Response of framed buildings to twin tunnelling in sand: a numerical investigation

Tunnel excavation induces ground movements and structural deformation, with the response of surface-framed buildings becoming particularly complex under twin-tunnel construction. This paper presents a parametric numerical study of the deformation responses of framed buildings on raft foundations induced by side-by-side twin tunnelling in sand. Two types of two-storey framed buildings with different transverse widths were considered, and effects of horizontal spacing between twin tunnels and eccentricity ratio were evaluated. Sand behaviour was simulated using a hypoplastic constitutive model. Soil displacement and foundation movement at soil-foundation interfaces and deformation patterns of framed buildings were examined. Structural shear distortion and corresponding modification factors were calculated and correlated with relative soil-building stiffness. The results demonstrate that horizontal spacing between twin tunnels governs settlement-trough evolution and structural response. At small horizontal spacing, twin-tunnelling settlement troughs overlap extensively, and foundation movement is governed mainly by overall settlement. With increasing horizontal spacing, settlement troughs gradually separate, resulting in more pronounced local differential settlement and bay shear distortion. Building width and eccentricity ratio significantly affect structural deformation patterns. Long frames are more susceptible to bay shear distortion, while eccentric buildings gradually shift from combined control by twin tunnels to dominance by the near-side tunnel as horizontal spacing increases. Second tunnel excavation does not simply amplify shear distortion in all bays. It alters settlement curvature and the distribution of differential foundation settlement, thereby redistributing shear distortion among bays. Empirical envelopes established for single tunnelling can support structural deformation assessment in twin tunnelling scenarios. The findings may provide useful guidance for risk assessment in urban tunnelling projects.