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.

The effects of stress path and loading direction in the triaxial test on strength and dilatancy of sand are investigated. It is shown that the unique relationship observed between peak friction angle and dilation rate at peak in conventional triaxial tests is followed regardless of stress path, confining stress at failure, relative density, and the mode of loading (compression or extension).

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A state-boundary surface defines a boundary in stress -void-ratio space above which no stress state can exist. The applicability of the state-boundary surface for sand has not gained widespread attention primarily because sand is not generally considered to be a difficult soil from a design point of view apart from liquefaction. Liquefaction is a phenomenon usually encountered in very loose cohesionless materials. An experimental study relating the drained and undrained behavior of very loose saturated sand is presented. It is shown that the post-peak portion of undrained stress paths travels along the state boundary and that the state boundary can be approximated by a straight line. The slope of this straight line appears to stay constant for very loose sand. There are potentially an infinite number of these lines, which form a three-dimensional surface in deviator stress -effective mean normal stress -void-ratio space. Previously published results by various researchers are used to confirm the existence of the state boundary. This surface is mathematically defined in deviator stress -effective mean normal stress -void-ratio space. Loose saturated sand samples loaded drained from a stress state on or very close to the state boundary surface essentially travel along the state boundary surface. . SUne surface d'Ctat limite d6finit une frontikre dans un espace contrainte-indice des vides au-dessus de laquelle aucun Ctat de contrainte n'existe. L'application du concept d'6tat limite aux sables n'est pas routinibre surtout parce que les sables ne sont pas considCrCs comme des sols difficiles 2 traiter d'un point de vue de conception mis 2 part les problemes de IiquCfaction. La IiquCfaction est un phCnomene qui est gCnCralement associk-aclx-matkriaux tres Iiches. Une Ctude expCrimentaie concernant le comporternent drain6 et non drain6 d h n sable saturC et tres liche est prCsentCe. I1 a Ct C montrC que les chemins de contraintes pour des conditions non-drainCes sont situCs sur la surface d'Ctat limite lorsque la rksistance au pic a Ct C mobilisCe. De plus, la surface d'6tat limite peut &tre repr6sentCe par une droite. La pente de cette droite semble &tre constante pour un sable tres Iiche. Il y a une infinit6 de droites, donc une surface tridimensionnelle dans un espace contrainte dkviatorique-contrainte principale moyenne-indice des vides. Des rCsultats publiCs par diffkrents chercheurs confirment I'existence de la surcface d'Ctat limite. Cette surface est dCfinie par une relation mathkmatique dans I'espace contrainte principale moyenne-indice des vides. Des Cchantillons liches de sable saturC dans un Ctat de contrainte sur ou 2 proximitC de la surface d7Ctat limite chargCs en conditions drainCes ont essentiellement des cheminements de contrainte sur la surface d'ktat limite.

In situ ground freezing to obtain undisturbed samples of a loose sand beneath the Duncan Dam in British Columbia is described. This was the first known use of ground freezing to assist in obtaining samples at depth greater than 10 m in Canada. Once frozen, the sand was cored and brought to the surface using a Cold Regions Research and Engineering Laboratory core barrel, which is used extensively in permafrost regions. The design, installation, and performance of the liquid nitrogen freezing systems are described in detail. The drilling, sampling, and preparation of the frozen core for transport to the testing facility are also outlined. The quality of the core retrieved during the sampling was judged to be excellent, and good agreement was found between the void ratios measured using both the frozen core and high-quality gamma–gamma density logging techniques, which were performed adjacent to ground-freezing and sampling locations. Key words : ground freezing, cohesionless soils, undisturbed sampling.