## 2 Materials and Methods  
During the study, a representative typical slope model and representative soil properties were used to investigate the impact of antecedent conditions on the slope stability. The slope stability analyses were carried out by using a numerical program, Fast Lagrangian Analysis of Continua (FLAC), which is a finite difference code software [26]. FLAC is able to run two-phase flow (tp-flow) of water and air in a 2D environment representing an unsaturated soil and computes the stability of a slope through deformation analysis. The slope geometry was also chosen as the one used in FLAC manual (see Figure 1). The model is 60 m wide and has a height of 30 m on the left side and 10 m on the right side. The relatively flat parts are both 20 m on the left and right sides. The highest slope angle is 45\({}^{\circ}\) with a former secondary 27\({}^{\circ}\) angle. The analyses were conducted by using two different soils. One of them was the soil given in the manual of FLAC (FLAC sample) and the other soil was obtained from a natural slope (this study). The average saturated hydraulic conductivity of the soil used in this study was measured by a flexible wall permeameter with falling head procedure [27]. The soil sample was taken directly from the natural slope origin and carefully trimmed to fit in the permeameter. During the tests, the confining stress was 35 kPa, the average hydraulic gradient was 10, and no backpressure was applied. In addition, the soil was left for prehydration for the first 24 h to obtain a homogeneous diffusion environment. Tests were terminated regarding the conditions of consecutive measurements of hydraulic conductivity being less than 25% and the ratio of the inflow and outflow being constant at least for the last 3 measurements. The suction measurements were carried out by using filter paper method [28]. A bio-incubator-type incubator (Santez SI-150K, Santez, Istanbul, Turkive) was used, and the equilibrium time was 1 week. Following the suction measurements, the unsaturated soil parameters were determined by constructing the SWRC of the soil in SWRC-Fit [29; 30]. SWRC-Fit is a general model for multimodal unsaturated soil hydraulic properties. It can fit several soil hydraulic models to laboratory- or field-measured soil water retention data. This study used the van Genuchten model and constructed the SWRC of the soil as matric  
Figure 1: The slope model. suction related to the volumetric water content. The constructed SWRC is given in Figure 2. To obtain the saturated mechanical properties of the soil, conventional direct shear tests were run for soaked conditions [31]. The geotechnical and hydromechanical properties of the soils used during the modeling study are presented in Table 1 as used in FLAC. Any relevant details of each test can also be found in the study of Durukan and Basar [32]. When studying with FLAC for rainfall-triggered slopes, the first case is to run the seepage calculations which give the pore water distributions, and the second case is to combine the case with deformation analysis, determining the FoS. For seepage calculations, the left, right, and bottom boundaries are set as impermeable boundaries. The surface of the slope is the rainfall infiltration boundary and set as the flow boundary. The rainfall infiltration boundary depends on the relation between the rainfall intensity and the soil infiltration capacity. If the rainfall intensity is lower than the infiltration capacity, then the boundary is flow boundary allowing the water to flow through the surface. However, if the rainfall intensity exceeds the infiltration capacity, then a runoff mechanism would ensue. For this condition, in order to prevent the accumulation of water at the surface, the boundary was fixed. The unsaturated parameters for seepage calculations were van Genuchten characteristics, and, for stress and strain calculations, model had the Mohr-Coulomb criterion, which allows for normal iterative computation.