Investigating mechanical properties of cemented gangue backfill materials subjected to static-dynamic combined loads

•Mechanical response of CGBM under static-dynamic combined loads was studied.•Real-time monitoring of crack activities and strain field evolution was achieved.•Typical Failure modes of CGBM regarding various loading paths were identified. This study aims to investigate the mechanical properties and failure mechanisms for the cemented gangue backfill materials (CGBM) of the roadside filling structures along the gob-side entry with no coal pillars under static-dynamic combined loading. Cuboid cemented gangue backfill samples (CGBS) with dimensions of 50 × 50 × 100 mm were first prepared, with the fractal dimension D = 2.6 characterizing the particle size distribution of aggregates, and then conducted static-dynamic combined loading tests in terms of various pre-static loads (5–11 MPa), amplitudes (2–5 MPa), frequencies (1–16 Hz) and dynamic loading cycles (20–320 cycles). The test results indicate significant differences in the mechanical properties, acoustic emission (AE) responses, maximum principal strain fields, and ultimate failure modes of CGBS under different static-dynamic combined loading paths. With an increasing pre-static load, dynamic loading amplitude and number of cycles, the peak stress decreases by 21.28 %, 12.48 % and 11.84 % respectively, while the peak stress first decreases and then increases with the dynamic loading frequency and reaches the minimum value at the “characteristic frequency” of 4 Hz. The irreversible strain generally experiences an “acceleration - constant speed” variation during the dynamic loading as a result of the gradually stable damage effects. The dynamic elastic modulus increases by 45.84% in the pre-static load range of 5–11 MPa, while decreases with the dynamic loading amplitude. The dissipated energy increases by a factor of 6.5 with the dynamic loading amplitude, and by 27.52% with the pre-static loads. The significantly stimulated cracking activities at the beginning of dynamic loading result in extremely violent AE responses, and then the AE signals weaken due to the dominate role of opening and closure of existing cracks. The RA and AF calculation results illustrate that shear cracks become active at the initial dynamic loading and macroscopic failure stages. As the degradation effects of dynamic loading intensify, three various failure modes of inclined shear failure, “Y” shaped shear failure and conjugate shear failure for the CGBS are respectively classified, especially the conjugate shear failure charact