The 12.5km TKE is divided into 13 civils contracts with seven stations, two of them double level. The two main tunnelling contracts on the TKE are Contract 603, the Black Hill tunnels; and Contract 611, the Pak Shing Kok tunnels. While similar in length at approximately 2km, the two contracts vary greatly owing to the ground conditions and the relative complexity of the layout of the underground structures demanded by the client’s masterplan. Blasting restrictions due to local residential developments and the presence of existing underground infrastructure near the drive alignment have added further complications.
Because of the multiple running tunnels specified, the overall tunnelled length will be 6km for Contract 603 and 6.4km for Contract 611, with a maximum gradient of 3%. Both tunnels have been let as design+construct contracts, which means that the final design of the two schemes varies. However, in principle the tunnels are 5.4m wide and horseshoe shaped, having a cross sectional area of around 30m².
The contracts are on the critical path for the entire extension programme and construction schedules are tight. Both contractors have therefore decided to maximise the productivity of the traditional drill+blast operations by employing state-of-the-art technology in the form of Atlas Copco’s latest generation of L2C Rocket Boomer drill rigs (Sept ’98). Both orders – for three rigs each – include complete packages comprising spares, drill steel, container workshops and service contracts. All machines are 2-boom units.
This is the first time in Asia that the new rigs have been used, and keen interest in their performance is expected. The two JVs have decided not to take the fully automated versions of the Boomers, opting for the Contour Control system with manual positioning by the operator using the targets and computer screen. However, the manufacturer will be supplying two fully automated, 3-boom machines to Contract DB 350 on KCRC‘s WestRail project later this year.
At the time of the visit, excavation on both contracts was at an early stage. However, work was sufficiently advanced to show ground conditions and the advance rates that could be achieved with the new drill rigs. According to both contractors, while there has been a steep learning curve, early indications are that, once mastered, the rigs will be capable of high productivity with minimal overbreak.
Black Hill Tunnels
Travelling from Hong Kong Island, the first contract encountered is 603, the US$75m Black Hill Tunnels, awarded to the Dumez GTM-Chun Wo JV. This is the first tunnelling contract in Hong Kong for the French contractor. Design was carried out by Halcrow China. Work on site began in Nov ’98, with tunnelling proper starting in April. Ninety-five per cent of the 1.8km long contract is in tunnel, with two sections of 50m and 20m in cut+cover. The contract comprises four single-line tunnels running from the western Yau Tong Portal to the Tiu Keng Leng station portal in the east.
New stations at Yau Tong to the west and Tiu Keng Leng to the east are being built by means of contracts let on either side of Contract 603. Both are designed as double-level stations to allow passenger interchange between the new KTE and the existing Kwun Tong Line, which is to be redirected and extended to run parallel to the TKE as far as a new station at Tiu Keng Leng. The resulting interchange will provide an eastern route for passengers from Kowloon to Hong Kong Island, but requires complex interweaving of tunnel alignments and elevations.
Ground conditions are expected to be good, comprising competent granite on the whole. The tunnel alignment incorporates cross passages, ventilation adits and crossovers, which, together with the client’s need for access to the depot at TKO, make for a very complex job to design, plan and execute.
Rock is Class 1 granite and small amounts of tuff up to 322MPa. Maximum overburden is just over 300m, with few faults and little water anticipated. Three L2C Rocket Boomers will be used altogether. Two second-hand Boomer 282 rigs will be employed for rockbolting, as the L2C machines’ feeds are too long to be used for rockbolting in the 30m² cross section tunnels.
As the western portal is in the middle of a built-up area, the contractor faces major constraints over vibration, noise and night-time operation. At the time of the visit in August, no blasting licence had been granted for this portal, although it was anticipated within a few weeks of the visit. Meanwhile, the contractor was building a noise mitigation structure around the portal and removing rock using expansion agents in drillholes.
Using a H175 boomer, Dumez/Chun Wo is drilling 2m holes which are then packed with Japanese manufactured S-mite. Although it is effective, this method is very slow, taking 12-24h/round and therefore expansion agents will only be used on the first 5m of the drive. In addition the rock in this western portion is expected to be of poorer quality than the rest of the line but will improve rapidly as the face advances. The third L2C rig, due to be delivered in August, was earmarked to take over drill+blast operations at this portal when permission for blasting was obtained.
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“This is the first time that the new Atlas Copco rigs have been used in Asia and a keen interest in their performance is expected” |
Driving from the eastern end
Partly as a result of the problems at the western portal, the contractor has decided to drive only 350m from this end. Most of the drive will be excavated from the eastern end of the contract. Here, four headings, stacked two on two, are being driven into the mountain from the huge rock excavation for the Tiu Keng Leng station box prepared by an advance works contract. The two sets of tunnels are separated by about 4m between crown and invert. Blasting in the lower tunnels began in April.
The L2C rigs are drilling 73 holes/face, each 4.5m long, with 500mm between contour holes. Bulk emulsion explosive is used except in the contour holes, where cartridges are employed. It takes 1h to charge each face, following 3h of drilling. The contractor aims to achieve one blast/tunnel/12h shift. It is planned to work 5.5, 24h days/week with no night shift at weekends, which should equate to a target advance rate of around 30m/day.
At the time of the visit to the eastern portal, two blasts/day on the lower tunnels and one on the upper were being achieved. A total of 900m had been driven, 780m of it on the lower two tunnels. Up to this point, the contractor had been using the smaller 282 rig in the upper drives because the turning platform outside the portals had proved too small to accommodate the larger L2C. However, with the completion of a steel ‘parking’ structure cantilevered over the lower portals, it was planned to introduce the more productive larger machine to the upper workings. The best week’s production so far is 86m, using two boomers in granite, although the contractor points out that the crews are still on the learning curve.
In addition to the running tunnels and cross passages, the contract calls for construction from the surface of a vertical ventilation shaft 350m from the western portal, with connecting adits to the tunnels. The circular shaft is 100m² in cross section by 45m deep. It is being formed by shaft sinking, a process which had just started at the time of the visit.
Existing structures had to be taken into account during the planning of the MTR extension. Dumez/Chun Wo has to cater for a high voltage cable, a road and a sewage pipeline along the alignment. Mucking out during the early part of the operation was carried out by Cat 966 wheeled loaders and trucks – a mixture of 10m³ Nissan road trucks and Astra 8m³ dual direction low profile haulers.
As the length of drive increases, it is proposed to use a single backacter Häggloader, loading via conveyor into trucks to remove spoil from the lower drives; the Cat wheeled loaders will be retained in the two upper drives. Ventilation on Contract 603 is being performed by Cogemacoustic fans feeding into 1.2m diameter ducts.
The largest excavated spans on Contract 603 are found at the junctions of cross passages and main running tunnels, where spans of 13.5m are achieved. Here, Swellex rockbolts every 1.5m and 150mm of wet shotcrete, with fibre reinforcement where necessary, are applied as primary support using two boom rigs with a basket. Elsewhere, rockbolts and shotcrete are used where conditions dictate, and the only area where steel arch support is required has been at the portals.
A mass concrete secondary lining with waterproof membrane will be installed to complete the running tunnels, unreinforced for most of the drained tunnel length, However, 20m will be reinforced at the portals because here the tunnels are undrained and the lining will have to withstand groundwater pressure.
As part of its design+construct brief, the contractor is proposing that shotcrete be used as final lining for the long span junction caverns in order to avoid the need for complex shuttering, but the client has yet to agree this proposal.
Pak Shing Kok Tunnels
Further east, Contract 611, comprising the Pak Shing Kok Tunnels, is being carried out under a $59m contract, awarded to a JV of Hyundai and Kier. This forms the final section of the southern branch of the extension, providing access to the depot at TKO, which is being constructed under another contract. Other contracts will see a northern branch line (before the Pak Shing Kok Tunnels) to Po Lam, plus a link to a planned station to the south, adjacent to the depot. This is the first time the two partners have worked together and it is recognised that the JV will have its work cut out to meet the tight construction schedules that will allow Contract 611 to be the first section of the extension to finish to accommodate track laying from the depot, starting in February 2001.
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“it is recognised that the JV will have its work cut out to meet the tight construction schedules” |
Ground comprising volcanic tuff is expected to be less good than on Contract 603. Highly fractured and decomposed materials are anticipated, with intact rock strengths of 200-400MPa.
Four headings will be driven on the two running tunnels at the north end of the contract, accessed from a 380m long ventilation adit 350m from the contract boundary. Five tunnels have their portals in the mixed and decomposed rock to the south, where a reversing siding and a pair of tunnels feeding the depot are required in addition to the main running tunnels. The design specifies that at least 6m be maintained between headings at all times.
Rock at the southern end of the contract is faulted, decomposed tuff which needs extensive primary support. Quality ranges from Grade 4 to Grade 2; maximum overburden is 80m and minimum 6m. No water has been encountered so far, but some is expected later. It is possible that jet grouting will be needed to control areas of water penetration and soft ground in the tunnels at the south end.
In the poor rock, steel ribs at minimum 750mm centres are used with up to 250mm of steel fibre reinforced shotcrete applied using MeycoSuprema shotcrete rigs. In better material, no ribs are required and shotcrete is reduced to 75mm. Rockbolting is used extensively, with 3m long Swellex rockbolts placed at minimum 1.5m centres in poor ground.
Drill + blast operations on Contract 611 also involve three L2C Boomers purchased for the job. Work on the contract started in November 1998. Blasting started in April ’99 and is expected to take about 18 months. Contract 611 involves several turnouts and junctions, including a major diamond crossover to be accommodated within a 23m span cavern.
However, while normal operations can be adopted from the unpopulated south, stringent restrictions have been imposed on working from the north portal because of the proximity of residential developments. Indeed, the slow process of issuing blast licences has meant that the contractor has had to concentrate on other aspects of the project to avoid falling behind schedule.
At the time of the visit, no blasting had been possible from the northern portal and the 30m driven in each tunnel had been achieved using excavator- mounted hydraulic hammers. In addition, the contractor faced several complications at the start of the project, including the diversion and subsequent re-instatement of a main road to allow construction of a 30m cut+cover section at the depot contract boundary.
The 7.5m dia. ventilation adit, 350m from the northern boundary is 380m long and branches to serve the running tunnels in 8m and 5.8m diameter adits. As this is a residential area, a noise attenuation structure has been built at the tunnel portal and only 16h working is permitted. Nevertheless, two rounds/day are being managed, and it is hoped to increase this as the excavation goes deeper and additional faces are opened in both directions along the main running tunnel alignments.
The contractor is hoping to work seven faces in total, with the possibility of nine if permission for blasting in the built-up area is granted. Working is six 24h days/week. Each face requires 80 x 4m deep holes and takes 1.75h to drill. According to drilling sub-contractor Kane Tunnelling, the accuracy of the contour control system has proved excellent, reducing overbreak to a minimum. The operators were learning quickly and productivity was improving. An estimated 1h saving/ per round was being achieved using the new rigs, even taking into account the more frequent changing of drill steels necessitated by this increased speed. The contractor’s target is for 8m advance/12h, or a minimum of three rounds/24h day/face. According to Hyundai, this corresponds to 150m/month/face.
At the time of the visit, 590m had been excavated on the five drives and ventilation adit. Mucking out was being achieved by Volvo L90C wheeled loaders and 8m³ trucks. Later, as tunnel lengths increase, it is planned to use low level AMCO trucks to remove spoil from the face. Ventilation is provided by Korfmann fans.
One problem faced by the Hyundai-Kier JV is the presence of an existing 2.4m diameter sewer tunnel, which crosses the alignment twice within 6m of the new MTR tunnels. Here, it is expected that reduced pulls or non-blasting techniques will be used, although inspection has shown the 400mm thick mass concrete lined sewer tunnel to be in good condition. According to the contractor, extensive monitoring has so far recorded vibrations which are well within guidelines. However, monitoring will continue and necessary modifications to drill+blast operations implemented where necessary. These tunnels will be finished with a permanent secondary lining of 250mm of unreinforced mass concrete. Short lengths will be reinforced to resist groundwater pressure in undrained sections.
Tunnelling work on both contracts is scheduled for completion in mid-2001, and client, the MTR Corporation, says that the first passengers will use the new extension in December 2002.
Related Files
Tunnel Layout
Location Map
Geological Section
