DAMAGE CHARACTERIZATION OF COAL MEASURE ROCKS UNDER UNIAXIAL COMPRESSION

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1 DAMAGE CHARACTERIZATION OF COAL MEASURE ROCKS UNDER UNIAXIAL COMPRESSION and Michael Alber Engineering Geology Group Ruhr University Bochum, Germany ABSTRACT The identification of burst-prone rock layers is always difficult. By the application of different testing techniques brittle rock layers in a mine in southwest Germany have been identified. Especially the use of acoustic emission monitoring during uniaxial testing has proven very useful. The combination of different fracture stages identified from stress-strain diagrams and cumulative AE curves show differences in the fracture behavior of coal measure rocks. A further interpretation of the acoustic emission data sets even allows distinguishing between differently behaving rock types by AE signatures. The use of this testing technique is a benefit to the identification of brittle rocks. 1. INTRODUCTION Mining induced seismicity is a worldwide phenomenon associated with deep mining. In southwest Germany the exploitation of Carboniferous coal deposits by the longwall mining method extends up to 1500 m below surface. For several years the area is being monitored for induced earthquakes that can be traced back to mining activities (Uhl et al. 2004). Recently many seismic events with local magnitude M L > 2.0 have been observed. Common theories such as published by Hasegawa et al. (1989) or Whittles et al. (2007) do not explain the observed seismic events which have been located some 100 m above and 250 m in front of the face. As the reason for these events is still unknown, a comprehensive mechanical testing has been executed on samples from the vicinity of the seam. The goal of testing was to identify brittle rocks that may be responsible for the seismic events. The intention behind the laboratory testing was to find some properties that are common in fast and violently failing rock supposedly representing the seismogenic strata. If possible, these laboratory tests should be easy to carry out, be affordable and seismogenic rocks should be easily recognizable. Towards this goal ultrasonic testing, uniaxial loading, and acoustic emission recording were employed. This paper reports on some results of the research done to date. 2 GEOLOGICAL SETTING The Saar-Nahe Basin in SW Germany is an intermontane sedimentary basin filled with fluvial and lacustrine Carboniferous and Permian sediments (Stollhofen1998). The mine, situated at the southern rim of the basin, exploits hard coal from a depth of overburden of some 1500 m. The original sedimentary cover was 2110 m thicker than at the present day and Page 369

2 was eroded during the Permian (Hertle & Littke 2000). The mine personnel executed doublecore drilling and provided two drill cores from Stefan A and B strata (Pennsylvanian) in the hanging wall of the seam with an overall length of 200 m. Lithological units of the cores consist of silty claystones, fine to coarse siltstones, fine, medium, and coarse sandstones as well as conglomerates as shown in Figure 1. Fig 1: Typical examples of rock samples from the Saar-Nahe Basin. A: poorly sorted Conglomerate, B: medium-grained sandstone, C: Siltstone with sandy layers. 3.1 ULTRASONIC TESTING Methods A rather easy way to estimate some mechanical properties of rocks exists via ultrasonic measurements. For the equipment used in this study and the method employed we used cylindrical samples with a length to width ratio of 2:1. The sample is then placed between a transducer and a receiver and a sonic wave is transmitted through the sample. This method generates parameters such as compressional wave velocity v P, shear-wave velocity v S, and Rayleigh-wave velocity v R. A computer program then calculates the elastic properties such as dynamic Young`s Modulus and Poisson`s ratio. The parameters bulk density and v P have in case of samples from the Saar mine proven to be good indicators towards the behaviour of the sample during uniaxial compression testing. The advantage of this method is that the sample remains unharmed and the test can be carried out quite fast. Page 370

3 3.1.2 Results The dynamic parameters proved to be good indicators of how the rock will behave under uniaxial loading. Especially bulk density, together with compressional wave velocity v P may be used to delineate rock type of very brittle behavior and violent failure behavior. A trend towards a group of very brittle behaving rocks which fail violently is discussed later. However, the use of ultrasonic measurements assumes isotropic rock properties which are often not the case with sedimentary rocks. In this case of coal measure rocks there exists anisotropic features such as interlayering of fine-grained sandstones with siltstones and similar. For those strongly anisotropic rocks the calculated dynamic Young`s Moduli are way off the reasonable range due to the distorted travel paths of the emitted wave which led to very low P- and S-wave results. Additionally, the dynamic Young`s Modulus calculated from the dilatational wave was also far too low compared to the static one derived from the stressstrain curve from uniaxial testing. The very brittle rocks with sometimes violent behavior in the post peak area have some common rock properties as shown in Table 1. However, a few anisotropic rocks with lower property values than those from Table 1 also showed very brittle behavior. Property Range of values Bulk density ρ [g/cm³] 2,67-2,8 Compressive wave velocity v p [km/s] 4,5-5,3 Dynamic Young`s modulus E [GPa] Static Young`s modulus E [GPa] 20,7-25,6 Uniaxial compressive strength σ c [MPa] Tab. 1: Summary of mechanical properties of very brittle rocks derived from different testing approaches. 3.2 UNIAXIAL TESTING Methods From the drill cores from the mine numerous cylindrical samples for compressive testing were prepared following the ISRM suggested methods (ISRM 1981). Uniaxial compressive testing is probably the most used method to determine mechanical properties of rocks. It is however, time consuming, leads to the destruction of the sample and the properties strongly depend on the loading system and loading method used. Uniaxial testing was carried out with a servo-controlled MTS Test-Star system using a TestStar IIm controller. The tests were executed according to the ISRM suggested method (Fairhurst & Hudson 1999). The sample was first loaded under axial strain control with 10-5 mm/mm/s. One unloading cycle was carried out after the sample had reached the elastic phase of deformation. After the reload, the control was switched to lateral strain with a rate of 10-5 mm/mm/s for better control of the possible fast and violent failure process in the post peak area. Page 371

4 3.2.2 Results Some 90 rock samples consisting of claystones, siltstones, sandstones and conglomerates have been tested, exhibiting very different failure behaviors. Few samples showed an explosive behavior during the final moments of uniaxial testing. This rapid and often violent loss of strength could not be avoided by the lateral strain control. This behavior is hitherto termed very brittle and includes the following rock types: (i) siltstones and fine grained sandstones, (ii) interbedded sandstones with silt lenses and (iii) a few of the medium grained sandstones. It is assumed that the violent and sudden failure originated from thin and fine-grained sedimentary layers within the samples. These layers react with a very small lateral strain upon axial loading, thus prohibiting stable fracture propagation within the other layers. This internal confinement led finally to the in a ductile transition. In contrast, the samples from massive sandstone layers behaved all rather docile. Figure 2 shows the stress-strain curves of a very brittle and a ductile rock specimen under axial compression in lateral strain control. Fig. 2: Combined stress-strain diagram for two very differently behaving samples. Black line: Slow failing conglomerate with a low Young`s modulus, undergoing severe deformation. Gray line: Fast failing brittle siltstone with a noticeably higher Young s modulus. The fast failing brittle rocks showed high uniaxial compressive strength between 65 and 116 MPa. The modulus of elasticity of these sedimentary rocks range from GPa. About 10% of the tested specimen failed in a violent way. Page 372

5 3.3 ACOUSTIC EMISSION MONITORING Methods During uniaxial testing acoustic emissions emitted by the rock sample were recorded. This was done by attaching a piezoelectric transducer to the rock sample with cable clip. To reduce noise generated by the transducer grinding against the specimen, a thin layer of silica gel was placed between the rock and the transducer. Seismic events were recorded with a frequency of 500 Hz. A picture of the test setup can be seen in Figure 3. The generated AE data sets are very large and difficult to handle. They were analyzed using the software package SigmaPlot Results Fig. 3: Test setup for acoustic emission monitoring during uniaxial testing. What knowledge can be developed from the recorded AE? Figure 4 shows an example of peak amplitudes recorded during a uniaxial test at a frequency of 500 Hz. There are small events, indicated by the small amplitudes, that are always present and which are classified as continuous background noise. This noise has a certain level which increases permanently with increasing stress and also decreases when the load is taken off the sample. Additionally, there are distinct acoustic emissions with high amplitudes called transient noise. The number and the peak amplitudes of those distinct AE, along with their occurrence during the compressive test may be used to classify the different behavioral rock types discussed in the previous section. The acoustic emissions reflect fracture initiation and propagation within the stressed rock sample. The basic research of the failure mechanisms in brittle rock deformation was done by Bieniawski (1967). His findings continue to be of great value until this present day. Many researchers combined Bieniawski`s findings about fracture propagation with their observations of AE (Lockner 1993, Eberhardt 1997). This approch was also taken here. The fracture stages identified by Bieniawski (1967) are: 1. Closing of cracks 2. Linear elastic deformation 3. Stable fracture propagation 4. Unstable fracture propagation 5. Forking and coalescence of cracks Page 373

6 These five fracture stages are somewhat difficult to identify when using stress strain curves of the sedimentary rocks under investigation. Figure 5 shows axial stress vs. axial strain and volumetric strain, respectively. Three fracture stages may be identified with these curves: (i) closure of microcracks, denoted by the flat stress strain curve. (ii) beginning of the linear elastic range during the tests, indicating stable facture propagation, and (iii) the onset of unstable fracture propagation as seen by the reversal of the slope of the volumetric strain curve. Fig. 4: Plot of peak amplitudes (db) and axial stress (MPa) against run time recorded during a uniaxial test. The figure shows all AE emitted during an uniaxial loading test. It may be however difficult to establish those fracture stages depending on the test control mode, the accuracy of the equipment used and on the rock type. The analyses of acoustic emissions may be used for easier identification of some fracture stages. The compressive test from Figure 5 is re-plotted in Figure 6 showing additionally the cumulative acoustic emissions. Those are computed by counting the events with amplitudes higher than the background noise and summing them up. Page 374

7 Fig. 5: Axial stress vs. axial strain/volumetric strain for a medium grained sandstone. Three different stages in fracture behaviour were identified as discussed in the text. Fig. 6: Axial strain vs. cumulative AE curve and axial strain vs. axial stress. The three stages identified in figure 5 correlate well with the slope of the cumulative AE curve. In stage 1 (crack closure) a large amount of AE occur, while in stage 2 (elastic region) only a few AE are emitted. With the beginning of stage 3 (unstable crack growth) the amount of AE increases substantially. Page 375

8 With this approach the following fracture stages may be more easily identified: Closure of microcracks causes numerous acoustic emissions. The amount of AE emitted during this phase depends upon rock properties such as grain size and grain contacts, bulk density (e.g. porosity) and v p. A stage, during which the deformation is purely elastic and no AE are recorded, is not present with this rock. Stable fracture propagation with rather few AE may be clearly delineated from cumulative AE plot. It will be shown later that the AE characteristics in this stage may be used to generally classify different behavioral rock types. The beginning of unstable fracture propagation (Stage 4 after Bieniawski 1967) is marked by an increase in AE. In some fine grained brittle rocks, especially siltstones, the onset of this stage coincides with the peak strength and violent fast fracture propagation leads to the disintegration of the sample. From the analyses of the AE signatures of all 90 tests is was found that with slow failing samples more acoustic emissions with low db values occur than in fast failing ones. On the other hand fast failing rocks exhibit some values with very high peaks which mark the fracture process, e.g. stress drop from peak strength to some lower value. Generally, fast failing samples exhibit less AE than slow failing ones. These observations are schematically shown in Figures 7 and 8. Figure 7 shows for two different rock types, a medium grained sandstone and a siltstone, the cumulative AE vs. axial strain and axial stress vs. axial strain, respectively. The medium grained rock reacts upon loading with many AE of mainly low amplitudes which occur throughout the test. This indicates continuous fracture initiation and propagation so that the sample fails slowly and non-violently. The fine grained siltstone shows after the initial closure of fracture few acoustic emissions, indicating that few fractures are formed during the main part of the test. Finally the stresses are released by a few fast propagating fractures which emit few AE with high amplitudes. Figure 8 depicts two histograms reflecting the frequency of the AE events ordered after their amplitude. The slowly failing rock shows much more AE events, but only a few in the high amplitude range over 30 db. The fast failing siltstone has rather few AE events, but a significant amount in the high amplitude range over 50 db, reflecting the sudden release of energy when failing violently. These findings may be used to delineate behavioral rock types with help of the AE signatures of the different specimens. Page 376

9 Fig. 7: Combined plot of stress strain-diagrams (dotted lines) and cumulative AE against axial strain (solid lines) of two different samples. The two specimen have similar Young`s moduli, but showed completely different failure behavior. Black lines: slow failing medium grained sandstone with many AE occuring during the entire test. Gray lines: Very brittle siltstone with visibly less AE. Note that in the fast failing sample (gray) no AE occur before the fracture. Fig. 8: Histograms of peak amplitudes from two rock samples with significantly different AE. Black columns: Medium grained sandstone specimen with an overall large number of low amplitudes and hardly any high amplitudes. Gray columns: Brittle siltstone with a relatively small number of low peak amplitudes and a few high amplitudes between 50 and 60 db as result of the final fracture. Page 377

10 4. CONCLUSION What benefits have these results for ground control in a coal mine? The tests show that some brittle rocks occur in the surrounding strata of the mining area and that they are clearly identifiable by uniaxial testing. But it is also possible to identify a larger range of possible dangerous rock layers by more easily determining parameters such as vp, bulk density, and a dynamic modulus of elasticity, allowing therefore the mine management to implement an inexpensive test program and to take counteractive measures. The acoustic emission monitoring during the uniaxial testing is also a good indicator if the samples tend to burst rather than to fail slowly. A classification of 3 fracture stages by the cumulative curve of AE allows identification of the significant phases of the fracture process, like unstable crack propagation. It is even possible to distinguish between different rock types by cumulative AE patterns and peak amplitude histograms. The very brittle rocks from the Saar-Nahe Basin tended to have no AE output before imminent failure while slow failing ones tended to generate a fair amount of AE before reaching their peak strength. The use of acoustic emissions during uniaxial testing therefore represents therefore a great step towards classification of damage characterization of coal measure rocks. REFERENCES BIENIAWSKI, Z.T. Mechanism of Brittle Fracture of Rock Part I, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1967, Vol. 4, pp EBERHARDT, E., STEAD, D.STIMPSON, D. & READ, R.S. Changes in acoustic event properties with progressive fracture damage, Int. J. Rock Mech. Min Sci., 1997, 34(3-4), Paper No.071B. FAIRHURST, C.E. & HUDSON, J.A. Draft ISRM suggested method for the complete stressstrain curve for intact rock in uniaxial compression, Int. J. Rock Mech. Min. Sci., 1999, Vol. 36(3), pp HASEGAWA, H.S., WETMILLER, R.J., & GENDZWILL, D.J. Induced Seismiscity in Mines in Canada An Overview., S.J. Gibowicz (ed.), Seismicity in Mines, 1989, pp HERTLE, M. & LITTKE, R. Coalification pattern and thermal modelling of the Permo- Carboniferous Saar-Basin (SW-Germany). Int. J. Coal Geol., 2000, 42: ISRM Suggested Methods for Rock Characterization, Testing, and Monitoring, E.T. Brown (Ed.), 1981, Pergamon, Oxford, 211p. LOCKNER, D. The role of acoustic emission in the study of rock fracture., Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1993, Vol. 30(7), pp Page 378

11 STOLLHOFEN, H. Facies architecture variations and seismogenic structures in the Carboniferous-Permian Saar-Nahe Basin (SW Germany): Evidence for extension-related transfer fault activity, Sedimentary Geology, 1998, Vol. 119, pp UHL,O.,POLYSOS,N.,FRITSCHEN,R.,LOSKE,B.,SROKA, A. Seismologische und geologische Untersuchungen der Erderschütterungen des Bergwerks Ensdorf. Tagungsband zur 44. Wissenschaftlichen Fachtagung des Deutschen Markscheider Vereins e.v., 2004, Bochum (in German). WHITTLES, D.N., REDDISH, D. J. & LOWNDES, I. S. The development of a coal measure classification (CMC) and its use for prediction of geomechanical parameters. Int. J. Rock Mech. Min. Sci., 2007, Vol. 44(4), pp Page 379

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