Light at the end of the tunnel
Old sewers and surface water drains are being replaced or repaired in Sudbury town centre. Anglian Water hopes the project will reduce flooding and pollution of the River Stour
A new main sewer is now being built to collect the offending sewage which will now be pumped west, below the north bank of the river, to Sudbury's STW which has also been upgraded by Laing and Purac. Many of the existing sewers and surface drains are brick-built and date from Victorian times. Some are still in good condition after 100 years of service but others are in urgent need of replacement or re-lining. Young added: "The engineering that went into these sewers was incredible. We have some original drawings from 1901 which themselves must have taken hours to create."
Today pipes which would have taken weeks to build can be installed in minutes. Where the old pipes can be saved they are being lined by Insituform and elsewhere replaced by Barhale using 1,200mm concrete jacking pipes supplied by Stanton Bonna Concrete and 600mm clay pipes supplied by Naylor Drainage.
Most of the drives for the new sewers have now have been completed. Barhale has been using machinery supplied by Japanese firm Iseki to jack up to 100m of pipe at a time. Much longer drives are possible - in Cambridge Barhale recently completed a drive of 307m. Staging pits are first dug at intervals along the route, with sheet piling and braces where necessary to hold back the saturated ground. In Sudbury groundwater is in many places just one metre below the surface.
The tunnelling machine is lowered into the staging pit along with the two main jacks, which push against anchored props rather like train buffers. As the tunnelling machine advances, sections of pipe are lowered into place behind it and pushed forward by the jacks which can each apply up to 750t. Young said: "Here we have not had to use more than 225t - there is really no need to worry until you have to apply more than 450t."
The orientation of the tunnelling machine is constantly monitored from a control room at the top of the staging pit and a laser is used to assess the accuracy of progress within a few mm. The technique has a tolerance of 1-1.50 but keeping a straight track is the key to a successful drive. In Sudbury tunnelling is being carried out at depths of 1-4.5m, which is relatively shallow for pipejacking, so accurate monitoring of progress is essential.
To help keep friction to a minimum, a bentonite-based lubricant mix is also injected under pressure between the pipes and tunnel wall. The larger Stanton Bonna pipes also have Denso seals to create a water-tight connection. Jacking pipes are made with a steel band which increases their cost but also increases their strength to cope with the enormous forces involved. Stanton Bonna's managing director Barry Cooper said: "We have been using machine-cast jacking pipes in Britain for three years. In Europe, where the standards for square ends are not as strict, they have been used for about 12 years." It is essential pipe ends are square to transfer loads down the jacked line and achieve water-tight joints.
The surface of a machine-cast pipe is not quite as smooth as a pipe made by the older spinning process but Cooper claims: "As long as the bentonite mix is applied properly this should not be a problem." As a precaution if pipes become jammed, interjacks are installed at points along each drive so that force can be applied to pipes nearer the drilling face. No problems apart from an unexpected brick wall have been encountered in Sudbury and so the interjacks have not yet been called into play.
Barhale is trying to coincide part of its work with the demolition of the Ballingdon bridge which crosses the river south-west of the town. This will change the route of the one-way system and should take pressure off the roads to be worked on. Despite efforts to minimise disruption local press have dubbed the project "Nightmare on East Street."
Several sections of new surface water drain are still to be jacked along East Street, one of the main shopping streets in the town. Barhale hopes to complete the project by Christmas instead of the official February 2002 deadline.
Throughout the project Barhale and Anglian have been in discussion with the council and providers of other services to ensure the work is completed as quickly and safely as possible. Young said: "Can you imagine the chaos if we'd had to open trench down to four metres? It doesn't really bear thinking about."
Ground settlement is being monitored at 500 points around the town and as Sudbury is a town of significant historical interest, archaeological investigations have been carried out. According to Anglian's spokeswoman Sarah Rowland: "Archaeologists have been attending all of the pit excavations, but so far they haven't found anything."
Barhale is one of a number of preferred contractors on a partnering panel with
Anglian, which also includes Miller Tunnelling. Barhale's job list includes
projects ranging from 0.5-7M in value. The contract at Sudbury is worth around
£3M to Barhale; a project recently completed in Oxford netted the company