Soil-rock mixtures (SRM) from mine overburden form heterogeneous dump slopes, whose stability relies on their shear strength properties. This study investigates the shear strength properties and deformation characteristics of SRM in both in-situ and laboratory conditions. Total twelve in-situ tests were conducted on SRM samples with a newly developed large scale direct shear apparatus (60 cm x 60 cm x 30 cm). The in-situ moist density and moisture content of SRM are determined. Particle size distribution is performed to characterize the SRM in laboratory. The bottom bench has the highest cohesion (64 kPa) due to high compaction over time while the other benches have consistent cohesion values (25 kPa to33 kPa). The laboratory estimated cohesion values are high compared to in-situ condition. It is further observed that for in-situ samples, the moist density notably affects the cohesion of SRM, with cohesion decreasing by 3 to 5 % for every 1 % increase in moist density. At in-situ condition, internal friction angles are found to be 1.5 to 1.7 times compared to laboratory values which is due to the presence of the bigger sized particles in the SRM. The outcomes of the research are very informative and useful for geotechnical engineers for slope designing and numerical modeling purpose.

Background Loess is prone to large deformation and flow slide due to natural and artificial interfaces inside. The strength of these interfaces controls the mechanical properties of loess. Obtaining their mechanical parameters through in-situ testing is essential for evaluating the mechanical stability in loess engineering with interfaces. Methods By developing a borehole micro static cone penetration system and creating various types of loess with interfaces, extensive borehole penetration model tests were conducted to observe changes in cone tip resistance during penetration. The response surface method was used to analyze the impact of various test conditions on the calculated resistance. A three-dimensional surface fitting method was employed to establish the relationship between penetration parameters and shear strength parameters, which was validated through in-situ testing. Results The developed borehole micro static cone penetration system achieves overall miniaturization while providing significant penetration power and ensuring an effective penetration distance. Cone tip resistance development during penetration can be divided into three stages: initial, rapid increase, and slow increase. The transition times between these stages vary for different soils. Calculated resistance is positively correlated with dry density and normal stress and negatively correlated with water content. A quadratic positive correlation was established between calculated resistance and shear strength parameters during penetration. In composite soils, the interaction between water content and normal stress is strong. Compared to intact soil samples, the shear strength parameters of composite soils are more prominently influenced by water content. Conclusion A system for testing interface mechanical parameters was innovatively developed, fulfilling the need to obtain interface shear strength parameters for deep soil. This study can provide support for ensuring the long-term stability of the loess slope or subgrade with interfaces.

Temporal variability in the macro-mechanics and microstructure induced by periodic water fluctuations during reservoir operation is widespread but adverse for slip zone soils. Herein, taking the slip zone soils of Huangtupo No. 1 landslide in the Three Gorges Reservoir area as a research case, the consolidation undrained (CU) triaxial tests coupled with wetting-drying cycles are organized to address macroscopic temporal variability of shear strength parameters. Then, quantitative microscopic characterizations are performed based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with mercury compression test (MIT). Eventually, the macro and micro connections are identified via gray rational analysis (GRA) and dynamic time warping (DTW) to be thus mathematized. Moreover, the weakened constitutive model is constructed. The test results show that the temporal variability of macroscopic shear strength parameters can be quantified as negative exponential decay. The wetting-drying cycles prominently contribute to the generation of intra-agglomerate pores (0.9-35 mu m). Besides, the inter-granular pores (0.007-0.9 mu m) and porosity are the connections to bridge microstructural parameters and macroscopic shear strength parameters. Furthermore, empirical equations for macro and micro connections are tentatively derived; the temporal variability of slip zone soils is invited to appropriately model the weakening laws of stress-strain. This study is expected to provide ingenious perspectives and promising references in stability evaluation and even disaster prevention of reservoir landslides.