For many rock engineering projects, the stress of surrounding rocks is constantly increasing and decreasing during excavating progress and the long-term operation stage. Herein, the triaxial creep behavior of dolomitic limestone subjected to cyclic incremental loading and unloading was probed using an advanced rock mechanics testing system (i.e., MTS815.04). Then, the instantaneous elastic strain, instantaneous plastic strain, visco-elastic strain, and visco-plastic strain components were separated from the total strain curve, and evolutions of these different types of strain with deviatoric stress increment were analyzed. Furthermore, a damage variable considering the proportion of irrecoverable plastic strain to the total strain was introduced, and a new nonlinear multi-element creep model was established by connecting the newly proposed damage viscous body in series with the Hookean substance, St. Venant body, and Kelvin element. The parameters of this new model were analyzed. The findings are listed as follows: (1) When the deviatoric stress is not more than 75% of the compressive strength, only instantaneous deformation, transient creep, and steady-state creep deformation occur, rock deformation is mainly characterized by the instantaneous strain, whereas the irrecoverable instantaneous plastic strain accounts for 38.02–60.27% of the total instantaneous strain; (2) Greater deviatoric stress corresponds to more obvious creep deformation. The visco-elastic strain increases linearly with the increase of deviatoric stress, especially the irrecoverable visco-plastic strain increases exponentially with deviatoric stress increment, and finally leads to accelerated creep and delayed failure of the sample; (3) Based on the experimental data, the proposed nonlinear creep model is verified to describe the full creep stage perfectly, particularly the tertiary creep stage. These results could deepen our understanding of the elasto-visco-plastic deformation behavior of dolomitic limestone and have theoretical and practical significance for the safe excavation and long-term stability of underground rock engineering.

Studying the effects of upper and lower joint wall strengths on shear behaviors is conducive to understanding the sliding problems of engineering masses, such as the contact surfaces of hard-soft composite strata. In this study, several types of soft-hard specimens with the same natural joint were cast using 3-D scanning technology and 3-D printing technology, and shearing tests were conducted on the specimens under different constant normal stresses. The results of the direct shear test showed that the deformation and shear strength characteristics of composite joints, including shear compaction, shear dilatancy, shear stiffness, peak shear strength, and failure characteristics, were affected by the compressive strengths of the upper and lower joint walls. The study also indicated that the shear damage of asperities occurred in both the hard and soft joint walls; the shear failure volume of the soft joint side accounted for a larger proportion of the total shear failure volume, which was strengthened with an increase in the wall strength coefficient (λσc). This finding explains why the joint roughness coefficient-joint compressive strength (JRC-JCS) formula has limitations in predicting the strength of soft-hard joints. To better predict the strength of soft-hard joints, a modified JRC-JCS formula for shear strength was proposed by considering the effects of upper and lower joint wall strengths on shear behaviors. Finally, we analyzed the mechanisms of three failure modes, including sliding wear, shearing failure, and sliding-shearing failure, and presented a reasonable explanation for different joint wall strengths causing different shear behaviors from a microscopic perspective.