# Influence of Suction on Capacity of Pile Foundation
## 1 Introduction  
Fully saturated conditions of the soil can be reached whenever voids are filled with water. On the other hand, if voids are partially filled with gas (mostly air), states are considered unsaturated [1, 2]. The thickness of the unsaturated soil zone depends on the specific area's climatic conditions, where the net moisture is calculated as the difference in precipitation and evaporation rate [3]. As stated by [PERSON] et al. [4], the zone in the middle of the ground surface and the water table is called the \"vadose zone\". It can be seen that the unsaturated zone is divided into a dry zone, two-phase, and capillary zones, where the degree of saturation differs with the depth [5, 6]. In the common practice of geotechnical design, the state of soil is considered fully saturated, while recent examples show the importance of the consideration of unsaturated conditions as well due to the location of the groundwater table at deeper levels [7, 8]. However, it is problematic to design a foundation and calculate bearing capacity, as the internal stress in unsaturated conditions needs to be quantified [9]. Therefore, this paper dwells on the challenges of the design in unsaturated conditions, as well as the different approaches for calculating the pile foundation's matric suction, bearing, and shaft capacity. The mechanics of unsaturated soils are critical in geotechnical engineering, especially when the case of the foundation design is required [7]. Unsaturated soils have four major difficulties that are the source of errors in making assumptions for the calculation of the bearing capacity of any foundation [10]. The challenge of quantifying the internal stress in these conditions, applying the obtained results on-site cases, and obtaining the laboratory data from a limited number of sources seriously affects the safety of the design [11]. The ending problem could be a tension crack, an unexpectedly large settlement, and even a failure of the foundation structure. It is an often-used practice to detect the v-shape and develop from the sides to the center cracks along the foundation in any residential (tall or small) building [12]. The problems with those cracks are the saturation levels of the expansive soil, namely expansive clay layers, below the building. The change in the volume occurs due to the change in the moisture content of the clay that can be triggered by rainwater infiltration. Subsequently, a non-uniform water content can cause destabilization of the soil and lead to an upward-downward movement of the foundation in cases when the swelling pressure is large [12; 13]. On the contrary, when the soil undergoes shrinkage, the foundation and the building subside, creating larger cracks in the foundation. Moreover, underground water flow is non-homogeneous [14; 15]. The practical challenge associated with this water movement in the ground is settlement. Uneven increases in the saturation of soil in different layers can lead to an upper layer's weight being greater, which leads to an increase in the total pressure of the soil [16; 17]. Within the lifetime of the building, creep deformation of the foundation in soft soils may gradually occur [18]. Soil creep causing slope instability and settlement may adversely affect the engineering construction [19]. On a microscopic scale, within a creep process, the breaking of particles is initiated, and the particles' spacing decrease, thereby decreasing the volume of pores and increasing their quantity [20; 21]. Additionally, the effect of the topographic irregularity should also be considered in soil-structure interaction (i.e., pile-soil interaction) analysis [22; 23] to reach the accuracy of future deformation of the foundation. Based on a review of the literature, there are only limited studies investigating the effect of suction changes on the bearing capacity of pile foundations utilizing common \(\alpha\), \(\beta\), and \(\lambda\) methods.