Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/560

 510

A Q U E

earth above them. Angles are turned by means of special bend pipes, the curves being made of as large a radius as convenient. In the case of the Thirlmere Methods aqueduct, double socketed castings about 12 Of laying. inc;hes long (exciusiVe 0f the sockets) were used, the sockets being inclined to each other at the required angle. They were made to various angles, and for any particular curve several would be used connected by straight pipes 3 feet long. As special castings are nearly double the price of the regular pipes, the cost was much diminished by making them, as short as possible, while a curve, made up of the slight angles used, offered practically no more impediment to the flow of water in consequence of its polygonal form, than would be the case had special bend pipes been used. In all cases of curves on a line of pipes under internal fluid pressure, there exists a resultant force tending to displace the pipes. When the curve is in a horizontal plane and the pipes are buried in the ground, the side of the pipe trench offers sufficient resistance to this force. Where, however, the pipes are above ground, or when the curve is in a vertical plane, it is necessary to anchor them in position. In the case of the Tansa aqueduct to Bombay, there is a curve of 500 feet radius near Bassein Creek. At this point the hydrostatic head is about 250 feet, and the engineer, Mr Clerke, mentions that a tendency to an outward movement of the line of pipes was observed. At the siphon under Kurla Creek the curves on the approaches as originally laid down were sharp, the hydrostatic head being there about 210 feet; here the outward movement was so marked that it was considered advisable to realign the approaches with easier curves (Proc. Inst. C. E. vol. cxv. p. 34). In the case of the Thirlmere aqueduct the greatest hydrostatic pressure, 410 feet, occurs at the bridge over the river Lune, where the pipes are 40 inches in diameter, and in descending from the bridge make reverse angles of 311-0. The displacing force at each of these angles amounts to 54 tons, and as the design includes five lines of pipes, it is obvious that the anchoring arrangements must be very efficient. The steel straps used for anchoring these and all other bends were curved to fit as closely as possible the castings to be anchored. Naturally the metal was not in perfect contact, but when the pipes were charged the disappearance of all the slight inequalities showed that the straps were fulfilling their intended purpose. At every summit on a line of pipes one or more valves must be placed in order to allow the escape of air, and they must also be provided on long level stretches, and at changes of gradient where the depth of the point of change below the hydraulic gradient is less than that at both sides, causing what may be called a virtual summit. It is better to have too many than too few, as accumulations of air may cause an enormous diminution in the quantity of water delivered. In all depressions discharge valves should be placed for emptying the pipes when desired, and for letting off the sediment which accumulates at such points. Automatic valves are frequently placed at suitable distances for cutting off the supply in case of a burst. At the inlet mouth of the pipe they may depend for their action on the sudden lowering of the water (due to a burst in the pipe) in the chamber from which they draw their supply, causing a float to sink and set the closing arrangement in motion. Those on the line of main are started by the increased velocity in the water, caused by a burst on the pipe at a lower level. The water, when thus accelerated, is able to move a disc hung in the pipe at the end of a lever and weighted so as to resist the normal velocity; this lever releases a catch, and a door is then gradually revolved by weights until it entirely closes the pipe. Reflux valves on the ascending

DUCT leg of a siphon prevent water from flowing back in case of a burst below them ; they have doors hung on hinges, opening only in the normal direction of flow. Due allowance must be made, in the amount of head allotted to a pipe, for any head which may be absorbed by such mechanical arrangements as those described where they offer opposition to the flow of the water. These large mains require most careful and gradual filling with water, and constant attention must be given to the air-valves to see that the gutta-percha balls do not wedge themselves in the openings. A large mass of water, having a considerable velocity, may cause a great many bursts by waterramming, due to the admission of the water at too great a speed. In places where iron is absent and timber plentiful, as in some parts of America, pipes, even of large diameter and in the most important cases, are sometimes made of wooden staves hooped with iron. A description of two of these will be found below. The Thirlmere Aqueduct, to which reference has been made above, is capable of conveying 50,000,000 gallons a day from Thirlmere, in the English lake district, to Manchester. Thirlmere. The total length of 96 miles is made up of 14 miles of tunnels, 37 miles of cut - and - cover, and 45 miles of castiron pipes, five rows of the latter being required. The tunnels where lined, and the cut - and - cover, are formed of concrete, and are 7 feet in height and width, the usual thickness of the concrete being 15 inches. The inclination is 20 inches per mile. The floor is flat from side to side, and the side-walls are 5 feet high to the springing of the arch, which has a rise of 2 feet. The water from the lake is received in a circular well 65 feet deep and 40 feet in diameter, at the bottom of which there is a ring of wire-gauze strainers. Wherever the concrete aqueduct is intersected by valleys, cast-iron pipes are laid ; at present only one of the five rows 40 inches in diameter has been laid, as the city does not yet require its present supply to be augmented by more than 10,000,000 gallons a day. All the elaborate arrangements described above for stopping the water in case of a burst have been employed, and have perfectly fulfilled their duties in the few cases in which they have been called into action. The water is received in a service reservoir at Prestwich, near Manchester, from which it is supplied to the city. The supply from this source was begun in 1894. The total cost of the complete scheme will not be far short of £5,000,000, of which rather under £3,000,000 had been spent up to the date of the opening. The Fyrnwy Aqueduct was sanctioned by parliament in 1880 for the supply of Liverpool from North Wales, the quantity of water obtainable being at least 40,000,000 gallons a day. Vyrnwv v A tower built in the artificial lake from which the supply is derived, contains the inlet and arrangements for straining the water. The aqueduct is 68 miles in length, and for nearly the whole distance will consist of three lines of cast-iron pipes, one of which, varying in diameter from 42 inches to 39 inches, is now in use. As the total fall between Vyrnwy and the termination at Prescot reservoirs is about 550 feet, arrangements had to be made to ensure that no part of the aqueduct be subjected to a greater pressure than is required for the actual discharge. Balancing reservoirs have therefore been constructed at five points on the line, advantage being taken of high ground where available, so that the total pressure is broken up into sections. At one of these points, where the ground level is 110 feet below the hydraulic gradient, a circular tower is built, making a most imposing architectural feature in the landscape. At the crossing of the river Weaver, 100 feet wide and 15 feet deep, the three pipes, here made of steel, were connected together laterally, floated into position, and sunk into a dredged trench prepared to receive them. Under the river Mersey the pipes are carried in a tunnel, from which, during construction, the water was excluded by compressed air. Denver Aqueduct.—The new supply to Denver City, initiated by the Citizens Water Company in 1889, is derived from the Platte river, rising in the Pocky Mountains. The first aque- _ duct constructed is rather over 20 miles in length, of enver. which a length of 16^ miles is made of wooden stave pipe, 30 inches in diameter. The maximum pressure is that due to 185 feet of water ; the average cost of the wooden pipe was $1.36| per foot, and the capability of discharge 8,400,000 gallons a day. Within a year of the completion of the first conduit, it became evident that another of still greater capacity was required. This was completed in April 1893 ; it is 34 inches in diameter and will deliver 16,000,000 gallons a day. By increasing the head upon the first pipe, the combined discharge is 30,000,000 gallons a day. An incident in obtaining a temporary supply, without waiting for the completion of the second pipe, was the construction of two wooden pipes, 13