Nearly 400,000 people live on the island of Hong Kong, where the natural water is insufficient for their needs. On the mainland of Kowloon were built a large dam and reservoir to store an adequate supply, which is carried across to the island in a main
THE SHING MUN DAM was built across a gorge in the valley of the Shing Mun River, in Kowloon, to create a large reservoir to supply the island of Hong Kong with water. The dam is 275 feet high and its summit is 635 feet above sea level. The upstream face is here illustrated.
GEOGRAPHICALLY, Hong Kong is one of the most peculiar places in the world. Here is a British colony of which the most important part is an island lying off the coast of the Chinese province of Kwang Tung. The island is only eleven miles long and from two to five miles wide, yet it contains a seaport city -Victoria - of considerable extent and world-wide importance. Opposite the island, on the mainland, lies Kowloon, also incorporated in the colony, and providing a railhead for the main line to Canton and the interior of China.
Although an island port can obtain its food supply and similar services from more or less remote sources, its water supply may present difficulties. The island of Hong Kong, it is true, is supplied with water by the periodical torrential rains peculiar to that part of the world, but this natural Water supply is insufficient for the needs of a city of nearly 400,000 inhabitants. The resources of the island itself had originally been tapped by an elaborate system of catchments and channels, leading to several impounding reservoirs of relatively small capacity; but as the city expanded, it fast outgrew the capacity of such a system. In spite of
the fact that about a mile of water lay between the island and the mainland, the mainland offered the only possibilities of augmenting the island’s water supply.
Water engineers extended the system of catchments and channels to the hills of the territory surrounding Kowloon, and conducted the rainwater from the various catchments to a system of reservoirs, which fed Hong Kong through a main passing under the intervening strait. They experienced considerable difficulty, even so, in finding suitable sites for their catchments within a reasonable distance of Hong Kong. Moreover, the system of reservoirs and elaborate conduits leading from the catchments in the hills was becoming unduly complicated.
As the years advanced, and the population of Hong Kong gradually increased, it became evident that a centralized supply dependent on a single large source provided the only satisfactory solution to the colony’s water problem. Surveyors went out into the hills to the north of Kowloon, and reported a number of possible sites for a centralized reservoir, all of which were carefully considered by those responsible for the water supply. Kowloon, opposite the island, lies on a peninsula, backed by the range of hills which provided water for the colony. Beyond these hills a sound or arm of the sea passes a long way inland, and receives the flow of the Shing Mun River. This river, with its source in a range of mountains rising to some 2,500 feet, flows first southwards and then eastwards to its mouth. It was the Shing Mun River which the engineers chiefly recommended as a potential source of Hong Kong’s water supply. The British firm of civil engineers called in by the Government advised that a large reservoir be created by a dam in the Shing Mun Valley at a point where it narrowed to a deep gorge, several hundred feet above sea level.
Various reasons governed the final position of the dam, besides the fortunate contraction of the valley into a narrow canyon. The underlying rock consisted of hard granite impervious to water and capable of supporting an enormous weight. By building a dam 275 feet high at this point, which meant that the summit of the dam would be 635 feet above sea level, the engineers calculated that they would be able to produce a reservoir with an ordinary capacity of no less than 3,000,000,000 gallons.
The natural catchment area of such a reservoir formed by the slopes of the surrounding mountains would come to approximately 3,000 acres. In the event of the yield of the reservoir having to be increased, it would be possible to include a catchment area of 2,000 acres on the western and 500 acres on the northern side by the simple expedient of building additional channels leading to the main catchment.
In normal conditions, a simple gravity dam of concrete would have been eminently suitable on this site, but the designers of the Shing Mun Dam found several considerations weighing against this. Concrete is an expensive commodity in Hong Kong and its adjacent territories. Moreover, the region in question was known to be subject to disturbance by earth tremors, which would render the simple rigid concrete dam a dangerous expedient. If, however, the builders adopted a stout rock-fill dam they had plenty of building material on the spot, besides enjoying all the advantages of cheap unskilled Chinese labour for constructing the fill. The rock fill, therefore, was adopted in the final designs for the Shing Mun Dam. Before work could be started on the building of the great barrier, special allowance had to be made for the passage of the river flow during construction. The engineers provided for this by driving a tunnel, 15 feet in diameter, below the future site of the dam, and allowing the water to flow through this until the dam was completed, or at any rate was far enough advanced to allow for the water to be headed up.
Inverted Siphon in Aqueduct
The river was diverted into this tunnel by a temporary dam thrown across the Shing Mun some way upstream. Thence the engineers drove their tunnel straight into the rock stratum of the mountainside at an angle of 45 degrees to the centre line of the future dam. After this tunnel had pursued a straight course for some way, they took it round a curve of 90 feet radius until it was running at right angles to the line of the dam. This tunnel, which was lined with concrete, descended gradually throughout its length, the outlet being 20 feet below the inlet and discharging into the original river bed farther downstream. Another complicated operation had also to be carried out in connexion, with carrying a water-flow past the dam during building. This arose out of the temporary measures which had been taken for conducting water from the Shing Mun River to Hong Kong, before the dam had been built. The water from the catchments higher up the valley was carried past the site of the dam in a special aqueduct. The level of this aqueduct on the hillside was below that which the summit of the dam would ultimately reach. It was not practicable to build the dam round the aqueduct, and then seal up the aqueduct as soon as the dam was completed.
DOWNSTREAM FACE of the Shing Mun Dam, Kowloon. The first important operation was the construction of a thrust block across the valley. Then came the building of the rock fill, an enormous bank of packed masonry rising on a slope of 1 in 1½. The slope is broken in two stages by transverse horizontal terraces, each 10 feet wide.
At the same time, it was essential that the water supply passing through this aqueduct should not be in any way interfered with. The builders of the dam accordingly put in an inverted siphon, that is, a section in the conduit resembling a much elongated letter U, the bottom of which passed under the dam. From a point on the aqueduct a little above the centre line of the dam they carried their pipe downwards to the lowest possible level. Then they took it through the tunnel under the darn, which had been originally built for carrying the diverted flow of the river. On the other side of the dam they carried their pipe up the mountainside again and reunited it with the main line of the aqueduct. Once this inverted siphon was in operation, they were able to take out the old intervening section of the aqueduct and seal up the ends of the broken conduit. This operation was carried out with comparatively little trouble beyond a relatively unimportant heading up of the water in the upstream end of the aqueduct.
The building of the dam presented various interesting features. The engineers’ first important operation was the construction of a huge “thrust block” across the valley. This consisted of an enormous wall of concrete in two parts, one above the other. They laid their foundations on the natural stratum of grey granite, which was capable of bearing a superimposed weight of 500 tons to the square foot.
On the upstream side, the face of this great concrete wall rose perpendicularly to a height of 453 feet above the sea. On the downstream side, the block had a batter of 1 in 9, that is, it decreased in thickness inwards from the base at a rate of 1 foot out of the perpendicular for every 9 feet of height. The width of the block at the top was 8 ft. 6 in. over the supporting buttresses. Above this a wall of impervious concrete blocks rose to the summit of the dam, at a height of 635 feet above sea level. This great concrete wall supported the reinforced concrete water-facing by buttresses, at intervals of 12 feet, faced with blocks of moulded concrete, and having a batter of 1 in 3·72.
Novel methods of transit were used for the handling of the cement needed in these and other features of the dam. The cement was packed into steel containers at the manufactory on the coast, each container holding 4 tons.
Inspection Pits and Cross-Galleries
These steel containers were carried up to the scene of operations in motor lorries, each lorry carrying two containers at a time. On arrival, the containers were lifted by cranes to the level of the cement silos, over which they were tipped. The contents of each container then passed down into the silos, whence they could be drawn off through outlet valves as required, passing directly by gravity feed from the silos to the mixing machines.
From the top of the dam the designers carried deep wells or inspection pits down to the level of the lower section of the thrust block. On the thrust block, cross-galleries run at right angles to the line of the dam, all of them communicating with a single long gallery running from one end of the structure to the other. This longitudinal gallery is accessible from either end by flights of steps running down into the interior of the dam, as well as from the inspection pits. Any leakage from the upstream face of the dam will cause water to find its way into one or other of these galleries. Such a leakage is thus easy to locate, and immediate steps can be taken to rectify it. Leaking water, however, does not accumulate in the galleries themselves, but finds a passage, by way of the cross-galleries, to the downstream face of the thrust block and thence by percolation through the rock fill.
The rock fill on the downstream side of the thrust block required but little preparation. In the main, the bottom of the valley was sufficient in itself, and only in a few places where boggy or other unstable patches existed was any special excavation required. The building of the rock fill was a lengthy task, but it did not prove unduly difficult.
CREST AND VALVE TOWER of the Shing Mun Dam. Inside the valve tower rises the 48-in. standpipe which governs the supply of water from the reservoir to the main, which runs to Hong Kong.
The rock fill consists of an enormous bank of packed masonry sloping upward from the bottom of the valley on the downstream side to the summit of the dam. Its surface rises on a gradient of 1 in 1½, but this slope is broken at two stages by transverse horizontal terraces, each with a width of 10 feet. The coolies employed on the building of the rock fill laid their materials - hewn boulders - in superimposed layers, each layer having a thickness of 2 feet, and sloping downwards towards the core of the dam on an incline of 1 in 12. The coolies covered these layers of boulders, forming the body of the fill, with a facing of squared blocks of granite, laying these in a shallow W or horizontal zigzag formation from one side of the dam to the other. The weight of the fill as a whole averages approximately 1·30 tons per cubic yard.
From ground level up to that of the inspection galleries the rock fill leans its weight against the concrete thrust block forming the heart of the dam; but above the galleries, 453 feet above sea level, the builders adopted a different formation. From the 453-feet level to the summit of the dam they erected a great perpendicular wall of rubble, set in strong mortar, against the boulders of the rock fill. This inner wall thus left a relatively narrow wedge-shaped gap between the upper parts of thrust block and rock fill, which the engineers filled with coarse, dry sand, covering it over at the top with concrete paving.
This huge wedge of sand, running right across the valley inside the dam served several purposes. It ensured an even degree of support between the rock fill and the thrust block throughout the length of the structure. Moreover, it acted as a sort of buffer, compensating for any possible settling of the rock fill or movement of the thrust block. The concrete paving above was designed to protect the sand-wedge from the torrential rains prevalent at certain times in this part of the world. Percolation into the sand-wedge from the sides was also guarded against by the provision of drainage wells or tubes filled with coarse granite chips.
The impervious upstream face of the dam is subjected to the full pressure of the impounded water. The designers of the structure arranged for this impervious face to be in two parts. Below the 453-feet level they erected a huge cut-off wall. They carried the foundations of this wall down to a depth of 70 to 80 feet below the level of the old river bed, until they reached the underlying stratum of impervious grey granite.
A Circular Weir
The top stratum on which rest the thrust block and rock fill is not impervious to water. The lower layer of granite, however, is impermeable and is capable of bearing a direct weight of 2,000 tons per square foot. In the construction of this lower part of the upstream face the builders used a special slow-setting cement which, on hardening, would be fully capable of withstanding the tremendous pressure of the artificial lake formed in the valley of the Shing Mun River. The blocks forming the upstream face were sealed with thin copper strips.
From the top of this lower section of the upstream face, the builders carried the upper section to the summit of the dam in the form of a series of huge reinforced concrete panels, each 25 feet wide by 20 feet deep, which they rested against the buttresses of the thrust block. The thickness of the panels decreased from 6 feet to 3 feet as they mounted upwards. They filled the intervening space between concrete panels and thrust block with a layer of bitumen, thus forming a sort of sliding joint. They made the panels on the spot, pouring each, in a fluid state, into a mould which already contained the grid of steel rods destined to form the reinforcing agent. Up to a level of 573 feet the outer face of the panels is built up to a batter of 1 in 3·4; above that level the inclination is 1 in 3·7 out of the perpendicular.
There is not, however, an absolutely even inclination of the outer face of the panels. The lower part of each panel is sloped to the general batter, but the upper part is perpendicular, so that its uppermost extremity projects from the main slope in the form of a step, or narrow terrace. This work was carried out during a dry season and, as. it was essential that the concrete panels should not dry too rapidly, they were sprayed with water over a period of three weeks to each block after the “forms” or moulds had been removed. Within twenty-eight days of setting, the concrete of the panels was capable of withstanding a breaking load of 260 tons to the square foot.
FILTER BEDS at Bowen Road, Hong Kong. The natural resources of the island on which stands the port of Victoria had been tapped by a series of catchments and channels, but the supply was inadequate for the growing population. Water is now supplied from the reservoir on the mainland. The reservoir is fed by the Shing Mun River.
An enormous quantity of rain can fall within a short time in this part of the world, and special provision had to be made from the outset to allow for this precipitation. In due course, the old tunnel built under the site of the dam to carry the flow of the Shing Mun during the initial stages of construction was blocked up towards its upstream end; but a new flood discharge tunnel, also with a diameter of 15 feet, was led into the old tunnel at an angle about halfway along its course. This now takes the water from an overflow governing the level of water above the dam.
To make provision for this overflow, the engineers built an enormous “bell-mouth” in the valley above the dam. The top of the bell-mouth is at the maximum level allowed for the impounded water. The bell-mouth was built of concrete, in the form of a huge funnel narrowing towards the bottom. It forms a circular weir, with a diameter at the top of 74 feet, and it is placed in shallow water. During construction, there was no water on the site at all.
From the bottom of this bell-mouthed weir the engineers carried the great overflow pipe down below the dam into the flood discharge tunnel. To guard against the formation of a whirlpool over the bell-mouth during a period of unduly high water, they placed a single vertical baffle across the top. This baffle automatically destroyed any tendency towards the formation of a vortex.
Additional provision for the discharge of flood water was made by means of six siphons built into the top of the dam and discharging into a tunnel leading to an outlet 460 feet above Ordnance datum.
Overflow water issues from the siphon outlets with considerable force, and it is necessary to break this to avoid erosion of the surrounding rocks in the bottom of the valley. Normally this is done by diffusing the overflow water on an area of flat benching, which spreads it out and breaks up its initial force. At the Shing Mun Dam, however, this benching was already provided, not by Nature, it is true, but by the quarrying operations which had preceded the building of the rock fill.
Inlets 537 feet Above Sea Level
The quarry floor was hard and flat, and provided an admirable “bench” or terrace on which to lead the water from the overflows, and all that was necessary in addition to this was a curved wall of masonry round the side of the benching to protect the adjacent “toe” or downstream edge of the dam.
One major feature which remains to be described is that which governs the supply of water from the artificial lake to the main running down to Hong Kong. The principal, though not the most spectacular part of this consists of a 48-in. standpipe originating in the artificial lake and passing underneath the dam. It was essential that this standpipe should receive its supply from several different levels. On the upstream side of the dam, therefore, the engineers built a concrete tower, rising from the bed of the artificial lake that was to be, to a level some way above its ultimate high-water mark. Within the tower they placed a valve chamber to control the flow of water into the standpipe, which rose perpendicularly inside the tower. Inlets were provided in the standpipe at various levels, the lowest being 537 ft. 6 in. above the sea.
From the base of the tower, the standpipe passes under the dam, following the same route as the old diversion tunnel. For the first part of its way it lies along the bottom of the diversion tunnel itself. Where the diversion tunnel is plugged before being united with the overflow tunnel from the bell-mouthed weir, the supply pipe passes downwards. It continues under the floor of the tunnel for the rest of its course beneath the foundation of the dam. Parallel to it, but lying below tunnel level all the way, is a scour pipe designed to relieve the action of the impounded water against the dam’s upstream foundations.
Having emerged beyond the dam, the supply pipe passes through a Venturi meter, which keeps a continuous record of the flow through it, into a pump-house, where the flow of the water to its ultimate destination is regulated.
The Shing Mun Dam is one of the most remarkable works of its kind in the Far East. It contains 166,000 cubic yards of concrete, and the rock fill consists of 500,000 cubic yards of stonework. The dam was completed in 1937.
THE OVERFLOW OPENING of the Shing Mun Dam takes the form of a concrete bell-mouth, with a maximum diameter of 74 feet, level with the maximum permissible height of the water in the reservoir. From the bottom of the bell-mouth the overflow pipe leads into a special flood discharge tunnel.