Assessment of Impact to Buried Pipelines due to Tunneling-Induced Settlement

Tunneling in urban environments is challenging due to the number of pipelines to cross, which may adversely impact the pipelines due to tunneling-induced ground settlement. An adequate vertical separation distance between the bottom of the pipeline and tunnel crown is not always possible, and a minimum volume loss may be difficult to achieve due to subsurface conditions encountered and tunneling methods available. Therefore, the impact to the existing pipelines from tunneling needs to be assessed to mitigate the risk. Considering gap formation beneath the existing pipeline explicitly and a limiting overburden loading acting on the pipeline, this study proposes a design-oriented analytical approach to estimate the upper bound of the maximum bending moment of the pipeline induced by tunneling. A dimensionless quantity, namely, the overburden loading factor defined as the ratio of overburden loading and the minimum required loading to eliminate the gap, is introduced to evaluate the extent of gapping, by which gap formation can be predicted. The effect of pipe-soil interaction, which reduces the maximum bending moment in the pipeline, is considered by conducting nonlinear Winkler-based finite element analyses with varying stiffness factors. The design chart and table are presented to provide a more accurate estimation of the maximum bending moment, especially for greater stiffness factors. It is demonstrated for small overburden loading and stiffness factors, i.e., wider gaps, a perpendicular crossing may not always be the worst-case scenario. As illustrated by the partially supported beam model, the effect of the increase in the unsupported span is greater than the effect of the reduction of the curvature of the settlement trough for these cases. Design examples are provided to demonstrate the analytical procedure step-by-step, and a comparison with previous studies is made. It is demonstrated with reasonable conservatism for small to moderate stiffness factors, the proposed design-oriented analytical approach can straightforwardly predict the maximum bending moment in pipelines.

It is common to cross beneath an existing buried pipeline in the proximity for any tunneling project in urban areas due to the restrictions of underground space, soil conditions, right-of-way, and construction costs, etc. Considering gap formation beneath and an ultimate overburden loading on the existing pipeline, this study can be used to estimate the maximum bending moment in the existing pipeline due to ground settlement induced by tunneling, especially in the preliminary stage when the soil information is unavailable. The analytical procedure can be easily implemented by hand calculation or a spreadsheet, and more accurate results can be found with the aid of a chart and table provided when the soil information is available. The results obtained can assist the tunnel practitioners to determine an adequate separation distance, intersection angle, and an appropriate tunneling method to minimize the impact to the existing pipeline. Three detailed design examples are provided in the Appendix to illustrate the procedure step-by-step.