754 THE BELL SYSTEM TECHMCAIv JOI'KNAL, MAY 1957 



ducted by H. Loreiiz/ Further tests conducted witli 500-lb weights 

 (h-opped from different heights demonstrated that the effect of dynamic 

 loads for clay backfill is of minor importance. 



FURTHER INVESTIGATION i| 



The tests previously conducted did not include investigations of the ' 

 effects of conduit flexibility, various conduit diameters, multiple arrange- 

 ments of the conduits in the trench, consolidation and compaction of the 

 backfill, and auxiliary protection of the conduit. Studies of these factors 

 are noAV in progress. 



SUMMARY 



Strength requirements of underground conduits depend on \'arious 

 factors. The most severe conditions were obtained with wet Georgia clay 

 as backfill for a trench width of 18 to 30 inches. The relationship between 

 the height of backfill, the load applied at the surface of the backfill, and 

 the equivalent two-point load for these conditions is shown in P'ig. 12. 

 For example, a round conduit of 4-inch mean diameter under 24 inches 

 of clay cover subjected to an applied load of 15,000 lb should be able to 

 carry a two-point load of 1,250 lb wdthout fracture of excessive deforma- 

 tion. Figs. 12 to 14 can be used to determine the required crushing 

 strength of 4-inch round conduits subjected to different applied loads, 

 buried at different depths with clay, sand, or gravel cover, but only for 

 trench widths of 18 inches or more. Although this investigation is not 

 completed, the results obtained to date may be used, within the indi- 

 cated limits of application, for the selection of acceptable conduit. 



ACKNOWLEDGEMENT 



The author wishes to acknowledge with thanks the invaluable assist- 

 ance given by R. G. Watling, W. T. Jervey, J. J. Pauer and Duncan M. 

 Mitchel in planning and conducting these tests; also to express his 

 gratitude for advice given by L V. Williams during the progress of 

 the investigation and preparation of this paper. 



REFERENCES 



1. Marston, A., The Theory of External Loads on Closed Conduits in the Light 



of the Latest Experiments, Bulletin 96, Iowa Engineering Experiment Sta- 

 tion. 



2. Spangler, M. G., Soil Engineering, International Textbook Company, Scranton, 



1951. 



3. Marquard, Rohrleitungen und geschlossene Kaniile, Handhucli fiir Eisenbeton- 



bau, 4, Aufl., B(l. 9, Berlin 1939. 



4. Lorenz, H., Der Baugrund als Federung und Dampfung srlnvingender Korper, 



Bauingenieur, p. 11, 1950. 



