Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/867569
10 WET NEWS SEPTEMBER 2017 Tunnelling proves a steel • The Burham Strategic Water Main scheme was one of South East Water's most challenging. Why? The M20, a railway line and the navigable River Medway were in the way. main with challenging crossings of the tidal River Medway, the Medway Valley Railway Line and the M20 motorway. The scheme was planned and designed by SEW's inte- grated team (including the assets, operations, environmen- tal, communications and cus- tomer services departments) and the framework consultant, Jacobs, all located on a single site. The close collaboration that this fostered was the catalyst for realising additional benefits beyond the original objective of replacing a leaky main. During the project appraisal stage, the design team devel- oped five route options varying in length, land-use, environ- mental and social impact and cost. Each option was designed to avoid the SSSI, cater for the area's predicted future popula- tion growth and increased tap water demand from 14.6Ml/d to 21.2Ml/d. But by drawing on the insights of the broader team, further value was added by rec- ognising opportunities to rein- force the supply to other areas and improve network connectiv- ity and resilience. One of the biggest design challenges was crossing the M20, the railway line and the navigable River Medway with- out disrupting commuter or boating traffic. A number of trenchless crossing methods were considered as part of the route selection, including micro-tunnelling, auger boring and directional drilling. To determine the most suitable method a geotechnical investi- gation was commissioned. The report indicated the presence of Hythe Formation overlain by a thin layer of Sand- gate Formation and River Ter- race Deposits. This guided the project team to opt for twin micro-tunnels, up to 20m below ground and 200m in length, for each crossing and NEC3 ECC Option C documents were drawn up accordingly for a design and build contract. Farrans Construction was awarded the contract following a competitive tender process and construction began in Janu- ary 2015. Farrans proposed a change in crossings arrangements, from ONSITE InfRaSTRucTuRE The aggressive nature of the marshland soil had led to nine bursts within five years The in-situ concrete caissons proved their worth, as the shafts could be sunk underwater or in the dry The closed-faced tunnel boring machine Lay-rates of up to 240m a day were achieved by each gang. Pictures courtesy of SEW The 600mm ID ductile iron pipes were jacked into the tunnel using the same jacking frame used for the tunnelling process its required depth. This system provided a fully reinforced con- crete structure with cast-in water-stops at the joints, signifi- cantly reducing the risk of leaks in the sha™. This system also provided better flotation pre- vention due to the sha™'s increased wall thickness result- ing in a shallower depth of base excavation. The sha™ pipework is designed to last 80 years and to reduce the safety risks associ- ated with future maintenance, the pipe is designed to ensure the majority of activities can be carried out from above ground. For the open-cut pipe laying, Farrans utilised excavators with GPS control. Uploading the pipe's 3D model to the excavator increased operator accuracy, reducing the need for rework or for personnel to enter the pipe trench excavation. This helped achieve lay-rates of up to 240m per day per gang. The first major challenge arose when the ground condi- tions encountered during sha™ and tunnel construction proved to be more variable than investi- gations suggested. Weak and fractured sandstone rock was uncovered, with groundwater flow of up to 150l/s. Dealing with this water required consul- tation with the Environment Agency (EA) and other organisa- tions to agree discharge pathways. The switch to in-situ concrete caissons proved their worth, as S outh East Water (SEW) has delivered one of its largest and most challenging pipeline schemes to date, secur- ing the water supply to thou- sands of homes and businesses in north Kent and providing the area with critical new infra- structure needed to promote development and growth for decades to come. The 6.5km Burham Strategic Water Main is designed to deliver nine million litres of water daily from Burham Water Treatment Works (WTW) to a population of more than 19,000 in Aylesford, New Hythe, Ditton and Snodland while also aug- menting supply to the Aylesford Reservoir which feeds Maidstone. The new pipeline has replaced an existing trunk main that crosses a flood plain and the Holborough to Burham Marshes Site of Special Scien- tific Interest (SSSI). The aggres- sive nature of the marshland soil had led to nine bursts within five years. Access for repair is difficult due to the marshland conditions and can cause significant damage to sen- sitive flora and fauna. The environmental impact, the substantial risk to custom- ers' tap water supply and an average repair bill of £250,000 meant a solution to protect this critical water supply was urgently needed. The solution turned out to be 6.5km of new ductile iron water twin micro-tunnels to a single, larger diameter tunnel to con- tain both pipes and the tunnel- ling sub-contractor, Ward & Burke, selected an Herrenkne- cht AVN 1600 AB closed faced tunnel boring machine. Similarly, the tender design for the tunnels' launch and reception sha™s was revisited. Initially conceived as precast segmental sha™s, these were redesigned as in-situ reinforced concrete caissons. A steel cut- ting shoe was installed at the base of the caisson and an 8m diameter concrete ring cast in- situ in a series of 2.5m li™s above ground. Excavating from inside and undermining the sha™ allowed it to sink under its own weight, the process being repeated until the sha™ reached