## 3 Results and Discussion
### Effect of Sand Content  
The effect of varying percentages of added sand (\(FS\)) on the elasticity modulus and unconfined compressive strength is investigated by setting the maximum dry density for each percentage of added sand. Figure 14A,B are scatter plots with a series of lines, with each line representing the trend for a specific degree of saturation (\(SR\)) within the soil mixture. Figure 14 displays the change in unconfined compressive strength and elasticity modulus as a function of the percentage of sand for each degree of saturation. The results indicate that an increase in the percentage of sand leads to an increase in both elasticity modulus and unconfined compressive strength up to a certain percentage related to the degree of saturation. However, this tendency of increase becomes less significant with the increase in the degree of saturation. Moreover, after reaching a specific percentage of added  
Figure 13: The change in \(q_{u}\) and \(E_{t}\) with \(SR\) according to \(Fs\); (**A**) the changes in \(q_{u}\) with \(SR\) according to \(Fs\); (**B**) the changes in \(E_{t}\) with \(SR\) according to \(Fs\).  
sand, the strength decreases. This percentage is about 30% for high saturation degrees, where the percentage of increase in \(E_{t}\) and \(q_{u}\) is about 75% and 65%, respectively. Notably, this critical percentage increases with a decrease in the degree of saturation.  
Figure 14: The change in \(q_{u}\) and \(E_{t}\) with \(F\)s according to \(SR\): (**A**) change in \(q_{u}\) with \(F\)s according to \(SR\): (**B**) change in \(E_{t}\) with \(F\)s according to \(SR\).  
This behavior can be explained by the suction forces within the soil. The increase in density increases suction forces, while an increase in saturation and sand percentage decreases suction forces. For instance, in Figure 14A,B, for 90% or greater saturation, both strength and stiffness increase with the percentage of sand due to the increase in maximum dry density. However, after about 30% of the added sand, there is a greater decrease in suction forces due to the increase in the percentage of sand, which overcomes the increase in suction forces caused by the increase in dry density. The study found that there is a substantial rise in suction forces when the saturation degree decreases, resulting in a continuous increase in unconfined compressive strength for larger percentages of the added sand.  
Unconfined compression tests on mixtures corresponding to the optimum moisture and maximum dry density according to standard Proctor's tests revealed that the strength and elasticity modulus increase with the percentage of sand. The reason for the continued increase in strength and elasticity modulus is that the saturation degree corresponding to the optimum moisture is less than 90% for added sand percentages greater than 30%. Additionally, it is worth noting that there is an inflection point between 20% and 30% of the added sand, after which the increase becomes less severe, as shown in Figure 15. As a result, a sand percentage of 30% can be considered the ideal percentage in clay soil. The percentage of increase in the unconfined compressive strength and elasticity modulus when the sand percentage increases from 0 to 30% is approximately 68% and 81%, respectively.