<p>This research conducted cyclic triaxial tests to develop a user-specified constitutive model of sand for evaluating interactive responses (i.e., moment and displacement) of the tunnel–sand–pile interaction (TSPI) model. For this purpose, the laboratory test results are validated against the previous experimental findings of Fuji River sand and constitutive laws to obtain variations of 2.9% and 1.64%, respectively. The tunnel moments of the proposed model are varied at rates of 26.6% and 15.8% from the experimental and numerical studies on the TSPI model. These differences are obtained for the 27% relative density of sand. The parametric study shows the incremental tunnel moment by varying relative clearance and cover for 27% and 41% relative densities. Also, the sand displacement shows a higher value for these relative densities. So, these densities may not be suitable for the design of the tunnel because of higher moment and displacment. On the other hand, the higher relative clearance and lower relative cover may be used for the design of tunnels and underground structures in sand due to the lower sand displacement. However, the user-specified constitutive model may help engineers to evaluate responses of the TSPI model.</p>