Miandouab, Sabr Akbarian; (1997) Directional shear cell tests on kaolinite powder. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The research was a laboratory investigation into the behaviour of dry Kaolinite powder which was subjected to controlled changes of both magnitude and direction of principal stresses. A stress controlled apparatus, the Directional Shear Cell (DSC), tested cubic samples under plane strain conditions. The DSC has the ability to apply both normal and shear stresses to four faces of a sample. By varying the combination of the shear and normal stresses on these faces the directions of principal stresses in the plane of strain could be changed in steps of any magnitude or continuously within a range of 0° to 90° relative to a sample face for any given stress path. Simple monotonic tests, those with directional changes and tests which included continuous rotation of the principal stress directions during shearing were conducted. The results show that shear strength can be described by two distinct strength envelopes. The greater strength for a given voids ratio relates to the Hvorslev envelope, while the smallest strength can be described by an envelope with the Critical State angle of friction and a cohesive intercept. The cohesive intercept of the Hvorslev envelope appears to be unique for a particular voids ratio, while that associated with the Critical State angle of friction is directionally variable with respect to the previous principal stress direction. DSC tests with large changes of principal stress direction gave smaller cohesive intercepts than those with small changes even though the elements were at the same voids ratio. For these tests volume reduction was prevented by ensuring that the stress on any plane was not increased more than the previous maximum stress on that plane. The findings could influence design procedure where it is known that principal stress direction changes occur, eg: design of hopper exits.

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