Yng Li,Yuqi Ren,*,Syd S.Peng,Hozhou Cheng,Nn Wng,Juno Luo

a School of Energy and Mining Engineering,China University of Mining and Technology (Beijing),Beijing 100083,China

b Department of Mining Engineering,West Virginia University,Morgantown,WV 26505,USA

Keywords:Overburden measurement GPR survey Close multiple coal seam Secondary roof break Periodic triangular cave Overburden caving

ABSTRACT In the Kaiping Coal field,mining of five coal seams,located within 80 m in the Kailuan Group,#5,#7,#8,#9 and #12 coal seam,is difficult due to small interburden thickness,concentrated stress distributions,high coal seam metamorphism,and complex geological conditions.By using the ZTR12 geological penetration radar(GPR)survey combined with borehole observations,the overburden caving due to mining of the five coals seams was measured.The development characteristics of full-cover rock fractures after mining were obtained from the GPR scan,which provides a measurement basis for the control of rock strata in close multiple coal seam mining.For the first time,it was found that the overburden caving pattern shows a periodic triangular caved characteristic.Furthermore,it is proposed that an upright triangular collapsed pile masonry and an inverted triangular with larger fragments piled up alternately appear in the lower gob.The research results show that the roof structure formed in the gob area can support the key overlying strata,which is beneficial to ensure the integrity and stability of the upper coal seams in multiple-seam mining of close coal seams.

1.Introduction

Due to the particularity of the coal seam occurrence conditions,there is often more than one recoverable coal seam in some major production mining areas in China.Among the recoverable coal seams,the interburden thickness between the seams can be as large as hundreds of meters,or as small as one to 2 m.At the same time,due to the continuous exploitation of coal resources and resource exhaustion,those coal seams which were not mineable previously were redefined as recoverable coal seams.Therefore,mining under such mining and geological conditions,there will be many occasions in which coal operations will be under the impact of gobs and pillars in close multiple-seam mining [1-3].

At present,many scholars have carried out research on the evolution of stress field,overburden movement,and fracture development during the close multiple seam mining [4-8].Kang et al.studied the characteristics of top rock layer migration,plastic zone range,and stress variation in upper and lower mining panels under different staggered conditions during simultaneous mining of thin coal seams at extremely close distances [9].Huang et al.applied a physical similarity simulation method to study shallow-buried and extremely close-distance coal seam mining [10].It was concluded that during the re-compaction of the lower coal seam after mining,the roof structure of the caved upper coal seam was ‘‘activated”,which means the formation of obvious step rock beams.Li et al.believed that the fractures and strata separations formed by the overburden under the influence of repeated mining in close multiple coal seams would undergo five dynamic changes[11].Lu et al.adopted a similar simulation test method,and believed that the immediate roof of lower coal seam would break as a V structure when the upper and lower thick and close multiple coal seams were kept at a certain distance and were mined at the same time[12].

During mining close multiple coal seams,after the lower coal seam has been mined,the caving characteristics of the immediate roof in the gob cannot be directly observed through field measurement.In addition,existing research often focuses on the feasibility of ascending mining.Research on the breakage characteristics of overburden rock in the mining area of the lower coal seam is limited.This paper presents a study on using geological penetration radar (GPR) and borehole camera to map the caving and fracture zone height of different seams that are closely located in the Qianjiaying Mine in Kailuan mining area.For the first time,this paper will use both the borehole measurement and GPR equipment to detect overburden caving characteristics after mining in multilevel gobs.It is proposed that during the ascending mining of multiple coal seams,the overburden of the gob after the lower coal seam mining will undergo secondary breakage,and the ‘‘inverted triangle”area formed by the secondary breakage will control the key overlying strata,which provide important protection for ascending mining sequence.

2.Project overview

There are five mineable coal seams located within about 80 m of coal-bearing strata in the Qianjiaying Mine in the Kailuan Coal field.In order from top to bottom,the seams are the #5,#7,#8,#9,and #12 coal seam (Fig.1).The average thickness of each coal seam is 1.1,4.0,1.4,1.9,and 3.5 m,respectively.The layers of coal seams have the characteristics of small interburden,high degree of metamorphism,and complex occurrence conditions.The coal quality of each coal seam is shown in Table 1.

Based on the production capacity,coal quality,and the mutual influence of mining between coal seams,an efficient multipleseam mining technology was established.The mining sequence of the coal seams is #7,#8,#12,#9,and #5 [13].The distribution of coal seams is shown in Fig.1.

3.Underground measurements

3.1.Measurement method

3.1.1.Geological penetrating radar technique

The ZTR12 series of ground penetrating radar the 100 MHz antenna (Fig.2) was used to measure the TG (tailgate) floor of the #9 panel in #9 seam and the gob of #12-1 panel in #12 seam in Qianjiaying Mine[14].The GPR(ground penetrating radar)relies on the transmission of high-frequency electromagnetic waves(1 MHz-2.5 GHz) through a transmitting shielded antenna and the acquisition of the corresponding reflected signals by a receiving antenna to store in and display the signals by a computer.The GPR antenna is the key component of the GPR system,and the performance of the GPR system largely depends on the performance of the antenna,which can be regarded as a sensor,radiation,converters and transducers.According to the design requirements,the center frequency of the ZTR12 antenna is designed to be 100 MHz,and the geometric size of a single detachable antenna is less than 1 m,so as to meet the needs of entreating and adapt to the mining application environment.The stepping delay output can be controlled within 2 ps.The output signal width is narrow(10 ns)with higher voltage(90 V),and the four-step avalanche circuit is adopted.The generated 100 MHz transmitting pulse can be seen that the amplitude ratio of pulse to ripple is significantly increased to more than 30 dB.The effective mapping depth for GPR measurement is within 30 m below the #9 coal seam.

(1) Working principle of GPR

An antenna is used to transmit broadband high-frequency electromagnetic waves to the ground.Reflection,transmission,and refraction occur when an electromagnetic wave signal encounters a dielectric interface with a large dielectric difference when propagating inside the medium.The greater the difference in dielectric constant between the two media,the greater the energy of the reflected electromagnetic wave.After the reflected electromagnetic wave is received by the receiving antenna that moves synchronously with the transmitting antenna,the GPR monitor accurately records the motion characteristics of the reflected electromagnetic wave.Then,a radar scan image will be formed by signal technology processing.Through the interpretation of the image,the engineering technicians judge the actual structure of the underground target.Because the hollow and water-rich anomalies detected by this project are largely and electrically different from the surrounding medium.The working principle of ground penetrating radar is shown in Fig.3.

The broadband high-frequency time-domain electromagnetic pulse wave reflection of GPR is used to detect the target.According to the interval between the transmitted wave and the reflected wave,the depth of the detected object can be obtained by using Eq.(1).

where Z is the depth;X the distance between the transmitted and the reflected waves;and V the wave speed.

Based on the measured radar wave trend,the depth Z and range of the reflector are automatically determined.

The electrical characteristics of common media are shown in Table 2.Considering the inhomogeneity of propagation medium in a certain range,the wave velocity of different media is set as a random value.A total of eight different types of media were considered:air,fresh water,limestone (dry),limestone (wet),sandstone (wet),shale (wet),dry coal,wet coal.

In the actual measurement,the radar antenna is attached to the measurement surface and continuously slides along the predetermined line from point 1 to point 2 (or point acquisition method).The GPR monitor records the time and amplitude of the reflected waves of each measuring point in real time to form a continuous radar profile.When the wave velocity of the local medium is known,the position and depth of the target can be obtained based on the measured two-way travel.

(2) Location of GPR survey line

The fracture development above the gob in #12-1panel of #12 coal seam was measured,and the measurement depth was 30 m.The measurement position is shown in Fig.4.The parameters of geological radar survey system are shown in Table 3.

3.1.2.Drilling measurement method

The probe and positioner of the drilling speculum were first inserted into the drill holes at the same time.The strata condition inside the borehole was observed on the borehole monitor.While slowly inserting the probe and positioner into a hole at a uniform speed,the camera of the speculum was used to photograph the rock formations at different depths of the hole.Four different locations were selected:

Table 1.Quality of each coal seam in Kailuan Coal Mine.

Fig.2.Schematic diagram of GPR layout.

Fig.3.Principle of ground penetrating radar.

Table 2.Electrical characteristics of common media.

Fig.4.Location of GPR survey.

Table 3.Parameters of GPR survey system.

Location one:two boreholes,vertical to the coal seam,were arranged on the floor of the panel#9 in#9 seam,and the development of fractures and the caving zone of #12 coal seams were observed.The drilling sites were located in the tailgate of the panel#9 TG in#9 coal seam(Fig.5).The #1 borehole is 105 m from the start of the tailgate and the hole depth is 8.1 m.The#2 borehole is 130 m from the start of the tailgate and the hole depth was 13.8 m.The detailed position is shown in Fig.5.

Location two:the drilling site was located in the tailgate of the#12-2 panel of the#12 coal seam to measure the caving and fracture development of the roof of#12 coal seam,the floor of#9 coal seam,and the floor and roof of #8 coal seam (Fig.6).Borehole #3 was drilled from,and to the roof of #12 seam with an inclination angle of 60°.The designed drilling length of the borehole was 70-80 m and the diameter was 65 mm.The detailed location is shown in Fig.6.

Location three:3 experimental boreholes were drilled in the main entry of the connecting roadway under the #12 seam to explore the failure depth of the floor of 12-2 panel in #12 coal seam (Fig.6).However,only borehole #4 was successful among the three boreholes.Borehole #4 was drilled from,and to the roof of#12 seam and passed through#9 coal seam with an inclination angle of 60°.The estimated drilling length was 80-90 m,and the hole diameter was 65 mm.The detailed location is shown in Fig.6.

Location four:the drilling site with 4 experimental boreholes was located in the roof of the tailgate of the #7-1 panel of the #7 coal seam (Fig.6).The panel was overlain by #5 coal seam.However there was only one boreholes(borehole 5)successfully drilled out of the four boreholes.Borehole 5 was available to monitor and survey the height and fracture development of the #7 coal seam mining with the inclination angle of 48.1°.The length of the borehole was 85-90 m and the diameter was 65 mm.The boreholes passed through the roof of the#7 coal seam gob and#5 coal seam in sequence.The detailed location is shown in Fig.6.

3.2.Measurement results

3.2.1.GPR survey results

Point acquisition method is implemented in the survey of #9 TG.GPR system with the 100 MHz antenna,and the reflective method is employed for data acquisition.Fig.7 shows the original survey results of the roof fracture development in #12 coal seam mining.The surveying lines are directly set on the #9 coal seam TG floor.For the original signal result in Fig.7,it can be seen that some abnormal regions are detected at 40-60 m from the starting point (see Table 4).

Fig.5.Plan layout showing the location of boreholes #1 .

Fig.6.Cross-section showing the location of boreholes #3,#4 .

Fig.7.Survey results of the roof fracture development in #12 coal seam mining.

By distracting the signal transmission capability in the anomalous zone,the damage position of the overlying strata after #12 coal seam mining can be obtained.The results of the enlarged view of the anomaly area in Fig.7b show that there are obvious separations in the roof rock formations,that is,the rock formations have obvious shifts in these areas.The signal transmission of rock formation is weaker in the yellow area due to less dielectric interfaces inside the medium which indicates that the caved rock blocks are less fragmented or larger.In the blue area,the signal transmission of the rock strata is strong.Thus there are larger dielectric interfaces of caved rocks,indicating the caved blocks are well broken or smaller.However,based on the original survey results,the location of the anomalies can be identified,but what the roof broken structure becomes cannot be obtained.So the acquired GPR data should be processed through a series of processing steps,such as zero calibration,background denoising,filtering and gain processing,to better show the fracture development as shown in Fig.8.

As shown in Fig.8,the shear movement of strata layers can be observed according to the processed GPR data.There are four anomalous areas from 35 to 70 m away from the starting point with regular phenomena in the main roof of #12 coal seam.There would be one area with less broken roof where the integrity of the roof is better at 35 m away from the starting point,beside area with severely broken roof where the integrity of the roof is worse at 50 m.This phenomena happened on and on along with the mining direction,and the interval between two areas is approximate to the periodical weighting interval.

3.2.2.Borehole measurement results

In borehole drilling operation,the caving zone boundary were determined by the water return,when there is no drilling water return in drilling process,it is indicating the drill has reached the boundary of caving zone.For borehole 1&2(Fig.9),after the panel of the #12 coal seam is mined,the drilling operation was stopped at 8 and 13.8 m deep when no drilling water return occurred.So the caving zone is within the range of 15.4-21.1 m,and the fracture zone is greater than 8-13.8 m above the caving zone.

Due to the inclination of the borehole 3,the drilling distance is converted to the vertical to coal seam as below.

Table 4.Summary of the locations of boreholes and target of measurements.

Fig.8.Processed GPR data detection results.

Fig.9.Observed wall strata conditions for borehole 1 &2 of panel #9 TG.

The borehole 3 was drilled toward the fractured zone above the gob (Fig.10).According to the measurement results (Fig.11a),the observation section is divided into two parts inside borehole 3.The first part is from the intact strata area beyond the caving zone with few and small cracks to the obvious fracture zone which is determined as the boundary of caving zone with the 23.4 m from the roof of the #12 coal seam.The second part is the micro-fracture area.This part is shale strata with the 31.2 m from the roof of the#12 coal seam.So the fractured zone is more than 7.8 m above the caving zone.Due to the collapse of the surrounding rock in borehole,the monitor probe of the borehole can only reach to this point.So the picture capture were stopped with the 31.2 m from the roof of the #12 coal seam.

For borehole 4,according to the measurement results(Fig.11b),the observation section is divided into two parts.The first part is the complete area of the rock formation with few cracks.The second part is the obvious fractured area:the fractures of the surrounding rocks are deep and some fractures are severe.The drilling operation was stopped at 10.3 m distance to the floor of#12 coal seam without water return in the borehole,which determined as drilling to the boundary of floor damage.

The borehole 5 was drilled toward the fractured zone over the gob the same as borehole 3.For borehole 5,the observation section is roughly divided into two parts(Fig.11c).The first part is from an intact zone to the severely broken area.This part is distributed within the range of 9.7-27.7 m from the roof of the #7 coal seam,which is determined as the boundary of caving zone.The second part is the micro fracture area.This part is from layer shale up to layer sandstone within 28-51.9 m distance from the roof of the#7 coal seam with the monitoring video of the borehole can only reach this point.So the picture capture were stopped at 51.9 m from the roof of the #7 coal seam.

Fig.10.Borehole 3 drilling toward to fractured zone above the gob.

Fig.11.Observed borehole wall strata conditions for boreholes.

Table 5.Comparison between borehole and GPR surveys under the #9 coal seam floor.

3.3.Analysis

3.3.1.GPR survey results

Fig.12 shows the roof fracture development after the original GPR data have been processed.It can be seen that caving has developed well in the immediate and main roofs as well as the top layer of siltstone,which is the floor of#9 coal seam.The#9 coal seam is less affected by the mining of the#12 panel.So the caving zone of#12 coal seam mining is developed to the bottom of siltstone with the height 16.8 to 20 m.By calibrating the results of the borehole 1&2 surveys,the GPR survey results are close to that of the borehole survey results,of which the caving zone is within the range of 15.4 to 21.1 m(Table 5).Both the borehole and GPR survey results corroborate that the caving zone of #12 coal seam mining develops and stops at the layer of mudstone,and it does not reach and damage the#9 coal seam.So mining of#9 coal seam after mining of the#12 coal seam below is feasible.

The caved rock fragments in the caving zone of #12 seam(Fig.12)exhibit certain pattern:larger fragments piled up to form the inverted triangular blocks,separated a trapezoidal block where the caved fragments are smaller.This is due to the periodic weighting of the roof as the dotted line shown in Fig.12 and the secondary breakage of the roof in the gob area during mining as the heavy line shown in Fig.12.Based on the GPR inversion shown in Fig.12,the survey area is divided into three zones,namely I,II,and III.In Zone Ⅰ,the lower rock blocks are severely broken and the degree of fragmentation is high,while the upper rock blocks are moderately broken with larger pieces and the degree of fragmentation is small,forming an inverted triangular area of the roof broken rock layer as relative to the intact rock layer.In Zone Ⅱ,the lower part of the rock block is severely broken,and the degree of fragmentation is higher,while the upper rock block is more moderately broken with smaller pieces,and the degree of fragmentation is also large,forming an upright triangular area of the roof broken formation.Zone III is basically the same as Zone I,and an inverted triangular area of the roof broken layer is formed in the gob of the #12 panel.

Fig.12.Roof fracture development by inversion of GPR data.

3.3.2.Borehole measurement results

The borehole measurement results are summarized in Figs.13-15.Although borehole 1,2,and 3 are in different location between#9 and #12 coal seams,it can be concluded that the#9 coal seam is in the fracture zone of the#12 coal seam,and the integrity of the#9 seam has not been damaged.The interburden between these two seams in the fractured zone has obvious delamination and fragmentation.The maximum caving zone height for mining of#12 coal seam as observed from boreholes 1,2,and 3 is 23.4 m from the roof of the #12 coal seam.

For the floor damage survey of the#12 coal seam from the borehole # 4 measurement (Fig.14b),the drilling operation was stopped at 10.3 m deep from the floor of #12 coal seam when the drilling water return was stopped,thereby determining the boundary of floor damage.So the floor damage for #12 coal seam is 10.3 m deep.

Fig.13.Summary of borehole strata condition for borehole 1 &2 of panel #9.

Fig.14.Summary of borehole strata condition for boreholes.

Fig.15.Summary of overburden movement based on borehole and GPR surveys.

From borehole 5,the caving zone is up to 27.7 m and the fractured zone is 51.9 m deep from the roof of the #7 coal seam(Fig.14c),reaching the full borehole length.The #5 coal seam is located in the fractured zone,where rock strata is fairly intact.The fractured zone is extended up to the fine sandstone which is the main roof of #5 coal seam.

4.Conclusions

By using the GPR survey combined with borehole observations in the same location of#9 coal seam,the overburden caving due to mining of the#12 coals seams was estimated.The caving zone has developed well in the immediate and main roofs as well as the floor of#9 coal seam.The#9 coal seam is less affected by the mining of the#12 panel.Both the borehole and GPR survey results corroborate that the caving zone of #12 coal seam mining does not reach and damage the #9 coal seam.So mining of #9 coal seam after mining of the #12 coal seam below is feasible.

The development characteristics of full-cover rock fractures after mining were obtained from the GPR scan,which provides a measurement basis for the control of rock strata in close multiple coal seam mining.For the first time,it was found that the overburden caving pattern shows a periodic triangular caved characteristic.It is proposed that an upright triangular block where the caved fragments are smaller and an inverted triangular with larger fragments piled up alternately appear in the lower gob.

Furthermore,the overburden caving characteristics of 5 minable coal seams within 80 m after mining in multi-level gobs were accurately detected by using the borehole observations.The results show that the height of the caving zone of#7 and#12 coal seam is 27.7 and 23.4 m,respectively.The damage depth of floor of #12 coal seam is 10.3 m.The # 5 coal seams and #9 coal seams have not been damaged after mining the lower coal seam.It provides a measurement basis for the ground control in close-coal mining.

Acknowledgements

The research is supported by National Key R&D Program of China(No.2017YFC060300204),National Natural Science Foundation of China (No.52074293),Hebei Province Natural Science Foundation of China(No.E2020402041),Yue Qi Young Scholar Project,CUMTB and Yue Qi Distinguished Scholar Project(No.800015Z1138),China University of Mining &Technology,Beijing.