At 5,464 km long, the Yellow River is the second longest in China and provides water to over 12 percent of the country’s 1.4 billion people. Its reach falls short, however, in chronically dry Shanxi Province—a region that only receives about 473 mm of rainfall annually, and has in recent years experienced severe droughts coupled with rapid economic growth.
Sprawling equally long at thousands of kilometers, the province’s Great Hydro Network (GHN) is a mind-boggling feat of human engineering in the making. The network of tunnels will source water from the Yellow River to benefit up to 24 million people in the drought-ridden region. Once complete the tunnels will supply 2.3 billion cubic meters of water per year, improving both capacity and reliability of water supply.
Tunnels throughout the GHN project are being excavated mostly by drill & blast, with four designated TBM-driven tunnels. About half of all the tunnels under construction are very deep underground. The terrain and geological structure in the area is complex; some tunnels cross coal seams and below protected areas, underground springs and other unique geological structures. The tunnels carry construction risks including methane gas, groundwater and rock bursting.
Robbins has supplied three Double Shield machines on various lots. Contractor China Railway 18 Bureau Group Co. Ltd. is responsible for Tunnel 1 (T1), a 26 km long drive through limestone, dolomite, mudstone, amphibolite, and gneiss. Tunnels 2 and 4 (T2 and T4) are using 5.0 m and 4.2 m Double Shields, respectively, on 25 km and 15.6 km long drives. Both sites are operated by Shanxi Hydraulic Construction Engineering Bureau. Tunnel 3 (T3) is using a TBM from another manufacturer.
All three Robbins TBM-driven tunnels are located in Class III to Class V rock, and excavation was expected to be challenging from the outset. In particular the rock at T4 tested as over 27% Class V and nearly 23% softer soils, with just 36% of the tunnel in Class III rock.
The trio of TBMs have been designed to meet the geological challenges and long tunnel lengths. All three machines were assembled at Robbins China in Shanghai, then reassembled at the jobsites. The TBMs were built with components from Italy, Germany, Switzerland, China, and the U.S.
The machines were optimized to transport large segments quickly and safely into position to form a segment ring. The hexagonal segments are built in rings of four, each 1.2 m long. No steel reinforcement is included in the design, but gaskets between each segment help to seal them. Afterwards pea gravel is pumped into the annulus through a port in the segments to backfill voids, while a layer of grout seals them into place.
The TBMs have a uniquely long back-up system design due to the inclusion of redundant support systems– each machine is trailed by between 45 and 50 decks. Each deck is 6 to 10 m long. The redundant systems include different grout systems and at least one additional pea gravel system. This was done if one system needs maintenance or is shut down, so that the contractor can continue to bore and not affect the production of the entire line. Additional extras include redundant air compressors, as well as a rescue chamber, cafeteria, and toilet. A specialized car mover on the back-up allows two empty muck cars to be brought in with each supply train. The cars can be slid into place without needing a locomotive so that downtime in the long tunnel is minimized. The two extra muck cars have enough capacity for about two machine pushes or five rings, and can be pulled out with the next muck train.
The different diameters of the machines also necessitated unique design features. On all of the machines, squeezing the internal elements, particularly the hydraulics, into a small diameter was a challenge. The T1 and T2 machines, at 5.06 m in diameter, were able to use conventional torque cylinders in their design. The T4 machine, however, at just 4.16 m in diameter, required a redux of an old design—the lattice cylinder arrangement. The design is reminiscent of earlier Double Shield designs and offers a distinct advantage for machines less than 6 m in diameter: space. Torque arms normally occupy a large amount of space on Double Shield TBMs, and the design opens up more area to position the motors on smaller machines. It also leaves the invert open, which can aid in maintenance such as cutter changes.
The small Double Shield for T4 was the first to launch in summer 2014. The machine began boring in two eight-hour shifts, with a third 8-hour shift dedicated for maintenance. The vast jobsite covers an area of 133 square kilometers and employs an army of workers. Obstacles presented themselves nearly from the outset, as ground changed quickly between soft, weak rock and hard, abrasive rock—a condition that caused frequent cutter changes. Clay clogged the cutterhead, while water inflows occurred from a lake overhead. Despite this, the machine achieved its best monthly advance of 840 m in April 2016, and by spring 2017 had excavated more than 70% of its tunnel.
The machine at T1 was the next to launch in early 2015—the assembly of which was a challenge due to timing. Winter temperatures reached -25℃ while assembling, testing and launching the TBM. Heating equipment was employed and a greenhouse was installed to keep the whole TBM warm. With those strategies the TBM testing and launch ran smoothly. Since startup, the TBM has encountered very hard rock–up to 160 MPa in some areas and progress has been slow. In areas of good ground the machine has achieved as much as 1,402 m per month and as of spring 2017 the TBM had excavated about 36% of the total tunnel length.
The last of the machines, for T2, was launched in spring 2015. About 320 people work and live on the jobsite that covers 20 square kilometers. The T2 machine experienced similar varying ground conditions to those at T4, vacillating between soft rock and hard, abrasive rock. Water inrushes have been encountered multiple times and have slowed down the excavation speed. Polyurethane grouting is being done to control the water inflows. The TBM has been able to achieve up to 720 m in one month, and as of Spring 2017 had excavated more than two-thirds of the total tunnel length.
Updates of this project will be posted as boring continues.