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The Influence of Soil Water Matric Potential on the Strength Properties of Unsaturated Soil¹

ABSTRACT

A study was made of the contribution that matric potential makes to effective stress and therefore the strength properties of two unsaturated soils. From an evaluation of the effective stress parameters C' and ø', and the unconfined strength, the shear strength, effective stress, and the factor were estimated as a function of soil water matric potential over the range –0.59 to –15.1 bars.

The strength properties of a Gretna clay increased rapidly with decreasing matric potential. In a Wellwood loam, however, the strength properties remained relatively constant over the range –0.59 to –15.1 bars.

The influence of matric potential on the strength properties of an unsaturated soil can be described in terms of the contribution that matric potential makes to the effective stress operating in the soil system. A measure of the proportion of the matric potential operative in the effective stress is given by the factor which is related to the degree of saturation. Therefore, the influence that matric potential has on the strength of an unsaturated soil is related to the moisture retention properties of the soil.

The implications arising from the contribution that matric potential makes to the strength properties of an unsaturated soil are briefly discussed in terms of soil-plant relations.

¹ Contribution from Dept. of Soil Science, Univ. of Manitoba, Winnipeg, Canada.

² Post-doctoral Fellow and Assistant Professor of Soil Physics, respectively.

Received for publication May 29, 1970. Accepted for publication June 24, 1970.