Mechanism of Uplift in Black Shale Fill Subgrade During Operational Phase: Integrating Field Monitoring with Numerical Simulation

The K166 embankment of the railway connecting Zhangjiajie to Huaihua experienced persistent uplift deformation during the operation state, posing a threat to traveling safety. This study systematically investigated the underlying mechanism through an integrated approach combining field monitoring, laboratory testing, and numerical simulation. Monitoring data confirmed continuous uplift progression. Laboratory analyses revealed that the oxidation of pyrite in the black shale fill generated an acidic environment, triggering acid-driven chemical reactions that caused significant expansion of the fill material. Numerical modeling successfully reproduced the deformation pattern. More importantly, a quantitative link between laboratory-scale expansion and field-scale uplift was established through equivalent parameter inversion. The results indicate that increasing the depth of replacement with non-expansive fill material effectively mitigates foundation uplift, and provides a basis for quantitative evaluation of remediation strategies. Furthermore, this effect becomes more pronounced as the replacement depth increases. It is concluded that in areas with black shale fill, monitoring environmental chemical conditions is essential, and remediation efforts should focus on controlling pyrite oxidation. This work provides both theoretical and practical foundations for addressing similar embankment failures and offers a simplified analytical framework for engineering predictions of uplift under varying conditions.