<p>This study investigates the deformation mechanisms of an existing tunnel adjacent to a deep basement excavation supported by circular beams in soft clay deposits. Initially, a well-instrumented basement in soft clay is back-analyzed using three-dimensional numerical modeling with the Hardening Soil model with small-strain stiffness (HSS). The computed lateral wall movements and ground surface settlements show good agreements with field measurements, validating the numerical model and model parameters. Subsequently, a comprehensive parametric study is conducted to examine the effects of tunnel cover depth (<i>C/H</i>_<i>e</i>), tunnel-basement clearance (<i>S</i>/<i>H</i>_<i>e</i>), and basement geometry (G/<i>H</i>_<i>e</i>) on tunnel responses. The results indicate that the lateral tunnel movements are more significant than settlements, with the maximum values typically occurring at the basement centerline. Tunnel movements increase with decreasing <i>S</i>/<i>H</i>_<i>e</i> to peak values when <i>C</i>/<i>H</i>_<i>e</i> is approximately 0.6. A linear relationship is observed between tunnel movements and basement geometry. A simplified design chart is developed to categorize tunnel deformation influence zones. This chart provides a practical tool for engineers to assess excavation-induced tunnel deformations and implement necessary protective measures during the design of deep basements adjacent to existing tunnels.</p>