„ " " „ su „ " " Sub-Foundation Drainage of Streets. The sub-foundation drainage of streets cannot be effected by transverse drains, because of their liability to disturbance by the introduction of gas, water, and other pipes.
Longitudinal drains must be depended upon entirely; they may be constructed of the same materials and in the same manner as road drains. The number of these longitudinal drains must depend upon the character of the soil. If the soil is moderately retentive, a single row of tiles or a, hollow invert placed under the sewer in the center of the street will generally be sufficient; or two rows of tiles may be em ployed, one placed outside each curb line; if, on the other hand, the soil is exceedingly wet and the street very wide, four or more lines may be employed. These drains may he permitted to discharge into the sewers of the transverse streets.
Surface Drainage. The removal of water falling on the street surface is provided for by collecting it in the gutters; from which it is discharged into the sewers or other channels by means of catch-basins placed at all street intersections and dips in the street grades.
Gutters. The gutters must be of sufficient depth to retain all the water which reaches them and prevent its overflowing on the footpath. The depth should never be less than 6 inches, and very rarely need be more than 10 inches.
Catch basins are of various forms, usually circular or rectangular, built of brick masonry coated with a plaster of Portland cement. Whichever form is adopted, they should fulfil the following conditions: (1) The inlet and outlet should have sufficient capacity to receive and discharge all water reaching the basin.
(2) The basins should have sufficient capacity below the outlet to retain all sand and road detritus, and prevent it being carried into the sewer.
(3) They should be trapped so as to prevent the escape of sewer gas. (This requirement is frequently omitted, to the detriment of the health of the people.) (4) They should be constructed so that the pit can easily be cleaned out.
(5) The inlet should be so constructed as not easily to be choked by leaves or debris.
(6) They must offer the least possible obstruction to traffic.
(7) The pipe connecting the basin to the sewer should be easily freed of any obstruction.
- The bottom of the basins should be 6 or 8 feet below the street level; and the water level in them should be from 3 to 4 feet lower than the street surface, as a protection against freezing.
The capacity and number of basins will depend upon the area of surface which they drain In streets having level or light longitudinal grades, gullies may be formed along the line of the gutter at such intervals as may be found necessary.
Catch-basins are usually placed at the curb line. In several cities, the basin is placed in the center of the street, and connects to inlets placed at the curb line. This reduces the cost of construction and cleaning, and removes from the sidewalk the dirty operations of cleaning the basins.
Catch-basins and gully-pits require to be cleaned out at frequent intervals; otherwise the odor arising from the decomposing matter contained in them will be very offensive. - No rule can be laid down for the intervals at which the cleaning should be done, but they must be cleaned often enough to prevent the matter in them from putrefying. There is no uniformity of practice observed by cities in this matter; in some, the cleaning is done but once a year; in others, after every rain storm; in still others, at intervals of three or four months; while in a few cities the basins are cleaned out once a month.