Abstract: Tunneling in urban areas might cause damage to the adjacent buildings, and mitigation methods are required to protect those buildings. The effect of separation pile and diaphragm wall is studied using 3D numerical simulation. A constitutive model for small strain of soils is implemented in FLAC^3D and then verified by simulating a small strain triaxial test and a centrifuge test. The main factors are analyzed, and it is revealed that the length of the pile and the wall has the most significant influence on the mitigation effect. To achieve satisfactory mitigation effect, the toe of the pile/wall should be located at least half of the tunnel diameter deeper than the bottom of the tunnel. The pile diameter, pile distance and wall thickness are found to have much less influence on the mitigation effect. Furthermore, the pile and diaphragm wall are found to have similar mitigation effect.