AN ELEVATED RESERVOIR.

Otago Witness, Issue 2997, 23 August 1911, Page 83

AN ELEVATED RESERVOIR.

HOW THE CITY OF CALCUTTA HAS SOLVED A DIFFICULT PROBLEM. By James Armstrong. The eye of the traveller wandering over the prevailing flatness as he approaches

the City of Calcutta is arrested -by a towering square structure which 'looms prominently above the sky-line. It is not an aesthetic contribution to the landscape, and its square lines and grim appearance of importance contrast vividly with the general evidences of Orientalism. It cannot fail to provoke curiosity, and when in response to the craving for further information one inquires as to what it is, the reply that " it is a water reservoir, a gigantic tank, if you like," dissipates wonder as readily as heat melts snow. The one million-odd people forming the population of the capital city of Bengal are dependent upon the River Hoogly for their drinking water. The waterway is tapped at PiUta, about miles above the city. Here are enormous settlingtanks, stretching over 21 acres, where the liquid is 'freed from all impurities and then despatched through two huge mains measuring 48in and 42in in diameter respectively, to Tallah, a suburb of the •ciity. Here it is collected in capacious underground reservoirs, which when full contain 8,000,000 gallons. At various parts of the city are four subsidiary reservoirs, three of which, situate in the heart of the metropolis, can carry six, four, and three million gallons if water, these being charged from Tallah by means of pumps. From these subsidiary reservoirs the water is pumped direct into the mains, distributing at the requisite pressure. But the demand for water during the 24 hours fluctuates. At one time during the day or night it reaches a certain figure, which is known as the maximum, and for which the facilities must be prepared. But the adjustment of the supply to the demand to meet all. necessities is a vexed question. It is obvious that the pumping plant should be capable of fulfilling this uttermost limit, but if this is provided, not only is a heavy capital outlay involved, accompanied by a long .period during the day and night when a certain proportion of the power ie not required, but from the engineering point of view it is an unsound policy. The average city, having an undulating situation, overcomes the difficulty by hollowing out the highest humps of ground for the purposes of storing water, which gravitates into the supply-mains, and consequently the supply is met under all and varying conditions. These reservoirs act as balancers, and the result is that the pressure always remains the same, instead of falling off when the demand is at its •highest, and becoming unduly great when the demand is at its minimum. —A Daring Proposition.— The civic authorities of Calcutta struggled with this problem for years. At . last the chief engineer to the municipality, Mr W. B. MacCabe, M.lnst. C.E., F.I.C, evolved a solution which he considered would meet the situation most economically and satisfactorily. The reservoir could not be placed underground, so he would erect it overhead. His suggestion was assailed most vigorously, but as the critics could not advance a workable alternative scheme, and the demand for some way out of the difficulty was becoming more acute every day, the chief engineer's project was accepted ultimately. It was a daring proposition. No effort had to be spaied to secure absolute safety, and to hold some 9,000,000 gallons of water in reserve in the air meant the support of a tremendous weight—about 40,000 tons. Steel was the only structural material which could be used, and, owing to the unique character of the undertak- j ing, tenders were invited from all parts of the world. The competition for the j contract was exceptionally keen, but a British firm came first easily both in cost j and the time required for the erection of j the reservoir, and consequently the task i was entrusted to them- The firm was j Messrs Clayton, Son, and Co. (Ltd.), of . Moor End Works, Hunslet, Leeds, who j have had prolonged experience with water-engineering in all its branches. —A Labyrinth of Steel. — Their task embraced the erection of the whole of the steelwork. The. foundations were to be prepared for them by local organisations, and an area measuring 342 ft square, representing, a superficies of 117,000 square feet, was acquired. Owing to the soil being a mixture of sand and clay, the preparation of the site was a somewhat delicate operation. First the "bad spots" of ground were piled with logs varying from 20ft to 25ft in , length and heavily ballasted, the latter j material being first rammed down tightly, j and then compressed finally by means of i steam rollers. A wall was built round i this enclosure to prevent the foundations from spreading, this barrier comprising | two rows of piles varying from 25ft to 30ft long and concrete. Upon this prepared i surface a massive solid plinth of reinforced concrete was laid measuring 30in in thick- j nccs, with the reinforcing steelwork dis- | posed in horizontal layers and running at j right angles. As the weight of the steel used in construction represented 8500 tons, which, | together with the water, gave an aggregate weight of 48,500 tons, this founda- i tion plinth had to be of tremendous j strength. The tank is supported on lofty i steel towers, built up of columns measur- j ing 18in by 12in, braced together in fours to'form trestles. These latter are each 20ft square. Altogether 289 of these j columns were erected, and they are secured firmly to the concrete foundation. | Bach column is 885 ft in height, and j weighs 4-£- tons. | Heavy girders were laid on the tops of ■ these towers to support the tank itself. There are two rows of these girders, one I layer being disposed at right angles to i the other. The lower layer comprises beams measuring 24in by 12in, placed in pairs 20ft apart; the upper layer consists of crirders measuring 15in by 6in, placed 26§in apart. On the top of this second layer teak beams 6in wide by 4in deep, . and steeped in hot tar, are laid, and upon these rests the floor of the

tank, which is built up of steel plates fin thick and riveted together. —Tank Sixteen Feet Deep.— The tank, which is square, measures 321 ft along the sides, and the -walls are 16ft high, the roof being 110 ft above the ground. The tank itself is divided into four quarters by steel partitions, so that for cleaning or repairing one or more of the divisions may be emptied without interrupting the supply of water. The reservoir is roofed in, the latter being supported upon steel pillars, and the covering is of such a character as to secure "absolute watertightness. In order to' prevent birds and insects from gaining access to the tank under the eaves, all such spaces are covered with braes gauze having a very fine mesh. As the tank is designed to act as a balancer, only one pipe is required to connect it to the pumping plant and to the mains. This pipe is sft in diameter. The action of the tank 'was follows: During the time of xnajritrum demand the water in this elevated reservoir comes to the assistance of the pumps, the pressure of. water being secured from _ its elevation. During' the period of minimum demand, when the pumps are exerting more power than is required, the excess is expended in charging the tank. Native workmen under white supervisors were pressed into service for constructional purposes, and so harmoniously and expeditiously was the task prosecuted that the engineers drove the last rivet several months within the contract time. Upon completion it was filled to overflowing with water to test watertightness, which was an essential feature in the work, and this was satisfactorily demonstrated —so much so that the contractors were congratulated by the designing engineer upon the thoroughness with which they had fulfilled their taskWorld's Work.