China is the largest coal producing nation in the world. For the past decade, the average coal production in China is approximately 3.48 billion tons a year, which represents 43.81% of the total coal production in the world [1
]. Coal has been a tremendous booster for China’s economic development, but has also brought considerable negative impacts on ecological environment, especially on water resource. On the average, 2.04 m3
water should be discharged for producing a ton of coal [2
]. Coal mining can cause the deterioration of regional ecological environment especially in arid and semi-arid mining areas which have vulnerable ecosystems [5
]. Achieving the conservation of water resource in coal mining process is of great significance to maintaining ecological balance in the mines [7
As early as the 1960s, with the aim of attempting the conservation of water resource in the coal mining process, Austrian scholars began to use tracer agents to examine the relation between subsidence and underground water [11
]. American researchers investigated the strata disturbance region while performing coal mining under a surface water, and established related mathematical models to predict water diversion performances of the mining-induced fractures [12
]. Chinese scholars proposed the concept of water conservation mining from the end of the 20th century. After the development over the past 30 years, systematical water conservation mining technical and theoretical system has been initially established in China [14
]. At present, the water conservation mining methods mainly include drift mining, slice mining (height-limit mining), room and pillar mining, and backfilling. Specifically, drift mining exhibits certain limitations, including low coal recovery rate and difficulty in recovery of stranded coal pillars; using slice mining (height-limit mining), the mining time and the effects on aquifer can hardly be determined; room and pillar mining also encounters some difficulties, such as low coal recovery rate and complex production processes. By contrast, the backfilling mining (also referred to mining with backfilling) becomes the most effective water conservation mining method; nevertheless, traditional longwall backfilling technology also shows some problems, such as insufficient constraints of backfilling space and great difficulties in mining activity coordination, thereby seriously restricting the development of water conservation mining [18
In recent years, by combining the advantages and concepts of the Wongawilli rapid mining method [21
] and pillar backfilling mining [23
], the writers proposed continuous excavation and continuous backfilling (CECB) in longwall mining and explored the preparation of filling materials [24
]. This method adopts the interval excavated MRs for coal mining. Once a MR is excavated, it will be filled immediately. At the same time, the next MR will be excavated until all MRs of the whole working face are excavated and filled, as shown in Figure 1
. This method can successfully overcome the limitations in traditional longwall backfilling mining and provide the backfilling face with sufficient time and effective space. Accordingly, the filling bodies can be solidified, and the requirements on bearing strength can be satisfied to effectively control mining-induced fractures (MIF).
It is one of the important scientific problems in water conservation mining that avoiding MIF conducting through aquifers [26
]. Scholars have undertaken considerable researches on the law of MIF and its influence on water resources when longwall mining. American scholars have studied the importance of aquicludes between aquifers and coal seam by studying MIF in longwall working face [27
]. Ukrainian scholars studied the distribution pattern and evolution law of MIF in a longwall working face of the Donetsk coalfield, and divided the overlying strata into caving zone, fracture zone and continuous deformation zone [29
]. At the same time, the caving zone and fissure zone were defined as MIF connected area of roof water. Chinese scholars have detected MIF distribution patterns of hundreds of longwall working faces under different geological conditions, and concluded that MIF in overlying strata are saddle shaped [30
]. In addition, scholars have laid a theoretical foundation for backfilling water conservation mining by studying the law of MIF under longwall backfilling and drift backfilling conditions [32
However, there are relatively few studies on the control of MIF in roadway backfilling, especially the characteristics and control of MIF under CECB are still unclear. The interval mining technology under CECB leads to alternate distribution between coal pillar (or filling body) and mined roadway [34
]. The supporting structures of strata are discontinuous, and the distribution patterns of MIF are also different. In addition, CECB is essentially a backfilling mining method. The main control principle of MIF is to fill before the goaf forms and occupy the subsidence space of strata, reduce the degree of strata migration, and control the development of MIF. The filling percentage is an important parameter to measure filling qualities and filling effects. Therefore, the control of MIF through the mining roadway (MR) filling percentages is one of the important scientific problems of CECB. At present, there is relatively little research in this area.
Based on the engineering background of CECB in the Wangtaipu Coal Mine, and on the basis of different boundary conditions of the main roof stress, this paper establishes the mechanical models of clamped-clamped beam, continuous beam and elastic foundation beam between the filling body, main roof, and strata. Moreover, the mechanism and characteristics of MIF are analyzed, and the controlling effects of filling rates on MIF are studied. The present results should provide theoretical and practical guidance for water conservation mining under extremely close distance aquifers and other practices of water conservation mining with extremely thin barriers.
(1) Advantages of CECB
The background of CECB is to solve the limitation of traditional longwall backfilling mining, which concludes:
(a) the exposed area of the roof in longwall mining is larger, and the overburden migration is relatively significant, which is not conducive to the support of the roof by the filling body; (b) the filling body is lack of lateral constraints after filling into the goaf, and its deformation resistance is weak; and (c) the mining activities and filling operations are mutually constrained, which not only affects the mining efficiency, but is difficult to ensure the full compaction of backfill.
The advantages of CECB are as follows:
(a) mining in the form of excavating MRs, the exposed roof area is smaller than longwall mining, and the roof is supported by coal pillars or filling bodies on both sides of MRs, so as to minimize the migration degree of overburden; (b) the filling body is filled into the confined space, which is always in the state of three-dimensional stress, and its deformation is relatively small; (c) the mining activities and filling operations are independent in space and do not interfere with each other, so the efficiency of mining and filling is high; and (d) both sides of the MRs just filled are not excavated immediately, which provide sufficient time and effective space for the solidification and full bearing of the filling body.
(2) Comparison the results by using strain-softening model and Mohr–Coulomb model
To check the feasibility by using Mohr–Coulomb model and rationality of the numerical results, a strain-softening model with 90% MR filling percentage is established and studied. The general parameters of each stratum are the same as those in Mohr–Coulomb model, such as bulk modulus, shear modulus, density, internal friction angle, cohesion, and tensile strength. Then the additional parameters, such as ctable, ftable, and ttable, are determined by the plastic zone characteristics of the total stress–strain curves through uniaxial compression tests. The numerical results are shown in Figure 14
It is determined that the density and the range of MIF obtained by the Mohr–Coulomb model and strain-softening model are similar. In strain-softening model, the height of MIF is 7.2 m (2.88 times the mining height), which is slightly higher than the 7.0m (2.8 times of mining height) calculated by using Mohr–Coulomb model. The differences between the two results are about 3%, but both are smaller than the theoretical calculation value. Therefore, it is also feasible to use the Mohr–Coulomb model to study the characteristics of MIF under CECB. In addition, the numerical results of the two models are smaller than the theoretical results. From the perspective of water conservation mining, the height of MIF is predicted by the theoretical calculation value, which can also avoid the influence caused by the selection of different numerical models.
(3) Selection of mining and filling parameters
The influence of MR filling percentages on MIF is affected by factors, such as mining technology, backfilling technology, and geometrical conditions. The influencing factors mainly include the strength of the backfilling materials, the MR filling percentage, the MR height-to-width ratio, and the interval MR width. This study performed single-variable analysis for examining the effect of the filing percentage on the MIF height, during which the effects of other factors were not taken into account.
A smaller MR height-to-width ratio, more mining phases, and slighter roof subsidence can contribute to controlling the MIF height, but simultaneously reduce the mining efficiency. Higher strength of the filling materials and higher MR filling percentages can more remarkably control the MIF height; however, the utilization rate of the backfilling materials dropped. A reasonable arrangement of the MR and setting of backfilling parameters can not only achieve the function of water conservation mining in extremely close distance aquifers, but can also ensure the effectiveness of mining and increase the utilization rate of materials.
(4) Methods of improving coal mining efficiency
In actual production, the MR filling percentage is generally greater than that of excavation. In order to achieve coordinated operation of mining and filling, parallel excavation of multiple MRs can be carried out, which is similar to the simultaneous cutting of multiple shearers in the same working face. On the premise of meeting the requirement of water conservation mining, filling materials are not limited to one kind. High water expansion materials, aeolian sand paste-like materials, and industrial solid waste can be selected.
(5) Advantages of promotion
Filling cost is an important issue affecting the promotion and application of CECB. One advantage of this method is that the filling of MR can be adapted to local conditions and the comprehensive benefits of coal mining can be maximized. For example, in the northwest mining area of China, filling materials are difficult to obtain in situ materials. On the premise of water conservation mining, the filling quantity of MRs can be reduced appropriately, so as to reduce the filling cost. In the eastern mining area of China, the filling materials of industrial solid waste residue mainly consisting of gangue are sufficient and cheap, and MRs can be fully filled, which not only realizes the water conservation mining in the conditions of extremely close distance aquifers, but also solves the pollution problem of industrial solid waste residue to the surface environment.
(6) Scientific and social significance
The scientific significance of this study lies in providing theoretical guidance for water conservation mining under the extremely thin barrier layers, including mining under the extremely close-distance aquifers, mining close-distance coal seams, and other geological conditions. At present, CECB has been widely used in Ordos, Jincheng, Xinwen, and Zaozhuang mining areas in China. On the premise of water conservation mining, more than 2.6 million tons of recovered coal has been increased in total in the above mining areas, which increased profits by more than 67 million dollars, and achieved good economic and environmental benefits. With a view of realizing efficient, safe, and green mining of coal resources, CECB will be gradually extended to the core coal production bases in Western China.
According to the different boundary conditions of gradual replacement of the main roof stress from coal pillars to filling bodies, the mechanical models of clamped-clamped beam, continuous beam, and elastic foundation beam are established. The horizontal tensile strains of the overlying strata are calculated, and the relationship between the MIF height and MR filling percentages satisfy a hyperbolic function.
The distribution patterns of MIF under CECB are studied. It is concluded that the distribution pattern is an isosceles trapezoid with the moving angle of the overlying strata as the bottom angle, and the upper and lower boundary of MIF as two parallel sides. Furthermore, the moving angle of the overlying strata does not change with MR filling percentages.
The controlling effects of MR filling percentages on MIF are studied. Taking the value of λ as the main index, the curve of the MIF height is divided into three ranges, including the stability control range, the critical range, and the lost control range. In addition, the calculating expression of the MIF height in the stability control range is given.
In the study area, the 90% MR filling percentage is used for CECB. The MIF height is 7.55 m (3.0 times mining height), and the main roof is not penetrated. The aquiclude III is effectively protected by the main roof, and its water-resisting property is not destroyed, thus realizing water conservation mining.