Light at the end of the tunnel

Sudbury in Suffolk is to benefit from a new network of sewers and surface drains,

many of which are being jacked to minimise traffic disruption.

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

£6M