An Expansive Soil Index for Predicting Shrink–Swell Potential

Abstract

Soil properties indicative of shrink–swell potential were studied for 12 soils encompassing several parent materials in Virginia. Soils are rated from moderate to very high shrink–swell potential. The mineralogy classes, soil series, and (parent materials) examined include: smectitic—Iredell (hornblende gneiss), Jackland and Waxpool (diabase); vermiculitic—Kelly (thermal shale); kaolinitic—Cecil (granite gneiss) and Davidson (diabase); and mixed—Carbo and Frederick (limestone), Craven and Peawick (Coastal Plain sediments), and Mayodan and Creedmoor (Triassic sandstones). Soil properties measured were swell index, coefficient of linear extensibility (COLE), particle‐size distribution, cation‐exchange capacity (CEC), liquid limit, plasticity index (PI), and clay mineralogy. Soils with estimated high or very high shrink–swell potential were clayey, with clay contents exceeding 60%. These expansive soils also exhibited high CEC (>15 cmolc kg−1 soil), high liquid limits (>70), and appreciable swelling 2:1 mineral content (>15% montmorillonite + 1/2 vermiculite on whole‐soil basis). An expansive soil rating system, termed the Expansive Soil Index (ESI), was developed using the soil properties most correlated with shrink–swell potential. The sum of swelling 2:1 minerals, swell index, liquid limit, and CEC gave ESI ratings for each soil series. The higher the ESI, the greater the shrink–swell potential. Where less‐detailed information is required, such as for initial feasibility studies, an ESI consisting of liquid limit and CEC is recommended. Finally, the soils were grouped into risk categories based on parent material to allow for classification of similar soils into the ESI rating system. Soils with restricted drainage formed from mafic rocks, carbonate parent material, and metamorphic shales are at high risk for expansive soil behavior.