Sewer rehabilitation saves money, time and disruption across city of Derby

July 23, 2007
During Year 1 of Severn Trent Water's (STW) AMP4 delivery period, an extensive sewer rehabilitation program was promoted for the City of Derby, UK. The majority of the sewers within the scheme were of critical status (Grade 4 & 5) and in urgent need of repair, as the majority were at the point of collapse. An additional problem was that the sewers in question were situated in traffic sensitive areas. The rapid repair of these sewers throughout Derby was essential to prevent any disruption to...

DERBY, UK -- During Year 1 of Severn Trent Water's (STW) AMP4 delivery period, an extensive sewer rehabilitation program was promoted for the City of Derby, UK. The majority of the sewers within the scheme were of critical status (Grade 4 & 5) and in urgent need of repair, as the majority were at the point of collapse. An additional problem was that the sewers in question were situated in traffic sensitive areas. The rapid repair of these sewers throughout Derby was essential to prevent any disruption to STW's level of service to its customers.

Having investigated various repair options, assessing cost and other program constraints, STW Engineering decided a 'No-Dig' approach would provide the best, least disruptive solution.

STW Engineering staff commenced work on the various projects in March 2005. Considering the volume of work to be completed and to ensure that the projects were completed within a year, it was essential that STW's contractors were involved in the early stages of the projects' development. Whitehouse Civil Engineering was selected as 'Principal Contractor' for the works and immediately made a site agent available, to help scope the project and then progress the works through to completion. Whitehouse's CIPP lining contractor, OnSite, also made an engineer available to work on the projects from feasibility through to completion.

The original feasibility brief highlighted a total of 10.6 km of Grade 4/5 sewers, which were thought to require immediate rehabilitation, covering nine individual projects. The Project Team comprising STW, Whitehouse and OnSite conducted a detailed analysis of these lengths, as well as further lengths, within the Derby City area. The analysis comprised surveying the necessary pipelines and conducting scoping meetings to determine the extent of work required at each location.

Following this detailed and swift analysis of these critical sewer lengths, the Project Team highlighted an actual total of 6.5 km of Grade 4/5 sewers that required immediate rehabilitation and renovation. The analysis reassessed some sewer lengths that were found to be Grade 3 and so could be eliminated from the critical sewer work program. This enabled a realistic magnitude of rehabilitation to be completed, taking into account the complex, traffic sensitive nature of the scheme locations and the timescales involved.

The use of a 'fixed price' contract meant that, for the project to be safe and successful, the full extent of work required had to be investigated and detailed, and all construction risks minimized during the feasibility/design period prior to construction work commencing. An agreement to commence construction on the first project was awarded in early July 2005, with all of the works being completed by the end of March 2006. The extent of the works consisted of some 6.5 km of sewer renovation carried out, with only 150 m of open-cut excavation being required. Within this work some 80 sewer lengths were relined in 50 critical locations, including 20 in Traffic Sensitive Streets.

As part of the program, OnSite's also invested in new robotic technology to further extend the advantages of 'No-Dig' technology and to eliminate the need to construct a 7 m deep manhole adjacent to some 'Listed' residential properties.

Environmental Initiative
At an early stage of the design process it was decided that the majority of the sewer repairs would be completed using sewer relining techniques. Initial discussions between the project team highlighted a number of possible problems associated with arranging for delivery of clean water, which would be used in the lining process, and the disposal of contaminated curing water following the completion lining cure. Due to the vast lengths of sewer across various sites in Derby, it was predicted that huge quantities of clean water would be needed in the lining process and similar volumes of contaminated curing water would require safe removal and disposal.

Two initiatives were proposed and ultimately implemented on all the projects which involved modifying the start and end processes of the lining method. The first initiative was the use of final effluent in the lining process. This involved utilizing final effluent from Derby Sewage Treatment Works, in place of clean (drinking) water in the installation operation. This meant approximately 4.6 million liters of treated, clean water would not be removed from the local supply network.

The second initiative involved disposal of the contaminated curing water directly into the Foul Sewer System and onto Derby Sewage Treatment Works. Severn Trent Water has not previously been in favor of disposing of curing water in this way because of the possible effects on the biological sewage treatment process. Previously high concentrations of curing water entering into Sewage Treatment Works have been known to completely wipe out the treatment process, as the bacteria involved in the breakdown of the sewage have been killed by the high styrene concentrations.

There was also potential for possible effects on a water supply abstraction point downstream of Derby Sewage Treatment Works. Styrene is a very robust chemical and is therefore only broken down in limited portions by the treatment process; however it can easily be diluted to harmless concentrations in clean water.

By involving STW's Technology and Design section in the planning process, maximum discharge rates were agreed that would prevent styrene levels reaching concentrations that would affect either process.

This initiative removed the need to tanker out the 4.6 million liters of contaminated curing water to Sheffield, a round trip of approximately 94 miles per tanker journey.

The areas involved on the Derby projects also covered some 220,000 STW customers whose living environment would be exposed to the sewer rehabilitation work. Therefore both STW Engineering and OnSite carried out a concentrated customer care exercise informing all members of the public of when the works were to proceed and what they would expect to see.

To minimize disruption, work on traffic sensitive sites were completed at night and an extensive Public Relations campaign was undertaken including monthly press releases, extensive customer care correspondence and regular updates on project developments. Over 14,000 Customer Care letters were delivered during the course of the works.

Whitehouse Construction successfully carried out all the enabling, and traffic management work prior to OnSite starting its sewer rehabilitation work.

Lining Process
The lining process used throughout the Derby works was OnSite's Premier-Pipe lining system.

Premier-Pipe is a CIPP (Cured in Place Pipe) renovation process, which is installed without the need for costly and disruptive excavation. Premier-Pipe liners are individually manufactured to suit the dimensions of the pipes to be lined, the lining thickness being determined by individual design requirements.

The lining tube is made of polyester felt with an outer coating of polyurethane, which is impregnated thoroughly with a liquid resin chosen to suit the working environment of the pipe.

Once impregnated with resin the Premier-Pipe is normally installed using the scaffold tower inversion method. However depending on the application in hand and the particular site access alternative inversion systems that may be utilized as necessary.

Across the various projects in Derby, sewers renovated ranged in diameter from 225 mm through to 1,200 mm, including several large Victorian brick-egg shaped sewers at depths of up to 7 m. Many sites were also located next to Grade II listed buildings. Part of the program included approximately 600 m of Premier-Pipe being installed into a main 900 mm diameter trunk sewer that ran through a railway engineering works. This section was completed without causing any problems or delays to production deadlines.

Project Successes
In the event, on completion of the works, both client and contractor were very pleased with the outcome. Financially the out-turn costs of the project were lower than the original Target Price, the works were completed to give a quality product within the specified contract period, there were no accidents and dangerous occurrences during the course of the operations and no environmental incidents. There were also no customer complaints.

Commercially, the whole scheme was very cost effective with a final spend of approximately £3 million, against an anticipated cost of £5million had the work had been carried out using traditional open-cut methods of repair. Open cutting would have also caused tremendous disruptions and delays to the whole city of Derby.

STW's working relationship with Derby City Council was also enhanced due to the high standard of workmanship, program planning and minimization of traffic disruption during the construction work.

The project also contributed to STW's British Safety Council Five Star Award and the works were presented with the STW Sustainability Award.


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