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Compression of Soil by Isotropic Stress¹

ABSTRACT

Loose packs of moist soil aggregates were encased in thin, cylindrical rubber membranes and subjected externally to air pressure in a triaxial test cell. The pressure was increased in increments, and the volume of the pack was determined from the axial strain after the rate of strain became less than 0.01% per minute at each step.

The stress-strain measurements revealed certain characteristics of the compaction process. The soil aggregates were deformed plastically during loading and had developed a condition of "plastic set" at the end of each step. However, the system expanded substantially when the pressure was released. This effect has been attributed to translocation of water within the aggregates as a result of differential stress. The mechanical behavior of moist soil aggregates is different in this respect from the behavior expected of perfectly plastic grains under the same conditions.

A new sequence of plastic deformations occurred when the precompacted samples were recompressed to pressures which exceeded those previously applied. The volumetric contraction of the pack resulting from pressure gave an almost continuous envelope which represented successive states of plastic equilibrium throughout the pack.

Contribution from Department of Soils and Plant Nutrition, University of California, Berkeley. Presented before Div. I, Soil Science Society of America, Cincinnati, Ohio, Nov 13, 1956. The support of the Western Regional Technical Committee W-30 is gratefully acknowledged.

² Research Assistant and Associate Professor of Soil Physics, respectively.

Received for publication December 17, 1956. Accepted for publication October 29, 1957.