Vapor Adsorption Index for Expansive Soil Classification

A laboratory index defined as gravimetric water content at ambient relative humidity of 75% $\left(w_{75}\right)$ is proposed for qualitatively classifying the swelling potential of clays and clayey soils (e.g., low, moderate, high). The methodology is calibrated by comparison with existing plasticity-based and suction-based classification methodologies for a series of natural clays from Missouri and Colorado and clay mixtures prepared to represent a wide range of swelling potential. Procedures are described for obtaining $w_{75}$ by placing samples in the headspace of an environmental chamber maintained under controlled humidity using a saturated NaCl solution. The proposed methodology has potential advantages over existing expansive soil classification methods because a large number of samples may be tested concurrently, no specialized testing equipment is required, and testing procedures may be readily automated. Measurements may be obtained in as little as $3–5\text{days}$ with coefficients of variation ranging from 5 to 25%.