744 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



BEDDING EFFECT 



Based on the theoretical considerations illustrated in Figs. 5 and 6, 

 comparison of Figs. 7 and 8 indicates a high load concentration at the 

 bottom of the tube buried in a trench with clay backfill. For a verifica- 

 tion of this assumption, a series of additional tests were conducted. The 

 test tube was placed upon (a) a flat steel plate and then covered with 

 moist clay, (b) a flat steel plate and then covered with dry sand, and 

 (c) carefully distributed moist clay and then covered with clay. The 

 external load was then applied. The moment distributions obtained for 

 cases (a), (b), and (c) are shown in Figs. 9, 10 and 11, respectively. 



A comparison of the data presented in Figs. 9 and 11 shows the effect 

 of the bedding condition on the moment distribution (steel plate versus 

 clay bedding). These data, as well as theoretical considerations (Figs. 5 

 and 6), indicate that the bedding condition affects mainly the bending 

 moment at the bottom of the tube and only to a lesser degree the bend- 

 ing moment at right angles to the vertical axis. Due to a change in 



LOAD DISTRIBUTION 



P = 2.22 PSI 



ttnntttttttttttittt p-2-«ps. 



? 6 



X 4 



m 



u 

 2 

 O 

 1-2 



e> 



z 

 5-4 



2 

 UJ 



<^-6 



MOMENT DISTRIBUTION 



M = 4- pr2cos2e 



Fig. 5 — Theoretical moment distribution in a thin-walled tube subjected to 

 uniformly distributed vertical pressure. 



