Pipe Sewers
179. The Trench Bottom.—It is customary to dig the bottom of the trench to conform to the shape of the lower 45 degrees to 90 degrees of the sewer if the character of the material will allow such construction. In soft material which will not hold its shape the sewer may be encased in concrete or a concrete cradle may be prepared for the pipe. In rock the trench is excavated to about 6 inches below grade and refilled with well-tamped earth so as to form a cradle giving bearing to 60 to 90 degrees of the pipe circumference. For large sewers to be constructed in the trench special foundations are sometimes built.
180. Laying Pipe.—Before the pipe is lowered into the trench the sections which are to be adjacent should be fitted together on the surface and the relative positions marked by chalk so that the same position can be obtained in the trench.
Small pipes are lowered into the trench and swung into position on a hook as shown in Fig. 124. Pipes up to 15 or 18 inches in diameter can be handled by the pipe layer and helper in the trench without assistance. Heavier pipes may be lowered into the trench by passing ropes around each end of the pipe. One end of the rope is fastened at the surface and the ropes are paid out by the men at the surface as the pipe is lowered. If the pipes have been fitted together and marked at the surface it is undesirable to use this method of lowering as the position in which the pipes arrive in the bottom of the trench can not be easily predicted. A cradle may be used for shoving the pipe into position as is shown in Fig. 125.
Fig. 124.—Hook for Lowering and Placing Sewer Pipe.
Fig. 125.—Cradle for Placing Sewer Pipe.
Pipes above 24 to 27 inches in diameter are too large to be handled from the side of the trench. A hook as shown in Fig. 124 is placed in the pipe so that it will be in the proper position when lowered. It is raised by a rope passing through a block at the peak of a stiff-legged derrick which spans the trench, or by a crane. If a derrick is used the rope passes to a windlass on the opposite side of the trench from the pipe. Mechanical power may be used for raising pipes too heavy to be raised by hand. The pipe is then lowered and swung into position while supported from the derrick. Excessive swinging is prevented by holding back on the guide rope as the pipe is raised and lowered.
Pipes are usually laid with the bell end up grade as it is easier to fit the succeeding pipe into the bell so laid and to make the joint, particularly on steep grades. The Baltimore specifications state:
The ends of the pipe shall abut against each other in such a manner that there shall be no shoulder or unevenness of any kind along the inside of the bottom half of the sewer or drain. Special care should be taken that the pipe are well bedded on a solid foundation.... The trenches where pipe laying is in progress shall be kept dry, and no pipe shall be laid in water or upon a wet bed unless especially allowed in writing by the Engineer. As the pipe are laid throughout the work they must be thoroughly cleaned and protected from dirt and water, no water being allowed to flow in them in any case during the construction except such as may be permitted in writing by the Engineer. No length of pipe shall be laid until the preceding length has been thoroughly embedded and secured in place, so as to prevent any movement or disturbance of the finished joint.
The mouth of the pipe shall be provided with a board or stopper, carefully fitted to the pipe, to prevent all earth and any other substances from washing in.
181. Joints.—Pipes may be laid with open joints, mortar joints, cement joints, or poured joints. Open joints are used for storm sewers in dry ground close to the surface. Mortar and cement joints are commonly used on all sewers except in special cases. Cement joints are more carefully made than mortar joints and result in a greater percentage of water-tight joints. Poured joints are used in wet trenches where it is necessary to exclude ground water from the sewer.
A specification used in some cities for open joints is:
Pipes laid with open joints are to be laid with their inverts in the same straight line and shall be firmly bedded throughout their length on the bottom of the trench. No cement or mortar is to be used in the joints. Not more than ⅛ inch shall be left between the spigot end of the pipe and the shoulder of the hub of the pipe into which it fits. The joints shall be surrounded with cheese cloth, burlap, broken pipe, gravel or broken stone.
The purpose of the cheese cloth, etc., is to prevent fine earth from sifting into the pipe until the cheese cloth or other material has rotted away, by which time the earth has become arched over the opening.
Mortar joints are specified by Metcalf and Eddy as follows:
Before a pipe is laid the lower part of the bell of the preceding pipe shall be plastered on the inside with stiff mortar of equal parts of Portland cement and sand, of sufficient thickness to bring the inner bottoms of the abutting pipe flush and even. After the pipe is laid the remainder of the bell shall be thoroughly filled with similar mortar and the joint wiped inside and finished to a smooth bevel outside.
In some work a wood block or a stone is embedded in the mortar at the bottom of the joint to bring the spigot in place concentric with the next pipe.
Cement joints are specified in the Baltimore specifications as follows:
Cement joints shall be made with a narrow gasket of hemp or jute and cement mortar, and special care shall be taken to secure tight joints. The gasket shall be soaked in Portland cement grout and then carefully inserted between the bell and the spigot, and well calked with suitable hardwood or iron calking tools. It shall be in one continuous piece for each joint, and of such thickness as to bring the inverts of the two pipes smooth and even. The remainder of the joint shall be filled with cement mortar all around, on the bottom, top and sides, applied by hand with rubber mittens, well pressed into the annular space and beveled off from the outer edge of the bell to a distance of two inches therefrom, or to an angle of 45 degrees. The inside of each joint shall be thoroughly cleansed of all surplus mortar that may squeeze out in making the joint; and to accomplish this some suitable scraper or follower, or form shall be provided and always used immediately after each joint is finished.
Cement joints so made, form the most satisfactory joint for ordinary conditions and are the most frequently used. They are not always water-tight and can be penetrated by roots. Some roots are able to penetrate holes of almost microscopic size and to form growths in the sewer or to split the joints.
Poured joints are made by pouring some jointing compound, while in a fluid state, into the joint in which it hardens, thus sealing the joint. Water-tightness in sewer lines to exclude ground water has also been attempted by using the ordinary cement joint and surrounding the pipe with a layer of cement or concrete. This has not always been successful as it is difficult to obtain the proper class of workmanship in wet sewer trenches.
The requisite qualities of a poured jointing material are:
(1) It should make a joint proof against the entrance of water and roots.
(2) It should be inexpensive.
(3) It should have a long life.
(4) It should not deteriorate in sewage which may be either acid or alkaline.
(5) It should adhere to the surface of the pipe.
(6) It should run at a temperature below about 400° F., as too high temperatures will crack the pipe.
(7) It should neither melt nor soften at temperatures below 250° F. in order to maintain the joint if hot liquids are poured into the sewer.
(8) It should be elastic enough to permit slight movements of the pipes.
(9) It should not require great skill in using as it must be handled ordinarily by unskilled workers.
The materials used for poured joints are: cement grout; sulphur and sand; and asphalt or some bituminous compound made of vulcanized linseed oil, clay, and other substances the resulting mixture having the appearance of vulcanized rubber or coal tar. The bituminous materials most nearly approach the ideal conditions.
Cement grout is made up of pure cement and water mixed into a soupy consistency. Its main advantages are its cheapness and ease in handling in wet trenches or difficult situations. The result is no better than a well made cement joint. There is no elasticity to the joint and a movement of the pipe will break it.
Sulphur and sand are inexpensive, comparatively easy to handle, and make an absolutely water-tight and rigid joint which is stronger than the pipe itself. It frequently results in the cracking of the pipe and is objected to by some engineers on that account. In making the mixture, powdered sulphur and very fine sand are mixed in equal proportions. It is essential that the sand be fine so that it will mix well with the sulphur and not precipitate out when the sulphur is melted. Ninety per cent of the sand should pass a No. 100 sieve and 50 per cent should pass a No. 200 sieve. The mixture melts at about 260° F. and does not soften at lower temperatures. For making a joint in an 8 inch pipe about 1½ pounds of sulphur, 1½ pounds of sand, ½ pound of jute, and 0.4 pound of pitch are used. The pitch is used to paint the surface of the joint while still hot in order to close up any possible cracks.
Among the better known of the bituminous joint compounds are: “G.K.” Compound made by the Atlas Company, Mertztown, Pa., Jointite and Filtite, manufactured by the Pacific Flush Tank Co., Chicago and New York, and some of the products of the Warren Brothers Co., Boston. These compounds fill nearly all of the ideal conditions except as to cost and ease in handling. They are somewhat expensive and if overheated or heated too long become carbonized and brittle. In cold weather they do not stick to the pipe well unless the pipe is heated before the joint is poured. On some work joints have been poured under water with these compounds, but success is doubtful without skillful handling. An overheated compound will make steam in the joint causing explosions which will blow the joint clean, and an underheated compound will harden before the joint is completed.
The materials should be heated in an iron kettle over a gasoline furnace or other controllable fire, until they just commence to bubble and are of the consistency of a thin sirup. Only a sufficient quantity of material for immediate use should be prepared and it should be used within 10 to 15 minutes after it has become properly heated. The ladle used should be large enough to pour the entire joint without refilling. There are other important points to be considered in pouring joints which can be learned best by experience.
The quantity of material necessary for making these joints, as announced by the manufacturers, is shown in Table 65.
| TABLE 65 | ||||||
|---|---|---|---|---|---|---|
| Quantity of Compound Needed for Poured Joints | ||||||
| Diameter of Pipe, in Inches | Quantity of Material in Pounds per Joint | |||||
| Standard Socket | Deep and Wide Socket | |||||
| Jointite | Filtite | G. K. | Jointite | Filtite | G. K. | |
| 6 | 0.82 | 0.72 | 0.42 | 1.46 | 1.28 | 0.72 |
| 8 | 1.06 | 0.95 | 0.73 | 1.82 | 1.60 | 1.25 |
| 10 | 1.30 | 1.15 | 0.89 | 2.26 | 1.98 | 1.52 |
| 12 | 2.08 | 1.82 | 1.42 | 2.65 | 2.32 | 1.80 |
| 15 | 2.52 | 2.20 | 1.74 | 3.20 | 2.80 | 2.20 |
| 18 | 3.02 | 2.64 | 2.58 | 3.75 | 3.29 | 3.25 |
| 20 | 3.44 | 3.00 | 2.86 | 4.30 | 3.78 | 3 60 |
| 22 | 3.62 | 3.16 | 3.13 | 4.62 | 4.07 | 3.97 |
| 24 | 4.03 | 3.50 | 3.41 | 4.91 | 4.31 | 4.27 |
In making a poured joint the pipes are first lined up in position. A hemp or oakum gasket is forced into the joint to fill a space of about ¾ of an inch. An asbestos or other non-combustible gasket such as a rubber hose smeared with clay is forced about ½ inch into the opening between the bell and the spigot and the compound is poured down one side of the pipe through a hole broken in the bell, until it appears on the other side, and the hole is filled. Occasionally the non-combustible gasket is wrapped tightly around the spigot of the pipe and pressed or tied firmly to the bell. In pouring cement grout joints a paper gasket is used which is held to the bell and spigot by draw strings. Greater speed in construction and economy in the use of materials are obtained by joining two or three lengths of pipe on the bank and lowering them into the trench as a unit. The pipes are set in a vertical position on the bank with the bell end up, one length resting in the other. The joint is calked with hemp and poured without the use of the gasket. The joint should always be poured immediately after being calked so that the hemp can not become water soaked. The asbestos gasket should be removed as soon as possible after the joint is poured in order to prevent sticking with resultant danger of breaking of the joint when attempting to pull the gasket free.
One man can pour about 33 eight-inch joints, and two men can complete about 26 twelve-inch joints per hour on the bank where conditions are more or less fixed.
182. Labor and Progress.—The labor required for the laying of pipe sewers, exclusive of excavation, bracing and backfilling, consists of pipe layers and helpers. For pipes 24 to 27 inches in diameter or smaller one pipe layer and one or more helpers are necessary, dependent on the size of the pipe and the depth of the trench. For larger pipes two pipe layers can work economically each working on one-half of the pipe and making half of the joint. The speed of pipe laying is ordinarily limited by the speed of the excavation, but on a job in Topeka, Kan.,[[100]] where the average day’s progress with a machine excavator was 200 to 500 feet of trench per day, the pace was limited by the speed of the pipe laying gang. This gang consisted of two pipe layers in the trench and two helpers on the surface. The sizes of pipes handled were from 8 to 27 inches.