A possible elastic–plastic framework for unsaturated soils with high-plasticity

The paper proposes a new elastic–plastic framework for unsaturated, high-plasticity, clayey soils and sand–clay mixtures. The framework considers possible coupling of stress- and suction-induced hardening, leading to a yield surface that is closed or "capped" as suctions increase. This produces a stress state boundary surface in three-dimensional p–q–s stress space (where p is the net mean stress, q is the deviator stress, and s is the matric suction) which differs from that of other conceptual models of its kind. Yielding, a hardening law, and failure criteria for saturated soils are incorporated into the stress state boundary surface. Two parameters, equivalent pressure pe and stress ratio ηs, are introduced to form the basis of the proposed elastic–plastic framework for highly plastic soils with high suctions. This provides an alternative for the stress variables net mean stress and matric suction that are commonly used in modeling unsaturated soils with lower plasticity and lower suctions. This framework has allowed results of experiments on an unsaturated sand–bentonite mixture to be successfully described using elastoplasticity. Yield and failure envelopes associated with the proposed state boundary surface in p–q–s space can be normalized using pe and ηs in such a way that they agree with a comparable envelope for saturated specimens. Key words: unsaturated, elastic–plastic, triaxial, matric suction, state boundary surface, sand-bentonite.

The effect of anisotropic elasticity on the yielding characteristics of overconsolidated natural clay

Quantitative application of elastoplastic theory to the yielding behaviour of natural soils has always been uncertain. Part of the reason is that the theory was developed for reconstituted materials with isotropic structure, in contrast to natural soils that are usually anisotropic. The approach considered in this study assumes that pre-yielding behaviour is governed by the theory of linear anisotropic elasticity and that yield loci in the mean effective stress ( p') – deviator stress (q) plane are aligned approximately along the coefficient of earth pressure (K0) line. The assumption of a rotated yield locus associated with anisotropic elastic behaviour within the state boundary surface indicates that the elastic wall within the state boundary surface is inclined. The form of the state boundary surface has been determined mathematically in terms of anisotropic elastic and Cam-Clay soil parameters. Stress path tests were conducted on samples of Belfast Upper Boulder Clay removed from a depth of 28 m below ground surface. Good agreement was found between predicted and measured yield loci. The study also examined the influence of subsequent isotropic compression on the yielding characteristics of the natural clay. The indications are that the anisotropy developed during deposition disappears when the sample is loaded to a stress level at least twice the stress generated during the original deposition process. The methods developed in the paper have also been applied to test results reported previously on Winnipeg clay, and good agreement was obtained.Key words: sampling, anisotropy, elasticity, suction, stiffness, yielding.

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. Key words : sand, collapse, liquefaction, stress path, state boundary, triaxial test.