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A study has been carried out to investigate the influence of fines and gradation on the skeletal collapse behaviour of loosely prepared sand samples. Loose sand samples, formed by moist tamping and consolidated to the same effective stress level, were prepared with varying percentages of both plastic and non-plastic fines (< 74 μm) and non-plastic fine sand (> 74 μm). Samples were isotropically consolidated and subjected to monotonic undrained triaxial compression. Results indicate that increased percentages of fines (< 74 μm) have a pronounced effect on the monotonic undrained behaviour at large strains (> 0.5%). This change is represented by a decreased strain softening behaviour and a shifting of the steady-state line. Variation of the gradation of the host sand appears to have little effect on the monotonic undrained behaviour. A supplemental study, involving use of the scanning electron microscope, was carried out to evaluate the findings of the test program in terms of sand fabric. Key words : collapse, monotonic undrained behaviour, scanning electron microscope, fabric.

Loose cohesionless materials can collapse during either static or dynamic loading, resulting in a rapid buildup of pore pressure and associated reduction in shear resistance. As the cohesionless material collapses, it rapidly looses resistance until the acting shear stress decreases to the available residual or steady-state strength. Specially designed stress-path testing has been performed on sand to investigate this collapse process. Results from this test program and previously published data show that a state boundary can be defined when a cohesionless material moves from peak to steady state along a constant void ratio stress path regardless of whether it is loaded drained or undrained. Further, it is demonstrated that the state boundary represents a surface in the effective mean normal stress–deviator stress–void ratio space. Hence, flow slides and liquefaction can be initiated when the stress path followed during either drained or undrained loading attempts to cross this state boundary surface. Key wordy : sand, collapse, liquefaction, stress path, state boundary, triaxial test.

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