132 



Bulletin 86 



weight of a wagon or tractor. Both theory and experiment have 

 shown that the pressure due to the backfilling is a function of the 

 width of the trench, and not a function of the width of the pipe. 



The downward pressure of backfilling can be approximated from 

 the following formula developed by Professor Anson Marston at 

 the Iowa State College*. 



L=C W B^ 

 in which 

 I L=the load on a pipe in a ditch, in pounds per lineal foot, 



from the weight of ditch filling. 

 C=coefficient, taken from a table or diagram. 

 W=weight of ditch filling material, in pounds per cubic 



foot. 

 B=the breadth of the ditch, a little below the top of the 

 pipe, in feet. 



The coefficient "C" depends upon the nature of the ditch filling 

 material and upon the ratio of the height of fill above the pipe, H, 

 to the breadth of ditch, B. Marston has provided a table of safe 

 working values of C, and also a convenient diagram from which 

 the values can be takenf . A much condensed table is given here in 

 Table XV for the convenience of designers. 



TABLE XV. APPROXIMATE SAFE WORKING VALUES OE "c" 



Trenches are dug about ten inches wider than the outside diame- 

 ter of the pipe. For 12-inch pipe and 20-inch pipe, therefore, the 

 trenches are 25 and 35 inches wide, respectively. 



The downward pressure for 12, 16, and 20-inch pipe under var- 

 ious depths of filling, as computed by the Marston formula, is given 

 in Table XVI. The filling is assumed to be sandy soil. 



♦Bulletin of Iowa State College of Agriculture and Mechanic Arts. Vol XII, 

 No. 34, p. 96. 



Tlbid., pp. 95-96. 



