80 HIGHWAY CONSTRUCTION 



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. 



FOUNDATIONS 



The stability, permanence, and maintenance of any pavement 

 depend upon its foundation. If the foundation is weak, the surface 

 will soon settle unequally, forming depressions and ruts. With a good 

 foundation, the condition of the surface will depend upon the material 

 employed for the pavement and upon the manner of laying it 



The essentials necessary to the forming of a good foundation are : 



(1) The entire removal of all vegetable, perishable, and yielding 

 matter. It is of no use to lay good material on a bad substratum. 



(2) The drainage of the subsoil wherever necessary. A per- 

 manent foundation can be secured only by keeping the subsoil dry; 

 for, where water is allowed to pass into and through it, its weak spots 

 will be quickly discovered and settlement will take place. 



(3) The thorough compacting of the natural soil by rolling with 

 a roller of proper weight and shape until it forms a uniform and un- 

 yielding surface. 



(4) The placing on the natural soil so compacted, a sufficient 

 thickness of an impervious and incompressible material to cut off all 

 communication between the soil and the bottom of the pavement. 



The character of the natural soil over which the roadway is to be- 

 built has an important bearing upon the kind of foundation and the 

 manner of forming it; each class of soil will require its own special 



