Even before commissioning the $1.2bn west Qinling railway tunnel, the Chinese Ministry of Railways (MOR) is pressing on with another large project. Once again Xian, China’s most ancient capital, is the start point – as for the just completed project – but this line runs three times further.
The first project was a twin track, electrified passenger link from Xian in the Yellow River basin southwards and west to Ankang 268km away in the Yangtse basin.
The new line, to be built over five years, will be 955km long running south and east to Hefei. It will also then be linked into a new line to Nanjing creating part of a strategic network of lines across this huge country.
The line is single track to begin with, though there are plans to upgrade it in the future to twin track. It will be a "Class One Electrified" line carrying both passenger and freight trains, and the estimated cost for the project is 23.15bn Yuan (US$2.80bn). Design is by the 1st and 4th Survey & Design Institutes of the Ministry of Railways.
Qinling does come into the equation for the new line. Its route passes through the Qinling mountains to the south of the city and two long tunnels are required which, if not quite as big as the twin bore 18.5km west Qinling tunnel which broke through in 1999, will still be large at over 6km each. There will be other tunnels in the entire Qinling line too, part of a total of 80 tunnels on the route, stretching 81.8km and forming approximately 8% of the total. There will also be 449 bridges totalling 95km of the line.
Use of western machinery
It would be unlike the Chinese not to make the best and fullest use of equipment, especially when it is complex western machinery, that is not only expensive for their economy but offers the chance to develope valuable technical experience. So the two large tunnels will take advantage of equipment bought for the west Qinling project. These are Wirth TBMs; open style hard rock machines, designated B 880 E. They are 8.8m in diameter and were delivered originally to China in 1997.
The machines were taken to workshops near Xian first of all for reconditioning and maintenance work. Repairs included work on cutterhead welds.
The two machines drove 6.0km and 5.5km, respectively, of very hard rock at west Qinling. They met substantial difficulties in the project with cutter head cracks reported, some gearbox difficulties and a faulty main bearing seal (T&TI December 1999). Wirth has been advising on the renovation.
Delivery to site was in spring last year and at another tunnel site, Taohapu, work started in August. Much of the machine for the second tunnel arrived at the Mougouling site in March, although the main bearing for the cutterhead did not arrive until later. The machine was assembled and tested on 9 July before beginning operations.
The backup trains and mucking out equipment from the original west Qinling drive has also been used for the new project. Wirth’s original contract for $60M of equipment included Mülhlhaüser muck cars, Schôma diesel locomotives, Howden Group ventilation equipment, Putzmeister shotcreting systems, grouting equipment from Häny, rockbolt drills and probe drills from Finland’s Tamrock, and other backup equipment from Rowa of Switzerland and Baoji in China. Segment moulds for invert segments are from CBE of France.
As well as moving over equipment, the two contractors for the works have moved over the engineers and workers who had extensive experience on the Qinling project. After going through such a steep and challenging learning process it would be a mistake to waste their now valuable skills of course.
That said, their skills must now be applied to somewhat different rock conditions further to the east in the mountains. The very hard granites and gneiss on the original project give way to schists and softer more mixed rock with fractures and faults. The mountains are better described as high hills in this area, reaching 824m.
Hard going
On the first of the two tunnels in particular, the Taohapu, the going has proved exceptionally difficult at the start; the machine had made only 300m progress after four months when T&TI was at the site in November.
Details were difficult to clarify because of translation difficulties but, according to Li Quanshe, a mechanical engineer from the 18th Construction Bureau of the China Railway Construction Corporation – one of two huge contractors owned by the MOR – there was short initial progress of about 70m from mid-August, when tunnelling began in earnest, and then the machine ran into falling rock problems in what he described as a fault zone. The rock here is schist but was expected to be of better quality.
Extensive support has been necessary to prevent small blocks falling from the crown. The contractor has used steel arches at as little as 0.9m centres, and in places reinforced this with longitudinal steel beams and even steel plate to prevent falls.
Considerable hand cleaning has also been necessary, particularly behind the cutterhead which has been prevented from being retracted by the fallen debris. The crews have also brought up hand shotcreting guns to the area immediately behind the head to provide early support with a 150mm layer of shotcrete over the ribs. The normal shotcreting equipment operates about 70m back along the back-up train from the face.
It was also pointed out that the rock was not sufficiently strong to make use of the full grippers force of the TBM, and this in turn was preventing applying full force on the cutterhead. The maximum gripper force maximum is 61,000kN and machine head thrust force is 21,000kN, but only around 70% was being used.
When T&TI was on site the team was using a hollow ribbed steel tube system of rock support anchors, developed by Mai in Australia. The tubes are 32mm in diameter and are perforated along the length. They are drilled between 5m-20m into the rock using a sacrificial drill bit and then injected with a cement grout which fills the drill holes and rock voids along the length.
Conditions are expected to improve fairly soon, T&TI was told, as the machine leaves the early section of the tunnel. Rock should be marble and quartz and the support will be mainly shotcrete and mesh. A permanent lining will be put in after the tunnel boring.
Mougouling tunnel
More information is available for the another tunnel slightly further east, the Mougouling tunnel. This one is being built by another major MOR contractor, the China Railway Engineering Corporation, using on this project its Tunnel Engineering Bureau (TEB).
According to vice commander Francis Liujun, a mechanical engineer of the TEB, the tunnel is 6,113m long with 5,762m to be bored by the TBM, including a 1,000m long curved section at the beginning. Bore diameter is 8.8m.
The entry portal is at 485.4m altitude and the exit at 481m. The first 1,936m has a rising gradient and then the tunnel descends for 4,176m. Maximum cover is 300m or so.
At the end of January he reported that progress was 1,482.7m since boring began in mid-July follow a week of tests starting 9 July. An initial 169m section of the tunnel had been formed by drill and blast.
The machine achieved 60m in July, 170m in August, 310m in September and a best result, in October, of 472.4m. Maximum daily advance was 30.4m, also in October. "Rock has been difficult," says Liujin "and it is completely different to west Qinling before where it was very hard. This is very soft and broken."
The overall geology working eastwards, he says, is a sequence of shale with mica and quartzite, shale with mica, dolomite-like shale with mica and quartzite, sandy ground and clastic ground.
There are fractured and broken areas and one of these has been reached, he says, in the current zone. Here the rock is "broken and soft, with instability of the surrounding rock and deformation of broken rock…" Progress has therefore slowed up.
According to the survey in August, he told T&TI, the average compressive strength of the rock should be around 50MPa.
The reason for the bad ground is a transition of almost 600m length between two massifs, one with a low grade marble and the other a quartz schist, which they hope will be in a much better condition, and a bit harder, once they are fully into it.
Support is in four categories. First is steel arches at 0.9m centres, second alternating at 1.8m and 0.9m centres, third 1.8m spacing only, and fourth mesh and anchors. The anchors are 3m long and used at seven per metre length. In all cases there is 120mm thickness of shotcrete.
The TEB also reports difficulties in using the grippers at full capacity. Using them beyond 75% and applying full head force causes "too much vibration and falls of rock".
Machine condition is monitored by the contractor which has created a special on-site clean laboratory with a variety of electrical and mechanical diagnostic instruments. Oil condition is monitored and metal particulate content inspected for wear of bearings and other components. Maintenance is done in one eight hour shift alternating with two tunnelling shifts.
Boring time for the tunnel is an estimated 21 months with another 15 months to go taking the project into 2002. After that the tunnel will get a 300mm permanent lining with a polyethylene waterproofing membrane.
