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Health check on Highlands dams

EXPLORATION Associates had to overcome difficult site and borehole access before investigation could start out on a recent project in the Scottish Highlands.

The contractor, working for Alcan Smelting and Power UK, drilled boreholes into the Blackwater, Spey and Laggan dams to obtain insitu data to confirm their strength and stability under different flow and loading conditions. The dams form part of Alcan's hydroelectric schemes at Fort William and Kinlochleven.

Blackwater Dam, in the mountains just north of Glencoe, is 325m above sea level and can only be reached by off-road vehicles along an 8km dirt track.

The 933m long dam was built in 1908 to provide hydroelectricity to the British Aluminium Company's aluminium smelter in Kinlochleven. It is a concrete gravity structure largely comprising large granite blocks in concrete made with igneous and metamorphic aggregate.

Although the dam is 3m wide, hand rails and a wave wall reduced the working area to only 1.2m, so the company used a mini Hydrec drilling rig that could be operated with the mast and drill head as one unit powered by a remote power pack and control unit. It was manhandled and winched into position by three people and was able to fit into the limited space available. The rig was attached to the concrete dam and hydraulic rams used to exert thrust on to the drill bit.

Spey Dam, built during the Second World War, is near Laggan Bridge to the north of Blackwater. Although the general dam structure was known, the investigation was commissioned to find out the materials used and their method of placement.

Access to site was easier, but the nature of the structure itself meant that locating boreholes was more difficult. Positions included one on the very narrow curved crest of the spillway and one halfway down the spillway face, both of which needed extensive scaffolding. Work was carried out using the same rig as at Blackwater.

Laggan Dam, built in late 1940s, is a concrete gravity dam built in a similar way to the Blackwater dam. It is 208m long and 54m high, with a spillway along most of its length below a roadway supported on masonry arches. The dam interior has a complex of drainage holes leading into a gallery system and the central part of the dam contains a series of syphons and a control house.

The original plan was to drill two boreholes from the middle of the spillway face and two from inside the galleries. But both had extremely difficult access so it was decided that drilling at an angle from the road on top of the dam would provide the required information and save money. Accurate drilling of the boreholes was needed to ensure that the piers of the masonry arches were penetrated without 'daylighting' on the spillway face.

A truck-mounted rig was used to drill the boreholes 5degrees from vertical with a gyroscopic survey tool used to prevent deviation. A thin wall core barrel matched with drill rods of similar diameter was also used to minimise the risk of the bit veering off line.

In all, eight boreholes were sunk through the dams and into the foundations, with three horizontal bores drilled to examine construction joints. Reservoir water was used as flush to reduce the risk of contaminating the water supply. Thin wall barrels were used to recover 76mm diameter cores of concrete and bedrock. Testing was carried out by Exploration Associates at its Newcastle laboratory to determine the compressive and tensile strength of the concrete and its composition.

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