Stability and Swell Pressure Characteristics of Compacted Clays

Abstract

This paper discusses some of the factors affecting the density vs. stability relationships of compacted clays and some of the limitations of the concept that the stability of a compacted soil increases with an increase in density.

A series of tests performed on silty clay, in which stability was measured by triaxial compression tests, are described. It is shown that the relationship between density and stability of a soil depends on the criterion used to define stability; the greater the permissible strain before a sample is considered unstable, the greater is the possibility that an increase in density will cause an increase in stability. For the silty clay, when the stability is defined as the load required to cause 10 percent strain or less, an increase in density at a given water content caused an increase or decrease in stability depending upon the water content and the range of densities involved; at constant degrees of saturation, however, an increase in density always caused an increase in stability.

The effect of compaction method on the density vs. stability relationship is illustrated. Test data are presented for two soils, a silty clay and a sandy clay, comparing the stabilities, at various water contents and densities, of partially saturated samples compacted by impact methods, static pressure and kneading action. It is shown that, at equal water contents and densities, samples compacted by static pressures exhibit higher stabilities than samples compacted by impact or kneading methods; also, that the stabilities of samples at equal densities and water contents, compacted by impact and kneading methods, are very similar, with kneading compaction producing somewhat higher stabilities at lower degrees of saturation, and impact compaction producing slightly higher stabilities at higher degrees of saturation. For static compaction, both soils showed an increase of stability with increase in density at all water contents, while for samples compacted by impact and kneading methods, an increase in density at a given water content did not necessarily lead to an increase in stability.

The effect of saturation on the stability of compacted clays is shown and the influence of water content at compaction on the stability of saturated samples having the same density and water content is illustrated.

Finally, a comparison is made of the stability and swell pressures developed at various densities of samples of the sandy clay compacted by kneading action and static pressure and subsequently saturated by exudation of moisture from the samples under static loads. It is shown that very much higher swell pressures and stabilities are exhibited by the statically compacted samples than by the samples prepared by kneading action.