# Bearing Capacity of Shallow Foundations on Unsaturated Silty Soils
## 1 Introduction  
The design of shallow foundations involves two main objectives: determining the ultimate bearing capacity (UBC) and ensuring compliance with allowable settlement limits. Traditional methods for determining UBC and settlement behavior on saturated soils can be divided into two groups: the Effective Stress Approach [1] (ESA) and the Total Stress Approach [2] (TSA). The ESA requires effective soil parameters and uses \(c^{\prime}\) (effective cohesion) and \(\varnothing^{\prime}\) (effective internal friction angle), whereas the TSA is based on the \(\mathcal{O}_{\text{u}}=0\) analysis and is used for undrained conditions. Until recently, UBC values for footings on unsaturated soil layers were determined based on the assumption of saturated soil [1, 2, 3]. However, this assumption is not always accurate and can lead to non-economical solutions. To ensure a realistic design in unsaturated the soil environments, the effect of soil saturation degree (SSD)/matric sution (MS) must be taken into account. MS, which is frequently used as a measure of suction, is a critical characteristic that determines the behavior and mechanical properties of unsaturated soil [4, 5, 6]. Therefore, in geotechnical applications, the the MS value must be taken into account when calculating soil strength and volume changes. Capillary tension is the ability of soil to absorb and retain water and is directly related to water content [7]. Research on the behavior of unsaturated soils, which make up a large portion of the Earth's land, has been important in recent years. The majority of shallow foundations are found in areas of unsaturated soil, depending on the depth of the groundwater table. The SS of unsaturated soils is important in the design of shallow foundations because the failure in these foundations is usually caused by exceeding the UBC value [8; 9; 10; 11]. Therefore, research on the behavior of unsaturated soils has been prioritized over the last ten years. In the literature, experimental studies have been conducted under both drained and undrained conditions [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. In general, research in this area has aimed to improve understanding of the mechanical behavior of unsaturated soils and to develop appropriate methods for assessing the bearing capacity of unsaturated soils. Studies have focused on factors such as matric suction, soil structure, and soil water content and their effects on the bearing capacity of unsaturated soils. Some studies have also investigated the use of various test methods to assess the bearing capacity of unsaturated soils, such as triaxial tests, direct shear tests, and plate load tests. Additionally, numerical models have been used to simulate the mechanical behavior of unsaturated soils. These studies have shown that the UBC values of foundations, calculated using traditional saturated soil mechanics concepts, are quite far from the UBC values of foundations on unsaturated soil conditions. Studies on the estimation of the UBC of foundations on unsaturated soil layers can be grouped under two main groups, which are similar to traditional soil mechanics groupings. These are the proposed approaches to estimate the SS of unsaturated soils under drained and undrained conditions. There are various structural models and modeling techniques that can be used for both approaches, but these models require a variety of soil parameters that necessitate complex and extensive laboratory tests. On the other hand, there are various numerical modeling techniques presented in the literature that are extended by modifying the ESA and the TSA to take into account the effect of MS [11; 12; 30; 31; 32; 33; 34; 35; 36; 37]. The approaches introduced in the literature are called the Modified Effective Stress Approach (MESA) and the Modified Total Stress Approach (MTSA). These methods, which can be estimated with the test results obtained for unsaturated conditions for the MSEA and the MTSA in the literature, are simple and fast.