# Influence of Degree of Saturation on Soil-Pile Interactions for Piles in Expansive Soils
###### Abstract  
Conventional designs of pile foundations for houses on expansive soils adopt conservative approaches by using swelling pressure measured in oedometer tests to compute pile uplift force. However, in practice, piles are often installed in unsaturated soils, where changes in moisture content influence soil behavior. Increasing moisture in expansive soils reduces matric suction, increases soil volume, and induces swelling pressure, all of which affect uplift shear stress. This study investigates the impact of varying degrees of saturation on pile uplift force through a series of laboratory tests on single-pile models. The results of the experimental investigation indicate that uplift force developed along the pile shaft due to the wetting of expansive soils exhibits a hyperbolic trend. A significant portion of the uplift force developed during the early stage of the heaving process. Back-calculation analyses using theoretical equations reveal that the coefficient of uplift, \(\alpha\), and the swelling pressure ratio, \(\beta\), increases as the initial degree of saturation of soil specimens increases, with a change of less than 10% within the tested range. These findings suggest that constant values of the \(\alpha\) and \(\beta\) parameters can be used for pile design in expansive soils, even under unsaturated conditions. Nonetheless, the influence of other factors, such as pile dimensions, pile materials, and soil properties, on the \(\alpha\) and \(\beta\) values should be investigated to improve the accuracy of pile design in expansive soil conditions. p 2025 12 June 2025 13  
methods [4]. The depth of the design active zone is defined as the depth (zone) of soil expected to become wetted by the end of the design life [5]. In the total stress method, the interface shear stress is equal to an empirical coefficient multiplied by the undrained shear strength of the soil [6; 7]. [PERSON] et al. (2022) [8] demonstrated that total stress analysis provides results that closely align with observed data. In the effective stress method, the interface shear stress is calculated based on the normal effective stress on the pile, soil cohesion, and the interface friction angle between the pile shaft and the soil [5; 7; 9; 10; 11; 12; 13]. The effective stress method is commonly used in geotechnical engineering practice and will be discussed in further detail in Section 2 of this paper. The analytical and finite element methods can be used to evaluate elastic solutions between the movement of the soil and the pile system. [PERSON] and [PERSON] (1980) [10], [PERSON] and [PERSON] (1992) [14], and [PERSON] et al. (2015) [5] presented design charts for evaluating the uplift force and movement of piles in expansive soils. The effective stress method commonly used in geotechnical engineering practice utilizes the swelling pressure measured in oedometer tests to compute the pile uplift force. In these tests, soil specimens are inundated with water in an oedometer to measure the swelling pressure of expansive soils. However, this conventional design practice, based on fully wetted conditions, may not fully capture the behavior of piles in unsaturated expansive soils. Changes in moisture content can significantly impact the soil's volume, swelling pressure, and matric suction, consequently influencing the uplift shear stress in pile design [15; 16; 17; 18]. Limited research has quantified the influence of saturation changes on pile design in expansive soils. To address this knowledge gap, this research paper presents a comprehensive investigation into the influence of moisture content on pile design for expansive soils. [PERSON] et al. (2024) [19] previously examined the behavior of pile-soil interaction in expansive soils subjected to water infiltration. On the other hand, the primary objective of this present study is to determine the relationship between moisture content, swelling pressure, matric suction, and pile design parameters under unsaturated conditions.