The swelling behavior of expansive clays is widely investigated in the literature, while fewer studies were focused on their collapse behavior under high-stress level during wetting. This paper investigates the development of one-dimensional volumetric strain ( ε ) (1) with time during soaking and (2) with gravimetric water content ( w ) during wetting over a wide range of initial densities ( ρ d ) and vertical stress ( σ v ) levels, for an expansive clay collected from Nanyang, China. It was found that (1) specimens with high ρ d tend to swell during wetting or soaking under low σ v . Swelling gradually transfers to collapse with increasing σ v and decreasing ρ d . The amount of swelling or collapse is sensitive to the σ v and ρ d ; (2) collapse and swelling ε –log( t ) relationships during soaking are similar and have three distinct stages. They are well described by a unique model; (3) the speed of the volumetric change during soaking is only sensitive to the σ v and ρ d when the ρ d is higher than a threshold value (1.5 Mg/m 3 for the investigated clay); (4) during soaking, relationships between the final ε , ρ d , and log( σ v ) are linear and continuous from swelling to collapse and are described using a linear equation; and (5) the collapse and swelling ε – w relationships during wetting have similar nonlinear characteristics. A simple model was proposed to predict all measured swelling and collapse ε – w relationships under various σ v and ρ d levels using only one set of parameters. Experimental data and models presented in this study contribute to better understanding and predicting the volumetric behavior of expansive clays for the long-term stability and serviceability evaluation of geotechnical structures.