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THE HELL SYSTEM TEf'HXirAL JOURNAL, MAY 1057 



tion.'- Ilowexcr, duo to the liir^v (lianu'ter.s of the coiiduits ii.scd and 

 the particular test eonditions employed, tJie test results obtained were 

 not directly applicable for the determination of strength reciuirements 

 for conduits for the Bell System. 



Underground conduits under service conditions are subjected to 

 external loads. These external loads are caused by: 



a. The weight of the backfill material. 



b. The loads applied at the surface of the fill. 



The magnitude and distribution of the external loads around the con- 

 duit are affected by: 



1. The properties of the backfill material. 



2. The magnitude of the applied load at the surface of the fill. 



3. The height of the backfill over the conduit. 



4. The trench width. 



5. The bedding condition. 



6. The diameter of the conduit. 



7. The flexibility of the conduit. 



8. Impact. 



9. Arrangements of conduits in the trench. 



10. Consolidation and compaction of the backfill material. 



1 1 . Auxiliary protection of the conduit. 



Fig. 1 — Thin-walled tube with SR-4 strain gages. 



