Interplay of Tunnel Geometry and Shallow Foundation Stability in Rock: Comparative Insights into Failure Modes

Abstract

This study delved into the intricate relationship between tunnel geometry and the stability of shallow foundations. It investigated the influence of various tunnel geometries on the behavior of a strip footing (width, B) subjected to uniform loading positioned above a single/dual-tunnel system. The analysis encompassed three different tunnel geometries, namely, circular, square, and horseshoe, for both single- and dual-tunnel configurations. The primary focus of the study was quantifying the stability number, Nv, as a measure of tunnel’s impact on footing stability using finite-element analysis. For the single-tunnel configuration, the study examined the vertical depth between the footing base and the tunnel crest (Z) and the horizontal distance between the central vertical axis of the footing and the center of the tunnel (X). Additionally, in the case of the dual-tunnel configuration, the influence of horizontal spacing between the two tunnels (S) on Nv and also the impact of asymmetrical shapes of tunnels with three different cases, i.e., Case I (circular and square-shaped tunnels), Case II (circular and horseshoe-shaped tunnels), and Case III (square- and horseshoe-shaped tunnels), were studied. The findings of this study emphasize that tunnel geometries have a substantial impact on Nv, especially when the Z/B ratio is less than 3 for both tunnel configurations. However, when Z/B exceeds 3, the influence of tunnel geometries on Nv becomes negligible. Furthermore, in the case of asymmetrical tunnel configurations, Case II consistently exhibits the highest stability number (Nv), followed by Case I and Case III, across all combinations of S/B and Z/B values. Additionally, the study identifies zones of plastic stress deformation associated with different tunnel geometries. It underscores the importance of implementing supplementary support systems in cases where these deformations are prominent, ensuring the overall stability of the footing–tunnel system. In the dynamic realm of underground space utilization, ensuring the safety of structures above and below ground is paramount. With the increasing construction of tunnels near existing buildings, it is crucial to prioritize their protection and assess their suitability for use. In response to contemporary challenges, this study investigated the impact of nearby tunnels on load-bearing capacity of structures. It delved into how the shape of tunnels influences the behavior of surrounding structures, taking into account the regions of the rock mass affected by such interactions. Additionally, the study provided insights into the practical implications of planning and constructing asymmetrical tunnels—a common scenario where new tunnels with varying shapes are built near pre-existing ones. The findings of this research offer practical applications in determining the load-carrying capacity of shallow foundations in the vicinity of tunnels with diverse configurations, shapes, and locations from the foundation. The special significance of this research lies in its ability to guide the design of surrounding structures and identify specific zones around tunnels that require attention, ultimately contributing valuable insights for establishing suitable support systems.