“Jinping-II is one of the most understated tunnelling projects in the world today. The planning and organization are probably equal to if not greater than the Channel Tunnel.”
Steve Chorley, Robbins field service manager at the jobsite, based his statement on the project’s 30+ years of development and planning and the remote location in China’s Sichuan Province – over two hours away from the nearest city. The Jinping-II Hydroelectric Project involves infrastructure planning and development on a massive scale, requiring four 16.7km long headrace tunnels, auxiliary tunnels, and powerhouse structures, all supported by a provisional town of 20,000 workers from China and around the globe.
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As of October 2008, work is ongoing on the powerhouse structures as well as the four headrace tunnels – two are being excavated by drill and blast and two by 12.4m diameter TBMs. A total of three TBMs, two of them Robbins, were launched in summer 2008 for work on headrace tunnel nos. 1 and 3 and an additional 15.3km long dewatering tunnel.
Project overview
Jinping-II will be the largest power station in an ambitious 21-station project for owner Ertan Hydropower Development Co. Ltd. The project will harness up to 25 million MW per year from the Yalong River for China’s West to East Electricity Transmission Project. Power from these stations and other resources in the west will be transmitted to Guangdong, Jiangsu, and Zhejiang Provinces, as well as the cities of Shanghai, Beijing, Tianjin, and other eastern locations in short supply. The entire scheme is envisaged to go online in 2030.
“This project has been ongoing for the past 40 years, as far as preliminary geological surveys, feasibility studies, and necessary approvals for large scale development,” said a senior technical engineer from Ertan. Beginning in 2000, work was completed on the Ertan Hydropower Station, with a 3300MW annual generation capacity. In 2003, work began on the 62km long main road leading up to the Jinping-II jobsite. By 2006, the site of the Jinping-I station, located 17km downstream, was dammed off. The Jinping-I station is slated for completion in 2014 and will have an annual generating capacity of 3600MW.
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By GlobalDataThe Jinping-II site is unique in that it will utilize a natural 180 degree bend in the Yalong River, a tributary of the Yangtze, to generate 4800MW annually. Four headrace tunnels will travel at a 3.65% downgrade from intake structures near Jingfeng Bridge to the underground Dashuigou powerhouse, which will utilize eight hydraulic turbine generators. The parallel headrace tunnels, each 16.7km in length, are separated by 60m from centreline to centreline. Two access tunnels and a drainage tunnel run parallel to the headrace tunnels on the southern side.
“The Chinese economy will benefit immensely from the Jinping-II project. There is a 320m head of water between the inlet and outlet areas of the tunnels, enabling a huge amount of power generation annually,” said the Ertan technical engineer. The owner elected to use two 12.4m TBMs, one Robbins Main Beam Machine (headrace tunnel no. 1), and one Herrenknecht machine (headrace tunnel no. 3). The tunnelling and D&B work is split into two contracts – China Railway 18th Bureau (Group) Co Ltd. is responsible for headrace tunnel nos. 1 and 2, while China Railway 13th Bureau (Group) Co Ltd are constructing headrace tunnel nos. 3 and 4.
The parallel 15.3km long dewatering tunnel being excavated by a 7.2m diameter Robbins TBM is being managed by Beijing Vibroflotation Engineering Company (BVEC), and will be excavated ahead of the four main tunnels in order to alleviate expected water inflows of up to 5m3/sec.
Challenging logistics
Much of the challenge at the jobsite has been a result of the remote location. Onsite First Time Assembly (OFTA) was elected due to project scheduling and difficulty in shipping large pre-assembled TBM components. The OFTA process, developed by Robbins, allows machines to be initially assembled at the jobsite without pre-assembly in a manufacturing facility. The overall process results in savings due to decreased shipping costs and man hours.
OFTA enabled large TBM components to be shipped via barge on the Yangzi River, ahead of the low water season between November and April. The area sees vastly different seasons, with the May to October rainy season accounting for as much as 95% of the annual rainfall.
Once at the site, the 12.4m machine was erected in an underground assembly chamber measuring 20m wide x 26m high. Limited space required that many of the TBM components be staged about 80km away in the town of Manshuiwan, where warehouse space and a large outdoor yard were located.
Assembly of the TBM and back-up system began in July 2008 and finished on September 17. Crews then walked the TBM and the first three back-up gantries 200m forward from the assembly chamber to a launch chamber. The vacated chamber was then used to erect the conveyor system and six more back-up gantries.
Both Robbins machines were successfully assembled despite record snowstorms, as well as this year’s devastating earthquake in Chengdu, which caused some delays to the schedule.
Designing for heavy water inflows
All four tunnels are located in relatively stable geology consisting of massive, blocky marble with limestone and sandstone between 50 and 85MPa UCS. A high overburden, with over 70% of the cover above 1,500m and a maximum of 2,525m, creates a risk of squeezing ground, however. In addition, preliminary core tests showed several faults and fractures, as well as karstic patterns with water bursts at a pressure of up to 2.4MPa in some sections. These same core tests revealed maximum water flows of up to 5m3/sec, with steady flows of 2 to 3m3/sec.
In response to the core tests, an aggressive ground support program has been developed based on the rock mass classification.
“These tunnels are under very high stress and high cover at a large diameter. This is a significant and challenging project for the Chinese tunnelling industry,” explained the Ertan senior engineer. In relatively stable rock, support is minimal including sparse rock bolts. In rock mass Class III, systematic rock bolts up to 6m long are installed, as well as steel-fibre reinforced shotcrete. Class IV and V sections are stabilized with a permanent concrete lining up to 70cm thick, as well as rock bolts and reinforced shotcrete.
Robbins specially designed the 12.4m diameter TBM for high water inflows and difficult ground conditions. The entire TBM, back-up, and continuous conveyor setup in the tunnel are raised 1.5m above the invert on a continuously installed steel framework, allowing the expected large water inflow to pass under the back-up. Portions of the back-up are also covered by an arched steel cover to further deflect inflows. In addition, a water discharge pump can relay water from the cutterhead support to the end of the back-up. Reserve saddles located under the gage area on the TBM cutterhead allow for over-boring up to 250mm if squeezing ground is encountered. TBM modifications in the dewatering tunnel are much the same.
Muck will be transported from all tunnels by continuous conveyor to a nearby storage site. For the headrace tunnels, a Robbins continuous conveyor system, 15.4km in length, handles muck from both headrace tunnels 1 and 2 before dumping onto a customer-supplied overland conveyor. The steel cable belt system utilizes a 1,200kW main drive in addition to a 1,200kW booster drive, and is capable of transferring 1,800 metric tons per hour. The dewatering tunnel utilizes a similar steel cable belt system 15.4km in length.
Current state of work
The assembly of both 12.4m machines was completed in Autumn 2008, while the 7.2m machine was launched earlier in May. As of late October, the Robbins machine at headrace tunnel no. 1 was undergoing continuous testing and had advanced more than 10m of its 2,000m long commissioning bore. Increased ground support at the interface between the starting chamber and the bored tunnel was still being fine-tuned, including ring beam installation every 900mm and a comprehensive 17-bolt pattern of rock bolts every 1.5m.
Excavation at the dewatering tunnel has advanced a total of approximately 1,400m, at rates up to 42m per 10-hour shift. Operations at the drill and blast tunnels were also underway and had advanced approximately 2km in headrace tunnel nos. 2 and 4.
The TBM for the dewatering tunnel is expected to finish in late 2009, while the machine at headrace tunnel no. 1 is slated for a mid-2012 breakthrough. Although Ertan’s entire hydropower scheme will not be finished until 2030, the construction of the Jinping-II hydropower station is a feat in its own right. “This is one of the biggest TBM-driven tunnelling projects in Chinese history,” said Biyue Li, Robbins Chief Operational Officer, Far East. “This is one of many ongoing projects for the country. China is currently one of the leading TBM markets, and will be for at least the next five to ten years.”
Fig 1 – Location map Fig 2 – Plan map of the tunnels The 12.43m diameter Robbins TBM began boring in mid-October 2008 An additional 7.2m Robbins machine is boring a dewatering tunnel ahead of the headrace tunnels
