<p>The rapid development of urban metro systems in China has prompted the high-intensity utilization of underground space. This trend has led to a continual reduction in the minimum separation between new and existing tunnels, making adjacent construction effects increasingly pronounced and the impact of new tunneling on existing structures inevitably more severe. This study supplements and refines the theory of loosening earth pressure under an incomplete soil arch effect. It proposes that for the theoretical calculation of overburden pressure on shield tunnels, particularly in sandy soils and stiff clays where the soil arch effect is considered, the loosening earth pressure theory under an incomplete soil arch effect can be adopted. Furthermore, based on numerical simulation experiments, this paper systematically analyzes the influence of new tunnel construction on the deformation at various locations of the wall of an adjacent existing tunnel, revealing the torsional characteristics exhibited by the existing tunnel. The results indicate that the torsion of the existing tunnel decreases as the spacing between the new and existing tunnels increases. The relationship between the rotation angle and the spacing follows a quadratic polynomial function, with a maximum rotation angle of 0.14°. Prediction models for the maximum horizontal and vertical displacements of the existing tunnel were established for scenarios where it is located horizontally, diagonally above, and directly above or below the new tunnel. The findings of this research provide theoretical guidance for assessing the impact of new tunnel construction on the stability of adjacent existing tunnels during the tunnel design process.</p>