Experiment on monitoring and early warning of karst collapses based on acoustic emission technology
The karst collapse is a dynamic geological process in which the damage of soil mass and development of cavity ultimately result in the instability of the overburden layer. Therefore,identifying the characteristics and evolution of the damage of soil mass are the important prerequisite to develop effective monitoring and early warning methods for karst collapses. However,the commonly used early warning methods that utilize hydrodynamic conditions and optical fiber monitoring technique often overlook both the process of microscopic damage of the soil mass and the effect of damage on karst collapses. Previous studies on karst collapses induced by groundwater or rainfall typically employ qualitative or semi-quantitative methods. These approaches appear to be one of the reasons why karst collapses-major and prevalent geological disasters in karst regions-have seen limited advancement in monitoring and early warning system. In this study,acoustic emission (AE) and fiber grating technology have been firstly applied to conduct model tests on monitoring and early warning methods of karst collapses. The research on dynamic characteristics of AE has been conducted under different rainfall conditions. In addition,the characteristics of key time domains and frequency domains of AE have been identified and selected to establish a spatial-temporal responding mechanism between AE and karst collapses through model experiment.The results show that as follows,(1) The total ringing count,amplitude,and energy of AE in the shallow overburden layer ranged from 1 times to 348 times,30.5 dB to 175.9 dB,and 0 PJ to 57×10-3,respectively,under heavy rainfall conditions. In contrast,the same parameters in deep overburden layer ranged from 1 times to 2,361 times,30.5 dB to 179 dB,and 0 PJ to 946.4×10-3 PJ,respectively. In addition,under rainstorm conditions,these parameters in shallow overburden layer ranged from 1 times to 89 times,30.5 dB to 140.9 dB,and 0 PJ to 22×10-3 PJ,respectively,while in deep overburden layer,they ranged between 1 times and 1,322 times,30.5 dB and 213.1 dB,0 PJ and 4,694.6×10-3 PJ,respectively. During the formation of karst collapses under heavy rain conditions,the ringing count of AE in the deep overburden layer increased by 6.78 times to 6.89 times compared to the shallow layer. Additionally,the amplitude of AE in deep overburden layer increased by 1.02 times to 1.12 times compared to the shallow layer. The energy of AE in deep overburden layer increased by 4.45 times to 16.6 times compared to the shallow layer. During the formation of karst collapses under rainstorm conditions,the ringing count,amplitude and energy of acoustic emissions in deep overburden layer increased by 14.85 times,1.51 times and 213.39 times,respectively,compared to the shallow layer. (2) Under heavy rain conditions,the karst collapses were defined as creep-failure collapses which were caused by the expansion and instability of soil caves. But the energy emitted by the final collapse of the soil cave was larger than the energy emitted by creep-failure collapse,which indicated higher activity of AE signals. Under rainstorm conditions,the karst collapse was defined as compression-shear fracture collapse caused by the whole fracture and collapse of soil mass. In this process,the strain energy in the soil layer was completely released in a short time. Hence,the AE signals showed a sharp increase in dynamic curve in the second rainfall. This result indicated a strong relationship between AE signal characteristics and types of collapse. (3) There are four types of signal waveforms in the formation of karst collapse,which can be divided as slippage of soil mass,dislocation of soil layer,development of cavity and layer collapse. The irregular up-down fluctuation of signals represented the slippage of soil mass,while concave triangle signals indicated that rainfall dislocated stability of soil layer. Additionally,wedge-shaped signals indicated that a tiny cave was developed in the overburden layer. The signals which were showed as combined form of equilateral triangle and concave triangle implied the occurrence of karst collapse. Therefore,the signal energy,rising and falling time and duration of signal waveforms were closely related to soil deformation in the process of karst collapse. (4) In the formation of karst collapse,the spectrum signal waveforms of AE fell into high frequency narrow pulse form,in which spectrum energy of the four signal types were concentrated in the high frequency range of about 50 kHz and 20 kHz,respectively. (5) The cumulative ringing counts of AE were closely related to pore water pressure,soil pressure and displacement of soil mass. The ringing counts of AE increased in sudden during soil deformation or collapse. This observation indicated that AE technology is feasible for monitoring and early warning of karst collapses.
karst collapserainfallacoustic emissiontime-frequency characteristicmonitoring and early warning
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