Bachy Soletanche enjoys the hillside views during Nab Scar project
Leading geotechnical specialist, Bachy Soletanche, is working on a £1.2M project to strengthen the surrounding ground of the Thirlmere Aqueduct at Nab Scar near Grasmere in the Lake District. Working from a great height, the Bachy Soletanche team is conducting a two phase permeation grouting, mini-piling and tension pile project alongside a fast track four week outage. The stabilisation project is due for completion within the next few weeks [end of December].
Hand built in the 1890’s the Thirlmere Aqueduct, which runs from the Lake District to Manchester, has some areas of concern. Subsequently over a five year period, the United Utilities owned aqueduct has been shut down for short periods of time as part of a detailed maintenance programme.
At Nab Scar, spoil originally arising from sections of the tunnel was used to cover the aqueduct at that section. 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 section of the aqueduct that required the expertise and some very careful forward planning from Bachy Soletanche. When presented with the project, the team had to not only use suitable techniques, but also get its machinery up a 440m fell and create a platform off a 40 degree cliff face. Daniel Barnard, contracts manager for Bachy Soletanche’s Anchor, Grouting and Mini-piling Division said:
“The greatest challenge we faced was the logistics and health and safety aspects of the project. We envisaged several ways to get the equipment up the fell, even briefly considering using helicopters. However our whole concept was based around the cost effective use of mini piling rigs and grouting equipment so with some very ingenious design solutions, we successfully got the equipment up the fell.”
Bachy Soletanche is using its permeation grouting technique to stabilise areas of rock which have been disturbed over the years. This will transfer the supporting loads to the minipile A frame groups also constructed. The team is installing, 34, 220mm diameter mini piles, drilled up to a depth of 16m into the hillside where the aqueduct crosses Nab Scar. Tension piles will also be constructed in conjunction with the mini piles. This combination of techniques is the best viable solution to successfully carry out the remedial work due to the restricted access of the aqueduct and getting the equipment up the fell.
The final stage of the programme was to be completed during the October 2010 outage, however Bachy Soletanche has been able to demonstrate to the client that works can be confidently completed in one extended visit rather than returning in 2010. This is thanks to a successful conclusion of the repair works and monitoring installations completed inside the aqueduct during the planned outage, as well as some very delicate piling operations.
The Thirlmere Aqueduct, completed in 1894, is 134km in length and is the longest gravity-fed aqueduct in the country. The tunnel transports water at three miles per hour from the Lake District to Manchester and takes just over a day to reach its final destination. At 2.1m diameters it would be possible to drive a small car through it in places.