To investigate the shear strength evolution of soil-rock material in waste dump induced by freezing and thawing cycle (FTC), the shear tests are conducted by a self-developed direct shear apparatus, and the effect of FTCs on the shear strength degradation curve of soil-rock material is obtained. Furthermore, the inner microstructure of soil-rock material induced by FTC is analyzed by computer topography (CT)scanning testing.Finally, the stability of waste dump after and before freezing and thawing is compared. The results show that the shear and displacement curves of soil-rock material present strain softening during low normal loading pressure. However, the shear and displacement curve development trend changes from strain softening to strain hardening, and the shear dilatancy happens during loading process. As the FTCs increase, the strain hardening behavior becomes more obvious and the shear dilatancy effect is weakened. In addition, both the cohesion and friction angles decrease as the FTCs increase. When the number of FTC increases to 15, the decrement of cohesion and friction angle is 18.45 % and 9.42 %, respectively. The reason is mainly attributed to the dislocation and regroup of fine rock particles induced by freezing crack.The change in porosity of soil-rock material mainly happens within the initial 5 freezing and thawing cycles. The safety coefficient decrement of waste dump is approximately 7.6 % as the FTCs increase from 0 to 15. The results can provide guidance for waste dump in cold regions.

This study proposes to explore the effect of interface angles β on the triaxial mechanical properties of backfill and surrounding rock combined bodies(backfill-rock combined bodies)by conducting a triaxial compression experiment of backfill-rock combined bodies with interface angles β of 0°, 15°, 30°, 45° and 60°. This paper compared and analyzed the mechanical properties, failure characteristics and strength evolution patterns of the combined bodies with different interface angles. The experimental results show that: When β≤30°, the stress-strain curve of the combined bodies can be divided into four stages: pore compaction, elastic deformation, plastic deformation and failure development. Its failure characteristics are mainly compression shear failure of the backfill. With the increase of the interface angles, the combined bodies stress drops steeply after reaching the peak strength, without significant plastic deformation or failure development stage. Its failure characteristics gradually change from compression shear failure inside the backfill to sliding failure along the interface. The peak strength of the combined bodies first increases and then decreases with the increase of the interface angles, and peak strength reaches a maximum value when β=30°. Based on the theory of single weak plane, the interface critical angles for sliding failure of the combined bodies is 57° to 68°. Where β=60°, the combined bodies occurred sliding failure surface along the interface, and the results from theoretical calculation confirm the experimental results.

The waste dump of Tibet Jiama Copper Mine, which has a steep and soft base, is taken as the engineering background to reveal the failure mode and mechanism with assistance of similarity simulation principle.Using a method combining bottom friction test and numerical simulation.The deformation evolution of the waste dump is obtained by the contactless displacement monitoring system.The experimental results show that: ①The deformation and instability of the soft steep base dump can be divided into three stages: unloading deformation at the slope toe, vertical tensile crack penetration and tensile crack propagation.②The soft soil layer at the base is compressed and the soft soil layer at the slope toe is subjected to shearing.The slope toe is the trigger zone for slipping deformation and instability.③The potential slip surface passes through the soft soil layer to form an approximate circular arc slip surface.The instability failure of the soft-steep base dump mainly presents the modes of settlement, cracking and slippage.