CONSTRUCTION NOTES 247 



additional width of excavation at these points. A grade stake should be 

 carefully set to one side of each inlet or catch basin as a reference for the 

 elevation of the top of the grating. It should be placed so that the top of 

 the grating can be reached by a straight edge not more than ten feet long. 



Excavating trenches. In the excavation of all drainage trenches, espe- 

 cially through lawn areas, great care should be exercised to excavate the 

 top soil to the full depth and pile it along one side of the trench. The sub- 

 soil should then be excavated to a point slightly above the finished grade 

 for the bottom of the trench, and this subsoil should be piled on the oppo- 

 site side of the trench. Great care should be exercised in excavating for the 

 bottom of the trench, and frequent checks should be made from the string 

 line stretched between batter boards. This grade at the bottom of the 

 trench is usually a flow line grade for the bottom of the inside of the tile, and 

 an allowance equal to the thickness of the tile should be made in determining 

 this surface for the bottom of the trench. It is very important that no part 

 of the trench should be excavated below the finished grade, because all 

 pipes for the best results should rest firmly upon solid, undisturbed soil. 



Laying pipe lines. The method of installing tile drainage is practically 

 the same in all cases. Tiles with open joints should be spaced and covered 

 as shown in figure 7 on Plate 108. Each tile as it is laid should be covered 

 and firmly supported on either side in order to prevent any possibility of 

 getting out of line. The approximate spacing between the ends of the tile 

 is one-eighth to one-fourth inch, and the joints are covered with a strip 

 of tar paper approximately three inches wide and long enough to cover 

 the upper two-thirds of the circumference of the tile. The object of the 

 tar paper is to prevent silt from getting into the tile at the top. Vitrified 

 pipe, having bells, needs no tar paper, but the soil must be cut out below 

 the pipe to fit the bell. 



Backfilling trenches. All trenches should be carefully backfilled in 

 layers not exceeding six inches in depth. Each layer should be thoroughly 

 tamped and preferably puddled except immediately around the pipe before 

 a succeeding layer is applied. The method of backfilling with cinders, sub- 

 soil and topsoil is shown in figures I, 2 and 3 on Plate 108. The great danger 

 in backfilling trenches comes from a subsequent settlement which causes 

 oftentimes irreparable damage to the surface conditions above the drains. 



All drainage trenches on all areas except wooded areas should be care- 

 fully backfilled with screened cinders up to a point where the proper depth 

 of topsoil is required. (See figures I, 2 and 3 on Plate 108.) 



In abnormally deep trenches where it seems advisable to conserve the 

 supply of cinders, the trench can be backfilled, using a board partition on 

 one side of which a backfill of cinders is made and on the other side of 



