Clever use of trenchless technology is making life safer and cheaper on construction of a new sewer, reports Jon Masters.
Two tunnelling innovations developed by specialist water contractor Barhale Construction are helping to improve operative safety and increase working efficiency on a series of sewer drives at Welwyn Garden City.
Both developments reduce operatives' exposure to working in confined spaces, leading to improved health and safety as well as a more efficient tunnelling process.
Welwyn Garden City's southern outfall sewer will increase capacity of the town's wastewater system.
Construction is being carried out by Barhale as part of its AMP3 framework agreement with Thames Water.
A total of 3.6km of new sewer is being driven to allow sewage to be transferred via gravity to the local main trunk sewer.
A 1,200mm diameter Iseki tunnel boring machine (TBM) is completing drives up to 320m in length from a total of 14 shafts sunk to an average depth of 25m.
The first innovation introduced at Welwyn is a tunnel drive lubrication system which removes the need for operatives to enter the tunnel to operate valves manually. A polymer based lubricant is remotely injected through valves in the tunnel pipe work, while a touch screen computer linked to pressure sensors behind individual pipes allows lubricant to be injected as friction levels demand.
'This system reduces exposure to work in confined spaces and increases efficiency as control of friction, lubrication and pressure is now more precise, ' says Barhale project manager Paul Salmon. 'On one recent drive, just 150 tonnes of jacking force was needed to push around 450 tonnes of pipe work, due to a more effective reduction of friction and pressure.'
The site team at Welwyn has also introduced curved sleepers with integrated neoprene rollers for use in tunnels, to aid easy removal of slurry pipes and cables following completion of a drive. Sleepers were placed at regular intervals behind the TBM as it moved forward, ultimately allowing pipes and cables to be removed easily with winch equipment in the reception shaft.
Use of the sleepers and rollers means there is no need for operatives to enter the tunnel to disconnect and remove individual pipes manually. Barhale's steel fabrication division, BCS, spent £36,500 developing and manufacturing the pipe rollers, and their use at Welwyn has saved around £124,000 compared to the cost of manual removal of slurry pipes.
'Use of the system has enabled particularly long drives to be achieved and has removed the need for one of the Welwyn shafts to be constructed, leading to an overall saving of £153,000 and delivery well ahead of programme, ' Salmon says.
Work to construct sewers at Welwyn Garden City totals £11.3M and is being carried out on behalf of Thames Water's Network North Alliance, which brings together the client, contractor Barhale and support services company Enterprise. This spring the Alliance entered the fifth and final year of Thames Water's AMP3 capital works programme.
'From a client point of view, the project represents the best in alliance working and microtunnelling construction, ' says Thames Water senior project manager Russell Waller. 'Network North Alliance contracts are based on Green Book target cost with Thames Water amendments that encourage early involvement and maximum input from contractors towards innovative, safe and cost effective designs.
'The use of trenchless technology at Welwyn has undoubtedly reduced disruption and environmental risk at surface level and the measures introduced by Barhale have produced a similar impact below ground.
All pipes are double sealed and an eco-friendly lubricant system is being used to minimise potential for contamination.'