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Into the; tunnels

Tai Lam tunnel DB-350 The first contract to be let was for Hong Kong's longest transport tunnel.

There is a sense of deja vu for Nishimatsu and Dragages on their West Rail tunnelling contract. 'Very seldom do contractors get the opportunity to build much the same thing twice,' says Nishimatsu Construction Hong Kong branch projects director John Porter, but this is one such occasion.

The 5.5km Tai Lam rock tunnel, the first West Rail contract to be let, runs through the same mountain range as a road tunnel the team built last year, less than a kilometre away. Nishimatsu and Dragages had formed a joint venture a few years ago for Route 3's 4km Country Park section, a private project put together with investors. 'It was very successful,' says Porter, 'and we decided to stay together for this job. From our point of view it was a happy marriage, and we're sure the client takes comfort from knowing the contractors have worked together on a similar project.'

East will meet west roughly in the middle of the tunnel, though the Japanese and French contractors have swapped direction this time round. 'Both parties said we had the harder section on Route 3, so we have changed ends,' jokes Porter. There will be a management team for each end of the tunnel, headed up by the firms' own staff, while a joint venture team headed by Nishimatsu's John Mundy will pull the two together as well as taking responsibility for electrical and mechanical works and co-ordination of follow on contracts.

Cost of the KCRC contract is HK$1.79bn (£143M), against £480M for the road tunnel. Although the rail tunnel is longer, it has a smaller cross section than the twin bore road tunnels and the scope of work is less. Route 3 involved more mechanical and electrical work, and the work included approach roads.

'Tai Lam is a long contract in terms of time,' says Mundy. 'But most of the civils work has to be completed by about the half way stage. After that, the civils contractors will be in attendance while KCRC's directly employed contractors carry out track work, signalling and catenery work and so on.'

The tunnel runs through a rock mountain. Ventilation buildings are at the end, with a small section of open cutting at the north, and a cut and cover section built within a diaphragm wall at the southern end.

Tunnelling will be fairly straightforward, he believes. The contract is design and build, and the joint venture is taking forward the client's general requirements.

It is very much a fast track approach says Mundy. 'There is a short learning curve. We can get straight into it from day one, which from our point of view is very important.'

The job started on 28 September. 'In the tendering stage, we were required to provide very detailed design, with an exact programme and estimate,' says Nishimatsu's deputy general manager H Ichikawa. Further detailed development has been needed, but in principle most of the design is already in place.

'The trick is to make it design and construction - sometimes it becomes construction and design,' says Porter. 'Once you start digging it is difficult to stop.' Being design and build, the contractors can influence the design, to make it as cost effective as possible, with benefits to the programme he adds.

The design team is the same as for Route 3 - Maunsell and Maunsell Geotechnical for the civils and Parsons Brinckerhoff Asia for the M&E. Independent checking is by Ove Arup & Partners, and environmental consultant is the Hong Kong Productivity Council.

The tender had suggested twin tubes, but a single tunnel with a cross section of 110m2 is being built - a total volume of some 650,000m3. It diverges only at the approach to Tseun Wan station to the south.

Building a single bore removes the need for cross adits at 60m centres for evacuation throughout the 5.5km length. Hardest problem to solve for a single tube was provision of a crossover in the middle, as a separation wall was needed in case of emergency. The solution has been to design a very big sliding door, as in the Channel Tunnel. This is a particularly stringent requirement, says Ichikawa, and is not used in similar situations for higher speed lines in Japan or France.

'We are going to use drill and blast,' says Porter. 'We did look for a short period at using a tunnel boring machine for this job, but with the change to one tunnel it would not have been practical.' The size would have been too big.

Drill and blast gives more flexibility adds Porter, and is a method traditionally adopted in Hong Kong.

The ground is fairly hard rock, granite with some faulting. 'Also we pass existing water supply tunnels and under a small reservoir, which obviously we are not keen on draining,' adds Mundy. No blasting is allowed within 60m, unless approved by the independent blasting consultant says KCRC chief resident engineer Simon Mui.

The target rate will be 150m/month from each end. The contractors have taken possession of the site, held the ground breaking ceremony and begun demolition of temporary housing and a six storey warehouse. 'Behind the scenes everyone is working feverishly developing the design,' says Porter, 'and holding regular meetings with the client, designer and checker.'

There will also be some tricky cut and cover work at southern portal. 'It will be quite exciting,' says Mundy. 'We are surrounded by high buildings, elevated roads and the sea.'

He explains that at the southern end, the portal is below ground level and a cofferdam is needed. Part of this is at the northern limit of the cut and cover section, and diaphragm walls have to be constructed as part of the temporary works. Excavation inside these will be needed for the tunnel itself. Diaphragm wall work is just starting and the walls will be left in place with the box built within.

'As I see it, the tunnelling side is not particularly difficult,' says Mundy. 'But we are very close to the sea, and the ground is saturated. And we are surrounded by large buildings and a flyover - and have to underpin one of its columns.' Utility and traffic diversions are also needed. 'It is critical to make an early start on that portal,' he adds. 'We are aware of the problems, and certainly it is more difficult to get to the starting point than in the north.' The joint venture has also been asked to consider a short but complex extension to the south to take the tunnel under a live eight cell drainage culvert, which would involve extending the diaphragm wall close to the sea wall.

Approval procedures for design and checking are based on those for build, operate and transfer schemes rather than the normal government systems. Adds Porter: 'The normal system of obtaining approvals from government can be laborious. There are timescales by which the government has to reply to design submissions but when in doubt, proposals are rejected.'

The process has however allowed government to have its say. ''Approval in principle' meetings were critical in this process,' adds Mui. 'Decisions were made there and then.'

One key difference to BOT is that the client does have the right to make variations. 'I hope any changes are minimal,' says Mundy. BOT tunnels such as Tate's Cairn, the Western and Eastern Harbour crossings and Route 3 all beat programme by at least three months.

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