Northeastern china’s Liaoning Province and its largest city, Shenyang, are experiencing the worst drought in over 60 years. With just 102mm of rainfall in the summer of 2014, the Chinese government resorted to what it calls ‘artificial rainfall’, consisting of planes flying over farmland and dispersing large amounts of water. Even with those measures, the larger problem of drought in the area requires long term planning to fix. The Chinese government has done just that, by investing in a massive network of water transfer tunnels totaling over 120km in length for irrigation and drinking water. Excavated entirely below ground, the mammoth conduit is testing the limits of TBM design and excavation.
Super-long Water Tunnels
The Chinese government commissioned the project, known as the Liaoning NOW Water Transfer Project, in nine lots, designated T1 through T9 (for Tunnel No. 1 to 9, etc.). Each lot, except for T7, is utilising a TBM to bore two sections of tunnel ranging from 6.5km to 8km in length each. Lot T7 is utilising drill and blast. Lots T1 and T2 purchased new Herrenknecht Main Beam machines. Contractor SinoHydro Bureau 3, responsible for lots T3 and T4, elected for new Robbins Main Beam TBMs, 8.53m in diameter. Similarly, T5 contractor Shanxi Hydraulic Engineer Construction Bureau ordered an 8.53m Robbins Main Beam. Chinese equipment supplier NHI contracted with Robbins to supply Main Beam machines of the same diameter for T6 and T8, and a rebuilt Robbins machine was provided for lot T9. All eight machines, including the Herrenknecht TBMs, were ordered with Robbins continuous conveyors for muck removal. The difficult and long tunnels were found to pass through mainly granite and migmatite geology of varying abrasivity. Mountainous terrain including valleys and rivers meant the machines needed versatile ground support. Cover was found to vary widely, from as little as 97m to as high as 590m at T6.
Long Distance TBM Design
In order to design machines for such conditions, consideration must be given to the harsh aspects of tunneling in hard rock over long distances. "This can be broken down into what can be done at the TBM design stage, and what can be done during TBM operation and maintenance," said Brian Khalighi, Robbins Vice President-Engineering.
Cutterhead and cutters
Khalighi emphasised that much of the design centers around areas directly in contact with the rock face, namely, the cutterhead and cutters. High strength materials, wear protection on the cutterhead, and cutter spacing all affect cutterhead wear in dramatic ways. The cutterhead should be designed with regular cutter inspections and changes in mind. In terms of rolling disc cutter design, larger diameters are manufactured with larger bearings capable of withstanding heavier loads while also offering more wear volume. Larger (19-inch or 20- inch) cutters are preferable to smaller cutter diameters, such as 17-inch.
Main bearing and seals
Large diameter main bearings, with the largest possible bearing to tunnel diameter ratio, are capable of withstanding more load impacts and give longer bearing life. Robust seal design is also essential. "We have a proven seal design using hardened wear bands," said Khalighi. "Many other manufacturers don’t use wear bands, and so as the TBM operates, it wears a groove into the seal lip contact zone. We have a sacrificial wear band that can be switched out or replaced, making repairs easier." The abrasion-resistant wear bands can be changed in the tunnel in the unlikely event of excessive wear, or can be relocated on the bearing to ensure that damage is not done to the TBM structure itself on long drives.
Load path
A uniform load path, from cutterhead to main bearing to cutterhead support, is always desirable. However for long distance tunneling, the load path can be crucial as high stresses occur wherever the load path shifts. A cutterhead with a cone-shaped rear section can help with this problem by evenly distributing the load across the circumference of the main bearing.
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The path of muck, from the muck bucket to the chute to the machine belt conveyor, must also be as smooth as possible. Smooth muck flow not only increases efficiency, but also prevents the problem of re-grind on the cutterhead. It can also reduce wear on both the external and internal surfaces of the cutterhead.
Similarly, a smooth muck flow path all the way out of the tunnel is critically important. Use of continuous conveyors limits downtime as compared to the downtime experienced when a locomotive and muck cars are used. As a tunnel gets longer, the time to transport muck cars in and out of the tunnel becomes less and less efficient.
Maintenance
Once the machine has been built, regularly scheduled maintenance based on tunnel length and geological conditions is key. Gear boxes, for example, may be designed for long tunnels but if it is known that the tunnel length will exceed the life of the gear boxes then planned refurbishment should occur during tunnelling. Similarly, planned cutterhead changes and inspections should be a regular part of maintenance.
One last important component of machine maintenance in long tunnels is oil. Fluids and hydraulics such as lube oil must be regularly checked for their quality and levels.
Versatile Ground support
A flexible ground support system was provided for all six Main Beam TBMs at Liaoning due to the long tunnel lengths and variable conditions. The machines are capable of installing a wide variety of ground support, from wire mesh and rock bolts to ring beams and McNally slats. The TBMs are the "first to be designed with McNally pockets during fabrication, and the first to use 20-inch disc cutters in China," according to Andy Ju, sales manager for Robbins China.
The McNally roof support system, developed by C&M McNally for exclusive use on Robbins TBMs, is a unique solution: In the past, roof support fingers provided limited protection to the crew working at the front of the machine and also to a degree prevented damage to cutterhead drive motors and other equipment installed on the front of the machine. However, when poorer or blocky ground conditions were encountered these fingers would simply bend out of shape and more often than not the contractor would end up removing them altogether.
With the removal of the roof support fingers, the bored tunnel is exposed at the back of the roof support where more effective ground support is easier and quicker to install. The fingers are replaced with a curved assembly of pockets known as McNally pockets, and as the TBM advances, workers load the pockets with steel slats that are then extruded during a TBM stroke. The slats are bolted in place, providing continuous and maximal support to the tunnel crown in difficult rock conditions.
Several other unique aspects were designed in order to accommodate multiple ground support options within an 8.53m diameter space. Materials handling takes place in the tunnel invert, requiring a 180-degree rotating backhoe scoop that can be moved out of the path of the cart. A bridge crane and jib crane pick up materials such as mesh panels, new disc cutters, etc. and transfer it to the bridge area. Invert cleaning is ongoing when the cart is not in place.
The ring beam erector and roof drill system are both mounted on the same rail system, but are capable of independent movement. The ring beam erector consists 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. Previous assembly methods required that the fully assembled ring beam be transported to a pocket before being expanded against the tunnel wall. The method is not as fast, and does not give the flexibility often needed in changing ground that may require steel straps.
TBM’s fly through hard rock
The machines were launched between October 2013 and February 2014 from adit tunnels, with the exception of the refurbished T9 machine, which was launched earlier in 2013. Robbins Field Service were at the site providing mechanical, electrical, and hydraulic system supervision, TBM operation during site assembly and commissioning, and training on proper operation and maintenance of the machine.
Each machine will break through into chambers on its first section of tunnel, and will ultimately break through into a disassembly chamber underground.
As of 23 August 2014, the T9 machine had broken through into its first adit chamber after having bored more than 7km. T5 had progressed the second farthest, at 5.8km or 36 per cent of the total tunnelling drive. Despite having started early in October 2013 and January 2014, the two drives at T1 and T2 had completed the fewest meters. Hundreds of bolts had to be replaced on the T1 and T2 cutterheads, slowing down the drives due to down time.
Cutter wear has been another issue due to a combination of abrasive ground, cutterhead design, and cutter spacing, with the T1 and T2 machines averaging 88 cubic meters and 338cu.m bored per cutter, respectively. Cutter life and stoppages for cutter changes have been so significant that the contractor has opted to test out Robbins 20-inch cutter discs in high wear areas of the cutterhead such as the gauge area along the outer circumference of the cutterhead.
Cutter wear at the other tunnels has been better – as high as 1,022cu.m bored per cutter at T5, where the machine is also averaging a healthy 618 m per month. Ground conditions have been difficult — T5 passes through four fault zones up to 10m wide, and passed under a river in a difficult section of hard rock. T6 has the most fault zones, with six faults measured at up to 20m wide.
At T5, fractured ground conditions have required the use of McNally slats to consolidate the ground in over 50 per cent of the tunnel. "This is the best and reliable ground support system we have ever used before," said an official with contractor Shanxi Hydraulic Engineer Construction Bureau. The contractor at T5 has taken steps to extend the coverage of the original McNally system to the gripper shoes area and both side supports in case difficult ground is encountered.
The contractor will reinforce tunnel walls under the gripper shoe position by using a combination of McNally slats, ring beams and a top layer of shotcrete so the gripper shoes can react against the reinforced shotcrete face. The process should allow for fast and continuous boring without the need for continual reinforcement of the grippers. Some issues with the shotcrete system design have also prompted Robbins engineers to work with the shotcrete equipment supplier to develop solutions.
Design specifications had called for a 6m long extendable boom, but the pulsation of the shotcrete pump during extension would cause deflection, resulting in inconsistent application to the tunnel walls. A cable pull-back system was designed as a temporary solution to solve the extension and retraction problem. Robbins engineers are now developing a brand new bridge-type system to solve all the problems permanently, which will be installed in mid-December 2014.
Overall, the machines are progressing well, with the continuous conveyors being a particularly important part of the overall system. Continuous conveyors transfer muck to adit conveyors between 600m to 1.3km long, which then load up radial stackers for temporary muck storage onsite. "The entire conveyor system is reliable, stable and efficient," said Li Xiao Han, a representative from the project owner. Tunnelling is expected to be completed on the massive project in October of 2015.
