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New tunnel to clear 57 properties from the flood register

TBM or not TBM; that was the question facing Damian McGirr, an associate at Donaldson Associates, when deciding the best way to build an 800m tunnel for the second phase of the Bristol City Centre Flood Alleviation scheme.

The aim of the Wessex Water project was to remove 57 properties from the flood register by April 2009.

Wessex Water is funding the £9.7M project to build a tunnel that will divert the overspill of water to a larger existing tunnel, and eventually away from the city.

A length of Victorian sewer running down Trenchard Street is at the heart of the problem. Here the sewer levels off for about 0.8km and is subject to pressure from rainfall running along a downhill section to it.

McGirr, who gives a talk at One Great George Street on the subject on 21 February, will be accompanied by Wessex Engineering & Construction Services contracts manager Richard Soloman and construction manager Mark Thomas.

The tunnel is being constructed by drill and blast techniques below Bristol University to connect to the existing Northern Foul Water Interceptor (NFWI) Sewer. It passes through interbedded sandstone, limestone, siltstone and mudstone from the Quartzitic Sandstone Group and the Hotwells Group (Cromhall Sandstone Group).

The new tunnel will take foul water from the problematic section into the NFWI waste water tunnel built 15 years ago and, it is hoped, remove properties from the at-risk register in the process.

The NFWI is a trunk sewer and has enough spare capacity to take the 2000l/s being transferred from the old sewer.

"My presentation will outline issues like choosing the route alignment, permanent works design, site investigation and the geotechnical issues," said McGirr.

So why was drill and blasting chosen? "Well, that's a long story," said the quietly spoken Tyrone man. "You'll have to come along to the presentation to find out," he joked.

- 'Drill and blast tunnelling Bristol City Centre Flood Alleviation Phase II' takes place at 6pm at ICE on 21 February. For more information, contact Ms Jade Donovan 020 7665 2233 or email jade.donovan@ice.org.uk

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