Enhancing Geotechnical Engineering Education Through Case-Based Innovation: A Predictive Modeling Framework for Cemented Sand in Strength Theory Teaching

The shear strength–suction (here suction induced from unsaturation) relationship is inherently challenging, yet this difficulty is compounded for cemented crushable sands, whose behavior fundamentally diverges from classical clay-centric theories. This paper presents an innovative teaching case study focusing on colloidal-silica-cemented calcareous sand, based on direct shear tests across a full saturation range (0–100%). Our experimental findings reveal two unconventional characteristics that challenge textbook models: (1) suction strength exhibits a positive dependency on normal stress—an inverse trend to conventional expectations; and (2) strength near desiccation drops below saturated values, contradicting the monotonic increasing function typically observed in clays. A review of 20 existing models confirms that none of them can simultaneously capture both features, highlighting a clear gap in both theory and instruction. To address this gap pedagogically, the core novelty of this work lies in the development of a classroom-friendly predictive model that introduces two physical innovations: first, it incorporates normal-stress-dependent suction strength by modifying capillary condensation probability—departing from constant-angle assumptions; second, it accounts for desiccation-induced strength deterioration through a gel crack size effect, which is absent in conventional unsaturated strength formulations. The model retains clear physical interpretability and demonstrates strong agreement with experimental data. By integrating unconventional behavior, model limitations, and novel physically inspired formulations into a coherent case study, this work equips students not only to recognize deviations from classical unsaturated strength theory but also to construct their own mechanistic models in geotechnical engineering education.