AQUEDUCTS. In the broad sense of the word, an aqueduct is an artificial channel for the conveyance of water. In a more restricted sense it is often understood to mean a bridge formed in a series of arches or spans for the conveyance of water across a valley. In the usual acceptance of the word in modern engi neering an aqueduct is a primary channel or conduit for the con veyance of water from a source to the principal point of distri bution or use, subsidiary distribution conduits being not generally classed as aqueducts.
Traces of remarkable aqueducts carried out by the Phoenicians have been found in Syria and in Cyprus, including tunnelling through rock and the construction of syphons across valleys for the conveyance of water to temples.
Jerusalem has from very early times been supplied by a series of conduits, the earliest of which probably date back to the times of the kings of Judah. The principal reservoir is formed by the Pools of Solomon from which two conduits delivered water to the city. The lower of the two, which is still complete, is about 20 miles long, and crosses the valley of Hinnom on arches.
In Europe the earliest aqueducts were made by the Greeks. Lake Copais in Boeotia was drained by a tunnel driven from 16 shafts, the deepest of which was 15of t. Eupalinus, a celebrated hydraulic engineer, carried out a water supply for Megara about 625 B.C. and later drove a tunnel 8ft. by 8ft. in section and 4,2ooft. long through a hill to convey a water supply to Samos. Conduits involving less difficult work brought in water from Hymettus and Pentelicus to Athens. The Hadrian Aqueduct was constructed A.D. 134-40 to augment the supply to that town. It consists of over 15 miles of tunnel lined with brick and masonry arranged to tap underground water-bearing strata by means of subsidiary galleries, wells and drains. After long years of disuse and disrepair this aqueduct has again been made good and brought into service to supply about 2,500,000 gallons per day.
The second aqueduct, known as the Anio Vetus, was con structed 4o years later to convey water from a source on the river Anio in the Apennines. It was a work of some magnitude, having a length of about 41 miles, formed mainly a covered con duit, and was carried out by contract, the price being the spoils obtained from the defeat of Pyrrhus. The waterway was of rectangular form with pointed roof and had a height of 8f t. i 2in. and width of aft. 7in.
In 145 B.C. Marcius the praetor constructed the first high level aqueduct to convey water of fine quality from various springs including those near mile 38 on the Via Sublacensis where it emerged from caves into a pool of deep green hue. His aqueduct, known as the Marcia, had varying cross-sections, the internal dimensions at the upper end being about 8ft. tin. high by 5ft. 7in. wide. Its length of about 58.4 miles included half a mile raised above ground on masonry walling, and about 61 miles on masonry arches mainly at the city end.
The construction of the fourth aqueduct, a comparatively small and short one, known as the Tepula, followed 18 years later in 127 B.C. Large use was made of concrete in its construction and for part of the way it was built on top of Marcia.
After an interval of 94 years Agrippa in 33 B.C. tapped another source near the 12th milestone on the Via Latina by the con struction of the fifth aqueduct called the Julia which joined the Tepula and ran on top of it. Its waterway had a rectangular section about Oft. 7in. high by eft. 4in. wide. Its total length was about 14.6 miles of which fully 6 miles were on arches. Near the Porta Tiburtina the aqueducts Marcia, Tepula and Julia were formed one above another on the same line of arches.
In 20 B.C., the sixth aqueduct, called the Virgo, about 13 miles long, was completed by Agrippa to convey water of excellent quality to Rome from a group of springs on the estate of Lucullus. The waterway was about 6f t. 7in. high by 'ft. i in. wide and fully one mile was raised above ground on walling and arches.
The Alsietina was the seventh important aqueduct and was constructed by Augustus to bring in a low level supply of rather unpalatable water from the small lake Alsietina situated about 20 miles north-west of Rome. Its length was 21 miles, of which one-third of a mile only was on arches.
In A.D. 36 Caligula initiated the construction of two additional aqueducts known as Claudia (the eighth) and Anio Novus (the ninth) respectively, to cope with the increasing demands of the public services and private luxury. The former was completed by Claudius in A.D. 5o, and conveyed water from some fine springs near milestone 38 on the Via Sublacensis. Its waterway was about 6f t. yin. high by 3f t. Sin. wide and the whole length of about 44 miles; nine miles were carried on arches generally of loft. span with piers 8ft. thick in elevation.
The Anio Novus was completed in A.D. 86 and drew its water from the river Anio, at a point near the 42nd milestone on the Via Sublacensis. The waterway was variable, one section being about 8ft. oin. high by 3f t. Sin. wide. The whole length was about 55.6 miles and the last seven miles were formed on the top of Claudia on walling and arches having a maximum height of 1o9ft. It was constructed principally of concrete with brickwork facing. Some 36 years later the route of this aqueduct was shortened by driving a tunnel about 3 miles long under Mount Affliano.
Towards the close of the first century A.D. the aqueducts, now nine in number, came under the care of Sextus Julius Frontinus, who had record plans prepared showing the valleys and rivers crossed, the raised and arched portions, and the places on the hill sides where particular care was required in maintenance. He states in his work De Aqueductibus Urbis Romae that he thereby reaped the advantage of having the works in which he was concerned directly before him. That he appreciated the practical importance of his charge may be judged from his observation, "Will anybody compare the idle Pyramids, or those other useless though much renowned structures of the Greeks with these many indispensable aqueducts?" He found a number of practices in vogue which were inimical to public service. The aqueducts were surreptitiously bled en route by proprietors of adjacent lands. At the city the water was measured into distributing tanks by a few bronze meter orifices of large capacity and distributed out again by a large number of small orifices to the various purchasers. The water men by making the inlet orifices larger than the nominal diameter and the outlet orifices smaller, were enabled to balance the quantities received and distributed while having a considerable actual surplus which they sold on their own account. Many of the citizens also were not averse to receiving water by illegal tapping of the State lead mains supplying the public institutions and fountains.
Based on his experience of the upkeep of the vast system of water supply conduits under his charge, developed by a succes sion of builders during a period of 400 years, he formed the opinion that the workmanship in the old aqueducts was better than in the new. The work of maintenance was heaviest on the above-ground portions of the aqueducts, particularly the arches and much less on the underground portions which were immune from the effects of heat and frost, but which on the other hand were liable to damage from penetration by the roots of trees. Major works of repair were usually undertaken on one aqueduct at a time, with the flow cut off, but were sometimes carried out without interruption of supply by erecting supports from the ground and by-passing the water in a lead trough.
The water supply system was further extended after the time of Frontinus by the construction of the Aqua Trajana in A.D. 109 which conveyed water from the springs near Lake Bracciano, 20 miles north-westwards from Rome and the Alexandrina in A.D. 226 which tapped some springs near the Via Praenestina about 14 miles from Rome.
Frontinus gives quantities of water delivered by the various aqueducts in "quinaria" but the equivalent discharge in modern measure is difficult to arrive at. A determination by Claudia di Fenigio is, one quinaria=o•48 litre per sec. which is equal to 9,400 imperial gallons per day. On this basis the total water delivered by the aqueducts in the time of Frontinus was about 130,000,000 gallons per day of which 92,000,000 gallons were used within the city.
The water supply per head in Rome was about twice as great as in Glasgow (in 1928), but allowing for the unavoidable loss from Rome's free flowing conduits, the utilizable supply to the two cities would be more nearly on a par.
What has been the fate of those great works? The Virgo and the Trajana were restored to use in 157o and 1611 respectively. The ugly Acqua Felice, constructed by Sixtus V. in 1585 takes the place of the Alexandrina, though on a somewhat different line. The excellent waters from the springs which fed the old Marcia are once again being conveyed to Rome by a new aqueduct 33 miles long, known as the Pia Marcia, constructed in 187o. The upper half is formed as a rectangular masonry conduit with arched roof having a capacity of 27,000,000 gallons per day, while the lower half is a pressure conduit formed of a number of lines of 24in. cast iron pipe.
The most impressive remains are the stretches of arches which still stand, indicating in monumental fashion the vast extent of the ancient structures provided for the conveyance of the high level supply across the last broad depression of the campagna to the city of Rome.
Roman Aqueducts Outside Italy.—The following examples are specially noteworthy : (1) The Pont du Gard at Nimes, France, which is still standing, and for boldness and gracefulness of design is unrivalled. It was built by Agrippa possibly in A.D. 18. It has three tiers of arches in its height of i 6of t., the lowest tier having six arches of 6oft. to 75f t. span, the middle tier 11 arches of 75f t. span, and the upper tier 35 small arches over which the waterway was constructed. (2) The aqueduct bridge at Segovia in Spain about 2, 700f t. long and 1 oaf t. high formed with 109 masonry arches in two tiers. The aqueduct, of which this splendid bridge forms part, is still in use.
Mediaeval Aqueducts.—The water supply to the cathedral city of Spoleto in central Italy is conveyed by a 7th century aqueduct about 700f t. long and 2 7of t. high. It is noteworthy for its light and graceful proportions and for the use of pointed arches, there being ten of about 66f t. span.
Several fine examples of arched construction exist on the con duit system of Constantinople, the most noteworthy being the aqueduct of Justinian which constitutes one of the most im posing monuments of its period. It is 7 2of t. long, 1 o8f t. high and has two tiers of pointed arches with 5 5f t. spans in the lower story and 40f t. spans in the upper. The piers are buttressed and lightened by having small arches pierced through them at dif ferent heights.
In the ancient aqueducts, with few exceptions, the conveyance of the water was accomplished by forming the aqueduct as a free flowing channel, usually built of masonry, with a continuous slight fall in the direction of the delivery point. If it were neces sary to cross a valley the channel was continued on its pre-deter mined level and gradient on a built masonry construction either of continuous walling or of piers and arches until the other side was reached when it again followed the contour of the ground. The advancement of engineering science has obviated the neces sity for the use on a large scale of such constructions in modern aqueducts, where the water can be conveyed when required in large closed pipes flowing full under pressure, and arranged within limits to follow the depressions and elevations of the surface of the ground.
There are, therefore, two distinct methods of flow which may be used according to the circumstances. In the one case the water flows with a free surface in a channel having a regular gentle slope, corresponding to the case of natural flow in a river, in the other case the water completely fills the closed conduit in which it is confined and exerts pressure on the whole of the interior surface tending to burst the walls.
Free Flowing Conduit.—The principal types of construction used for free-flowing conduits are the following : (a) Open canal formed in the earth, with or without an impervious lining; (b) covered conduit built of brickwork, masonry or concrete; (c) tunnel, unlined or with a smooth lining of brickwork, masonry or concrete.
Free-flowing conduits take many shapes in cross-section such as rectangular, horse-shoe and circular. In the case of open canals the most usual form has a straight flat floor and sloping sides.
Open Canals.—Open canals in Britain are not in favour for con veying domestic water supplies on account of their liability to con tamination and interference. They are principally used on a large scale for conveying water to low head water power stations and for the irrigation of large cultivable areas in dry countries. When unlined the gradient must be fixed so that the velocity does not become great enough to cause erosion of the bed or banks, say from 22 to 5ft. per second according to the firmness and cohesion of the material. Considerable loss of water from leakage may be expected with unlined canals. In lined canals velocities up to 'oft. per second or more are possible.
An unlined canal at the Humber-Arm Hydro-Electric Works, Newfoundland, to convey a minimum water flow of 5,000 cusecs, has a bed width of 1 oof t. and water depth of 2I ft. as shown in fig. 1. The gradient is I in 7,450 and the maximum velocity about 3f t. per second.
A concrete-lined (fig. 2) canal for conveying water to the low head power station at Beaumont-Monteux on the Basse-Isere, France, is 1 o5f t. wide on the bottom with 15f t. maximum depth of water, side slopes of I to ' and cement concrete lining I2in. thick on floor and sides, finished with a surface rendering of cement mortar i din. thick. The lining is laid in sections with a day from the Elan Valley to Birmingham and the Catskill conduit for water supply to New York respectively. The latter is one of the largest aqueducts ever constructed for conveyance of domestic water supply.
Tunnels.—Continued improvements in mechanical drilling, in the power and reliability of explosives, and in the methods of handling and transporting the excavations, have contributed to a decided increase in the speed and relative economy of tunnel construction during the first three decades of the loth century. The result is that tunnelling is used in aqueduct-construction to a much larger extent than formerly. It may be the only way for conveying water from one valley to another through a ridge which cannot be circumvented, and in other cases it may be chosen in preference to covered conduit on account of shorten ing of the route, greater security, les sened maintenance cost, and obviation of surface damage. In very sound unfissured rock, lining of masonry, brickwork or concrete may be dispensed with, but it is very seldom that lining of the invert is omitted; where reliability of service under determinate hydraulic conditions is essential, a smooth lining on the whole of the wetted surface should be provided.
The new Loch Katrine aqueduct of the city of Glasgow has about 19 miles of lined tunnel of horseshoe shape with a height of 9ft. and width at springing of roof arch of 'oft. It has a fall of I I din. per mile and a capacity of 76,000,000 gallons per day. Tunnelling on a very large scale has been used in America in aqueducts supplying certain of the large cities such as New York, San Francisco, and Los Angeles. A long concrete lined tunnel on the Catskill aqueduct for New York has a capacity of 500 million U.S. gallons per day with a horseshoe shaped cross-section having a width of 13f t. 4in., a height of ' 7f t., and an effective waterway of 165 sq.ft. The gradient is .37 per I,000.
Covered Conduits.—Built aqueducts for the conveyance of domestic water supplies in Britain are almost invariably roofed and covered over with earth. Numerous examples have been con structed in masonry and brickwork, but concrete is now generally preferred. Such conduits in suitable circumstances are more economical than pipes where the quantity of water is large, but are only applicable where a route can be located on the ground having the desired fall along a contour gradient tending in the direction of the point of delivery. Figs. 3 and 4 show typical cross-sections of the conduit for supply of 75,000,000 gallons per of resisting high tensile stresses and is most effective when dis posed in circular form. The most usual types of construction are: (a) Cast Iron Pipes, (b) Steel Pipes, (c) Wood Stave Pipes, (d) Reinforced concrete pipes or conduit, (e) Tunnel.
Cast Iron Pipes.—Cast iron pipes came into use about the beginning of the 19th century as an alternative to lead and wooden pipe, for the distribution of water in towns. With rapid devel opment of the iron-founders' craft it became available for aque ducts and has been so used in sizes up to 54-in. diameter and over.
Cast iron pipes are usually made in lengths of 9 to i3ft. with end joint of spigot and socket form made watertight with lead. Pipes with flanged ends to bolt together are also used. Pres sure conduits of cast iron pipe may follow any irregular pro file imposed by the route chosen, ascending here and descend ing there, and crossing summits and hollows provided always that none of the summits rise above the "hydraulic gradient" which for a pipe of uniform diameter is the straight line join ing the inlet and outlet ends on the longitudinal profile. Should air collect at any of the summits the flow will be interfered with. Air valves are therefore installed on top of the pipe at such places and automatically discharge the air as it collects without allowing water to escape. Sluice valves fixed on scour branches at the bottom of the pipe are required at the lowest points of the pipe for the periodical discharge of any sediment which may col lect there and restrict the flow. Should a burst occur on a large pipe great damage and loss of water may occur before the water can be cut off. Automatic valves for stopping the flow when a burst occurs are desirable at the upper end of an important pipe and also at the principal summits in the case of a long pipe, and commonly take the form of a circular disc valve on a horizontal axis which passes through glands in the walls of the pipe and is connected to hydraulic operating gear in a chamber formed over the pipe. The operating gear is set in motion by a valve con nected to and controlled by a small paddle within the pipe. The flow of water in the pipe imposes pressure on the paddle, which becomes greater with increase of velocity, and matters are so adjusted that a definite increase of velocity above the normal will move the paddle, thereby setting the operating gear in motion and closing the main valve.
In Britain pipes are secure from the effects of frost if laid with a cover of eft. 6in. of earth, whereas in more severe cli mates greater cover up to 5 or 6ft. as a maximum may be neces sary. Where the foundation is unsatisfactory or where excep tional loads have to be resisted or special forces dealt with as in changing direction at a bend, the pipe should be reinforced by bedding it on, or surrounding it with concrete. In normal cir cumstances cast iron pipe has long life and is generally more liable to deterioration inside than outside, so that some loss of capacity takes place with lapse of time.
Steel Pipes.—The advantages of steel are its lightness, re liability, watertightness, and security from cracking, and not least the fact that it can be used in much larger sizes than cast iron. Experience also shows that well coated steel pipes when buried in normal soil will endure for many years. Riveted pipe is suitable for moderate heads, a good example of such construc tion being the new pipe line from Lake Tansa for the Bombay water supply. The pipe has diameters from 57in. to 72in. and is formed of rings 7ft. 4in. long alternately in and out, each made from a single plate $in. thick, with the longitudinal joints double riveted and the circumferential joints single riveted. The pipes are uncovered and laid direct on the graded surface of the ground.
On the Catskill aqueduct of New York there are many miles of large riveted pressure pipe from 7ft. 4in. to i i ft. Sin. diam eter. The pipes are made in ring courses of 71ft. alternately in and out with one or two plates to the circle according to the size. The thickness varies from Ain. in. and the longitudinal joints are lapped for the thinner plates and butted and double strapped for the thicker. The pipes are surrounded with a casing of concrete having a minimum thickness of 6in. placed in position after the pipe had been tested and made watertight and while it was full of water under pressure. They were afterwards fur nished with a smooth interior protective lining of cement mortar having a thickness of tin.
In Britain and elsewhere there has been considerable develop ment in the use of smooth welded steel pipes which have definite advantages over riveted pipes in that the weight is less for the same strength, the capacity is greater for the same size and instal lation in the field involves less work from the fewer number of joints required. Such pipes are formed of mild steel plates, heated and bent to circular form by means of large bending rolls and having the longitudinal joints lap welded by heating with water gas. The pipes are commonly made in lengths of 20 to Soft. and one or two plates are required to form the circle according as the diameter is under 3ft. or between 3ft. and 6f t. Still larger pipes may be made with three plates to the circle. The steel used requires to be of extra mild ductile and weldable quality such that when allowance of about 9o% effi ciency for the welded joint is made a working tensile stress of about 5 tons per square inch is appropriate. Joints of welded pipes for water supply aqueducts are commonly formed with spigots and sockets, either close fitting for riveting or with an annular gap for filling with lead.
Considerable effort has been directed towards the application to metal pipes of a smooth in terior lining of substantial thick ness which will adhere perma nently and prevent rusting, pit ting and incrustation. Experience is being accumulated with two such linings, having portland ce ment and bitumen respectively as the basic material, which are applied by a process of centri fugal spinning. Protection of ex terior surfaces of steel pipes is usually effected by a coating of bituminous material which may be reinforced by one or more wrappings of bitumen-impregnated hessian.
Wood Stave Pipes.—Wood stave pipes formed in the manner shown in fig. 5, with machined staves bound at close intervals with mild steel hoops, are economical for low heads in districts where suitable timber is abundant and cheap. At the Humber-Arm works, Newfoundland, there are seven lines of such pipe, 9ft. 6in. diameter, used for a maximum head of 167ft.
A wood stave pipe i 6f t. diameter and 1,318f t. long to carry 3,00o cusecs was constructed in 1925 to serve the Hydro-Elec tric Plant No. 2 of the California-Oregon Power Co.
Reinforced Concrete Pressure Conduits.—Closely spaced hoops of steel, along with a system of longitudinal bars are con tained in a cylindrical concrete shell and serve to prevent burst ing and fracture. The chief advantages of this method arise from small cost of maintenance, security against collapse, and fre quently saving in first cost as compared with other methods. For moderate diameters and pressures, pipes may be precast in lengths up to i aft., transported to the site and connected together with joints of the "collar," "lockjoint," or other special type. On the Spavinaw aqueduct for the water supply to Tulsa, Okla homa, there are 53 miles of reinforced concrete pressure conduit. Large conduits require to be manufactured in situ. Fig. 6 shows a cross-section of the huge conduit 19.7f t. diameter of the Drac Romanche Hydro-Electric Development, near Grenoble, France, designed to carry 2,800 cusecs with a fall of i in 1,500 under a maximum head of 5oft.
Pressure Tunnels.—Low pressure tunnels are largely used in mountainous districts to convey water from a reservoir or intake to the head chamber of a hydro-electric station. In hard, sound unfissured rock, such tunnels may be unlined, but a smooth lining of rich concrete in conjunction with sealing of fissured places by the injection of cement through drill holes is generally desirable.
Specially interesting examples of pressure tunnels exist on the Catskill aqueduct of New York. A concrete-lined pressure tunnel, about 4 miles long, is formed in shale deep down below the valley of the Wallkill river, thus obviating construction with pipes on the surface. It is i4ft. 6in. in diameter with a capacity of 500 million U.S. gallons per day and connects with free flow ing conduit at each end by means of vertical shafts. Many miles of similar tunnel have been formed in rock at a depth of 5ooft. below the streets of New York.
New York city began work in 1937 on a $273,000,000 aqueduct to increase its water supply by 5o%. The new project will bring water from the Delaware river by a tunnel to the existing Croton and Kensico reservoirs. The Catskill aqueduct is capable of delivering 500 million gallons per day to New York city. (X.) Modern Italy again furnishes one of the most remarkable water supply systems of the world in the Apulian aqueduct which conveys water from the moist western slopes of the Apennines to an area of 8,000 square miles of semi-arid territory in the south-eastern corner of the country. The intake is at the perennial Caposile springs so that, as in the case of the aqueducts of ancient Rome, a storage reservoir is not required. The main conduit, which has a capacity of about II o million imperial gallons per day is carried through the ridge of the Apennines in a tunnel 92 miles long and extends westwards and southwards for a distance of 152 miles, terminating at Taranto. Altogether some 67 miles of the conduit were formed in tunnel, a typical cross section having a horseshoe shape 8f t. gin. wide by 9f t. 5in. high. Water is deliv ered to a population of three millions in 266 communities by a system of 841 miles of main and branch pipes and 55o miles of distribution pipes.
Of surpassing interest in some respects is the aqueduct con structed in 1903 for supplying the gold mining centres of Cool gardie and Kalgoorlie in Western Australia with water. The source is near the western coast and the point of delivery is some I,3ooft. higher in the arid heart Gf the country 35o miles away. The aqueduct is formed as pressure pipe of the locking bar type, 3oin. diameter with steel plate fin. thick. The water is forced up stage by stage by means of a series of eight pumping stations with balancing tanks at each station.
Two forms of riveted joint are shown in figs. 7 and 8. Fig. 7 shows a spigot and socket form made to slight taper to ensure perfect fit and watertightness. Fig. 8 shows the so-called "bump" joint, with coned and swelled ends, connected by riveting. For the smaller sizes of pipe, flanged and bolted joints of various types are used, generally with a packing ring to ensure water tightness. Typical forms of two classes of flanged joint are shown in figs. 9 and Io. Where great strength is required, welded pipe, with weldless steel bands shrunk on at intervals, has been used.
Two contrasting methods of installation are in use, which may be designated the Swiss and the French method respectively. In the Swiss method a heavy concrete anchorage surrounding the pipe is provided at every bend, horizontal or vertical, the pipe line being thus divided into a series of straight lengths, each of which is provided with an expansion joint near its upper end. The anchorages are designed to resist the maximum forces trans mitted by the section of pipe-line attached thereto. In the French method main anchorages are provided at top and bottom only, there are no expansion joints, subsidiary intermediate anchorages are provided to hold the pipe in position and take care of the stresses produced by local variations of temperature and generally bends and elbows are left free to spring somewhat under extremes of temperature.