THE LOCHABER WATER POWER DEVELOPMENT .sx The Works Comprise the Driving of a Main Tunnel 15 Miles in Length and a Smaller Tunnel of 2 Miles in Addition to Two Dams .sx Above is a View of one of the Small Intake Dams on the Loch Laggan Section of the Work .sx THE Lochaber Water Power Development was undertaken by the British Aluminium Co. , Ltd. , in order to obtain electric power for their new factories near Fort William , Inverness-shire .sx The scheme involves the utilisation of the flood waters of the River Spey above Laggan Bridge and of the run-off from the catchment areas of Lochs Laggan and Treig and the northern slopes of the Ben Nevis range of mountains , making a total catchment area of 303 square miles .sx ( See map) .sx A dam to be built across the River Spey ( which flows to the North Sea ) at a point two miles above Laggan Bridge will divert the water through an open conduit crossing the watershed to the River Pattack which flows into Loch Laggan and thence to the sea on the West Coast .sx Loch Laggan , which will form one of the two principal reservoirs , will be extended by the construction of a dam across the River Spean at a .sx point 4 miles downstream from the Loch .sx The river will be deepened to allow a drawoff of 16 feet of water from the reservoir .sx Water impounded by this dam will be conveyed through a tunnel , 2 miles long and 14ft .sx 6in .sx diameter , to Loch Treig which will be the main storage reservoir .sx This loch is to be raised 35 feet and can be drawn down 85ft .sx , thus giving an available depth of storage of 120 feet .sx The loch is 5 miles .sx long by about mile wide .sx From Loch Treig the water passes through a tunnel 15 miles long and 15 feet diameter driven through the solid rock of the Ben Nevis .sx range of mountains to a point overlooking Loch Linnhe .sx The tunnel and power house are connected by a steel pipe line and from the power house the water passes through a tail race to the mouth of the River Lochy at sea level .sx With Loch Treig reservoir full the gross head on the turbines is 800 feet .sx The works necessary to divert Loch Laggan to Loch Treig are in hand and those to the west of Loch Treig were completed two years ago .sx The Ben Nevis or main tunnel is not straight in plan but follows roughly the contour of the hills , thus allowing the centre line to be reached most economically at a number of points by working adits .sx and shafts .sx In all , seven adits were driven varying in length from 424 feet to 1,384 feet , and four shafts from 447 feet to 357 feet deep sunk thus providing 22 working faces in addition to that at the west end .sx Access to the various working points was obtained by the construction of a 3 feet gauge light railway , 23 miles long , from the service pier built at the head of Loch Linnhe near Fort William to Loch Treig .sx The men employed numbered about 2,500 and were housed in camps situated near the adits and shafts .sx Power was obtained from a temporary hydro-electric power station , using a fall of approximately 100 feet in the River Spean , near Monessie and this was distributed to the required points by a transmission line 25 miles long .sx It was used principally for driving air compressors , rock crushing plants , pumps , concrete mixers , and for lighting .sx On the light railway steam loco-motives were used .sx The method of excavating the tunnel varied , but that most commonly utilised was the short heading and bench system in which the heading was approximately 12 feet aheadof the bench .sx Drilling was accomplished by the use of four pneumatic drills of the percussive type in each main heading , whilst two were required for drilling the bench .sx For one round , between twenty and thirty holes , each 8 to 10 feet long .sx were required .sx The spoil was removed by means of either-side tip waggons , drawn by electric battery locomotives on 2 feet gauge tracks and dumped in tips close to each adit or shaft mouth .sx There being two headings to each adit and shaft , the hours of working were so arranged that whilst drilling was taking place in one heading , mucking was proceeded with in the other .sx Work was carried out day and night for six days a week .sx Very little water was encountered and that was dealt with by natural drainage so far as possible .sx For this reason the adits were driven on a slight up gradient to the tunnel .sx Pumping had to be resorted to in some of the downstream headings and also up the working shafts .sx Ventilation was obtained by forcing fresh air up to the heading through thin galvanised sheet iron piping by means of electrically driven fans at each adit or shaft .sx The foul and dust-laden atmosphere was thus forced back through to the tunnel .sx In the majority of headings , the lining of the tunnel was started before the headings actually joined and special steel shutters carried on steel frames were employed to allow an easy passage for trains proceeding to and from the headings .sx The lining was built up in four stages ; first the haunches were put in , followed by the sides and roof and lastly the invert .sx Concrete blocks were set to finished invert level along each side of the tunnel at 10 feet intervals , and on these , steel running rails were laid at 8ft .sx 6in .sx centres to carry the frames which ran on flanged wheels .sx This track was also used to carry a trimminggantry which preceded the shuttering frames , and on which was mounted a light timber frame shaped to the lines of the in-side limit for excavation .sx The track was lifted immediately prior to the laying of the invert , which operation was one of the last .sx to be executed in the tunnel .sx The concrete was mixed in electrically operated mixers situated close to the form work .sx The con- .sx crete was placed principally by hand although a gun was used in parts of the roof .sx The invert was laid in 40 feet lengths alternately and screeded to shape , the intermediate lengths being filled in later .sx Each adit was sealed by a mass concrete plug 24 feet long , but access to the tunnel is provided for by a 42-inch steel pipe through each plug .sx Six-inch scour pipes , complete with valves , were also fitted in each adit .sx As the driving of the plain tunnel progressed eight small vertical shafts were sunk to permit the tapping of mountain streams crossing the line of the tunnel .sx These streams all flow from the northern slopes of the Ben Nevis range of mountains , which have the highest annual rainfall in the whole catchment area , and it has been estimated that during a very heavy rainfall it is possible for these streams to supply water in excess of the total required at the power house .sx The balance of the yield will then flow up the tunnel to Loch Treig reservoir .sx The shafts for these intakes were sloped upwards from the tunnel to permit the spoil being dropped and removed via the main tunnel and to avoid the necessity for pumping .sx The lining consisted of pre-cast concrete pipe sections .sx The details of the arrangements for taking the water from the streams varied , but the general principle was the same in all cases .sx A concrete dam was built in the stream diverting the water either direct or through .sx a conduit to the vertical shafts leading to the tunnel .sx The dams are equipped with scour sluices and the conduits are protected by in-take screens .sx The water intake to the tunnel from Loch Treig is controlled by two sets of gates situated 373 feet from the reservoir .sx In order to accommodate these and also two screens , the main tunnel is split into two tunnels , 18 feet by 12 feet , which run parallel in way of the gates and which rejoin again .sx The gates and screens were designed and supplied by Messrs .sx Ransomes and Napier , Ltd. .sx The main tunnel was driven into Loch Treig at a depth of 100 feet below the surface , the last barrier of rock being removed by blasting in one operation .sx When the main heading had reached a point 85 feet from the Loch , a small pilot heading , at floor level , was continued to within feet of the water .sx This was followed by the full heading 17 feet by 17 feet which at the extreme end was deepened by 5 feet to form a sump to catch rock falling back into the tunnel after the explosion .sx In the remaining face 131 holes were drilled to within 2 feet of the water and charged with specially prepared blasting gelatine .sx A concrete bulkhead was constructed across the tunnel at a point 100 feet downstream of the gates and the tunnel above this bulkhead was flooded with water after which the charge was fired by electrical means from .sx ground level .sx The gates were kept suspended clear of the tunnel until after the blast when they were closed to allow of the removal of the concrete bulkhead .sx The surge chamber which is sunk in solid rock is situated near the top of the pipe line .sx It is cylindrical in shape with a diameter of 30 feet and a height of 240 feet .sx Two short tunnels connect the surge chamber with the top of two groups of pipes , one of which has still to be built .sx The tunnels are 2 feet in diameter and increase in size to form a chamber 32 feet wide by 8 feet high at the commencement of the pipe line .sx The pipe line already built is 3,240 feet in length and consists of two lines having an average diameter of 5ft .sx 6in .sx Each line .sx is made up of 30 feet lengths of welded mild steel pipes having spigot and socket ends which were electrically welded together in situ by the Quasi-Arc process .sx The thickness of the metal increases from -inch at .sx the top to 1-inch at the lower end .sx Heavy mass concrete anchor blocks secure the pipe line at six points and expansion joints are provided immediately below each anchorage .sx The pipe line is supported on mass concrete .sx pedestals fitted with lubricated bearing .sx plates .sx At the power house the two lines divide into five 12-inch diameter pipes for .sx supplying five main turbines and two auxiliary sets .sx In order to obtain the maximum head , the foundations for the power house were sunk 40 feet below ground level , the excavation for which was mainly in rock .sx Each turbine consists of a two-jet pelton wheel developing up to 10,000 h.p. and driving two electric generators all mounted on a common horizontal shaft .sx The aluminium factories adjoin the power house and are served by the Company's own pier in Loch Linnhe to which they are connected by a 3 feet gauge railway and also by a siding from the .sx L. & N.E. Railway .sx The tail race which is lined throughout with concrete is approximately 3,200 feet long , the first 936 feet being in tunnel and the remainder in open cut .sx Three bridges , of the concrete arch type , were constructed over the tail race to carry the West Highland section and the Mallaig branch line of the L & N.E. Railway , and also the main road from Fort William to Spean Bridge .sx Allthese were built without interrupting the traffic .sx The pier at the head of Loch Linnhe consists of a head 200 feet long by 63 feet wide and an approach viaduct 900 feet long by 26 feet wide carrying two 3 feet gauge railway tracks and a footpath .sx The structure is built wholly in reinforced concrete and is founded on groups of piles encased in cylinders .sx The works now under construction include the dams across the River Spean to form the Laggan reservoir and across the River Treig to raise the level of the Loch .sx The former is of the gravity type in concrete and will be 140 feet high .sx The latter which will be founded on a glacial moraine is of the earth embankment type with a concrete core wall founded on rock .sx