The Construction of Underground Conduits
Your committee in deciding to take up the all important question of underground construction and to have the same discussed at this time, has proven beyond a doubt that the association is keeping abreast the times of the twentieth century. It is the duty of this association, not only to care for and construct electrical conduits for the transmission of electrical energy in the form of signals, light, heat, telephone and telegraph, but to sec that the corporations and individuals encaged in the business of the transmission of electric energy for any purpose, shall cause the public the least possible inconvenience, and expose their lives and property to little or no danger. It will not be necessary to give you the history of the underground conduits as you all are acquainted with the same, the writer will therefore confine himself to the different classes of construction and material used in this work. The ideal conduit or housing for underground conductors must have smooth, clear, jointless, fireproof duct openings, free from pockets or depressions in which water may lodge; should be laid to true grade to drain to the manholes, must be conMracted of a material of high electrical resistance, and which will not deteriorate with age; must be strong and able to sustain itself anti a considerable load over fairly long spans. While there arc about six different kinds of material used in the construction of conduits, namely: the clay duct, cement lined pipe iron pipe, fibre ducts, wood ducts, and cement ducts, but three of these arc used to any extent, they tire the cement duct, the clay duct and the fibre duct. The clay duct is most generally used. These ducts arc made in the single duct in inches long, with the hole 2 inches or i inches in diameter; they are also made in t. ti or t) multiple, that is to say that in the one piece there will be -4. or !) holes funning through the tile, these holes are 3½ inches square, the length of the duct being from 3 to ti feet in length; they are made in this way to save space ;md in laying the same they can be kept in line better and also save concrete. The fibre duet is made up of paper, etc., and passes through a process which preserves it. They are made in lengths of 10 feet, and the holes are made of different sizes, from 2 inches to ti inches itt diameter. The concrete ducts arc made in the trench where they are moulded and constructed as the trench is being made. In laying the ducts, it is necessary to use care to obtain perfect alignment and sealing. Each new section of duct must be fitted to the preceding one. after the ducts are laid no absolute inspection of them is possible, and the only information the owner can obtain relative to the presence of foreign matter, alignment and sealing, is derived front the passage of a short mandrel through the various ducts. Drawing a mandrel through a duct can do no more than demonstrate that there exists a hole sufficiently large and straight to permit the mandrel to pass through it. It is desirable in any conduit that the ducts be completely isolated from each other, and that they be free from water pockets nr depressions, and present a large and smooth surface as a bed for the cable, in order that ground currents may be equally distributed over the greatest possible area of the cable sheathing, so that the local electrolytic action may be minimized. Conduits built up of single duct tile or pipe offer greater isolation of individual ducts, but are more expensive to construct than conduits built up of multiple duct. The first step taken in the construction of an underground conduit system is the digging of the trench. 1 his must be made some 6 or 8 inches wider than the width of the ducts, depending on the thickness of the concrete to he placed for the walls. After the trench is dug the first thing is to place the concrete in the bottom of the trench, usually 6 inches. This is levelled and left to partly set. The ducts are then placed on this concrete, the joints are wrapped with burlap and are tarred or cemented; they in turn are levelled and made as true as possible; after the bottom concrete is set the sides are lied in with concrete which is usually 3 inches thick, also the top is covered with about 3 inches. The trench is then refilled and your conduit is about ready for use. While it is necessary that the ducts are laid level, it is also important that there be no sharp turns in the line. The ducts should be laid straight from one manhole to the other. In the construction of these underground ducts the manhole is of as much importance as the ducts, as it is in these that the actual work is done by the company that is operating the same, all the cables are spliced here and, should something happen to the cables, they are opened at one of these holes and are repaired there. The manholes are constructed sometimes of brick, and others of concrete. The walls are usually about 9 inches in thickness. The other dimensions depend on the number of ducts that will come into the hole. They are, however, never less than 5 feet square. This size hole will accommodate about 6 ducts, but in a hole where there are corning into it from all sides 30 to 90 ducts the holes sometimes are made from 12 to 18 feet square and from 7 to 12 feet deep. This all depends upon the conditions that are met with in the construction of the ducts. There are also built what is known as service boxes. These are used where it i. wanted to feed a large building. These holes are also made of different sizes from 3 to I feet square, and the same depth. All manholes are then covered with a casting with a square or round cover. The round cover is more generally used, the diameter of the opening being from 21 to 36 incites. The manholes should also be ventilated. This is done by means of pipes running from the manhole to a pole. Then in the construction of the conduit you must have poles. They are known as distributing or terminal poles. These are connected to the manholes by means of pipes which are run up the pole some 8 or in feet to protect the cables from injury.