Experimental study on monitoring and early warning of cover collapse sinkhole based on acoustic emission technology
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摘要: 岩溶塌陷是土体损伤孔洞发育并最终导致盖层失稳的动力地质过程，故查明土体损伤特征及演化过程是提出有效监测预警方法的重要前提。本文利用声发射与光纤光栅传感技术对岩溶塌陷形成过程展开模型试验，甄别与筛选声发射关键信号特征，并建立其与岩溶塌陷的响应机制。试验结果表明：（1）大雨条件下岩溶塌陷的形成演化过程里，覆盖层深部声发射信号振铃计数较浅部增大6.78~6.89倍，幅度增大1.02~1.12倍，能量扩大了4.45~16.6倍。在暴雨条件下，覆盖层深部声发射信号振铃计数较浅部增大14.85倍，幅度增大1.51倍，能量扩大了213.39倍；（2）大雨试验工况下是以土洞扩展并失稳破坏的蠕变破坏型岩溶塌陷，暴雨试验工况下是土层整体错断坍塌的压剪断裂型岩溶塌陷，不同塌陷类型的声发射信号特征有明显差异；（3）岩溶塌陷过程中会出现土体滑移、层面错动、孔洞发育和塌陷四类信号波形，信号波形释能幅值、上升与下降时间、波形持续时间等与岩溶塌陷演化过程土体变形密切相关；（4）岩溶塌陷过程中声发射频谱信号波形为高频窄脉冲，其中土体滑移、层面错动、孔洞发育和塌陷等四类信号频域能量分别集中在50kHz和20kHz左右的高频区段；（5）声发射累计振铃计数与覆盖层孔隙水压力、土压力和土体位移的变化过程存在紧密的关联性，在土体变形与塌陷时会导致声发射振铃计数增加或突发性跃迁现象，因此证明声发射技术用于岩溶塌陷监测预警是可行的。Abstract: 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.
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图 1 试验仪器与监测系统展示图
Figure 1. General diagram of the experimental equipment and monitoring system
图 2 监测仪器布设与试验装置示意图
Figure 2. Schematic diagram of the experimental equipment and monitoring apparatus
图 4 大雨条件下土体深层声发射参数特征
Figure 4. Characteristic of acoustic emission parameters in deeper overburden layer under heavy rain condition
图 5 暴雨条件下土体浅层声发射参数特征
Figure 5. Characteristic of acoustic emission parameters in upper overburden layer under rainstorm condition
图 6 暴雨条件下土体深层声发射参数特征
Figure 6. Characteristic of acoustic emission parameters in deeper overburden layer under rainstorm condition
图 3 大雨条件下土体浅层声发射参数特征
Figure 3. Characteristic of acoustic emission parameters in upper overburden layer under heavy rain condition
图 9 大雨条件下覆盖层多场耦合关联特征
Figure 9. Multi-field coupling characteristics of overburden layer under heavy rain condition
图 10 暴雨条件下覆盖层多场耦合关联特征
Figure 10. Multi-field coupling characteristics of overburden layer under rainstorm condition
表 1 试验材料基本物理力学指标
Table 1. The basic physical and mechanical indexes of the experimental material
最优含水率
$ \omega $/%最大干密度
${\rho }_{d}/g\cdot {cm}^{-1}$液限
$ {W}_{L} $/%塑限
$ {W}_{p} $/%塑性指数
$ {I}_{p} $土粒比重
$ {G}_{s} $压缩模量
$ {E}_{s} $/Mpa30.21 1.46 51.24 34.34 19.9 2.725 10.11 
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