Stiffness of unsaturated compacted clays at small strains

The objective of the research presented in this dissertation is to investigate the effects of compaction conditions and the state of stress on the small-strain stiffness of unsaturated specimens of a low plasticity clay. An experimental study was conducted to address this objective. A pressure plate apparatus was developed to study the effects of compaction water content, dry unit weight, and degree of saturation on soil matric suction. Matric suction decreases as the compaction water content increases regardless of the dry unit weight and degree of saturation.; A triaxial cell equipped with piezoelectric transducers was developed to investigate the effects of compaction water content, dry unit weight, and degree of saturation on soil stiffness under undrained test conditions. Shear and compression wave velocities generally decrease as the compaction water content increases. The variation of shear and compression wave velocities with dry unit weight depends on the compaction water content. For specimens compacted at similar water contents, the initial dry unit weight does not seem to have a significant effect on shear and compression wave velocities. Accordingly, soil stiffness was judged to be inapplicable to determine dry unit weights for compaction control purposes.; Results from pressure plate tests were used to estimate the initial matric suction for specimens tested in the undrained triaxial cell, which was then used to investigate the effects of matric suction (ua--u w) on soil stiffness. For specimens compacted at similar dry unit weights or compactive efforts, soil stiffness increased as the matric suction increased and tended towards a nearly constant value beyond which the stiffness did not appear to increase further for the range of matric suctions being investigated. For similar degrees of saturation, soil stiffness increased as matric suction increased for the range of matric suctions being investigated. Relationships between soil stiffness and matric suction are not unique and depend on factors such as compaction dry unit weight and degree of saturation, which essentially relate to the soil fabric, and overconsolidation ratio. For specimens compacted at similar dry unit weights, the relationship between soil stiffness and matric suction was investigated to determine the applicability of using stiffness and matric suction to infer dry unit weight for compaction control purposes. The variation observed in this relationship seems to be beyond what can be used to determine dry unit weights for compaction control purposes.; A suction-controlled triaxial cell equipped with piezoelectric transducers was developed to investigate the effects of matric suction (ua--u w) and net stress (sigmac--ua) on soil stiffness. Constant water content tests were performed on specimens compacted at similar degrees of saturation and compactive efforts. Three-dimensional diagrams were plotted to illustrate the variation of soil stiffness with matric suction and net stress. Generally, soil stiffness increases as matric suction increases, but at a decreasing rate. Stiffness also increases as net stress increases. Relationships between stiffness, matric suction, and net stress are not unique and depend on factors such as compaction degree of saturation and compactive effort, which essentially relate to the soil fabric, and overconsolidation ratio. Several empirical equations reported in the literature were evaluated with respect to their validity for modeling the variation of shear modulus with stress for the soil tested in this study.