HIGHWAY CONSTRUCTION 79 



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 rnay l>e em- 

 ployed, one placed outside each curl) line; if, on the other hand, the 

 soil is exceedingly wet and the street very wide, four or more lines 

 may l>e employed. These drains may be 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. 



