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Title: Time effects and progressive failure in clay slopes
Author: James, Peter Michael
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1970
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The stability of overconsolidated fissured clays and clay shales is a central problem in Soil Mechanics. In this thesis the problem has been approached using field evidence; the actual failure surfaces of slips being analysed by the generalised stability method rather than the most critical circle or wedge for a failed slope being determined. A different emphasis on slip development has resulted. Case records of over 50 slips have been analysed in a spectrum of argillaceous sediments ranging from the Oligocene to the Carboniferous. The London Clay, Oxford Clay and Lias Clays provided the bulk of the data, with the support of several Cretaceous Clays, and mention is made of the Coal Measure shales, bentonitic shales and shales weakened by flexural slip. Laboratory tests were undertaken. These included classification tests on nearly all slips, except those in London Clay, and several sets of direct shear tests on Oxford Clay, two of which illustrated the importance of directional (sedimentary) properties on the path to the residual strength. From the analysis the time effects on the stability of soils were obtained. Decay in strength with time was found to be predictable in the London Clay and, to a lesser extent, the Oxford Clay, although the Lias Clays were more erratic. In general, it was found that the initial failure of a slip was explainable with the average angle of friction equal to, or greater, than the laboratory peak value, and that the decay in strength - at least over periods of the order of 100 years - was a drop in the effective cohesion term only, ie. a softening process. This general trend may not apply where any form of progressive failure occurs. The modes of progressive failure do, however, appear to be very limited and deformation, due either to lateral expansion towards a cutting or simple swelling of susceptible layers, needs to be severely constrained in order to develop strains of sufficient magnitude to produce residual conditions. Some information on the average drop in strength due to movement of slips is given, and it has been found that a slip will generally need to be reactivated once or twice to give average conditions near to the residual. An attempt has also been made to show the time development of progressive failure mechanisms in some London Clay slides. Generally good agreements were obtained between the residual angle of friction of reactivated slides and that found by conventional laboratory tests, e.g. direct shear or tests on cut planes. However, most slides take place at very much smaller values of the average effective normal stress than are used in laboratory tests and the implications of these two facts, combined with the possibility of curvature of the failure envelope, are mentioned. Occasional wide discrepancies between the field and laboratory values of the residual angle of friction were also encountered; these perhaps being due to bands of different clay minerals in the slope or excessive movements. In Section I of the thesis the generalised stability analysis is described and the effect on the results of several variables and assumptions is investigated. Comparisons are also made between this method and the other conventional (hand) methods of stability analysis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available