Determining tunnel stability across fault zones under seismic loading based on load/unload response ratio theory

Abstract

Geological faults impair tunnel stability during earthquakes. This study establishes a tunnel dynamic stability evaluation index based on load/unload response ratio (LURR) theory. It considers a seismic wave as a load/unload parameter and tunnel structure strain response as a response parameter. The rationale behind this evaluation index and the factors affecting tunnel stability across fault zones under seismic conditions are investigated. Compared to the traditional dynamic instability criterion, the LURR accurately measures the degree of structural deviation from the steady state and better determines the potential destabilization region of the structure. As the peak value of the input seismic wave increases, the LURRs of the more unstable parts increase, while the LURRs of the stable parts remain unchanged. According to LURR theory, the size of the range affected by the fault on the tunnel during an earthquake depends mainly on inherent fault properties (i.e., the dip angle, strike, and thickness), independent of the earthquake intensity. Because the LURR can theoretically be infinite, its dynamic instability threshold cannot be determined accurately.