Abstract:

Along with the development of underground engineering in recent decades，the underground en-gineering excavation depth and the construction scale is increasing.In the deep-layer rock engineering activities，the researchers observed that deep rocks are subjected to huge in-situ stress in all directions，which has a non-negligible effect on the mechanical properties of the rock.In order to ensure the safety of underground engineering construction，technician must solve the deep strata of rock “high geostress” and so on.With the deepening of underground engineering，due to the different stress conditions of rocks at different depths，there is a difference between the horizontal axial pressure and the surrounding rock pressure under different depths.In addition，the implementation and development of underground engineering activities are often carried out through drilling and blasting methods，explosive blasting，mechanical disturbance，manual excavation，etc.，each time the development of the project will have different degrees of disturbance and damage to the surrounding rock.In this experiment，the improved SHPB test device was used to study the mechanical properties of a complete granite rock specimen with transverse holes under the same axial pressure level and different axial pressure levels.Unequal amplitude cyclic impact tests on rock samples under the combined action of axial compression and impact load was conducted.The research results are of great significance to protect and maintain the stability of surrounding rock around the roadway in the future underground engineering construction.In the impact test，a certain level of axial pressure was applied to each sample in advance，and the axial pressure level was 0 MPa，0.3 σ_f，0.4 σ_f，0.5 σ_f，0.6 σ_f and 0.7 σ_f(σ_f is the uniaxial compressive strength, MPa), respectively.During the implementation of axial pressure，the axial pressure is controlled by a manual oil pump to adjust the axial pressure to achieve a constant axial pressure load.In order to avoid the next impact failure of the samples under different coaxial pressure，a lower pressure of 0.45 MPa was defined in this test through several adjustment tests.Each sample was impacted by the initial pressure once.After the end of one cycle，the pressure of the previous one was increased by 0.05 MPa，until all samples were damaged by impact.Test results show that under different impact load step by step，the peak stress increased with the increase of the number of shock load increases first，then decrease，and increases with the increase of average strain rate.Elastic modulus increase first and then decrease with the increase of impact times performance.Under different axial pressure levels，the impact failure modes of the specimens are all axial splitting failure modes.As the strain rate increases，the fracture surface gradually increases，the degree of fragmentation increases，and the fragment size decreases.When the strain rate is small，the crushing effect is not obvious，and when the strain rate is large，the crushing effect is significant.

Key words: cyclic impact, load failure mode, strain rate, SHPB, mechanical characteristics, failure patterns