The surrounding rock of underground engineering is subjected to the superposition of multi strain rate loads such as in-situ stress static load, far-field stress wave transmission dynamic load, near field strain energy release dynamic load. In order to study the physical and mechanical characteristics of rock under the above complex mechanical environment, the key technology of rock mechanics experiment with multi strain rate dynamic and static load superposition was proposed. This key technology has solved four technical problems such as single-sided superposition of dynamic and static loads with multiple strain rates and rapid compensation of static loads, automatic balanced loading of creep loads, high-frequency cyclic loading of impact loads, integrated control of loading devices and coupling acquisition of multiple information. Thus, multi strain rate dynamic and static loads such as creep load (< 10^-5 s^-1), hydraulic static load (10^-5 ~ 10^-2 s^-1), pulse dynamic load (10^-2 ~ 10⁰ s^-1) and cyclic impact dynamic load (10⁰ ~ 10² s^-1) can be superimposed on one side of the rock; the static load can be compensated quickly in the process of rock collapse and instability to simulated the rapid release process of elastic energy of surrounding rock in elastic zone; the lever can be leveled automatically during the continuous creep of rock to keep the gravity creep load constant. Through the above technology, a multi strain rate dynamic and static superposition rock mechanics experimental system was developed, and the dynamic and static superposition experiment of siltstone was carried out. The experimental results show that the greater the static load stress or the smaller the impact loading frequency, the greater the cumulative damage of rock, the smaller the peak strength, the greater the final strain and the smaller the failure duration. There is a strain rate effect on the damage evolution and peak strength of rock under dynamic and static superposition. The greater the static load, the greater the elastic energy accumulated at the rock crack tip, and the dynamic load enhances the rock brittleness. Therefore, with the increase of static load stress or the decrease of impact frequency, the rock failure form undergoes the transformation of "inclined shear failure-vertical tensile failure-overall burst failure", and the rock burst position expands from the bottom to the whole.
 

rock mechanics; multi strain rate load; superposition of dynamic and static loads; experimental technique; System development and application