or half that contracted. The two drives were completed in late 2018 and early 2022, respectively.
REFERENCES ● Chorley, S. and Brinkerhoff, E. 2022. Unprecedented In-Tunnel
Diameter Conversion of the Largest Hard Rock TBM in the US. North American Tunneling (NAT) conference.
● Smading, S. (2017) . Large-Diameter 20-Inch Disc Cutters: A Comparison of Tool Life and Performance on Hard Rock TBMs. RETC
● Roby, J., Willis, D., and Askilsrud, Odd. G. 2012. Extreme Machine Modifications in Difficult Ground at the World’s Second Deepest Civil Works Tunnel. NAT
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