750 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



LOADING CONDITION 



r- 1000 LBS 



z 

 z 



OD 



Z 

 bJ 



o 



s 



o 



U) 



o 



■^WOODEN BOX 



Fig. 11 — Moment distribution in a thin-walled tube in moist clay. 



BACKFILL MATERIAL AND HEIGHT OF BACKFILL 



Backfill provides the main protection for conduits against loads ap- 

 plied at the surface of the fill. The type of backfill used and the height 

 of the backfill over the conduit are therefore extremely important. Figs. 

 12, 13, and 14 show the relationship between the bending moments in 

 the conduit and the height of backfill for different applied loads. Fig. 

 12 shows this relationship for clay as backfill, Fig. 13 for fine sand, and 

 Fig. 14 for gravel. The maximum bending moment is here expressed in 

 terms of the "equivalent two-point load." The equi^'alent two-point 

 load is the two-point load that will cause the same maximum bending 

 moment in the conduit wall as the maximum bending moments measured 

 in the field. The relationship between the two-point load and the bend- 

 ing moments in the walls of the conduit was given by (2). The equivalent 

 two point load is obtained for = and becomes 



