Instability and Plastic Flow of Soils. II: Analytical Investigation

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Abstract
Based on the condition that a stable material is able to sustain a small perturbation in load, the stability of an elasto‐plastic granular material with non‐associated flow is analyzed, and necessary conditions for mechanical stability of saturated and partly saturated specimens are proposed. Using experimental data obtained from drained tests, the local yield and plastic potential surfaces are established. Their position shows the existence of a wedge where plastic loading occurs, while the second increment of plastic work is negative. Under drained conditions where strains are free to develop without restraint, instabilities cannot develop, and the material will be stable, even when Drucker's postulate is violated. For an elasto‐plastic material under undrained conditions with full or partial saturation, the proposed conditions show that instability may occur when the yield surface opens in the outward direction of the hydrostatic axis, and the material tends to compress. The experimental data supported the suitability of the proposed stability conditions to predict the instabilities created under undrained conditions.

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