Effects of wetting–drying cycles on soil strength profile of a silty clay in micro-penetrometer tests

• Micro-penetrometer is capable to detect inner soil strength with high resolution.
• Soil strength decreases with increasing W–D cycles and exhibits delayed effect.
• Soil structure heterogeneity is intensified by W–D cycles.
• Effect of W–D cycles and water content on soil bearing capacity was highlighted.

A good understanding of the soil strength profile under wetting–drying (W–D) cycles is essential when studying the hydro-mechanical response of soils under the effects of precipitation and evaporation. In this investigation, initially saturated slurry specimens were prepared and subjected to three W–D cycles. A micro-penetrometer was used to characterize the effect of W–D cycles on soil mechanical behavior. Based on the obtained penetration curves (penetration resistance versus depth), the temporal–spatial evolution of soil strength under each drying path was analyzed and discussed. The results show that the developed micro-penetrometer is a simple, quick, economical and reliable tool for identification of soil strength profile. Upon drying, as the water content is relatively high, the strength of soil is low and uniform along the depth. The influence of evaporation is insignificant in the initial stage of drying even though a large amount of water is lost. After a low critical water content is reached, the strength of the upper soil layer increases rapidly, and the overall strength generally increases exponentially with decreasing water content while decreases exponentially with increasing depth. The drying induced strength exhibits obvious delayed effect in profile. With increasing W–D cycles, the strength tends to decrease. The penetration curves change from typical mono-peak pattern to multi-peak pattern after the third W–D cycle, suggesting that the W–D cycles create more defects in soil microstructure and intensify the heterogeneity of strength in profile.

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