In situ investigation of the excavation-loose zone in surrounding rocks from mining complex coal seams

Journal of Applied Geophysics 168 (2019) 90–100

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In situ investigation of the excavation-loose zone in surrounding rocks from mining complex coal seams Maolin Tian a,b, Lijun Han a,b,⁎, Qingbin Meng a, Yuhao Jin a, Lingdong Meng a a b

State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China

a r t i c l e

i n f o

Article history: Received 11 September 2018 Received in revised form 21 March 2019 Accepted 10 June 2019 Available online 18 June 2019 Keywords: Soft coal seam with soft roof and floor Ground-penetrating radar (GPR) Picture segmentation Excavation-loose zone (ELZ) Local fractured zone

a b s t r a c t The excavation-loose zone (ELZ) is a key factor for the design and construction of underground engineering. In this study, ground-penetrating radar (GPR), non-invasive and effective method, is utilized to estimate the ELZ around an excavated soft coal seam with roof and floor consisting of soft rock seams. The picture segmentation method based on the 2D maximum between-cluster variance is conducted to optimize the radargrams obtained GPR, and then the ELZ can be accurately estimated by analyzing the radargrams. The results show that the magnitude of the ELZ different along different survey lines and is distinct at different positions along the same survey line. A large ELZ is concluded to surround the two sides of the coal walls and most of the floor. Meanwhile, due to complicated geological structure, local fractured zones exist widely within the ELZ. This case study is quite useful for understanding the range of the ELZ around a soft coal seam with soft rock seams in the coal wall and floor. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Soft coal seams with soft roofs and floors are widely distributed in the underground coal mines of China (Zhang et al., 1999). The soft roofs and floors contain numerous fractures so that these rock seams are generally unstable in a mining process. If this type of the roof is not promptly and effectively supported, it will cave soon after excavation, which makes coal mining difficult. In mining a soft, unstable and fractured coal seam, the soft coal wall very easily spalls, which may further influence the stability of the roof and floor strata. The soft floors can be divided into two types, i.e., soft coal floors and soft rock floors. The former have properties similar to those of the soft coal seam whereas the latter have low compressive strength, and many fractured rocks are easily softened by water injection. In mining this type of soft coal seams, the range of the excavation-loose zone is greater than in other types of coal seams. In situ investigations (Wang et al., 2015; Yang et al., 2016; Yang et al., 2017) have revealed that coal wall spalling, roof caving and floor heave happen more easily with increasing range of the excavation-loose zone. The excavation of coal seams causes deformations and failures of rock seams so that the stability of the roadway decreases and the risk of failure increases. According to the disturbed degrees, ⁎ Corresponding author at: State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (L. Han).

https://doi.org/10.1016/j.jappgeo.2019.06.008 0926-9851/© 2019 Elsevier B.V. All rights reserved.

the rocks around a roadway can be classified into several different zones (e.g., Kelsall et al., 1984; Egger, 1989; Dong et al., 1994; Martino and Chandler, 2004; Tsang et al., 2005; Perras and Diederichs, 2016). Generally, these zones include excavation-loose, excavation-influenced and undistributed zones, as shown in Fig. 1, and the definitions of the excavation-loose zone and excavationinfluenced zone used in this paper are from (Diederichs et al., 2013; Wang et al., 2015): Excavation-loose zone is defined as the region with irreversible deformation and numerous continuous macro-fractures and/or opening or slip along existing joints due to unloading, where the physical, mechanical and hydraulic properties of the rock have been non-negligibly altered. Excavation-influenced zone is defined as the region with reversible deformation and hydro-mechanical and geochemical modifications without open macro-fractures, where no significant changes in the mechanical and hydraulic properties of the rock have occurred. In the ensuing, the excavation-loose zone and excavation-influenced zone are referred to as the loose zone (or ELZ) and influenced zone (or EIZ), respectively, for simplicity. The rock seams in the ELZ are broken, and their physical, mechanical and hydraulic properties obviously vary. The evolution of the ELZ is a progressive process. In this case, any unreasonable supports result in ELZ enlargement. Moreover, severely broken rocks form permeable pathways for groundwater that further influences the long-term safety of deep mines. The understanding of the ELZ properties and range is particularly significant for roadway support design and long-term mining safety in underground engineering.

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As a noninvasive and effective method, ground-penetrating radar (GPR) has a wide range of applications for shallow subsurface investigation in engineering geophysics, such as mineral exploration and geological, geotechnical, and environmental studies (Xia et al., 2004; Francke, 2012; Elkarmoty et al., 2017; Lai et al., 2018). The thickness of the surrounding rock ELZ in a tunnel under unsymmetrical loading were measured by Guo et al. (2011) using GPR, and the feasibility of testing the ELZ in the tunnel were verified with actual measurement, operation and application in the field. Walton et al. (2015) collected and analyzed a variety of survey parameters, such as earth resistivity, induced polarization, and GPR data, to develop models for the highly damage zone and excavation damage zone at both sites. Their results indicated that GPR data are more useful in the estimation of the excavation damage zone dimensions. This paper gives a detailed description of an experimental setup designed to detect the range of the ELZ with GPR when mining soft coal seams with soft roofs and floors. We then optimize the radargrams obtained in field investigations using the picture segmentation method based on the 2D maximum between-cluster variance and analyze the optimized radargrams to assess the ELZ range and destruction degree. Moreover, combined with geological conditions and in situ stress, the evolution mechanism of ELZs can be analyzed. Fig. 1. Schematic diagram of failure zones.

At present, several methods have been developed to detect the ELZ. Among them, the borehole digital image technique has been widely used to analyze the evolution and depth of ELZs (Tan et al., 2013; Yu et al., 2015; Xu et al., 2017). Li et al. (2013) proposed the International Society of Rock Mechanics (ISRM) suggested method for rock fractures observations using a borehole digital optical televiewer. Cai and Kaiser (2005) presented a method to characterize ELZs based on microseismic monitoring data. ELZ evolution investigations were conducted by Wang et al. (2015) using the microseismic monitoring and traditional testing methods including multipoint extensometers, acoustic wave testing and borehole TV. Felix et al. (2014) suggested that seismic travel-time and attenuation tomography can be used to characterize ELZs and their adjacent rocks. In addition, acoustic emission monitoring (Cai et al., 2001), borehole radar reflection surveys (Kwon et al., 2009) and the ultrasonic wave technique (Maxwell et al., 1998; Zhang et al., 2011) are used to quantify rock damages and assess ELZs. The ELZ methods are found to be very limited, somewhat inefficient, and unable to achieve noninvasive detection.

2. Mining conditions and experimental design 2.1. Mining conditions at the roadway The south air-return roadway in the west wing of the Zhaozhuang Mine, in Jincheng, Shanxi Province, China was used for the in situ measurements. The section of the rectangular roadway is 5.2 m wide and 4.6 m high. The roadway is located along the roof and floor of No.3 coal seam with a depth of 482–490 m (Fig. 2). The seam being extracted at the roadway has an average thickness of approximately 5.5 m and a dip of 1–8°. As shown in Fig. 3, the immediate roof of the mined coal seam is composed of sandy mudstone, and the main roof is composed of sandstone, sandy mudstone and siltstone. In addition, the groundwater level elevation of Ordovician limestone fissure aquifer is higher than that of No.3 coal seam, and both sides and floor of roadway are wet and groundwater seeps out. Table 1 presents the mechanical parameters of rocks around the experimental roadway. The saturated uniaxial compressive strength decreased by 40–58% compared with the unsaturated strength. Hence, the roof and floor are easily softened by groundwater. There are two high stress concentration zones along the east-west direction and five faults around the experimental roadway, indicating

Fig. 2. Location of monitoring roadway in the Zhaozhuang mine.

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Fig. 3. Stratigraphic and lithological description of the roadway.

that the overlying strata are fractured, soft and broken rock, forming unstable roofs. There are a large number of joints in No.3 coal seam. Moreover, this coal seam has a low compressive strength (7.32 MPa). The immediate floor is composed of mudstone and its uniaxial compressive strength is less than 25 MPa. Thus, this roadway is a representative soft coal roadway with soft roofs and floors.

2.2. Design of the GPR experiment The geological conditions of the strata around the experimental roadway are complicated. To accurately detect the ELZ, six surveying lines were positioned on two sides of the coal walls and the floor of the experimental roadway (Fig. 4(a)). Surveying line No. 1 was located at the crossing of the experimental roadway and roadway No. 33024 for detecting the ELZ of the south side coal wall. Meanwhile, surveying lines Nos. 2 and 3, and surveying lines Nos. 4 and 5 were arranged on the south and north side coal walls, respectively. The surveying lines on the coal wall were located at different positions to probe the ELZ in different sections of the roadway. Surveying line No. 6 was placed at the middle position of the experiment roadway. All the detection started from the crossing of the experimental No. 33024 roadways. Due to the existence of faults and groundwater, the roadway floor seriously heaved. Hence, surveying line No. 6 advanced for a distance of 250 m along the experiment roadway to accurately detect the ELZ at different positions in the floor. The GPR detection is carried out between primary support and secondary support. The primary support scheme of experiment roadway is to install anchor cables and anchor bolts on the roof and both sides, respectively. But no supporting measures are adopted

on the floor. After completing the primary support, surfaces of the floor and both sides of coal wall are relative flat and suitable for GPR survey activities. In addition, to avoid the influence of metal bolts, surveying lines were arranged in the middle of two rows of bolts, as shown in Fig. 4(b). An RIS-K2 ground-penetrating radar system manufactured by the IDS Company from Italy was used for in situ investigations, as shown in Fig. 4(c). The RIS-K2 system included one computer, one FastWave radar host and one TR 200 shielded antenna is especially suited for middle and shallow detections. The TR 200 shielded antenna is dipole shielded antenna with a frequency of 200 MHz, which integrates transceiver and receiver. In practice, a range of 256 ns into the rock medium were set to record GPR data and the number of individual sample in each scan curve is 512. However, considering that the range of ELZ is usually less than 5 m, GPR data in the range of 100 ns are selected for analysis. And the instrument was close to the surface along the survey lines to obtain high-quality information data. In addition, in order to obtain more accurate and better visual representations of geophysical signature, data processing to enhance the signal of the GPR profiles was conducted by Greswin2. Greswin2 is a software tool manufactured by the IDS Company, and can be used to process the GPR data. The raw data were processed according to the following steps: vertical band-pass filtering, move start time, background removal, subtract mean (dewow), linear gain and smoothed gain. And based on the principle of the GPR signal behavior, the time to depth conversion can be performed, using Eq. (1)

h¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi v2 t 2 −x2 2

ð1Þ

where t is two-way travel time, v is the propagation velocity in the rock and x is the distance between the transmitter and the receiver. Due to using TR200 shielded antenna with integrating transceiver and receiver, x can be negligible. The propagation velocity v in the rock mass can be determined by the following equation (Davis and Annan, 1989) c v ¼ pffiffiffiffiffi εr

ð2Þ

where c (0.300 m/ns) is the propagation speed of electromagnetic wave in vacuum and εr is the relative dielectric constant of rock. The vertical resolution is important in this research, which determines how large fractures and discontinuities need to be in order to be picked-up by the GPR. In general, the vertical resolution is primarily based on the wavelength (Apel and Dezelic, 2005):

rv ¼

λ 4

ð3Þ

where rv is the vertical resolution and λ is the wavelength.

Table 1 Mechanical parameters of the roof and floor strata of the extracted coal seam. Strata

Elastic modulus (GPa)

Poisson's ratio

Uniaxial compressive strength (MPa)

Saturated uniaxial compressive strength (MPa)

Tensile strength (MPa)

Cohesive force (MPa)

Frictional angle (o)

Siltstone Sandstone Sandy mudstone Coal seam Mudstone

18.35 21.17 11.70

0.21 0.28 0.29

21.36 24.23 19.86

10.25 14.54 10.33

3.14 5.21 2.88

9.54 13.85 6.47

27.75 28.00 35.40

2.97 8.67

0.32 0.23

7.32 14.00

3.07 8.4

0. 21 2.70

2.05 3.07

35.30 18.60

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Fig. 4. The test of GPR (a) the layout of survey lines for the south air return roadway (b) the diagrammatic sketch of GPR detection and (c) the RIS-K2 GPR system.

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Table 2 Relative dielectric constants of some medium. Medium

Air

Water

Mudstone

Coal(dry)

Coal(wet)

Relative dielectric constant εr

1

81

9

4.5

8

The wavelength λ can be determined by the following equation λ¼

v f

ð4Þ

where f is the antenna frequency. Both sides of coal wall and floor are wet, and partial fractures in the ELZ are filled by water. And the relative dielectric constant of rock has increased due to water content increase (Conroy and Guy, 2005; Strange et al., 2005). Thus, the relative dielectric constant of rock is higher than that of dry rock in this roadway. The relative dielectric constants of coal and mudstone are selected as 8 and 9, respectively, as shown in Table 2 (Du et al., 2014). The propagation velocity can be determined by using Eq. (2), and the propagation velocity in coal and mudstone are 0.106 m/ns and 0.100 m/ns, respectively. Then, the detection depth within 100 ns of coal and mudstone that can be determined by using Eq. (1), and are 5.3 m and 5.0 m, respectively. In addition, vertical resolutions in coal and mudstone can be obtained by using Eqs. (3) and (4), and are 13.25 cm and 12.50 cm, respectively. The GPR system utilizes different characteristics of electromagnetic signal in different mediums to detect different targets. Generally, discontinuities, fractures and cracks in rock medium cause the change of the relative dielectric constant and the electrical conductivity. In this research, the medium of separation in discontinuities and fractures is usually air and water. It can be seen from the Table 2 that the relative dielectric constant between the rock medium and discontinuities or fractures is very different. GPR electromagnetic waves reflect well from the contact surface between rock medium and the fracture. Thus, detection of the fracture by GPR is theoretically practicable (Yang et al., 2016; Guo et al., 2011; Wu et al., 2013). 3. Methods and results for optimizing radargrams 3.1. General The coal and rock mass is broken in the ELZ where many macrofractures and discontinuities, which increases porosity and changes the parameters of electrical conductivity and dielectric constant. The original GPR pulse signal can be reflected in the location of fractures,

joints and discontinuous interfaces. However, the reflection is slight in relatively homogeneous and intact rock masses. Therefore, the ELZ is considered to correspond to a zone with large amplitude and high reflector density, particularly for chaotic wave forms. Otherwise, the EIZ exists as close microfractures caused by excavation, without open macro-fractures, and is a relatively homogeneous and intact zone. Notably, there are a distinct fracture interfaces between ELZs and EIZs that influence the distribution of reflections. By analyzing the reflection signature, the extent of the ELZ depth can be estimated. 3.2. The picture segmentation method based on 2D maximum betweencluster variance For interpreting the signal of the GPR detection, the picture segmentation method based on 2D maximum between-cluster variance was used to segment the radargram. This method was originally proposed by Otsu (1979), and it was widely used in picture segmentation. The picture segmentation algorithm based on 2D maximum betweencluster variance uses the picture gray and its neighborhood average

j

Neighborhood average gray

Fig. 6. The comparison between the original and optimized radargrams (a) the original radargram and (b) the optimized radargram.

0, L-1

4

Table 3 The segmentation thresholds of different radargrams.

3

Locations

s, t

South side coal wall

0,t 2

1

North side coal wall

0,0 s,0

L-1,0 Gray

Fig. 5. 2D gray histogram segmentation scheme.

i

Floor

Segmentation thresholds DB (s*,t*) No.1 survey line No.2 survey line No.3 survey line No.4 survey line No.5 survey line No.6 survey line 0–50 m No.6 survey line 50–100 m No.6 survey line 100–150 m No.6 survey line 150–200 m No.6 survey line 200–250 m

(128, 203) (128, 202) (127, 203) (127, 201) (127, 202) (127, 197) (124, 192) (126, 196) (124, 192) (126, 197)

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Fig. 7. Optimized radargrams of the south side coal wall (a) the radargram of survey line No.1 (b) the radargram of survey line No.2 (c) the radargram of survey line No.3.

gray to construct a 2D histogram and to divide the picture histogram into four parts by choosing a segmentation threshold. The method supposes that the pixel of the picture is divided by a threshold combination (s, t). The picture could be divided into four zones, i.e., zone 1, 2, 3 and 4 by the 2D gray threshold (s, t). The 2D gray histogram segmentation scheme is shown in Fig. 5. If the target gray is smaller than the background gray, it is supposed that zones 1 and 3 correspond to the target and background, respectively, and zones 2 and 4 are ordinarily correspond to the noise and margins in the picture. In general, the probabilities of zones 2 and 4 can be ignored.

Thus, the between-cluster variance of the target and the background can be defined as:    2   2  Dðs; t Þ ¼ ω1 ðμ 1i −μ i Þ2 þ μ 1 j –μ j þ ω3 ðμ 3i −μ i Þ2 þ μ 3 j –μ j ð5Þ where ω1 is the probability of the target and ω3 is the probability of the background. μ1i and μ1j are components of the target average value vector, μ3i and μ3j are components of the background average value vector,

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and μi and μj are components of the total mean vector. The detail of these coefficients can be found in Otsu (1979). To achieve the optimal effect of picture segmentation, the best segmentation threshold DB(s⁎,t⁎) should satisfy the following equation: DB ðs ; t  Þ ¼ maxfDðs; t Þg

ð6Þ

3.3. Optimized radargram The picture segmentation algorithm can be implemented by MATLAB, and is used to segment the radargram. Firstly, the original radargram plotted by Greswin2 software can be called by the picture segmentation program. Secondly, the picture segmentation program analyses the gray and automatically chooses the optimal threshold of the radargram based on 2D maximum between-cluster variance. Finally, the program plots the optimized radargram according to optimal threshold. Generally, the zone where reflectors with large amplitude are densely distributed corresponds to the compact district of low gray pixel points; conversely, the zone with low density of large amplitude corresponds to the sparse zone of low gray pixel points. Thus, it is concluded that the compact district and the sparse district of low gray pixel points are, respectively, the fracture zone and the reflection noise zone or background. The picture segmentation algorithm can effectively distinguish the target from the background noise and automatically recognize the optimal threshold to quickly and accurately segment the picture by analyzing the gray information of the picture, and then eliminate the background noise. Thus, fractures or discontinuous can be clearly distinguished from the whole rock in the optimized radargram

by the picture segmentation algorithm, which help us to analyze ELZ more accurately and quickly. To confirm the picture segmentation effect, the original radargram of the No.1 survey line was optimized by the method (Fig. 6), and the segmentation threshold was (128, 203). Compared to the original radargram, the fracture zone and the reflection noise zone or background can be more clearly identified in the optimized radargram. Therefore, all radargrams were optimized by the picture segmentation method for analyzing characteristics of the ELZ, and the different segmentation thresholds and optimized radargrams are shown in Table 3 and Figs. 7, 8 and 9. 4. The ELZs in coal walls and floor strata 4.1. The ELZ of the south side coal wall Fig. 7 shows the optimized radargrams of the south side coal wall during the field investigations, and the depth of ELZ edge can be delineated in Fig. 10. Figs. 7 and 10 show that: 1. In the No. 1 survey line radargram, reflectors with large amplitude are densely distributed in the strip-type zone located at 1.9–2.3 m depth indicating that the coal and rock are broken in that zone, and the rock damage degree is distinctly different in both sides of the zone. Accordingly, the red line can be considered the fractured interface, i.e., the edge of the ELZ. Furthermore, at distance of 8–9.5 m and depth of 2.3–5.0 m, the reflectors with large amplitude are dense, which means that the coal and rock are fractured, and it can be deemed the local fracture zone. Thus, the ELZ extent ranges from

Fig. 8. Optimized radargrams of the north side coal wall (a) the radargram of survey line No.4 (b) the radargram of survey line No.5.

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Fig. 9. Optimized radargrams of the floor (a) the radargram of survey line No.6 0–50 m (zone A) (b) the radargram of survey line No.6 50–100 m (zone B) (c) the radargram of survey line No.6100–150 m (zone C) (d) the radargram of survey line No.6150–200 m (zone D) (e) the radargram of survey line No.6200–250 m (zone E).

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depths of two investigation zones are larger than the ELZ critical value, they belong to the large ELZ (Dong et al., 1994). 4.2. The ELZ of the north side coal wall Survey line No. 4 was placed at a distance of 100 m on roadway No. 33024, and survey line No. 5 was adjacent to roadway No. 33024 (Fig. 4). Fig. 8 depicts the optimized radargrams of the north side coal wall during the field investigations, and the depth of the ELZ edge can be delineated in Fig. 11. Figs. 8 and 11 show that:

Fig. 10. A comparison of the ELZs for different survey lines in the south side coal wall.

1.9 to 2.3 m in the investigation zone, but it is nearly 5.0 m in a local fracture zone. 2. In the survey line No.2 radargram, the strip-type zone with high density of large amplitude is distributed centrally in 1.9–2.5 m depth. Moreover, at distance of 5.7–9.4 m and depth of 2.7–4.5 m, reflectors with large amplitude are also dense. This means that coal and rock is broken by excavation in two zones. The former zone has a distinct fractured interface, shown as the red line (Fig. 7 (b)), and the latter is the local fracture zone. Obviously, the extent of the ELZ ranges from 1.9 to 2.5 m in this investigation zone, but is nearly 4.5 m in the local fracture zone. 3. In the survey line No. 3 radargram, reflectors with large amplitude are distributed centrally at 1.6–2.9 m depth inside the coal wall, so the location of the fracture interface can be delineated by the red line. In addition, the extent of the ELZ is in the range of 1.6 m and 2.9 m. 4. Because the survey line No. 2 is adjacent to the survey line No. 1, their measurement data can be combined for the drawing in the Fig. 10. It can be found that the ELZ extent is not only different at different survey lines, but also distinct at different distances along the same survey lines. The extent of the ELZ for survey line No. 3 has greater span than other survey lines. Furthermore, because the ELZ edge

Fig. 11. A comparison of the ELZs for different survey lines in the north side coal wall.

1. In the zone along the No. 4 survey line, the high density of large amplitude is distributed in two different positions. The first zone whose trace resembles a strip-type is located at 1.7–2.7 m depth. Due to the distinctly different density of reflectors with large amplitude in both sides of the zone, the red line can be used to indicate the fracture interface. The second zone is located at 2.5–5.1 m depth and distance of 1.0–7.6 m and is considered the local fracture zone. 2. In the zone along the survey line No. 5, the strip-type broken zone is located at 1.8–2.7 m depth, and the fracture interface can be approximately depicted by the red line in the strip-type broken zone. Moreover, the occurence of two local zones with large amplitude reflectors indicates that they are local fracture zones. 3. The extent of the ELZs in the two survey line zones are roughly identical, and are located at 2.1–2.5 m depth. However, the variation range of ELZ extent in the survey line No. 5 zone is greater than the other one. Furthermore, considering the local fracture zones, the extent of ELZs can extend to 4.8–5.3 m in local areas. Because the extents of two ELZs are more than 1.5 m, they belong to the large ELZ (Dong et al., 1994). 4.3. The ELZ of the floor Survey line No. 6 advanced 250 m along the floor of the experimental roadway (Fig. 4). To accurately and succinctly detect the ELZ, survey line No. 6 was divided into 5 portions of 50 m each. Optimized radargrams of the floor are shown in Fig. 9, and the ELZ edge depth can be delineated in Fig. 12. Figs. 9 and 12 show that: 1. The rock surrounding the floor is seriously broken. In comparison, the fracture zone extent is relatively small at distance of 0–50 m (zone A) along the survey line No. 6, but at distance of 50–100 m (zone B), 100–150 m (zone C), 150–200 m (zone D) and 200– 250 m (zone E) along the survey line No. 6, the fracture zone is extremely widespread. Considering the distribution of reflectors with large amplitude in optimized radargrams, the floor rock of the latter 4 portions is very broken. 2. According to the location of strip-type zones, the fracture interfaces can be approximately depicted by the red lines (Fig. 9), and the

Fig. 12. A comparison of the ELZ for different distances along survey line No. 6 in the floor.

M. Tian et al. / Journal of Applied Geophysics 168 (2019) 90–100

depth of the ELZ edge can be confirmed. The ELZ types of zones B, D and E whose edges are greater than 1.5 m belong to the large ELZ. However, in zones A and C, at some local zones, the ELZ edges range from 0.4 m to 1.5 m, which indicates a medium ELZ (Dong et al., 1994). 3. In zone A, the density of reflectors with large amplitude is relatively small at distance of 10–40 m indicating that the destruction of the excavation-influence zone rock is lighter in this zone. However, there is a local fracture zone at the distance of 0–10 m in zone A (Fig. 9 (a)), and the ELZ extent can be extended to 5.0 m. In zone B, there are 4 local fracture zones where the ELZ can extend to 4.5– 5.0 m (Fig. 9 (b)). In zones C and E, local fracture zones are located at 122–127 m, 135–144 m and 211–227 m along the survey line (Fig. 9 (c) and (e)). In zone D, there are two local fracture zones located at 158–167 m and 187–195 m along the survey line (Fig. 9 (d)). Due to local fracture zones, the ELZ extent may enlarge to 4.0– 5.0 m in these zones. 5. Discussion First, the extent of the ELZ is greater than 1.5 m, and the type is large ELZ in the two side coal walls and most of the floor zone during the field investigations. The borehole digital image was used to measure the ELZ in the No. 5105 roadway with similar geological conditions to experimental roadway (Li, 2017). The borehole digital image results showed that the coal wall is severely broken and continuously broken in the range of 0 to 2.20 m, and in the range of 2.20 to 5.47 m, the coal wall is relatively complete without macroscopic cracks but local zones were severely damaged. These results approximately consistent with the result of GPR that the edge ELZ ranges from 1.9–2.9 m in the coal wall and there are local fracture zones in the EIZ. Due to the roadway location in a soft coal seam with soft roof and floor, the surrounding coal and rock are soft with low strength and numerous fractures, joints and interfaces. Additionally, there is a large amount of groundwater in the surrounding No. 3 coal seam that softens the broken coal and rock, particularly the sandy mudstone and mudstone. Furthermore, the surrounding rock stress and the mining dynamic stress are both high in a deep mine. Thus, wall spalling and floor heave easily occur, leading to extension of the ELZ and to large ELZ. The large ELZ is a severely damaged zone and needs a support method that can bear great bulking force and produce proper deformation. Second, the ELZ extent is not only different at different survey lines, but also distinct at different distances along the same survey line. Based on evidence from the optimized radargrams, the local fracture zones in the EIZ can be clearly delineated. The experimental roadway is located in a region with complex geological structures including faults and collapse columns, and there are two high stress concentration zones along the east-west direction. Hence, at the fault, the surrounding rock was severely affected by high concentrated stress and mining dynamic stress, and it led to a greater ELZ extent. Influenced by the fault and collapse column, the local fracture zones are widespread in the two side coal walls and the floor. In addition, the floor mudstone was severely broken under the interaction of the increased permeability from excavation and the mudstone being easily softened by groundwater. Hence, the floor rock at the distance of 0–250 m along survey line No. 6 has multiple local fracture zones as shown in Fig. 10, and the ELZ extent can be extended at the local fracture zones. 6. Conclusions The radargram of the Zhaozhuang Mine that was optimized by the picture segmentation method based on the 2D maximum betweencluster variance was obtained to understand the influence of the excavation and mining dynamic stress on the extent of ELZ around the soft coal roadway with soft roofs and floors. The results of the investigations are:

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1. Optimizing the radargrams by the picture segmentation method based on the 2D maximum between-cluster variance is useful for interpreting the radargrams. The fracture zone and the reflection noise zone or background can be more clearly identified in the optimized radargrams. Optimization provides an effective method to optimize and interpret the radargrams. 2. The extent of the ELZ in the investigation zone around soft coal roadways with soft roofs and floors can be determined by optimized radargrams, and the ELZ size and type of each investigation zone is presented. The ELZ extent in the south side coal wall, north side coal wall and the floor vary in the range of 1.9–2.9 m, 2.0–2.5 m and 1.3–2.2 m, respectively. However, the ELZ extent can extend to 4.0–5.0 m in local area due to local fracture zones. The reasons for different ELZs from different survey lines and at different distance along the same survey line are discussed. In addition, the influence of the excavation and mining dynamic stress on the extent of the ELZ around soft coal roadways with soft roofs and floors has been identified. 3. Local fracture zones can be located by analyzing the distribution of reflectors with large amplitude in the optimized radargrams. Local fracture zones are widespread in the floor but rare in the two side coal wall. The geological structure, concentrated stress and mining dynamic stress have been discussed to analyze their influence on local fracture zones.

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