Ground Support for High Cover Tunnels: What is the Best Method?
Conditions in high cover tunnels are often challenging. At the Olmos Trans-Andean tunnel, crews face 2,000 m (6,500 ft) of hard quartz porphyry, andesite, and other volcanic rocks under high stress. At two rail tunnels in China’s Gansu Province, open-type machines are boring beneath 1,400 m (4600 ft) of weak phyllite and sandstone.
Ground support, particularly in deep tunnel projects, is a highly debated topic. However, recent successes have pointed to several systems that work in a variety of situations. The Robbins Main Beam at Olmos was launched in 2007, and began to experience significant rock bursting and cathedralling under sections of high cover. “We encountered a 1,200 m (4,000 ft) section of highly fractured ground, with over 8,500 recorded rock bursts. Cavities also formed during tunneling that had to be injected with grout,” said John Simm, Robbins Field Service Superintendent.
To better contain the fractured rock, the machine’s roof shield fingers were removed and replaced with the McNally Support System, supplied by C&M McNally Engineering of Toronto, Ontario, Canada. The McNally support System provides the benefit of continuous support along the roof area of the tunnel, which protects workers from falling rock. The system works by replacing the curved finger shield plate for a curved assembly of pockets with rectangular cross-sections. Steel or wood slats are extruded from the pockets and bolted to the tunnel crown using a steel strap, effectively supporting and minimizing collapsing ground. As the machine advances, slats are reloaded into the pockets.
The below video shows the crew loading McNally slats into the pockets:
The system successfully contained numerous high-pressure rock bursts at Olmos, as seen in the below video courtesy of Dean Brox from Hatch Mott MacDonald:
For the machines at China’s West Qinling tunnels, the answer is a versatile ground support system that can be adapted depending on the ground conditions. The two 10.2 m (33.5 ft) diameter Main Beam TBMs are the first Robbins Main Beam machines built with specialized ground support and no finger shields. Instead of the roof fingers, the machines are designed with mesh pockets beneath a protective shield for workers to safely install mesh panels.
The machines can also use the McNally Support System if needed. Modified pockets can be bolted inside the mesh windows, allowing space for short slats of steel or wood.
In addition, streamlined materials handling and separate roof and probe drills allow for improved progress in high cover conditions. Redesigned ring beam installers allow for loose assembly and expansion against the tunnel walls, and can be easily converted to install steel straps rather than full rings.
Both machines are currently excavating 16.6 km (10.3 mi) long rail tunnels.
For more information on ground support in large diameter tunnels, check out this recent technical paper: Large Diameter TBMs_WTC2010
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