A tunnel fifteen miles long, under Ben Nevis, is but one of the major feats of engineering involved in the building of the huge hydro-electric undertaking to give power for the production of aluminium in the Scottish Highlands

WONDERS OF WATER POWER - 5

UNDER THE FLANK OF BEN NEVIS, the highest mountain (4,403 feet) in Great Britain, are the power station and aluminium factory, where the metal is extracted from alumina, or aluminium oxide, by an electrolytic process. Two and a half kilowatt years of direct current are required to produce one ton of aluminium.

AN abundant supply of electricity is essential to the production of aluminium. To produce electricity economically enough for the smelting of aluminium ore on a large scale the first necessity is water power. Stored up in the mountainous districts of Europe, in the snows, in the lakes and in the rivers, there are masses of latent power waiting only to be tamed and harnessed by the engineer. Such natural forces, however, are not easily to be tamed. Years may be spent surveying the locality, devising means to subjugate the waters which, when harnessed, will serve man, but, when un-c necked, may sweep man and his works to destruction.

Switzerland, Scotland and Scandinavia are the most suitable parts of Europe for the production of hydro-electric power. In all these regions there are schemes supplying power specifically for the smelting of aluminium ore. but in Scotland, particularly in Lochaber, conditions are probably the most favourable.

Little more than a hundred years ago aluminium was a virtually unknown element. To-day it ranks fifth in the world production of metals. The Danish scientist Oersted was the first successfully to isolate the metal, in 1825; but another sixty years passed before discoveries were made that enabled the metal to be produced on a commercial scale.

Curiously enough, these discoveries were made simultaneously, in America and in France, by two young scientists who were both only twenty-two years old. In 1886 Charles Martin Hall, working in a woodshed at Oberlin, Ohio, found that aluminium could be reduced from its ore by an electrolytic process. In the same year, Paul Heroult, working in France, came upon the same solution to the problem. Thus the process which is used to-day on a gigantic scale is sometimes known as the Hall-Heroult process.

The ore from which aluminium is obtained is known as bauxite. Resembling clay, bauxite is a moderately hard rock and occurs in France, Italy, Hungary, Ireland and Dutch and British Guiana. In these countries it is mined extensively, in Europe by quarrying and mining proper, in the New World by surface excavation.

Aluminium in its pure state exists in the ore only as a metallic oxide. This aluminium oxide, or alumina, as it is called, is the source of the metal. For the extraction of the metal, alumina is subjected to electrolytic treatment in conjunction with a mineral known as cryolite.

ACROSS THE RIVER SPEAN, a few miles below Loch Laggan, was built the Laggan Dam, a mass concrete structure, slightly curved in plan, with a length of 700 feet. The dam carries a road along a series of arches built above the crest. From the downsteam face project the mouths of the six siphons which take away excess flood water.

Cryolite is a white mineral which closely resembles rock salt. Its chemical name is sodium aluminium fluoride. This mineral, so important for the manufacture of aluminium, occurs in commercial quantities in only one place in the world, at Ivigtut, on the west coast of Greenland. Here there is a large pocket of the mineral, enclosed in granitic rock and penetrating to an unknown depth.

Molten cryolite has the property of taking alumina into solution. Thus when an electric current is passed through a solution of alumina and cryolite, metallic aluminium is precipitated and oxygen is liberated. The electric furnaces in which the process is carried out are lined on the inside with carbon manufactured from petroleum coke. This carbon lining forms the cathode, the anode being formed by blocks of similar carbon suspended in the solution.

The current passing between cathode and anode fuses the alumina and the cryolite, causing the aluminium to fall to the floor of the furnace in a molten state, whence it is tapped and run off into ingots. The liberated oxygen combines with the carbon of the anodes to form carbon monoxide, which burns into carbon dioxide. The cryolite acts only as a flux and is not consumed.

By this process 2½ kilowatt years of direct current are required to produce one ton of aluminium. A cheap but copious supply of electricity, therefore, is needed and aluminium works are almost always situated in districts where hydro-electric power can be obtained. Such districts, however, are generally hundreds, if not thousands of miles from the natural deposits of bauxite. Ore for the Canadian aluminium industry, for instance, is for the most part obtained from British Guiana. The ore is shipped to the St. Lawrence, thence up the Saguenay River to Port Alfred, on Ha Ha Bay, where the refining and smelting plants are situated. Thus the ore has to travel nearly 4,000 miles from the mines to the source of electricity.

Sea transport, therefore, is an important consideration, and it is most convenient, if possible, to find hydro- electric power near the sea, to obviate additional transport expense. France is fortunate in having aluminium ore deposits and hydro-electric power in close proximity, but all aluminium ore for Great Britain has to be shipped by sea to the source of power.

In the Western Highlands long arms of the sea extend for miles into the heart of the mountain ranges. One of the longest of these sea lochs is Loch Linnhe. At the head of this loch Ben Nevis, the highest mountain in Great Britain, rises directly from sea level to a height of 4,406 feet. Behind Ben Nevis is a well-watered plateau. The configuration of the land and the character of the rock favoured the establishment of a huge hydro-electric power plant for the production of aluminium in works on the edge of the sea at Fort William.

LINING THE BEN NEVIS Tunnel with concrete. First the haunches were lined to a height of 3 feet above the level of the invert. Then the sides were finished to a height of 12 feet. Rails along the invert took the travelling carriages from which the roof was concreted.

At Fort William, therefore, a town served by the West Highland Railway (L.N.E.R.) and by the Caledonian Canal, the British Aluminium Company, Limited, decided to locate a huge plant. Its first plant was built at Foyers, on Loch Ness, where the Falls of Foyers were harnessed to generate the electric power. In 1908 a hydroelectric scheme was started at Kinlochleven, Argyllshire, at the head of Loch Leven, which joins Loch Linnhe about ten miles south of Fort William. This is one of the finest instances in Great Britain of the way in which the engineer has conquered Nature.

The entrance to Loch Leven is a narrow strait, at the foot of Glencoe, one of the wildest and most picturesque parts of the Highlands. From the dark waters of the loch the mountains rise sheer on either side. With the greatest difficulty a road was made along the mountainside. The road was blasted out of the rock high above the calm waters of the loch. For about eight miles the road clings to the side of the mountain, crags on one side, a precipice on the other. Then a turn brings the head of the loch into sight. Enormous pipe lines run down the. mountain opposite, feeding water to the turbines in the power house below.

Round the power house has been built a township, industrial in character, with rows and rows of brick houses in well-planned formation. Hundreds of feet below, at the head of the water, is a jetty for the steamers that bring the alumina for smelting. The road descends steeply to the town, past the power house and the roar of the water that is passing out of the turbines into the tailrace.

Tunnel Through Ben Nevis

All this, on a much larger scale, is reproduced at Fort William by the Lochaber Power Scheme, by far the largest hydro-electric scheme in Great Britain. Parliamentary sanction for this great feat of engineering was obtained in 1921. C. S. Meik and Halcrow were the engineers. From a catchment area of 303 square miles — elevated land with heavy rainfall — a scheme was planned to generate electric power at a continuous output of 120,000 horse-power. Over the range dominated by Ben Nevis there are two passes, one used by road, the other by rail. The road follows the valley of the River Spean, past Loch Laggan, a sheet of water seven miles long, over the pass and down to Kingussie and the Spey Valley, nearly fifty miles from Fort William. The West Highland Railway also follows the Spean Valley for a distance, but then turns southwards up a subsidiary valley past Loch Treig into the desolation of the Moor of Rannoch.

Loch Treig is the main storage reservoir for the Lochaber Power Scheme. The reservoir is fifteen miles from the power house at Fort William, and the task of conveying the huge volume of water for this distance involved one of the biggest engineering feats of its kind in the world.

It was decided to drive a tunnel right through the mountain mass from Loch Treig to Fort William. The Ben Nevis Tunnel, fifteen miles long, is one of the longest tunnels in the world. In length it is surpassed by two American tunnels which supply water to New York and Los Angeles respectively. The Ben Nevis Tunnel has a width of 15 feet, and a height of 14 ft. 8 in. from invert to crown. Longer by miles than the famous Alpine railway tunnels, it ranks as an outstanding achievement of British engineering.

A CATCHMENT AREA OF 303 SQUARE MILES is tapped by the Lochaber Power Scheme, by which hydro-electric power is supplied for the production of aluminium at Fort William. The fully-developed scheme includes a dam, or weir, across the headwaters of the River Spey, which are diverted into Loch Laggan by a conduit. Loch Laggan, 820 feet above Ordnance Datum, is dammed and its waters are diverted through a tunnel to Loch Treig. From there a tunnel, fifteen miles long, conveys water to the pipe lines leading to the Fort William turbines. The power house is on the shore of Loch Linnhe, an arm of the sea.

The tunnel was excavated from twenty-three faces simultaneously. This was possible because the tunnel does not take a straight line between Loch Treig and the power house. The line of the tunnel approximately follow’s the contour of the northern side of the Ben Nevis range. In this way it was possible to take in additional supplies of water from the streams that flow down this face of the range. It was also thus possible to drive seven nearly horizontal adits from the surface to the line of the tunnel.

Narrow-Gauge Railways

Four vertical shafts also were driven along the line of the tunnel for the same purpose. The shafts had a section of 18 by 12 feet and varied in depth from 147 to 357 feet. The adits, 9 by 8 feet in section, varied in length from 421 to 1,384 feet. The tunnel was excavated in either direction from each shaft and adit, and also from the portal at the Fort William end of the tunnel.

Before the work of excavation could be begun, some years of preliminary work were necessary. First, there was the surveying. Then there was the problem of accommodating the 3,000 workmen in an inaccessible and mountainous region. Large camps had to be built and provision made for the thousands of men who would have to live in the region for about ten years.

The difficulty of transporting men and materials was increased tenfold by the mountainous nature of the land. In addition to the boring of the tunnel, other parts of the scheme were being carried out many miles away from Fort William and from the site of the power house. First a special railway was built from Fort William to connect the points where adits and shafts were being driven. This railway, with a gauge of 3 feet, was twenty-three miles long, starting at sea level and reaching a height of 1,200 feet. Further narrow-gauge railways were built to connect Lochs Treig and Laggan, to connect the various camps and the works, and to link with the L.N.E.R. and with the quarries where material for building the dams was obtained. The engineers were engaged on developing an enormous scheme for the production of power from the forces of Nature; but for driving the tunnels and building the dams that would develop this power they needed power themselves.

BUILDING LAGGAN DAM, one of the dams designed in connexion with the Lochaber scheme for the provision of hydro-electric power for the production of aluminium at Fort William, Inverness-shire. The dam is a mass concrete structure 700 feet long at spillway level, and is 175 feet high. The first operation was to build a bridge for the contractors’ railway across the gorge just above the site of the dam. The central 200-feet span of this bridge was carried by two tall steel towers, on top of which were mounted the cranes that handled the concrete used in the dam.

About twelve miles above Fort William the River Spean passes over what are known as the Monessie Falls. Here were built a short tunnel and a pipe line connecting with a temporary power house. The falls were harnessed to generate 4,800 horse-power at 11,000 volts, the power being transmitted by overhead lines to the various points where it was required.

Thus was electricity obtained for driving the compressed-air plant installed at each of the adits and shafts along the line of the Ben Nevis Tunnel. At the entrance to each adit and shaft was a two-stage air compressor delivering air to the working faces by 6-in. pipes. At the headings groups of rock drills bored holes 8 feet or 10 feet deep into the blank wall of rock ahead.

When about twenty or thirty of these holes had been drilled, about 180 lb. of gelignite was inserted. The explosion normally brought down about 150 tons of rock, which had then to be cleared away before further drilling could take place.

Various kinds of rock were encountered from time to time, and thus the methods of excavation were varied slightly to suit the ruling conditions. Some of the rock was so abrasive that the bits of the rock drills required sharpening after having penetrated a depth of only 6 in. Special drill sharpeners, therefore, installed at the entrance to each adit and shaft were in constant use.

NATURAL GRANITE FOUNDATIONS were available for the Laggan Dam, built across a deep gorge through which the River Spean flows. The mass concrete dam was built in blocks, or sections. Slightly upstream of the site the contractors’ railway was carried across the stream on a bridge with a central span of 200 feet.

Three thousand men were employed on the excavation of the tunnel. The maximum rate of advance was more than 900 feet in four weeks, with a record advance of 91 feet a week in one heading. At the busiest periods compressed air was being used at the rate of 17,250 cubic feet a minute.

As the headings advanced railway tracks were laid in the tunnel and battery-driven locomotives hauled trains of spoil to the surface entrances. Later these trains were used to convey the concrete with which the tunnel was lined.

The invert in which the rails were laid was concreted last of all. The haunches were lined first to a height of 3 feet above the level of the invert. Then the sides were finished to a height of 12 feet. Next, with the help of travelling carriages, the ceiling was lined, and finally, as the rails were removed, the invert was concreted.

Eleven streams which crossed the line of the tunnel were dammed and their waters were diverted into the tunnel through shafts. As in all tunnels of this nature, a surge shaft was necessary to relieve the pressure in the event of a sudden stoppage of the turbines in the power house. The surge shaft was sunk through solid granite about 350 feet from the Fort William end of the tunnel. The shaft is 240 feet deep and has a diameter of 30 feet. The top of the surge shaft opens into a concrete basin equipped with an overflow tunnel.

From the surge chamber at the base of the shaft the main tunnel divides into two branch tunnels with a diameter of 12 feet. Across the entrances are screens and sluice gates which, although normally kept open, can be used in an emergency or to isolate the pipe lines.

3,200-Feet Pipe Lines

It was not necessary for the engineers to complete both these branch tunnels at the same time, for some years were to elapse between the completion of the tunnel and the final development of the water conservation in the Spean and Spey watersheds. One of the branch tunnels was, therefore, completed first, being part of the first stage in the development of the Lochaber Power Scheme.

From the surge chamber the branch tunnel has a diameter of 12 feet until, at a point 240 feet from the surface, the diameter is expanded until the tunnel is large enough to contain three pipes with an outside diameter of 70½-in. These pipes emerge from the mountainside to enter a valve house, in which each is connected to a 54-in. valve. Should the velocity of the flow in any pipe exceed a predetermined figure, a 69½-in. butterfly valve, fitted beyond the 54-in. valve, automatically closes. In the first stage of development, only two of the massive pipe lines which connect the valve house with the turbines were completed. Work on the second branch tunnel and three further pipe lines, however, was begun early in 1937.

The enormous pipe lines which run down the slopes of Ben Nevis to the great power house at the foot are monumental evidence of the great feat of engineering involved. Each of the pipe lines has a total length of about 3,200 feet. They were made in lengths of 30 feet and vary in diameter from about 65 in. to 70 in.

The joints of the pipe line were welded on the site, this being the first example of such work in Great Britain. The quasi-arc process was used (see the chapter “The Craft of the Welder”). Current was supplied for the lighter work by two portable generators driven by petrol engines, although lower down the slope more powerful sets were used, driven by electric motors.

AN EARTH AND ROCK FILL DAM built to retain the waters of Loch Treig and to form the main storage reservoir of the Lochaber hydro-electric scheme. Slabs of reinforced concrete were used to face the downstream side and form the spillway, which has a pitch of 1 in 3. Two concrete guiding walls direct the water on to a standing wave wall at the toe of the dam.

The pipe lines are anchored at either end and at four intermediate points by enormous anchor blocks of concrete. At intervals of 30 feet between the anchorages the pipes rest on concrete pedestals provided with curved saddles of mild steel. Expansion joints are provided below each anchorage to allow a movement of 6 in.

Another army of men was engaged on the great conservation schemes on the other side of the Ben Nevis range. The main features of the scheme were the building of two great dams below Lochs Laggan and Treig and the driving of a tunnel, nearly three miles long, between the two reservoirs.

About five miles below Loch Laggan the River Spean flows through a deep and narrow gorge, in which granite is frequently exposed and is never more than a few feet below the surface. Thus there were firm foundations for a structure which, because of its position, would need to be built to a considerable height, namely 175 feet above the foundations. Laggan Dam is a mass

concrete structure, slightly curved in plan, with a length of 700 feet at the level of the spillway.

The preparations for the building of this dam included the laying of a railway about four miles long. This line, with a gauge of 3 feet, crossed the gorge of the Spean a short distance above the site of the dam on a bridge having a central span of 200 feet. The span was carried by two steel towers, on which derrick cranes were mounted. Situated about 150 feet above the river, these cranes had a lifting capacity of 7 tons at a radius of 100 feet.

Problem of Flood Water

Excavation was carried out, first on the sides of the gorge, and then in a steel sheet cofferdam built out into the river. Meanwhile a concrete mixing plant was erected near the railway bridge, so that materials for making the concrete could be tipped from wagons on the railway and skips of the mixed concrete could be handled by the derrick cranes on the tower of the bridge.

The dam was built in a number of sections, or blocks, three of which extended across the bottom of the gorge. The concrete base of one of these blocks was laid first. This projected a short distance into the stream and was provided with two temporary openings through which the stream could be passed while the bed was being blocked by the central sections of the dam. When the first block had been raised to a sufficient height and the openings were ready to take the flow of the Spean, the cofferdams were built right across the river so that the bed could be excavated for the foundations of the central part of the dam.

CENTRAL CONCRETE CORE WALL of the Treig Dam. For the foundations of the wall a trench 703 feet long and 10 feet wide was excavated down to hard rock at a maximum depth of 100 feet. Behind the steam roller can be seen the toe of the downstream face. A fill of earth and rock was laid to complete the dam, as shown on the opposite page.

When the work on the foundations had been completed, the concrete monolith was built up, each section being added to in turn to allow time for the poured concrete to cool and to avoid the formation of cracks, such as would appear if each layer of concrete had been given insufficient time in which to cool. The concrete was laid in lifts of about 3 ft. 6 in., that is, in layers about 3| feet thick. Every lift was covered with heavy coconut matting, which was kept wet until it was removed for the next lift. Despite all the precautions taken to prevent cracks in the structure caused by the contraction of the concrete on cooling, in a dam of this size small cracks were bound to appear. To stop the gradual percolation of water, therefore, the upstream, or vertical face was covered with “Gunite” rendering (see the chapter “The Mersey Tunnel”).

Interesting steps were taken to deal with the problem of flood water. The rainfall in this district is considerable and floods are frequent. The length of the spillway, too, was limited by the width of the gorge and was considered insufficient to cope with a heavy flood. Additional accommodation, therefore, was provided for flood waters by the provision of six siphons of special design in the body of the dam.

On the upstream face of the dam, at crest level, are six openings rectangular in section. These are the inlets of the siphons. The openings are continued downwards through the structure, curving over into a vertical position. Sixty or seventy feet lower down the openings curve into the horizontal, ending in outlets which project out of the inclined downstream face of the dam.

The. principle of a siphon is simple. Its action can be demonstrated with a glass of water and a piece of rubber tubing. A siphon is formed by placing one end of the tubing in the water and leading the tubing over the rim of the glass so that the other end is lower than the immersed end. If air is first drained from the tubing, say by suction, atmospheric pressure on the surface of the water will force it up the tubing and over the rim until it discharges from the other end. This siphonic action will continue automatically until the water in the glass falls to the level of the higher end of the tubing.

The action of the siphons in the Laggan Dam, on an immense scale, is similar, with the important exception that the air in the “tubing” is not withdrawn by suction to prime the siphon, or to start the siphonic action.

Siphonic Action

The upper bend of the siphon and its inlet form what is known as the throat. The point where the inside of the curved pipe reaches its highest level is known as the lip. If the lip is at the same level as the crest of the dam, when water starts to flow over the crest it will also start to flow over the lip, running down on the inside of the pipe without touching the opposite wall. Air is still free in the pipe. As, however, the water level continues to rise, when it covers the lip to a depth corresponding to one-third of the diameter of the pipe, the water cascades away from the inner wall and touches the outer wall of the vertical section of the pipe. This has the effect of entrapping the air in the upper part of the throat and preventing the passage of air upwards from the lower end of the siphon. The force of the water, still pouring by gravity, drags part of the entrapped air down with it, thus forming a vacuum, which is immediately filled by more water from the reservoir. Once the throat is completely filled with water the siphonic action has started.

The action will continue until the flood subsides and the water in the reservoir falls below the level of the inlet to the siphon, thus admitting air in place of water. Breaking the siphonic action by this means, however, is apt to be slow. Air comes into the pipe in gulps, causing some vibration. This difficulty was overcome in the Laggan Dam by the provision of air valves in the throat of the siphon. The valves are operated automatically by the fluctuations in the level of the water in the reservoir. The rising water closes the valves, sealing the throat so that priming will take place. As the water recedes the valves open and admit air to the throat, thus breaking the siphonic action.

150 FEET ABOVE THE RIVER, situated on the steel towers which supported the contractors’ railway, two derrick cranes were used to handle the concrete poured into the Laggan Dam. The cranes had a lifting capacity of 7 tons at a radius of 100 feet.

The siphons are formed in the concrete without ally lining, except for the steel inlets, throats and outlets. The outlets are bent slightly upwards to throw the jets of water clear of the toe of the dam. Concrete wing walls on the downstream sides of the gorge prevent erosion by the water flowing over the spillway. Further discharge from the reservoir is effected through a needle valve situated in the base of the central portion of the dam. A gate on the upstream side of the dam controls the admission of water to the valve. The gate is operated from a valve house on top of the dam. Arches built over the spillway support a road which is 12 feet wide.

The main storage reservoir of the Lochaber Power Scheme is formed by Loch Treig. Water from the Laggan Reservoir is conveyed to Loch Treig by a tunnel which opens at the eastern end of the Laggan Dam. Entry of water from the reservoir to the tunnel is controlled by a sluice gate operated by a petrol engine in the valve house above the portal.

The Laggan-Treig Tunnel has a length of 14,600 feet, or nearly three miles. It is similar to the Ben Nevis Tunnel, except that for the most part it was driven through solid granite. For a distance of only about 250 feet, glacial gravel and sand were encountered. This necessitated shoring the sides of. the tunnel during excavation and lining the section with cast-iron segments. The last 80 feet at the Loch Treig end were built by the cut-and-cover system. Three shafts admit water which is diverted from three streams that cross the line of the tunnel. The outlet into Loch Treig is situated upstream, of and slightly below the spillway level of the Treig Dam.

Loch Treig, the main storage reservoir, has a capacity of 7,800 million cubic feet. Originally, about five miles long and little more than half a mile wide, the loch is exceedingly deep, as are many other Scottish lochs. The dam across the loch, however, reaches a height of only 39 feet above the original level of the site. The dam is an earth and rock fill structure, only the central vertical core wall being of concrete.

Along the eastern side of the loch runs the West Highland Railway. The raising of the loch necessitated the diversion of the track for a distance of nearly one and a half miles. The new course involved the driving of a tunnel through a rocky shoulder on the side of the mountain. Similar diversion work had to be undertaken before the building of the Laggan Dam. In this instance the road to Kingussie had to be rebuilt at a higher level for about a mile of its length.

Standing Wave Wall

For the foundations of the core of the Treig Dam a trench 700 feet long and 10 feet wide was excavated down to hard rock. The trench was taken down to a maximum depth of 100 feet before the rock was reached. Excavated material was removed by jib cranes travelling on rails. Before the mass concrete was poured the rock was grouted, but it was so sound that only four of the 140 grout-holes that were drilled took any appreciable quantity of the neat cement grouting.

Against the core wall on the upstream face a fill of clay was first applied. Gravel and earth constituted the remainder of the upstream fill, which was faced with granite at a pitch of 1 in 3. The pitch of the downstream side is the same but the composition of the fill is slightly different, being of rubble. The rubble was tipped and consolidated in layers by steam rollers. The fill was faced with slabs of reinforced concrete, forming the spillway. The toe of the dam is supported by 15-feet piers of concrete with sheet steel piling between. Slantwise across the downstream face are. two concrete guiding walls and across the bottom of the slope is what is known as a standing wave wall, to break the force of the water flowing down over the spillway.

One of the most spectacular feats of engineering in connexion with the Lochaber scheme was the opening of the Ben Nevis Tunnel portal into Loch Treig, more than 100 feet below the surface of the waters. This part of the work was not carried out until part of the plant in the power house at Fort William had been operating for some time with water introduced into the Ben Nevis Tunnel at intermediate points.

LAYING THE PIPE LINES between the portal of the Ben Nevis Tunnel and the power house at Fort William. Each pipe line is 3,200 feet long, and varies in diameter from 65 in. *o 70 in.

For some time, therefore, the fifteen miles tunnel was left closed from the waters of Loch Treig by a wall of natural rock. About 400 feet back from the loch the tunnel bifurcates. From each of the two tunnels thus formed a vertical shaft leads to the surface, which at that point is about 40 feet higher than the natural level of the loch. These two shafts, 38 feet apart, have a depth of 146 feet each.

Across the tunnels at the base of the shafts were installed screens and sluice gates 18 feet high and 12 feet wide. These are raised by winches at the surface and, though normally kept open, serve to seal the tunnel from the entrance of water from the loch should occasion for this arise.

Beyond the shafts the two tunnels reunite, and after the completion of the shafts, excavation was continued towards the loch. About 50 feet away from the loch the concrete lining was discontinued and the size of the tunnel was increased to a certain extent. Gradually the engineers penetrated farther into the wall of solid rock which separated them from Loch Treig.

When the curtain of rock was reduced to a thickness of only 20 feet excavation was stopped and preparations were made for blasting the final barrier and admitting the waters. Drill holes were bored into the rock and 3,435 lb. of gelignite were inserted.

Meanwhile, however, measures had to be taken to prevent the sudden inrush of water from destroying the tunnel when the final barrier was blasted away. So great would be the sudden inrush of water that the lowering of the sluice gates would have caused their destruction, although they were situated 400 feet down the tunnel. The only way to stem the sudden release of water, it was decided, was to counter it with another mass of water. Some way behind the sluice gates, therefore, a great plug of concrete was keyed into the tunnel walls. Between this bulkhead and the rock barrier water was admitted into the tunnel from the shafts.

Weir Across the Spey

On January 3, 1930, the blast was fired. A deep, hollow rumble was heard on the surface as the barrier of rock was shattered and then the surface of the loch was disturbed as bubbles of gas rushed upwards. The fall in the level of the water in the shafts indicated that the blast had had the desired effect, and this was later confirmed by the reports of divers who descended into the loch to examine what was now the inlet portal of the Ben Nevis Tunnel.

The third and final stage in the development of the water conservation scheme for the Fort William turbines is the diversion of water from the River Spey into the Laggan Reservoir. For this purpose it was decided to build a weir across the headwaters of the Spey to divert a part of the flow through a conduit three miles long to discharge into a stream which feeds Loch Laggan.

The Spey Weir is 880 feet, above sea level and the Fort William turbines are only about 20 feet above sea level. This gives a difference in level of 860 feet, the reservoirs having a combined storage capacity of nearly 10,000 million cubic feet. Thus sufficient water power can be harnessed for a continuous output of more than 120,000 horse-power.

The building which houses the turbines will be, after full development, 600 feet long and 65 feet wide. The tailrace, . which discharges into the River Lochy, just before its outflow into Loch Linnhe, is 3,000 feet long, of which a length of 1,000 feet is formed by a concrete-lined tunnel. The factory and power house are connected to the pier on the shores of Loch Linnhe by a double-track railway. The pier and its approaches are nearly 2,000 feet long.

In this way the natural water power of the Scottish Highlands has been harnessed to supply the electricity needed for the large-scale production of aluminium, a metal which is rising in importance from year to year because of its value to the engineer.

FORT WILLIAM END of the Ben Nevis Tunnel during its construction. Battery-driven locomotives hauled trains carrying material and spoil from the working faces to the surface. Three thousand men were employed on the excavation of the tunnel.

You can read more on “Alpine Tunnels”, “Britain’s Electric Power Supplies” and “Power From Scotland’s Lochs” on this website.

You can read more on the West Highland Railway in Railway Wonders of the World