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TUNELLING - The first drive for two major water tunnels began this month in Vancouver. Adrian Greeman reports.

Work is under way on western Canada's biggest tunnelling project for 25 years with two 7.2km-long tunnels each measuring 3.88m in diameter.

German contractor Bilnger Berger is building the Hatch MacDonald design for the Greater Vancouver Regional Authority (GVRD) at a bid price of C$99.6M (£48M).

The tunnels will service a new water processing facility under construction in the mountains just behind the Vancouver North Shore area, which will supply more than 1.2M people with water. A filtration plant takes supplies from two reservoirs: the Seymour, upstream from its location; and Capilano, one valley away.

The Seymour water goes on downstream into an exisiting distribution system. Capilano water will be pumped up to the higher level plant via a raw water tunnel and clean water will return in a second bore to go into the existing Capilano distribution system. Some of the pumping energy needed will be recovered by a generator on the clean water line.

Main excavation started on 1 July when the assembly of the first of two new Robbins hard rock TBMs was complete and it moved out of its starter chamber at the bottom of a 180m deep, 11.5m diameter shaft at the ltration plant location.

A second machine is almost assembled and due to be under way in September from a second starter tunnel. The two machines will diverge initially until about 100m apart, when they will follow parallel routes to the Capilano water reservoir, converging again at the end.

'That is to ensure complete separation of the raw water and nished water return paths and remove any possibilities of diffusion through the rock and therefore contamination, ' explains Tom Morrison, client GVRD senior project engineer for the tunnels.

For the same reason, adds Andrew Saltis, GVRD's area manager for the tunnels, 1.2km of the tunnel length will be steel lined at the far end by the Capilano reservoir where rock may be fractured, along with the 270m deep water return shafts there. Some lining will also be used at the treatment works shaft.

'In fact we considered doing all this as a pipeline, but it would have to follow mountain contours on a much longer route. The cost was astronomical, ' says Morrison.

Initial work on the tunnel has been difficult. An early location for the starter shaft had to be abandoned during design because of running sand problems; it was within the boundaries of the 16ha ltration plant which is primarily on glacial till and sands.

Ground freezing was ruled out for the shaft, adds Morrison, because the groundwater ow is too fast and so several new locations were investigated. By chance a 'buried hill' was found in the bedrock, bringing hard rock to within 18m of the surface. Unfortunately it lay just outside the site boundary in a national park area of preserved forest. Use of the new location was eventually negotiated with the local park authority. 'They get some money, ' explains Morrison, who adds that the rest of the contractor's facilities have to lie on the main site.

The harder bedrock horizon was better but has proved difcult for the contractor, which needed six months over the originally scheduled time to complete the 180m deep drill and blast excavation.

The contractor had problems mobilising quickly, and then difficulties with the shaft excavation, both in the overburden and in the drill and blast excavation, says Christian Genscher, Bilnger's project manager. 'The overburden was supposed to be sand and gravel for 30m and then 20m with small boulders but it had a lot of, sometimes very large, boulders in it.' Drilling below was complex because of extreme variability in the rock, which ranged from hard granites to brittle metamorphics, he told GE. 'It would change quite rapidly and therefore we had to drill the closest blast hole pattern - for harder rock - all the time. It cost time.' But work has now nished on a 'Y' shape of two 60m long starter tunnels and a back tunnel stub - for manoeuvring mucking out trains at the shaft base - where the TBMs have been put into place and assembled. A 10-month schedule is planned for the machines to finish the drives, which will pass under a mountain, to the Capilano valley. Each tunnel will use one of the 270m deep vertical shafts there to complete the water line to the surface. Site workers will use raise boring to build these shafts while the TBMs return to the starter shaft at the plant to be dismantled.

The drives will be through granite and granitic materials using 1200kW power machines with 19 inch [483mm] disc cutters, says Saltis. An unusually steep downward gradient of 2.5% will be needed for much of the length to take the tunnels even deeper than the started shaft.

'There is a sediment-lled buried valley in the harder rock once you are past the mountain, ' says Saltis.

At least an 18m clearance is needed to pass beneath it.

The tunnel will atten out once it is at a sufficient depth to avoid the soft ground, to a nearly at 0.15% gradient for the nal drive.

Mucking out for the main drives is a rail system that uses five railcars of just under 6m 3 capacity, says Genscher. 'Together they match the spoil from one TBM advance stroke of 1.5m.' Spoil from these will fill two 30m 3 hoist buckets to serve the two machines, with the buckets lifted by an unusual shaft head hoisting system developed by the contractor in conjunction with crane maker Liebherr. It is assembled from various crane components to support a horizontal trolley mechanism on five tower pairs.

The hard work will undoubtedly prove worthwhile: the new £290M Seymour-Capilano works will upgrade Vancouver's water treatment to new standards imposed by the federal government following contamination problems in eastern Canada in the 1990s.

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