Kamhawi, KZR; (1992) The shear strength of two resedimented clays at low and very low stress levels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Two main means of measuring shear strength were used: the first was the direct shear apparatus with internal pore water pressure measurement and the second was plane strain tilting of a submerged, initially horizontal, 20mm layer in a tank. The lower limits of shear strength for the two clays, which exclude the strong effect of thixotropy at low stress levels, are the same as values reported in published studies covering stress levels three orders of magnitude higher (namely 0:----- = 22.5 °for Speswhite Kaolin and cp -=- = 21 °for London clay). The pore fluid was London tap water for most experiments with some variations also tried including added salt and coagulant. Up to an effective consolidation stress of 5kPa, a large thixotropic component of strength was measured which was pseudo-frictional in that it was proportional to effective normal stress. Various experimental factors suggest that thixotropic strength may result from unsustainable dilatancy as flock bonds are broken. Any small degree of overconsolidation made this dilatancy sustainable within this stress range. Linear relationships between lower limit shear strength and overconsolidation ratio have been established. Clear relationships were established between shear strength and voids ratio for each clay when overconsolidated. The correlations with pre-existing shear data and a wide variety of direct shear and tilting test data show there are two alternative rupture plane orientations: maximum stress obliquity and a no extension direction associated with coincident axes of stress and strain increment. The absence of noticeable creep beyond 7 days after sedimentation was gratifying in that there is a possibility of simulating natural clay formation in a reasonable time frame.

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