Abstract: Defected rock mass seriously threatens the safety construction and long-term operation of underground engineering. It is significant to investigate the physical variables before and after the instability, which provides useful information to identify the precursor of rock mass instability failure. This study is to conduct experiments on the gypsum in combination with crack elements, and further to investigate its space-time evolution process of the failure-instability. Before the instability failure, the changes in the corresponding physical field are captured and compared at critical moments. The processes from the independent activity to overall synergistic movement of rock mass are discussed as well. Experimental results indicate that the complete destruction of the crack combination can be divided into three stages: strong deviation from the linear, static sub-instability and dynamic sub-instability. In the non-linear stage, the relatively strong segment become the main bearing body. The isolated strain accumulating area and releasing points appeared firstly in the weak segment, and then acted on the interface between strong and weak segments. Strain accumulating areas in the weak segment rapidly increase, extend and migrate. Different lithologies of rock mass present strain synergistic transmission with unitary and regional characteristics. Finally, strain releasing points are mutually connected. When both the synergistic effect and strain accumulation degree in the strong segment reach their limits, the stage of rapid destruction of rock mass into full instability emerges. A stope fault model is established to further verify that the results are scientific and rational. This model was based on the engineering background of water inrush in a coal mining. The two-axial loading experiment was carried out to investigate the process of water inrush and trigger conditions caused by the fault instability. The mechanism of synergetic failure in the sub-instability stage is revealed. Experimental results indicate that the sub-instability stage of jointed rock mass makes the beginning of synergistic failure.

Key words: sub-instability stage, strain release, defected rock mass, synergistic destruction