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Healing the Lake District's scar

At Nab Scar in the Lake District geotechnical contractor Bachy Soletanche is helping to guarantee Manchester’s water supply.

Thirlmere Aqueduct is one of the unsung heroes of Victorian engineering. Built in the 1890s the 134km-long aqueduct runs from Thirlmere Reservoir in the Lake District and Heaton Park Reservoir, bringing fresh water to the thirsty population of Manchester.

But recently the 2.1m diameter structure has been showing signs of age and is causing its owner water company United Utilities, some areas of concern. As part of a detailed maintenance programme it has been closed down for short periods so that work can be carried out on the predominantly cut and cover built structure.

“The greatest challenge we faced was the logistics and health and safety aspects of the project”

Richard Piggin

One particular area around the rocky outcrop of Nab Scar, which towers above Rydal Water near Grasmere, is causing particular concern and Bachy Soletanche was drafted in under a £1.2M scheme to help stabilise and strengthen ground around the section.

The Bachy team used a twophase permeation grouting, mini-piling and tension pile project on a fast-track scheme that has seen the work carried out in just four weeks.

Spoil originally arising from sections of the tunnel was used to cover the aqueduct at that section when it was first built. As time has gone by this spoil has weakened posing a danger of the slope moving, subsequently causing damage to the aqueduct.

It was this area of the scheme that required the expertise and some very careful forward planning from the site.

On the ground at Nab Scar

On the ground at Nab Scar

The difficulty wasn’t just in using the technique most suitable for installing piles at the site but also in getting its plant and equipment up a 440m fell and creating a platform off a 40-degree cliff face. Even helicopters were considered to lift in the team’s equipment.

It was, admits Richard Piggin, contracts manager for Bachy Soletanche’s anchor, grouting and mini-piling division, the biggest challenge over the whole project.

“The greatest test we faced was the logistics and health and safety aspects of the project,” he says. “We looked at several ways to get the equipment up the fell.

“Our whole concept was based around the cost effective use of mini piling rigs and grouting equipment.”

“However, our whole concept was based around the cost effective use of mini piling rigs and grouting equipment so with some ingenious design solutions, we got the equipment up the fell.”

The team used its permeation grouting technique to stabilise areas of rock which have been disturbed over the years. This transferred the supporting loads to the mini-pile ‘A’ frame groups also constructed. Some 34, 220mm diameter mini piles were installed, drilled up to a depth of 16m into the hillside where the aqueduct crosses Nab Scar. Tension piles were also used in conjunction with the mini piles.

Initially, the final stage of the programme was to be completed during a planned outage in October, but thanks to the work it had already completed the Bachy Soletanche team has been able to demonstrate that the works can be completed in one extended visit rather than returning later in the year.

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