In the construction of deep excavations in urban areas, the safety of adjacent ground and structures becomes major concern for engineers. In soft clays, the main reason for occurrence of large deflections of soil support systems in excavations is instability of the excavation base. This paper will focus on analyzing and comparing design of an excavation with and without jet grout improvement applied to the excavation base by employing the finite-element code PLAXIS. A well-documented case study is analyzed, and soil properties in the model are calibrated using available field data from inclinometers. Then the same analysis is repeated for the improved case and results are compared. The results reveal the effectiveness of jet grout blocks in decreasing lateral wall movement, base heave and surface settlement on the retained side.