One of the most important issues in urban and suburban civil engineering projects is the problem of stabilizing vertical and inclined soil slopes. Nowadays, soil nailing technique is considered as a very popular method for increasing stability of soil masses because of the main reasons such as ease of construction, economic efficiency, and fast installation in very civil engineering projects. The proper design of these structures requires an estimation of lateral earth pressure, which is a function of several factors, such as (a) the type and amount of wall movement, (b) the shear strength parameters of the soil, (c) the depth of excavation and (d) the unit weight of the soil, etc. In this dissertation with use of the finite difference software, FLAC3D6.0 and applying 3-D numerical method, a study has been conducted on soil nail walls in the following fields: · Defining design pressure diagrams for soil nail walls in sandy soils. In this section two categories of design diagrams based on lateral earth pressure for designing structural elements of soil nail walls, has been created and introduced. · Defining a distribution of earth pressure acting on soil nail walls. · Evaluation of the design procedure proposed by the reference manual of Federal Highway Administration of America (FHWA). · Defining an optimum arrangement for soil nails for cases when after the construction of a soil nail wall has been completed, inevitably the excavation need to be continued. In this study, soil nail walls are stabilized using one or two long strengthening nail(s) and the optimum location of them is defined, considering which arrangement gives the highest factor of safety. · Defining an optimum arrangement for soil nails, considering which arrangement gives the highest factor of safety using one or two long strengthening nail(s) under pseudo-static loading. In this study various accelerations applied on six different walls modeled using FLAC3D6.0. In most cases the proposed diagrams were more compatible with the results obtained from modeling using FLAC3D6.0 than other methods. The highest maximum forces of nails belong to the 2 nd and 3 rd rows of nail from the bottom of the wall and the least value always belongs to the lowest row of nail. The lateral earth pressure acting on shotcrete in upper two third of wall’s height estimates the overall force of head nails in this area, 10 times more precise than FHWA’ reference manual. Also the other remarkable results showed that installing the long strengthening nails in lower region of wall (almost 0.8 wall’s height) under pseudo-static loading and in the middle area of wall’s height in two-phase excavation, can yields the most values of factor of safety. Key Words: Lateral earth pressure diagram, distribution of lateral earth pressure, Pseudo-static loading, Finite diference method, Optimum arrangement of soil nails