Allt Na Lairge, Dam

Built 1957. First and only pre-stressed concrete dam in UK.

Information from R Paxton and Jim Shipway 2007b, 'Civil Engineering heritage: Scotland - Highlands and Islands' .

(Name cited as Allt-na-Lairige, and date as 1959). This pre-stressed gravity dam was built as part of the Glen Shira [2nd] phase of the Loch Sloy scheme; there was a tunnel to a power station on the River Fyne. The contractors were Maples, Ridgway and Partners Ltd. The average annual output of the Allt na Lairige scheme is 20 million units.

J Miller 2002.

The Allt-na-Lairige Dam has the unique distinction being the first (and so far the only) large pre-stressed concrete dam constructed in Britain. Others, pioneered by the French engineer Andre Coyne, exist on the Continent and elsewhere.

The dam is part of the Glen Shira Hydro-Electric Scheme and is situated in a valley in the hills some 8 miles northeast of the head of Loch Fyne. The structure was designed by J. A. Banks of Babtie, Shaw & Morton, Glasgow, and built by Marples, Ridgway & Partners. It was completed in 1957.

It is 73 ft high at its maximum and some 1360 ft long. The slender section was achieved by groups of pre-stressing anchors each consisting of 28 high-tensile steel bars, each 11/8 in. in diameter, located at 21 ft centres and extending downward from the crest of the dam to a depth of 27 ft into the rock below its base. The associated power station is about 2 miles downstream on the east bank of the River Fyne.

R Paxton and Jim Shipway 2007b

Reproduced from 'Civil Engineering heritage: Scotland - Highlands and Islands' with kind permission from Thomas Telford Publishers.

Alt-Na Lairge Dam provides storage for Alt Na Lairge power station (see separate item) and forms part of the second phase of development in the Sloy catchment. The dam was constructed using pre-stressed concrete and is amongst the earliest examples of the use of this technology in the UK. Its slender section was created using groups of pre-stressing anchors. They are located at 21m centres along the length of the dam and each consists of twenty-eight 300mm diameter high tensile steel bars. The anchors start at the dam top (or crest) and travel down through the height of the structure. They are buried 8m into the base rock below the dam. The dam has a central dispersal valve located at the base of the single central buttress with a small control tower above. Water leaves the dam via a tunnel to reach the power station. This is a relatively small scale dam with some additional interest gained from the early use of pre-stressed concrete construction methods. It is unique in Britain but other large pre-stressed concrete dams can be found in Europe and elsewhere, having been pioneered by the French engineer Andre Coyne. P L Payne, 1988, 5; E Wood, 2002, 38; J Miller, 2002.

The Loch Sloy hydro electric scheme

This was the first scheme to be tackled in 1945. Serious problems arose due to the post-war shortages of manpower and materials. The first of these was addressed by using hundreds of German prisoners of war. Access problems required the building of roads and conveyors for materials landed from Loch Lomond. Severe weather conditions delayed progress and the project was not completed until 1950.

A dam of massive buttress type was built at the south-eastern end of Loch Sloy leading to a tunnel through Ben Vorlich, followed by four circular pipelines leading to to the Loch Sloy power station at Inveruglas on the side of Loch Lomond. A surge shaft was incorporated in Ben Vorlich.

The generating sets consist of four vertical shaft Francis turbines each coupled to a 32,500 kW alternator. Delays due to adverse weather continued and transport problems for the heavy equipment which was finally installed by 1950. When fully loaded the turbines pass a million gallons of water per minute.

Information from NRHE item catalogue number WP007424 compiled by George Walker in 2005.