## 1 Introduction  
The failure of slopes leads to extensive social and economic consequences and is accompanied by the degradation of the natural environment. A soil nailing system is a proven effective and economic method used to stabilize earth slopes from the external (factors increasing the shear stress) and internal (factors decreasing material strength) failure causes. A review of the literature related to the susceptibility of slope failure is presented by [PERSON] et al. [1]. The comprehensive review of ground improvement techniques using earth nailing systems, including system installation and construction, failure modes of soil-nailed structures and effects of various construction parameters on the nailed soil slope design method, is presented by [PERSON] et al. [2]. The guidelines are suggested by [PERSON] [3] to improve the design procedures for soil-nailed structures considering the lateral pressures generated by soil expansion. The soil nailing system is used to stabilize earth slopes by installing closely spaced nails in the ground in situ. A soil-nailed slope essentially consists of three elements, namely, the soil mass, the slope facing and steel reinforcement (nails). The deformations behavior of the soil nailing system depends on the composite interactions between the soil nailing system elements. The slope facing provides the local stability of the ground between the nails and limits its decompression in the soil nailing system [4]. [PERSON] et al. [5] have proposed a measurement procedure for the crest displacement of the soil-nailed wall and concluded that effective soil cohesion has the greatest effect on crest displacement. [PERSON] and [PERSON] [6] have studied the influence of bending stiffness of facing material, installed on a nailed slope surface of dense sand, on deformation and bearing capacity characteristics by conducting the tests on rigid and flexible models of footings. Various experimental and numerical models of nailed soil structures have been studied for the evaluation of structural performance and the development of reliable design methods for nailed soil structures. [PERSON] et al. [7] have developed numerical models based on the material point method to investigate the application of plate anchors for landslide stabilization. The numerical model has been validated with the results of some small-scale laboratory tests. [PERSON] et al. [8] have carried out the shaking table tests to study the seismic behavior of nailed soil slopes. A simplified limit equilibrium method is presented by [PERSON] et al. [9] to analyze the stability of slopes reinforced with anchor cables considering nonlinear Mohr-Coulomb strength criterion for the shear failure of the slip surface. The laboratory models have been developed by [PERSON] et al. [10] to study the effects of surcharge loading and nails characteristics in Nailed Soil Slope behavior. They found that the increase in length and inclination of soil nails decreases the vertical, horizontal stress and footing settlement, while the increase of spacing of nails increases the vertical and horizontal stress behind the soil mass. [PERSON] and [PERSON] [11] have studied the lateral displacements, nail forces and failure modes of nailed soil vertical slope under both static and seismic conditions. Numerical analysis of stabilizing mechanisms of loose-fill slopes using hybrid nail arrangement has been carried out by [PERSON] et al. [12], who studied the influence of hybrid nail orientations on the behavior of the ground nailing system and concluded that a hybrid nail arrangement would enhance the robustness of the ground nailing system. [PERSON] [13] has applied [PERSON]'s Simplified method to evaluate and rank the sensitivity of the soil cohesion, unit weight and internal friction angle with respect to the global factor of safety using Monte Carlo simulation in a soil nailing design.