Commenting on some details of the paper the writer said that Mr Kennison claimed for pre-stressed concrete embedded cylinder pipe that the resistance of the mortar coating to cracking was greatly improved. This was presumably because the coating was better able to bond to the core concrete than to a steel cylinder and the tensile strain was, therefore, distributed uniformly, preventing the development of cracks of appreciable size at working pressures, despite the coating having been substantially without pre-stress.
It had been, shown that in the highveld areas of South Africa, where the problem was most acute, movements of structures showed a general upward trend on which was superimposed a seasonal variation, as opposed to the purely seasonal variation reported in England.1 It was therefore interesting to examine movements recorded in an area of South Africa which might approach English conditions. Fig, 1 showed the results of movement records on a badly cracked hall in Rosebank, Cape Town. The records were commenced approximately four months after completion of the hall, by which time severe cracks, of the order of 1 in wide had opened up, been repaired and had re-opened to the same extent
In Southern Rhodesia the old granite and gneiss did not, in general, give rise to expansive soil types, although certain sandveld conditions were troublesome in road construction. The extensive greenstone schists of the Primitive or Basement Complexes, such as occurred at Bulawayo, Gwelo and the Goldfield Belt north of Salisbury, had given rise to expansive soils. Epidiorite schists could be seen, in areas of sparse topsoil, to be slickensided.
The writer had listened to the above paper and while he appreciated the information furnished considered it unfortunate that a joint report by the various firms engaged in drilling for under-reamed piles was not possible, as various types of drilling and under-reaming equipment had been specially developed in South Africa for this purpose. The writer's department had been responsible for erecting eleven major schools and Government buildings in the new O.F.S. Goldfields area. These services, which had cost over 1,000,000, were founded on under-reamed piles, and no defects had become evident as yet. It should be noted that joints had been provided at various points in these structures to compensate for any differential movement in the piles.
Mr Lewis's statement 'short-comings of any design method ... have led design engineers to place considerable weight upon other factors in addition to C.B.R. values of saturated soil samples' was unhappily true, but surely engineers were ultimately interested only in strength, other tests being merely a measure of past experience of known strength. It would therefore appear that either the C.B.R. test was invalid, or, as the speaker preferred to believe, the accepted methods of performing the test needed revision.
Mr Kleyn must have carried out considerable testing with the C.B.R. methods to obtain all the data which he had reported so simply. Referring to Fig. 1 were the isolines of C.B.R. obtained by plotting the C.B.R.s of soaked samples at their moulding (or pre-soaked) moisture contents? If this was the case, by obtaining C.B.R. density curves of soaked samples all moulded slightly dry of optimum, the Author was setting out to test materials compacted at their best or strongest condition. This was a laboratory condition not obtained in the field where the soil was initially compacted, say, at the optimum moisture content for the plant used, and finally by traffic over a range of moisture conditions.
The Author's practice was to dry soils in his central laboratory at 50-60C. How did he dry out soils in his field laboratories and what correlation did he obtain? He stated that there had been cases where compaction had weakened soils. This the speaker had found to be true with schistose materials which broke down on compaction, and with fine semipermeable materials such as slightly plastic sand-clays and, gravels where pore pressure might be readily set up.
Mr Strongman had mentioned the existence of large deposits of ash, dust and pumice, which materials were presumably similar to fly ash, and had been mixed with lime with great success by the Romans! The possibilities of lime-ash stabilization of soils in East Africa would seem to be great and the speaker felt that lime could be imported at a lower cost than the Author quoted for the local product. Surely lime could, however, be produced on a large scale for much lower prices. Again could Mr Strongman elaborate on aniline-furfural stabilization in more detail?
The Author's paper brought to every member of the Conference some knowledge of the little-known field of Slimes Dam design and construction. The writer asked how control could be exercised over the seepage water by keeping the water in the pond away from the badly drained sections of the dam; it seemed that this state could only exist if the badly drained portions were not also at the lowest points of the dam.
The writer confessed that, as a Rhodesian engineer, and in ignorance of the soil moisture conditions in the Congo, he had read the paper with much interest but many misgivings as to the general applicability of the Author's methods. Did he encounter gravelly or laterised formations? Surely his cone resistance values would vary considerably with the moisture of the formation. Was this relatively constant in the Congo and, in the southern tip of the Congo, where conditions were probably similar to those in Northern Rhodesia, did he use this method of subsoil exploration?
In Fig. 1 the Author showed a series of what he called Mohr envelopes representing the shear strength of unsaturated soils. The writer felt that a Mohr envelope could be drawn only in terms of intergranular stresses at failure and that a better title would be 'plot of shear strength versus total stress.' In addition, the assumption that the curve was tangent to the circles, as for example in graph 1, was surely wrong, and in the case of stability investigations, slightly on the unsafe side.
The Authors mentioned that resistivity measurements should be made in all boreholes drilled for foundation exploration and they added that the resistivity log of a borehole was used to determine variations in porosity or changes in formation with depth. They were asked please to explain how this resistivity log was obtained, and whether these methods could be used above the water table.