Water & Wastewater Treatment

12 | APRIL 2014 | WWT | www.wwtonline.co.uk Project focus T he city of Bristol is said to be the eighth largest city in the UK, with a core population of over 400,000. However, it serves in excess of 1M people in the greater urban zone around the city. S o it is not surprising that Bristol, with its great international trading history and engineering heritage has some of the oldest and most interesting infrastructure serving it. Whilst many people know of Brunel's Great Western Railway and the famous Cli„on Suspension Bridge, it is not always notoriety that makes infrastructure important. THe provision of drinking water to Bristol has long been of great importance to the success of the city and this requirement is the responsibility of Bristol Water, an independent private water company established in 1846, which serves some 1.2M population in the Bristol area. Aqueduct maintenance As part of its responsibility to the 1.2M population it serves, Bristol Water has to maintain the Mendip aqueduct, which, at almost 170 years old, is an excellent example of large-scale Victorian engineering. The aqueduct has been carrying untreated water from the source of the River Chew at Chewton Mendip to the reservoirs and water treatment facility at Barrow Gurney since it was constructed dur- ing the mid-1840s and was one of the ● Mixed trenchless technologies applied ● Specialist man-entry techniques ● Mock emergency rescue scenario first structures to supply fresh drinking water to the city. As might be expected of a structure of this age, it has reached a point where it is in need of some refurbishment. In a project known as the Line of Works Rehabilitation & Improvement Scheme, Bristol Water, in association with the design engineering company Black & Veatch (B&V), developed a rehabilitation programme designed to ensure that the historic aqueduct was able to continue operating for the people of Bristol for the foreseeable future. Contract award Once the design was confirmed, the contract to complete the works was awarded to main contractor Kier May "Both wet/dry spray techniques were used depending on the surface condition prevailing at the location of the installation" MAtt Durbin MAnAgIng dIRecToR, MATT duRBIn ASSocIATeS gRP sectional liners are used in some parts of the aqueduct during 2012 ● the age of the 170-year old aqueduct meant that it had reached the point where it was in need of refurbishment and strengthening ● Desire improved access and maintenance facili- ties, extend its functional life and improve the flow of water through the structure • Drivers Bristol aqueduct lining Access constraints overcome in £8M refurbishment www.wwtonline.co.uk | WWT | APRIL 2014 | 13 Gurney (KMG). As a significant part of the planned works was to use trench- less lining techniques, these works were subcontracted to rehabilitation specialist contractor Matt Durbin As- sociates. The £8M project will add a structural lining within the aqueduct in order to strengthen it, extend its functional life and improve the flow of water through the structure. The work is designed to be confined to small areas around the shaƒs that give access to the aqueduct and is largely confined to the Litton Area and in East Harptree (located at Harptree Coombe) as well as West Harptree and Chew Stoke. Phased construction Whilst the majority of the 19km-long aqueduct runs underground at depths of up to 40m through predominantly sandstone and limestone strata, it does run on surface at Harptree Coombe, crossing Watery Lane near Winford and finally as a backdrop to the village of Winford itself. The whole rehabilitation programme was planned to be completed over three years in 2012, 2013 and a final stage in 2016. It involves using a variety of techniques including: • The installation of glass- reinforced plastic (GRP) liners • Concrete spray lining • Grouting of the existing tunnel Access sha s In order to provide adequate access to the aqueduct, it was also necessary to construct three new shaƒs onto the structure, in Litton, East Harptree and near Winford. These were successfully completed in 2012. In addition to the lining works, Bristol Water is also taking the opportunity to improve existing facilities on the Line of Works system to help operate and maintain the network more safely and efficiently. The techniques listed above were ultimately chosen aƒer careful consideration of the rehabilitation available on the market. The depths and access restrictions associated with the refurbishment works prohibited cost effective use of remote refurbishment techniques and this led to the selection of the specialist man-entry techniques throughout the aqueduct. Apart from the new shaƒs, access from the surface was generally restricted to the use of existing A section of the overland aqueduct route • Challenges 1 ● Given the location of the various works and the access requirements there were a number of logistical problems associated with the pro- jects undertaken to date. These included dimen- sional constraints within the aqueduct with limited cross-section dimension. This was combined with depth and distance between surface access points, 620m being the longest, which restricted effective pumping distances for the micro- concrete spraymix, par- ticularly for the wet spray technique. Although dry spray tech- niques overcame this specific difficulty with improved pumping per- formance, the extremely confined access condi- tions within the aqueduct substantially increased non-productive opera- tional time such as clear- ance of excess material and surface finishing. ● Further to this, all access sha s were located on agricultural land and one section of work was to be carried out within a Sites of Special Scientific Interest (SSSI) area at Harptree Coombe. This meant that close liaison with landowners and all other interested parties had to be maintained and supported at all times. • Collaboration ● Due to the location and critical nature of these works, the contractors collaborated to organise a mock emergency rescue scenario, involving the specialist Hazard Area Response Team (HART) and Urban Search & Rescue (USAR) team from the emergency services. This involved extraction of a casualty from a depth of 14m and some 100m along the aqueduct, using specialist rope-access techniques. ● The trial run created good communication between all parties and provided the emergency services with an excellent chance to sharpen their skills in a live situation. With health and safety being at the forefront throughout, no accidents or incidents have occurred during the project.

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