A simplified method to estimate three-dimensional tunnel responses to basement excavation

For the convenience of shoppers and users, there is an increasing demand for the construction of basements in close proximity to existing tunnels. To ensure the safety and serviceability of the existing tunnels, attention has been paid to the basement-tunnel interaction. However, a simplified method for direct estimation of tunnel heave and tensile strain due to basement excavation is not available. In this paper, a systematic numerical parametric study is conducted to develop a simplified and approximate method to enable practicing engineers to estimate tunnel response due to overlying basement excavation. This method only considers the effects of excavation geometry, excavation depth, cover-to-tunnel diameter ratio and sand density on tunnel response to basement excavation. An advanced soil model, a hypoplastic sand model, is adopted to simulate soil behaviors. The soil model and soil parameters are calibrated and verified by centrifuge test results. For the tunnel crown located 0.5-1.5 D (i.e., diameter) below the formation level of the basement, calculation charts of excavation geometry versus tunnel heave and tensile strain at two relative sand densities (i.e., 68% and 90%) are developed for estimating tunnel responses due to basement excavation. Because denser sand has larger soil stiffness, excavation-induced tunnel heave and tensile strain are found to decrease almost linearly with relatively sand density (Dr) when it varies from 30% to 90%. Thus, excavation-induced tunnel heave and tensile strain at other soil densities can be linearly interpolated by using the proposed calculation charts. An example is provided to illustrate the application of the calculation charts. The tunnel heave and tensile strain predicted from the proposed method are found to be in good agreement with the numerical results.