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WATCH POINT

GEOENVIRONMENTAL ENGINEERING

Remote structural and ground monitoring is keeping a close watch on improvement works to Lewes' sewerage system.

Lewes, near Brighton in East Sussex, has a problem. When it rains heavily, diluted sewage can enter the River Ouse and other watercourses in the town.

'There is a combined system of sewers, some Victorian, in the old part of the town, which means a single pipe takes away both rainwater and sewage for treatment, ' explains Southern Water business manager Wayne Middleton.

'During heavy storms, the sewers are not big enough to cope with the increase in flows, so to prevent flooding the stormwater has to be released into local rivers and watercourses through 20 overflows.'

Now work is under way on a solution.

Southern Water's £16M Lewes CSOs Improvement Scheme includes 1.1km of new 1.2m diameter tunnels beneath the town, capable of storing up to 2M litres of water; upgrading of sewers to relieve localised flooding; and two new underground pumping stations.

The result will be a big improvement in water quality in the River Ouse and its associated catchments, the Winterbourne Stream and Upper Pells.

The construction team is a joint venture of designer Black & Veatch (B&V) and contractor Costain.Work began earlier this year.

Middleton says: 'By constructing a single, large tunnel, closing off all these unsatisfactory overflows, and building just a single overflow to release stormwater into the Ouse, we can substantially reduce this problem and meet stringent Environment Agency water quality standards.'

B&V lead design engineer Richard Walton says the retained flows will be transferred to Newhaven Sewage Treatment Works for treatment.

'This scheme is not intended to reduce the risk of flooding from the River Ouse [Lewes was hit by severe flooding in winter 2000 and 2001];

that is being dealt with for the Environment Agency by another part of Black & Veatch, ' he adds.

Three shafts up to 20m deep have been sunk into the Chalk bedrock by caisson techniques. A Herrenknecht TBM started tunnelling the first drive on June 10, and an Iseki machine began the second drive on June 17.

Walton says extensive ground and hydrogeological investigations, including consultation with Chalk expert (and Lewes resident) Professor Rory Mortimore of the University of Brighton, gave the design team confidence in their proposals.

However, he adds, the tunnel will be driven beneath some historic buildings as well as the London to Eastbourne main railway, and the possibility of unexpected ground conditions can never be ruled out.

B&V geotechnical director Raymond Coe says a comprehensive monitoring regime was specified because of work's sensitivity.

'This will keep the design and build teams informed at all times of the ground movements resulting from site operations as well as helping to allay the concerns of Lewes residents, ' he says.

A combination of automated and manual monitoring devices will record building movements and ground surface subsidence and the observations will be used to validate design assumptions.

The site includes a number of listed and residential properties spread over a wide area, so traditional instrumentation, either manually read or cabled to data acquisition units, would have been too costly and too intrusive, explains Nick Slater of subcontractor Instrumentation Testing and Monitoring.

'Manual surveying was considered but the frequency of data capture required, coupled with the need to access private gardens to take levels, ruled this out, ' he says.

ITM proposed a network of cable-free sensors from Soil Instruments for the primary monitoring regime, complementing the manual survey which could be carried out in roads and more accessible areas.

'The cable-free sensors are discrete devices which require no external power supply, transmitting for up to 10 years on their standard internal battery, ' Slater says.

'Operating on the new EU-wide 868870MHz frequency range, each sensor contains the sensing element, excitation, signal conditioning, analogue to digital converter and a unique ID code.'

Data is transferred every 15 minutes.The sensor sends a digitally encoded signal containing the measured data and the ID code to a receiver/logger in a convenient location, which sends it on to the office using a built-in GSM modem.

For the Lewes contract data is collected and processed by ITM and posted to a web page that is automatically updated every hour.

'If a sensor passes an individually selected blue, amber or red alert threshold, then the duty engineer receives a text message and can access the data from any internet-enabled PC by using the project log-in and password, ' Slater says.

'The cable-free system has delivered high quality stable data at a lower cost than any other means, including manual readings, ' he claims.

Tunnelling is due to finish in November and the improved sewerage system is due to become operational in June next year.

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