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.
Fig. 147.—Diagrams used in Estimating Depreciation of Brick Sewers Due to Age, Manhattan Borough, New York City.
a. Proportionate deterioration from various causes.
b. Percentage of depreciation based on examination of sewers, use of deterioration curve (Fig. a), and age of sewers examined.
Eng. News, Vol. 71, p. 84.
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.