Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | FEBRUARY 2015 | 21 Wimbleball Dam pictured from the air A major project to repair the foundations of Wimbleball Dam made use of a temporary water treatment solution to protect the Haddeo Valley in Exmoor National Park while the work was taking place, safeguarding the environment and drinking water supplies. With a storage volume of 21.5M cubic metres, Wimbleball Reservoir is a strategic water resource for both South West Water and Wessex Water, and was formed in 1979 by the construction of a 49m high concrete buttress dam. During dry summers, water is released from the dam to support abstractions at downstream Water resources Securing the future of Wimbleball Dam water treatment works serving 150,000 people in Tiverton and Exeter, while water may also be pumped directly from the dam to the water treatment works serving Taunton and Central Somerset. However, since Wimbleball Dam was commissioned, water has seeped through its foundations, particularly at its southern end. A limited pro- gramme of grouting in 2003 reduced the flow rate passing through the bed rock, but did not eliminate the prob- Project focus ● Grouting work repairs foundations of 35-year old dam ● Effective waste management system required to protect watercourse in Exmoor National Park ● Temporary treatment solution addresses solids and pH issues James Brockett EDITOR, WATER & WASTEWATER TREATMENT ● Grouting work was required at Wimbleball Dam because of seepage through its foundations evident since it was com- missioned in 1979 ● environmental protec- tion was crucial to the project because of the sensitivity of the area, with recreational water us- ers and a nearby fish ● pH and turbidity needed to be strictly controlled to meet ea regulations • Drivers lem. For this reason, the decision was taken in AMP5 to complete an exten- sive programme of grouting works, to reduce the flow passing through the dam's existing grout curtain. With CH2M Hill (formerly Halcrow) as the designers and geotechnical specialists Bachy Soletanche as the contractor, the grouting work began in April 2013 and was completed by June 2014. The works consisted of 103 grout injection holes, 5,500m of drilling and the injection of 670 cubic metres of grout. The intensive drilling and grouting programme involved three drilling rigs and two self-contained and automated grout injection units (each capable of up to six simultaneous injections) enabling up to 50 grout stages to be completed in a week. "The majority of the works were conducted through descending stage drilling and grouting," says Chris Olley, Project Manager at Bachy Soletanche. "This consists of drilling a 4m 'stage' in the rock and then injecting grout, utilising Bachy's Grout Intensity Number. The following shiž will involve drilling through that stage, progressing another 4m in rock and repeating the exercise." The drilling and grouting went as deep as 80m beneath the dam. As well as its role in supplying drink- ing water, the Haddeo Valley is part of a designated National Park and is popular with recreational water users, so the impact on the environment was a concern from an early stage. "As we are drilling through a micro

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