Correlation between Atterberg Limits and Soil Adsorptive Water

The Atterberg limits are empirical indices of the critical water contents defining mechanical states of soil-water mixtures between the semisolid state and plastic state (the plastic limit or $w_P$), and between the plastic state and liquid state (the liquid limit or $w_L$). They reflect semiquantitatively a fine-grained soil’s ability to resist external loading. They are controlled by four fundamental factors: soil mineralogy, particle size distribution, pore fluid chemistry, and pore structure. However, general quantitative relationships among the Atterberg limits and these controlling factors are yet to be established. The authors hypothesize that the total amount of adsorptive water content for a given soil is directly related to the soil’s $w_L$ and $w_P$. A broad suite of 35 soils with measured soil-water retention (SWR) and the Atterberg limits from the literature were synthesized to explore these relationships. Using the measured SWR data and interpreting them through a generalized SWR model, the adsorptive water contents of all 35 soils were quantified. The authors demonstrate that the Atterberg limits, including $w_P$, $w_L$, and plasticity index $I_P(=w_L-w_P)$, were all correlated to mechanisms of soil-water interaction, specifically to a soil’s total adsorptive water content in terms of gravimetric water content, confirming the hypothesis. Further, the correlation between the Atterberg limits and a soil’s volumetric water content was poor, indicating the Atterberg limits’ independence from capillary water retention mechanism. The correlations provide a new pathway to move beyond the Atterberg limits to classify soil directly using more representative soil physical properties like adsorption suction stress, which could be linked to all four fundamental factors of soil mineralogy, particle size distribution, pore fluid chemistry, and pore structure.