Multiaxial Strength Characteristics of Cement-Stabilized Mining Waste from Taiwan’s Jinguashi Gold–Copper District: Anisotropy, Yield Behavior, and a Unified Constitutive Framework

Understanding the mechanical behavior of cemented mining waste under multiaxial stress conditions is critical for sustainable tailings management in Taiwan’s mining districts, which are in a seismically active region. This study investigates the strength anisotropy of tailings from the Jinguashi gold–copper mine and local river sand stabilized with ordinary portland cement through low- and high-pressure true triaxial testing. A novel constitutive framework is developed incorporating the effects of cementation degradation, Lode angle dependency, and stress-induced anisotropy. Mixtures are designed using the porosity-to-volumetric cement content index ( η / C iv 0. 16 ) to achieve equivalent initial conditions across varying dry unit weights and cement contents. The experimental program comprises unconfined compressive strength tests and true triaxial tests covering multiple stress paths, mean effective stresses, and loading orientations. The results demonstrate that cementation effectively suppresses compaction-induced anisotropy below the yield threshold, promoting near-isotropic behavior. However, anisotropy reemerges when cement bonds degrade beyond yielding, with extension-dominated stress paths exhibiting greater strength reduction than compression paths. Jinguashi tailings achieve higher normalized yield surfaces compared with natural sand, which is attributed to their finer matrix, higher specific surface area (22.3 m 2 /g), and acidic pH conditions (4.2) that are inferred to enhance cement bonding potentially through pozzolanic reactions based on geochemical affinity and strength development trends; direct microstructural evidence was not obtained in this study. A new elastoplastic model with three evolution regimes is derived incorporating cementation degradation through internal variable evolution. The model successfully predicts the transition from isotropic to anisotropic behavior. These findings validate the η / C iv 0. 16 index as a practical design parameter for optimizing compaction effort and cement content, enabling engineers to balance construction costs with performance targets under complex field stress paths prevalent in Taiwan’s mountainous terrain.