Calcareous sand, a distinctive granular material in geotechnical engineering, has garnered significant interest due to its irregular particle shapes, internal porosity, susceptibility to breakage, and critical role in island and offshore construction. Despite its importance, the influence of loading paths on its mechanical behavior and particle breakage remains underexplored. This study addresses this gap through an extensive experimental program, including isotropic consolidation and both drained and undrained triaxial compression tests, systematically varying loading paths and initial densities. The findings demonstrate that the strength and deformation characteristics of calcareous sand are profoundly affected by loading paths, initial densities, and particle breakage. A novel breakage evolution model is proposed, effectively capturing gradation changes under diverse testing conditions. Furthermore, the study quantifies the impacts of these factors on critical mechanical properties, including peak friction angle, dilatancy, secant modulus, and critical state parameters. These results provide a robust theoretical foundation for the development of constitutive models that integrate particle breakage and initial density effects. The insights are essential for optimizing geotechnical designs, enhancing stability, and improving infrastructure reliability in coastal and marine environments, particularly in island and reef development projects.

Currently, the mechanical properties of calcareous sand are mainly studied through triaxial tests, as traditional uniaxial compression tests fail to capture real loading conditions and soil strength anisotropy. To address this, true triaxial tests were conducted to examine the effect of the intermediate principal stress parameter (b) on the three-dimensional strength and deformation behavior of calcareous sand. In the constant b and sigma 3 tests, as the b value increased, both the strength and peak friction angle (phi ps) of calcareous sand were increased, while the tangent slope of the dilatancy curve showed a gradual rise.. The phi ps of calcareous sand was found to be higher compared to silica sand and coarse-grained soils. In the constant mean effective stress (p) and b test, the strength was increased with higher values of both b and p. The Matsuoka-Nakai 3D strength criterion proved more effective in fitting the 3D strength of calcareous sand in pi plane. As the b value increased, the critical stress ratio (Mc) was decreased. A quadratic function can better represent the Mc of calcareous sand in the pi plane under varying confining pressures. Furthermore, the Mc of calcareous sand was higher than that of silica sand and completely decomposed granite soil. This study provides a valuable experimental basis for understanding the 3D strength and deformation characteristics of calcareous sand in oceanic engineering infrastructure.

The anisotropic mechanical behavior of rocks under high-stress and high-temperature coupled conditions is crucial for analyzing the stability of surrounding rocks in deep underground engineering. This paper is devoted to studying the anisotropic strength, deformation and failure behavior of gneiss granite from the deep boreholes of a railway tunnel that suffers from high tectonic stress and ground temperature in the eastern tectonic knot in the Tibet Plateau. High-temperature true triaxial compression tests are performed on the samples using a self-developed testing device with five different loading directions and three temperature values that are representative of the geological conditions of the deep underground tunnels in the region. Effect of temperature and loading direction on the strength, elastic modulus, Poisson 's ratio, and failure mode are analyzed. The method for quantitative identi fication of anisotropic failure is also proposed. The anisotropic mechanical behaviors of the gneiss granite are very sensitive to the changes in loading direction and temperature under true triaxial compression, and the high temperature seems to weaken the inherent anisotropy and stress-induced deformation anisotropy. The strength and deformation show obvious thermal degradation at 200 degrees C due to the weakening of friction between failure surfaces and the transition of the failure pattern in rock grains. In the range of 25 degrees C-20 0 degrees C, the failure is mainly governed by the loading direction due to the inherent anisotropy. This study is helpful to the in-depth understanding of the thermal-mechanical behavior of anisotropic rocks in deep underground projects. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).