A review of the research and writing since the early work of Collin (21)2 about 1846 shows that evaluating the dynamic and static resistance of soil has always been recognized as a problem of major importance by engineers and research workers in soil mechanics. In retrospect, it would appear that Collin was many years ahead of his time in his clear perception of the basic elements of the plastic resistance of cohesive soils. His recognition of the transient nature of temporary dynamic reactions and the primary importance of permanent static resistance is just beginning to be recognized by many research workers of the present era of modern soil mechanics. In the author's opinion, it is more than coincidence that Collin found that the permanent resistance of clay soil in his investigations bore approximately the same ratio (24 to 34 per cent) to the instantaneous or temporary resistance that has now been found as the ratio (25 per cent) between the yield value from the ring shear test and the ultimate shearing resistance from the 5-min unconfined compression test.
While there is general recognition of the increased resistance of plastic clay soils under rapid rates of loading and the control methods necessary to evaluate time effects, there are very few test procedures which clearly exercise such systematic control of variables. The interpretation of shearing resistance tests is further complicated by a failure to define and separate the incompatible soil reactions due to cohesion and internal friction and to eliminate consolidation effects caused by application of normal pressures in shear testing.
In a review of the research program at the University of Michigan, three methods of measuring the permanent static shearing resistance of soils were presented with corroborative data cited from practical applications. These methods were: (a) incremental loading at constant time intervals, followed by extrapolating rates of deformation back to a zero rate to determine the yield value; (b) the determination of minimum and maximum soil resistance coefficients derived from settlement and stress reactions in linear equations for bearing capacity; and (c) the determination of the elastic properties of piles and supporting soil to determine loads carried at the elastic limit. Both of the last two determinations have been correlated with yield values determined by extrapolation of settlement rates with good agreement.
All three methods have been correlated with field observations by practical application in design and construction over a period of years, with results of this experience having been reported in references cited and summarized in the paper. Further observations of the dynamic resistance of soil are presented in connection with stability analyses of differential loading of soil masses where progressive displacement on a large scale is involved. While typical examples were given, the large volume of such data requires more space for presentation than is available in this paper and is a subject for future publication.