A Novel Prediction Model for Estimating Ground Settlement Above the Existing Tunnel Caused by Undercrossing

A new tunnel undercrossing an existing tunnel not only affects the deformation and stress response of the existing tunnel but also triggers ground settlement due to secondary excavation disturbances. By combining the equivalent layer method with the mirror method and incorporating corrections from numerical simulations based on actual intersection projects, a novel prediction model is developed to consider the impact of the existing tunnel on estimating ground settlement caused by a new tunnel that undercrosses it in an orthogonal manner. The influence of geological conditions, tunnel dimensions, and spatial layout on ground settlement patterns was investigated. The elastic moduli of smaller strata correlate with greater surface settlement. Larger existing tunnel diameters result in reduced settlement within a 15 m area near the new tunnel axis. Conversely, new larger tunnel diameters yield more pronounced settlement. A consistency assessment method was introduced to quantitatively measure the consistency between the prediction model and numerical simulations. The results indicate that the prediction model exhibits high consistency (CI > 0.9) under various conditions. Based on an actual engineering case, indoor similarity model tests were designed. When the new tunnel is directly located beneath the existing tunnel, ground settlement begins, with a maximum settlement of 0.17 mm. After the new tunnel traversed the existing one, ground settlement continued to increase within approximately 50 m on both sides of the new tunnel’s axis, ultimately reaching a value of about 0.765 mm. The CI between the predictive model and the model test results reached 0.8, confirming the model’s robust predictive capability.