High capacity ground anchors are helping secure the long-term future of a 100-year-old dam in The Trossachs in Scotland.
Loch Arklet is a remote and beautiful loch in the Loch Lomond and The Trossachs National Park in Stirlingshire, Scotland, lying between Loch Katrine to the east and Loch Lomond to the west.
Impounded by a masonry faced concrete gravity dam, the loch is 4km long and 0.8km wide and is popular with walkers and those fly fishing for brown trout. The area is well known for its wildlife, including Ospreys and feral goats, and Corrie Arklet Farm, on the loch’s shore, was where Scottish folk hero Rob Roy married Mary Helen MacGregor.
Commissioned for the Corporation of Glasgow, Loch Arklet Dam was built by Charles Brand & Son over five years and completed in June 1914. It provides water to Glasgow. After 100 years of service, owner Scottish Water reviewed the dam’s future, assisted by Jacobs Engineering as part of its role in Scottish Water Solutions. Several options were considered and it was decided to repair and strengthen the dam by installing high-capacity ground anchors to post-tension the structure.
Work, which began in July 2014, was let to one of Scottish Water’s framework contractors, Expanded Group (part of Laing O’Rourke). The ground anchors are being installed by BAM Ritchies, which has extensive dam ground anchor experience and has, in recent years, successfully completed similar projects at Clunie Dam and Spey Dam in Scotland, as well as Llyn Alaw on Anglesey, Wales.
BAM Ritchies is undertaking the drilling, installation and testing of the 64 vertical ground anchors through the concrete core of the dam and into the underlying bedrock. With 22, 23, 24 or 27 strands and capacities rising from 3,000 to 4,000kN, these are some of the highest capacity ground anchors ever installed in the UK.
Site access is a particular challenge. BAM Ritchies general foreman Patrick Mayberry explains: “At Clunie Dam, we were able to use floating plant to install the anchors, which limited the number of movements on the narrow dam crest.
“However, at Loch Arklet the water level is too low to use floating plant. Additionally, one section of the viaduct over the spillway section has had to be removed owing to its poor condition, which meant we could not access the site from the northern end of the dam.”
As a result, the logistical arrangements for the three primary operations of drilling, anchor installation and stressing were critical, since there is only one, very restricted, access route from the southern end of the dam.
Anchors are being supplied by Dywidag-Systems. Specialist handling is needed because the anchors are up to 54.5m long and can weigh 2.2t. A protective nose cone is fitted to the base of the anchors and each is delivered to site on special ring frames, which fit onto BAM Ritchies’ anchor spooling wheel. Owing to the narrow and undulating single track road from nearby Aberfoyle, articulated lorries cannot be used, so only four anchors in their frames can be delivered at a time.
Expanded constructed manhole chambers in the dam using large diameter coring and saw cutting in advance of the anchor hole drilling. Bam Ritchies’ Hutte KBR 203 drilling rig with a Numa RC100 Challenger down-the-hole hammer is being used to drill the 311mm diameter anchor holes into the schist bedrock, which includes quartz veins.
BAM Ritchies senior geotechnical engineer Gilson Gaston says that one of the challenges has been the variation in rockhead across the site.
“So far, we have seen the rock level suddenly dropping by about 7m for a 20m section of the dam,” explains Gaston. Anchor holes reach between 11m and 25m into rock, giving overall depths of between 36m and 56m. And, as work has progressed towards the area of deepest bedrock, the amount of water entering the drill holes has also increased.
“The variation in rock depth, along with seepage inflows, is making drilling, cuttings handling and disposal far from straightforward. We have had to install a special hole-top safety device and are using high pressure capacity wash piping to cope with the high pressure compressed air we use during drilling.”
Reverse circulation flush
To ensure high levels of environmental control, reverse circulation flush is being used, with disposal of the cuttings and water arisings handled by Expanded through a complex system of dewatering, skips and tankers. The anchors are double corrosion protected, in accordance with BS8081, provided by 235mm external diameter corrugated plastic sheathing.
Using BAM Ritchies’ specialist hydraulically-powered anchor spooling device, the anchors are installed by flooding the sheathing internally to overcome buoyancy. Since the anchors need to be spooled, they have been supplied as grout insitu anchors, where the fixed length is not factory grouted (as would be the case in many unrestricted installations). Instead, bond length grout is placed on site, displacing the internal water by pumping via a tremie line. The bond lengths vary between 8m and 9.5m.
The external grout is also placed by tremie. All grout has a design strength of 40N/mm2 but to achieve high early strengths and enable quick stressing, a higher strength grout – 50N/mm2 – is being used on site.
Since each anchor has a large head block with up to 27 wedges, the strands are required to splay out at the proximal end: therefore control of the wash back of primary grout and secondary grout is very important before making up the head termination. A specialist suction pump is also being used, so that the primary and secondary grout levels can be controlled precisely.
In advance of the main works, the team installed and tested a trial anchor to prove BAM Ritchies’ in-house design. Three of the working anchors are also being “suitability” tested and all anchors have “acceptance” tests at 1.5 times design working load to BS EN 1537.
A 6,800kN long stroke jack with internal gripping wedges has been brought in from Dywidag in Germany to test and stress the anchors, which are locked off at 110% of working load. This is thought to be the highest capacity anchor stressing jack in the UK at the moment and, since it operates differently from conventional strand anchor jacks, the site team had to develop modified stressing and testing procedures.
All anchor heads are fitted with sealed corrosion-protected “top hats” within the manholes. A major benefit to Scottish Water is that the anchors have re-stressable head details, in other words, long caps to accommodate 450mm protruding strands. This will enable re-stressing of the entire anchor or individual strands in the future, if any drop off of load is detected.
All activities are being recorded by geotechnical engineer Peter Brum, using a rugged tablet device on site. Data and photographs are fed into the BIM model being used on the project.
“It is great to progress a high capacity anchoring job on a 20th century structure using 21st century BIM,” Brum says. “With rapid advances in technology, I am sure that there is much more we can develop and do with BIM on ground engineering projects.”
The site team report that a collaborative approach, with Scottish Water Solutions, Expanded and BAM Ritchies working closely together, is helping work progress towards successful completion in spring 2015.