## 3 Experimental Results  
The testing program produced a comprehensive dataset that characterizes the anisotropic swelling behavior of the expansive soil under varying stress conditions. Here, we present the key findings from the analysis of vertical swelling strain and lateral pressure responses measured across a range of controlled suction reductions.  
Figure 4(a) illustrates the variations in the vertical swelling strain with matric suction reduction observed across all net normal stress conditions for the OVS and OHS test series. Several consistent trends were observed. Firstly, the swelling strains increased with decreasing suction as the soil moisture content rose for both test orientations. This behavior is consistent with the typical mechanisms of expansive soils gaining volume as water enters the diffused double layer of smectite clay plates.  
Secondly, the strains in the vertical direction were significantly larger compared to the horizontal direction. At full saturation under zero net stress, the maximum vertical swell measured 8.54% in OVS, whereas only 4.99% horizontal swell occurred in the OHS tests. The vertical swelling strain in OVS was about 1.71 times that in OHS. This confirms the presence of intrinsic anisotropy, where clays expand more readily out of the plane of their layered structure due to the influence of depositional packing and consolidation stresses.  
Thirdly, higher net normal stresses resulted in reduced vertical swell strains as confinement restricted volume changes. However, strains progressively increased with desaturation across all stress conditions, indicating a complex interaction between the applied stresses and changing suction states.  
Figure 4(b) illustrates the variations in the measured lateral confining pressure with suction reduction. Once again, suction decreases induced growing pressures across all test  
Figure 5: Variation in the (**a**) vertical swelling strain and (**b**) lateral pressure under different net normal stresses with matric suction.  
series as soil expansive forces developed against the constant lateral confinement. Peak pressures often occurred within the initial high suction range above 50 kPa as the volume occupied by the adsorbed water layers increased the thicknesses of the diffused double layers.  
Interestingly, lateral pressures exhibited the opposite behavior to vertical strains, with higher initial responses developing under elevated net stresses. For instance, maximum pressures of 44.7 kPa and 70.0 kPa were obtained at full saturation under zero net stress for OVS and OHS tests, yet these grew substantially larger under confinement. This confirms that confinement amplifies both vertical and lateral swelling capacities through stress-suction coupling effects. In addition, it should also be noted that the lateral pressures measured under zero net stress can be seen as pure swelling pressure, where the swelling pressure in the vertical direction (OHS-0) was \(1.57\) times that in the horizontal direction (OVS-0), also demonstrating an obvious swelling anisotropy as with swelling strain.  
Notably, the OHS tests consistently produced higher lateral pressures than OVS runs across all suction-stress conditions, highlighting the greater development of expansive forces in the horizontal direction related to microstructural anisotropy.  
In summary, the test results clearly demonstrated that the expansive soil exhibits anisotropic volumetric swelling strongly influenced by the stress state, moisture condition, and orientation of the layered mineral structure. These behaviors have significant implications for properly assessing and accommodating earth and swelling pressures in geostructure designs situated within expansive ground.