<p>When excavating foundation pits in karst areas, karst caves can destabilize them. This study presents the ZSI-R (unit safety state index considering karst characteristics). It captures the residual state of brittle rock after failure, offering new ideas and quantitative methods for stability assessment. Based on this index, it can fully describe the elastic, yielding, failure and residual states of brittle rock units, making up for the traditional methods’ inaccuracy in quantifying caves’ impact on geotechnical stability. Using the Yanjiaping foundation pit project of Hubei Yilai Expressway as a case, the study explores the impact of cave location on foundation pit stability. By combining numerical simulation with ZSI-R, it refines the karst cave treatment scope and develops practical engineering solutions. The results show that the influence of the caves decreases from the passive to the active zone, which is highly significant for optimizing support design and construction strategies, and reducing engineering risks and costs. Furthermore, it analyzes the rock deformation and failure during excavation, and assesses foundation pit stability via ZSI-R values of different sections. Field application confirms that this approach can clearly describe stability quantitatively, providing references for research and practice in foundation engineering stability analysis in karst areas.</p>