field distributed displacement monitoring of the coal roadway...

American Journal of Engineering, Science and Technology (AJEST) Volume 5, 2020

1

Field Distributed Displacement Monitoring of the Coal Roadway

Roof Underground in Yinying Coal Mines TaoHu,

1Gongyu Hou

1, SuBu,

2Yan Wang,

3 Ziyi Hu,

3Zixiang Li

1,Jinping Liang

1,Yuliang

Zhou1,Haoyong Jing

1

1 School of Mechanics and Civil Engineering, China University of Mining and Technology,

Beijing100083, China. 2

School of Laws and Humanities, China University of Mining and Technology,

Beijing100083, China. 3Shan Dong Energy Group Co., Ltd., Jinan 250000,China.

Correspondence should be addressed to TaoHu; [email protected]

Abstract

The roof displacement monitoring of coal mine roadways is the most common means to

prevent the roof accident, and is the key to ensure the coal mine safety production. But the

monitoring blind zones or leakage points caused by traditional monitoring methods are one of

the main reasons for that roof accidents still are the most frequent among coal mine accidents

nowadays. So, a creative approach of distributed settlement displacement monitoring of coal

roadways roof was put forwards which combined distributed optical fiber sensing (DOFS)

and pre-stretched optical fiber fixed-point layout (POFFL) technology in this paper. Through

the detailed study of optical fiber strain monitoring of DOFS, and the theoretical analysis of

POFFL on the roof of coal roadways, the mathematical expression of fiber strain quantifying

roof displacement was obtained. Appling the approach in the indoor roof settlement model

experiment and the field monitoring experiment of roof displacement in an air intake coal

roadway of 150313 working face in Yinying Coal Mine during the period of the working face

retreating, the experiment results showed that optical fiber strain not only accurately

presented the range of roof deformation, but also qualitatively and quantitatively

characterized the roof settlement, the displacement quantified by the optical fiber strain was

consistent with measured value of other traditional measuring methods. The indoor and field

experiments results verified the feasibility and validation of distributed roof displacement

monitoring method based on DOFS and POFFL technology for coal roadways roof. The

research in this paper has important reference significance for the distributed roof monitoring

and the support design of coal roadways under similar geology and production conditions.

This roof monitoring method will also be of a great help and promotion role for the safety

production of coal mines.

1. Introduction

As the coal roadways roof collapse is the most common and frequent accident, for example,

over ten years of statistical data (State Administration of Coal Mine Safety (SACMS)) show

that the proportion of coal mine roof accidents in the total number of occupational accidents

was 51.2%, and the number of deaths caused by coal mine roof accidents accounted for

36.5% of the total deaths1

in China, coal roadways roof collapse is one of the major causes of

mine personnel injuries, equipment damage and production losses2. Therefore, it is the

primary guarantee and prerequisite for coal mine safety production to eliminate and reduce

the roof accidents of coal mine roadways. It has been demonstrated that the catastrophic roof


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collapse accidents are mainly the results of excessive roof subsidence and separation3Error!

Reference source not found.. It has been validated and demonstrated that roof separation

monitoring and roof displacement monitoring are currently the most direct and effective

prevention countermeasures in reality mining activities underground. They have been not

only commonly widely adopted by personnel at all levels in coal mining all over the world,

but also the mandatory requirements of current coal mine safety Regulations4-6

. Because the

roof settlement displacement is equal to or greater than the roof separation amount4, roof

settlement displacement monitoring has the better function of early warning and alarm. This

requires that the roof settlement displacement of coal roadway is to be monitored timely for

detection of even the slightest sign of roof disaster, and then corresponding countermeasures

would be taken.

Although technically, roof monitoring has undergone the development progress from

early manual measure-rods and measure rulers etc., then to mechanical or electronic

extensometers, finally to FBG optical fiber gratings sensors of intrinsic explosion-proof and

electromagnetic-proof7 etc., the occurrence of coal mine roof accidents still cannot be curbed.

There are two important reasons, one is that the sensors currently used are all point sensors

with a large spacing distance, for examples8, two-anchor wire extensometers (tell-tales) are

installed at every 20 m in roadways and in all intersections in UK and Canada underground

coal mines; China stipulates that a roof separation meter shall be installed every 50 m for

ordinary bolt-net-supported coal roadways; the other one is that the monitoring mode is a

point monitoring mode, and there are inevitably monitoring blind areas and monitoring leaks

between sensors existed, resulting in the data of monitoring of roof separation and

displacement is too scarce to correctly reflect the roof deformation characteristics, therefore,

many of the corresponding early warning and the alarm effect were ignored. The cruel reality

and severe coal mine safety production situation make it very urgent and imperative that a

new distributed monitoring technology without monitoring blind area and monitoring

omission point must be explored to effectively monitor the safety of coal roadway roof.

It is only the distributed optical fiber sensing (DOFS) technology which emerged in the

1980s is just the kind of techniques the coal mines need utmost urgently. It can make up for

the shortcomings of traditional monitoring technology with its distributed, long-distance and

large-scale continuous strain monitoring features. Moreover, the medium of the sensing and

transmission of DOFS are optical fiber, which also has the following intrinsic advantages:

passive (all dielectric); anti-electromagnetic interference; high temperature performance;

explosion proof; corrosion and oxidation resistance; light weight, small size and convenient

layout. Therefore, DOFS breaks through the limitations of traditional mechanical and

electronic sensors and has been widely used in the various structure health monitoring9-10

,

especially in the civil tunnels11-12

and roadways of underground metalliferous mine13-14

.

Unfortunately, there is seldom research reported about coal roadways roof monitoring based

on DOFS in scientific papers nowadays.

In this paper, a new approach based on DOFS combined with POFFL technology is

proposed for realizing the distributed settlement monitoring of coal roadways roof. Through

detailed research on the principle of DOFS optical fiber strain monitoring and the theory of

POFFL, the theoretical study of optical fiber strain quantification to characterize roof

displacement was carried out, and the quantification was applied in the indoor roof settlement

model experiment and the field coal roadway roof monitoring. The research in this paper is of


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significant reference to improve the safety management of coal roadways roof and promote

the sustainable development of coal mine safety production in China.

2. Introduction of distributed optical fiber sensing technology (DOFS)

DOFS uses low-cost telecom-grade optical fiber cable as the inherent sensing material for

sensing and transmission purposes. DOFS uses the laid one-dimensional optical fiber as the

sensor to monitor the changes of various physical measurands along the axial direction of the

optical fiber according to the optical properties of the scattered light, such as temperature,

strain and vibration. According to the layout principle of sensors, it is divided into point

sensors, quasi distributed sensors and fully distributed sensors16

; according to the sensing

principle16

, it is mainly divided into: distributed vibration sensing technology (DVS or DAS)

based on Rayleigh scattering, distributed temperature sensor (DTS) based on Raman

scattering principle and distributed temperature / strain sensor (DTSS) based on Brillouin

scattering. For our research purpose, the principle of backscattering light and Brillouin

optical time domain reflectometry (BOTDR) are introduced in details in this paper.

2.1. Backscattering light of DOFS

The working principle of DOFS technology is mainly based on the scattering phenomenon of

light, which can be described by the interaction between optical signal and propagation

medium. The light emitted by the light source will be backscattered during transmission in

the optical fiber. According to the scattering mechanism, the backscattered light in the optical

fiber can be divided into three categories: Rayleigh scattering light, Brillouin scattering light

and Raman scattering, as shown in the Fig.1.

Figure 1: Light scatterings in optical fiber.

Fig. 1 shows the main components of backscattered light when a section of optical fiber

is deformed, including Rayleigh scattering, Raman scattering and Brillouin scattering.

Rayleigh light scattering is an elastic scattering, Raman scattering and Brillouin scattering are

the inelastic scattering of the interaction between light and phonon; Rayleigh and Brillouin

scattering can detect changes in temperature or strain, which are modulated by the intensity

and frequency of scattered light respectively. Raman scattering is only caused by temperature

change and modulated by light intensity. It is the Brillouin scattering that widely utilized for

strain measurement in structure health monitoring .The principle of Brillouin scattering

sensor has been widely discussed in references17

. There are mainly three broad categories of

Brillouin sensing systems: Brillouin optical time domain reflectometry (BOTDR), Brillouin

optical frequency domain analysis (BOFDA) and Brillouin optical time domain analysis


4

(BOTDA). A BOTDR instrument launches an input pulse from one end into a single mode

fibre and observes the spontaneous backscattered light generated by the pulse at the same end

of the fibre, and the others techniques require a looped fibre configuration to fulfil a whole

test. Therefore, BOTDR has unique advantages in the case of dangerous and inconvenient

maintenance, such as underground coal mines.

2.2. Principle of BOTDR strain measurement

BOTDR has been widely used with its advantage of single end test. It is a system that uses

spontaneous scattering light to measure the return time of optical pulses entering the fiber

under test (FUT) and the intensity of scattered light at each sampling point. By analysing the

measured time and strength, BOTDR calculates the physical quantities of distribution, such

as temperature and strain. The basic configuration of BOTDR is shown in Fig. 2.

Figure 2: Schematic diagram of strain monitoring of BOTDR

In the measuring of BOTDR, the relationship between the frequency shift and the

longitudinal strain and temperature changes in the core / cladding can be approximately

expressed by a linear function, as shown in formula (1):

0 0 0 0B B TT T C T T C （ , ） , + - + - (1)

Where, B T, （） is the optical frequency shift at temperature T and strain ε; 0 0B T （ , ） is the

Brillouin frequency shift at the initial temperature T0 and initial strain ε0 of the optical fiber;

CT is the temperature coefficient of the optical fiber; Cε is the strain coefficient of the optical

fiber; where the standard single-mode fiber SMF-28 at room temperature CT≈ 1MHz / ℃ and

Cε≈ 500MHz /%.

The position of fiber where temperature or stain changed can be determined by the

time of light propagation in the optical fiber, the time T from the transmitting end to the

position of the measured optical fiber, and then back to the transmitting end, as shown in

formula (2):

2

C TZ

n

(2)


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Then, combining with the formula (1) ~ (2), we can process the signal of the spectrum

and transform the frequency shift of Brillouin scattering spectrum into the strain distribution

in distance.

Because the temperature and strain all affect the frequency shift of the Brillouin

scattering light. For eliminate the temperature effect in strain monitoring, temperature

compensation must be implemented. According the reality practice experience, the common

used compensation equation18

below is adopted in this paper.

cor rel T (3)

Where rel is the measured strain; cor is the corrected strain; is the proportional

coefficient,with the unit of με /℃, taken as 30 με / ℃in this study; T is the temperature

difference.

3. POFFL techniques of the roof in coal roadways

In China, more than 90% of the coal mines adopt the longwall mining technology19

. The

longwall mining process usually includes two gateroads, which are called haulage gateroad

(air-intake gateroad) and air-return gateroad. The mining gateroads account for more than

80% of the total roadway footage in coal mines20

. The mining gateroads of longwall caving

mining technology mainly are coal roadways in rectangular shape which the roof and two

sides are all composed of coal, the roof of coal roadways in top coal caving mining needs to

collapse synchronously after the working face mining in order to improve the recovery rate of

coal resources. Therefore, this kind of roadway usually only needs the primary support of

bolts etc. It was found that the most frequent mine instabilities are roof falls in coal roadways

ahead of the longwall face, this kinds of accidents are most likely to cause significant

economic loss because they are the critical access to the longwall operations5.

Because the coal roadways only need primary bolts supporting, and would be

abandoned after the working face passes, the roof of the coal roadways must be in stable state

before the working face passes, and the roof must collapse as quickly as possible in time after

being mined for preventing the gas accumulating forming gas accidents20

. So, the coal

roadways roof should try to avoid the secondary or multiple supporting that existed in civil

tunnels or roadways in metalliferous mines such as reinforced concrete precast segments

support and secondary concrete shotcrete lining etc. It is very clear that the roof surface of the

coal roadways is not smooth and neat, and it mostly present discontinues broken coal rocks

and irregularly unsteadily fractured roof surface due to the original sedimentary geological,

tectonics conditions and high overburden stress. With the cutting effect of coal rocks and

loosening effect of irregular unsteadily roof strata, the optical cables cannot be stick to the

roof surface or imbedded in the fractured coal roof along the roadway strike. It is imperative

and urgent to explore a method of optical fiber layout in line with this worse situation and

condition of coal roadway roof.

Because the role of bolts support is to keep the stability of anchorage body composed

of bolts, anchor cables and surrounding rock; and the main function of anchor cables is to

suspend the unstable part of lower rock layers on the upper stable rock layers. Therefore, the

displacement change of rock bolts (or anchor cables) not only reflects the settlement

displacement of anchorage body, but also reflects the separation change of roof strata. So,on

one hand it not only can distributed monitor the roof displacement that using the existing

bolts on the roof serve as fixed point to lay out the stretched optical fiber cables, it can avoid


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the optical fiber cables being cut or loosen by the broken coal rocks owing to a certain

distance between the out-end of the rock bolts and the surface of coal roof as well.

3.1 Pre-stretched optical Fiber Fixed-point Layout (POFFL) technology

Pre-stretched Fiber is to give a certain initial pre-tightening force to the relaxed optical fiber

before laying out, so a certain tensile strain change occurs and maintains in the stretched

fiber, which is defined as the initial strain. Later, when the optical fiber is compressed, the

fiber strain decreases compared with the initial pre-stretched strain, that results in a negative

relative strain, namely compressive strain, which the negative value decreases with the

increment of fiber shrinkage; on the contrary, a positive relative strain generates when the

fiber is elongated, called tensile strain, and the tensile strain increases with the elongation

increases.

The fixed-point layout is based on the principle that two fixed points determine a

straight line. The optical fiber is fixed by two fixed points. The optical fiber is stretched or

contracted in a straight line between the two points, and the fiber strain changes uniformly.

The POFFL of optical fibres on a roof can be simply illustrated as figure 3 below.

A BL

d d d

Anchor cables （rock bolts）

Top coal

Stretched Optical Fiber

Overburden

Lock of bolts

Coal roadway roof

B'

Point B is

stretched to B

Figure 3: The diagram of the POFFL on the roof of a coal roadway

Assuming that the distance between adjacent fixed points is d and the uniform strain of

optical fiber between fixed points is ε, it can be seen that the stretched amount (compressed

amount) δ of optical fiber between fixed points is expressed by formula (4), and the straight

line length L of optical fiber after deformation between fixed points is expressed by formula

(5). Needless to say, it also can be obtained from the strain of equal spatial resolution

sampling interval of BOTDR if the uniform ε is unknown.

=

= zB

A

d

dz

or, (4)

= (1 )= (1 z )B

AL d d d dz (5)

Where is the uniform strain of fiber AB; dis the interval space length of adjacent fixed

points, such as from Point A to Point B; is the deformation amount of the deformed fiber

AB; z is the average strain between the neighbouring sampling points, that is to say,

between A and B. The sampling interval of the BOTDR analyzer was set as 0.20 m, so there

were 5 sampling points in the 1 m-long sensing fiber.

3.2 Deformation characteristics of coal gateroads roof

Under the combined support of bolts, anchor cables, wire mesh and W steel strips, the broken

roof of coal roadways can be effectively delayed and restricted. The roof of coal roadways

presents irregular settlement in different position, different area and different damage degree.


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Production practice shows that from the macroscopic and overall view of roof surface

morphology, this kind of coal roadways roof subsidence deformation can be divided into

"point" type and "zone" type: “point” type refers to the part with the largest settlement is very

small compared with the whole deformation settlement area, and this part can be regarded as

a point of the whole deformation area; “zone" type refers to that the largest settlement part is

a platform compared with the whole deformation settlement area ,The platform can be

described as a contour platform on which there are the same displacement. From the strike

section and cross section of rectangular coal roadway, the “point” type looks like inverted

triangle and the “zone” type looks like inverted trapezoid as shown in Figure 4a below.

Overburden

Point type Triangle uniform settlement

Zone type

Trapezoid uniform settlement unevenly irregular triangle Optical fiber

Anchor bolts

(cables)

Floor FloorThe strike of roadway

a

Uniform Triangle

settlement

Uniform trapezoid settlement

Unevenly

settlement

Unevenly

settlement

Fixed

Point

b

Figure 4: The diagram of the settlement types of coal roadways roof laid out of pre-stretch cables; (a) Schematic

diagram of uniform settlement of two Types ;(b) Deformation diagram of POFFL, the blue solid line represents

the uneven settlement fiber, the black solid line represents the uniform settlement fiber, and the red solid line

represents the original fiber state..

The two waist deformation of the triangle and trapezoid can either be uniform

deformation of straight line forms, or also can be non-uniform deformation of polygon forms,

as shown in Fig.4 b. accordingly; the optical fiber laid through bolts (cables) also presents the

deformation of approximate inverted triangle and trapezoid. Under the influence of gravity

balance, the final equilibrium state of roof settlement is the shape of symmetrical settlement

deformation.

3.3 Calculation of deformation of the optical fiber laid out by fixed points

According to the aforementioned discussion, no matter what types of roof settlement is, the

deformation amount of laid optical fiber cables can be calculated by the fiber strain according

to deformation mode, that is to say, uniform deformation and uneven deformation. Uniform

deformation refers to the deformation of optical fiber along a straight line, shown as Fig.5a,

and uneven deformation refers to the polygonal deformation of optical fiber under the

constraint of fixed points, shown as Fig.5b.

(1) Uniform deformation

Shown as Fig. 5(a), supposing that the optical fiber between the fixed points is simply

subjected to the action of linear axis force along the fiber, there are n fixed points among AB,

the original interval between adjacent fixed points is d. when the optical fiber is stretched

from AB to AB1, because the strain of optical fiber between the adjacent fixed point is

uniform, the optical fiber strain of the whole stretched AB1 is also uniform, and the

deformation between adjacent points is the same as δ.


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A B

A B1

A B C

εL

d

0

εAB

εBC

Initial state of the optical fiberFixed Point

stretched optical fiber

Optical fiber strain curve

.........

.........

dd

d+δ d+δ d+δ

a

Initial fiber

Deformed fiber

Fixed point

G

G

G

A B

B1

l

(1

)i

l

ih H

l

b

Figure 5. Schematic diagram of stretching (or shrinkage ) of pre-stretched fixed-point optical fiber:

(a) the optical fiber is stretched (or contracted) under the action of axial linear force; (b) the

horizontal fixed-point optical fiber is subject to irregular settlement deformation under the action of

gravity.

It is very easy to get the stretched length AB1 through simple calculation using the known

uniform strain AB of the whole length from Point A to Point B, or through the uniform strain

FP of every adjacent fixed points, or even through the complex strain ( )i z of every sampling

point along fiber AB ,So,

1 (1 ) (1 )= ( )

B

AB FP

A

AB AB n d AB z dz (6)

Where AB is the initial length of the fiber; AB1is the length of the stretched fiber; AB is the

uniform strain of the whole fiber AB; FP is the uniform strain between any adjacent fixed

points;dis the initial distance of any adjacent fixed points; ( )z is the strain of sampling point

along the fiber from point A to point B, According to different monitoring requirements,

there are the following sampling intervals: 1 m, 0.5 m, 0.2 m, 0.1 m, 0.05 m.

(2) Uneven deformation

It is assumed that the pre stretched optical fiber is fixed on the roof of coal roadways, and the

fixed point is affected by gravity, and the vertical displacement occurs. The optical fiber

appears irregular and unevenly deformed, which is usually reflected in:

a) Restricted by W steel strips and rhombic metal mesh, adjacent fixed points will never

change from horizontal arrangement to vertical arrangement or turn over inversely.

b) When the roof subsidence occurs, the fiber strain in the deformation area is restricted by

the fixed points in the un-deformed area at both ends, which only shows tensile strain;

c) Under the action of gravity, the fixed points only move vertically. With the restriction of

the vertically movement of fixed points, the optical cable is only subjected to the inclined

stretching action towards the maximum displacement point. As shown in Fig. 5 b.

The optical fiber between adjacent fixed points deform in an irregular polygonal shape

rather than in a straight line shape. The linear optical fiber AB is deformed to polygonal AB1,

there are n fixed-points between AB, and the deformed height of whole AB1 can be obtained

by sampling resolution intervals.


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Because BOTDR measures the strain i of distributed sampling points according to the

set sampling resolution interval of l . Therefore, the optical fiber AB with the length of L

laid out by POFFL can be divided into numerous sections il according to the sampling

resolution l ; when the fiber deformation occurs, each il is stretched to (1 )il by a

deformation amount( il ), then the vertical displacement of any interval of adjacent

sampling points, ih , can be obtained by the formula (7) .

2

2 2(1 ) 2i i i i

h l l l (7)

Then, the whole deformation height of optical fiber AB1 can be derived as equation (8).

2

1 1

( 2 ), ( )n n

AB

i i i

i i

LH h l n

l

, (8)

Where the parameters aforementioned are known ahead or can be measured.

For coal roadways, optical fiber is laid horizontally through the bolts (cables) on the roof, the

inverted triangle and inverted trapezoid deformation of optical fiber geometric deformation

are formed due to the plastic settlement displacement of roof deformation. On the one hand, it

is convenient to calculate the roof settlement displacement, on the other hand, the optical

fiber keeps a certain distance from the broken roof which cannot contact and destroy the

optical fiber during the settlement and deformation of the roof failure process, so the optical

fiber is protected and the smooth implementation of roof settlement monitoring is ensured.

3.4 Quantitative characterization of coal roadways roof settlement displacement

Next, it is discussed in detail about the roof displacement calculation of inverted triangle and

inverted trapezoid settlement in “Point” and “Zone “types of settlement of coal roadways

below.

(1) Inverted triangle settlement deformation of “Point” type settlement

The main feature is that the settlement displacement of the middle point of the deformation

section is the largest, the settlement displacement to both sides gradually decreases, and the

overall appearance is similar to inverted triangle, which can be divided into two cases.

A. The optical fiber between the anchors on both sides of the maximum settlement point is stretched in a straight line with uniform deformation, as shown in the black solid

line of Fig. 6. It is assumed that there are n bolts in the deformation section AD and

DG respectively, and the No. n bolt in the middle has the largest settlement

displacement, and the overall deformation is an inverted triangle.

X

Y

A FB

B1

C

C1

D

D1

E

E1

F1

G

L=n d

d

Anchor bolt

Uniform deformed fiber

Initial fiber

hi

d

i

ih

D1

C1

B1

E1

F1

l

i ih

l

l

l

ih+1

i

l

（

）

l

Sampling resolution

Initial

Horizontal

optical fiber

Deformed

optical fiber

Enlarged diagram of the selected unit


10

Figure 6.Quantitative characterization diagram of fiber strain of triangle settlement displacement of

roof: black solid line indicates that the fiber is triangle deformation of uniform straight line

stretching; green solid line indicates that the fiber is subjected to uneven tension and presents polygon

deformation of approximate triangle.

Assuming the rock bolts spacing is d, the uniform strain of the optical fiber is , the

oblique length of the triangle form can be got from equation (6), that is 1 (1 )AD n d

.The vertical height of triangular deformation of roof can be obtained, the height is the

largest when i n :

2 2 ,( 1, )H i d i n , (9)

B. When the roof settles unevenly and the optical fiber between the fixed points deform

non-uniformly too, the roof will deform as a triangle polygon. The fiber deformation

is shown in the green solid line in Fig. 6.

Due to the pre-stretching of optical fiber between anchor bolts, each sampling

interval l produces a settlement displacement ih with the strain i when the roof

deforms under the action of gravity, it is shown as the green solid line in Fig.6. ih is

obtained by Equation (7); the maximum settlement displacement DD1 can be derived

from Equation (8), that is the Equation (10) :

2

1

2 ,m

i i

i

LH l m

l

（）, (10)

Where l is the sampling resolution; i is the strain change at the sampling point; Lis the

length of the optical fiber AD,all are known or measurable. In the same theory, the

displacement of any point among AD can be calculated too.

(2) Inverted trapezoidal settlement deformation of “zone” type settlement

The main feature of trapezoidal settlement of roof is that the maximum settlement position is

not a point, but a relative big platform in the whole deformation region. The displacement of

each point in this settlement platform is basically the same as the maximum displacement

value. From the maximum settlement platform to both sides, the displacement of roof

deformation gradually decreases; theoretically there is no strain change in the platform of

maximum roof settlement, and the geometric shape of the roof settlement deformation is

similar to the inverted isosceles trapezoid, and the two waists of the trapezoid are strain

deformation segments. It can be divided into two situations:

A. In the first scenario, the fiber is stretched linearly between the bolts in the deformation

section, the fiber is uniformly strained, as the black solid line shown in Fig. 7.


11

x

y

A B

B1

C

C1 D1

D

Anchor bolt

dFE

Initial state of fiber

B1

C1 D1

Δl

Δhi

Δl

L=nd

Maximum settlement zone

Maximum settlement zone

G

Figure 7: Quantitative characterization diagram of fiber strain of trapezoidal settlement displacement of roof:

black solid line indicates that the fiber is uniform straight line stretching; green solid line indicates that the fiber

is subjected to uneven tension and presents polygon deformation of approximate trapezoid.

It is assumed that the roof bolts spacing of coal roadway is d. According to the

Equation (9), the displacement of any point from point A to C or point G to E can be

calculated easily. The accurate deformation location can be obtained from the optical fiber

strain distribution curves as well.

B. The second scenario is the uneven deformation of optical fiber. Under the gravity of overlying strata, when the uneven and irregular settlement of the roof is approximately

trapezoidal polygonal deformation, the optical fiber between n anchors on both waists

side is not linear tensile deformation, but polygonal deformation, it is shown as the green

solid line in Fig.7. The strain of the fiber in the strain section will form a non-uniform

strain. According to the sampling resolution from the fiber-optic strain curve, the

displacement of any point on the broken line AC1 can also be obtained by formula (10).

The maximum displacement is the displacement of the settlement platform, and the

specific position of the deformation section can be obtained from the strain distribution

curve.

In order to verify the effectiveness and feasibility of quantitative characterization of roof

displacement by optical fiber strain, the indoor roof settlement displacement model

experiment of pre-stretching fixed-point optical fiber and field coal roadway roof

displacement experiment based on DOFS monitoring were carried out.

4. Field monitoring of the roadway roof displacement based on BOTDR

The long wall fully mechanized top coal caving technology is adopted to mine 15 coal seam

with an average thickness of 7.31 m in 150313 working face of Yingying coal mine ,Shanxi,

China. The direct roof is mudstone with an average thickness of 1 m, and the basic roof is

12.5 m thick hard K2 limestone. The overburden is mainly composed of various hard

sandstone strata, as shown in Fig. 8. Roof management adopts comprehensive caving

method.


12

No. 13 coal

Medium mudstone

sandy mudstone

K1 sandstone

2.95 K3 Limestone

0.61

7.5 Medium grain sandstone

5

8

6 Fine si ltstone

6.39 Shale

12.5

1

7.31

1.31

6

Thickness/m

Lithology

fine grain sandstone

K2 limestone

Medium grain sandstone

800

24

00

80

08

00

80

0

800

10°

10

°

Rock bolt:φ22×2400Model:

BHRB500

Anchor cable:1×19,φ22×8000

Interval space:800×1000

800×1000Interval space:

30

00

5400

Air intake gateroad (haulage gate)

W strips on roof

Steel mesh support on roof，pillar

and wallNo. 15 coal

the unit measurement of Length：mm

Figure 8:The lithology histogram of 150313 working face and the support diagram of the air-intake gateroad.

The position of 998 rows of W steel strips is the open-cutting of the working face, and

the stopping line is located at 50 rows of W steel strips. According to the gateroad strike, the

optical cables were fixed at an interval of 1 m between Point A (424 rows of W steel strips)

and Point B (698 rows of W steel strips) of the roof via the exposed ends of the

corresponding anchor cables as fixed points. The total monitoring length is 274 m, as shown

in Fig. 9.

150313

working face

Goaf

大巷

N

150313 air-return gateroad

150311 working face Goaf

150315 working face to be mined next

Main

road

way

Protective coal pillar

Protective coal pillar

Mining direction

15

03

13w

ork

ing

face

Protective

coal

pillar of

mining

area

boundary

150313 air intake gateroad

698 rows of W strips424rows of W strips

Pre-stretched 5 mm SS cables layout on fixed points

Ordinary multicores

of single mode

optical fiber cables

B

O

T

D

R

B A

150315 air-return gateroad

Figure 9:Layout of the distributed roof settlement monitoring of in the 150313 working face.

4.1 Selection of sensing optical fiber and setting parameters of BOTDR in the field

experiment

With the influence of the excavation and working face advancing, the roof of coal roadways

is often in an unstable state of plastic failure, and the coal seam on the roof surface is

seriously broken, which often leads to uneven, regional and sudden settlement. Therefore, the

high-strength 5mm Steel Strand (SS) cables were selected to avoid the risk of being broken

by fractured coal rocks in the field experiment, the detailed introduction of the SS optical

cables can be referred to the reference21

. The single-end mode testing BOTDR analyser

AV6419 was used in the field experiment for optical fiber strain data collection and strain

data processing and analysis, the test parameter settings are shown in Table 1


13

Table 1: Parameter setting of BOTDR in the field experiment.

Measuring

range

\km

Sampling

resolution

\ m

Strain

coefficient

\ MHz/με

Initial

frequency

\ GHz

Pulse

Width

\ ns

Starting

frequency

\ GHz

Stop

frequency

\ GHz

5.0 0.2 0.05 10.82 20 10.45 12.00

4.2 POFLF of the coal roadway roof in 150313 working face in Yinying coal mine

According to the aforementioned analysis, in order to use the existing bolts and anchor cables

to layout optical fiber on the roof of coal roadways, a special clam is designed as shown in

Fig. 10a. A high-strength steel tray with right angle bending to prevent deformation is tightly

fixed on the anchor cables, and the long axis direction of the tray is vertical to the direction of

the coal roadway; two metal clips are tightened by four screws at the lower end of the tray,

and there is a groove in the middle of the clips, and the 5mm SS optical cable is placed

exactly in the groove.

Metal clip

Tray with right angle anti bending

Simulated bolt (cable) with screw rod

5 mm SS optical cable

(a)

Anchor cables

Rock bolts

W steel strips

Cable locks

Special clam for optical cable

(b)

Coal pil lar

Coal wall

Point ASS cable

(c)

Point A

Point B

Anchor cableRock bolt

Convey belt

W strip

(d)

Figure 10: SS optical fiber fixed point layout diagram: (a) Fixture diagram; (b) actual effect diagram of optical

cable layout with anchor cable as fixed point; (c) Actual supporting effect drawing of the air intake roadway; (d)

Overall actual effect drawing of anchor cable layout from fixed point A to B..

When laying out, the optical fiber laid at fixed points between bolts (cables) shall be

kept as horizontal as possible. The specific laying processes of optical cables along the roof

are as follows:

(1) Selecting the monitoring area and initial laying out of optical cable.

(2) Fixing the fixture to the exposed end of the anchor cables with anchor cable locks.

(3) Fastening clamps with the optical cable tightly compressed to the tray of the fixture.

(4) Adjusting the screw and anchor lock and pre-stretching the fixed cable horizontally.


14

(5) Testing the passing path of the optical fiber; later fusing the SS optical cables with the

common telecom optical cables; finally laying out the cables to the main roadway’s

observation where it was connected to the BOTDR AV6419 through an optical fiber

jumper. The complete optical fiber layout is shown in Fig. 16 b-d.

4.3 Field monitoring of roof settlement displacement

Only the roof monitoring during mining retreating was studied in the paper. The roof

displacement monitoring test of optical fiber strain quantification began on June 1, 2019, at

this time, the distance between the open-cutting and the fixed point B of 698 rows W steel

strip is 300 m. in the meantime, the optical fiber strain was initialized again, and applying the

simple ways to encrypt the measuring points of cross observation method (COM).

(1) Setting of roof settlement displacement observation by simple COM

Because the spacing of the original designed roof separation instrument is 50m, it is difficult

to monitor the roof deformation timely and accurately. In order to verify the BOTDR

monitoring of roof subsidence by POFFL, COM stations and measuring points were set up

every 5 m from point A to point B by using simple COM. The layout of station measuring

points is shown in Fig. 11

The base

point of roof

The base point of floor（wood

wedge identification）

R

F

The base

point of

coal pillar

C D

O

5.4 m

3.0 m

（steel bolt in coal pillar）

（resin bolt

in coal wall）

（anchor bolt or

cable on roof）

The base

point of

coal wall

Figure 11Layout of measuring points for COM of roadway roof displacement.

The principle of simple COM is the same as the traditional com measurement method, but

the latter uses permanent pegs as monitoring points23

. In this simple COM method, the stable

exposed ends of rock bolts or anchor cables were used as temporary measuring points, and a

total of 55 COM measuring stations were established.

In the actual measurement of roof displacement using COM, it is assumed that there is

none the vertical displacement of coal pillar and coal wall. Like the convergence

measurement of roof and floor24-25

, CD and RF remain vertical, and RO measured for the first

time is taken as the initial value of roof settlement position, as shown in Fig. 17. Then the

continuous measurement displacement of the measuring point was obtained through

subtracting the subsequent measurement value of RO from the initial value measured by

telescopic measuring rods. In the early stage, COM was used to measure the settlement

displacement of the roof once a week, and the optical fiber strain was measured at the same

time. When the working face was close to point B of the test optical cable, special personnel

was arranged to measure the roof displacement with COM every day along with the optical

fiber strain monitoring.


15

(2) Optical fiber strain data analysis of pre-stretched fixed-point lay out optical fiber

Due to the large buried depth of the coal gateroad, the average depth from the ground was

about 400 m, which resulting in the temperature of the coal roadway was basically constant

during the test, and the influence of temperature on the strain was ignored in the actual

experiment unless the temperature difference exceeded 5 °, the temperature compensation

was carried out according to formula (3).

From the Fig. 12 below. It can be seen from the figure that the pre-tensile strain value

of 5 mm SS optical fiber used in this test was between 2500 με and 5000 με. The original

strain curve of optical fiber is irregular and jag-saw like, and the relative strain curve is a

nearly horizontal wavy line near zero. This demonstrated that the roof of the gateroad was

stable all the time before the working face advanced within 35 m of tested cable.

In the afternoon of August 27th, 2019, when the working face was - 35m ahead of the

test optical fiber point B, The original fiber strain curve began showing two anomaly tensile

strain zones, namely abnormal strain regions (i) and (ii), which deviated significantly from

the previous tested data, as shown in Fig. 18 a. The varied strain of optical fiber in the

abnormal strain zones was stead and did not change with the advance of the working face any

more until August 31st, when the field test was terminated, this can be seen from the

zooming-in view of the strain abnormal region, Fig. 12 b ~ c as well.

The relative strain curve obtained by the difference between the strain curve and the

initial strain curve is shown in Fig. 12 (d). It can be seen that there is a single peak strain peak

curve in the strain anomaly zone (i) and a double peak strain peak curve in the strain region

(ii). It can be seen from the enlarged relative strain difference curves that the strain anomaly

zone (i) is located in the section between 52 – 61m away from point A, and the maximum

peak value is about 250 με, as shown in Fig. 12 (e); the range of strain anomaly zone (ii) is

within 151-173m away from point A, and the strain change in the middle flat section between

the two peaks is very small which is 159-165m away from point A, and the strain value of

double peak is about 300 με, as shown in Fig. 12 (f). There is no obvious change of optical

fiber strain in other zones, and the variation is less than 50 με except for the two ends of the

optical cable.

In the actual underground observation from August 27th to 31st, there was an

approximate triangle settlement zone in the roof, which was located in the range of abnormal

zone (i); there was an approximate isosceles trapezoid polygon settlement in the abnormal

zone (ii); the roof in other areas remained stable. It can be seen that the strain distribution of

the optical fiber laid out on the roof could better qualitatively reflect the deformation range

and failure degree of the Roof26

.

50 52 54 56 58 60 62 64 66 68 70 72 743500

3600

3700

3800

3900

4000

4100

4200

Ori

gin

al o

pti

cal

fib

er s

trai

n \

με

Distance to Point A\ m

2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

0 50 100 150 200 250 3002000

2500

3000

3500

4000

4500

5000

B

Ori

gin

al F

iber

str

ain

\ μ

ε

Distance to Point A \ m

2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

Face advancing

A

150 152 154 156 158 160 162 164 166 168 170 172 174 176 178 1803200

3300

3400

3500

3600

3700

3800

3900

4000

4100

4200

4300

4400

Ori

gin

al t

este

d o

pti

cal

fib

er s

trai

n \

με


2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

(ii) (i)

Abnormal region (i) Abnormal region (ii)


16

b a c

50 52 54 56 58 60 62 64

-50

0

50

100

150

200

250

300

350

rela

tive

opti

cal

fiber

str

ain \

με


2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

0 50 100 150 200 250 300

-100

0

100

200

300

400

500

B

Rel

ativ

e opti

cal

fiber

str

ain \

με

Distance to Point A \m

2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

A

Face advancing

150 152 154 156 158 160 162 164 166 168 170 172 174

-50

0

50

100

150

200

250

300

350

400

450

Rel

ativ

e opti

cal

fiber

str

ain \

με


2019/6/1 2019/7/3 2019/7/10 2019/7/20 2019/8/2 2019/8/20 2019/8/22 2019/8/23 2019/8/24 2019/8/25 2019/8/26 2019/8/27 2019/8/28 2019/8/29 2019/8/30 2019/8/31

e d f

Figure 12:The distributed optical fiber strain curve of roof deformation: (a) the whole original fiber strain curve

tested; (b) the enlarged original fiber strain curve of abnormal region (i); (c) the enlarged original fiber strain

curve of abnormal region (ii); (d) the whole relative optical fiber strain curve; (e) the enlarged relative fiber

strain curve of abnormal region (i); (f) the enlarged relative fiber strain of abnormal region (ii).

4.4 The optical fiber strain quantification of the gateroad roof displacement

Through the above analysis, the range of settlement position and displacement change can be

obtained by formula (10), and the calculation results are shown in Figure 13 below.

51 52 53 54 55 56 57 58 59 60 61 62300

250

200

150

100

50

0

-50

-100

-150

-200

Roof

abnorm

al r

egio

n (

i) d

ispla

cem

ent

\mm

Distance to Point A \m

Aug 27th Aug 28th Aug 29th Aug 30th Aug 31st

(a)

150 155 160 165 170 175300

250

200

150

100

50

0

-50

-100

-150

-200

Roof

sett

lem

ent

dis

pla

cem

ent\

mm

Distance from Point A to the calculated points\ m

Aug 27th Aug 28th Aug 29th Aug 30th Aug 31st

(b)

Figure 13: The change curve of roof settlement displacement calculated by optical fiber strain value: (a) The

zone of strain anomaly (i) approximates to triangle region; (b) The region of strain anomaly (ii) approximates

trapezoid region.

As shown in Fig. 13a, the displacement curve of sampling points in anomaly zone (i)

is in the shape of an inverted triangle during the period from August 27th to August 31st, its

spatial position is among the range of 51.88 - 61.07 m away from Point A, and its maximum

displacement varying range is 61.89-66.29 mm.

As shown in Fig.13 b, the settlement displacement in the abnormal zone (ii) is

inverted trapezoid from August 27 to August 31, which is within the range of 151.33 ~

172.16 m away from point A. The results show that the two waist sides were polygonal, and

the maximum displacement range of left waist line is 135.17 ~ 140.23mm; the maximum

displacement of right waist line varies from 131.02mm to 139.66mm. The maximum

Abnormal region (i) Abnormal region (ii)


17

displacement range of the trapezoid bottom edge between the two waists of trapezoid is

131.02 ~ 140.23 mm.

4.5 Cross observation method (COM) manual measurement of roof displacement

In order to validate the calculation results of quantitative characterization of roof

displacement by optical fiber strain, the simple COM was used for continuous observation

and measurement of roof displacement at No.1 ~No.55 measuring points in the air intake

gateroad of 150313 working face. Through actual COM measurement, the change curve of

roof settlement displacement from point A to point B of coal gateroad roof with working face

advancing is obtained, as shown in Fig. 20a. At the same time Fig.20 b shows the roof

settlement displacement curve quantitatively characterized by optical fiber strain, and the two

are used as comparison. It is shown there was no any obvious displacement within the whole

monitored roof before 27th Aug. Only on and after 28th Aug., there were the manifested roof

settlement deformation in two zones respectively, it was shown in Fig.20a.Similarity, when

the working face is - 35m away from the testing optical fiber point B on Aug. 27th, there are

two large settlement deformation zones on the roof, one zone was irregular triangle

settlement deformation located 50 ~ 60 m away from point A, the other one is irregular

Trapezoid settlement deformation located 150 ~ 175m away from point A, as shown in

Fig.20 b.

0 50 100 150 200 250 300400

350

300

250

200

150

100

50

0

-50

-100

-150

-200

Point BMea

sure

d r

oof

dis

pla

cem

ent

by C

OM

\ m

m

Distance from Point A to the measuring stations of CRM \m

June 6th July 10th July 20th Aug.2nd Aug.25th Aug.26th Aug.27th Aug.28th Aug.29th Aug.30th Aug.31st

Point A

working face advancing direction

a

0 50 100 150 200 250 300400

350

300

250

200

150

100

50

0

-50

-100

-150

-200

Point AQu

atif

ied

d

isp

lace

men

t b

y f

iber

str

ain

\ m

m

Distance from Point A to the Sampling Points\m

Aug 27 Aug 28 Aug 29 Aug 30 Aug 31

working face advancing direction

Point B

b

Figure14: Actual measured value curve of roof settlement displacement, listed as: (a) Displacement

measurement curve of roof using COM in coal gateroad test section; (b) Comparison curve of settlement

displacement of roof fiber strain.

Fig. 14 a shows that the roof displacement measured in the triangle settlement zone,

which is 55 m away from point A, developed from 40 mm to 44 mm in 5 days from August

27th to the night of August 31st. Due to the actual observation, the real maximum

displacement is located at 56 m away from point A, with a difference of 1m from the

designed observation station. So, the measured value at 55 m was taken as the benchmark,

the displacement 56 m away from point A was measured using COM, and the manual

measured maximum displacement of the triangle deformation was 63 mm.

Figure 14a also shows that when the working face is - 35m away from the optical

cable point B in the trapezoidal settlement zone, the roof subsidence displacements of 69

mm, 133 mm, 137 mm and 32 mm appeared at the four measuring stations which are 155 m,

160 m, 165 m and 170 m away from the optical cable’s point A, respectively. The settlement


18

displacement of the measuring points were basically stable from Aug. 27th to the night of

Aug.31st, and did not change anymore with the advancing of the working face. The real

position of maximum measured displacement was nearby 165m away from point A, it has

developed from 135 mm to 137 mm and stabilized at 137 mm within 5 days since the

deformation.

Compared with Fig.14a and 14b, the settlement location range and the settlement forms

of the COM measuring results were in accordance with that of the fiber strain quantitative

characterization from the beginning to the end of the field monitoring experiment.

4.6 Discussion on roof settlement displacement

In the evening of August 31st, when the ahead supporting bracket of the working face arrived

at the point B of the testing SS optical cables, the field monitoring test of roadway settlement

terminated because the roof of the roadway needs to cave in completely in time after mining.

Nonetheless, during the working face is retreating, compared with the roof displacement

quantitatively characterized by optical fiber strain and the roof displacement measured by

COM, as shown in Tab. 2, the change relationship between the two can still be obtained, and

then the stability of roof surrounding rock of air intake coal gateroad in 150313 working face

was analysed and discussed further.

Table 2. Comparison of displacement measured by COM and that of quantitative characterization.

Measuring items COM strain

quantification

Error analysis

Triangle Def. range \ m 55 ~ 60 51.88~61.07 5.6% ~1.75%

Dis. of 55m M.P. from P. A\ mm 44 45.43 3.15%

Dis. of 56m M.P. from P. A\ mm 63 66.29 4.96%

TRAP. Def. range \m 150 ~ 175 151.33 ~172.16 0.88%~1.62%

Scope of Trap. Sett platform \m 160 ~ 165 159 ~ 165 0 %~0.6%

Dis. Of 165m M.P. from P. A \mm 137 140.23 2.3%

*Def. =Deformation; Dis. =Displacement; M.P. =Measuring Point; P. =Point; Trap. =Trapezoidal; Sett. =settlement.

It can be seen from the analysis in the table that the maximum error is 5.6% between

the actual roof deformation range measured by COM and the quantitative characterization of

the optical fiber strain; the maximum error of roof displacement is 4.96%. The absolute

different value of vertical displacement is less than 4 mm compared to the vertical height of

3000 mm in the gateroad, this means that the COM measurement and optical fiber strain

quantitative characterization show a high degree of consistency. The optical fiber strain

quantitative characterization in the coal gateroad roof settlement measurement meets the

requirements of coal gateroads roof measurement, it also validate the feasibility of the

distributed measuring approach of coal roadway roof settlement based on DOFS and POFFL.

The field BOTDR monitoring results fully showed that during the working face

advancing, under the protection of 12.5 m hard K2 limestone basic roof and overlying hard

sandstone, under the traditional bolts-cables-W strips-mesh active support combined with the

20 m advanced mobile reinforced bracket supports, in the case of manual roof caving in time

after mining , the overburden mining pressure is unloaded in time, so the abutment stress or

the advanced mining support stress had no effect on the coal gateroad roof under monitoring.


19

It can be deduced that the abutment stress was within 20 ahead of the working face beyond

doubt. The roof only affected by the original geology tectonic factors in overburden, such as

in-situ gravity stress and other tectonic stresses , which only resulted in two roof settlement

deformation zones with relatively small displacement and small area during the field

monitoring experiment . The maximum deformation range was smaller than 30m, and the

maximum settlement displacement was only 140mm in the observation process. The results

showed that the cross-section and support design of the air intake coal gateroad in 150313

working face are reliable and rational, the surrounding rock stability of the gateroad is high

stable, the support design meets the geological conditions and production requirements of the

working face, and it realized the support design principle: successful primary support, the

gateroad maintains the support stability before mining, and the roof caves completely and

quickly in time after mining.

Combined with POFFL technology, the field BOTDR monitoring showed that the

distributed optical fiber strain can not only accurately present the settlement range and

qualitatively indicate the failure degree of roof, but also quantitatively characterize the

settlement displacement of coal roadways roof. This kind of distributed settlement

monitoring of roof is feasible and validated in the coal roadways roof safety monitoring; The

research in this paper has important reference significance for the support design of coal

roadway under similar geology and production conditions, and for the distributed monitoring

of coal roadway roof under similar conditions and the structural health monitoring of similar

geotechnical or civil engineering objects.

5. Conclusions

In this paper, through the theoretical analysis, indoor roof settlement model experiment, and

the field settlement displacement monitoring of coal roadway roof in 150313 working face of

Yinying Coal Mine based on DOFS and POFFL technology, some conclusions are achieved

as follows.

(1) Under the influence of plastic failure on the roof strata of coal roadways supported by

bolts, two kinds of regional failure forms are often formed, that is the “point “type and the

“zone “type.

(2) For the roof deformation of coal roadways, pre-stretched optical fiber can be fixed-point

layout through the exposed end of the existing roof rock bolts or anchor cables. On the one

hand, it ensures the survival rate of the optical fiber cables; on the other hand, it ensures the

coupling deformation between the optical fiber cables and the corresponding roof.

(3) By laying out optical fiber horizontally via POFFL technology along the strike of

rectangle coal roadways roof, when the roof subsidence occurs, the strain of optical fiber not

only qualitively ,but also quantitively characterize the settlement deformation and

displacement of the roof.

(4) The displacement of the coal roof can be calculated using the optical fiber strain of

sampling resolution intervals based on BOTDR when the optical fiber is laid out POFFL

horizontally on the coal roof.

(5) The optical fiber strain characterization analysis showed that the influence range of

abutment stress caused by mining was within 20 m in front of the working face. On the

whole, the surrounding rock of the air intake roadway was in a high stable state, which was in

line with the principle of bolt primary support, and the support design was reliable. It can


20

provide important reference for roadway design under similar geological and production

conditions.

(6) It is feasible and validated to quantify the roof displacement by means of optical fiber

stain based on DOFS and POFFPL technology. It is helpful to realize the distributed

settlement displacement monitoring of coal roadways roof underground, which plays a great

role in preventing coal mine roof accidents and ensuring the sustainable safety production of

coal mine.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

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