<p>Urban spaces are becoming increasingly congested, and excavations are frequently performed close to existing underground structures such as tunnels. Understanding the mechanical response of proximal soil and tunnels to these excavations is important for efficient and safe underground construction. However, previous investigations of this issue have predominantly made assumptions of plane-strain conditions and normal gravity states, and focused on the performance of tunnels affected by excavation and unloading in sandy strata. In this study, a 3D centrifuge model test is conducted to investigate the influence of excavation on an adjacent existing tunnel in normally consolidated clay. The testing results indicate that the excavation has a significant impact on the horizontal deformation of the retaining wall and tunnel. Moreover, the settlements of the ground surface and the tunnel are mainly affected by the long-term period after excavation. The excavation is found to induce ground movement towards the pit, resulting in prolonged fluctuations in pore water pressure and lateral earth pressure. The testing results are compared with numerical simulations, achieving consistency. A numerical parametric study on the tunnel location shows that when the tunnel is closer to the retaining wall, the decreases in lateral earth pressure and pore water pressure during excavation are more pronounced.</p>