Slope Stabilisation of a Section of the Thirlmere Aqueduct by the Use of Spaced Micropiles and Permeation Grouting-Article

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The 50 m Nab Scar section of the Thirlmere Aqueduct, built from unreinforced concrete, had developed significant cracking and joint separation. Investigations linked the damage to torsional movement caused by unstable tunnelling waste placed over and around the conduit. The steep hillside was close to its limit of stability, while construction had to avoid direct loading, vibration and drilling within 2 m of the aqueduct.<br /><br />The stabilisation design used two rows of spaced micropile pairs, positioned uphill and downhill of the conduit. Each pair formed an A-frame comprising a vertical compression pile and a raking tension pile, socketed into competent volcanic tuff. The uphill row was designed to provide the principal slope-restoring force and raise the calculated factor of safety to at least 1.3. Targeted permeation grouting between the conduit and downhill piles filled voids and improved support while preserving groundwater flow.<br /><br />Lightweight drilling equipment operated from scaffold platforms anchored to stable rock. A comprehensive monitoring system tracked hillside and conduit movements, inclinometers and strains across existing cracks, with trigger levels to halt work if necessary.<br /><br />Monitoring showed that crack movement largely stopped after the downhill piles and grouting were completed. Subsequent measurements indicated the conduit remained stable, with movements within the specified 5 mm limit. The work was completed in February 2010, ahead of schedule and without the planned second aqueduct outage, saving approximately £330,000.

Keywords

Thirlmere Aqueduct

Nab Scar

micropiles

slope stabilization

unreinforced concrete

cracking

joint separation

permeation grouting

ground monitoring

tunnelling waste