TBMs engaged for rail project at Qinling May 1998

Qinling is the critical path element on the new 264km electrified railway line being built between Xi'an in Shaanxi Province and Ankang in Sichuan Province. The new line is scheduled to open in 2001 and will cut the existing 800km rail journey between the two cities from 19h to only 6h. Some 49%, or 130km, of the new line runs through 90 tunnels of which Qinling is the longest and the only one excavated by TBM. The rest are drill+blast excavations which are also currently under construction.

Drill+blast pilot portal with the TBM assembled and ready to launch in the parallel portal

The line will eventually be doubled, but under this first phase only one track is being built. The second phase will require excavation of parallel single-track tubes for all the current single-track tunnels, as well as parallel viaducts and bridges where necessary.

The two 8.8m hard-rock TBMs and their entire support systems were ordered from Wirth by the China National Technology Import Corporation (CNTIC) on behalf of China's Ministry of Railways in September 1996 and were shipped from Antwerp in June 1997. They arrived at the remote working sites in the isolated foothills of the Qinling mountain range about three-hour drive from Xi'an in August 1997 and spent the next four months in assembly. Transport of the TBMs was by road from the seaport to the Xi'an railhead and then by road through the countryside and up the narrow, unpaved mountain roads of the foothills.

As the critical path element, the National Railway Ministry of China, client for the project, and the Number 1 Survey and Design Institute of Railways as the design engineers, concentrated on the Qinling tunnel, deciding early in the planning phase that it would have to be a hard-rock TBM rapid excavation operation.

In further recognition of Qinling's importance and critical status it was decided that a drill+blast pilot section of the parallel tunnel would be started ahead of TBM launch to identify and confirm geological conditions and provide access for any rock stabilisation required on the TBM alignment.

One of the two Wirth 8.8 m-diameter hard-rock TBMs

Work on this pilot tunnel started from both ends in January 1995 and new, modern drill+blast equipment was engaged. Two Hl78 rail-mounted Boomers from Atlas Copco, each with three booms and a basket, were imported for the job, together with two sets of Hagglund muck-trains. These have performed exceptionally well and the crews were showing competent management and operation of the systems when a site visit was made to the project in November 1997. At that time, the converging pilot tunnel headings had advanced 8.8km from one portal and 7.7km from the other, and the 9.5km contractual meeting point was expected to be reached by January and February 1998. Latest reports from China confirm that this was achieved and the pilot broke through on 10 March 1998.

Management of the tunnel's excavation is divided into three separate contracts. The north portal end of the drill+blast pilot tunnel was completed by the Number 1 Construction Bureau of the Railway Ministry. The south portal drill+blast drive, as well as management of the TBM from the south portal, is in the hands of the Number 18 Construction Bureau of the Railway Ministry which is headquartered near Beijing. The north portal TBM drive and overall responsibility for the TBM project is held by the Tunnel Construction Bureau of the Railway Ministry.

Supervision of both the drill+blast and the TBM operations is by the South West Research Institute of China's Academy of Railway Sciences which is based in Chengdu in Sichuan Province.

Geological advantage

The 18.5km tunnel passes beneath a maximum overburden of 1,600m through the gneiss and granite of the Qinling mountain range. Rock conditions are expected to be favourable for TBM excavation; however, there are several identified faults zones in which water ingress, unstable fractured rock and perhaps squeezing conditions are anticipated. These conditions have been confirmed by the pilot tunnel excavation in which there were incidents of roof falls, rock bursts and unstable conditions in the fault and fractured zones.

Drill+blast in the 30m² pilot tunnel is based on a rail-mounted, gantry type HJ78 Atlas Copco Boomer

The faster than expected advance rates in the drill+blast pilot tunnel, however, are attributed to a higher percentage of more favourable rock than originally anticipated. During the visit to the north portal site in November 1997, the Atlas Copco Boomer was working through a zone of competent, dry massive granite of about 250MPa with 10-20% quartz content and no obvious joint sets. In these conditions, the proficient jumbo operators and tunnelling crews were achieving a best of 17m in a 24h production day. It was taking approximately 8h with the three-boom jumbo to drill the 80 x 4.6m long x 50mm diameter charge holes in the 30m² face together with five large 102mm diameter cut holes and four smaller cut holes of 50mm diameter. Charging the face with China-produced Nonel detonators and cartridges of explosive was taking a further 90min with one of the five large cut holes also filled with 4-stick bundles of explosive. This charge pattern produced a pull of 4.2m using some 3-4kg of explosive/m³.

After a 30min ventilation period, mucking out of the blast was taking a further 3h on average, using two 14m³ Hagglund shuttle cars in each train set. The 4.9m wide invert of the pilot tunnel carries twin track. A 1.3m diameter flexible ventilation duct forces 1,400cfm to the face area. An intermediate ventilation adit 420m long and 10m² crosssection is located at chainage 4.2km to assist the main ventilation installation which has a maximum effective distance of 6.3km.

In the competent granite of the geological report's Classes 5 and 6, the crews were achieving three blasts/day and had achieved a best advance of 456m in one month working three 8h shifts/24h day, 7 days/week. Less than 10% of the pilot tunnel drives to date had been in the poorest conditions of Classes 1 and 2 and average advance rates to November 1997 were 260m/month.

Where necessary, immediate rock support comprised 3m long, cement grouted, non-tensioned rebar rockbolts and layers of shotcrete. Work in the drill+blast pilots started with dry-mix shotcrete applied using hand-held nozzles and silica and aluminate based liquid accelerators. This system was in the process of being changed to a thin-stream, wet-mix shotcrete system developed in China by Professor Wang Jianyu, Director of the Southwest Research Institute. Unlike the rotary compressed- air thin-stream wet-mix system, Professor Wang's system is based on rotating piston pump technology. When the new systems arrived in late-1997, Qinling was only the second application of the prototype systems, which had taken five years to develop and had shown commendable performance on their first application in mid-1996.

lTC Schaeff track mounted loaders

TBM designs

Although the adjacent pilot drive confirms that more than a third of the TBM tunnel alignment is expected to be in good massive gneiss and granite, the TBMs are designed to cope with squeezing, as well as faulted, rock conditions. Steel arches and rockbolts can be installed within 4.5m of the tunnel face and a drill rig on the top deck of the first trailing gantry allows hole drilling through the cutterhead for either probe holes or consolidation grouting. The cutterhead shields on the TBMs are expandable by 10cm to cope with suspected squeezing conditions and there is a set of trailing fingers to provide protection over the immediate support installation area. Bulk shotcreting is applied from the shotcreting station on the second gantry of the back-up while flash shotcrete can be applied immediately behind the cutterhead using hand-held nozzles.

The cutterheads of the two 8.8m diameter Wirth TBMs are driven by eight two-speed electric motors and at speeds of 2.7 or 5.4rev/min to provide a breakout torque of 8.7kNm. An auxiliary hydraulic motor attached to each electric motor gearbox assembly provides low-speed, reversible cutterhead rotation to increase breakout torque to a maximum 10.5kNm for driving through broken and fractured rock. The two gripper sets provide a lateral clamping thrust of 60,000kN and the forward thrust cylinders exert a force of 21,000kN. The 65 x 432mm single-disc Wirth cutters on each cutterhead are designed for a maximum cutter load of 250kN each.

Depending on rock conditions, the two TBMs are designed for advance rates of between 1m/hand 3.5m/h. Operators of the machines work in airconditioned cabins on the top deck of the backup sledges about 45m behind the cutterhead and a ZED 260 system assists guidance and steering.

Once excavated, and only in areas where necessary, the TBM driven tunnel will be finished to a 7.7m internal diameter with an in-situ concrete lining cast from either side of the integrated invert segment. The latter is cast on-site and is placed concurrently with TBM advance.

Professor Wang Jianyu (right) with the site project manager of the Qinling TBM excavation programme Zhuo Zhengguo

Conspired timing

Unfortunately, the best intentions of using high-speed TBMs on the project have been somewhat undermined by circumstances. For reasons not fully discussed during the visit, the TBMs are about one year later in starting than originally programmed. In that time, and through better rock than anticipated, the parallel drill+blast pilot tunnel has been completed. It had been intended that the pilot would progress just ahead of the TBMs.

The pilot, however, cannot be completed and used as the first operating tunnel on the twin tube tunnel facility since it is on the second of the two tracks of the new twin track railway line to be completed and it is too small. The full drill+blast tunnel profile for the electrified railway line is 50m2 whereas the pilot is 30m². It was considered at one stage to increase the size of the pilot to the full profile, but after much discussion this was considered uneconomic. The project was committed to the TBM operation and the pilot had already started at the smaller cross section. The better rock quality than anticipated will speed excavation of the TBM tunnel, bringing it to completion close to the finish of all other elements on the first track of the line to go into operation. The completed pilot also has the advantage of providing access to allow drill+blast excavation of particularly difficult sections along the TBM tunnel alignment. The TBMs will then be pulled through any such sections. Programme risk on the TBM project schedule is thus reduced significantly, but the pressure will remain for the contractor and the design and supervising organisations to achieve high production rates and have the TBM-driven tunnel completed as quickly as possible.

TBMs in China

On the Qinling project, the Chinese authorities feel they have eliminated potential problems and system downtimes by procuring the entire TBM tunnelling equipment package from one source and consider that the equipment is the best on offer from the international market place.

North portal site of the Qinling tunnel project

The DM90 million (about £36 million or US$60 million) order for the two TBMs and all the associated equipment is the largest ever stand-alone procurement order for WIRTH and includes 72 x 20m³ Mühlhäuser muck cars with 32 units of Mühlhäuser remixers, manriders and segment cars; ten 45 and 35t diesel locomotives from Schoma; ventilation fans from Howden in Scotland; shotcreting systems from Aliva; grouting equipment from Häny; Tamrock probe and rockbolting drill rigs; and invert segment moulds from CBE of France. The backup systems were designed by Wirth and manufactured in collaboration by Wirth Howden in Scotland, Rowa in Switzerland, and by the Chinese manufacturing company Baogi.

When invited to bid for the order, China's Ministry of Railways required manufacturers to demonstrate a track record of at least sevenyears experience in the design and fabrication of open-type hard rock TBMs. To satisfy the requirement, Wirth listed, among others, the Vereina TBM, which recently completed its 10.5km section of the 19km long railway tunnel in Switzerland and the two Wirth machines used on the headrace tunnel of the Cleuson-Dixence hydroelectric scheme, also in Switzerland. Delivery on schedule of the two 11.72m diameter telescopic TBMs currently working on the Pinling project in Taiwan (an order worth DM66 million) also demonstrated Wirth's ability to manage large, complex and complete procurement contracts.

The DM90 million Qinling order also includes a supply contract for consumables and spare parts including oils, greases and disc cutters, as well as a machine service and operator training agreement. In addition, the 13 Wirth technicians engaged on site to assemble and commission the TBMs will stay on to operate them and train the local Chinese crews through the first 1,000m for each machine. Nine employees of the Amberg Consulting company of Switzerland are also engaged over the first 1,000m of each drive to train the local engineers in the specific rock quality and support concerns associated with TBM projects. After the first 1,000m, the TBMs will be turned over to the Railway Ministry's Tunnel Construction Bureau for the remaining 16km.

Acknowledgments

The author thanks Wang Jianyu of the Southwest Research Institute and his assistant Yan Jinxiufor arranging the visit to the Qinling project site and for arranging discussions with project managers especially Zhuo Zhengguo. Technical details about the Wirth TBMs and their order were supplied during a visit to see the TBMs at the Wirth factory in Erkelenz.