State-boundary surface for very loose sand and its practical implications

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.