Swelling, shrinkage and shear strength of unsaturated expansive clays

  • Ali Khakbazan

    Student thesis: Doctoral Thesis


    The swelling, shrinkage and shear strength properties of unsaturated expansive clays are examined in this thesis. This research was instigated to provide the engineer with improved techniques for predicting the swelling behaviour encountered in unsaturated expansive clays. To achieve this objective, a double-walled triaxial cell was developed. This was necessary to eliminate the effects of creep on the perspex walling of the inner chamber as pressures within the cell varied. Swell and consolidated drained tests were conducted on the soil samples. They were sheared after expansion had taken place, without expansion and without prior expansion or the use of back pressures.The clay samples used for this study were a mixture of kaolinite and sodium montmorillonite. They were artificially constituted to produce identical test specimens prior to testing.

    Three series of swell and swell pressure tests were conducted under controlled environmental conditions. The test series consisted of samples containing 10%, 20% and 30% sodium montmorillonite.

    The data from the experimental programme were analysed and the results showed:
    a) The water intake and total sample volumes generally exhibited an initial large and rapid increase and then tended to equilibrium.
    b) The samples were fully restrained axially during testing. However when the loads were removed, free axial expansion, due to the stress release, took place.
    c) The affinity of clay particles for water was demonstrated when the water intake by the soil samples showed a greater gradient as montmorillonite content increased.
    d) A time delay was found between the water intake by the soil samples and the reaction time with montmorillonite.
    e) The axial swell pressures increased rapidly in the first hour from zero to approximately 220 kN/m2. Further increases were recorded in the following twenty hours and then tended to a near constant value.
    f) The volume increase between samples within the ranges of 10% to 30% montmorillonite was non-linear. Where free expansion was taken into consideration, the volume increased curvi-linearly. However, under test conditions, a reduced rate of expansion was experienced.
    g) The values of the angles of internal friction, whether total or effective, decreased as the amount of montmorillonite in the soil samples increased.
    h) The values of Φb were found to be consistently less than Φ'' with respect to pore air back pressure. This further emphasised the role that matrix suction plays in the behaviour of unsaturated soils. Therefore, it is not sufficient to determine values of Φ'' alone in the case of unsaturated soils.
    Date of Award1995
    Original languageEnglish

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