Microstructure evolution of collapsibility of thick loess treated with DDC piles: an in-situ experimental study

Downhole Dynamic Compaction (DDC) piles are the most widely used technique for the mitigation of loess foundation collapsibility. While previous studies have extensively verified their macro-mechanical improvement performance, the understanding of the underlying micro-scale mechanisms remains largely insufficient. This study focuses on the thick self-weight collapsible Malan loess (Q3) in Fu County, and adopts an integrated approach combining field compaction tests, laboratory collapsibility tests under water immersion, quantitative microstructural analysis, and Pearson correlation-random forest (RF) coupled modeling to reveal the evolution law of loess macro- and micro-properties under DDC compaction and elucidate the intrinsic mechanism of collapsibility elimination. The results show that, for a hole-forming diameter d = 0.4 m, a pile spacing H ≤ 1.4 m (i.e., ≤3.5 d) can reduce the collapsibility coefficient δs of inter-pile soil to below 0.015, achieving complete elimination of loess collapsibility. In contrast, a pile spacing of 1.5 m results in the failure of collapsibility elimination at multiple test points, which cannot meet the engineering requirements. From a microstructural perspective, the intensification of compaction effect drives the coating of skeletal particles by clay aggregates, the transition from unstable point contacts to stable face-to-face contacts, and the conversion of macropores (>32 μm) into mesopores (8–32 μm) and small pores (2–8 μm). Multivariate statistical analyses identify that total pore area is the dominant factor controlling loess collapsibility, followed by mesopore area ratio and void ratio. On this basis, the process of collapsibility elimination by DDC piles is conceptualized as three sequential stages: the compression and destruction of the original pore structure, followed by particle rearrangement and soil matrix densification, and finally the regularization of pore morphology and degradation of pore connectivity. These findings offer scientific guidance for geotechnical engineering practices in loess areas, contributing to the mitigation of water-related geotechnical risks and the promotion of sustainable construction development.