TAT 5,464KM LONG, the Yellow River is the second longest in China and provides water to over 12 per cent of the country’s 1.4 billion people. Its reach falls short, however, in chronically dry Shanxi Province – a region that only receives about 473mm of rainfall annually, and has in recent years experienced severe droughts coupled with rapid economic growth.

Sprawling equally long at thousands of kilometres, 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 bene_ t up to 24 million people in the drought-ridden region. Once complete the tunnels will supply 2.3 billion cubic metres of water per year, improving both capacity and reliability of water supply. "Shanxi Province has rich coal resources, but the economic and social development is mainly restricted by the shortage of water resources. The GHN project will reallocate the mainstream con_ guration of stored water as well as the water from the Yellow River to where it is needed most. It will greatly improve the water resource usage and help economic growth, so people in China look forward to the project’s success very eagerly," said Weihao Li, project manager for Robbins China.

The project is a continuation of efforts in the region to transfer water – the Shanxi Wanjiazhai Yellow River Water Diversion Project, initiated in 2001, spanned over 10 years and 300km of tunnel. The network transports water from Wanjiazhai Dam to far-reaching industrial areas in Taiyuan, Pingsuo and Datong.

On that project, multiple Double Shield TBMs excavated long reaches of tunnel and erected a honeycomb segmental lining in all of the tunnels. In that sense, the Great Hydro Network takes a cue from past projects in the region, but aspects of TBM design and treatment of the dif_ cult ground on the vast project are all new.

A NEW GENERATION OF WATER TUNNELS

Tunnels throughout the GHN project are being excavated mostly by drill and 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," said Lijuan Xu, production control planner for Robbins China. She continued: "The construction area is about 115,000km2. The coordination required for land acquisition for construction sites, demolition, compensation and resettlement has been a huge task."

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 26km long drive through limestone, dolomite, mudstone, amphibolite, and gneiss. Tunnels 2 and 4 (T2 and T4) are using 5m and 4.2m Double Shields, respectively, on 25 km and 15.6km 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 per cent Class V and nearly 23 per cent softer soils, with just 36 per cent of the tunnel in Class III rock.

The jobsites are located in mountainous areas and logistics of site preparation ahead of the TBMs was another challenging task. "The T4 jobsite is quite remote, about 1.5 hours from the nearest town," explained Robbins project manager Mark Belli. "The contractor had to finish an access road to get to the site before we could prepare the portal."

The other sites, T1 and T2, are located about 45 minutes from towns, but even those required major jobsite preparation work.

"On T2 they had to reconstruct the access road to go around the camp, because the original road would have gone right through the portal." All three of the sites required onsite work camps that were built in greenfield-type areas. Structures including segment plants, spares inventory, and even convenience stores for the workers were all built from the ground up.

Workshops were built near each portal to repair and maintain equipment, such as cutter discs, allowing for quicker access to supplies in what will become very long tunnels.

TBM Design

The trio of TBMs have been designed to meet the geological challenges and long tunnel lengths. All three machines were assembled in Shanghai, then reassembled at the jobsites. The TBMs were built with components from Italy, Germany, Switzerland, China, and the U.S.

The tunnel is lined with precast concrete segmental rings. The segments are of hexagonal type and built in rings of four, each 1.2m 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 some redundant supporting equipment systems that have caused us to lengthen the back-up on each machine to between 45 and 50 decks. Each deck is 6 to 10m long. We have two different grout systems and at least one additional pea gravel system," said Belli. Additional extras include redundant air compressors, as well as a rescue chamber, cafeteria, and toilet.

A specialised 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 minimised. 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, continued Belli. 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 5.06m in diameter, were able to use conventional torque cylinders in their design. The T4 machine, however, at just 4.16m 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 space advantage for machines less than 6m in diameter. 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 lattice cylinder design allows for greater control of steering in a smaller machine. However the drawback is that it gives so much steering freedom and there are so many features, the operator has a larger learning curve when steering the machine," says Belli.

In the early startup and boring phase, Robbins drove the TBMs with the customer, providing special training to speed up the learning curve. Modifications were later added to the hydraulics of the lattice cylinders and the skew ring, which corrects roll on the TBM. The modifications allow for easier steering control on the T4 TBM.

Challenging ground Conditions

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 eighthour shift dedicated for maintenance. The vast jobsite covers an area of 133 square kilometres and employs an army of workers. "We have 180 people all living at the T4 jobsite. We have a good environment here, with people working hard at muck removal, TBM operation, and segment casting," said Jinping Li, Onsite Director for T4, Shanxi Hydraulic Construction Engineering Bureau.

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 the challenges, the contractor is hopeful that the 15.6km tunnel will be completed in 2017.

The machine at T1 was the next to launch in early 2015 — the assembly of which was a challenge due to timing. "It was winter and the temperatures reached -25oC while we were assembling, testing and launching the TBM. "It brought great difficulties to the normal operation of the TBM, so we had to use heating equipment and a greenhouse was installed to keep the whole TBM warm. With those strategies the TBM testing and launching ran smoothly," said Xingkun Yang, Project Manager for T1, China Railway 18 Bureau Group Co.

Nearly 300 people work and live onsite in a schedule that begins with a four-hour maintenance shift at 7:00 AM, followed by an eight-hour day shift and 12-hour night shift. Since its launch the TBM has bored over 2km.

Design changes were made due to the tunnel being bored on a decline. Yang added: "As this lot is a decline boring tunnel, all the construction wastewater is discharged by a pump. We worked with the manufacturer’s field service and design teams to come up with a better solution. We decided to reuse some of the TBM equipment cooling water for equipment cleaning, belt washing and grout making for cement grouting, which reduces the needed water supply and drainage for energy saving."

As the TBM progresses further into the tunnel drive, logistics will be of increasing concern. "There is only one adit (for ventilation and drainage) in the middle of the 26km tunnel. Therefore, we require the highest standards for the water supply, power supply, ventilation, materials supply, water discharge and muck removal. It is a great challenge for us," said Yang. The contractor expects the long tunnel to be completed in 2019.

The last of the machines, for T2, was launched in spring 2015. About 320 people work and live on the jobsite that covers 20sq.km. The T2 machine experienced similar varying ground conditions to those at T4, vacillating between soft rock and hard, abrasive rock.

"There are still some problems. Currently, we have water bursting in the tunnel, so we need to use probe drilling to detect the geology ahead of us," said Zhimin Li, deputy project manager of T2 for Shanxi Hydraulic Construction Engineering Bureau. Other challenges include the decline nature of the tunnel — the jobsite opted for rubber-tired vehicles to haul muck as the gradient exceeded the limit of rail cars. The 25km long tunnel is expected to be completed in 2017.

A look ahead

As the project continues to advance, the benefits of the water tunnels remain clear. About two-thirds of all surface rainwater that the arid province receives annually are not captured, but flow out of Shanxi. The GHN will improve water storage capabilities as well as improve the reliability of the water supply. The vast network of tunnels will increase the province’s total water supply by 2.3 billion cubic meters, and its scope of supply applies to an area roughly 76,600sq.km, about the same size as the Czech Republic.

Roughly 72 per cent of the province’s population, or about 24 million people, will see a benefit from the tunnels, including 11 large cities and 70 counties.

To put it simply, GHN is tunneling on a grand scale, and it will be a project to follow over the next years as work progresses