ResearchArticle ...downloads.hindawi.com/journals/amse/2020/9016583.pdf · ResearchArticle AnalysisofResistivityAnisotropyofLoadedCoalSamples XiangchunLi,1,2,3ZhenxingAn ,1QiZhang

Research ArticleAnalysis of Resistivity Anisotropy of Loaded Coal Samples

XiangchunLi123 ZhenxingAn 1QiZhang 1XiaolongChen1Xinwei Ye1 andSuye Jia1

1School of Emergency Management and Safety Engineering China University of Mining amp Technology Beijing 100089 China2State Key Laboratory of Coal Resources and Safe Mining Beijing 100083 China3State Key Laboratory Cultivation Base for Gas Geology and Gas Control Jiaozuo 454000 China

Correspondence should be addressed to Zhenxing An anzhenxingz163com

Received 6 December 2019 Accepted 11 May 2020 Published 4 June 2020

Academic Editor Candido Fabrizio Pirri

Copyright copy 2020 Xiangchun Li et al is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

In this paper an experimental study of the variation of resistivity of coal samples in different bedding directions at 1MHzfrequency was performed by establishing an experimental system for resistivity testing of coal under triaxial stress e low-pressure nitrogen gas adsorption (LP-N2GA) experiment and scanning electron microscopy (SEM) were obtained to analyze thepore-fracture structural characteristics of coal samples and the influence on resistivity anisotropy Furthermore the fundamentalcause of anisotropy of coal resistivity is expounded systematicallye results show that the resistivity of loaded coal decreased firstbefore increasing e ionic conductance and the high degree of metamorphism slow down the decrease of resistivity edistribution of pore and fracture structures is anisotropic e connected pores and fractures are mainly distributed along theparallel bedding directione weak plane of bedding diagenetic fractures and plane fracture structures of parallel bedding resultin the increase of fractures in the direction of vertical bedding so increasing the potential barrier erefore the resistivity in thevertical bedding direction is higher than that of the parallel bedding Loading coal resistivity anisotropy degree is a dynamicchange trend the load increases anisotropy significantly under axial pressure and the degree of anisotropy has a higher dis-creteness under confining pressure It is mainly the randomness of the internal pore-fracture compaction closure and de-velopment of the heterogeneous coal under the confining pressure the more rapid the decline in this stage the larger the stressdamage degree

1 Introduction

In recent years Chinarsquos demand for coal energy keeps in-creasing which leads to the stage of deep mining e in-crease of mining depth and intensity increases the possibilityof coal-rock composite dynamic disasters such as rock burstand coal and gas outburst [1 2] Research on electricalparameter changes based on the electrical and physicalproperties of coal is the physical property prerequisite forconducting electrical exploration in underground coal mines[3 4] Dynamic prediction without damage based on re-sistivity testing principle plays a positive role in monitoringand forecasting coal and rock dynamic disasters

e development of cracks is also synchronized with theadvancement of the working face and there are verticalsubsidence and horizontal deformation of the rock strata [5]e response position of abnormal high resistance of loaded

coal is consistent with the fracture development position [6]By monitoring the local resistivity and its anisotropy changethe coal and gas outburst can be predicted and forewarned[7] erefore scholars at home and abroad have studied theresistivity change characteristics of loaded coal Differentpore size distribution and connectivity lead to distinctresistivity and pore structure has a significant effect onresistivity [8] e change of electrical parameters of loadedcoal is obviously consistent with the stress e electricalparameters can better reflect the formation developmentand penetration of microfracture as well as the character-istics of changes in the internal structure in the failureprocess of coal and rock [9 10] e direction of the an-isotropic principal axis of the rock with the largest change inthe apparent resistivity is basically the same as the directionof the main fracture of the rock ere are significant an-isotropy of measured points in cracks and fractured zones

HindawiAdvances in Materials Science and EngineeringVolume 2020 Article ID 9016583 13 pageshttpsdoiorg10115520209016583

[11 12] Xu et al [13] explored the anisotropy change of rockunder uniaxial compression by defining the anisotropyfactor which proposed that the anisotropy change tends torise at the first macrofracture time Chen et al [14] putforward the concept of series-parallel dominance and ana-lyzed the law that the resistivity of anthracite increases atdifferent temperatures as the sampling angle increases Ananisotropic multilayer model was established by Shen toanalyze the apparent resistivity of anisotropic formation Itwas found that the apparent resistivity of the perpendicularformation is greater than that of the parallel formation [15]e permeability of parallel bedding sample is higher thanthat in the vertical bedding sample [16] e permeability ofcoal is highly anisotropic even on a small scale which iscontrolled by cleat properties [17] Liu et al [18] found thatthe dispersion degree of tensile strength in the verticalbedding direction of coal is smaller than that in the parallelbedding direction and the anisotropy of mechanicalproperties is affected by the strip distribution of macrocoalcomponents and the directionality of the distribution of cleatsystem Uniaxial compressive strength of parallel and ver-tical bedding coal samples is different which is controlled bybedding structure [19]erefore bedding and pore-fracturestructure may also be the key to affecting the anisotropy ofresistivity

Previous studies have focused on the various charac-teristics and anisotropy of coal resistivity in different bed-ding directions under uniaxial compressione influence ofpore-fracture structure characteristics charge transfer andloading path on resistivity anisotropy remains to be furtheranalyzed It is necessary to comprehensively and system-atically understand the influence of coal and rock beddingon resistivity anisotropy characteristics erefore an ex-perimental system for resistivity testing of coal under triaxialstress is established e change laws and mechanisms ofresistivity anisotropy under different loading paths are an-alyzed by experimental and theoretical research to provide acertain theoretical basis for the prediction of coal and rockdynamic disasters

2 Experimental System and Method

21 Experimental System As shown in Figure 1 the systemis composed of a three-axis pressurization device a resis-tivity measuring device a temperature control device and acomputer for data collection e following briefly intro-duces the structure and function of each device e three-axis pressurization device is designed by the laboratory torealize the axial and confining pressure loading of the elasticcavity through water pressure e pressure control range is0ndash16Mpa and measurement control accuracy is 001MpaResistivity test device uses HIOKI 3532-50 LCR tester in-strument connected to PC and then collects data continu-ously in its software It can display parameters such asimpedance phase angle inductance capacitance and re-sistance at the range of 5MHz e temperature controldevice employs ES-III temperature controller manufacturedby Changzhou Easy-to-Use Technology Company to achieveconstant temperature control of the coal samples

22 Samples and Method e coal samples were fromDashucun Sihe Yuwu Coal Mine in Shanxi province inChina Large coal samples with good integrity were pro-cessed into a Oslash25mmtimes 50mm cylinder e core samplingdirection of the drilling sampler is vertical or parallel to thecoal bedding direction and test specimens with differentbedding directions are drilled in the same coal sampleEnsure that the stress of the coal samples is uniform toprevent the damage of the coal body caused by the stressconcentration reduce the visible cracks as much as possibleand timelymark sample with different bedding directions Inorder to eliminate errors caused by sample irregularities andthe test electrode well coupled with specimens the surface ofsamples is smooth and cylindrical end face evenness error isless than 002mm Finally complete dense and no obviouscrack test specimens were selected as shown in Figure 2 Itshould be noted that the vertical bedding refers to the axis ofthe sample being perpendicular to the bedding plane andthe parallel bedding refers to the axis being parallel to thebedding plane

e experimental procedures are as follows

(1) Industrial analysis low-pressure nitrogen gas ad-sorption (LP-N2GA) and scanning electron mi-croscopy (SEM) tests were carried out on DashucunSihe and Yuwu coal samples to study and analyze themicrostructure of coal

(2) e cored coal samples were placed in the electricalparameter test chamber the copper electrode and theend face were coupled with conductive glue andenameled wire was connected with the LCR testerBefore loading in different paths a certain preloadwas applied to fix the specimen in the elastic cavityand the coal samples with different bedding direc-tions were increased the axial pressure to 9Mpa stepby step and the computer recorded the change ofelectrical parameters of the loaded coal sample Afterthe axial loading was completed the above steps wererepeated and the confining pressure of coal sampleswith different bedding directions gradually increasedto 8Mpa

According to the working principle of LCR tester theresistivity calculation formula is as follows

|Z|cos θ R

ρ |Z|cos θS

L R

S

L

(1)

where Z is the impedance absolute (Ω) θ is the phase angle(deg) ρ is the resistivity (Ωmiddotm) R is the resistance (Ω) S is thesample cross-sectional area (m2) and L is the sample length(m)

In order to facilitate the analysis of experimental resultsSDLA618 industrial analyzer was used to measure the basicparameters of coal samples in different mining areasaccording to GBT212-2008 Industrial Analysis of Coal Vdafis used to quantify the degree of coal metamorphism e

2 Advances in Materials Science and Engineering

higher the degree of metamorphism the smaller the Vdafand the results are shown in Table 1

3 Experimental Results on ResistivityAnisotropy of Loaded Coal Samples

e variation of the resistivity of coal samples with differentbedding direction was measured under axial pressure andconfining pressure during 1MHz frequency e tempera-ture of the coal samples was controlled at 30degC to reduce theinterference of temperature on resistivity changes

According to the experimental results of specimens withdifferent bedding directions the change curves of coalsamples resistivity under different axial pressure and dif-ferent confining pressure were drawn as shown inFigures 3(a)ndash3(f) Under the axial pressure process theresistivity of coal samples in different mining areas does nothave to vary synchronism as stress increases e change ofcoal samples resistivity in Dashucun is characterized byldquorapid decline-gentle rise-rapid declinerdquo Sihe is character-ized by ldquoslow decline-rapid decline-sudden increaserdquo and isirregular ldquoVrdquo shape [20 21] Yuwu is characterized byldquoserrated fluctuations-abruptly decreasedrdquo ere are

obvious differences in the resistivity changes of coal samplesin different mining areas under axial pressure ere is noinflection point of resistivity sudden increase in Dashucunand Yuwu coal samples during the loading process It isconsidered that the data recording is incomplete and thestress has not reached the value of expanding the coal bodywhich was related to the pore structure and sedimentationmetamorphism of the coal body Under confining pressurethe resistivity of coal samples in different bedding directionshas obvious consistency as stress increases e resistivitydecreases rapidly at the beginning and the rate of declinedecreases gradually After the confining pressure increases toa certain value the resistivity starts to fluctuate and rise

Although the range of resistivity measured in experi-ments is large it can be seen from Figure 3 that the resis-tivities of different bedding directions in all coal samples are

(a)

Axis

Beddingplane

(b)

Figure 2 Parallel and vertical bedding direction coal samples (a) Experimental coal samples (b) e bedding direction

Table 1 Industrial analysis basic parameter

Samples number Coal rank M ad V daf A d FCad

DSC Lignite 045 5118 265 4572SH Lean coal 192 739 1336 7733YW Lean coal 238 479 1459 7824

LCR tester

Temperature controller

Pressure gauge

Electrical parametertest cavity

Computer for data collection

Manual loading device

Sample

Electrode plate

Axial pressure Confining pressure

(a) (b)

Figure 1 Diagram of loaded coal resistivity test system (a) Schematic diagram (b) Physical diagram

Advances in Materials Science and Engineering 3

200

190

180

170

160

150

140

130

120

110

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)

Vertical bedding DSCParallel bedding DSC

(a)

Resis

tivity

(Ωmiddotm

)

70

75

65

60

55

50

45

40

350 2 4 6 8 10

Stress (MPa)

Vertical bedding SHParallel bedding SH

(b)

Resis

tivity

(Ωmiddotm

)

200

180

160

140

120

100

800 2 4 6 8 10

Stress (MPa)

Vertical bedding YWParallel bedding YW

(c)

Resis

tivity

(Ωmiddotm

)140

120

100

80

60

40

20

0 2 4 6 8Stress (MPa)


(d)

Resis

tivity

(Ωmiddotm

)

70

65

60

55

50

45

400 2 4 6 8

Stress (MPa)


(e)

Resis

tivity

(Ωmiddotm

)

140

120

100

80

60

40



(f )

Figure 3 Resistivity curves in different bedding directions (a) Dashucun coal samples under axial pressure (b) Sihe coal samples underaxial pressure (c) Yuwu coal samples under axial pressure (d) Dashucun coal samples under confining pressure (e) Sihe coal samples underconfining pressure (f ) Yuwu coal samples under confining pressure


not equal e resistivity of coal has obvious anisotropiccharacteristics e resistivity in the vertical bedding di-rection is greater than that in the parallel bedding Coal ishighly anisotropic and heterogeneous [17 22] affecting coalresistivitye high degree of coal heterogeneity results froma combination of many geological factors including sedi-ment sources depositional environments tectonic settingsdiagenesis climate and hydrological conditions [23 24]e heterogeneity of coal increases the uncertainty of thedevelopment and connection of the pores and cracks of theloaded coal erefore the resistivity in the parallel beddingof singular points is greater than that of the vertical beddinge variation curves of coal resistivity of different beddingdirections under the same loading path are similarerefore it is considered that the bedding structure has noimpact on the total variation characteristics of the resistivityResistivity first decreases and then increases during coalsamples loading and the resistivity can better respond to therelationship between load and pore-fracture e change ofbulk structure is still the determinant of resistivity changeduring the loading process pore-fracture deformation crackinitiation and propagation are the main factors

4 Discussion and Analysis

41 Effect of Pore and Fracture Structure and Its Anisotropy onResistivity e coal metamorphism degree and porestructure are different in different geological areas roughLP-N2GA and SEM of coal samples the characteristics ofpore-fracture structure and its influence on coal resistivityanisotropy were investigated Smashing and screening coalsamples with a particle size of 60ndash80 mesh were carried outfor vacuum degassing conducting low-temperature nitro-gen gas adsorption When the relative pressure of con-densation and evaporation of a pore is different the so-calledadsorption loop is formed LP-N2GA isotherms as shown inFigure 4

Based on the experimental data of low-temperaturenitrogen gas adsorption of coal samples the specific surfacearea of coal samples was calculated according to the BETmultilayer adsorption theory and the pore size distributionwas calculated by BJH model as shown in Table 2

If the pore structure is different the adsorption capacityis different e adsorption loop reflects the distribution ofdifferent pore structure of coal samples in different miningareas [22 25] so as to understand the pore structure type ofcoal Dashucun coal samples showed capillary condensationat a relative pressure of 05MPa which demonstrates thatthe pore morphology is relatively simple ere are opencylindrical pores and bottle-shaped pores whose connec-tivity is general Sihe coal samples adsorbed the least N2 atthe highest pressure and the adsorptiondesorption iso-therms are relatively close which illustrates the limitation ofpores number and poor porosity e nonconnected cy-lindrical pores and the slit-shaped pores account for themajority and the pores with poor connectivity are domi-nant e pore structure of Yuwu coal is complex theconnectivity in the pore is very good and the microporeclusters are developed so the adsorption capacity is strong

and the hysteresis loop is highly separated SEM can directlyvisualize the micro-nanoscale solid surface micromor-phology and analyze the pore structure of coal with differentmetamorphic degrees through the development character-istics of pore as shown in Figure 5 e deep part of theimage represents pores while the shallow part is nonporesUnder the same magnification ratio the microsurfacemorphology of coal with different metamorphism degrees isobviously different Under the SEM 5000 times magnifica-tion the surface of the Dashucun coal is rough with manypores and fractures the surface of Sihe is flat with rarefractures the surface of Yuwu has many pores andmicrofractures and the original structure of the coal body isclear Besides the microstructure of the coal samples underSEM is consistent with the experimental results of LP-N2GA

e pore size of Dashucun is dominated by macroporeswhich above 50 nm [26] is 5163 and the proportion ofmesopore is 2309 e macropores and mesopores ac-count for more than 70 of the pore structure the con-ductivity is poor with low compactness and developmentpore-fracture so the resistivity value before loading is large[27] With the deepening of the degree of metamorphismthe polycondensation of coal macromolecular structure isincreased and tends to be ordered [28] e total porevolume of Sihe is the smallest the compactness is high andthe conductivity is stronge Yuwu coal samples have largeporosity and well connectivity microcracks develop obvi-ously and the conductive channel is destroyed to a certainextent so the resistivity before loading is large e responselaw of acoustic emission parameters of loaded coal can bedivided into the initial loading stage the slow increase stagethe sharp increase stage and the damage instability stage[29] e response law of resistivity is also divided into fourstages densification stage elastic stage plastic stage andbreakage stage [30] At the early stage of loading themicrocracks and pores of the coal body are compressed andclosed the contact between the particles inside the coal bodyis tighter the conductive channel gets better the electricalconductivity increased and resistivity shows descendingtendency With the loading of pressure pores and crackshave been almost completely enclosed to the elastic stagee interaction between the intermolecular electron cloudsin the compact coal body is remarkable the electron mo-bility increases and the resistivity further descends [31]With the occurrence of the inflection point of coal resistivitywith dilatation microfracture has occupied a dominantposition the conductive channel suffers from breakage andthen causes resistivity to rise Finally when the pore fracturedevelops into a large fracture zone the conductive channel isseverely damaged and the resistivity is far higher than theinitial value

Coal is a complex porous medium composed of poresfissures and coal skeletons the degree of pores and fracturesdevelopment in different directions is not the same underthe action of sedimentation and geological structure Cleatsare fractures that are mutually perpendicular and alsoperpendicular to bedding which are confined to individualcoal beds Vertical connectivity of cleat networks is com-monly limited by the termination of small cleats at interfaces


between coal types so connectivity is poor in verticalbedding direction [32] Based on the nuclear magneticresonance (NMR) method it was found that the pores ofparallel bedding coal samples were tightly closed afteruniaxial compression some of the fractures were deformedradially and the crack closure effect in the vertical beddingdirection was significant [33] Fissures in the coal body arethe main channels for gas flow and the permeability is large

in the direction of parallel beddinge connected pores andfractures are mainly distributed along the bedding directionthe pores and fractures in the vertical bedding direction arepoorly connected and the tortuosity of the seepage channelis large [34 35] Previous studies showed that the calciteparticles uniformly distribute on the bedding planes [19 24]and a large number of pores almost parallel to the beddingare formed on the edges of the minerals due to compaction

35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

00 02 04 06 08 10Relative pressure (PP0)

AdsorptionDesorption

(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10

Relative pressure (PP0)


(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)

Figure 4 LP-N2GA isotherms for three coal samples (a) Dashucun (b) Sihe (c) Yuwu

Table 2 Microscopic parameters of coal samples

Sampling sites Total pore volume (cm3middotgminus1)Pore size distribution Specific surface

area (m2middotgminus1) Porosity ()Micro (lt2 nm) Meso (2sim50 nm) Macro (gt50 nm)

DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82


by sedimentation e bedding fractures can be formedduring diagenesis In addition the fractal theory is applied tothe study of coal fracture development and distribution efractal box dimension can be used to characterize the de-velopment degree of coal fracture It was found that thefractal box dimension of parallel bedding direction fractureis larger than that of vertical bedding [36]e surface fissurestructure of the parallel bedding is developed and thedistribution of the fissures shows a certain dominant ori-entation [16 37] In summary there are anisotropic featuresin the pore-fracture structure distribution in differentbedding directions e pores and fractures develop alongthe bedding direction and the structure parallel to beddinghas well connectivity Bedding planes and fissure channelsare the dominant channel for gas migration as shown inFigure 6 e distribution characteristics of pores andfractures make the charge transfer difficult to be differentMigration along the direction of bedding is easy to thevertical bedding erefore the resistivity in the verticalbedding direction is higher than that of the parallel bedding

42 Effect of Charge Transfer on Resistivity AnisotropyOne of the important characteristics of coal as a sedimentaryrock is the existence of bedding structure e difference ofvertical bedding direction resistivity and parallel beddingresistivity is one of the concrete manifestations of resistivity

anisotropy e difference in conductivity of coal sampleswith different bedding directions under load conditions isobvious e conductivity of the vertical bedding sample islower than that of the parallel bedding e resistivity of theloaded sample is 686Ωmiddotm higher than that of the parallelbedding sample

e anisotropic characteristics of resistivity can be an-alyzed from both microscopic and macroscopic aspectsMicroscopically electrons must overcome coal moleculesbarrier from one atom to another Coal is a macromolecularstructural unit composed of an aromatic ring as a core whenelectronsmove in an aromatic layer the barrier mainly refersto the binding of large molecules of coal to electrons butwhen electronsmove between the aroma layers the electronsnot only have to overcome the binding force of the coalmacromolecules to electrons but also overcome the force ofthe electrons penetrating the aromatic parallel stack estructural characteristics of the weak plane of the beddingmake it difficult for carriers to move between the beddingand there is a barrier to the bedding e total barrier in thedirection of parallel bedding is mainly the standard barrierwhile vertical bedding direction is composed of the standardbarrier penetrating the potential barrier and bedding bar-rier It is difficult for electrons to migrate in a parallelbedding than vertical bedding so the conductivity in theparallel bedding direction is higher [38] In a macroscopicview the plane of bedding has a weak cementation degree

Slit-shaped pores

CracksBottle-shaped pores

Cylindrical pores

BCPCAS4800 50 kV 151 mm times 500 k SE(M LA30) 100 um

(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)

Figure 5 e SEM image at 5000 magnification (a) Dashucun (b) Sihe (c) Yuwu


Micro and medium fractures are relatively developed underthe combined action of internal and external stress [39] econtact between bedding is not dense under the samebedding and usually has different dielectric constant andconductivity e anisotropy of pores and fractures distri-butionmakes the connected pores and fractures develop intodominant channels along the bedding direction and thetotal fissure degree of vertical bedding sample is greater thanthat of parallel bedding [16] e migration of charge be-tween the bedding plane and the fracture channel will behindered In the parallel bedding the fracture zone that thecharge needs to pass through is relatively reduced and theelectrical conductivity is high Also the free charge tends tomove along the bedding column under the external electricfield and continues to move toward the lower potential endwhen it is in equilibrium e current density on differentbedding columns is different When the charge accumulateson the bedding plane a new electric field Ersquo is formed whichhinders the movement of the charge to the low potentialdirection and the sample resistivity increases [15] as shownin Figure 7 e distribution of the bedding plane and thefracture structures makes the resistivity of the verticalbedding equivalent to the series connection of multipleresistancese resistivity of the parallel bedding is similar tothe parallel connection erefore e bedding resistivityhas obvious anisotropy characteristics

43 Effect of Loading Path onResistivityAnisotropy It can beseen from Figure 3 that the change law of resistivity ofloaded coal samples under different loading paths echanging trend of resistivity under different loading pathsis different the rate of resistivity decrease under confiningpressure is greater than axial pressure at the initialloading It is considered to be related to the variation ofthe loaded fracture and the large pore size under differentloading paths Based on the bending deformation theoryof simple support the deformation of pore and fractureunder confining pressure loading was discussed [40] eeffect of stress on pores and fractures strain is differente fracture is higher than macropores mesopores and

micropores the larger the fracture width and the largerthe pores the higher the deformation degree At thebeginning of confining pressure loading fracture andmacropores are closed rapidly the conductive channelincreases and the resistivity decreases rapidly e in-fluence of confining pressure on micropores is limitedresistivity reduction tends to flatten out after 2Mpa andthe resistivity tends to increase with the expansion de-velopment and transfixion of microcracks Comparedwith axial pressure the confining pressure has a bettercompaction effect on the pore-fracture structure [41] andthe deformation in the early stage is the largest esample has a stronger sensitivity to confining pressure thepore-fracture closure rate is faster after loading and thesample is also the most damaged

e anisotropy coefficient is defined as follows

λ

ρnρt

1113970

(2)

where ρn is vertical bedding direction resistivity and ρt isparallel bedding direction resistivity

e closer the anisotropic coefficient λ value is to 1 thelower the resistivity anisotropy characteristic appears andthe more the deviation from 1 is the more obvious theanisotropic characteristic is Calculate the anisotropy coef-ficient and the degree of deviation of the resistivity an-isotropy coefficient during the loading process as shown inFigure 8

e λ deviation value reflects the degree of resistivityanisotropy of coal rock It can be seen from Figure 8 that theresistivity anisotropy is obvious during the loading processere are obvious dispersions of the λ deviation values underdifferent loading paths but the dispersion degree is higherunder confining pressure When the stress increases duringaxial pressure the λ deviation value of coal samples indifferent mining areas changes significantly As the stressincreases the degree of anisotropy increases e dispersionof λ deviation under confining pressure loading is heavy andaxial pressure has a better influence on resistivity anisotropythan confining pressure Affected by the heterogeneity and

Face

Butt

(a)

Beddings

Fissure channels

(b)

Figure 6 Schematic diagram of fracture distribution (a) Cleat-trace patterns in plan view (b) Distribution of fissure channel


the direction of confining pressure the compaction de-velopment and transfixion of the pore-fracture are randomand uneven so anisotropic sudden increase or sudden de-crease occurs during loading and the variation law is notclear It is not hard to find that the degree of anisotropy ofthe resistivity of the loaded coal is dynamically changingedevelopment of coal pore-fracture is the main factor in thechange of resistivity anisotropy e overall feature is thatthe resistivity anisotropy is significant with the stress in-creases under axial pressure

44 Influence of Mineral Elements on Electrical ResistivityAccording to the solid dielectric theory [42] solid con-ductivity is divided into ionic conductivity (intrinsic con-ductivity and impurity conductivity) and electronicconductivity e intrinsic conductivity is affected by thethermal movement of ions in the crystal lattice so the ionicconductivity is dominant at room temperature e generalexpression of conductivity is

σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)

Figure 8 λ deviation degree under different loading paths (a) Axial pressure (b) Confining pressure

Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)

Figure 7 Schematic diagram current conduction (a) Vertical bedding (b) Parallel bedding


where σ is conductivity ni is carrier concentration ei ischarger charge and microi is carrier mobility

e conductance mechanism of coal when loaded is alsoionic conductivity and electronic conductivity [9 21] It isfound from Figure 3 that the resistivity of Sihe and Yuwucoal samples under axial pressure did not decrease signifi-cantly with the increase of stress which is considered as theresult of the interaction between ionic conductivity char-acteristics and compact coal structure e mineral elementsof coal samples from different mining areas were analyzed byEDS spectrometer as shown in Table 3

Dashucun coal samples are rich in macropores meso-pores and fissures the internal moisture and mineral ele-ments are scarce so the electronic conductivity is absolutelydominant during the process of axial pressure coal

resistivity decreases rapidly after loading e metamorphicdegree intrinsic moisture and mineral elements of othercoal samples are higher Under stress loading coal particlepores and cracks are closed the intermolecular distance onthe microscopic level is also reduced the free space availablefor ion transition is reduced the ion mobility is reduced andthe ion conductivity is reduced resulting in an increase inresistivity [43] At the same time the close contact betweenthe coal particles facilitates the circulation of electrons theelectronic conductivity is enhanced and the resistivity isdecreased erefore the initial resistivity of the loaded coalis slowly decreased and fluctuated and there is no significantdrop and rise e bedding plane is mixed with differentmineral compositions ese channels change the electricalproperties of the coal body and the interlayer channels

Table 3 EDS element analysis table

Coal sample Element Mass percentage Atomic percentage

DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)

ǁDSCǁSHǁYWperpDSCperpSHperpYW

ǁDSC fitting curveǁSH fitting curveǁYW fitting curveperpDSC fitting curveperpSH fitting curveperpYW fitting curve

(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)



ǁDSH fitting curveǁSH fitting curveǁYW fitting curveperpDSH fitting curveperpSH fitting curveperpYW fitting curve

(b)

Figure 9 Resistivity fitting curve under stress (a) Axial pressure (b) Confining pressure


weaken the migration barrier of ion carriers [44] e dis-tribution of mineral elements on coal samples in vertical andparallel bedding has a positive effect on electricalconductivity

45 Influence of Anisotropy on Stress-Resistivity RelationshipWhen the temperature is constant and the coal body is underpressure the formula [31] for expressing the conductivitycan be described as follows

c A exp(Bσ) (4)

where c is the resistivity σ is stress and A and B are thecoefficients

By fitting the original data it is found that the resistivityof coal fitting relation of axial pressure and confiningpressure is different as shown in Figure 9 e relationshipunder the axial pressure fitting is as shown in (5) and thefitting relationship under the confining pressure is as shownin (6)

y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)

where x is stress y is the resistivity and b1 b2 b3 A R0 andR1 are the coefficients associated with coal

It is consulted by the fitting relationship under differentloading paths that the loading paths have a significant in-fluence on the stress-resistivity relationship e correlationof the cubic polynomial fitting under axial pressure is higherand the fitting degree of the formula (6) under confiningpressure loading as well (R2 is much larger than 80) isgood Coal is highly anisotropic and heterogeneous beddingstructure is one of its important characteristics and thedistribution of pore-fracture also has anisotropic charac-teristics Samples in different regions have a high degree ofheterogeneity and the pore-fracture structure of the samplesis completely different under the microcosm so there are nosynchronization of resistivity changes under loading Forcoal samples with different bedding directions there is ananisotropy in pore-fracture structure distribution but thedegree of coal metamorphism and pore development are thesame e experiment and fitting results show that theanisotropy has basically no effect on the changing trend ofthe resistivity of the loaded coal samples under the sameloading path e resistivity of coal samples with differentbedding directions has the same trend with stress and theresistivity in the vertical bedding direction is greater thanthat in the parallel bedding

5 Conclusions

Based on previous studies the microstructure loading pathresistivity change and anisotropy of coal samples withdifferent bedding directions were experimentally studiedand the following conclusions were obtained

Coal is highly heterogeneous and anisotropic e porestructure of coal with different metamorphisms variesgreatly e distribution of pore and fracture structure is

anisotropic e connected pores and fractures are mainlydistributed along the parallel bedding direction the poresand fractures in the vertical bedding direction are poorlyconnected Compressive deformation crack initiationpropagation and coalescence of coal samples lead to re-sistivity undergoing the process of descending-to-ascendingtrend Macropore mesopore and fracture development ofcoal samples make the resistivity even larger

ere are anisotropic characteristics of coal resistivitye weak plane of bedding diagenetic fractures and planefracture structures of parallel bedding result in the increasedof fissures degree in the direction of vertical bedding evertical bedding direction has a large total potential barrierwhich electron migration is difficult e resistivity of coalsamples in the direction of vertical bedding is larger thanthat of parallel bedding

e resistivity change of loaded coal under axial pressureand confining pressure have significant difference the porefracture is quickly closed at the beginning of confiningpressure loading so the degree of stress damage is thelargest e anisotropy factor is introduced to explore theresponse characteristics of resistivity anisotropy in thefailure process of the samples and find its dynamic changetrend On the whole the degree of resistivity anisotropyincreases with stress under axial pressure the anisotropydispersion is higher under confining pressure

Data Availability

All data used to support the study are present in the article asfigures or tables

Conflicts of Interest

e authors declare that they have no conflicts of interestregarding the publication of this paper

Acknowledgments

is study was supported by the Natural Science Foundationof Beijing Municipality (Grant no 8192036) the NationalKey Research and Development Program of China (Grantno 2018YFC0808301) Youth Foundation of Social Scienceand Humanity the Ministry of Education of China (Grantno 19YJCZH087) the State Key Laboratory Cultivation Basefor Gas Geology and Gas Control (Henan PolytechnicUniversity) (Grant noWS2018B04) and the FundamentalResearch Foundation for the Central Universities (Grant no2009QZ09)

References

[1] X Kong E Wang S Hu R Shen X Li and T Zhan ldquoFractalcharacteristics and acoustic emission of coal containingmethane in triaxial compression failurerdquo Journal of AppliedGeophysics vol 124 no 1 pp 139ndash147 2016

[2] G Cheng T Ma C Tang H Liu and S Wang ldquoA zoningmodel for coal mining-induced strata movement based onmicroseismic monitoringrdquo International Journal of Rock


Mechanics and Mining Sciences vol 94 no 4 pp 123ndash1382017

[3] N Bai-Sheng H Xue-Qiu and Z Chen-Wei ldquoStudy onmechanical property and electromagnetic emission during thefracture process of combined coal-rockrdquo Procedia Earth andPlanetary Science vol 1 no 1 pp 281ndash287 2009

[4] J E Chambers P B Wilkinson A L Weller P I MeldrumR D Ogilvy and S Caunt ldquoMineshaft imaging using surfaceand crosshole 3D electrical resistivity tomography a casehistory from the East Pennine Coalfield UKrdquo Journal ofApplied Geophysics vol 62 no 4 pp 324ndash337 2007

[5] X Yang G Wen L Dai H Sun and X Li ldquoGround sub-sidence and surface cracks evolution from shallow-buriedclose-distance multi-seam mining a case study in buliantacoal minerdquo Rock Mechanics and Rock Engineering vol 52no 8 pp 2835ndash2852 2019

[6] T Zhu J Zhou and J Hao ldquoResistivity anisotropy and itsapplication in earthquake researchrdquo Progress in Geophysicsvol 24 no 3 pp 871ndash878 2009

[7] H Lu and Y Jia ldquoExperimental study on change law ofrestivity of coal under compressive loadrdquo Mining Researchand Development vol 29 no 4 pp 36-37 2009

[8] L Sima D Huang S Han et al ldquoEffectiveness evaluation ofpalaeo-weathering crust-type karst reservoirs in the SouthernJingbian Gasfield Ordos BasinrdquoNatural Gas Industry vol 35no 4 pp 7ndash15 2015

[9] W Yungang W Jianping and Y Song ldquoExperimental re-search on electrical parameters variation of loaded coalrdquoProcedia Engineering vol 26 pp 890ndash897 2011

[10] L Qiu D Song E Wang et al ldquoDetermination of hydraulicflushing impact range by DC resistivity test methodrdquo Inter-national Journal of Rock Mechanics and Mining Sciencesvol 107 pp 127ndash135 2018

[11] F Chen J An and C Liao ldquoDirectionality of resistivitychange of anisotropy rock with original resistivityrdquo ChineseJournal of Geophysics vol 46 no 2 pp 381ndash395 2003

[12] D Song Z Liu E Wang L Qiu Q Gao and Z XuldquoEvaluation of coal seam hydraulic fracturing using the directcurrent methodrdquo International Journal of RockMechanics andMining Sciences vol 78 pp 230ndash239 2015

[13] X Xu B Liu S Li et al ldquoExperimental study on conductivityanisotropy of limestone considering the bedding directionaleffect in the whole process of uniaxial compressionrdquo Mate-rials vol 9 no 3 p 165 2016

[14] L Chen Y Zhang Z Huang et al ldquoEffects of bedding planeon anthracite coal resistivity under different temperaturesrdquoChinese Journal of Engineering vol 39 no 7 pp 988ndash9952017

[15] J Shen and NminusC Guo ldquoStudy on apparent resistivity andmagnetic field response in anisotropic layered mediardquo ChineseJournal of Rock Mechanics and Engineering vol 51 no 5pp 1608ndash1619 2008

[16] B Deng X Kang X Li et al ldquoEffect of different beddingdirections on deformation and seepage characteristics of rawcoalrdquo Journal of China Coal Society vol 40 no 4 pp 888ndash894 2015

[17] Y Tan Z Pan J Liu et al ldquoExperimental study of impact ofanisotropy and heterogeneity on gas flow in coal Part IIpermeabilityrdquo Fuel vol 230 pp 397ndash409 2018

[18] K Liu Q Liu Y Zhu et al ldquoExperimental study on Braziliansplitting and uniaxial compression of coal-rock consideringthe direction direction effectrdquo Chinese Journal of Rock Me-chanics and Engineering vol 32 no 2 pp 308ndash316 2013

[19] Z Zhang R Zhang G Li H Li and J Liu ldquoe effect ofbedding structure on mechanical property of coalrdquo Advancesin Materials Science and Engineering vol 2014 Article ID952703 7 pages 2014

[20] P Chen E Wang X Chen Z Liu Z Li and R ShenldquoRegularity and mechanism of coal resistivity response withdifferent conductive characteristics in complete stress-strainprocessrdquo International Journal of Mining Science and Tech-nology vol 25 no 5 pp 779ndash786 2015

[21] L Meng M Liu and Y Wang ldquoExperimental study on thechange law of resistivity under uniaxial compression ofstructural coalrdquo Journal of China Coal Society vol 35 no 12pp 2028ndash2032 2010

[22] B Nie X Liu L Yang J Meng and X Li ldquoPore structurecharacterization of different rank coals using gas adsorptionand scanning electron microscopyrdquo Fuel vol 158 pp 908ndash917 2015

[23] B Fu G Liu Y Liu S Cheng C Qi and R Sun ldquoCoal qualitycharacterization and its relationship with geological process ofthe early Permian Huainan coal deposits southern NorthChinardquo Journal of Geochemical Exploration vol 166 no 7pp 33ndash44 2016

[24] Y Cai D Liu Z Pan Y Yao and C Li ldquoMineral occurrenceand its impact on fracture generation in selected QinshuiBasin coals an experimental perspectiverdquo InternationalJournal of Coal Geology vol 150-151 pp 35ndash50 2015

[25] P Chen and X Tang ldquoe research on the adsorption of ni-trogen in low temperature and micro-pore properties in coalrdquoJournal of China Coal Society vol 26 no 5 pp 552ndash556 2001

[26] S J Gregg ldquoSixty years in the physical adsorption of gasesrdquoColloids and Surfaces vol 21 pp 109ndash124 1986

[27] F Lu J Gao J Liu X Zhang C Wang and Y LiuldquoCharacterization of the coal pore system by resistivity andNMRmethodsrdquo Energy Sources Part A Recovery Utilizationand Environmental Effects pp 1ndash14 2019

[28] G Zhang X Tan X Xie and Y Du ldquoExperimental study onthe relationship between coal conductivity and coal macro-molecular structurerdquo Coal Conversion vol 17 no 2pp 10ndash13 1994

[29] S Liu X Li D Wang MWu G Yin andM Li ldquoMechanicaland acoustic emission characteristics of coal at temperatureimpactrdquo Natural Resources Research vol 28 no 3pp 1755ndash1772 2020

[30] E-Y Wang P Chen Z Li R-X Shen J-K Xu andY-F Zhu ldquoResistivity response in complete stress-strainprocess of loaded coalrdquo Journal of China Coal Society vol 39no 11 pp 2220ndash2225 2014

[31] J Kang ldquoStudy on the relationship between coal conductivityand crustal stressrdquo Journal of Henan Polytechnic University(Natural Science) vol 24 no 6 pp 430ndash433 2005

[32] S E Laubach R A Marrett J E Olson and A R ScottldquoCharacteristics and origins of coal cleat a reviewrdquo InternationalJournal of Coal Geology vol 35 no 1ndash4 pp 175ndash207 1998

[33] F Lu Y Zhang and L Jiang ldquoAnisotropic characteristics ofnuclear magnetic resonance of pores and fractures in coalunder uniaxial loadingrdquo Coal Geology amp Exploration vol 46no 1 pp 66ndash72 2018

[34] Y Chen X Su J Wang et al Petroleum Geology and RecoveryEfficiency vol 26 no 5 pp 66ndash72 2019

[35] Y Wan Z Pan S Tang L D Connell D D Down andM Camilleri ldquoAn experimental investigation of diffusivityand porosity anisotropy of a Chinese gas shalerdquo Journal ofNatural Gas Science and Engineering vol 23 pp 70ndash79 2015


[36] S Peng ldquoStudy on the mechanism of failure and instability ofgas bearing coal and its applicationrdquo Chongqing UniversityChongqing China PhD dissertation 2011

[37] L Zhong ldquoe cause of internal cracks in coalrdquo Coal Geologyof China vol 16 no 3 pp 6ndash9 2004

[38] Y Du Z Ren X Xian et al ldquoRelationship between theanisotropy of coal and its macromolecular structurerdquo CoalConversion vol 18 no 4 pp 63ndash67 1995

[39] X Fu Feory and Method for Predicting Permeability ofCoalbed Methane Reservoirs in Multiphase Media ChinaUniversity of Mining and Technology Xuzhou China 2003

[40] Y Liu D Tang H Xu et al ldquoCharacteristics of the stressdeformation of pore-fracture in coal based on nuclear mag-netic resonancerdquo Journal of China Coal Society vol 40 no 6pp 1415ndash1421 2015

[41] H Deng W Wang J Li et al ldquoExperimental study on an-isotropic mechanical properties of layered sandstonerdquo Chi-nese Journal of Rock Mechanics and Engineering vol 37 no 1pp 112ndash120 2018

[42] W Jin Dielectric Physics Science Press Beijing China 2003[43] P Chen E-Y Wang and Y-F Zhu ldquoExperimental study on

resistivity variation regularities of loading coalrdquo Journal ofChina Coal Society vol 38 no 4 pp 548ndash553 2013

[44] S Bi H Zhang Y Zhang et al ldquoExperimental study on coalresistivity changing anisotropy during uniaxial compressionrdquoSafety in Coal Mines vol 47 no 12 pp 35ndash38 2016


[11 12] Xu et al [13] explored the anisotropy change of rockunder uniaxial compression by defining the anisotropyfactor which proposed that the anisotropy change tends torise at the first macrofracture time Chen et al [14] putforward the concept of series-parallel dominance and ana-lyzed the law that the resistivity of anthracite increases atdifferent temperatures as the sampling angle increases Ananisotropic multilayer model was established by Shen toanalyze the apparent resistivity of anisotropic formation Itwas found that the apparent resistivity of the perpendicularformation is greater than that of the parallel formation [15]e permeability of parallel bedding sample is higher thanthat in the vertical bedding sample [16] e permeability ofcoal is highly anisotropic even on a small scale which iscontrolled by cleat properties [17] Liu et al [18] found thatthe dispersion degree of tensile strength in the verticalbedding direction of coal is smaller than that in the parallelbedding direction and the anisotropy of mechanicalproperties is affected by the strip distribution of macrocoalcomponents and the directionality of the distribution of cleatsystem Uniaxial compressive strength of parallel and ver-tical bedding coal samples is different which is controlled bybedding structure [19]erefore bedding and pore-fracturestructure may also be the key to affecting the anisotropy ofresistivity

Previous studies have focused on the various charac-teristics and anisotropy of coal resistivity in different bed-ding directions under uniaxial compressione influence ofpore-fracture structure characteristics charge transfer andloading path on resistivity anisotropy remains to be furtheranalyzed It is necessary to comprehensively and system-atically understand the influence of coal and rock beddingon resistivity anisotropy characteristics erefore an ex-perimental system for resistivity testing of coal under triaxialstress is established e change laws and mechanisms ofresistivity anisotropy under different loading paths are an-alyzed by experimental and theoretical research to provide acertain theoretical basis for the prediction of coal and rockdynamic disasters

2 Experimental System and Method

21 Experimental System As shown in Figure 1 the systemis composed of a three-axis pressurization device a resis-tivity measuring device a temperature control device and acomputer for data collection e following briefly intro-duces the structure and function of each device e three-axis pressurization device is designed by the laboratory torealize the axial and confining pressure loading of the elasticcavity through water pressure e pressure control range is0ndash16Mpa and measurement control accuracy is 001MpaResistivity test device uses HIOKI 3532-50 LCR tester in-strument connected to PC and then collects data continu-ously in its software It can display parameters such asimpedance phase angle inductance capacitance and re-sistance at the range of 5MHz e temperature controldevice employs ES-III temperature controller manufacturedby Changzhou Easy-to-Use Technology Company to achieveconstant temperature control of the coal samples

22 Samples and Method e coal samples were fromDashucun Sihe Yuwu Coal Mine in Shanxi province inChina Large coal samples with good integrity were pro-cessed into a Oslash25mmtimes 50mm cylinder e core samplingdirection of the drilling sampler is vertical or parallel to thecoal bedding direction and test specimens with differentbedding directions are drilled in the same coal sampleEnsure that the stress of the coal samples is uniform toprevent the damage of the coal body caused by the stressconcentration reduce the visible cracks as much as possibleand timelymark sample with different bedding directions Inorder to eliminate errors caused by sample irregularities andthe test electrode well coupled with specimens the surface ofsamples is smooth and cylindrical end face evenness error isless than 002mm Finally complete dense and no obviouscrack test specimens were selected as shown in Figure 2 Itshould be noted that the vertical bedding refers to the axis ofthe sample being perpendicular to the bedding plane andthe parallel bedding refers to the axis being parallel to thebedding plane

e experimental procedures are as follows

(1) Industrial analysis low-pressure nitrogen gas ad-sorption (LP-N2GA) and scanning electron mi-croscopy (SEM) tests were carried out on DashucunSihe and Yuwu coal samples to study and analyze themicrostructure of coal

(2) e cored coal samples were placed in the electricalparameter test chamber the copper electrode and theend face were coupled with conductive glue andenameled wire was connected with the LCR testerBefore loading in different paths a certain preloadwas applied to fix the specimen in the elastic cavityand the coal samples with different bedding direc-tions were increased the axial pressure to 9Mpa stepby step and the computer recorded the change ofelectrical parameters of the loaded coal sample Afterthe axial loading was completed the above steps wererepeated and the confining pressure of coal sampleswith different bedding directions gradually increasedto 8Mpa

According to the working principle of LCR tester theresistivity calculation formula is as follows

|Z|cos θ R

ρ |Z|cos θS

L R

S

L

(1)

where Z is the impedance absolute (Ω) θ is the phase angle(deg) ρ is the resistivity (Ωmiddotm) R is the resistance (Ω) S is thesample cross-sectional area (m2) and L is the sample length(m)

In order to facilitate the analysis of experimental resultsSDLA618 industrial analyzer was used to measure the basicparameters of coal samples in different mining areasaccording to GBT212-2008 Industrial Analysis of Coal Vdafis used to quantify the degree of coal metamorphism e








(a)

Axis

Beddingplane

(b)





LCR tester


Pressure gauge




Sample

Electrode plate


(a) (b)



200

190

180

170

160

150

140

130

120

110

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)


(a)

Resis

tivity

(Ωmiddotm

)

70

75

65

60

55

50

45

40

350 2 4 6 8 10

Stress (MPa)


(b)

Resis

tivity

(Ωmiddotm

)

200

180

160

140

120

100

800 2 4 6 8 10

Stress (MPa)


(c)

Resis

tivity

(Ωmiddotm

)140

120

100

80

60

40

20



(d)

Resis

tivity

(Ωmiddotm

)

70

65

60

55

50

45

400 2 4 6 8

Stress (MPa)


(e)

Resis

tivity

(Ωmiddotm

)

140

120

100

80

60

40



(f )














35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)



(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10



(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)





DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82






Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References























































(a)

Axis

Beddingplane

(b)





LCR tester


Pressure gauge




Sample

Electrode plate


(a) (b)



200

190

180

170

160

150

140

130

120

110

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)


(a)

Resis

tivity

(Ωmiddotm

)

70

75

65

60

55

50

45

40

350 2 4 6 8 10

Stress (MPa)


(b)

Resis

tivity

(Ωmiddotm

)

200

180

160

140

120

100

800 2 4 6 8 10

Stress (MPa)


(c)

Resis

tivity

(Ωmiddotm

)140

120

100

80

60

40

20



(d)

Resis

tivity

(Ωmiddotm

)

70

65

60

55

50

45

400 2 4 6 8

Stress (MPa)


(e)

Resis

tivity

(Ωmiddotm

)

140

120

100

80

60

40



(f )














35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)



(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10



(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)





DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82






Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References

















































200

190

180

170

160

150

140

130

120

110

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)


(a)

Resis

tivity

(Ωmiddotm

)

70

75

65

60

55

50

45

40

350 2 4 6 8 10

Stress (MPa)


(b)

Resis

tivity

(Ωmiddotm

)

200

180

160

140

120

100

800 2 4 6 8 10

Stress (MPa)


(c)

Resis

tivity

(Ωmiddotm

)140

120

100

80

60

40

20



(d)

Resis

tivity

(Ωmiddotm

)

70

65

60

55

50

45

400 2 4 6 8

Stress (MPa)


(e)

Resis

tivity

(Ωmiddotm

)

140

120

100

80

60

40



(f )














35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)



(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10



(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)





DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82






Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References




























































35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)



(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10



(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)





DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82






Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References



















































35

30

25

20

15

10

05

00

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)



(a)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

14

12

10

08

06

04

02

0000 02 04 06 08 10



(b)

N2

adso

rptio

n ca

paci

ty (c

m3 g

STP

)

7

6

5

4

3

2

1

000 02 04 06 08 10



(c)





DSC 0002153 2528 2309 5163 1188 93SH 0001735 5535 2431 2034 0375 61YW 0002510 6989 1514 1396 1165 82






Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References





















































Slit-shaped pores


Cylindrical pores


(a)

Cylindricalpores

CracksSilt-shaped

pores


(b)

Bottle-shapedpores

Wedge-shapedpores

Microcracks

Crack

pores


(c)







λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References





















































λ

ρnρt

1113970

(2)




Face

Butt

(a)

Beddings

Fissure channels

(b)





σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References



















































σ 1113944 σi 1113944i

nieiμi (3)

030

025

020

015

010

005

000

Dev

iatio

n va

lue

0 2 4 6 8 10Stress (MPa)

DashucunSiheYuwu

(a)

030

025

020

015

010

005

000

Dev

iatio

n va

lue


DashucunSiheYuwu

(b)


Eprime

Bedding plane

(a)

Eprime

Bedding plane

(b)









DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References























































DSCAl 3654 1829Si 3128 1519Ga 0364 0121

SH

Al 3458 1734Si 3690 1840Ga 1020 0382Fe 0262 0061

YW

Al 2879 1441Si 3863 1926Ga 1829 0611Fe 9102 2118Na 0632 0321Cl 0501 0210

200

180

160

140

120

100

80

60

40

Resis

tivity

(Ωmiddotm

)

0 2 4 6 8 10Stress (MPa)



(a)

140

120

100

80

60

40

20

Resis

tivity

(Ωmiddotm

)




(b)





c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References



















































c A exp(Bσ) (4)



y y0 + b1x + b2x2

+ b3x3 (5)

y A exp R0x( 1113857 + R1 (6)



5 Conclusions






Data Availability




Acknowledgments


References








































































































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ResearchArticle ...downloads.hindawi.com/journals/amse/2020/9016583.pdf · ResearchArticle AnalysisofResistivityAnisotropyofLoadedCoalSamples XiangchunLi,1,2,3ZhenxingAn ,1QiZhang