Effect of confining pressure unloading on strength reduction of soft coal in borehole ... · 2020....

ORIGINAL ARTICLE

Effect of confining pressure unloading on strength reductionof soft coal in borehole stability analysis

Qingquan Liu1,2,3 • Yuanping Cheng1 • Kan Jin1 • Qingyi Tu1 • Wei Zhao1 •

Rong Zhang1

Received: 1 August 2016 /Accepted: 14 February 2017

� Springer-Verlag Berlin Heidelberg 2017

Abstract Underground borehole drilling usually causes

instability in the surrounding coal due to in situ stress

redistribution (including stress concentration and stress

release). However, the mechanisms of unloading-induced

coal strength reduction are still poorly understood. The

primary objective of this study is to investigate the effect of

confining pressure unloading on soft coal strength reduc-

tion for borehole stability analysis. A series of mechanical

tests were conducted on both the traditionally and newly

reconstituted coal samples under two different experi-

mental stress paths, including conventional uniaxial/triaxial

compression and triaxial compression with confining

pressure unloading. The unloading stress path was obtained

by analyzing the stress redistribution around a borehole, to

capture a more accurate coal mechanical response.

According to our experimental results, plastic deformation

generated before failure under the unloading stress path is

smaller than that generated under the conventional loading

stress path. Furthermore, the cohesion of the traditionally

and newly reconstituted samples diminishes approximately

by 44.77 and 29.66%, respectively, with confining pressure

unloading, indicating that there is a significant reduction in

coal strength due to confining pressure unloading. The

mechanism for unloading-induced coal strength reduction

comes from confining pressure unloading-induced increase

in shear stress on the fracture surface and a decrease in

shear strength. This effect increases the shear slipping

potential, whose driving force generates tension fractures at

both ends of the preexisting fractures.

Keywords Soft coal � Borehole stability � Strengthreduction � Loading condition � Unloading condition

List of symbols

P0 Initial in situ stress (MPa)

rr Tangential stress (MPa)rrp Tangential stress in the plastic zone (MPa)

rre Tangential stress in the elastic zone (MPa)

rh Radial stress (MPa)rhp Radial stress in the plastic zone (MPa)

rhe Radial stress in the elastic zone (MPa)

r Distance from the center of a borehole (m)

ra The borehole radius (m)

Rp The radius of the plastic zone (m)

c Cohesion (MPa)

u Internal friction angle (�)r Principal stress (MPa)s Shear stress (MPa)rp Peak strength (MPa)r1 Compressive strength (MPa)r3 Confining pressure (MPa)r30 Initial confining pressure (unloading condition) (MPa)

r3p Confining pressure measured at the peak strength

(unloading condition) (MPa)

e1p Axial strain measured at the peak strength

w Fitting coefficientf Fitting coefficient

& Yuanping [email protected]

1 National Engineering Research Center for Coal Gas Control,

China University of Mining and Technology,

Xuzhou 221116, China

2 State Key Laboratory of Coal Resources and Safe Mining,

China University of Mining and Technology, Beijing 100083,

China

3 School of Civil, Mining & Environmental Engineering,

University of Wollongong, Wollongong, NSW 2522,

Australia

123

Environ Earth Sci (2017) 76:173

DOI 10.1007/s12665-017-6509-9

http://crossmark.crossref.org/dialog/?doi=10.1007/s12665-017-6509-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1007/s12665-017-6509-9&domain=pdf

Abbreviations

CMM Coal mine methane

ISRM International Society for Rock Mechanics

AE Acoustic emission

UCS Unconfined compressive strength

SEM Scanning electron microscope

Introduction

Capturing coal mine methane (CMM) by drainage bore-

holes is not only important for improving safety in coal

mines but also enables the beneficial recovery of a clean-

burning fuel resource and reduction of greenhouse gas

emissions (methane) at the same time (Karacan et al. 2011;

Liu et al. 2014a, b). However, with increasing mining

depth, soft coal seams are widely distributed under high

in situ stress conditions in many Chinese coal mines,

generating stability problems in drainage boreholes and

further mitigating the effectiveness of CMM capture (Kang

et al. 2010; Liu et al. 2014a, b).

Stability of drainage borehole and other underground

openings has drawn a lot of attention, leading to the

development of various empirical, analytical and numerical

methods for stability analysis and design (Meier et al.

2015; Tao et al. 2012; Whittles et al. 2007; Zhang 2013).

But proper estimation of in situ strength of coal is required

to implement these approaches. In addition, coal strength

also plays a significant role in various mining related

engineering activities, including the evaluation of coal and

gas outburst dangers, pillar design, hydraulic fracturing

design (involving enhanced gas drainage), coalface sup-

port, coal seam CO2 sequestration and other activities.

The primary method for determining coal strength is by

conducting a series of uniaxial/triaxial compressive tests in

the laboratory, whose procedure has been standardized by

the International Society for Rock Mechanics (ISRM).

Because the ISRM suggested method provides us with a

relatively straightforward standard to determine coal

strength, many coal strength factors have been studied.

Poulsen et al. (2014) reported that coal loses significant

strength when it becomes water saturated due to its

absorption of water. Ranjith et al. (2010) investigated the

weakening effect of CO2 on Australian black coal by

conducting acoustic emission and uniaxial tests, which

have also been studied by a number of researchers (for

example, Perera et al. 2013; Ranjith and Perera 2012; Viete

and Ranjith 2006). Medhurst and Brown (1998) conducted

a series of triaxial compression tests on samples with dif-

ferent diameters to investigate scale effects on the

mechanical behavior of coal, and based on these experi-

mental results, Poulsen and Adhikary (2013) developed

and calibrated a numerical Bonded Particle Model to study

the scale effect on coal strength. Li et al. (2015)

demonstrated that coal strength first increases and then

decreases with increase in the loading rate during triaxial

compression tests. Recently, due to innovations in tech-

nologies for deep underground mining at great depths

([1000 m) or tunneling underneath mountains, effects ofhigh confining pressure and temperature on coal (rock)

strength have drawn a lot of attention (Alshayea et al.

2000; Brotóns et al. 2013; Cai and Kaiser 2014; Haimson

and Chang 2000; Renshaw and Schulson 2007). Based on

studies in excavation engineering, Kaiser and Kim (2015)

reported that the strength characteristics of massive rock in

the direct vicinity of excavation differ significantly from

that which is remote with higher confinement. Further-

more, Xu et al. (2011) reported that temperature generates

a significant weakening effect on the compressive strength

of coal.

In addition to the above-mentioned factors on coal

(rock) strength properties, it should be noted that stress

path can also significantly impact coal (rock) strength.

Underground coal or rock is under a three-dimensional

stress equilibrium condition. However, excavation of an

underground mine disturbs the original in situ stress state,

leading to stress redistribution around the created under-

ground cavity. With rock removal, the stress redistribution

is characterized by the unloading process, under which the

mechanical properties of rock (coal) are different from that

of the loading process as the confining pressure is kept

constant (Wu and Zhang 2004; Zhou et al. 2008). More-

over, the unloading-induced stress release and its related

deformation relaxation during excavation can drastically

influence the stability of underground openings and might

induce rock destruction or rock burst under the high in situ

stress conditions (Tao et al. 2012; Zhao et al. 2014). Ding

et al. (2016) investigated the mechanical behavior of

sandstone under unloading conditions and found that its

load bearing ability decreased gradually due to unloading

of the confining pressure. Chen et al. (2016) reported that

the volume deformation of halite under unloading condi-

tions was greater than that under the triaxial loading con-

ditions. To study rock burst processes, which usually occur

on the underground excavation surface, He et al. (2010)

conducted single-face dynamic unloading tests on lime-

stone under true-triaxial conditions and showed that the

acoustic emission (AE) energy release increases rapidly

from the onset of unloading to sample failure.

While rock strength has been widely studied, consider-

ing the physical structural differences between rock and

coal, insight into the unloading-induced strength reduction

mechanism of coal is still poorly understood. Accordingly,

an experimental study was undertaken in this paper to

analyze and compare the strength characteristics of soft

coal under triaxial loading and triaxial confining pressure

unloading conditions, with the latter being called unloading

173 Page 2 of 11 Environ Earth Sci (2017) 76:173

123

condition in short in the following. The primary objective

of this paper is to investigate the effect of confining pres-

sure unloading on strength reduction of soft coal for

borehole stability analysis. The principal features of this

work include the attainment of the unloading path by

analyzing the stress redistribution around a borehole and

the mechanical tests on two types of reconstituted coal

samples to capture a more accurate mechanical response in

soft coal.

Experimental procedures

Coal sample preparation

It is well known there are difficulties in cutting soft coal to

a standard size, and the representativeness of a natural soft

coal sample is poor due to the low cutting success rate.

Moreover, the highly heterogeneous nature of coal some-

times makes it difficult to interpret the results of laboratory

experiments. Therefore, a homogeneous reconstituted coal

sample with properties reproducible in the laboratory pro-

vides significant advantages, especially in understanding

the effects of various mechanical factors on the properties

of coal (Jasinge et al. 2011b; Ranjith et al. 2012). However,

while reconstituted coal samples have been widely used to

investigate the mechanical and permeability properties of

coal, there is still no standard particle size for their

preparation. In general, reconstituted coal samples are

made in the laboratory by compressing fine coal particles

with some additives in a specially designed steel mold

(Jasinge et al. 2011a). As there is no standard for coal

particle size or additive, Xu et al. (2010) investigated the

influence of varying the particle size on the mechanical

properties of coal; Jasinge et al. (2009) made reconstituted

coal samples with cement as an additive to study the

geomechanical properties.

In this paper, two types of reconstituted coal samples

were prepared: a traditional sample and a reconstituted coal

sample using a new method. The traditionally reconstituted

sample was made in the laboratory by the following four

steps: First, small coal blocks were crushed and sieved

using a milling machine and a vibrating screen to select

only coal particles of a small size (\1 mm). Second, thesieved fine coal particles were compressed using a spe-

cially designed steel mold, with a diameter of 50 mm, and

a mechanical servo press. The compression of coal parti-

cles was stress-controlled with a loading rate of 300 N/s,

and the compression was maintained for three hours when

the load reached 100 MPa. Third, the coal sample was

extruded from the steel mold and trimmed to an approxi-

mate length of 100 mm using a diamond cutter. Fourth, the

sample was polished with a grinder to a reasonable

uniformity and smoothness in accordance with ISRM

standards (Christiansson and Hudson 2003). Finally, the

sample was dried in a vacuum oven at 60 �C for 24 h.The newly reconstituted samples were made from a

mixture of naturally distributed small coal blocks and coal

powder instead of crushed coal particles. The naturally

distributed small coal blocks and coal powder were

obtained from an underground coal mine using a number of

specially designed core barrels at the same site. The

preparation process for the newly reconstituted samples

was the same as that for the traditionally reconstituted

samples. The basic physical properties of both the tradi-

tionally and newly reconstituted samples are listed in

Table 1. As shown in Fig. 1, the internal structures of the

two types of coal sample were examined optically using

SEM (Scanning Electron Microscope, FEI QuantaTM 250)

at various magnifications. It can be found there is little

difference in sample mass between the newly and tradi-

tionally reconstituted samples. The primary difference is in

their source material.

Testing facility

As shown in Fig. 2, all the mechanical tests were con-

ducted using a coupled ‘‘mechanical- permeability’’ sys-

tem. The system primarily consists of two modules, i.e., the

loading and fluid modules. The loading module is a

hydraulic servo-controlled coal (rock) test system that can

work independently to conduct uniaxial/triaxial compres-

sion tests. When coupled with the fluid module, the

Table 1 Physical parameters of coal samples

Samples Diameter (mm) Length (mm) Mass (g)

T1 50.6 103.0 248.42

T2 50.8 102.6 249.13

T3 50.8 102.4 248.85

T4 50.8 103.6 251.03

T5 51.0 103.5 253.32

TX1 51.0 109.5 265.69

TX2 51.1 109.8 268.56

TX3 50.8 102.3 248.68

TX4 50.8 104.5 254.04

N1 50.5 102.8 257.25

N2 50.2 103.1 254.83

N3 50.5 102.0 255.11

N4 50.8 102.2 258.46

N5 50.5 103.5 259.02

NX1 51.5 102.0 265.19

NX2 50.8 103.1 261.07

NX3 50.6 105.3 264.55

NX4 50.8 104.1 263.61

Environ Earth Sci (2017) 76:173 Page 3 of 11 173

123

integrated system can be used to test both the coal per-

meability and its mechanical properties when exposed to

different gases (CH4, CO2, N2, He, etc.).

The loading module consists of a loading frame, two oil

hydraulic pumps (one for axial loading and another for con-

fining pressure), a servo-system, a triaxial pressure cell, a

Fig. 1 Photomicrographs of internal structures of traditional and newly reconstituted coal samples

Fig. 2 Photograph andschematic diagram of the testing

facility


123

temperature control unit and a data measurement and

recording unit. Based on the hardware, the loading module

works in either a load-controlled mode or displacement-con-

trolled mode with a seamless real-time changeover ability. In

the load control mode, the designed maximum axial loading

and confining pressures are 300 and 60 MPa, respectively. In

the displacement control mode, the designed maximum dis-

placement of the two oil hydraulic pumps is 230 mm. The

control accuracies for the load-controlled mode and dis-

placement-controlled mode are B±1 and B±0.5%, respec-

tively. Using the temperature control unit, the temperature of

the triaxial pressure cell can be controlled between ambient

temperature and 90 �C with an accuracy B±0.2 �C. Whenconducting a mechanical test, the sample is placed inside the

triaxial pressure cell on the base plate as shown in Fig. 3. The

axial strain and radial deformation aremeasuredusing an axial

strain gauge and a radial strain gauge and can be automatically

recorded during experiments. The designed maximum

deformation of the axial strain gauge and radial strain gauge is

8 and 4 mm, respectively.

Unloading path design and testing procedure

It iswell known that accurate coalmechanical response can be

captured only if the unloading path is accurately represented

(Cai 2008). In this study, the unloading path was obtained by

analyzing the stress redistribution around a borehole. A typi-

cal stress redistribution around a borehole (when the initial

in situ stress is P0, the hydrostatic pressure state) is shown in

Fig. 4. Both the tangential stress (rr) and radial stress (rh)change with distance from the center of a borehole. The tan-

gential stress is lower than the initial stress and increases with

distance (r). The radial stress first increases from the residual

coal strength to the maximum concentrated stress and then

decreases with the distance (r) to the initial stress. The coal

surrounding a borehole is characterized by elastic–plastic

secondary stress distribution, and the thickness of the plastic

zone is Rp - ra (Zhang et al. 2003). According to the Mohr–

Coulomb Criterion, coal in the plastic zone fails to bear the

maximumprinciple stress and loses its bearing strength. Thus,

it can be concluded that with borehole drilling, the maximum

principle stress increases prior to failure and the minimum

principle stress decreases.

The uniaxial test was performed on two samples (one

traditional sample and one new sample); the triaxial tests

(with confining pressures 2, 4, 6 and 8 MPa) were per-

formed on eight samples (four traditional samples and four

new samples). Conventional uniaxial and triaxial com-

pression tests were conducted to obtain peak strengths,

which was used to determine the unloading point. The

Fig. 3 Photograph of a samplewith the sensors


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corresponding loading procedure was divided into two

stages. In the first stage, the hydrostatic strength (confining

pressure) was loaded at a rate of 10 N/s to some prede-

termined values (i.e., 2, 4, 6 and 8 MPa; in the confining

pressure-controlled mode). In the second stage, the axial

strength was loaded at a rate of 50 N/s until failure

occurred (in the axial load-controlled mode), while keeping

the confining pressure constant.

As shown in Fig. 5, the unloading path was designed

based on stress redistribution characteristics, with the

corresponding loading procedure divided into three stages.

In the first stage, the hydrostatic strength was loaded at a

rate of 10 N/s to predesigned values 2, 4, 6 and 8 MPa, in

the confining pressure-controlled mode. In the second

stage, the axial strength was loaded at a rate of 50 N/s until

it reached the predesigned unloading point (80% of the

corresponding peak strength), while keeping the confining

pressure constant. In the third stage, the confining pressure

was gradually unloaded at a rate of 10 N/s, and the axial

strength was loaded at a rate of 50 N/s simultaneously until

failure occurred. All conventional compression and

unloading tests were conducted at 30 �C to avoid theinfluence of potential temperature fluctuations.

Results and discussion

The internal structures of the two types of samples are

shown in the SEM images obtained by scanning the

surfaces of the two types of coal samples (Fig. 1). There

are numerous fractures present at different scales (ranging

from 469 to 8009 magnification) found in the new type

coal sample. In contrast, few microfractures were

observed in the traditional coal sample (6009 and 12009

magnifications). Based on the microscopic observations of

internal structures, we conclude that the new type of

reconstituted sample partially retained the structural

characteristics of soft coal and is more representative of

the natural ore body.

Deviatoric stress–strain relationship under

multi-triaxial loading conditions

Ten conventional uniaxial and triaxial compression tests

were conducted to determine the peak strengths of the two

types of reconstituted coal samples. Of the ten tests, five

were carried out on the traditionally reconstituted samples,

and the other five on the newly reconstituted samples.

Figure 6 shows the deviatoric stress–strain diagrams for the

two types of reconstituted samples, tested under different

confining pressures. As observed from the figures, the two

types of samples have some similarities, such as their peak

strength increasing with increasing confining pressure,

unconfined compressive strengths (UCS) being far lower

than the triaxial compression strength and undergoing

through a significant plastic deformation before reaching

the corresponding peak strengths. We therefore conclude

that confining pressure helps to close the preexisting coal

fractures, thereby inhibiting the generation of new fractures

and the extension of initial ones. Moreover, confining

pressure is also helpful in enhancing internal friction

between mineral components, thus improving the bearing

capacity of coal.

Fig. 4 Illustration of elastic–plastic stress redistribution around aborehole (Liu et al. 2014a, b) (where s represents shear strength, rrepresents principal stress, rh

p and rrp represent radial stress and

tangential stress in the plastic zone, rhp and rr

p represent radial stress

and tangential stress in the elastic zone, ra is the borehole radius, Rp is

the radius of the plastic zone)

Fig. 5 Illustration of the confining pressure unloading stress path(where rp represents the peak strength, r3 represents the confiningpressure, r1 represents the compressive strength)


123

However, there are still some differences between the

two types of coal samples. The peak strength (rp), the axialstrain measured at the peak strength (e1

p) and the corre-

sponding confining pressure (r3) are summarized inTable 2. As indicated in Table 2, at the same confining

pressure, the peak strength of the traditional sample is

greater than that of the new sample and the axial strain (e1p)

of the traditional sample is smaller than that of the new

sample (except at a 8 MPa confining pressure).

Preexisting fractures can strongly influence the

mechanical properties of coal (Ding et al. 2016). The pri-

mary reason for the observed differences is that the new

samples can manage to maintain their preexisting soft coal

fractures, while the preparation of traditional samples

damaged the natural coal fractures (due to the crushing of

the coal blocks). Thus, at the same confining pressure, the

traditional sample can bear greater axial load than the new

sample. With a lower bearing capacity, the new sample will

generate plastic deformation even at a relatively low axial

load.

Based on the obtained peak strengths for the two

types of samples, eight unloading tests were conducted

with different initial confining pressures. Of the eight

tests, four were carried on the traditionally reconstituted

samples and the other four on the newly reconstituted

samples. Figure 7 shows the deviatoric stress–strain

diagrams for the two types of reconstituted samples,

tested under different initial confining pressures. As

observed in the figure, the peak strengths of the two

types of samples also increase with increasing initial

confining pressure; the peak strength of the traditional

sample is also greater than that of the new sample under

the same initial confining pressure. However, it is clear

that the deviatoric stress–strain curves of the two types

of samples obtained under the unloading stress path are

quite different from those obtained under the conven-

tional loading stress path. The initial confining pressure

(r30), the peak strength (rp) and the corresponding con-

fining pressure measured at the peak strength (r3p) are

summarized in Table 3.

As listed in Table 3, the peak strengths of the two types

of samples are lower than that obtained under the con-

ventional loading stress path. One reason for this result is

that in the unloading path, the confining pressure measured

at peak strength is lower than the corresponding initial

confining pressure. In contrast, the plastic deformation

generated at peak strength under unloading stress path is

smaller than that generated under the conventional loading

stress path, indicating that coal generates shear damage

more easily under an unloading stress path.

Fig. 6 Deviatoric stress versus strain for the coal samples obtained under conventional compression condition

Table 2 Conventional uniaxial and triaxial compression testingresults

Samples r3 (MPa) rp (MPa) e1p (91E-3)

T1 0 1.66 16.3

T2 2 13.72 30.8

T3 4 21.90 38.8

T4 6 28.57 50.7

T5 8 36.14 97.2

N1 0 0.95 24.0

N2 2 11.64 35.9

N3 4 19.56 53.2

N4 6 27.17 64.6

N5 8 33.76 79.6


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The strength reduction effect of confining pressure

unloading

It is difficult to produce tensile stress under the high

compressive stress present in the earth; therefore, during

deep drilling and geological faulting, shear failure pre-

dominates (Maurer 1965). The Mohr–Coulomb strength

criterion is one of the most widely used strength criterions

in geomechanical engineering, which describes a linear

relationship between normal stress and shear stress at

failure. According to the Mohr–Coulomb criterion, the

shear strength of coal (rock) is made up of two parts, a

constant cohesion and a normal stress-dependent frictional

component (internal friction angle) (Zhao 2000). In gen-

eral, cohesion and the internal friction angle are indirect

measurement parameters but can also be calculated using

the statistics of uniaxial and triaxial compression test

results. For an accurate regression analysis of the com-

pression results, the Mohr–Coulomb criterion is translated

into the following principal stress form (Li et al. 2007):

r1 ¼ wþ fr3 ð1Þ

where

w ¼ 2c cosu1� sinu ð2Þ

and

f ¼ tan2 45� þ u2

� �ð3Þ

where w and f are fitting coefficients, c is cohesion, and uis internal friction angle.

In particular, coal failure under uniaxial compression is

due to shear dilatancy, rather than shear failure (Liu et al.

2014a, b; Su et al. 2006). The uniaxial compression

strength is far lower than the triaxial compression strength

(as shown in Fig. 8) and will introduce some non-negligi-

ble errors when calculating the shear strength parameters if

incorporated. Thus, in this research, uniaxial compression

strength was not incorporated in the regressive calculations

of shear strength parameters.

Fig. 7 Deviatoric stress versus strain for the coal samples obtained under confining pressure unloading condition

Table 3 Triaxial confining pressure unloading testing results

Samples r30 (MPa) r3

p (MPa) rp (MPa)

TX1 2 1.69 10.58

TX2 4 3.02 17.04

TX3 6 4.40 22.92

TX4 8 5.75 27.56

NX1 2 1.61 9.54

NX2 4 3.66 15.01

NX3 6 4.74 21.62

NX4 8 6.31 26.13

Fig. 8 Mohr–Coulomb circles of traditionally reconstituted coalsamples obtained under conventional compression condition


123

As shown in Fig. 9, compressive strength increases with

confining pressure, i.e., there is a clear enhancing effect of

confining pressure on strength. The results also suggest that

the strength criteria closely confirm the Mohr–Coulomb

criterion, as linear r1–r3 curves are clearly observed.Natural coal (rock) is a heterogeneous material, and the

strength test plots are usually scattered. This characteristic

is significantly influenced by internal structures, which also

depend on the confining pressure (You 2014). As the new

coal sample partially maintains the structural characteris-

tics of soft coal and the influence of confining pressure

unloading, the compressive strengths test plots are a little

scattered. The four linear r1–r3 curves represent fourexperimental conditions and are obtained by the linear

least-squares regression method. The R-square of three

linear regression results is higher than 0.99 and about 0.964

for the new samples under the unloading condition, indi-

cating that the standard errors in the four linear curves are

low and the fitting results are reliable. By using Eqs. (1)–

(3) and based on the linear regression results, cohesion and

internal friction angle of the coal samples tested under two

stress paths are calculated and listed in Table 4.

As indicated in Table 4, for the conventional compres-

sion stress path, the cohesions of traditionally and newly

reconstituted samples are 1.72 and 1.18 MPa, respectively;

for the triaxial unloading stress path, the cohesions of

traditional and newly reconstituted samples are 0.95 and

0.83 MPa, respectively. The cohesion of traditionally

reconstituted samples is higher than that of the newly

reconstituted samples under both loading and unloading

stress paths. As cohesion represents the intermolecular

force of coal, the comparison of cohesion between the two

types of coal samples indicates that the preparation of

traditional samples damaged the coal matrix (due to the

crushing of the coal blocks), while the new type of samples

maintained some of their internal structures.

Furthermore, for both of the traditionally reconstituted

sample and newly reconstituted sample, the cohesion

obtained under the conventional compression stress path is

higher than that obtained under the triaxial unloading stress

path. Comparing the conventional compression stress path

and triaxial unloading stress path, the cohesion of the tra-

ditionally reconstituted sample and newly reconstituted

sample reduced by 44.77 and 29.66%, respectively, with

confining pressure unloading. These results show that there

is a significant reduction effect of confining pressure

unloading on coal strength.

It is well known that during conventional compression, the

process of shear failure consists initially of fracture closure

followed by elastic deformation, then fracture initiation

Fig. 9 Relationships between r1 and r3 obtained under multi-triaxial loading conditions

Table 4 Computation results ofstrength parameters of the two

types coal samples

Samples Stress paths w (MPa) f c (MPa) u (�) R2

Traditional samples Conventional compression 6.60 3.70 1.72 35.06 0.998

Triaxial confining pressure unloading 3.91 4.21 0.95 38.03 0.992

New samples Conventional compression 4.54 3.70 1.18 35.06 0.997

Triaxial confining pressure unloading 3.18 3.65 0.83 34.74 0.964


123

followed by fracture damage. This phenomenon is known as

stable fracture growth; if the compressive load is removed

generation of fracture growth will stop. Finally, unstable frac-

ture growth occurs resulting in sample failure. However, con-

fining pressure unloading leads to an increase in shear stress on

the fracture surface and decrease in shear strength, thus

increasing the potential of shear slipping whose driving force

would generate tension fractures at both ends of the original

fracture (Huang andHuang 2014). Thus, stable fracture growth

transforms relatively quickly into unstable fracture growth,

degrading the bearing capacity (cohesion) of coal.

Conclusions

Due to the poor representation of natural soft coal samples

from the low cutting success rate, a new method has been

developed to make reconstituted coal samples. This method

partially maintains the structural characteristics of soft

coal. By using both the traditionally and newly reconsti-

tuted coal samples, a series of mechanical tests were con-

ducted to investigate the effect of confining pressure

unloading on strength reduction of soft coal in borehole

stability. Based on the comparison and analysis of strength

characteristics between conventional uniaxial/triaxial

compression tests and triaxial unloading tests, some fun-

damental conclusions can be drawn:

1. To more accurately capture coal mechanical responses

in borehole stability analysis, the unloading path was

obtained by analyzing the stress redistribution around a

borehole, which can be divided into three stages. The

third stage was unique as the confining pressure was

gradually unloaded at a rate of 10 N/s while loading

the axial strength at a rate of 50 N/s until failure

occurred.

2. Because preexisting fractures can strongly influence

the bearing capacity of coal, and the new samples

maintain the preexisting fractures of soft coal, in

conventional uniaxial/triaxial compression tests (at the

same confining pressure), the traditional sample can

bear a greater axial load than the new sample which

generate plastic deformation even at a relatively low

axial load.

3. Before failure occurs, the plastic deformation gener-

ated under the unloading stress path is smaller than that

obtained under the conventional loading stress path,

indicating that coal generates shear damage more

easily under the unloading stress path.

4. The cohesion (primary parameter) and internal friction

angle are used to quantitatively evaluate the shear

strength of coal. Comparing the conventional com-

pression stress path and triaxial unloading stress path,

the cohesions of the traditionally reconstituted sample

and newly reconstituted sample degrade approximately

by 44.77 and 29.66%, respectively, with confining

pressure unloading. These results show that confining

pressure unloading produces a significant reduction in

coal strength, primarily because the confining pressure

unloading leads to an increase in the shear stress on the

fracture surface and decrease in shear strength. This

effect increases the potential of shear slipping whose

driving force could generate tension fractures at both

ends of the preexisting fractures.

Acknowledgements The authors are grateful to the financial supportfrom projects funded by Natural Science Foundation of Jiangsu

Province (No. BK20160253), China Postdoctoral Science Foundation

(No. 2016M590519), the State Key Laboratory of Coal Resources and

Safe Mining (No. SKLCRSM16KFB01) and the Priority Academic

Program Development of Jiangsu Higher Education Institutions and

the Fundamental Research Funds for the Central Universities (No.

2013QNA03).

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Effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysisAbstractIntroductionExperimental proceduresCoal sample preparationTesting facilityUnloading path design and testing procedure

Results and discussionDeviatoric stress--strain relationship under multi-triaxial loading conditionsThe strength reduction effect of confining pressure unloading

ConclusionsAcknowledgementsReferences

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