198. Work Involved.—The principal effort in maintaining sewers is to keep them clean and unobstructed. A sewerage system, although buried, cannot be forgotten as it will not care for itself, but becoming clogged will force itself on the attention of the community. Besides the cleaning and repairing of sewers and the making of inspections for determining the necessity for this work, ordinances should be prepared and enforced for the purpose of protecting the sewers from abuse. Inspections to determine the amount of the depreciation of sewers with a view towards possible renewal, or to determine the capacity of a sewer in relation to the load imposed upon it are sometimes necessary. The valuation of the sewerage system as an item in the inventory of city property may be assigned to the engineer in charge of sewer maintenance.

The work involved in the inspection and cleaning of sewers in New York City for the year ending May, 1914, included the removal of 22,687 cubic yards of material from catch-basins, and 14,826 catch-basin cleanings. This made an average of two and one-half cleanings per catch-basin per year, or 1½ cubic yards removed at each cleaning. The 6,432 catch-basins were inspected 71,890 times. There were 4,112 cubic yards of material removed from 517 miles of sewers, or about 8 cubic yards per mile. Inspection of 194 miles of brick sewers were made, 4.4 miles were flushed, and 27 miles were cleaned. Inspections of 198 miles of pipe sewers were made, 80 miles were examined more closely, 37 miles were flushed, and 91 miles were cleaned. The field organization for this work consisted of 17 foremen, 8 assistant foremen, 29 laborers, 71 cleaners, 13 mechanics, 7 inspectors of construction, 3 inspectors of sewer connections, 13 horses and wagons, and 28 horses and carts.^[105]

199. Causes of Troubles.—The complaints most frequently received about sewers are caused by clogging, breakage of pipes, and bad odors. Sewers become clogged by the deposition of sand and other detritus which results in the formation of pools in which organic matter deposits, aggravating the clogged condition of the sewers and causing the odors complained of. Grease is a prolific cause of trouble. It is discharged into the sewer in hot wastes, and becoming cooled, deposits in thick layers which may effectively block the sewer if not removed. It can be prevented from entering the sewers by the installation of grease traps as described in Chapter VI. The periodic cleaning of these traps is as important as their installation.

Tree roots are troublesome, particularly in small pipe sewers in residential districts. Roots of the North Carolina poplar, silver leaf poplar, willow, elm, and other trees will enter the sewer through minute holes and may fill the sewer barrel completely if not cut away in time. Fungus growths occasionally cause trouble in sewers by forming a network of tendrils that catches floating objects and builds a barricade across the sewer. Difficulties from fungus growths are not common, but constant attention must be given to the removal of grit, grease, and roots. Tarry deposits from gas-manufacturing plants are occasionally a cause of trouble, as they cement the detritus already deposited into a tough and gummy mass that clings tenaciously to the sewer.

Broken sewers are caused by excessive superimposed loads, undermining, and progressive deterioration. The changing character of a district may result in a change of street grade, an increase in the weight of traffic, or in the construction of other structures causing loads upon the sewer for which it was not designed. The presence of corrosive acids or gases may cause the deterioration of the material of the sewer.

200. Inspection.—The maintenance of a sewerage system is usually placed under the direction of a sewer department. In the organization of the work of this department no regular routine of inspection of all sewers need be followed ordinarily. Attention should be given regularly to those sewers that are known to give trouble, whereas the less troublesome sewers need not be inspected more frequently than once a year, preferably during the winter when labor is easier to obtain.

The routine inspection of sewers too small to enter is made by an examination at the manhole. If the water is running as freely at one manhole as at the next manhole above, it is assumed that the sewer between the manholes is clean and no further inspection need be given unless there is some other reason to suspect clogging between manholes. If the sewage is backed up in a manhole it indicates that there is an obstruction in the sewer below. If the sewage in a manhole is flowing sluggishly and is covered with scum it is an indication of clogging, slow velocity and septic action in the sewer. Sludge banks on the sloping bottom of the manhole or signs of sewage high upon the walls indicate an occasional flooding of the sewer due to inadequate capacity or clogging.

If any of the signs observed indicate that the sewer is clogged, the manhole should be entered and the sewer more carefully inspected. Such inspection may be made with the aid of mirrors as shown in Fig. 140 or with a periscope device as shown in Fig. 141. Sunlight is more brilliant than the electric lamp shown in Fig. 141, but the mirror in the manhole directs the sunlight into the eyes of the observer, dazzling him and preventing a good view of the sides of the sewer. The observers’ eyes can be protected against the direct rays of the electric light, which can be projected against the sides of the pipe by proper shades and reflectors. It is possible with this device to locate house connection, stoppages, breaks of the pipe, and to determine fairly accurately the condition of the sewer without discomfort to the observers.

Sewers that are large enough to enter should be inspected by walking through them where possible. The inspection should be conducted by cleaning off the sewer surface in spots with a small broom, and examining the brick wall for loose bricks, loose cement or cement lost from the joints, open joints, broken bond, eroded invert, and such other items as may cause trouble. An inspection in storm sewers is sometimes of value in detecting the presence of forbidden house connections.

Certain precautions should be taken before entering sewers or manholes. If a distinct odor of gasoline is evident the sewer should be ventilated as well as possible by leaving a number of manhole covers open along the line until the odor of gasoline has disappeared. The strength of gasoline odor above which it is unsafe to enter a sewer is a matter of experience possessed by few. A slight odor of gasoline is evident in many sewers and indicates no special danger. A discussion of the amount of gasoline necessary to create explosive conditions is given in Art. 206. In making observations of the odor it should also be noted whether air is entering or leaving the manhole. The presence of gasoline cannot be detected at a manhole into which air is entering.

As soon as it is considered that the odors from a sewer indicate the absence of an explosive mixture, a lighted lantern or other open flame should be lowered into the manhole to test the presence of oxygen. Carbon monoxide or other asphyxiating gases may accumulate in the sewer, and if present will extinguish the flame. If the flame burns brilliantly the sewer is probably safe to enter, but if conditions are unknown or uncertain, the man entering should wear a life belt attached to a rope and tended by a man at the surface. Asphyxiating or explosive gases are sometimes run into without warning due to their lack of odor, or the presence of stronger odors in the sewer. Breathing masks and electric lamps are precautions against these dangers, the masks being ready for use only when actually needed. More deaths have occurred in sewers due to asphyxiating gases than by explosions, as the average sewer explosion is of insufficient violence to do great damage, although on occasion, extremely violent explosions have occurred. During inspections of sewers there should always be at least one man at the surface to call help in case of accident and the inspecting party should consist of at least two men.

It must not be felt that entering sewers is fraught with great danger, as it is perfectly safe to enter the average sewer. The air is not unpleasant and no discomfort is felt, but conditions are such that unexpected situations may arise for which the man in the sewer should be prepared. It is therefore wise to take certain precautions. These may indicate to the uninitiated, a greater danger than actually exists.

The inspection of sewers should include the inspection of the flush-tanks, control devices, grit chambers, and other appurtenances. A common difficulty found with flush-tanks is that the tank is “drooling,” that is to say the water is trickling out of the siphon as fast as it is entering the tank, and the intermittency of the discharge has ceased. If, when the tank is first inspected the water is about at the level of the top of the bell it is probable that the siphon is drooling. A mark should be made at the elevation of the water surface and the tank inspected again in the course of an hour or more. If the water level is unchanged the siphon is drooling. This may be caused by the clogging of the snift hole or by a rag or other obstacle hanging over the siphon which permits water to pass before the air has been exhausted, or a misplacement of the cap over the siphon, or other difficulty which may be recognized when the principle on which the siphon operates is understood. Occasionally it is discovered that an over zealous water department has shut off the service.

Control devices, such as leaping or overflow weirs, automatic valves, etc., may become clogged and cease to operate satisfactorily. They should be inspected frequently, dependent upon their importance and the frequency with which they have been found to be inoperative. An inspection will reveal the obstacle which should be removed. Floats should be examined for loss of buoyancy or leaks rendering them useless. Grit and screen chambers should be examined for sludge deposits.

Catch-basins on storm sewers are a frequent cause of trouble and need more or less frequent cleaning. Cleanings are more important than inspections for catch-basins for if they are operating properly they are usually in need of cleaning after every storm of any magnitude, and a regular schedule of cleaning should be maintained.

A record should be kept of all inspections made. It should include an account of the inspection, its date, the conditions found, by whom made and the remedies taken to effect repairs.

201. Repairs.—Common repairs to sewerage systems consist in replacing street inlets or catch-basin covers broken by traffic; raising or lowering catch-basin or manhole heads to compensate for the sinking of the manhole or the wear of the pavement; replacing of broken pipes, loosened bricks or mortar which has dropped out; and other miscellaneous repairs as the necessity may arise. Connections from private drains are a source of trouble because either the sewer or the drain has broken due to careless work or the settlement of the foundation or the backfill.

202. Cleaning Sewers.—Sewers too small to enter are cleaned by thrusting rods or by dragging through them some one of the various instruments available. The common sewer rod shown in Fig. 142 is a hickory stick, or light metal rod, 3 or 4 feet long, on the end of which is a coupling which cannot come undone in the sewer. Sections of the rod are joined in the manhole and pushed down the sewer until the obstruction is reached and dislodged. Occasionally pieces of pipe screwed together are used with success. The end section may be fitted with a special cutting shoe for dislodging obstructions. In extreme cases these rods may be pushed 400 to 500 feet, but are more effective at shorter distances. Obstructions may be dislodged by shoving a fire hose, which is discharging water under high pressure through a small nozzle, down the sewer toward the obstruction. The water pressure stiffens the hose, which, together with the support from the sides of the conduit, make it possible to push the hose in for effective work 100 feet or more from the manhole. A strip of flexible steel about ½ inch thick and 1½ to 2 inches wide is useful for “rodding” a short length of crooked sewer.

Sewers are seldom so clogged that no channel whatever remains. As a sewer becomes more and more clogged, the passage becomes smaller, thereby increasing the velocity of flow of the sewage around the obstruction and maintaining a passageway by erosion. This phenomenon has been taken advantage of in the cleaning of sewers by “pills.” These consist of a series of light hollow balls varying in size. One of the smaller balls is put into the sewer at a manhole. When the ball strikes an obstruction it is caught and jammed against the roof of the sewer. The sewage is backed up and seeks an outlet around the ball, thus clearing a channel and washing the ball along with it. The ball is caught at the next manhole below. A net should be placed for catching the ball and a small dam to prevent the dislodged detritus from passing down into the next length of pipe. The feeding of the balls into the sewer is continued, using larger and larger sizes, until the sewer is clean. This method is particularly useful for the removal of sludge deposits, but it is not effective against roots and grease. The balls should be sufficiently light to float. Hollow metal balls are better than heavier wooden ones.

Plows and other scraping instruments are dragged through pipe sewers to loosen banks of sludge and detritus and to cut roots or dislodge obstructions. One form of plow consists of a scoop^[106] similar to a grocer’s sugar scoop, which is pushed or dragged up a sewer against the direction of flow. As fast as the scoop is filled it is drawn back and emptied. The method of dragging this through a sewer is indicated in Fig. 143. At Atlantic City the crew operating the scoop comprises five men, two are at work in each manhole and one on the surface to warn traffic and wait on the men in the manholes. The outfit of tools is contained in a hand-drawn tool box and includes sewer rods, metal scoops for all sizes of sewers, picks, shovels, hatchets, chisels, lanterns, grease and root cutters, etc., and two winches with from 400 to 600 feet of ?-inch wire cable.

Another form of plow or drag consists of a set of hooks or teeth hinged to a central bar as shown in Fig. 144. A root cutter and grease scraper in the form of a spiral spring with sharpened edges, and other tools for cleaning sewers are shown in Fig. 145. A turbine sewer cleaner shown in Fig. 146 consists of a set of cutting blades which are revolved by a hydraulic motor of about 3 horse-power under an operating pressure of about 60 pounds per square inch. The turbine is attached to a standard fire hose and is pushed through the sewer by utilizing the stiffness of the hose, or by rods attached to a pushing jack as shown in the figure. This machine was invented and patented by W. A. Stevenson in 1914. Its performance is excellent. The blades revolve at about 600 R.P.M., cutting roots and grease. The revolving blades and the escaping water also serve to loosen and stir up the deposits and the forward helical motion imparted to the water is useful in pushing the material ahead of the machine and in scrubbing the walls of the sewer. In Milwaukee four men with the machine cleaned 319 feet of 12–inch sewer in 16 hours, and in Kansas City 7,801 feet of sewers were cleaned in 14 days.

Sewers large enough to enter may be cleaned by hand. The materials to be removed are shoveled into buckets which are carried or floated to manholes, raised to the surface and dumped. In very large sewers temporary tracks have been laid and small cars pushed to the manhole for the removal of the material. Hydraulic sand ejectors may also be used for the removal of deposits, similar to the steam ejector pump shown in Fig. 97. The water enters the apparatus at high velocity, under a pressure of about 60 pounds per square inch, leaps a gap in the machine from a nozzle to a funnel-shaped guide leading to the discharge pipe. The suction pipe of the machine leads to the chamber in which the leap is made. In leaping this gap the water creates a vacuum that is sufficient to remove the uncemented detritus large enough to pass through the machine, and will lift small stones to a height of 10 to 12 feet. Occasionally barricades of logs, tree branches, rope, leaves, and other obstructions which have piled up against some inward projecting portion of the sewer, must be removed by hand either by cutting with an axe or by pulling them out. Projections from the sides of sewers are objectionable because of their tendency to catch obstacles and form barricades.

Little authentic information on the cost of cleaning sewers is available. A permanent sewer organization is maintained by many cities. The division of their time between repairs, cleaning, and other duties is seldom made a matter of record. From data published in Public Works^[107] it is probable that the cost varies from $3 to $15 per cubic yard of material removed. From the information in Vol. II of “American Sewerage Practice” by Metcalf and Eddy the combined cost of cleaning and flushing will vary between $10 and $40 per mile; the expense of either flushing or cleaning alone being about one-half of this.

203. Flushing Sewers.—Sewers can sometimes be cleaned or kept clean by flushing. Flushing may be automatic and frequent, or hand flushing may be resorted to at intervals to remove accumulated deposits. Automatic flush-tanks, flushing manholes, a fire hose, a connection to a water main, a temporary fixed dam, a moving dam, and other methods are used in flushing sewers. The design, operation, and results obtained from the use of automatic flush-tanks and flushing manholes are discussed in Chapter VI.

The method in use for cleaning a sewer by thrusting a fire hose down it can also be used for flushing sewers. It is an inexpensive and fairly satisfactory method. There is, however, some danger of displacing the sewer pipe because of the high velocity of the water. An easier and safer but less effective method is to allow water to enter at the manhole and flow down the sewer by gravity. Direct connections to the water mains are sometimes opened for the same purpose.

Sewers are sometimes flushed by the construction of a temporary dam across the sewer, causing the sewage to back up. When the sewer is half to three-quarters full the dam is suddenly removed and the accumulated sewage allowed to rush down the sewer, thus flushing it out. The dam may be made of sand bags, boards fitted to the sewer, or a combination of boards and bags. The expense of equipment for flushing by this method is less than that by any other method, but the results obtained are not always desirable. Below the dam the results compare favorably with those obtained by other methods, but above the dam the stoppage of the flow of the sewage may cause depositions of greater quantities of material than have been flushed out below. A time should be chosen for the application of this method when the sewage is comparatively weak and free from suspended matter. The most convenient place for the construction of a dam is at a manhole in order that the operator may be clear of the rush of sewage when the dam is removed.

Movable dams or scrapers are useful in cleaning sewers of a moderate size, but are of little value in small sewers. The scraper fits loosely against the sides of the sewer and is pushed forward by the pressure of the sewage accumulated behind it. The iron-shod sides of the dam serve to scrape grease and growths attached to the sewer and to stir up sand and sludge deposited on the bottom. The high velocity of the sewage escaping around the sides of the dam aids in cleaning and scrubbing the sewer.

A natural watercourse may be diverted into the sewer if topographical conditions permit, or where sewers discharge into the sea below high tide a gate may be closed during the flood and held closed until the ebb. The rush of sewage on the opening of the gate serves to flush the sewers and stir up the sludge deposited during high tide. Other methods of flushing sewers may be used dependent on the local conditions and the ingenuity of the engineer or foreman in charge.

In some sewers it is not necessary to remove the clogging material from the sewer. It is sufficient to flush and push it along until it is picked up and carried away by higher velocities caused by steeper grades or larger amounts of sewage.

204. Cleaning Catch-basins.^[108]—Catch-basins have no reason for existence if they are not kept clean. Their purpose is to catch undesirable settling solids and to prevent them from entering the sewers, on the theory that it is cheaper to clean a catch-basin than it is to clean a sewer. If the cleaning of storm sewers below some inlet to which no catch-basin is attached becomes burdensome, the engineer in charge of maintenance should install an adequate catch-basin and keep it clean. Catch-basins are cleaned by hand, suction pumps, and grab buckets. In cleaning by hand the accumulated water and sludge are removed by a bucket or dipper and dumped into a wagon from which the surplus settled water is allowed to run back into the sewer. The grit at the bottom of the catch-basin is removed by shoveling it into buckets which are then hoisted to the surface and emptied.

Suction pumps in use for cleaning catch-basins are of the hydraulic eductor type. The eductor works on the principle of the steam pump shown in Fig. 97, except that water is used instead of steam. The material removed may be discharged into settling basins constructed in the street, or may be discharged directly into wagons.^[109] In Chicago a special motor-driven apparatus is used. This consists of a 5–yard body on a 5–ton truck, and a centrifugal pump driven by the truck motor. In use, the truck, about half filled with water, drives up to the catch-basin, the eductor pipe is lowered and water pumped from the truck into the eductor and back into the truck again, together with the contents of the catch-basin. The surplus water drains back into the sewer. The Chicago Bureau of Sewers reports a truck so equipped to have cleaned 1013 catch-basins, removing 1763 cubic yards of material, and running 1380 miles, during the months of August, September and October, 1917. The cost, including all items of depreciation, wages, repairs, etc., was $1,393.89. Orange-peel buckets, about 20 inches in diameter, operated by hand or by the motor of a 3½ to 5–ton truck with a water-tight body, are used for cleaning catch-basins in some cities.

Catch-basins in unpaved streets and on steep sandy slopes should be cleaned after every storm of consequence. Basins which serve to catch only the grit from pavement washings require cleaning about two or three times per year, and from one to three cubic yards of material are removed at each cleaning. The cost of cleaning ordinary catch-basins by hand may vary from $15 to $25, but with the use of eductors or orange-peel buckets the cost is somewhat lower. In Seattle the cost of cleaning large detritus basins by hand is said^[110] to vary from $45 to $60. With the use of eductors this cost has been reduced to one-third or one-fifth the cost of cleaning by hand.

205. Protection of Sewers.^[111]—City ordinances should be wisely drawn and strictly enforced for the protection of sewers against abuse and destruction. The requirements of some city ordinances are given in the following paragraphs.

Washington, D. C.,^[112] sewer ordinances provide that:

No person shall make or maintain any connection with any public sewer or appurtenance thereof whereby there may be conveyed into the same any hot, suffocating, corrosive, inflammable or explosive liquid, gas, vapor, substance or material of any kind... provided that the provisions of this act shall not apply to water from ordinary hot water boilers or residences.

The following extracts from the ordinances of Indianapolis are typical of those from many cities:

2950. No connection shall be made with any public sewer without the written permission of the Committee on Sewers and the Sewerage Engineer.

2953. No person shall be authorized to do the work of making connections until he has furnished a satisfactory certificate that he is qualified for the duties. He shall also file bond for not less than $1,000 that he will indemnify the City from all loss or damage that may result from his work and that he will do the work in conformity to the rules and regulations established by the City Council.

2955. It shall be unlawful for any person to allow premises connected to the sewers or drains to remain without good fixtures so attached as to allow a sufficiency of water to be applied to keep the same unobstructed.

2956. No butcher’s offal or garbage, or dead animals, or obstructions of any kind shall be thrown in any receiving basin or sewer in penalty not greater than $100. Any person injuring, breaking, or removing any portion of any receiving basin, manhole cover, etc., shall be fined not more than $100.

2962. No person shall drain the contents of any cesspool or privy vault into any sewer without the permission of the Common Council.

The Cleveland ordinances are similar and contain the following in addition:

1251. Rule 4. All connections with the main or branch sewers shall be made at the regular connections or junctions built into the same, except by special permit.

Rule 16. No steam pipe, nor the exhaust, nor the blow off from any steam engine shall be connected with any sewer.

Evanston, Illinois, protects its sewers against the additions of grease and other undesirable substances as follows:

1444. It is unlawful for any person to use any sewer or appurtenance to the sewerage system in any manner contrary to the orders of the Commissioner of Public Works.

1446. Wastes from any kitchen sinks, floor drains, or other fixtures likely to contain greasy matter from hotels, certain apartment houses, boarding houses, restaurants, butcher shops, packing houses, lard rendering establishments, bakeries, laundries, cleaning establishments, garages, stables, yard and floor drains, and drains from gravel roofs shall be made through intervening receiving basins constructed as prescribed in par. VIII of this code.

Receiving basins suitable for the work required in the code are illustrated in Chapter VI.

206. Explosions in Sewers.—Disastrous explosions in sewers were first recorded about 1886.^[113] Up to about 1905 explosions were infrequent and were considered as unavoidable accidents and so rare as to be unworthy of study. For a decade or more after 1905 explosions occurred with increasing violence and frequency causing destruction of property, but by some freakish chance, but little loss of life. A violent and destructive explosion occurred in Pittsburgh on Nov. 25, 1913,^[114] and another on March 12, 1916. The property damage amounted to $300,000 to $500,000 on each occasion, but there was no loss of life. Two miles of pavement were ripped up, gas, water, and other sewer pipes were broken, buildings collapsed and the streets were flooded. The streets were rendered unserviceable for long periods during the expensive repairs that were necessary. In recent years the number of explosions in sewers has been smaller, due probably to the gain in knowledge of the causes and intelligent methods of prevention.

The three principal causes of explosions in sewers are: gasoline vapor, illuminating gas, and calcium carbide. It is probable that gasoline vapor is by far the most troublesome. Explosions caused by these gases are not so violent as those caused by dynamite or other high explosives, as the volume of gas and the temperature generated are much less. The violence of sewer explosions may be increased somewhat by the sudden pressures that are put upon them.

Gasoline finds its way into sewers from garages and cleaning establishments. A mixture of 1½ per cent gasoline vapor and air may be explosive. It needs only the stray spark of an electric current, a lighted match, or a cigar thrown into the sewer to cause the explosion. As the result of a series of experiments on 2,706 feet of 8–foot sewer, Burrell and Boyd conclude.^[115]

One gallon of gasoline if entirely vaporized produces about 32 cubic feet of vapor at ordinary temperature and pressure. If 1½ per cent be adopted as the low explosive limit of mixtures of gasoline vapor and air, 55 gallons or a barrel of gasoline would produce enough vapor to render explosive the mixture in 1,900 feet of 9 foot sewer provided the gasoline and the air were perfectly mixed. Many different factors, however, govern explosibility, such as: size of the sewer, velocity of the sewage, temperature of the sewer, volatility and rate of inflow of the gasoline. Only under identical conditions of tests would duplicate results be obtained. A large amount of gasoline poured in at one time is less dangerous than the same amount allowed to run in slowly. With a velocity of flow of about 6½ feet per second it was evident that 55 gallons of gasoline poured all at once into a manhole rendered the air explosive only a few minutes (less than 10) at any particular point. With the same amount of gasoline run in at the rate of 5 gallons per minute, an explosive flame would have swept along the sewer if ignited 15 minutes after the gasoline had been dumped. With a slow velocity of flow and a submerged outlet the gasoline vapor being heavier than air accumulated at one point and extremely explosive conditions could result from a small amount of gasoline. Comparatively rich explosive mixtures were found 5 hours after the gasoline had been discharged. High-test gasoline is much more dangerous than the naphtha used in cleaning establishments, yet on account of the large quantity of waste naphtha the sewage from cleaning establishments may be very dangerous.

Illuminating gas is not so dangerous as gasoline vapor as it is lighter than air and it is more likely to escape from the sewer than to accumulate in it. It requires about one part of illuminating gas to seven parts of air to produce an explosive mixture.

Calcium carbide is dangerous because it is self igniting. The heat of the generation of gas is sufficient to ignite the explosive mixture. The gases are highly explosive and cause a relatively powerful explosion. Fortunately large amounts of this material seldom reach a sewer, the gas being generated in garage drains or traps and escaping in the atmosphere.

A hydrocarbon oil used by railroads in preventing the freezing of switches, if allowed to reach the sewers, may cause explosions therein.^[116] The oil crystallizes and in this form it is soluble in water. It will thus pass traps and on volatilization will produce explosive mixtures.

Methane, generated by the decomposition of organic matter, is a feebly explosive gas occasionally found in sewers. Its presence may add to the strength of other explosive mixtures.

Sewer explosions may be prevented by the building of proper forms of intercepting basins to prevent the entrance of gasoline and calcium carbide gases, and by ventilation to dilute the explosive mixtures which may be made up in the sewer. There are no practical means to predict when an explosion is about to occur, and after an explosion has occurred it is difficult to determine the cause as all evidence is usually destroyed.

207. Valuation of Sewers.—The necessity for the valuation of a sewerage system may arise from the legal provisions in some states limiting the amount of outstanding bonds which may be issued by a municipality to a certain percentage of the present worth of municipal property. The investment in the sewerage system is usually great and forms a large portion of the City’s tangible property. It may be desirable also to determine the depreciation of the sewers with a view towards their renewal.

The most valuable work on the valuation of sewers has been done in New York City^[117] by the engineers of the Sewer Department. The committee of engineers appointed to do the work recommended: (1) that the original cost be made the basis of valuation, and that (2), in fixing this cost the cost of pavement should be omitted or at most the cost of a cheap (cobblestone) pavement should be included. Trenches previously excavated in rock were considered as undepreciated assets.

The present worth of sewers depends on many factors aside from the effects of age, such as the care exercised in the original construction, the material used, the kind and quantity of sewage carried, the care taken in maintenance, and finally the injury caused by the careless building of adjoining substructures. During the progress of the inspections the examination of brick sewers, due to their accessibility, yielded better results than the examination of pipe sewers. The routine of the examination of the brick sewers consisted in cleaning off the bricks with a short broom, tapping the brick with a light hammer to determine solidity, and testing the cement joints by scraping with a chisel. In addition, measurements of height and width were taken every 30 feet. The bricks in the invert at and below the flow line were examined for wear.

A study of the reports of these examinations disclosed that the following defects were noticeable:

1.

Cement partly out at water line.

2.

Cement partly out above water line.

3.

Depressed arch and sewer slightly spread.

4.

Large open joints.

5.

Loose brick.

6.

Bond of brick broken.

7.

Distorted sides, uneven bottom, joints out of line.

Inspection of pipe sewers from manholes, the pipe being illuminated by floating candles, was found to be unsatisfactory. Reliance was placed on the reports of men experienced in making connections and repairs to the sewers. Early pipe sewers in New York were laid directly on the bottom of the trench. Under these circumstances a small leak at a joint was sufficient to wash the earth away and to drop the pipe, causing serious conditions along the line. No wear or deterioration of pipe sewers were noted, the only defects being cracking of the pipes at the center line due to poor foundation and to defects in the pipe itself.

The depreciation of brick sewers as studied in New York, is shown graphically in Fig. 147. At zero the sewer is in good condition and at 100 it is in such a state of dilapidation as to require instant rebuilding. Repairs are not considered economical in this condition. In the preparation of this diagram each condition on the list above was given a certain number of points, which when added together represented the state of depreciation of the sewer. These sums were plotted as ordinates and the corresponding ages of the sewer were plotted as abscissas. The various points were taken cumulatively, and where the bond of the brickwork was broken (given a value of 72) plus other defects gave a total of 164 the sewer was considered as valueless and not worth repair. The scale of 164 was later reduced to a percentage basis as shown on the right of the figure. Fig. 148 shows a similar diagram for the depreciation of pipe sewers.

It was concluded that the life of a brick sewer in New York is 64 years. Some of the sewers examined were over 200 years old. The total original cost of 483 miles of brick, pipe and wood sewers was figured as $23,880,000 with a present worth of $18,665,000 and an average annual depreciation of 2.2 per cent. In figuring these amounts no account was taken of obsolescence. The deterioration of catch-basins proceeded at about the same rate as for brick sewers.