CHAPTER XIII LARGE MAINS Examples of long pipes and aqueducts Conspicuous examples of American cities which have gone far afield for water are Los Angeles, which brought in a new supply from 235 miles away; San Francisco, 154 miles; New York, 120 miles; Tulsa, Oklahoma, 60 miles; Pheonix, Arizona, 32 miles; Butte, Montana, 27 miles; Denver, 25 miles; and Norfolk, and Portsmouth, Virginia, about 20 miles each. In Canada the city of Winnipeg brought in a new supply a total distance of 98 miles, while Victoria went 38 miles for its water. In Australia the Coolgardie pipe line is 351 miles in length, while the Apulian Aqueduct now being constructed to supply 266 communities in Southern Italy will have 152 miles of main trunk conduit and 841 miles of main and subsidiary branches leading therefrom. The near future will doubtless see a very large amount of capital invested in supply mains, so that the economic design of these mains is a live and important problem. Aqueducts The largest of these supply mains, commonly known as aqueducts, are generally, except at valley crossings, constructed of masonry built in trench or on enbankment or are masonry lined tunnels. They are generally placed at the computed level of the hydraulic gradient so as not to flow under pressure. Inverted syphons At crossings of valleys, inverted siphons, often called just siphons, are required, flowing under pressure. Some recent large siphons have been tunnels, called pressure tunnels, excavated in rock beneath the valleys. Where pipes are used for siphons they are essentially like other supply mains which form the subject of this chapter. Types of pipe available As a result of developments in modern manufacturing the engineer charged with the design of a supply main has at the outset four principal materials to choose from; viz., cast iron, steel, reinforced concrete, and wood. In the case of each material, there are two or more types of pipe available. In cast iron there are bell and spigot pipe, made in accordance with the standard specifications of the American Water Works Association; or, what is known as the "high tensile strength" bell and spigot cast iron pipe. For submarine work many types of cast iron ball and socket jointed pipe are available and these are sometimes used also under railroad tracks to reduce leakage. It is also possible to secure cast iron pipe with a mortar lining. In steel pipe the principal types are riveted, spiral, lock-bar and hammer-weld pipe, with various types of joints. Mortar lining has sometimes been used for steel pipes of the larger sizes. In reinforced concrete, there are available pipe cast in place and several types of precast concrete pipe, with joints at intervals of from 3 to 12 feet. The field of wood stave pipe offers the machine banded and continuous stave types. The staves themselves might be redwood, fir or pine. Period of use The use of cast iron pipe dates back over two hundred years; of steel pipe approximately forty-five years; of reinforced concrete pipe, about thirty-five years; of wood stave pipe, about forty or fifty years. Pipes of wrought iron plate were in use before steel pipe. Sheet iron pipe lined and covered with cement mortar has seen upwards of fifty years of use. Physical properties of various types of pipe Carrying capacity Cast iron pipe, lock bar and hammer-weld steel pipe, reinforced concrete and mortar-lined steel and cast iron pipe, and wood stave pipe all have smooth interior surfaces and comparatively high coefficients of discharge when new. For equal sizes it is doubtful if there are any strictly comparable gaugings which will prove con clusively superiority of one of these types over another when new. Rivetted pipes are rougher and have coefficients of discharge about 20 to 25 per cent less than equal sizes of the smoother types of pipes. The carrying capacity of all types of pipe becomes impaired with age. The first impairment is from filamentous vegetable organisms which quickly grow on the surfaces of pipe lines, particularly those parts nearest the sources, where the water is richer in food for plant life. More slowly there follows in the case of metal pipes, in sections of the country where the water is soft, a grosser impairment due to the formation of tubercles. These tubercles have a connection with the metal through small holes in the coating and are composed partly of metal, but mostly of calcareous and organic matter. Blisters of raised coating are also not infrequently formed. Tuberculation and blistering are always much less pronounced and sometimes almost entirely lacking in sections of the country where the water is comparatively hard. In most locations concrete and cement lined pipes suffer less impairment in carrying capacity than do metal pipes. In softer waters and in aggressive filtered waters most of the tar and asphalt coatings for metal pipe have been found ineffective in preserving carrying capacity and preventing tuberculation. Improved coatings having a refined coal tar base have shown decidedly better results than coatings previously tried when inspected seven or eight years after being placed in service. Such coatings are applied by a brush which gives a comparatively rough surface, the coefficient of which has not yet been measured. Similar coatings made with a refined asphalt base and applied as a dip so as to produce smooth coatings have been in use for several years. These seem promising, but have not been inspected after much service. Durability Steel and cast iron pipes. With the exception of pipes laid in salt water, in salt marshes or in other soils causing rapid corrosion, very few pipes have deteriorated to the point of permitting a measure of their ultimate life. Pitting under coating often progresses to a considerable proportion of the thickness of metal but these pits are too far apart to affect strength materially. It seems probable that steel pipe, coated in accordance with the best current practice, can be expected to carry its pressure for a period of sixty to seventy five years in localities where the water or the soil have no unusually destructive influence. Cast iron pipe doubtless has a longer life than steel in similar locations. Reinforced concrete pipe. Reinforced concrete pipe lines as usually built and covered are not subject to frost action, which is so frequently a deteriorating agent for exposed concrete surfaces. Good concrete buried in the ground is generally regarded by engineers as practically safe against deterioration. The dense concrete of the best quality of pressure pipe made today permits little seepage and actual observations show this to become less as time goes on. The so-called cylinder pipe, commonly used for the higher heads, has some similarity to the old cementlined pipe, but is much sturdier. A welded steel cylinder enclosed in the reinforced concrete checks seepage. The concrete furnishes for the steel a protection of great permanence. Reinforced concrete pipe of the cylinder type has been in successful use in Europe for the past thirty or thirty-five years, although of comparatively recent origin in this country. Estimates of seventyfive or one hundred years life for well made reinforced concrete pipe appear to be reasonable. Wood stave pipe. The life of the larger wood stave pipe depends on the nature of the soil, kind of wood and thickness of the steel bands. It is important also that the pipe be kept full of water to prevent decay of the wood. There are wood stave lines that are still giving service after twenty-five or thirty years of use. A continuous wood stave pipe, if kept full of water under pressure, may fairly be assumed to have a life of forty years. Strength By proper proportioning, each of the types of pipe may be safely used up to heads of about 250 feet for wood pipe, 400 feet or more for cast iron pipe and for reinforced concrete pipe of the so-called cylinder type and much higher heads for steel. The upper limit for reinforced concrete is not so well established in this country owing to comparative newness of that type here. Steel pipe is practically free from damage in handling. In rare cases short lengths of large steel pipe lines have collapsed where unprotected by air valves. Large wood stave pipes must also be protected from this danger. A comparatively small percentage of cast iron pipe will become cracked on the spigot end during shipment and these split ends must be cut off. Occasionally a projecting ledge or stone carelessly left in the bottom of the trench or some other more obscure cause will occasion a break. Concrete pipe carefully cured and handled and laid under expert supervision has so far proved very free from defects in strength and soundness. Wood stave pipe becomes leaky when insecurely founded. Leakage The leakage from all types of supply mains when well laid is comparatively small, although some leakage is unavoidable. A commonly prescribed upper limit for leakage is an amount, variously expressed, equivalent to 250 gallons per twenty-four hours per inch of diameter of pipe per mile. Thus a 60-inch pipe would be accepted, if its leakage were 15,000 gallons per mile per day. Choice of type of pipe Carrying capacity, cost and ability of purchaser to finance, durability and strength must all be given due weight in economic studies for determining the type of pipe for use in a given location. Buyers are increasingly aware that permanent carrying capacity is the thing purchased rather than mere metal or concrete to be buried in the ground. Increased diameter. in one type of pipe may be made to compensate for smoothness in another type. Each case requires special comparative estimates and sometimes alternative bids to determine the most truly economic type for the particular conditions. Nearness to steel, cement or lumber markets, corrosiveness of water or ground and other special conditions vary in different locations and no constant differences in cost are reliable. Care in locating pipe and investigating harmful soil conditions Careful surveys and test pits or borings to determine the best location of a pipe line will undoubtedly always pay for themselves. The final location should only be selected after a thorough study of its effect on the hydraulic gradient of the line, and of the nature of the difficulties encountered-such as rock excavation, abnormally wet soils, river, creek and railway crossing; and other things which |