TBMs Get Underway at Jinping II Hydropower in ChinaThe 1,571 km Yalong river, a major tributary on the upper reaches of the Yangtze river, is estimated to have 34.62 GW in hydropower resources. The Yalong is a massive, fast flowing and boiling river that races through an extremely steep gorge in a remote and mountainous region in Sichuan province, Southwest China. Just getting to the project site is an adventure. Road conditions range from double-laned highways to narrow, rutted and muddy roads in the rural areas. The mountainous terrain is extremely unstable, making travel sometimes impassable and dangerous. See photos here.For a 60 km stretch, the river runs north then curves directly south, separated in-between by steep mountains. But it comes back relatively close to where it started. The plan is to build a concrete arch dam - Jinping I - to store energy and generate power before directing the flow through four tunnels in the side of a mountain to Jinping II, which is 17.5 km further downstream. Jinping I hydropower station is slated for completion in 2014 and will have an annual generating capacity of 3,600 MW (6 x 600 MW). As a temporary measure during dam construction at Jinping I, water will be diverted into the east and west banks through large 1.2 km tunnels. At commissioning, Jinping I arch dam will be among the highest in the world at 305 metres. Though the two stations are different projects, they have a unique relationship.Jinping II is designed to cut the 150 km river bend by a group of power tunnels to use the natural drop created by the bend. The Jinping II hydropower station will produce 4,800 MW (8 x 600 MW) annually, using a natural 180° bend of the Yalong river, resulting in a 60 m drop in elevation. The project will harness up to 25 million MW per year.The Jinping I & II hydropower projects are just two stations in an ambitious 21-station programme planned along the main stream of the river by Ertan Hydropower Development Company (EHDC). In addition to the Jinping scheme, the Yalong river hydro development includes the construction and commissioning of the Guandi, Tongzilin and Lianghekou hydropower projects by 2025. Jinping II will be the largest power station in EHDC's programme. 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. Visit www.ehdc.com.cnMain Jinping II componentsA total of four parallel 16.7 km-long headrace tunnels, with excavation diameters ranging from 12.4 m to 13 m, will bring the water to the Jinping II station, diverting the water of the upstream Jinping I hydro station. After lining, the inner diameter at the tunnels is approximately 11.8 m. The spacing between the centre line of each tunnel is 60 m. Tunnels 1 and 2 formed one bid package awarded to one contractor while tunnels 3 and 4 were commissioned to another contractor.The Jinping auxiliary tunnels (2 x 17.5 km) serve as access tunnels between Jinping I and Jinping II and also as the exploratory and construction adits for the headrace tunnels of Jinping II. A drainage tunnel, approx. 17 km long and 6 m in diameter, runs downstream of the headrace tunnels on the south side. The drainage tunnel will divert a large water inflow from the work site during excavation of the headrace tunnels.There are several connection tunnels among the four headrace tunnels for drainage purposes. The underground powerhouse complex is located at Dashuigou on the other side of the river bend.Geology and groundwaterAll four tunnels are located in a complex geology of marble, shale and limestone with up to 80 MPa UCS. The high overburden, 1,500 m in average and up to 2,500 m in height, creates a risk of squeezing ground, while faults and fractures as well as karst patterns revealed in the adjacent access tunnel and probe tests indicate the potential for high water ingress. The bedrock provides relatively good conditions for excavation. Severe rock bursts have actually been encountered during excavation of the access tunnels and an adit. Therefore, slight to extremely intensive rock bursts are expected during construction of the headrace tunnels.During tunnelling of the headrace tunnels, the contractors must be prepared to encounter potential high pressure water ingress. Probing ahead of the tunnel face before excavation will investigate the geological conditions and the potential water inflow. A control approach will either stop inflow or allow the water in. Without control, a large water inflow would take time to seal and cause construction delays. Construction methodsThe four tunnels have been divided into west and east sections, so that there are two drives per tunnel to accelerate the excavation time. Headrace tunnels 2 and 4 and the west sections of headrace tunnels 1 and 3 will be excavated by drill and blast in a horseshoe shape. These tunnels will have an excavation diameter of 13 m. The length of the west sections of all four tunnels is 4,200 m.TBMsThe east side of headrace tunnels 1 and 3 will be excavated by two TBMs (one Robbins and one Herrenknecht) to a total length of about 21 km. The final tunnel will have a diameter of 11.8 m after lining. TBM excavation is expected to reduce the disturbance to the surrounding rock mass and the risk of rock bursts. TBM tunnelling can also increase safety of crews in case of rock bursts during excavation.On 18th September, 2008, a 12.43 m-diameter Robbins machine was launched on headrace tunnel 1. The large TBM will join another 7.2 m-diameter Robbins machine, which was launched earlier in 2008, to bore the drainage tunnel on the same project. Visit www.therobbinscompany.comAssembly of the main beam TBM and back-up system began in mid-April 2008 and finished on 11th August. Crews then walked the TBM and the first three back-up gantries 200 m forward to a launch chamber, where the conveyor system and six more gantries were assembled. The machine assembly was completed on schedule and no-load testing of all critical systems has gone smoothly. Crews are now prepared to begin a 1 km-long test boring programme to fully commission the equipment.The machine was assembled in an underground launch chamber pre-excavated by drill and blast using on-site first time assembly, without pre-erection in a manufacturing facility, which eliminated many shipping risks to the remote jobsite. The accelerated assembly schedule meant most of the heavy TBM structures were already shipped from the manufacturing facility in Dalian, ahead of the low water season on the Yangtze river, when such shipments are not possible. This method cuts the overall delivery time of the machine as well as project costs.The machine was specially designed to cope with specific ground conditions including heavy water inflows. In order to bore successfully in the challenging ground conditions, Robbins designed the machine for a variety of situations. The entire TBM, back-up, and continuous conveyor setup in the tunnel have been raised in order to allow the expected large water inflow of 5 cu m/sec to pass under the back-up. In addition, a water discharge pump will relay water from the cutterhead support to the end of the back-up. Reserve saddles located under the gage area on the TBM cutterhead will allow for overboring if squeezing ground is encountered. CutterheadDavyMarkham fabricated and shipped the 320 tonne cutterhead for the Robbins TBM. Fabricated and test assembled at DavyMarkham's Sheffield works prior to dispatch, the cutterhead has a contract valued at GBP1.15 million. Visit www.davymarkham.comDavyMarkham applied precision engineering standards to the heavy machining and fabrication. In order to improve competitiveness when dealing with such large steel pieces, the company employed a newly developed cutting tool for rough machining, which resulted in a metal removal rate five times faster than normal, and deployed the latest carbide U-drill technology, which cut holes eight times faster.After test assembly, the cutterhead was disassembled and shipped in manageable sections from Southampton to Shanghai, for onward freighting to Sichuan province. DavyMarkham fabricated the machine's main cutterhead in six separate sections, for ease of shipping and lowering below ground: one inner head weighing 60 tonnes, another at 52 tonnes and four outer segments each weighing 38 tonnes, all incorporating free-issue 48.3 cm cutters supplied by Robbins. DavyMarkham's cutterhead for the Robbins TBM on headrace tunnel 1 Special back-upSpecial features on the TBM and back-up will accommodate a high expected water inflow of 4,000 litres/second. The top of the back-up is arched over its entire length to allow for installation of water shields if high-pressure inflows are encountered. A water discharge pump will relay any inflow from the cutterhead support to the end of the back-up. Two TBM conveyors between the cutterhead and the tunnel conveyor were also designed for wet conditions. The conveyors are completely flat, rather than inclined, in order to prevent spillage of muck with high water content. The Robbins back-up system for the 12.43 m TBM is arched over its entire length to accommodate heavy water inflows The other TBM, manufactured by Herrenknecht (machine S-405), will bore 14.7 km. Its diameter is 12.40 m. This is is the largest gripper TBM ever manufactured by Herrenknecht. The back-up and rolling stock has been subscontracted by Herrenknecht to SELI. Due to water inflow, the back-up will be covered by a roof on its entire length, with the exception of the shotcreting area, to protect the personnel and equipment from potential rock bursts. The SELI supplies also include other ancillary equipment and services. Visit www.herrenknecht.com and www.selitunnel.comDrilling and blastingTo mitigate the risk of rock bursts in the sections prone to this phenomenon, smooth face blasting is adopted to replace normal blasting and optimize the blasting process. In sections subject to intensive rock bursts, smooth face blasting is combined with pre-fissure blasting.Rock reinforcementThe rock support has been designed based on the rock mass classification (Classes I, II, III, IV and V). Reinforcement is made by shotcrete reinforced with steel fibres or not and applied immediately after the excavation, rock bolts and wire mesh or steel beams. Grouting will be carried out along the tunnel to reinforce the surrounding rock masses.Class III requires systematic rock bolts and steel fibre-reinforced shotcrete. On a total length of about 8.65 km for all the tunnels, concrete lining up to 70 cm will be installed as permanent rock support, in addition to temporary rock support with systematic rockbolts and reinforced shotcrete. This corresponds to rock mass Class IV and V. Concrete lining up to 40 cm will be applied to rock masses Class II and III.Rock haulageIn the west portions of tunnels 1 and 2, tunnel 2 is the main transportation tunnel during the construction period, while tunnel 4 is the main transportation for the west portions of tunnels 3 and 4.Truck transportation will be used in the blasted tunnels while project planners have opted for conveyor belts in the TBM-driven tunnels. Robbins supplies conveyor belts behind its TBMs working in the drainage tunnel and in headrace tunnel 1.Marti Technik supplies a 14.7 km conveyor belt behind the Herrenknecht TBM in headrace tunnel 3. Powered at 2,800 kW and driven by a booster and tail drive, its capacity is for 1,800 tonnes/hour. The system will evacuate 4.44 million tonnes of rock.In addition, Marti Technik supplies another belt conveying system from the start of the headrace tunnels and drainage tunnel to collect all the excavated material. The system will negotiate a 1,100 m curve radius. Its capacity is for 6,200 tonnes/hour. It is driven by 12,000 kW and will transport 11 million tonnes. In detail, this system consists of two 1 km conveying lines installed in the access tunnels that lead to the headrace and drainage tunnels, a 7 km external belt in the valley and a belt in a 2 km tunnel built by Marti until the final dump site. At the dump site, there is a set of four conveyors per line. A 7 km conveyor line is used to transport aggregates on the return run until the access tunnel portal. Another conveyor goes until the concrete plant. At the end of the 7 km belt, the material can be placed in a temporary stock pile before recycling into concrete aggregates. The two lines have the advantage to transport up to 2,800 tonnes/hour each and it is possible to switch the initial loading. Visit www.martitechnik.chConclusionThe Jinping II headrace tunnels combine many challenges, including the uneasy access to the site, complex geological and hydrogeological conditions, high overburden, large cross-sections of the tunnels and long lengths, which make the scheme a prominent feat of engineering. The plant is scheduled to be commissioned in December 2012. 40/08.