Article Analysis and Monitoring Technology of Upper Seam Mining in Multiunderlayer Goaf

Based on the background of close coal seam mining in the Qianjiaying coal mine, Tangshan, China, the feasibility of the upper seam mining in complex underlaying goaf is analysed using the roof caving analysis and numerical method. The deformation of the mining seam and roadways is monitored and analysed by ﬁeld measurement and 3D laser scanning. The deformation characteristics of #5 seam after mining 1378P, 2071P, 2072P, and 2091P working panels with a depth of 39–54m below the #5 seam are analysed using roof caving analysis and numerical method. Results show that the maximum deformation of #5 seam in the superimposed area of the lower goafs reaches 2.5 m and the maximum deformation in the single coal goaf is 1.5m. The maximum seam inclination is 1.9 ° . The subsidence of the ﬂoor of 1359P roadways is obtained by ﬁeld measurement, and the result is consistent with numerical calculation. ZEB-HORIZON 3D laser scanner was used to measure and model the roadway deformation. Based on the analysis of multiple scanning data, the deformation of the 1359P roadways was obtained. Results show that the deformation of the surrounding rock of the roadway is not great, the maximum displacement of the roof fall is 30 mm, and the maximum rib convergence is 25mm. It can be concluded that the #5 seam can be recovered in this complex underlying lower seams’ mining condition.


Introduction
Close multiseams mining refers to the coal seams with mutual influence. As excavation has greater impact on the roof than the floor, the downward mining method is often used to reduce the influence on adjacent seams [1]. In practice, due to uncertainty of geological exploration or unsatisfied mining conditions, the lower seam may be mined firstly, namely, upward mining. Upward mining faces the challenges of upper seam and surrounding rock subsidence, fracturing and bending, gas imitation, and stress concentration directly above lower seam pillar [2][3][4][5][6][7]. Considering the timed effect of lower goaf compaction, the situation of upward mining for close multiple coal seams is complex; thus, it is often studied case-by-case in practice. e lower seams' mining causes upper seam movements and ground stress unloading, which introduce fractures influencing the stability of surrounding rock of upper seam [8][9][10][11][12][13][14][15]. e research of the upward mining method mainly focuses on the overburden damage analysis caused by lower seam mining and the deformation monitoring and strata control technology while upper seam mining. e former addresses the feasibility of upper seam mining. For example, based on the statistics of more than 200 upward mining cases, Wang and Li [8] suggested that the feasibility of upward mining can be determined using three zones' discrimination and surrounding rock equilibrium methods. Han et al. [9] classified the extent of damnification of upper seam caused by lower seam mining and established the criterion for the upward mining of close seams using the multivariate regression analysis. Wang et al. [16] identified the overburden stress distribution and fracture field for upward mining. In a case study, Zhang et al. [17] found that the fracture in upper seam located above the caving zone was recovered after a certain period, indicating that the upper seam could be mined. Jiang et al. [18] obtained roof stress distribution using the continuous beam model under uniformly distributed load, which can be used to study the feasibility of upper seam mining above large goaf area.
Another key of upward mining is strata deformation monitoring and related ground control technologies in upper seam development and excavation [19][20][21][22][23]. Suchowerska et al. [19] studied the vertical stress redistribution in close seam mining. Shao et al. [20] conducted simulation experiment study on the overlying rock fracture and stability of interlayer strata in upward mining, and result showed that the upper seam working face could pass through the fracture zone safely under proper support technique. Based on the theory of composite overburden structure, Huang et al. [24] considered the fracture zone inclined to the inner side of the goaf and proposed two schemes of upward mining, namely, the overlaying method and the staggering pillar method. Zhang [25] monitored the overburden movement during upward seam excavation, and the roof caving angle, overburden displacement, and large and small periodic weighting intervals were studied. Zhao et al. [26] calculated the range of upper seam influenced by lower seam mining in Qianjiaying and proposed a roadway reinforcing scheme for passing through lower seam pillars. Based on theoretical analysis and numerical calculation, Wang et al. [27] studied the upper seam floor subsidence upon lower seam open cut, and it showed that the working face was unstable while passing through the lower seam open cut, and special support should be implemented.
Qianjiaying colliery, located in Tanshan, China, is a typical close multiseams' coal mine. After mining out two lower seams, it is necessary to determine the possibility of upward mining [28]. In this paper, the influence of the lower seams' mining on the overlying strata is determined based on the roof caving analysis. en, the subsidence and bending angles of the upper seam due to lower seams mining are calculated using numerical method, so as to provide a criterion for the feasibility of upward mining. After roadway development of the upper panel, the actual subsidence of the upper seam floor is measured based on elevation data. Before panel excavation, multiple 3D laser scanning was carried out in the roadways using the ZEB-HORIZON 3D laser scanner, and the deformation of the upper roadways beyond lower seam coal mass, goaf, roadway, and coal pillars was evaluated to provide a basis for monitoring and strata control in the excavation. is paper conducts a case study of upper seam mining over complex lower seam conditions using theoretical, numerical, and field monitoring methods, which provide a reference for feasibility judgement and premining confirmation of upward mining under similar geocondition.

Field Conditions.
Qianjiaying coalmine is 17 km east of Tangshan, China, with a mining area of 42.6 km 2 . e minable coal includes #5, #7, #9, and #12 seams with total recoverable coal 600 Mt and annual production of around 6.0 Mt. e seams have medium thickness with composite overlaying strata; the bury depth is about 600-800 m underground. In situ stress measurements suggest that the major and medium principle stresses of the Qianjiaying coalmine are in horizontal direction, and the major principal stress to vertical stress ratio is around 1.6. e seams are mainly formed by clarain and semiclarain. e average thickness of #5, #7, and #9 seams is 1.4, 3.6, and 1.8 m, respectively; their dipping angle is 1°-10°. e distance between #5 and #7 seam is 39 m and is 54 m between #5 and 9 seam. e comprehensive stratigraphic column of related seams is shown in Figure 1. e mining method of #7 and #9 coal seams is full roof caving longwall mining.
Before mining of 1359 panel (shorten as 1359P hereafter) in #5 seam, 1378P, 2071P, and 2072P in #7 seam were mined out in 2006, 2009, and 2012, respectively; 2091P in #9 seam was mined out in 2017. e layout of the related panels is shown in Figure 2.
e roadway and mining direction of 1359P is nearly vertical to lower seams. e roadway development and face retreat of 1359P need to cross 4 gobs and 2 pillars. e aim of this study is to evaluate the excavation possibility of 1359P. Before roadway excavation of 1359P, the influence of lower seams' mining towards #5 seam should be studied quantitively. During roadway excavation, in situ measurement and data analysis should be conducted to confirm the prediction result. Before panel retreat, the deformation behaviour of 1359P roadways over different underlaying strata conditions should be monitored and evaluated to guarantee a safe environment of working face retreat.

Overlaying Strata Caving Analysis.
After panel excavation, the overlying strata form caving zone, fracture zone, and subsidence zone from the bottom to the top. For upward mining, if the upper panel is within the caving zone of lower seam gob, the seam and surrounding rock are seriously broken and cannot be driven or mined. If the upper panel is within the fracture zone, the roadway is unstable as the fracture cannot be restored for a long time, i.e., the upper seam cannot be mined. If the upper panel is beyond the middle section of the fracture zone or within the subsidence zone, the upper seam keeps certain integrity although there are a large number of fractures developed in the coal and surrounding rock. After a certain period of compaction, the strata can restore stability and the seam can be safely mined [29][30][31][32][33]. e height of each zone is related to geological condition and mining method, which are often evaluated using the semitheoretical and semiempirical method [1,8]. According to the geological characteristics of Qianjiaying (Figure 1), the height of one single coal seam caving zone can be calculated using the following equation: And, the height of the fracture zone can be calculated using where M is the excavation thickness, W is the roof subsidence, K is the volumetric expansion coefficient of crushed rock, normally 1.1∼1.4, and α is the seam dipping angle. e mining height of 2071P is 3.4 m and the seam dipping angle is 8°. e roof fall is around 400 mm and the volumetric expansion coefficient of crushed rock is supposed to be 1.4. en, the height of the caving zone can be estimated as 7.57 m; the height of the fracture zone can be calculated as 32.0-43.2 m and 46.9 m based on equation (2) or (3), respectively. Based on safety consideration, the height of the fracture zone of 2071P can be estimated as 46.9 m. e heights of the caving zone and the fracture zone of each lower seam panel can be calculated in a similar manner, and the result is summarized in Table 1.
e average distance between #5 and #7 seam is 39 m, i.e., 1359P is located in the upper position of fracture zone of #7 seam gob. Accordingly, #7 seam gob has less influence on the deformation of #5 seam. e distance between #5 and #7 seam is 54 m, so #5 seam is within the subsidence zone of #9 gob. e deformation of #5 seam due to #9 seam mining is small. e gob of 2091P overlaps with 1378P and 2071P gobs, and the deformation of #5 seam over this area is expected large.   Advances in Civil Engineering

Numerical Calculation.
To quantify the deformation of #5 seam induced by lower seams' mining, a FLAC 3D calculation model is constructed to calculate the subsidence consequence of #5 seam due to lower seams' mining. e model size is 1223(L) × 566(W) × 114(H) m using a total of 1193600 elements, as shown in Figure 3. e mechanical properties of the surrounding rock used in the model are shown in Table 2.
e calculation procedure followed the consequence of lower seam mining [34]. After 2071P is mined out, the deformation nephogram and numerical simulation of #5 seam floor are shown in Figure 4. It shows that the subsidence of the middle sections of 2071P gob and 1378P gob is large, up to 1.22 m, and gradually decrease towards mining boundary. Figure 5 shows side view of vertical displacement of A-A cross section. e maximum subsidence is 3.1 m, occurs at the #7 seam roof, and gradually decreases upwards. e subsidence of #5 seam floor is around 1.0-1.2 m.
After all lower panels are mined out, the subsidence of #5 seam floor is shown in Figure 6. It shows that, after excavation of #7 and #9 seams, the subsidence of #5 seam floor over the gob areas is 1.0-2.5 m, and the maximum deformation is on the overlapped area of 1378P gob and 2091P gob. Figure 7 shows the subsidence of middle axial of 1359P (A-A in Figure 2). e maximum subsidence is 2.5 m. e bending angle of the seam is calculated based on the subsidence geometry. It shows that the maximum bending angle is around 1.9°, according to studies related to deformational behaviour of coal mass [31,[35][36][37], and it suggests that the rock integrity of 1359P is favourable.

Field Measurement.
e results of roof caving analysis and numerical calculation suggest that the 1359P is minable. After roadway development, the subsidence of #5 seam should be measured to confirm pre-excavation prediction. In reality, due to the coal seam pinch out, the final length of the tailgate is 544. e subsidence contours of the roadway floor are obtained by the comparison of the elevation data before and after lower seams' excavation, as shown in Figures 8 and 9. e dashed line is the floor elevation before lower seams' mining. Figures 8 and 9 show that the maximum subsidence of the 1359P tailgate is about 2.5 m, which is located in the overlapping area of 1378P gob and 2091P gob. e maximum subsidence of the 1359P maingate is about 2.7 m, which is located in the overlapping area of 2071P gob and 2091P gob. Figure 10 shows the measured and calculated subsidence data of the 1359P roadway floor. e numerical calculation is in good agreement with field measurement.

Roadway Deformation
Monitoring. 3D laser scanning can obtain precise chamber profile data and quick modeling, which has been tested and trialed in deformation monitoring of underground roadway in the coal mine [38][39][40][41][42]. ZEB-HORIZON 3D laser scanner is used to analyse the roadway deformation of 1359P. ZEB-HORIZON is a mobile 3D laser scanner based on SLAM algorithm. It can dynamically measure and record 3D spatial information without GPS. Its scanning range is 30 m, the data acquisition rate is 43,200 points/s, the accuracy is 4 mm/100 m, and the angle accuracy is 60 mrad. It is the IP64 level that can be used in harsh environment. Its weigh is 3.5 kg and can be carried personally.
e deformation of roadway can be obtained by multiple scanning. e 3D laser scanning model of the tailgate and maingate of 1359P over 2072P is shown in Figure 11. Figure 12 shows the outline of the maingate of 1359P, and they are (a) underlying coal mass, (b) over 2072P gob, (c) over pillar between 2072P and 2071P, (d) over 2071P gob, (e) over pillar between 2071P and 1378P, and (f ) over 1378P gob. e red and blue lines are two measurements, after roadway development and before 1359 panel excavation, respectively. e calculation results of roof fall and rib convergence are shown in Table 3.
For large deformation of surrounding rock of the roadways, a special ground support scheme should be implemented based on the reinforcement principles and field conditions [43][44][45][46]. e maximum roof fall is 30 mm, and the maximum rib convergence is 25 mm. e overall deformation of the roadway is small. At the same time, it was observed that the roof, floor, and ribs of the roadway remained in good condition. Combining with result of theoretical analysis, it can be concluded that the roadway of 1359 panel in #5 seam is stable, and the panel can be mined as normal.     Figure 9: Subsidence of 1359P tailgate floor. 6 Advances in Civil Engineering Advances in Civil Engineering 7

Conclusion
Based on the upward mining engineering background of the Qianjiaying coal mine, this paper comprehensively uses roof caving analysis and numerical calculation to analyse the mining feasibility of 1350P. In the stage of the roadway development, the roadway subsidence is measured and the roadway deformation is modeled. e following conclusions can be drawn: (1) e results of caving analysis and numerical calculation show that, under the condition of 2071P. 2091P, 1378P, and 2091P are mined out, and the subsidence of #5 seam 5 is 1.0-2.5 m. e maximum bending angle of #5 seam is 1.9°. e subsidence and bending angle results of #5 seam show that 1359P roadway can be developed.
(2) rough field measurement, the floor subsidence of 1359P roadway is obtained. e maximum subsidence of 1359P tailgate is about 2.5 m, located in the overlapping area of 1378P gob and 2091P gob. e maximum subsidence of the maingate is about 2.7 m, which is located in the overlapping area of 2071P gob and 2091P gob. e numerical results are in good agreement with the field measurements.
(3) e roadway deformation is monitored using the multiple 3D laser scanning method. e maximum roof fall and rib convergence is 30 and 25 mm, respectively. So, 1359P can be mined as normal.

Data Availability
All data used to support the findings of the study are included within the article.