It is always recommended to improve the properties of the soft soil beneath the railway tracks to increase its ability in bearing different applied loads and to control the expected generated settlements. The most methods used to improve the soil are in using a ballast layers with or without reinforcement by a single geogrid layer or a geogrid layer at different depths. This study presents a three-dimensional finite element analysis for soft soil underneath a ballast railway track by using a finite element program (ANSYS v.11.0) which considered efficient used in many engineering applications and most completeness. Twenty four models were selected using a nonlinear three- dimensional finite element to study the effect of ballast thickness, mechanical properties of soft soil undrained shear strength and modulus of elasticity E, reinforced using geogrid layer to improve the soft soil. Using the element called Solid65 of 8- nodes brick element with three degree of freedom to represent the ballast. And using element called Solid 45 of 8- nodes brick element with three degree of freedom this element is in ANSYS to represent the soft soil and steel plate. Shell element 4- nodes with six degree of freedom (using three degree of freedom only) (Shell 181 in ANSYS) to represent the geogrid layer under and within ballast layer. The results show that increasing the undrained shear strength Cu and modulus of elasticity E will decreasing the settlement of soft soil and increasing the ultimate load. Increasing ballast thickness lead to decreasing the settlement of soft soil and increasing the ultimate load this mean that modulus of elasticity play main role to controlling settlement of soft soil (Ultimate displacement under plate loading) and ultimate load . The results show that the load capacity increase about 7.63% , 7.88 % , 13.80 % for Cu= 9KPa and 3.18 % , 3.27 % ,5.29 % for Cu=25 KPa comparing with reference model ,the settlement decreases about 0.86 % , 0.9% , 1.55 % for Cu =9 Kpa and 0.62% , 0.64% , 1.33% for Cu=25 KPa
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