The metro’s underground construction was originally offered as a package deal of stations and bored tunnel contracts. But, prices for the main stations came in way above budget and the client reissued separate tenders for the six city centre station packages. These are now being evaluated and contracts will be awarded in time for box excavation to start next summer.
For the single tunnelling contract, the joint venture Saturn – formed of Dywidag, Züblin and Vermeer – has already been chosen as preferred bidder. If confirmed, the JV will start its 18 month tunnelling contract by the end of 2004.
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However, the JV and preferred subcontractor, German TBM manufacturer Herrenknecht, are already closely involved in the client led and funded state-of-the-art research project into an ‘intelligent’ TBM. The goal of the eight-year project is to create the first full face earth pressure balance machine capable of controlling and minimising sub soil disturbance, and therefore reducing surface settlement.
Existing bentonite EPBMs are competent at equalising face pressures – it is in the rear tail void area behind tunnel lining segments where greater control is needed.
The aim now is to develop a more flexible machine capable, through a series of grout pressure sensors mounted in the tail, of accurately varying grout pressures and volumes to match prevailing ground and water pressures. The prototype design suggests a compact machine half the length of a comparable TBM for the same diameter. It would be able to accommodate either slurry or compressed air in its sealed head with adaptable muck removal systems.
Engineers, working with the machine operator, then need to analyse – and be able to counteract through measured grout releases – surface settlement that the TBM is causing directly above. Soldata will provide real time settlement readings which the client converts to a 3D visualisation of building movements instantly displayed in the machine cab. But there is a snag, the time delay between subsoil disturbance and its migration 30m up to the surface to trigger settlement in buildings. Here lies the clever bit, the 3D virtual model will have been fed with soil and TBM characteristics; the calculated movement of the building above; and the real reaction of similar buildings passed along the route. This information is fed to the operator who, with reference to the computer, estimates and dispenses the amount of grout needed before actually driving under the next building.
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By GlobalData"Our model should bridge the time gap, reducing it to under an hour, allowing us to minimise damage," predicts Kaalberg. "We are already getting very close on the drawing board to what we want and I see this type of interactive machine as the way forward for a wide range of future tunnelling projects where surface settlement poses challenges."
