Research ArticleEnergy Evolution and AE Failure Precursory Characteristics ofRocks with Different Rockburst Proneness

Feng Pei ,1,2 Hongguang Ji ,1,2 Jiwei Zhao,1,2 and Jingming Geng1,2

1School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China2Beijing Key Laboratory of Urban Underground Space Engineering, University of Science and Technology Beijing,Beijing 100083, China

Correspondence should be addressed to Hongguang Ji; 1078938768@qq.com

Received 7 April 2020; Revised 16 May 2020; Accepted 25 May 2020; Published 15 June 2020

Academic Editor: Hailing Kong

Copyright © 2020 Feng Pei et al.-is is an open access article distributed under the Creative CommonsAttribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Mastering the precursory information of rock failure is the basis of scientifically predicting rockburst, and AE technology is aneffective means to solve this problem. -e conventional uniaxial loading and cyclic loading/unloading tests of metagabbro andgranite were carried out with GAW-2000 uniaxial electrohydraulic rigid testing machine to evaluate rockburst proneness. -eenergy evolution and AE characteristics of rocks with different rockburst proneness during loading are revealed. -e results showthat the rockburst proneness of granite is obviously stronger than that of metagabbro based on the comprehensive evaluationmethod of multiple rockburst proneness index.-e reasons for different rockburst proneness are analyzed from the perspective ofmineral composition and microstructure. Rockburst proneness is positively correlated with energy storage capacity. -e elasticenergy ratio of granite is obviously larger than that of metagabbro before peak stress. -e intensity of AE signals generated in thefailure process of strong rockburst rock (granite) is significantly higher than that of moderate rockburst rock (metagabbro).However, the peak frequency bands and amplitude all increase obviously before failure.-e b-value andmemory characteristics ofrock with different rockburst proneness have obvious similar change rules.

1. Introduction

With the depletion of shallow resources, a great number ofcountries have gradually entered the deep mining stage.Rockburst disaster occurred frequently, which posed agreat threat to the safety of equipment, life, and property.Rock is a heterogeneous material due to its petrographic,mineralogical, and internal microstructure features thataffect the engineering properties of this rock. Rock storeselastic energy in the form of elastic strain, while it con-sumes energy in the form of crack propagation anddamage. When the rock is loaded to its ultimate bearingcapacity, aggregated strain energy in rock mass is releasedrapidly, thereby causing massive attack. In the meantime, alarge number of AE signals are released in the form ofelastic waves. AE signals contain a lot of information aboutcrack propagation and coalescence. It could infer internalstructure alteration and failure mechanism under different

stress levels, and then the rockburst could be effectivelypredicted based on AE signals.

Considerable research on rock energy evolution has beenundertaken, and scholars have achieved many importantresearch results. Xie et al. [1, 2], for example, reveal theessential characteristics of rock instability and failure fromthe energy point of view, which lays a foundation for energyevolution analysis of rock mechanics. Nonlinear charac-teristics and energy distribution proportion of red sandstonewere investigated by Zhang and Gao [3, 4] through uniaxialand triaxial cyclic loading/unloading testing. -e relation-ship between energy conversion and rock fragmentation wasclarified [5]. Li et al. [2] carried out uniaxial loading/unloading tests of rocks at different loading speeds andobtained the variation of releasable strain energy and dis-sipation energy. In addition, a large number of scholars havestudied the relationship between rock damage and energyevolution [6–8]. However, little attention has been paid to

HindawiAdvances in Civil EngineeringVolume 2020, Article ID 8877901, 12 pageshttps://doi.org/10.1155/2020/8877901

study the differences in the energy evolution of rocks withdifferent rockburst proneness.

-e deformation and failure of rocks have been inves-tigated by analyzing AE signals, and studies have shown thatinformation from the AE signals can be used to monitor thedevelopment of internal cracks in rocks and serve as an earlywarning for the instability and failure of rocks. Scholars havedone a lot of research studies on AE signals from charac-teristic parameters and spectrum characteristics. On the onehand, AE signals are used to study the evolution process ofrock fracture development by characteristic parameters suchas ring counting rate, event rate, and energy rate. On theother hand, the combination of characteristic parameterssuch as RA (rising time/amplitude) [9, 10] and AF(counting/duration) is used to study the variation of pa-rameters in the loading process. In the research of spectrumcharacteristics, Ji [11, 12] pointed out that the sudden in-crease of high-frequency and high-amplitude AE signalsindicates that granite will be destabilized with the increase ofstress level. -e main frequency distribution band of AEsignals in the process of rockburst is revealed [13–16].

Rockburst proneness is the inherent property of rock.-eresearch of rockburst proneness is the basis of predicting andpreventing rockburst and has important guiding significancefor dynamic disasters. Engineering practice shows that rock iscommonly loaded cyclically during construction. However,there are few comparison studies on energy evolution and AEcharacteristics of rocks with different rockburst pronenessduring cyclic loading/unloading. In this study, uniaxial cyclicloading/unloading testing is conducted on rocks with dif-ferent rockburst proneness, and energy evolution and AEcharacteristics of rocks with different rockburst proneness areobtained. Felicity ratio was used to characterize the damagememory characteristics of two kinds of rocks during loading/unloading. At the same time, the development law of AE b-value is revealed, which provides a theoretical basis for theprediction of rock failure. -e relationship between damageevolution, energy accumulation and release, andAE precursorinformation before the failure of rocks with different rock-burst proneness is clarified.

2. Comprehensive Evaluation of RockburstProneness Based on Multiple Criteria

Rockburst must have two necessary conditions [17]: (1) rockhas the ability to store high strain energy, which releasesenormous energy in the failure process, and (2) excavationdisturbance brings about local energy concentration, whichprovides energy for rockburst. In this work, a large numberof laboratory experiments have been carried out for the firstnecessary condition. In view of the limitation of a singleindex to evaluate rockburst proneness, the rockburstproneness is evaluated by brittleness coefficient B, maximumelastic energy density Es, and rockburst tendency indexWet.

2.1. Brittleness Coefficient B. -e strength brittleness coef-ficient could be defined as the ratio of uniaxial compressivestrength (σc) to uniaxial tensile strength (σt):

B �σcσt

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where σc is the uniaxial compressive strength and σt is thetensile strength. -e average tensile strength of granite andmetagabbro is 7.82–9.14MPa and 6.75–8.64MPa,respectively.

2.2. Maximum Elastic Energy Density Es. -e maximumelastic strain energy density before the peak strength indi-cates the energy storage capacity of rock, which providessufficient energy support for the dynamic impact of rockafter failure. Several typical uniaxial compression curves andfailure patterns are shown in Figure 1:

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where σc is the uniaxial compressive strength (UCS), MPa; Eis Young’s modulus, MPa.

2.3. Rockburst Proneness Index Wet. -e rockburst prone-ness index is a quantitative parameter, which corresponds tothe ratio between the storage energy and the consumptionenergy in the loading stage, i.e., the loading measures80–90% of the peak strength and the unloading measures 5%of the peak stress. -e rockburst proneness index is the ratiobetween the elastic strain energy recovered during theunloading process and the dissipation energy in the loadingprocess. -e larger the rockburst proneness index is, themore likely an impact occurs in case of failure. Typicalloading and unloading curves and failure patterns are shownin Figure 2.

Based on the comprehensive evaluation result in Table 1,it could be concluded that although there are small differ-ences in rockburst proneness discriminant results betweenthe different criteria, metagabbro shows a weak or moderaterockburst proneness, while granite shows a strong rockburstproneness. -e impact intensity of granite is obviouslystronger than that of metagabbro.

3. Microstructure Characteristics

Many research studies have clarified that the mechanicalbehavior of rock and energy storage capability are essentiallydetermined by mineral composition, micromorphology, andcombination mode [18–25]. -e mineral composition andmicromorphology characteristics of two kinds of rocks areobtained by combining powder X-ray diffraction (XRD) andbinocular transmission polarization microscopy. -e resultsare shown in Figures 3 and 4.

-e metagabbro is grey-green, with scaly-columnar andfibrous metamorphic textures and banded and massivestructures. It is mainly composed of amphibole (30%),feldspar (45%), quartz (15%), biotite (5%), kaolin (3%), andchlorite (2%).-e composition is not uniform, whichmainlyshows the difference of altered mineral content caused bydifferent alteration intensity. -e minerals have the char-acteristics of directional arrangement under the microscope.

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-e particle size distribution is shown in Figure 3(d), and theequivalent radius is mainly distributed in the range of0.04–0.2mm.

Comprehensive analysis shows that the granite has aporphyry-like structure with obvious schistosomiasis. -eporphyry is feldspar (60%) with a grain size of about 0.5mm.-e matrix is mainly quartz (30%). -e dark minerals aremainly biotite (5%) with directional arrangement. Otherminerals include chlorite (about 3%) and pyrite (about 2%).-e equivalent radius is mainly distributed in the range of0.2–0.75mm.

Quartz has compact structure, high stiffness, goodmechanical properties, and small dispersion. -e texture offeldspar is relatively soft than that of quartz, and its me-chanical properties are relatively discrete. Mica is a softmaterial with obvious pore structure and great dispersion ofmechanical properties. Quartz and feldspar with stable

mechanical properties in granite are obviously higher thanthat in metagabbro, which is the key element affecting rockimpact proneness. Meanwhile, the crystallization degree isalso the key factor affecting rockburst proneness.

4. Energy Evolution Characteristics

4.1. SpecimenPreparation. Samples used in the experimentsare granite and metagabbro, which were collected from themain shaft geological exploration hole of Shanling goldmine in Laizhou, Shandong Province. According to themethod recommended by ISRM, samples were cylindricalcores drilled with diameters of 50mm and lengths of100mm. To reduce the dispersion of experimental resultscaused by the natural differences in rock specimens,specimens with fewer cracks and no obvious defects wereselected for experiments.

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4.2. Test Procedures. -e testing schemes were as follows:granite and metagabbro specimens were loaded/unloaded ata rate of 0.01mm/min in steps of 30 kN. In order to preventthe specimen from sliding off the compression head, theloading was only unloaded to 2 kN in each cyclic step. -eloading/unloading cycles were repeated until the specimenfailed. AE signals were recorded with a processing systemusing a gain of 40 dB and a trigger amplitude threshold of40 dB. AE monitoring systems and the loading system wereturned on simultaneously to acquire the AE data and themechanical parameters.

4.3. Energy Evolution. -e relationship between energydensity and strain in uniaxial cyclic loading/unloading isdepicted in Figure 5.

It can be seen that energy density has an obviousnonlinear relationship with strain and the energy evolutionof rock with different rockburst proneness demonstratesobvious similarity characteristics.

-e input energy density and elastic strain energydensity almost coincide, and storing elastic strain energy isdominated in the elastic deformation stage. Due to internalcrack initiation and propagation with stress level

enhancement, input energy density deviates from elasticstrain energy gradually. Small-scale crack propagation isdominated, and dissipation energy density increases slowly.Small-scale cracks are generating continuously and large-scale cracks are starting to develop, more energy was con-suming when entering the unsteady crack propagation stage,and the closer to the peak strength is, the greater the increaseof dissipated energy density is until destroyed. -e storageelastic energy density of granite and metagabbro at the peakstrength is 0.248MJ·m−3 and 0.152MJ·m−3, respectively. -estronger the impact proneness is, the larger the energystorage capacity is.

In order to characterize the relationship between energydistribution ratios in the loading process, the energy storageratio is defined as the ratio of elastic strain energy density toinput energy density, and the ratio of dissipated energydensity to input energy density is energy dissipation ratio.Energy storage ratio and energy dissipation ratio are cal-culated in Figure 6. As shown from Figure 6 that the dis-sipation ratio could be divided into four stages: decreasing,stabilization, slow increase, and sharp increase. In Figure 6,the dissipation ratio of metagabbro in the compaction stageis 0.267;however the dissipation ratio of granite in thecompaction stage is 0.133. -e dissipation ratio of

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Table 1: Rockburst prediction results based on multiple criteria.

Lithology Depth (m)Maximum elastic strain

energy density Brittleness index Rockburst pronenessindex

Es Level B Level Wet LevelMetagabbro 761 172.9 III 14.9 III 3.76 IIIMetagabbro 965 145.5 III 14.7 III 3.47 IIMetagabbro 1090 146.8 III 13.4 III 3.82 IIIGranite 1415 247.0 IV 16.77 III 7.98 IVGranite 1524 321.0 IV 19.5 IV 6.32 IVGranite 1526 410.59 IV 21 IV 5.16 IVII: weak rockburst proneness; III: moderate rockburst proneness; IV: strong rockburst proneness.

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metagabbro is obviously higher than that of granite at low-stress level.

-ere are many defects and fractures in the metagabbro,and the compaction stage is more obvious, so the inputenergy is used to improve the stiffness of the rock. -edissipation energy ration of metagabbro and granite is stablein 0.06–0.1 and 0.05–0.09 during the elastic deformationstage. Distribution ratio is approximately the same, and theratio of elastic energy density to dissipation energy density isabout 9 :1 in the linear development stage. -e dissipationenergy ratio increases with the stress level increase, and thecloser to the peak strength is, the larger the increasing extentis. -e dissipation energy ratio at the peak strength ofmetagabbro and granite is 0.15 and 0.13, respectively. -eextension of the elastic stage and reduction of the plasticstage lead to obvious improvement of rock energy storagecapacity, and rock energy is released quickly at the momentof failure, which then causes strong impact failure.

5. AE Characteristics

5.1. AE Parameter Characteristics at Different Stress Levels.Given that AE signals are very sensitive to the initiation andgrowth of cracks in materials and structures, it has beenwidely used to evaluate damage mechanisms of rock. AE

transducers were attached to the specimen to record AE dataand to avoid background noise, and the trigger thresholdwas set to 40 dB, where the peak definition time (PDT), hitdefinition time (HDT), and hit locking time (HLT) were setto 50, 100, and 500 μs, respectively, for the test. A layer ofultrasonic gel and tape were applied to provide better contactbetween the transducer and surface sample. -e relationshipamong AE count, cumulative counts, AE energy, and loadwith time is depicted in Figure 7.

Compared with conventional uniaxial loading, rockunder cyclic loading/unloading also undergoes compaction,linear development, stable crack propagation, unstable crackpropagation, and rapid decline after peak. On the whole, theAE signals showed a slow increase in the elastic stage, asignificant increase in unstable crack propagation, and asharp increase in the failure stage.

Under the first cycle, the specimens are mainly in thecompaction stage. -ere are fewer AE events as a result ofcrack closure and compaction. -e cumulative countingcurve shows the first rising step, but AE signals of themetagabbro in the compaction stage are obviouslystronger than those of the granite, and even high-energyAE signals appear locally. -e occurrence of local high-energy signals may be related to the rapid adjustment ofthe internal structure of metagabbro. At relatively low

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stress, the stress did not play a primary role in the extension ofmicrocrack intensity; microcrack propagation was related tothe initial microcrack size, position, and direction.

Afterwards, the rock samples enter the elastic defor-mation stage corresponding to 47.4% and 65.7% stresslevels, respectively. Elastic deformation is predominantand almost no new cracks occur. Although the cumulativeringing count curve of granite increases like a step, thegradient is small, and the cumulative ringing curve ofmetagabbro is nearly horizontal. During 47.4%–63.2%and 65.7%–76.4% stress level corresponding to graniteand metagabbro, respectively, AE signals are graduallyactive, but the energy release is still at a lower level. Asshown in Figure 7(a), the AE signals are almost sym-metrically distributed at the maximum stress of this cycle,indicating that crack propagation stops immediately afterunloading to last peak stress, and the rock sample is still ina weak damage state.

During 63.2% and 76.4% stress level to peak strength, theAE ringing count and energy release increase suddenly, andthe cumulative ringing count increases abruptly. Obviously,the quiet phase of AE signals in the prepeak phase was notobserved. Even at a lower stress level, the AE signals are stillactive in the unloading stage. In the unloading process,because of the elastic recovery of the element in the elasticstate, there must be a friction effect between the elastic-plastic parts (elements or particles). -is friction should alsobe one of the physical mechanisms of AE during unloading.

New cracks and original cracks will continue to grow inrock under low stress level. When rock sample reaches its peakstress, a large amount of elastic energy stored in the rocksample will suddenly release along the direction most vul-nerable to damage and then produce high-energy AE signals.

Before the failure of samples, the AE ringing count rateof granite is obviously higher than that of metagabbro. -eAE energy released at the peak strength of granite is

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2×106 aJ, and that energy released by metagabbro is about6.5×104 aJ. -e maximum release energy at the peakstrength of granite is 30.77 times that of metagabbro, whichindicates the severity of rock destruction.-erefore, it can beconcluded that the rockburst proneness of rock is positivelyrelated to the AE energy released during failure.

5.2. AE Spectrum Characteristics. AE amplitude is a directcharacterization parameter of crack propagation strength.-e AE spectrum characteristics are determined by differentcrack growth modes. -erefore, amplitude and spectrumcharacteristics are of great significance to analyze themechanism of rock crack, and AE spectrum characteristicsunder cyclic loading and unloading are depicted in Figure 8.

AE amplitudes of granite and metagabbro generated bycracks closure are concentrated in 40–60 dB and 40–45 dB,respectively. AE amplitudes are generally less than 50 dB inthe elastic development stage. High-amplitude AE signalsappear at each peak stress of loading/unloading and the AEamplitudes increase gradually with the stress level increase.However, at the same stress level, the AE amplitude ofgranite is generally higher than that of metagabbro. -eamplitude of granite at the crack propagation stage changeswith time in the form of arch bridge, which results in theabsence of 40 dB–45 dB AE signals. When loading stressexceeds a certain stress level, the crack propagation isdominant in samples, and some crack initiation signals areconcealed. But metagabbro did not occur above phenomena,and this is related to internal structure adjustment. -e

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maximum amplitude of granite and metagabbro at peakstrength is 91 dB and 72 dB, which also shows the intensity ofreleased energy when samples are destroyed.

-roughout the whole cyclic loading/unloading process,the peak frequency of granite is mainly concentrated in the20 kHz and 50kHz bands. -e frequency bands near 50 kHzare dominant due to crack closure, and local high-frequencysignals of 155–165 kHz appear. New cracks are constantlygenerated in rock, and large-scale crack propagation occurslocally in the stable and unstable crack propagation stage, andthe frequency range is 60–80kHz. -erefore, it can be inferredthat the large-scale crack propagation is concentrated in thefrequency range of 60–80kHz. AE signals of 155 kHz high-frequency band appear near the peak strength of granite, andthe high-frequency AE signals appear earlier than the peakstrength. -erefore, the sudden increase of high-frequencysignals indicates that macrofracture surface will occur in rockand it is about to enter the instability stage. -e signal char-acteristics in the frequency band of metagabbro are similar togranite. In the unsteady crack propagation stage, 60–80 kHzfrequency band also appears, and the AE signals in 155kHzhigh-frequency band also appear at the peak strength. During80–100% stress level, the internal cracks expand fully and theAE signals increase obviously in the low-frequency range.

5.3. AE Felicity Ratio. -e discovery of Kaiser effect was firstmade in metals by Dr Joseph Kaiser and reported in hisdoctoral thesis.-en it was extended to rock materials, and a

lot of research studies on rock memory effect were carriedout. Kaiser effect refers to the ability of rock to remember themaximum loading history. It is affected by loading history,loading mode, time effect, rock structure, and rock com-position. Among these factors, loading history has the mostsignificant effect on the AE characteristics of rock.

Kaiser effect is not obvious with the stress level increase,and rock memory deteriorates with the damage increase,and this phenomenon is called Felicity effect.-e accuracy ofKaiser effect can be measured by the Felicity ratio, which isinversely proportional to the internal damage:

RF(i) �σAEσ(i−1)

, (3)

where RF(i) is the Felicity ratio in i-th cycle. σAE is the stresslevel when the effective AE is restored during i-th loadingprocess; σ(i−1) is the maximum stress level reached by (i-1)thloading. In view of the ambiguity of the definition of“significant increase” of AE signals, in this paper, the stressthreshold for σAE is set at AE counts greater than 100 for thegranite and metagabbro specimens.

-e variation in AE Felicity ratio of rocks with differentrockburst proneness was basically consistent with cyclenumbers (Figure 9). -e RF of granite and metagabbro canbe divided into three phases, Phases I, II, and III. In Phase I,RF is greater than one under low stress level (correspondingto below 58.3% and 77.8% stress levels, respectively), indi-cating that rocks have an obvious memory as a result of lessdamage. Rock samples are repeatedly compacted under

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loading, only the load exceeds previous maximum stress,and the significant AE signals will be generated. Accordingto the energy distribution theory, the input energy is mainly

stored in the form of elastic energy, while the dissipatedenergy by internal crack propagation is less.

In Phase II, 0.4<RF< 1, the RF gradually declines as aresult of increased damage. With the further increase ofstress level, the cracks parallel to the loading direction ex-panded. RF of metagabbro decreased by 0.6 from 0.98 to0.38, while RF of granite decreased by 0.41 from 0.9 to 0.49.-e decline extent of RF can be used to indicate the damagedegree. -e maximum drop of metagabbro is significantlygreater than that of granite, stating clearly that the damagedegree of metagabbro with the same stress increase is greaterthan that of granite. -e internal damage of rock samples ismainly caused by the initiation and expansion of internalcracks, and the dissipation energy of rock samples increasesgradually.

In Phase III, the RF of granite and metagabbro decreasedlinearly after the eighth and seventh cycles, respectively, andthe internal cracks were further extended due to the action ofhigh-stress level. Owing to accumulated damage in rocksamples, the cracks could expand continuously even at low-stress level and produce violent AE signals.

Overall, the changes in RF for granite are generally thesame as the changes in metagabbro. However, in theloading and unloading processes, the AE signals of granite

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freq

uenc

y (k

Hz)

Peak

freq

uenc

y (k

Hz)

200

160

120

80

40

0

(b)

Figure 8: (a) AE amplitude and (b) AE peak frequency (left: granites; right: metagabbro).

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Felic

ity ra

tio

2 4 6 8 10Cycle number

GraniteMetagabbro

I II III

Figure 9:-e relationship between Felicity ratio and cycle number.

Advances in Civil Engineering 9

with strong storage energy capacity are stronger thanthose of metagabbro. Due to the release of elastic energy,the stress level of rapid RF decline of granite is lower thanthat of metagabbro. However, the definition of RF indi-cates that its value can only be used to determine thedamage degree of rocks in the previous cycle, while thiscycle damage can only be reflected in the next cycle, with acertain hysteresis.

5.4. AE b-Value. In consideration of the hysteresis of usingFelicity ratio to characterize damage, this section studies theevolution of AE b-value during loading to discuss on theprecursory information of rock failure.

In the field of earthquake seismology, it is well knownthat small-magnitude earthquakes frequently occur, whereaslarge-magnitude earthquakes occur rarely. Gutenberg andRichter [26] conducted a statistical analysis of a largenumber of seismic activities and obtained the followingrelationship between earthquake magnitude and frequency[27–30]:

lgN � a − bM, (4)

where M is the magnitude, N is the earthquake frequencywithin ΔM range, a and b are constants, and b-value is animportant parameter to evaluate the seismic activity level ina certain region. Since there is no concept of magnitude inAE, the magnitude M is usually replaced by the monitoredAE amplitude during the experiment:

ML �m

20. (5)

ML represents the magnitude in the testing process, b-value was calculated by using the least square method, andthe amplitude interval was equal to 0.5. In order to avoid theexperimental error caused by too few AE events at a certainstress level, this paper took 1000 acoustic emission events asa set of data and 100 acoustic emission events as sliding forcalculation and obtained the variation law of b-value atdifferent stress levels.

b-value is the crack growth scale, reflecting the propa-gation and damage of cracks during loading. -e decrease ofb-value means that the proportion of major events in rockmass increases, so the large-scale microcracks increase. -echanging trend of AE b-value is the result of the game ofcrack scale development.

-e b-value variation of granite and metagabbro duringloading/unloading is investigated and shown in Figure 10,and the overall evolutionary law is similar. -ere is a cleardownward trend with loading stress level increase, but the b-values of granite and metagabbro show their own charac-teristics at the same loading stress level. -e AE b-values ofgranite and metagabbro are calculated from the 5th and 6thcycles, respectively. -e AE signals produced at the lowerstress level in the early stage are less, in order to ensure thecalculation accuracy, and the AE b-values are not calculatedat the lower stress level. AE b-values under medium stresslevel are relatively large, which concentrates in the range of0.8–1.2 and 0.62–1.12, respectively. In the elastic develop-ment stage, the crack growth level and scale are limitedmainly with low energy release rate. Under stable and un-stable crack growth stage, high-stress level promotes someparticles reaching its maximum bearing capacity, and thelocal stress is highly concentrated, leading to cracks growingand expanding. -erefore, AE signals with high amplitudeand high energy are generated, and the AE b-values decreasecontinuously. -e b-values of granite and metagabbro de-crease rapidly after ninth cycles and eighth cycles, while b-values fluctuate but remain at a low level.-e nearer the peakstress is, the lower the b-value is.

In conclusion, the trend of b-value decrease with loadinglevel increase also indicates that crack growth in differentscales is a gradual development process from small scale tolarge scale. -e damage degree of granite after peak stress ismore severe than that of metagabbro, which is mainly due toless damage in front of the peak and rapid release of energyat the peak strength.

For each cycle, the b-value change tendency showsV-shape, and the V-shape is obvious under the condition ofmedium stress level, but V-shape gradually disappears with

400

350

300

250

200

150

100

50

0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Load

ing

(kN

)

b

3000 4000 5000 6000 7000 6000 9000Time (s)

(a)

300

250

200

150

100

50

0

Load

ing

(kN

)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

b

4000 5000 6000 7000 6000Time (s)

(b)

Figure 10: b-value variation with time of (a) granite and (b) metagabbro.

10 Advances in Civil Engineering

the increase of stress level. AE b-value is a function of crackgrowth, and crack propagation and coalescence lead to adecline of b-value. However, crack grown stops immediatelywith stress drop under low-stress level, and almost no large-scale cracks occur, so the b-value rises subsequently.However, with the cycle number increase, the cumulativedamage will also generate obvious AE signals even at low-stress level. Large-scale cracks will be generated continu-ously through the internal cracks interaction, and AE b-value will continue to decline. Accumulative internaldamage results in new and existing crack expansion even atunloading stage, so the AE b-value does not increase sig-nificantly, adversely continuing to decline. Under the lastloading cycle, both granite and metagabbro show a rapiddecrease of b-value. At this time, the internal cracks de-velop rapidly, macrofracture surface is produced, anddissipation energy increases sharply. However, the b-valueof granite at the peak strength is obviously higher than thatof metagabbro.-e elastic stage of strong impact pronenessgranite takes up a relatively long proportion. Rock failureoften concentrates in an instant. -e overall loss of rockstrength tends to be concentrated in a relatively shortperiod of time.-e large size crack monitored by AE signalsbefore peak stress is smaller, so the b-value is larger. -elower the impact proneness is, the more fully the prepeakcrack expands and the lower the sudden release of energy atthe peak strength is. Although stress-induced changes inthe b-value intuitively indicate the state of damage, onedifficulty related to the use of the b-value to predict failureis that detectable changes in the b-value are generally theresult of large changes in stress.

6. Discussion

With the stress increase, the crack size and numbers enlargeand then AE b-value decreases obviously, showing fluctu-ating decline characteristics. However, with the furtherincrease of stress level, the local stress concentration is easyto occur at the edge of large-scale cracks, and thenmicrocracks are further developed at the edge of large-scalecracks. -e number of microcracks is obviously larger thanthat of small-scale cracks, and then the low-amplitude andlow-energy AE signals are released. At this time, the AE b-value increases slightly, which is the main reason for thefluctuating decline under higher stress level.

Under a higher stress level, the internal damage of rocksample accumulates, and the obvious AE phenomenonoccurs during the unloading stage.-is indicates that even atunloading stage, rock damage continues to increase anddamage caused by unloading cannot be neglected.According to the definition of Felicity ratio, the Felicity ratiois the memory of the cumulative damage of rockmass, whichcannot reasonably reflect the damage to rock mass caused bythe last loading to the maximum point, and its value shouldbe higher than the calculated value. -e calculation defi-nition of Felicity ratio illustrates that its value can only beused to determine the damage degree in the previous cycle,while the damage in this cycle needs to be reflected in thenext cycle, which has a hysteretic effect.

-e single b-value or Felicity ratio cannot accuratelypredict rock failure, so it is of great significance to improvethe accuracy of rock failure prediction by considering thechange rule of both before peak strength.

7. Conclusion

(1) Based on the comprehensive evaluation method ofmultiple rockburst proneness index, granite ismainly prone to strong rockburst and metagabbro ismainly prone to moderate rockburst. -ere is noobvious plastic deformation stage before the peakstress of two kinds of rocks, and the stress dropsinstantaneously after the peak strength.

(2) Rockburst proneness is positively correlated withmineral components with stable mechanical prop-erties. Quartz and feldspar with stable mechanicalproperties in granite are obviously higher than thatin metagabbro, which is the key element affectingrock impact tendency.

(3) Rocks with different rockburst proneness havesimilar energy evolution characteristics. -e energystorage and dissipation capacity of rock are the directreasons for rock impact in failure. -e stronger therockburst proneness is, the more the elastic strainenergy is stored before the rock peak, and the less thedissipation energy is.

(4) -e AE ring counts and energy of rocks with strongrockburst proneness and moderate rockburstproneness have similar evolution law. In quantity,rocks with strong rockburst proneness are obviouslymore than those with moderate rockburst proneness.In the aspect of AE spectrum, the peak frequencydistribution of rocks with strong rockburst tendencyis wider than that of rocks with moderate rockbursttendency.

(5) -e peak frequency distribution of AE signals be-fore the critical main fracture of rock shows thephenomenon of frequency band increase. Beforethe critical main fracture of rock with strongerrockburst proneness, the frequency distribution ismore dispersed and the number of frequency bandsis more.

(6) -e single b-value and Felicity ratio can characterizethe rock fracture process but both cannot accuratelypredict rock failure. Combining the change rule ofboth before peak strength is an effective way toimprove the criterion of discrimination.

Data Availability

-e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

-e authors declare that there are no conflicts of interestregarding the publication of this paper.

Advances in Civil Engineering 11

Acknowledgments

-is paper was supported by the National Key Research andDevelopment Plan (no. 2016YFC0600801), Key Program ofNational Natural Science Foundation of China no.(51534002), and Major Scientific and Technological Inno-vation Project of Shandong Province (no. 2019SDZY05).

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