Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/468709
1 0 WET NEWS march 2015 The project has an exemplary safety record with 120,000 man hours worked without a reportable incident the Environment Agency (EA) is focusing on reducing nitrate concentration in a major aquifer – one of the drivers for the scheme. Part of the brief was to seal and repair existing sewers and manholes and build new overflow sewers to contain the flows, protecting the aquifer long into the future. The team also built new ─ and enlarged existing ─ man- holes to make access easier and safer. The project has seen a num- ber of innovative techniques used. These have included the development of bespoke trench- less techniques that had a small site footprint, reducing disrup- tion and limiting the environ- mental impact. Initially, the headings needed to be fully surveyed to assess the extent of the prepara- tion works and several chal- lenges were encountered. These included collapsed sections in the crown of the headings, cre- ating voids that needed timber supports. Additionally unchar- tered headings and sewers were located where the sewer records did not always match the net- work that has developed over the years. Chalk collapses To overcome these issues, the project team continually reviewed the design require- ments and network hydraulic model to provide re-scoping solutions. Specialist technicians, who are trained to work in deep tun- nels, prepared the headings for the installation of the new pipe by clearing debris and chalk col- lapses, repairing and removing damaged sections of the exist- ing sewer pipes, laying a con- crete base for the installation of a cured-in-place-pipe (CIPP) liner and installing timber sup- port around the headings, where required. Great care was taken to use techniques that avoided the use of any substances that may have a detrimental effect on the aquifer. Another innovation was the development and adoption of a stand-alone CIPP lining system for the chalk headings. Consultation with the Envi- ronment Agency provided clear guidelines on the materials that could be used in contact with the chalk aquifer and it was important to avoid the use of chemical or cementitious grouts that could flow within the cracks. This presented a chal- lenge as traditionally the lining would have been created by the installation of a standard pipe surrounded by grout infill. To address this, CIPP liners were designed and tested that negated the need for any grout infill. This liner is a composite, seamless closed liner, compris- ing layers of unwoven polyester and glass fibre webs made with an ultraviolet, light-sensitive polyester resin. The liners have an internal and external poly- ethylene foil layer and an exter- nal HDPE protective grid foil sheath which together provide a completely sealed liner. Shock impacts To provide added protection to the liner a PVC-coated polyester foil is wrapped around the liner prior to installation. The liners have been reinforced to have sufficient strength to be self- supporting and the grid foil is designed to prevent over-infla- tion and provide protection against the rough chalk walls of the headings. The liners were subject to rig- orous testing to ensure they could withstand any shock impacts from chalk blocks fall- ing from the heading soffits. Engineers are working deep underground, refurbishing the sewers and building new ones where necessary ONSITE sEwEr maINTENaNcE