A Scientific Basis for the Use of Index Tests in Identification of Expansive Soils

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Courtesy of ASTM International

The paper examines the scientific basis in the use of index properties, such as liquid limit, plasticity index, and activity, for estimating expansivities of natural kaolinitic and montmorillonitic soils by consideration of the fundamental mechanisms governing the soil swelling and plasticity characteristics. Results showed that the physicochemical mechanisms governing the swelling ability and index properties are similar in case of montmorillonitic soils; hence the use of index tests in estimating expansivities of montmorillonitic soils is mechanistically justified. The index properties of kaolinitic soils are not a function of diffuse double layer repulsion (the seat of swelling forces), and hence their employment in prediction of soil expansivities has no scientific basis. Comparison of classifications based on index properties with those obtained from oedometer results for the kaolinitic and montmorillonitic soils indicated that the index tests are better suited for prediction of expansivities of montmorillonitic soils. The results also illustrated that the free-swell procedure earlier proposed by the authors is a more reliable means of estimating expansivities in comparison to index properties and is applicable to both kaolinitic and montmorillonitic soils.

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