Experimental study on monitoring and early warning of cover collapse sinkhole based on acoustic emission technology

The cover collapse sinkhole is the dynamic geological process in which soil damage of overburden layer developed and eventually lead to surficial collapse. Therefore, identifying the characteristics and evolution of soil damage are the important prerequisite for laying out effective monitoring and early warning methods for cover collapse. The frequently-used hydrodynamic condition and optical fiber monitoring technique easily ignore not only the process of microscopic damage in soil layer but also its effect applied on cover collapse. The possibility analysis on cover collapse sinkholes induced by groundwater or rainfall usually adopted qualitative or semi-quantitative methods. And it seems to be the one of the reasons of which cover collapse, a major and common geological disaster in karst region, has obtained few progresses in monitoring and early warning of collapses. In this paper, acoustic emission (AE) and fiber grating technology have been firstly applied to conduct model test on monitoring and early warning methods of collapse sinkhole. The research on dynamic characteristics of acoustic emission have been conducted under different rainfall condition. In addition, the important time and frequency domain features of acoustic emission have been identified and selected to establish the spatial-temporal responding mechanism between AE and collapse sinkhole through model experiment. The results show that: (1) The total ringing count, amplitude and energy of acoustic emission in upper cover layer is varied from 1 to 348 times, 30.5 to 175.9 dB and 0 to 57×10-3 PJ respectively under heavy rainfall condition, while the ringing count, amplitude and energy of acoustic emission in deeper cover layer is varied from 1 to 2361 times, 30.5 to 179 dB and 0 to 946.4×10-3 PJ respectively. Otherwise, in rainstorm condition, the ringing count, amplitude and energy of acoustic emission in upper cover layer is ranged between 1 and 2361 times, 30.5 and 179 dB, 0 and 946.4×10-3 PJ respectively. By contrast, the ringing count, amplitude and energy of acoustic emission in deeper cover layer is ranged between 1 and 1322 times, 30.5 and 213.1 dB, 0 and 4694.6×10-3 PJ respectively. During the formation process of collapse sinkhole under the heavy rain condition, the ringing count of acoustic emission in deeper layer was increased by 6.78 or 6.89 times than upper ones, respectively. Also, the amplitude of AE in deeper layer increased by 1.02 or 1.12 times than upper ones, respectively. On the other hand, the energy of AE in deeper layer increased by 4.45 or 16.6 times than upper ones, respectively. Furthermore, during the formation process of collapse sinkhole under the rainstorm condition, the ringing count, amplitude and energy of acoustic emission in deeper layer was increased by 14.85, 1.51 and 213.39 times than upper ones, respectively. (2) Under the heavy rain or rainstorm condition, the types of collapse sinkhole were defined as creep-failure sinkhole or compression-shear fracture sinkhole, respectively. The formation process of creep-failure sinkhole was characterized by soil cavity expanded and finally resulted in instability failure of cover layer. The emitted energy, which is much large in collapse stage than creep-deformation stage, implied that higher activity in AE signal condition. However, the formation process of compression-shear fracture sinkhole was characterized by the soil layer collapsed suddenly. It represented that strain energy in soil layer was completely released in a short time. Hence, the AE signals showed a sharp increase in dynamic curve, which indicated that the signal characteristics of acoustic emission had strong relationship with sinkhole types. (3) There are four types of signal waveforms in the formation process of collapse sinkhole, which were divided as soil slip, soil layer dislocation, cavity development and layer collapse. When signals showed as irregular up-down fluctuation, it represented as soil slip. While concave triangle signals indicated that rain fall dislocated stability of soil layer. On the other hand, wedge-shaped signals indicated that tiny cave developed in the overburden material. The signals which showed as combined form of equilateral triangle and concave triangle implied that cover collapse sinkhole. Therefore, the signal energy, rising and falling time and duration of signal waveforms were closely related to soil deformation in the process of collapse sinkhole. (4) In the formation process of collapse sinkhole, the spectrum signal waveform of acoustic emission was high frequency narrow pulse form, in which spectrum energy of four signal types such as soil slip, soil layer dislocation, cavity development and layer collapse were concentrated in the high frequency range of about 50 kHz or 20kHz, respectively. (5) The cumulative ringing counts of acoustic emission were closely related to pore water pressure, soil pressure and displacement of overburden material. The ringing counts of acoustic emission increased in a period of time or in a sudden during the process of soil deformation or collapse, respectively, which indicated that acoustic emission technology is feasible for motoring and early warning of collapse sinkhole.