The grand scale of China and everything that it does has become an axiom for our times. If its economic growth falls below double digits, the rest of the world panics about another recession.

Much of this growth is underpinned by substantial investment in domestic infrastructure, especially since its 2008 stimulus package provided USD 586bn for new projects.

Rail and metro lines feature prominently among the projects currently under construction in the People’s Republic, and US-based TBM manufacturer Robbins has its machines deployed at multiple sites across the country.

While most of the machines are making their way through soft ground conditions, Robbins does have four main beam TBMs boring through hard rock.

These four TBMs are deployed on two projects with scales befitting China’s role as a modern behemoth:
• The excavation of twin 12km long tunnels for the first phase of Chonqing’s metro Line 6, one of three lines under construction in this mega city of 35 million people.

• The boring of twin 16.6km tunnels in West Qinling, Gansu Province, as part of an USD 11.5bn railway that will link the province’s capital Lanzhou with Chongqing.

The Chongqing Metro Line 6 tunnels were the first new lines to be excavated for the city’s metro system, which is planned to include a network of nine rail routes and approximately 30km of tunnels excavated by both TBM and drill and blast.

The routes will be both above and below ground to accommodate the hilly terrain of the city.

Excavating the Line 6 tunnels are two 6.4m diameter Robbins Main Beam TBMs, launched in winter 2009-2010. The two machines are boring under sections of very low cover ranging from 10 to 60m in sandstone from 12 to 50 MPa UCS.

As of December 2010 the machines were 6,664m into their twin drives and were stopped at the site of the fifth station (there will be 11 underground stations in total). Work on the station is still underway, and the machines are expected to start up again in March.

The machines have been achieving advance rates of up to 47m per day, and one machine has achieved a best month of 1,148m.

On the West Qinling scheme, being built by contractor China Railways 18th Bureau, the new railway will shorten transport times from 17.5 hours to 6.5 hours and enable an annual freight capacity of 100Mt. Trains will run on the double track lines at 160 km per hour, with a 50-train daily maximum. The entire railway is expected to open to traffic in 2014.

Two 10.2m diameter Robbins Main Beam TBMs are excavating 16.6km long tunnels from parallel launch chambers just 40m apart, with their back-up and muck haulage systems starting from bridges across a deep valley.

The jobsites are located on the lower reaches of Qinling Mountain, about 1,000m above sea level.

Each machine has advanced just over 2km, with an advance rate of up to 423m per month.

Difficult ground
Both projects for the most part use ground support methods traditionally associated with main beam TBMs: on Chongqing ring beams are being placed every 1.5m in soft sandstone and mudstone rock, along with a layer of shotcrete 100mm thick.

Though the sandstone is abrasive, and can wear the tips of the disc cutters to a point, the 19-in disc cutters on the machine have experienced relatively little wear. Each machine is going through about four cutters per month.

However, on both schemes, and West Qinling in particular, the contractor was concerned that it would encounter some extremely difficult ground conditions.

Ground in the West Qinling tunnels consists of 30 to 80 MPa UCS sandstone and phyllite rock beneath more than 1,400m of cover. Broken, fractured rock is expected in a 915m wide section of tunnel, with three large faults specifically identified. The faults, ranging in width from 190m to 310m in width, consist of fragmented limestone and sandstone with gravels and breccias. Some ground water and karstic features are also expected during the tunnel drives.

The solution was that on both projects the TBMs were supplied without finger shields.

Instead, there is a roof shield canopy containing mesh windows, for installation by workers under the safety of the shield structure.

Mesh windows, installed in the roof shield, allow workers to slide panels of steel mesh in the annular space between the shield and the tunnel crown from the safety of the shield structure. The panels are then pinned or secured with rock bolts. Traditional ground support includes no specific provisions for mesh installation and little cover from falling rock.

High cover tunnels such as West Qinling are host to a range of problems from squeezing ground to falling rock. In cases of severely unstable ground, tunnelling is hindered by structures such as the finger shield. The structures allow the rock to fall several millimetres before they are stopped, creating substantial problems with bolting back the falling rock.

In addition to the mesh and rock bolts, ring beams have been installed every 180 to 900mm, depending on the severity of ground conditions.

Ring beams are installed using an erector consisting of the assembly ring and expander. The rotating assembly ring is fixed axially and used to loosely assemble five ring beam components. Once the components are loosely assembled and pinned to the assembly ring, the expander, which moves fore and aft, expands the components to a preset pressure against the tunnel wall.

A sixth Dutchman piece is installed in the resulting space, and the ring beam with tightened connections is bolted to the tunnel wall. The assembly and expander can also be easily converted for installation of steel straps, rather than full rings.

In addition to the rock bolts, mesh and ring beams, the contractor opted to also install a 300mm thick continuous concrete lining for the length of each tunnel.

Both tunnels have commenced lining works, which consist of nailing a waterproofing membrane to the tunnel walls, followed by a 120m long concrete pour.

Ground support components are loaded onto the back-up using a crane, and placed onto a carriage riding on an electric transport car. The carriage is designed to hold a stack of mesh panels, ring beams, rock bolts and other materials. The remotely operated trolley carriage transports the materials to a rack located in front of the ring beam guide rollers where they can be easily placed.

McNally System
Should extremely poor ground be encountered, the mesh windows can be relatively easily modified to use the McNally Support System, patented by C&M McNally Engineering of Toronto, Ontario, Canada for exclusive use with Robbins TBMs. The McNally System utilises steel or wood slats to provide continuous support along the roof area of the tunnel, protecting workers from falling rock. Crews would bolt McNally pockets inside the mesh pocket structures, allowing a space to slide short slats of steel or wood into the area where roof drills can operate.

“By capturing the loose rocks that other systems allow to fall, we created a circular arch that was otherwise unavailable,” says Mike McNally, the man behind the system.

“The difference is that you can’t tighten a rock support system against rock using finger shield because the fingers are in the way.”

Being able to tighten rock bolts while loose rocks are still held up by the shield or metal slats traps those rocks that may have otherwise fallen. It leads McNally to make a bold claim.

“[The system] pretty much makes shotcrete unnecessary,” he says.

“Our slat drillage is an alternative membrane to shotcrete.”

Despite this claim, shotcrete is still being employed on both West Qinling and Chongqing Metro Line 6, for reasons which Robbins Far East Operations vice president Biyue Li says are purely financial.

“The McNally system has been technically well accepted, but on Chongqing it has yet to be used,” he says.

“When operating, the system requires a lot of rebar to be put into the pockets, which can become expensive. The Chinese contractors do not want to use it unless they have to.”

At West Qinling, where unstable ground is far more likely, the system has been used, but sparingly so as the machines are only 2km in to their 12km drive and the cover is still relatively low: 250 to 300m.

McNally came up with the system when working on a combined sewer overflow tunnel in Toronto in 2000, and it has been used on numerous projects in Canada since. The first time it was used outside Canada was in 2007 on the Olmos Trans- Andean tunnel in Peru.

“I am certainly a proponent of the McNally system,” says Dean Brox of tunneling consultant Hatch Mott MacDonald, who worked on the Olmos tunnel project.

“The system can be easily modified to comprise different capacity support components, making it ideal for varying rock conditions. In shallow tunnels with horizontal geology, for example, the system can comprise relatively low capacity support components that are easy to install. For deep tunnels where high stress conditions exist with possible rock bursting, the system can be enhanced with high capacity, larger components that can still be practically installed with TBM excavation.”

On West Qinling in particular, the system is likely to see more action later this year as the main beam TBMs progress in to the higher cover section with broken, fractured rock and large faults. It is here where the cost of rebar is likely to prove favourable to halting the drives to deal with rock bursts.

Li says he is confident the system will be used on further hard rock projects in the region – Robbins is currently in negotiation with China railways for it to be fitted to a main-beam TBM they have ordered for a project in Vietnam.

However, for it to be widely accepted, there needs to be a change of attitude in designers, not contractors, as the key challenge the system represent is to design principles, rather than cost.

“The system doesn’t really cost much more to put on the TBM – it only becomes really expensive if you are using a lot of rebar,” says Li.

“If the system is to be used more in China then designers have to be happy with the concept of allowing loose rocks to stay in the ground. Traditionally in China they have not allowed loose rocks in the ground, and contractors are required by the designers to take the loose rocks out.”


Two 10.2m diameter Robbins Main Beam TBMs with newly designed ground support systems were designed for China’s West Qinling Rail Tunnels The Robbins machines were launched on parallel 16.6km tunnels at China’s high cover West Qinling project in Spring 2010 Both Robbins machines were walked into 2,000m launch chambers, with the back-up and muck haulage systems starting from bridges across a deep valley Ground support in the high cover tunnels consists of ring beams, wire mesh, and rock bolts, with the McNally support system optional in unstable ground Final lining with concrete is taking place behind both Robbins TBMs at West Qinling Ground support on the Robbins Main Beam TBMs at West Qinling foregoes roof shield fingers for mesh installation windows in the roof shield and streamlined materials handling on an automated cart Workers can install mesh panels into the tunnel crown from below the safety of the roof shield The McNally System, which involves steel or wood slats being installed to hold back unstable ground, can be used on the Robbins TBMs at both West Qinling and Chongqing Metro Two 6.4m diameter Robbins Main Beam TBMs are currently excavating China’s Chongqing Metro Line 6, following their March 2010 launch Current ground support in the Chongqing Metro tunnels includes ring beams, rock bolts, and wire mesh, but the two Robbins TBMs can utilise a modified version of the McNally roof support system if unstable ground is encountered