# Influence of Suction on Capacity of Pile Foundation
## 4 Results
### Matrix Suction Variation  
Matrix suction is the most significant physical variable comprising unsaturated soil behavior. It is the energy change in the water condition transferred from its soil to a free state. In other words, matric suction can be defined at the soil-water-air representative elementary volume and represented as the difference between pore air pressure and pore water pressure (Equation (27)) [32].  
\[\psi_{m}=u_{a}-u_{w} \tag{27}\]  
The increase in soil moisture content clarifies the effect of rainfall infiltration on matric suction. Values of matric suction decrease with the increase in the rainfall volume. According to the in situ test conducted by [PERSON] et al. [33], rainfall significantly affected the matric suction, mostly in the upper layers of the soil.  
Although the experiment was performed to show slope stability, the definition of decreasing matric suction can also be implemented in this study. A decrease in matric suction leads to a decrease in bearing capacity, which can negatively affect the stability of the foundation.  
To show the effect of the variation in matric suction, the original data were decreased by 100%, 75%, 50%, and 25%. The reduction in the matric suction is related to a total cohesion method, where the matric suction is considered as a \"percentage of hydrostatic pore water pressure\" [34]. The infiltration of the rainwater can decrease the matric suction in the upper layers and affect the factor of safety [34]. Figures 5-10 show the comparison of the resultant values for shaft bearing capacities.  
Reduction in the matric suction tends to decrease the pile capacity in both Type 1 and Type 2 cases. For the Type 1 pile, the effect of suction decrease does not severely affect the Qf except the great decrease in the values at the depth of 2 m for the modified \(\alpha\) and \(\lambda\) methods. For Type 2 soil, the changes have the same trend and are noticeable at the depth of 4 m for the modified \(\alpha\) and \(\lambda\) methods. According to the obtained results, changes in suction mostly affected the clay layers, where the capacity of the pile in Type 1 soil using the modified \(\alpha\) and \(\lambda\) methods decreased by 24% and 22%, respectively, on average for every 25% decrease in suction. The same trend is noticed for the Type 2 soil at 4 m depth,  
Figure 5: Pile bearing capacity in unsaturated soil Type 1 using modified \(\alpha\) method vs. depth with variating matric suction.  
pile capacity dropped by 21.5% using the modified \(\alpha\) and 17% using the modified \(\lambda\) for every 25% change in matric suction. The sudden change in the pile capacity is explained by a fitting parameter \(\mu\). Estimations of \(\mu\) were made according to [PERSON] and [PERSON] [35], where the relationship of the fitting parameter and plasticity index was derived based on limited data and usage shall be considered only with supporting evidence [35].  
Figure 6: File bearing capacity in unsaturated soil Type 1 using modified \(\beta\) method vs. depth with variating matric suction.  
Figure 7: File bearing capacity in unsaturated soil Type 1 using modified \(\lambda\) method vs. depth with variating matric suction.