A unified van Genuchten-type water retention model for compacted bentonite

Water retention behavior of bentonite is essential for the analysis of engineered barrier systems in deep geological repositories for high-level radioactive waste. Despite being a popular choice, the van Genuchten model requires labor-intensive calibration for each material and dry density condition and cannot propagate engineering-scale uncertainties from in-situ buffer evolution and mineralogical heterogeneity. This study proposes a unified van Genuchten-type model in which the fitting parameters are expressed as empirical functions of effective water retention density (EWRD). EWRD, defined as effective montmorillonite dry density normalized by specific surface area, incorporates the combined effects of dry density, montmorillonite content, and microstructure within a single porosity framework. A comprehensive set of over 200 confined wetting data points for seven Na- and Ca-type bentonites revealed that the van Genuchten parameters α and n collapse onto unique trends when plotted against EWRD, confirming its dominant control on water retention. For validation, the predictive ability of the unified model for dry density variation was first tested by successfully reproducing the unconfined wetting of FEBEX bentonite, after a simple correction of bias calculated from initial test conditions. Second, additional data were generated for two batches of Bentonil-WRK differing in montmorillonite content by ~10% for cross-validation. Excellent agreement between model prediction and experiments was observed, demonstrating reliable extrapolation across mineralogical heterogeneity. By preserving the form of the classical van Genuchten model, the proposed approach can be readily implemented in existing hydro-mechanical codes, providing informed estimates of water retention curves across various buffer designs and operation scenarios.