Effective stress paths for a loessial soil subject to collapse during confined compression have been determined from the results of a testing program consisting of (1) confined compression tests on natural samples of loess with initial water contents ranging from air-dry to saturation, (2) negative pore-water pressure measurements to -300 psi during these tests, and (3) K₀-tests in which the lateral stress ratio was measured for one-dimensional strain. Before collapse, coefficient of earth pressure at rest (K₀) was found to average 0.23, an extremely low value for a loose soil, whereas after collapse, K₀ increased to 0.54, which is consistent with values for other soils. Because of the low K₀-values before collapse, the effective stress path for loading in confined compression initially approaches the failure envelope. At collapse the stress path intersects the failure envelope and thereafter it changes direction as a consequence of the higher K₀-value after collapse. From the stress path interpretation of the results, it is demonstrated that the collapse mechanism of loess in confined compression and during wetting is a shear phenomenon and subject to analysis in terms of effective stresses.
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