A new microtunnelling machine is making short work of difficult ground on a sewer project in the Rhondda Valley in South Wales.
Boulders are bad news for tunnel boring machines and even worse for microtunnelling and pipejacking. These random but hefty obstacles can turn simple soft ground into a nightmare of deflections and stalled drives, and sometimes machines have to be rescued.
So small scale tunnelling did not seem the obvious choice for the Taff 's Well sewer installation, which is being built as part of a project to double the capacity of a main sewer line down the Rhondda Valley to a new treatment works in Cardiff.
Large hard sandstone boulders up to 2m 3are scattered throughout the dense glacial sands and gravels of the South Wales valleys. One option was the more cautious approach using traditional methods, chosen for the first phase of the project through the Welsh capital, where cut and cover techniques were used.
But this second, 1. 6km section of the sewer line brought it into the lower end of a narrow valley.
Open cut would have sliced through the small town of Taff 's Well on the north-west outskirts of Cardiff, so it was decided to use microtunnelling and pipejacking.
Another factor in the decision was the purchase of the first large Euro Iseki 'Super' Unclemole in Europe. The Japanese manufacturer believes the machine 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, ' explains Euro Iseki's Paul Wilkinson. 'These can break down larger boulders before they are brought inside. '
Like the well tried standard Unclemole, the TCS Super uses a slurry support at the face in combination with earth pressure balance, and material is taken into a cone-shaped crushing chamber.
Wilkinson says the machine can bore through medium hard rock and, in principle, would carve right through a boulder bigger than itself. There is, however, a differently profiled rock head to give more speed in purely rock conditions. German firm Kolk supplies the cutter discs.
Contractor Amec, part of a contractor/designer alliance with client Welsh Water on AMP3, a fiveyear £1130M asset upgrading programme, wanted mixed ground capacity from any microtunnelling equipment used. Project manager Howell James says the firm was seeking 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 - about 250 of the Super machines have been sold there - convinced the company that it would be a workable option.
Design and construction alliancing on AMP3 meant Amec was discussing the scheme early enough to influence the design process and it was able to work through costings and constructability with designer Hyder Consulting. The microtunnelling option looked good.
'Nothing is more disruptive than a deep sewer, ' says Richard Swift of Welsh Water. 'These days that is more and more an issue. '
Potential compensation for the village would be high, he says, as a number of small industries depend on the single main valley A-road connecting with the M4. Unthinkably, new rugby pitches would have been uprooted.
Welsh Water decided to buy the £0. 5M machine itself, allocating it first to Amec and seeing potential for it on similar future contracts.
Amec and five other alliance contractors in AMP3 have £650M of work to do. Amec has 70 projects totalling £134M.
The 1650m drive at Taff 's Well has eight legs of varying lengths, working out of shafts up to 13m deep and 6m diameter, sunk by wet caissons. The route crosses a railway and the main road twice, including at a point where the 800mm Cardiff freshwater main passes only 3m above.
'But settlement in the dense ground like this is very little, ' says James. 'Perhaps just a couple of millimetres. '
Control of the machine is 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 beginning of June and was finished early last month. Progress started slower than hoped, partly because a cluster of boulders was found in the initial shaft area, and also because there is a 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 of 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 with suppliers about polymers and mixes.
'We are hoping to install three and a half pipes per shift on average, ' says Ingle. 'Initially we achieved 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. '
Even on this drive the crew hit a maximum six pipes per shift and four at the end. He is confident the team will exceed the programme.