Due to the traffic congestion in a main street within the downtown of some cities, a roundabout has been changed to tunnel as a best solution. The objective of the current research is to evaluates the performance of an open tunnel part. The open tunnel part consists of two cantilever retaining walls possessing a combined foundation. The tunnel under study has a width of 25.6 m and a depth of 7.2 m. The open tunnel has been simulated utilizing Plaxis 2D FEM program. The cantilever walls retained and rested on silty sand soil. The excavation of the tunnel has also been idealized. The retaining walls sustains from somewhat high values of lateral displacement at the top. As well as the combined foundation implies high vertical displacement and internal forces values. The geogrid materials have been used as reinforced layers in the backfill soil and in the foundation soil. The reinforced soil with geogrid causes only a small reduction on the foundation vertical displacement. But once applying vertical loads on the foundation, the lateral displacement of the walls, and vertical displacement and the internal forces of the foundation have been minimized. The own weigh of the tunnel floor and the vehicles weight are represented the applied loads. Therefore, the applied loads enhance effectively the performance of the open tunnel part.