Studies of stress and displacement distribution and the evolution law during rock failure process based on acoustic emission and microseismic monitoring
Introduction
In the rock mechanics experiment, the stress and deformation are commonly recorded using the pressure transducer, strain gauge, displacement meter or other measuring methods. The stress-strain curve is the most basic information used to analyze the changes of stress and deformation. However, in most cases, this information only reflects the average stress and the overall deformation within the rock. As a typically anisotropic and inhomogeneous material, rock contains a great number of natural defects at various scales, such as micro-cracks, pores, fissures, joints inclusions, and precipitates. For this reason, the stress and deformation are not uniformly distributed in a rock specimen during the loading process. Therefore, the distribution and evolution law of stress and deformation is highly important in studies of the rock failure mechanism. When rock is subjected to loading conditions, micro-cracks are gradually generated, propagated and coalesced, accompanied by elastic strain energy release, i.e., acoustic emission (AE). Because the AE technique can continuously monitor the temporal-spatial evolution behaviors of micro-cracks, it can reveal the deformation and failure process in the rock. Many achievements have been reported, e.g., AE location algorithms, temporal-spatial evolution behaviors of micro-cracks in rock bodies, and the changes in many AE parameters.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 Acoustic emission can be considered as a form of micro-seismicity, and this is the reason why the acoustic emission technique is so similar compared to seismological techniques. Both approaches address the same concept but at a different scale.12 Far-field seismology investigates earthquakes at a distance of thousands of kilometers, and near-field seismology is applied at distances of several hundred kilometers. Acoustic emission technique is commonly applied for a source receiver distance of up several meters or even smaller, and the specimen can be quite small, reaching the millimeter scale. Rock mass failures in the mining process at a certain range of distances (i.e., the fracture plane is larger than one meter and may reach several meters) are known as microseisms. Therefore, the analyzed methods for stress and deformation from seismology theory can be applied to study the rock (mass) failure process.
Apparent stress is a ratio of the total radiated seismic energy to seismic moment, assessing the amount of energy released per unit of deformation. Increasing stress conditions store more energy while simultaneously increasing confining forces, which reduce deformation and result in high apparent stress. This enables apparent stress as a proxy for increasing stress conditions in a rock mass.13 High apparent stress values are indicative of high and increasing stress conditions within the rock mass, and vice versa.14,15 During the process of earthquake preparation, regional stress is constantly adjusted and redistributed. The relationship between regional stress evolution and earthquake preparation can offer deeper insight to the fracture mechanism of the rock mass and is significant for prediction of earthquake occurrence. Many research results have indicated that the apparent stress increases on average with increasing seismic moment and shows local concentrations to the hypocenter prior to strong earthquakes.16,17 By conducting studies on the distribution of the apparent stress in global or selected local regions, the apparent stress was verified as important in earthquake prediction.18, 19, 20 In deep metal mines, the dynamic rupture processes of these ground pressure disasters are similar to those of larger earthquakes.21,22 With the development and application of microseismic (MS) monitoring technology, many seismic source parameters, including the seismic moment, radiated seismic energy, apparent volume, apparent stress, etc., were introduced to evaluate the rock mass stability. Similar to apparent stress, apparent volume depends on seismic potency and radiated energy, and because of its scalar nature, it can easily be manipulated in the form of cumulative or contour plots.23 Therefore, studies of the stress and displacement distribution and evolution law during rock mass failure process are feasible. Based on analysis of the MS data, the stress and displacement distributions and evolutions have been widely used to assess the rock mass stability and forecast dynamic hazards to better manages disastrous rock failures.24, 25, 26, 27, 28, 29, 30, 31, 32
Considering that the stress and deformation obtained from external measurements cannot reflect they actual changes within rock, this study conducted stress and displacement analysis according to seismology. After the spatial-temporal evolutions of AE events during the rock failure process were obtained based on the simplex location algorithm by conducting laboratory experiments on a ‘Z’ shape specimen, the feasibility of stress and displacement analysis based on AE location and seismological theory was verified. The stress and displacement evolution for an intact rock specimen was analyzed, especially the relationship between the apparent volume and the strain. Based on laboratory results, a project case was introduced to examine the spatial-temporal distributions of MS events as well as the stress and displacement evolution before and after caving process of a deep stope in the Ashele copper mine (China). The results of this study are expected to deepen our understanding of the evaluation of rock stability and risk assessment of loose-type hazards.
Section snippets
Experimental process
Two types of granite specimens were used in these experiments. The ‘Z’ shape specimen shown in Fig. 1(a), which is 220 mm in length, 100 mm in width and 300 mm in height. The stress is easily concentrated and the fractures are prone to occurred in the middle of this shape specimen, which is applied to verify the feasibility of stress and displacement analysis based on AE location and seismological theory. For this type of specimen, uniaxial loading was adopted. The cubic specimen shown in Fig. 1
Theory for stress and displacement calculation
Determination of the P-wave arrival time is based on the assumption that the waveform can be viewed in the normal state before and after arrival, and the best cut point is used as the P-wave arrival time. The auto-regression models (AR model) and Akaike information criterion (AIC) were used to determine the P-wave arrival time.33 After the P-wave arrival times were determined, the simplex method34,35 was used to calculate the AE hypocenter position. The pencil lead break test result showed that
‘Z’ type specimen
Because the unit of the AE signals detected during the experiment is voltage, i.e., mV, the AE source parameters, including seismic moment, radiated seismic energy, apparent stress and displacement, are all relative values, rather than absolute values. Therefore, the display results of the size of AE events, the apparent stress, and the displacement cloud diagram are also relative values. For the ‘Z’ type specimen, micro-cracks were mainly concentrated in the middle of the rock, as shown in
Stress and displacement evolution in rock mass during deep stope caving
The Ashele copper mine is located in the Xinjiang uygur autonomous region of China. Currently, the development depth and mining depth are 1200 m and 900 m, respectively, and the daily ore output is 6000 t. The ground stress is controlled by tectonic stress in deep mining and increases with mining depth. According to the stress test results, the maximum principal stress exceeds 30 MPa when the mining depth reaches 800 m underground. At this mining depth, the horizontal stress is nearly 1.5 times
Discussion
- (1)
In calculation of the stress field and displacement field, the source parameters obtained, such as energy and seismic moment, all use the location information of AE and MS events, and thus the location accuracy of AE and MS events is highly important. Therefore, before stress field and displacement field analysis, the location accuracy of AE and MS events should be guaranteed to meet the requirements.
- (2)
Analysis of the stress field and displacement field from MS data is based on statistical
Conclusion
Studies on the evolution of stress and displacement within rock in laboratory experiments based on AE monitoring and the differences in the distribution and evolution characteristics of the stress field and deformation field before caving in a deep stope based on in-situ MS monitoring were conducted. The results are described as follows.
- (1)
The experimental results from a ‘Z’ type rock specimen indicate that the distributions of micro-cracks, apparent stress and deformation inside the rock are
Declaration of competing interest
The authors declare that the work described has not been published before; that it is not under consideration for publication anywhere else; that its publication has been approved by all co-authors; that there is no conflict of interest regarding the publication of this article.
Acknowledgements
This work was financially supported by the National Key Research and Development Program of China (2017YFC0602904, 2016YFC0801605), The Project supported by National Natural Science Foundation of China (51974059), the Fundamental Research Funds for the Central Universities (N180115010), the China Postdoctoral Science Foundation funded project (2017M612302) and the Postdoctoral Creative Funding of Shangdong Province.
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