Unstable behaviour of model Jamuna micaceous sand

  • Georgiannou V
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The instability of loose sands under undrained loading conditions has often been studied in the past. Sladen et al. (1985) observed that the undrained stress paths of contrac- tant (loose) sand specimens consolidated isotropically to the same void ratio, but sheared at different initial confining pressures, converge to the same ultimate steady state at large strains. The peak deviatoric stresses of all the stress paths fall on an approximately straight line that passes through the steady-state strength of the specimens. They referred to this line as the collapse line. Lade (1993) also noted the unstable behaviour of loose contractant soils, and defined the instabil- ity line that connects the peak deviatoric stress of the undrained stress paths and the origin of the stress space. Sasitharan et al. (1993) demonstrated that the post-peak portion of a constant-void-ratio stress path defines the state boundary surface. They presented results from drained tests to show that above this surface no stress state can exist. Any type of stress path, undrained or drained, would initiate collapse when it attempts to cross the bounding surface. Although collapse was induced under fully drained condi- tions in the drained tests, catastrophic failure took place under undrained loading conditions, leading to a complete loss of the specimens’ strength. Chu et al. (2003) performed drained tests at constant deviator stress (q ¼ a  r) tests to study the instability behaviour of a contractant loose sand. They observed instability under fully drained conditions, with the sudden development of large plastic strains in the vicinity of the instability line defined from undrained tests according to Lade (1993). However, the specimens did not show complete collapse, and the stress paths moved further towards the failure line of the very loose sand.

Author-supplied keywords

  • laboratory tests
  • sands
  • shear strength

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  • V. N. Georgiannou

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