An expansive soil index for predicting shrink-swell potential

Soil properties indicative of shrink-swell potential were studied for 12 so ils 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 (hornbl ende gneiss), Jackland and Waxpool (diabase); vermiculitic-Kelly (thermal s hale); kaolinitic-Cecil (granite gneiss) and Davidson (diabase); and mixed- Carbo and Frederick (limestone), Craven and Peawick (Coastal Plain sediment s), and Mayodan and Creedmoor (Triassic sandstones). Soil properties measur ed were swell index, coefficient of linear extensibility (COLE), particle-s ize distribution, cation-exchange capacity (CEC), liquid limit, plasticity index (PI), and clay mineralogy, Soils with estimated high or very high shr ink-swell potential were clayey, with clay contents exceeding 60%, These ex pansive soils also exhibited high CEC (>15 cmol, kg(-1) soil), high liquid limits (>70), and appreciable swelling 2:1 mineral content (>15% montmorill onite + 1/2 vermiculite on whole-soil basis). An expansive soil rating syst em, termed the Expansive Soil Index (ESI), was developed using the soil pro perties most correlated with shrink-swell potential. The sum of swelling 2: 1 minerals, swell index, liquid limit, and CEC gave ESI ratings for each so il series. The higher the ESI, the greater the shrink-swell potential. Wher e less-detailed information is required, such as for initial feasibility st udies, an ESI consisting of liquid limit and CEC is recommended. Finally, t he soils were grouped into risk categories based on parent material to allo w for classification of similar soils into the ESI rating system, Soils wit h restricted drainage formed from mafic rocks, carbonate parent material, a nd metamorphic shales are at high risk for expansive soil behavior.