###### Abstract  
Expansive soils exhibit directionally dependent swelling that traditional isotropic models fail to capture. This study investigates the anisotropic swelling characteristics of expansive soil with a medium swelling potential through the use of modified oedometric testing. Vertical swelling strains can reach up to 1.71 times that of the horizontal movements, confirming intrinsic anisotropy. A nonlinear elastic constitutive model incorporates vertical and horizontal elastic moduli with respect to matric suction to characterize anisotropy. Three elastic parameters were determined through the experiments, and predictive equations were developed to estimate the unsaturated moduli. The constitutive model and predictive techniques provide practical tools to better assess expansive soil pressures considering anisotropy, offering guidelines for utilization and design. The outcomes advance understanding of these soils' directionally dependent behavior and stress-strain-suction response. Acticle Laboratory Test and Constitutive Model for Quantifying the Anisotropic Swelling Behavior of Expansive Soils  
Zhengnan Liu, Zheng, Rui Zhang  
retaining walls, basements, and other structures, leading to structural cracking and failures if not adequately considered in their design [10]. Conversely, during dry periods, expansive soils shrink, contributing to differential settlement issues [11]. Therefore, there is a need to evaluate the swelling behavior of expansive soil and accurately estimate the earth pressure exerted by expansive soil against retaining structures, which is crucial for analyzing slope stability and designing retaining structures in expansive soil areas. Previous research has primarily focused on characterizing the swelling behavior and stress-strain response of expansive soils through laboratory testing and constitutive modeling. Various empirical and mechanistic models have been developed to describe the volume change phenomena of expansive clays, capture hydromechanical coupling behavior, and enable numerical analysis and design applications [12; 13; 14]. For instance, [PERSON]'s model [15] is popular due to its simplicity, as it relates stress and strain conditions to net stress and matric suction states. The highlights are also focused on the acquisition and prediction of the elastic modulus [16; 17]. However, many traditional models assume that expansive soils exhibit isotropic mechanical properties despite evidence that anisotropy is intrinsic to their fabric and swelling at the microstructural scale. The directional arrangement of clay minerals is known to influence the development and magnitude of swelling strains and pressures. Recent studies have observed distinct anisotropic characteristics between vertical and horizontal swelling of intact expansive soil specimens [18; 19; 20]. From the microstructure analysis, flaky clay particles within expansive soils are inclined to orient horizontally after compaction; this preferred orientation phenomenon becomes more apparent with an increase in static pressure, consequently leading to significant differences in swelling behavior [21]. Several researchers have developed experimental methods for determining lateral swelling pressure by modifying traditional oedometer and hydraulic triaxial apparatus [22; 23; 24; 25; 26]. By using the modified oedometer apparatus, researchers have consistently found that measured lateral pressures exceed those in the vertical direction under inundation conditions. Additionally, the ratio of swelling pressure in the vertical and lateral directions varies with the surcharge, moisture, and density of the soil specimens [27; 28]. This inherent material anisotropy is not fully considered in common isotropic constitutive formulations. Simplified elastic analysis of earth pressures acting on retaining walls may produce non-conservative estimates if true directional deformation behavior is not properly characterized.