Quantitative study on the effects of test pit dimensions during immersion testing of thick loess strata

The collapsible deformation of loess is significantly influenced by test pit dimensions, but the mechanisms and quantitative relationships remain unclear. This study combines engineering surveys and theoretical analysis to explore how pit dimensions affect collapsible deformation. Based on unsaturated soil mechanics, a coupled model integrating sidewall friction and collapsible deformation was developed. A method for calculating critical pit dimensions and a correction formula for measured self-weight collapsible deformation were proposed. Validation using engineering cases showed that when the pit radius exceeds the critical threshold, sidewall friction’s impact becomes negligible. Corrected deformation measurements then align better with theoretical predictions, reducing dimension-related errors. These results provide a theoretical basis for designing immersion test pits and correcting empirical data in collapsible loess regions.