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Estimating Critical Stress in Unsaturated Soils from Changes in Pore Water Pressure During Confined Compression¹

ABSTRACT

The objective of this study was to describe the changes in pore water pressure as a function of initial soil water content and applied stress. A hypothesis is developed that relates pore water pressure of unsaturated soils during application of applied mechanical stress to changes in soil structure.

During compression of unsaturated agricultural soils, the pore water pressure decreases to a minimum (becomes more negative), and then increases as the applied mechanical stress increases. The pore water pressure-log applied stress curves for a given soil with different water contents are similar in shape but vary in scale. The degree of water saturation at which minimum pore water pressure occurs appears to be a constant for a given soil. This degree of saturation increases with soil clay content up to about 33% clay and then remains nearly constant at 0.60. The pore water pressure-log applied stress curves for different water contents of a given soil, when normalized, fit a common curve. For different soils, the log of applied stress corresponding to the minimum pore water pressure, when normalized with respect to the log of stress at saturation, increases with clay content up to about 33% clay and then remains constant at 0.78. It appears that, as the applied stress increases beyond the point of minimum pore water pressure, soil aggregates are sheared and their integrity destroyed. The stress corresponding to the point of minimum pore water pressure is, we think, the maximum stress that should be applied to a soil during cultivation or by vehicular traffic. Finally, this paper describes a procedure for estimating the critical stress for other soils from a soil's clay content using the relationships developed in this study.

¹ Contribution of the Soil and Water Management Research Unit, North Central Region, Science & Educ. Admin., Agric. Res., USDA, St. Paul, Minn., in cooperation with the Minnesota Agric. Exp. Stn., Paper no. 11,161, Scientific Journal Series.

² Soil Scientists, USDA, SEA, AR, and Univ. of Minnesota, St. Paul, MN 55108.

Received for publication April 21, 1980. Accepted for publication July 28, 1980.

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