A Classification Tree Guide to Soil-water Characteristic Curve Test for Soils with Bimodal Grain-size Distribution

Abstract

Soil-water characteristic curves (SWCCs) can be unimodal or bimodal. However, insufficient SWCC data points may cause a bimodal SWCC to be erroneously interpreted as a unimodal SWCC. Suggested suction levels to determine SWCC in ASTM 6836-02 (2008) are excessive and can be reduced if the type of SWCC that a soil may have can be identified prior to the SWCC test. Bimodal grain-size distribution is a pre-requisite for soils to have a bimodal SWCC but not all soils with bimodal grain-size distribution have bimodal SWCCs. Models have been proposed to estimate bimodal SWCC of soils with bimodal grain-size distribution. However, the criteria used by these models identify soils with bimodal SWCC are not accurate. In this paper, a classification tree to identify bimodal grain-size distribution (GSD) soils with bimodal SWCC is proposed so that SWCC tests can be better planned to obtain sufficient data for correct interpretation of the SWCC. The classification tree was developed using a database of 226 soils with bimodal GSD. An independent data set consisting of 60 SWCCs from extant literature was used to evaluate the classification tree and the criteria proposed by others. The classification tree was shown to outperform the criteria proposed by others for identifying bimodal GSD soils with bimodal SWCC. Recommendation on suction levels for SWCC tests to obtain unimodal and bimodal SWCCs was made for the test methods in ASTM 6836-16 (2016).