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Water special: Bristol tunnel − Under control

Bristol’s oldest and best loved buildings are now safe from flooding following the completion of a £9M project to build a new sewer under the historic city’s streets.

The construction of a 805m long sewer tunnel to protect the historic centre of Bristol from flooding does not sound like much, but the project has been one of the most ambitious and prestigious engineering projects of its kind to have taken in place in the city for more than a decade.

The new sewer brings long awaited flood relief to some of the city’s oldest and best loved buildings in and around Denmark Street, St Augustine’s Parade and Frogmore Street. These include Bristol’s Hippodrome theatre and Hatchet Inn. Wessex Water commissioned Donaldson Associates chairman David Donaldson in 2007 to design the link from Frogmore Street in the east to Woodlands Avenue in the west.

The project has been one of the most ambitious and prestigious engineering projects of its kind to have taken in place in the city for more than a decade.

Construction work began in early 2008. The project proved challenging for engineers who had to tunnel through 18,000t of the second strongest rock in the UK. The initial tunnel was driven as a 3.5m by 3m rectangular structure by Doncaster-based subcontractor SDS.

It used drill and blast with steel frame supports, with around 500 controlled explosions needed to build the tunnel beneath some of Bristol’s most famous landmarks. Six enlarged passing bays with turning shunts were incorporated into the drive at intervals of 150m.

The job of lining the tunnel fell to Barhale Construction’s specialist tunnelling business in a £1.2M, 14 month long project.

To provide a final 2.2m diameter sewer, Barhale was keen to use its Tunneline system which uses a sectional steel shutter that is pretensioned in the host tunnel before concrete is pumped under pressure into the void. But at the Bristol tunnel, the host structure was too large for this approach. Instead Barhale designed and installed the lining in two sections.

Two sections

First, an initial lining of a three-quarter circle was cast to a diameter of 2.7m using a conventional shutter system of steel ribs and timber laggings. These were advanced at an average rate of 35m per day, working day and night to complete the 805m long tunnel.

Following on 120m behind this the Tunneline shutter was fixed and pretensioned inside the initial cast and a second crew was engaged to carry out the concreting work. In total some 5,563m³ of concrete was needed, which was quite a logistical challenge.

In total some 5,563m³ of concrete was needed, which was quite a logistical challenge.

“The main issues on the contract were resolving the logistical issues to provide an average of 400m³ of concrete a day into the two sections of tunnel that were being worked on,” says Barhale head of tunnelling services Peter Marchant.

“This had to be placed over an eight hour period so that there was sufficient time to achieve a set, then strip, move and reposition the shutters for the next day’s pour.

“Having a directly employed workforce who were used to the demands of the operation proved to be a key factor.”

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