High Capacity Strand Anchors Strengthen Loch Arklet Dam

Loch Arklet is a remote and beautiful loch north-west of Glasgow. In order to provide water to the city, an 800m wide masonry and concrete dam was built there 100 years ago. Due to increasing leakage, the dam had to be strengthened using high capacity ground anchors.

In total, 64 vertical DYWIDAG Strand Anchors were installed from the crest of the existing dam into the underlying bedrock to act as tie-backs. The double corrosion protected anchors have 235mm external diameter corrugated plastic sheathing. The ground anchors supplied by DSI UK have 22, 23 or 27 strands and load-bearing capacities between 3,000 and 4,000kN; the strongest anchors reach an ultimate strength of 8,100 kN.

Load testing was carried out to a maximum of 1.5 x working load, i.e. at a maximum of 6,000kN (for the 27 strand anchors), with suitability tests applied to three working anchors initially, followed by acceptance testing on all anchors. The anchors were locked off at 110% of working load.

Logistics was a major challenge for the contractors involved. The only access to the job site was via a narrow and winding single track road south of the dam on which only four DYWIDAG Strand Anchors could be delivered on each vehicle. Installation of the anchors had to be carried out from the top of the dam, as there was no possibility of working from a barge mounted crane due to the loch level having been drawn down for safety reasons.

The 311mm Ø boreholes for the strand anchors were drilled into bedrock consisting of schist with quartz veins using reverse circulation flush. The deeper the boreholes were drilled, the more water entered the drill holes. The maximum anchor length of 54.5m and their weight of 2.2t required specialized handling for installation on the narrow dam crest. A protective nose cone was provided at the base of the anchors, with each anchor delivered to site on special ring frames, which fit onto a specialized anchor spooling wheel of a fork lift.

In order to overcome buoyancy, the internal anchor sheathing was filled with water. Since the anchors had to be spooled for delivery to the jobsite, the grout for the 8 to 9.25m long bond lengths had to be placed in situ. The high strength grout was pumped into the anchors using a tremie line, thus displacing the water previously injected. The external grout needed for bond in the borehole was also placed by tremie. Following the stressing procedures, special sealed corrosion protected caps were installed on the anchor heads.

For both the anchor tests and the final stressing, a special 6,800kN long stroke jack was used that had been supplied by DSI Germany. It is thought that this is the highest capacity anchor stressing jack in the UK at present. Furthermore, the DYWIDAG Strand Anchors supplied by DSI UK are some of the highest capacity ground anchors that have ever been installed in the UK.

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Scottish Water, Great Britain