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Sending flows down the drain

Diverting waste and stormwater into Leicester's sewerage system should leave the Grand Union canal clear and clean, reports Andrew Mylius.

Leicester dog walkers, joggers and boaters are less than impressed with the flotsam and jetsam appearing on the Grand Union Canal towpath each time the city experiences a downpour. Flooding causes sewage to spew from the city's Victorian drainage system across the canal bank and into the canal itself.

But now, 110 years after the original egg-section brick sewer was built, the design flaw that leads to this unpleasant chain of events is being eradicated, as the old sewer is to be replaced. Severn Trent Water has embarked on the construction of a new 1.4km long, 1,500mm internal diameter pipeline, diverting combined waste and stormwater discharges into Leicester's Abbey sewer.

Work on the new £3.5M concrete lined sewer is being carried out to a tight 44 week schedule.

Consultant Haswell, responsible for designing the scheme, specified tunnel excavation by backhoe to achieve necessary productivity rates in the homogenous mudstone underlying Leicester.

Nine access shafts have been constructed along the alignment since work got under way in August. Now, two weeks into tunnelling proper, contractor DCT is making progress of just over 1m/h using a Herrenknecht SM2 shield mounted backhoe excavator.

The tunnel is relatively shallow - the invert is only 11m below ground. 'We could have done a cut and cover scheme, ' says Haswell project manager Steve Feeley. But he goes on to explain that as the sewer is running beneath Leicester's congested city centre, anything other than full-face excavation would have created street-level mayhem.

As it is, the access shafts used to install the excavator and for mucking out have been sited where they will cause least disruption. Leicester City Council has ordered a six week halt to work in the run up to Christmas so that the little work there is above ground will not disrupt festive shopping.

Alignment of the tunnel itself has to a large extent been determined by structures in its path. It has to skirt round the piled foundations of a 22 storey office block, and passes beneath the footings of two highway flyover structures and a pedestrian subway. Feeley says there is also a plethora of major services to negotiate.

Ground movement is being monitored with multiple rod extensometers sited along roads adjacent to the route, but as tunnelling advances Feeley doubts stabilisation will be needed, conditions are so good.

The precast concrete lining will be installed in 2.5m lengths by pipe jacking, with the excavator propelled forward by rams pushing against the leading lining element. It will be grouted solid on completion of jacking.

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