An Elasto–Plastic Hydro–Mechanical Model for Analyzing the Swelling Behavior of Heterogeneous Clay–Sulfate Rocks

ABSTRACT Swelling of clay‐sulfate rocks is a concern in geotechnical engineering that can damage infrastructure. In the historic town of Staufen in southwest Germany, geothermal drilling into a swellable anhydrite‐bearing clay formation resulted in water inflow and subsequent swelling, triggering ground heave and damage to more than 250 houses. To better understand and assess this phenomenon, which results from the interaction between groundwater flow and mechanical deformation, a coupled hydro–mechanical model was developed. The hydraulic behavior was described using Richards' equation for transient unsaturated flow, while the mechanical response of the swelling formation was represented by the Modified Cam‐Clay model, capturing its nonlinear elasto–plastic behavior. The model also accounts for spatial variations in Young's modulus and permeability within the swelling layer, reflecting the natural heterogeneity associated with variable clay content and permeability zones observed at the site. Model calibration was performed using field measurements from 2008 to 2011, optimized through a Particle Swarm Optimization algorithm to minimize the deviation between simulated and observed surface heave. The results demonstrate that incorporating both mechanical nonlinearity and geological heterogeneity can accurately reproduce the deformations observed at Staufen. The model was validated against independent monitoring data up to 2024, confirming its predictive reliability. It was subsequently used to forecast long‐term surface heave development until 2030, reproducing both the initial rapid uplift and the later gradual stabilization observed at the site. We conclude that the presented approach offers a practical and physically consistent tool for long‐term risk assessment in areas prone to rock swelling.