<p>This study examined the influence of new excavations on adjacent tunnels in structured clays subjected to varying degrees of prior degradation, using a constitutive model that incorporated both structural strength and its degradation. Numerical analyses demonstrated that greater tunnel construction disturbances caused more extensive and severe structural degradation in the surrounding clay. Consequently, larger prior disturbances led to greater excavation-induced ground surface settlement, horizontal displacement of the supporting structure, and segment deformation within the existing tunnel, but smaller segment internal forces after the new adjacent excavation. The effects of prior disturbance were more significant in clays with higher initial void ratios (<i>e</i>₀) than in those with higher structural yield stresses (<i>p</i>’_y). For clays with higher <i>e</i>₀, larger maximum settlements were observed, whereas clays with higher <i>p</i>’_y exhibited smaller maximum settlements under comparable disturbance levels. Structural degradation reduced the mobilized shear strength of the clay between the tunnel and the foundation pit, thereby increasing lateral earth pressure on the supporting structure and amplifying its horizontal displacement and associated ground settlement. These results emphasize the necessity of explicitly accounting for structural degradation induced by prior construction disturbances when assessing excavation impacts on adjacent tunnels in structured clays.</p>