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A method of computing the loads imposed on an underground pipeline by several simultaneous wheel loads acting on the surface, proposed by Spangler and Hennessey2 in 1946, and to which only a brief reference could be madei n a previous Paper1 by the present Authors, is now described in detail. A modification of that method is developed by the Authors to compensate for the nonuniformity of load distribution across the pipe breadth, which occurs in some wheel positions, and adversely affects the bedding factor for wheel loads compared with fill loads. The modification takes the form of correction factors for the conversion of the non-uniform loads to equivalent uniformly distributed loads. Their use enables the same bedding factor as that adopted for the fill load to be used for concentrated surcharge loads. Graphs of these values for the usual classes of bedding are given. The modifled method, which is applicablteo any wheel arrangement, has been used to compute the maximum loads (or load coefficients) imposed on pipes of various sizes at various depths with various beddings by:- (i) two equal wheel loads 3 ft apart (appropriate for pipes under access roads); (ii) the B.S.153, type HB, road load train (appropriate for pipes under roads, and carriageways, fields, and gardens); and the results are presented in the form of design graphs. The maximum foreseeable railway loading in England and Wales is given. It proves to be less severe than the B.S.153, type HB road loading. The design graphs may, therefore, be used for single or multiple rigid pipes of any normal size at any normal depth in trench, under roads, fields, or gardens, across railways, or under embankments, in any sort of soil and with any standard bedding and, it is hoped, will save much tedious computation.

DiscussionThe Chairman (Sir Herbert Manzoni) said that the Paper described a piece of pioneering work. It was probable that less was known about the many factors that governed the strength of pipelines than about the majority of civil engineering projects, for very little research work had been done on the subject. The Authors, however, had extended the work that had been done at Iowa. Pipelines were designed by what might be described as Roman methods, involving crude strength and bulk rather than by scientific analysis; there had always been, and still weie, so many difficulties and imponderables, such as the adequate packing and shaping of concrete round the pipes in the bottom of a narrow trench, or the determination of the characteristics of the backfilled earth. For these reasons, it was likely that the present methods would continue until much more basic information was made available.70. On two previous occasions Sir Herbert had taken the Chair when Papers on this subject were presented, and on each occasion criticism had been made of the suggestions arising from careful investigation. Because of the practical difficulties, certain assumptions and seemingly uneconomic methods might have to be continued on the basis of experience, but it was much better if experience were added to a known basis of actual research and ascertained facts.71. If nothing else were to result from the Authors' research, at least their insistence on the necessity for a new type of joint was most valuable. There was more trouble with the old-fashioned joints than with almost anything else, and the introduction of an agreed type of flexible joint would be a monument to their work. 72. Mr Clarke had spent, the Chairman believed, about 30 years on practical work and he was, therefore, able to bring to his research a knowledge of most of the practical difficulties, and the results of the Authors' work might well be that pipelines in the future could be laid more economically and with less risk of damage.

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