As part of the Derry Drainage Area Plan (DAP) capital works programme contracted to Lagan Construction Group on behalf of Northern Ireland Water, sewer rehabilitation schemes were identified and incorporated as two separate contracts – East and West of River Foyle.
The West contract included the rehabilitation and replacement of pipe work identified as being subject to extensive structural and serviceability defects. The City side of Derry and the East contract addressed the sewers in poor structural condition or suffering from serviceability defects, located Waterside of the of the River Foyle, with the aim significantly reducing the risk of out of sewer flooding and removing properties from the DG5 Flood Register.
The contracts were split into 2 stages; Stage 1 – “Soft Rehab” and Stage 2 “Hard Rehab”. The “soft rehab” involved the Closed circuit Television (CCTV) survey and the rehabilitation of 15,000 meters of existing sewers by internal root cutting, refurbishment of intruding lateral connections, removal of obstructions, encrustations and silt, using trenchless techniques. Within the “soft rehab” stage, a recommendation report and a set of drawings were produced based on CCTV survey reports. A defect calculation was done to decide the extent of rehab on each section of sewer and this formed the basis for the “hard rehab” scope of works. The “hard rehab” works involved the upgrade of the existing sewers mainly by the use of Cured in Place sewer relining and to a lesser extent, traditional open cut methods.
The main quantities involved were as follows:
• 186 No. Localised Patch repairs
• 7,781m of Cured in Place Pipes (CIPP) – combination of Hot Water Cured Needle Felt and UV Cured GRP, (Range in diameter 150-1,050mm)
• 39 No. Top Hats (Trenchless Sewer connections)
• Brick sewer removal and replacement via “Man Entry Repairs”
• Remove concrete and other obstructions remotely
• Remote opening of over 30,000 sewer connections
• 30 No. New manholes installed
• 504m of on line open cut pipe replacement (Range in diameter 150-600mm)
• Manhole re-benched
• Pipes wrapped to prevent root ingress