# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
## 3 Results and Discussion  
### Direct Shear Test  
Samples subjected to the direct shear test were reconstituted at 90% relative compaction and optimum moisture content to mimic the soil condition at maximum dry density closely. A fully saturated condition was simulated on these test specimens to identify stress-strain behavior at its weakest or most compressible form. In this state, moisture may exert pore water pressure on soil particles, thus influencing the deformation and strength of soil. Although pore water pressure dissipates with time, this depends on the specimen's permeability. Coarse soils such as sands and gravels create void spaces between grains, facilitating easier moisture flow. In contrast, slightly impermeable media such as expansive and other cohesive soils may dissipate pressure significantly. The direct shear apparatus cannot measure pore water pressure within the shear box; hence, it is necessary to perform a slow loading test to prevent the development of these pressures. ASTM D3080-11 provides equations for determining the shearing rate required for complete pressure dissipation. The elapsed time for failure may be determined at 50% or 90% consolidation, depending on whether log time or root time curve was used. The calculation identified a shearing rate of 0.151 mm/min.  
The stress-strain behavior and volumetric change observed in the soil sample and its treated variants are shown in Figures 4-6. Positive values in the volumetric strain plots indicate compressive volume change. All specimens subjected to the direct shear test manifested a direct correlation between effective normal stress and shear stresses at failure. In this study, trials with an applied effective stress of 54.50 kPa obtained the most significant stress values for each additive proportion. Samples also exhibited a progressive increase in shear stress, which resulted in a nearly constant value; hence, no definitive peak stress was exhibited. The absence of a peak is most prominent among normally consolidated clays. This type of stress-strain constitutes ductile failure in soils.  
Similar levels of increase in shear strength were observed with both gypsum-only and gypsum + RHA samples as shown in Tables 7 and 8. The rise in gypsum content among these specimen results corresponds to more compact samples. The gypsum-only samples exhibited a steady increase in strength. The 15% gypsum samples attained the highest shear strength values among all tested specimens. However, samples with RHA slightly lowered in strength compared to their gypsum-only counterparts. The same trend in values was observed with the specimen's failure friction angle. The friction angle increased in value for all gypsum-only specimens and up to 5% gypsum + 10% RHA. The friction angle was 33.9\({}^{\circ}\) for untreated soil and rose to 41.7\({}^{\circ}\) for soil + 5% gypsum + 10% RHA. However, the friction angle decreased significantly for the remaining samples with RHA, reaching a friction angle of 35.2\({}^{\circ}\) for soil with 15% gypsum + 10% RHA. In theory, more compact samples should be able to obtain higher strength values as expected from the specimens with RHA. Still, non-cohesive porous rice husk ash introduced more moisture in the specimen during the saturation process, offsetting the expected increase in strength by adding gypsum. Moreover, samples with RHA have less expansive particles, which may have improved cohesion and, consequently, the strength of the specimen.  
Figure 4: Comparison of the experimental and model-predicted stress-strain and volumetric strain-shear strain plots of fine-grained soil without treatment in a saturated condition.  
Figure 5: Experimental and model-predicted shear stress vs. shear strain and volumetric strain vs. shear strain behavior for samples of fine-grained soil with gypsum as the primary additive.