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Between a rock and a hard place

Microtunnelling : Taff's Well sewer

A 'universal' microtunneller from Japan is speeding work through variable ground in the Welsh valleys.

Boulders are bad news for TBMs and even worse for microtunnelling and pipejacking. These random obstacles can turn simple soft ground into a nightmare of deflections and stalled drives, sometimes leaving machines needing to be rescued.

So small scale tunnelling did not seem the obvious choice for the Taff's Well sewer installation, part of a project to double capacity of a main sewer line down the Rhonda valley to a new treatment works in Cardiff. Hard sandstone boulders up to 2m 3are scattered like sultanas in pudding through the dense glacial sands and gravels of the South Wales valleys. A more cautious approach using traditional methods might be the way: the first phase through the Welsh capital had used cut and cover.

But this second 1.6km section brought the sewer line into the lower end of a narrow valley.

Open cut would have sliced through the small town of Taff's Well and it was decided to go ahead with a pipejack system.

Just eight shafts were needed.

The decision was helped by the purchase of the first large size Iseki 'Super' Unclemole in Europe. It is a machine that the Japanese manufacturer believes can truly be called a 'universal' microtunneller, capable of handling everything from sands and clays through to medium hard rock with one head.

'This mixed ground head also has button rollers and cutter discs on the face which can break down larger boulders before they are brought inside, ' explains Paul Wilkinson of Euro Iseki. Like the already well tried standard Unclemole, the TCS Super uses slurry support at the face in combination with earth pressure balance, with material taken into a cone shaped crushing chamber.

Wilkinson says the machine will bore through medium hard rock if it meets it and, in principle, would carve right through a boulder bigger than itself. But there is a differently profiled rock head, which would give more speed in pure rock conditions. German firm Kolk supplies the cutter discs.

Contractor Amec, part of a contractor/designer alliance with client Welsh Water on AMP3, a five year £1,130M asset upgrading programme, wanted mixed ground capacity. Project manager on site Howell James says the company wanted a machine to tackle large cobbles and boulders up to 30% of its 1450mm outer diameter. But it also had to handle relatively high silt levels.

Seasoned microtunneller Amec had plenty of experience with standard Unclemoles so it was responsive to Iseki's claims.

A visit to Japan to see machines in operation - around 250 of the Super machines have been sold there - convinced the company it was a workable option.

The alliance of design and construction on AMP3 meant the contractor was discussing the scheme early enough to influence the design process and it was able to work through costings and constructability with the designer, Hyder Consulting.

Above all, by avoiding disruption the microtunnel looked a good choice.

'Nothing is more disruptive than a deep sewer, ' says Richard Swift, Welsh Water contracts manager. 'These days that is more and more an issue.'

Potential compensation for the village would be high, he says, because a number of small industries work there, dependent on the single main valley Aroad connecting to the M4. New rugby pitches would, unthinkably, have been uprooted.

The client decided to buy the machine at a cost of almost £0.5M, seeing a potential for it on similar future contracts in the area and allocating it first to Amec. Amec's five other alliance contractors in AMP3 have some £650M of work: Amec alone has 70 projects all told totalling about £134M.

The 1,650m drive has eight legs of varying lengths, working out of up to 13m deep, 6m diameter shafts sunk by wet caissons.

A railway has to be crossed once and the main road twice, including at a point where the huge 800mm Cardiff freshwater main passes just 3m above.

'But settlement in dense ground like this is very little, perhaps just a couple of millimetres, ' says James. Machine control is also highly sophisticated, with a completely computerised above ground guidance system tied into a Zed Systems laser.

The first drive, a relatively short 193m, began at the start of June and was finished in early August. Progress was initially a little slower than hoped partly because a cluster of boulders was found in the initial shaft area, and also due to the learning curve.

'Working out the exact mix density to use for the slurry system needs experimentation, especially as the ground is variable, ' says Dave Ingle, national contracts manager for all Amec's microtunnelling work. More density and viscosity is needed to carry heavier pieces of rock to the cleaning separators above.

Amec has been talking to suppliers about polymers and mixes.

'We are hoping for 3.5 pipes per shift on average but we were initially getting less than that here. But on some of the drives there should be mainly sand and gravel and we are likely to get up to five a shift comfortably, ' says Ingle. But even on this drive the crew has hit a maximum of six of the 2.5m pipes per shift and an average of four at the end. He is confident the team will exceed the programme.

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