# Bearing Capacity of Shallow Foundations on Unsaturated Silty Soils
## 2 Materials and Methods
### Method  
In this study, the ultimate bearing capacity (UBC) of shallow foundations on unsaturated soils was investigated. The basic physical properties of silt samples taken from Adapazarn were determined and void ratios were selected for use in experiments. The natural void ratios of silts in Adapazarn were examined [54], and samples with void ratios of 0.65, 0.70, and 0.75 were used in the experiments. The sample SSDs were set at 65%, 70%, 75%, and 85% to account for seasonal groundwater level changes in Adapazarn. The shear strength angle and cohesion values of the silt samples were obtained by using direct shear tests. The SS parameters of unsaturated soils can be measured using a modified MS-controlled triaxial apparatus or a direct shear box (DSB) test [53; 55; 56; 57]. However, these methods can be costly and time-consuming, and the results are not as practical as those from the saturated state. A more widely accepted method is to combine the application of the targeted suction magnitude with a DSB test. Studies have shown that this method is consistent with the results obtained using a modified DSB test capable of MS control [58; 59; 60; 53]. In this study, the shear strength of unsaturated Adapazarn silt was determined using a conventional direct shear test. The samples were prepared at the targeted SSD, and MS values were measured using a pressure plate and filter paper. The shear box test was carried out on a fully automatic direct shearing apparatus with a square shear box \(6\,\times\,6\) cm in diameter and 2.5 cm in height. During the test, the samples were carefully isolated from air to maintain their saturation degree/suction values. The \(\mathcal{O}^{b}\) values of silt soil were calculated from the test results and MS values, and the UBC was determined using these values. The relationship between the WC/SSD and MS is important in evaluating the behavior of unsaturated soils. In this study, the MS values of the samples were determined using the filter paper method and pressure plate method to create the SWCCs. The filter paper method, which was proposed in the 1930s, is an economical but slow method for measuring suction, and it can measure total and the MS values up to 30 MPa [61]. This method uses a filter paper whose calibration curve is known and places it in full contact with the sample to determine pore water flow. The measurement period is 6-8 days. After the moisture balance is achieved, the wet and dry weight of the filter paper is weighed on a balance with a sensitivity of 0.0001 g, and the corresponding MS value is found in the calibration curve according to the filter paper water content. The SWCC of the sample is formed by repeating these processes at different WCs from the saturated state to the air-dried state [28; 62]. In this study, the MS values were determined with the filter paper method by using the calibration curve recommended for the Whatman No. 42 filter paper in ASTM 5298-92 [63]. The pressure plate method is used to apply suction using the axis translation technique [64]. Air pressure (u\({}_{\mathrm{a}}\)) is applied to the samples placed on a ceramic with the high AEV in the steel cell coming from the inlet at the top of the cell, and pore water pressure (u\({}_{\mathrm{w}}\)) is applied trough the inlet connected to the ceramic. This process must be continued until the water flow is balanced [28; 65]. When the water at the output reaches equilibrium after the applied suction, the sample weight/volume is taken and the next suction stage is started. The MS (u\({}_{\mathrm{a}}\)-u\({}_{\mathrm{w}}\)) values were applied in incremental steps from 5 kPa to 1500 kPa. A 1500F1 pressure plate extractor with a 15-bar capacity was used to conduct the pressure plate experiments. The UBC of foundations on unsaturated silt soils was examined by model tests.