Floor Heave Mechanism of Gob-Side Entry ... - Semantic Scholar

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Floor Heave Mechanism of Gob-Side Entry Retaining with Fully-Mechanized Backfilling Mining Peng Gong 1 , Zhanguo Ma 1, *, Xiaoyan Ni 1 and Ray Ruichong Zhang 2 1

2

*

State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou 221116, China; [email protected] (P.G.); [email protected] (X.N.) Department of Mechanical Engineering, Colorado School of Mines, Golden, CO 80401, USA; [email protected] Correspondence: [email protected]; Tel.: +86-516-8388-5487

Received: 15 October 2017; Accepted: 5 December 2017; Published: 8 December 2017

Abstract: Serious floor heave in gob-side entry retaining (GER) with fully-mechanized gangue backfilling mining affects the transportation and ventilation safety of the mine. A theoretical mechanical model for the floor of gob-backfilled GER was established. The effects of the mechanical properties of floor strata, the granular compaction of backfilling area (BFA), the vertical support of roadside support body (RSB), and the stress concentration of the solid coal on the floor heave of the gob-backfilled GER were studied. The results show that the floor heave increases with the increase of the coal seam buried depth, and decreases with the increase of the floor rock elastic modulus. The development depth of the plastic zone decreases with the increase of the c and φ value of the floor rock, and increases with the increase of the stress concentration factor of the solid coal. The development depth of the plastic zone in the test mine reached 2.68 m. The field test and monitoring results indicate that the comprehensive control scheme of adjusting backfilling pressure, deep grouting reinforcement, shallow opening stress relief slots, and surface pouring can effectively control the floor heave. The roof-floor displacement is reduced by 73.8% compared to that with the original support scheme. The roadway section meets the design and application requirements when the deformation stabilizes, demonstrating the rationality of the mechanical model. The research results overcome the technical bottleneck of floor heave control of fully-mechanized backfilling GER, providing a reliable basis for the design of a floor heave control scheme. Keywords: theoretical model; floor heave; gob-side entry retaining; gob-backfilled mining

1. Introduction In gob-side entry retaining (GER) of underground coal seam mining, the headgate (entry 2 front of the active panel #1) of the current mining panel (panel #1) is retained by constructing the roadside support body (RSB) and serves as the tailgate (entry 2 behind of the active panel #1) to the subsequent adjacent panel (panel #2) [1,2] (Figure 1). For the gob-backfilled GER technology, a roadside artificial wall was built to maintain the roadway based on the backfilled gangue supporting the roof. This technology has the following advantages: (1) Backfilling mining can consume a large amount of waste gangues piled up on the ground, control the surface subsidence, turn waste into usable materials, and alleviate environmental stress; (2) The compacted gangues in the backfilling area (BFA) can limit the roof deformation, reducing the load during roof weighting and avoiding rock burst accidents of the working face [3–7]; (3) Owing to the support of gangues in the BFA, the support pressure of the RSB is reduced, which is favorable for stabilizing the RSB, thus improving the success rate of entry retaining. Therefore, the GER technology with fully-mechanized backfilling is adaptable to complex geological conditions such as deep roadways, high stress, large mining height, and weak surrounding rock, Energies 2017, 10, 2085; doi:10.3390/en10122085

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the success rate of entry retaining. Therefore, the GER technology with fully‐mechanized backfilling  the success rate of entry retaining. Therefore, the GER technology with fully‐mechanized backfilling  is  is adaptable  adaptable to  to complex  complex geological  geological conditions  conditions such  such as  as deep  deep roadways,  roadways, high  high stress,  stress, large  large mining  mining  expanding application rangerock,  of GER technology However, it of  is in the engineering height,  and  weak  surrounding  expanding  the  application  range  GER  [8–10].  height,  and the weak  surrounding  rock,  expanding  the [8–10]. application  range  of found GER technology  technology  [8–10].  practice of the gob-backfilled GER that the floor heave in the retained entry is larger than that in the However, it is found in the engineering practice of the gob‐backfilled GER that the floor heave in the  However, it is found in the engineering practice of the gob‐backfilled GER that the floor heave in the  GER with roof management using the caving method. In GER with fully-mechanized backfilling, retained entry is larger than that in the GER with roof management using the caving method. In GER  retained entry is larger than that in the GER with roof management using the caving method. In GER  the amount of floor heave accounts for more than of  70% of the roof-floor displacement, to serious with  fully‐mechanized  backfilling,  the  amount  heave  accounts  for  than  of  with  fully‐mechanized  backfilling,  the  amount  of floor  floor  heave  accounts  for more  more leading than 70%  70%  of the  the  convergence of roadway sections, thus affecting the normal transportation (Figure 2a) and seriously roof‐floor  roof‐floor displacement,  displacement, leading  leading to  to serious  serious convergence  convergence of  of roadway  roadway sections,  sections, thus  thus affecting  affecting the  the  threatening the ventilation safety of the roadway. Moreover, the local roadway floor is seriously normal transportation (Figure 2a) and seriously threatening the ventilation safety of the roadway.  normal transportation (Figure 2a) and seriously threatening the ventilation safety of the roadway.  damaged, breaking the floor aquifer, resulting in athrough  water inrush accident of the Moreover,  the  roadway  floor  is  seriously  breaking  the  sandstone  Moreover,  the local  local through roadway  floor  is sandstone seriously damaged,  damaged,  breaking  through  the floor  floor  sandstone  roadway floor (Figure 2b), as well as seriously affecting the safety of the roadway and the normal aquifer, resulting in  a water inrush accident of the roadway floor (Figure 2b), as well as seriously  aquifer, resulting in  a water inrush accident of the roadway floor (Figure 2b), as well as seriously  production of the mine. affecting the safety of the roadway and the normal production of the mine.  affecting the safety of the roadway and the normal production of the mine. 

Coal Coalseam seam

Hydraulic Hydraulicsupport support  

Panel Panel#1 #1

Backfilling Backfillingarea area  

Panel Panel#2 #2 Entry Entry33  

Entry Entry22  

Entry Entry11  

BFA BFA  

Backfilling Backfillingmaterial material  

RSB RSB  

High High  

Compactness Compactness  

Coal Coal  

Low Low  

  

Figure 1. Working face  face layout of  of gob-side entry  entry retaining (GER)  (GER) with fully‐mechanized  fully-mechanized gangue Figure  Figure 1.1. Working  Working  face layout  layout  of gob‐side  gob‐side  entry retaining  retaining  (GER) with  with  fully‐mechanized gangue  gangue  backfilling mining. BFA: backfilling area; RSB: roadside support body. backfilling mining. BFA: backfilling area; RSB: roadside support body.  backfilling mining. BFA: backfilling area; RSB: roadside support body. 

(a)  (a) 

(b) (b)

  

Figure 2. Field floor heave of gob‐backfilled GER. (a) Floor heave affects the roadway transportation;  Figure 2. Figure 2. Field floor heave of gob‐backfilled GER. (a) Floor heave affects the roadway transportation;  Field floor heave of gob-backfilled GER. (a) Floor heave affects the roadway transportation; and (b) floor water inrush.  and (b) floor water inrush.  and (b) floor water inrush.

Under the condition of gob‐backfilled mining, the stress and deformation characteristics of the  Under the condition of gob‐backfilled mining, the stress and deformation characteristics of the  Under the condition of gob-backfilled mining, the stress and deformation characteristics of the surrounding rocks are radically changed due to the supportive effect of BFA on the roof. At present,  surrounding rocks are radically changed due to the supportive effect of BFA on the roof. At present,  surrounding rocks are radically changed due to the supportive effect of BFA on the roof. At present, the research on backfill mining is more focused on the mechanical properties of filling materials and  the research on backfill mining is more focused on the mechanical properties of filling materials and  the research on backfill mining is more focused on the mechanical properties of filling materials and roof deformation of stope. Sheshpari [11] holds that the gob‐backfilled mining method increases the  roof deformation of stope. Sheshpari [11] holds that the gob‐backfilled mining method increases the  roof deformation of stope. Sheshpari [11] holds that the gob-backfilled mining method increases stability  stability of  of the  the mined  mined area  area and  and decreases  decreases land  land subsidence  subsidence and  and rock  rock bursts  bursts due  due to  to stress  stress pattern  pattern  the stability of the mined area and decreases land subsidence and rock bursts due to stress pattern changes. Jehring and Bareither [12] analyzed the tailings composition effects on the shear strength  changes. Jehring and Bareither [12] analyzed the tailings composition effects on the shear strength  changes. Jehring and Bareither [12] analyzed the tailings composition effects on the shear strength behavior of co‐mixed mine waste rock and tailings. The study by Huang et al. [13] shows that the  behavior of co‐mixed mine waste rock and tailings. The study by Huang et al. [13] shows that the  behavior of co-mixed mine waste rock and tailings. The study by Huang et al. [13] shows that the   

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fracture of the main roof is mainly controlled by the filling ratio and is non-correlated to the shield supporting pressure. In recent years, some scholars have studied the deformation mechanism of surrounding rock in GER and floor heave control in dynamic pressure roadways, achieving many positive results. He et al. [14] analyzed the stress distribution characteristics of deep coal roadways, and proposed an integrated method of controlling floor heave by establishing a numerical calculation model. Lin et al. [15] believed that the pressure of floor heave in GER of deep swelling soft rock comes from the transfer of the “compression mold effect” in the backfilling body and solid coal, lateral pressure generated by main roof rotation, compaction of caving gangue in the gob area, deep high stress, and soft rock swelling pressure. Sun et al. [16] obtained the dangerous location of asymmetric floor heave in the slightly-inclined coal seam roadway through physical simulation tests, explaining the formation of floor heave in a slightly-inclined coal seam roadway combined with infrared images, video pictures, and monitoring results of strain. Tang et al. [17] proposed a numerical simulation method based on humidity diffusion, and conducted a numerical simulation of floor heave of swelling rock tunnels under high humidity condition. Time-dependent deformation and the failure process of the tunnel under high humidity conditions were discussed. Li and Weng [18] analyzed the influence of dynamic loading on the fracture process of surrounding rock in deep roadways. Zhang et al. [19] studied the effect of different thicknesses of immediate roof on the deformation and fracture characteristics of roof in gob-side entry, and deduced a formula of rational support resistance of RSB. Gong et al. [20] analyzed the effect of initial compaction of BFA on the deformation of retained entry roof and the stability of RSB by using physical simulation, which demonstrated the decisive effect of BFA granular gangue on the control of main roof deformation. Meanwhile, a RSB structure containing a flexible cushion was proposed, which can maintain a certain support force while adapting to the bending deformation of the roof, isolating crushed gangue from the gob, and limiting the deformation of the immediate roof. Zhang et al. [21] used the double-yield and strain-softening models to simulate the gob area and RSB, respectively, to establish a GER calculation model, obtaining the reasonable width of RSB. Han et al. [22] established a mechanical model of the lateral fracture of cantilever beam, obtaining the roof deformation equation and balance criterion for broken blocks. Song et al. [23] believed that the RSB cannot withstand the great pressure exerted by the roof in GER engineering, causing an increase in the RSB width and backfilling costs. Under the great support pressure of the two sides, the floor suffers bending deformation and local damage. Chen et al. [24] found that the abutment pressure is a fundamental contribution to the serious floor heave of GER. Han et al. [25] observed that the GER floor heave has stage and asymmetrical characteristics. The current studies on the surrounding rock deformation mechanism of GER focus on the RSB stability and gob side roof caving. The studies on the floor deformation mechanism focus more on the soft rock floor in GER, but less on other rock floors, resulting in a certain one-sidedness. Although there are few studies on the GER floor heave mechanism, many scholars and engineers have carried out a variety of experiments and explorations on floor heave control of GER due to the urgent need for engineering technologies. Zhang et al. [26] suggested that deep-hole pre-split blasting should be conducted on the GER roof with a thick and hard roof in the mining face to release the pressure of the surrounding rock, thus reducing the floor heave of GER. Jiang et al. [27,28] tried to use grouting reinforcement to reduce the effect of water on floor deformation. Cao et al. [29] developed a support scheme focusing on the improvement of flexural rigidity of the floor. Sun [30] used floor slotting technology to reduce local horizontal pressure and successfully applied it in a roadway with high horizontal stress. Based on the above research results, we recognize that the factors causing floor heave in the roadway are varied under different geological conditions and engineering background, including horizontal stress distribution, floor rock properties, mining pressure, and geological structure. Research should be focused on the floor heave caused by mining methods and stope stress distribution after first eliminating the effects of the geological structure, floor lithology, softening, and expansion when

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geological  conditions.  Moreover,  the  floor  heave  of  gob‐backfilled  GER  has  stage  characteristics.  After  an  entry  floor  heave  mainly  occurs  at  the  compaction  stage  of  BFA  4in  the  first  Energies 2017,is  10,retained,  2085 of 19 mining process and the advanced influence stage in the second mining process. The floor heave at  the  advanced  influence  stage  in  the  second  mining  process  is  mainly  controlled  by  temporary  mixing with water in gob-backfilled GER engineering under different geological conditions. Moreover, support.  The  floor of heave  at  the  first  mining  stage  needs  be  controlled  by  heave permanent  the floor heave gob-backfilled GER has stage characteristics. After to  an entry is retained, floor reinforcement, and the support design principle is important.  mainly occurs at the compaction stage of BFA in the first mining process and the advanced influence stage in the second mining process. The floor heave at the advanced influence stage in the second A theoretical calculation model for floor heave of gob‐backfilled GER was established, based on  mining process is mainly controlled by temporary support. The floor heave at the first mining stage the field pressure monitoring results of the working face. The influence of the mechanical properties  needs to be the  controlled by permanent reinforcement, thevertical  support design principle is important. of  floor  strata,  granular  compaction  of  BFA, and the  support  of  RSB,  and  the  stress  A theoretical calculation model for floor heave of gob-backfilled GER was established, based on concentration of solid coal on floor heave in the gob‐backfilled GER was studied. The main factors  the field pressure monitoring results of the working face. The influence of the mechanical properties of affecting the GER floor heave were analyzed from a theoretical point of view, and the principles of  floor strata, the granular compaction of BFA, the vertical support of RSB, and the stress concentration controlling  the  heave  against  each  influencing  factor  were  necessary  of solid coalfloor  on floor heave in the gob-backfilled GER was studied. Theproposed,  main factorsproviding  affecting thea GER basis for the design of a floor heave control scheme.  floor heave were analyzed from a theoretical point of view, and the principles of controlling the floor heave against each influencing factor were proposed, providing a necessary basis for the design of a floor heave control scheme. 2. Engineering Property of Gob‐Backfilled GER  2. Engineering Property of Gob-Backfilled GER

2.1. Mining Geological Conditions 

2.1. Mining Geological Conditions

The test mine is in the Shandong Province of China (Figure 3). The ground elevation of the test  The test mine is in the Shandong Province of China (Figure 3). The ground elevation of the test zone was +33 m; the underground elevation was −642 to −636 m; the coal seam strike approximates  zone +33 m; south;  the underground wasto  −642 mThe  to −main  636 m; mining  the coal seam approximates east–west,  was trending  the  true elevation dip  is  0°  3°.  coal strike seam  is  seam  3lower,  ◦ ◦ east–west, trending south; the true dip is 0 to 3 . The main mining coal seam is seam 3lower, Protodikonov’s hardness coefficient is 1 to 2, thickness is at 3.08 to 4.10 m. Seam 3lower of the coal  Protodikonov’s hardness coefficient is 1 to 2, thickness is at 3.08 to 4.10 m. Seam 3lower of the coal floor floor consists of mudstone and siltstone, and Seam 3lower of the coal roof consists of mudstone and  consists of mudstone and siltstone, and Seam 3lower of the coal roof consists of mudstone and siltstone. siltstone. Through the field sampling and laboratory test, the cohesion c of the floor rock sample is 2  Through the field sampling and laboratory test, the cohesion c of the floor rock sample is 2 MPa, MPa,  and  the  internal  friction  is Protodikonov’s 25°.  The  Protodikonov’s  hardness  coefficient  and the internal friction angle φangle  is 25◦ . φ  The hardness coefficient of the mudstone and of  the  siltstone 4 to 6. Theis mining roadway is 4 m wide and 3.6is m4 high. The strata log is as strata  mudstone  and issiltstone  4  to  6.  The  mining  roadway  m  wide  and  geological 3.6  m  high.  The  shown in Figure 4. geological log is as shown in Figure 4. 

Shandong 

 

  Figure 3. Location of the test coal mine.  Figure 3. Location of the test coal mine.

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Thickness(m)

Lithology

Lithology description

20

Medium sandstone

Containing debris and feldspar and mafic minerals; hard

20

Siltstone

Local containing fine sandstone and mudstone; horizontal bedding; fracture developing

3.6

Coal

Black; glass luster

3

Mudstone

Black and gray; containing plant fossils

18

Fine sandstone

Deep gray; fracture developing

  Figure 4. Stratigraphic column and geological description.  Figure 4. Stratigraphic column and geological description.

2.2. Source of Floor Stress in Gob‐Backfilled GER  2.2. Source of Floor Stress in Gob-Backfilled GER In  the the gob-backfilled gob‐backfilled  GER GER engineering  (Figure 5a), initially  compacted  gangues gangues  were  used  to to  In engineering (Figure 5a), initially compacted were used backfill the gob at the right side after the right working face mining, forming the BFA. The RSB was  backfill the gob at the right side after the right working face mining, forming the BFA. The RSB was constructed at  the gob gob side side  the  roadway.  strengthened  anchors  the  constructed at the ofof  the roadway. TheThe  RSBRSB  waswas  strengthened withwith cross  cross anchors on theon  basis basis of the original support with bolt and cable in the roadway. The monitoring results show that  of the original support with bolt and cable in the roadway. The monitoring results show that the large the  large  curved asymmetric fold‐type  asymmetric  floor  heave  absence  of  water  structure and  geological  curved fold-type floor heave occurs in theoccurs  absencein ofthe  water and geological when structure when the field retained entry stabilizes (Figure 5b). In addition, this phenomenon was also  the field retained entry stabilizes (Figure 5b). In addition, this phenomenon was also observed in the observed in the physically similar simulation test of strata structure and mining conditions of the  physically similar simulation test of strata structure and mining conditions of the test mine without test mine without considering the effects of water and geological structure (Figure 5c). Therefore, it  considering the effects of water and geological structure (Figure 5c). Therefore, it can be assumed that can be assumed that the floor heave of gob‐backfilled GER is mainly caused by the load on the floor  the floor heave of gob-backfilled GER is mainly caused by the load on the floor strata when the stresses strata when the stresses are redistributed.  are redistributed. The loads that can directly affect the floor heave of gob-backfilled GER mainly include (Figure 6): (1) The vertical load of solid coal on the floor under the influence of mining. The measured data show that the stresses at the solid coal side are nonlinearly distributed in acable certain range when the GER Anchor Main roof enters the stable stage. The area where the stressesBolt are distributed from the surface of the roadway to deep solid coal can be divided into the stress concentration zone (L3 , L4 ) and the original stress zone (L5 ). The originalImmediate stress is expressed by γH, where γ is the rock volume weight and H is the coal seam roof depth. In the stress concentration zone, the stress tends to increase (q3 ) and decrease (q4 ) from shallow to deep, then gradually restores to the original stress (q5 ). The concentration factor BFA of the stress peak Coal seam point in the stress concentration zone is K; (2) The vertical load (q2 ) exerted by the BFA (L2 ) on the floor under compaction. In the gob-backfilled GER, the BFA plays aRSB major role in supporting the roof, Floor which determines the bending subsidence of main roof “large structure”. The area where the stresses are distributed in the BFA can be divided into stress relief zone and stress restoration zone from left to right. The gob can quickly enter the stable compaction state under the backfilling mining conditions.   stress Compared to the condition that roof is managed using caving method, the distribution of the (a) relief zone is very small; (3) Transmission of supporting reaction force of RSB (L1 ) in the direction of the floor. Owing to the support of gangue in the BFA, the main roof is not broken and the overlying strata pressure is transferred down through the main roof. On one hand, the immediate roof adapts to the rotational deformation of the main roof. On the other hand, it suffers a certain bed separation and subsidence. The engineering practice results show that it is not feasible to limit the rotation of

 

18

Fine sandstone

Deep gray; fracture developing

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  Figure 4. Stratigraphic column and geological description.  the main roof by the support of the RSB. Therefore, a 200-mm flexible cushion is established at the upper part of the RSB, forming a soft-hard RSB structure. On the one hand, it can adapt to the given 2.2. Source of Floor Stress in Gob‐Backfilled GER  deformation of the main roof, and on the other hand, it can block gangue, prevent the immediate roof fromIn  separating, and control the convergence deformation of the roadway. When the roofwere  load used  exceeds the  gob‐backfilled  GER engineering  (Figure 5a), initially  compacted  gangues  to  the strength of the flexible cushion, a large yield deformation occurs in the flexible cushion, preventing backfill the gob at the right side after the right working face mining, forming the BFA. The RSB was  the load from transferring toof  the lower rigid support The working load (q1 ) of anchors  the lower rigid constructed at  the  gob  side  the  roadway.  The  RSB body. was  strengthened  with cross  on  the  RSB can be retained within a reasonable range to avoid the overload damage to the RSB during roof basis of the original support with bolt and cable in the roadway. The monitoring results show that  subsidence, whichfold‐type  is critical to maintaining the completeness RSB; (4) The action of the the  large  curved  asymmetric  floor  heave  occurs of in the the  absence  of squeezing water  and  geological  horizontal ground stress. The engineering practice under deep mining conditions demonstrated that structure when the field retained entry stabilizes (Figure 5b). In addition, this phenomenon was also  the lateral pressure coefficient λ is approximately 1.0, and the high horizontal ground stress aggravates observed in the physically similar simulation test of strata structure and mining conditions of the  the bending deformation of floor strata. The parameters of vertical stress distribution in the roadside test mine without considering the effects of water and geological structure (Figure 5c). Therefore, it  can be detected (for the convenience of calculation, q1 is expressed by multiples of γH) by arranging a can be assumed that the floor heave of gob‐backfilled GER is mainly caused by the load on the floor  borehole stress meter at the roadside of the working face of the test mine. See Table 1. strata when the stresses are redistributed. 

Anchor cable

Main roof

Bolt

Immediate roof BFA

Coal seam RSB

Floor

 

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  (b) 

(c)

Figure  Figure 5. Diagram of  5. Diagram of the floor heave in gob‐backfilled GER. (a) Diagram of the  the floor heave in gob-backfilled GER. (a) Diagram of the gob‐backfilled GER  gob-backfilled GER section; (b) Floor heave in the field GER; and (c) Floor heave of GER in a similar model.  section; (b) Floor heave in the field GER; and (c) Floor heave of GER in a similar model.

The loads that can directly affect the floor heave of gob‐backfilled GER mainly include (Figure  Table 1. Parameters of vertical stress distribution in the roadside. 6): (1) The vertical load of solid coal on the floor under the influence of mining. The measured data  L1 /m L3 /m L4 /m d/m K q1 L1 /m show that the stresses at the solid coal side are nonlinearly distributed in a certain range when the  GER  enters  the  stable  stage.  The  area  where  the  stresses  are  distributed  2 4 8 4 1.53 1.3γH 2 from  the  surface  of  the  roadway to deep solid coal can be divided into the stress concentration zone (L3, L4) and the original  stress zone (L5). The original stress is expressed by γH, where γ is the rock volume weight and H is  the coal seam depth. In the stress concentration zone, the stress tends to increase (q3) and decrease  (q4) from shallow to deep, then gradually restores to the original stress (q5). The concentration factor  of the stress peak point in the stress concentration zone is K; (2) The vertical load (q2) exerted by the  BFA (L2) on the floor under compaction. In the gob‐backfilled GER, the BFA plays a major role in  supporting the roof, which determines the bending subsidence of main roof “large structure”. The  area where the stresses are distributed in the BFA can be divided into stress relief zone and stress  restoration zone from left to right. The gob can quickly enter the stable compaction state under the  backfilling  mining  conditions.  Compared  to  the  condition  that  roof  is  managed  using  caving 

Energies 2017, 10, 2085   

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Energies 2017, 10, 2085

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L5

q5

L4

L3

q4

q3

d 

L1

L2

q1

q2

KγH 

γH 

γH 

 

 

Energies 2017, 10, 2085   

Figure 6. Source of the floor stress of gob-backfilled GER. Figure 6. Source of the floor stress of gob‐backfilled GER. 

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3. Establishment of Mechanical Model for Floor Heave of Gob-Backfilled GER Table 1. Parameters of vertical stress distribution in the roadside. 





 ∫







As shown in Figure 7, the space coordinate system is established by setting the junction of the p 1 L 11-/m   R12  L2  L 1/m  1 L3/m  LL4/m  1d/m  K  p 1q L 1  L original stress and the stress concentration zonedin the solid coal as the origin, horizontal w  zone d   y  ln ( )   right -L (4)  2 E 4  - L 8  4  1 1.53  1.3γH  2  2 2 π π E R  L L 2 as the positive direction of x-axis, outward axially the roadway as the positive direction of the 2 1 R1  y along y-axis, and vertical down as the positive direction of the z-axis. As the load is evenly distributed in the 3. Establishment of Mechanical Model for Floor Heave of Gob‐Backfilled GER  direction themining  y-axis, the mechanical model of in  thethe  floor can be simplified to the plane problem. the  length  of  where  L is ofthe  influence  range  axially  direction  of  the  roadway;  R1  is strain As shown in Figure 7, the space coordinate system is established by setting the junction of the  Through calculation, the vertical linear load p(kN/m) distributed in the y direction and the stress 2 2 Oʹ N ,  R1 = x + z ;  p   is the evenly‐distributed linear load in the y direction;  μ   is the Poisson’s  original stress zone and the stress concentration zone in the solid coal as the origin, horizontal right  caused by the floor at any point N in the XZ plane can be obtained. Under the point load, the stress ratio of the floor rock; and E is the elastic modulus of the floor rock.  as the positive direction of x‐axis, outward axially along the roadway as the positive direction of the  and deformation at any point of the floor can be obtained: y‐axis, and vertical down as the positive direction of the z‐axis. As the load is evenly distributed in  the direction of  the y‐axis, the mechanical model of the floor can be simplified to the plane strain  problem. Through calculation, the vertical linear load  p(kN m)   distributed in the y direction and 

∫





the stress caused by the floor at any point N in the XZ plane can be obtained. Under the point load,  the stress and deformation at any point of the floor can be obtained:  The  distribution  of  stress  at  the  point  N  under  the  evenly‐distributed  linear  load  in  the  y  direction is as follows: 

2 Pz 3    z ∫-∞ d z  πR14 ∞

∞

 x ∫-∞ d x 

2 Px 2 z   πR14

(1) 

(2)  

Figure 7. Figure 7.Mechanical analysis of the floor in gob‐backfilled GER.  Mechanical analysis of the floor in gob-backfilled GER. 2

2 Pxz The distribution of stress at the point N linear load in the y direction The excavation of the roadway and the mining of the coal seam change the stress distribution in     under   zxthe  evenly-distributed (3)  xz 4 π R is as follows: the strata, thus causing the deformation of the surrounding rock. For the floor strata, the stress in the  1 Z ∞ 2Pz3 vertical direction of the mined coal seam is changed. The distribution of load in the vertical direction  σ = dσ = (1) z z The vertical displacement of the floor surface (z = 0) under the evenly‐distributed linear load in  −∞ πR1 4 on the floor can be simplified as shown in Figure 7 by detecting the pressure in the surrounding rock  the y direction is as follows:  Z The  in  the  horizontal  direction  of  the  roadway.  vertical  in  the  plane  XZ  is  regarded  as  ∞ 2Px2stress  z σ = dσxq =  are  simplified  x approximate  linearly  distributed  stress.  q3  −and  as  a  triangular  load  and  (2) a  4 4 ∞ πR 1

trapezoidal load, respectively, and  q2   is simplified as an evenly‐distributed load. The displacement  and  the  stress  state  of  the  floor  surface  at  any  point  are  the  result  of  the  superposition  of  various  loads  of  its  upper  part  at  this  point.  Therefore,  the  deformation  and  the  stress  distribution  of  the    floor after entry retaining are caused by the difference of load before and after coal seam mining.  

Stress distribution of the floor  The additional stress in the floor caused by the triangularly distributed strip load in the stress 

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τxz = τzx =

2Pxz2 πR1 4

(3)

The vertical displacement of the floor surface (z = 0) under the evenly-distributed linear load in the y direction is as follows: Z L

p(1 + µ)(1 − µ) w= dω = πE −L

Z L −L

p p(1 + µ)(1 − µ) R1 2 + L2 + L p ) dy = ln ( 1 πE R1 2 + L2 − L ( R1 + y2 ) 2 1

(4)

where√L is the mining influence range axially in the direction of the roadway; R1 is the length of O0 N, R1 = x2 + z2 ; p is the evenly-distributed linear load in the y direction; µ is the Poisson’s ratio of the floor rock; and E is the elastic modulus of the floor rock. The excavation of the roadway and the mining of the coal seam change the stress distribution in the strata, thus causing the deformation of the surrounding rock. For the floor strata, the stress in the vertical direction of the mined coal seam is changed. The distribution of load in the vertical direction on the floor can be simplified as shown in Figure 7 by detecting the pressure in the surrounding rock in the horizontal direction of the roadway. The vertical stress in the plane XZ is regarded as approximate linearly distributed stress. q3 and q4 are simplified as a triangular load and a trapezoidal load, respectively, and q2 is simplified as an evenly-distributed load. The displacement and the stress state of the floor surface at any point are the result of the superposition of various loads of its upper part at this point. Therefore, the deformation and the stress distribution of the floor after entry retaining are caused by the difference of load before and after coal seam mining. •

Stress distribution of the floor

The additional stress in the floor caused by the triangularly distributed strip load in the stress concentration zone of the solid coal side (OA section) is expressed as follows: σz1 = σx1 = τxz1 =

2(K − 1)γHz3 R a πa 0

ξdξ 2

[( x − ξ )2 + z2 ] 2(K − 1)γHz R a ( x − ξ )2 ξdξ 2 πa 0 [( x − ξ )2 + z2 ] 1)γHz2

2( K − πa

Ra

( x − ξ )ξdξ

0

[( x − ξ )2 + z2 ]

(5) 2

The additional stress in the floor caused by the triangularly distributed strip load in the stress concentration zone of the solid coal side (AB section) is expressed as follows: (b − ξ )dξ 2KγHz3 R b π(b − a) a [( x − ξ )2 + z2 ]2 R b (b − ξ )(x − ξ )2 dξ σx2 = π2KγHz (b − a) a [( x − ξ )2 +z2 ]2 2 R b ( b − ξ )( x − ξ )dξ τxz2 = π2KγHz (b − a) a [( x − ξ )2 + z2 ]2

σz2 =

(6)

The additional stress in the floor caused by the evenly-distributed load in the RSB (CD section) is expressed as follows: 2(q − γH )z3 R h dξ σz3 = 1 π 2 f [( x − ξ )2 + z2 ] 2 R 2(q − γH )z h ( x − ξ ) dξ σx3 = 1 π (7) 2 f [( x − ξ )2 + z2 ] 2(q1 − γH )z2 R h ( x − ξ )dξ τxz3 = 2 π f 2 [( x − ξ ) + z2 ]

The additional stress in the floor caused by the evenly distributed load in the original stress zone (AC section) is expressed as follows:

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σz0 = σx0 = τxz0 =

2γHz3 R f π a

dξ 2

[( x − ξ )2 + z2 ] ( x − ξ )2 dξ 2γHz R f 2 π a [( x − ξ )2 + z2 ]

2γHz2

R

π

f a

(8)

( x − ξ )dξ [( x − ξ )2 + z2 ]

2

The additional stress distribution in the floor after entry retaining is as follows: σz = σz1 + σz2 + σz3 − σz0 σx = σx1 + σx2 + σx3 − σx0 τxz = τxz1 + τxz2 + τxz3 − τxz0

(9)

The main stress at any point within the range of the floor can be obtained from Equation (9): σ1 = σ3 =

σx + σz 2 σx + σz 2

+ −

q q

2

( σx −2 σz ) + τxz 2

(10)

2

( σx −2 σz ) + τxz 2

where K is the stress concentration factor of the solid coal side, γ is the rock volume weight, H is the coal seam depth, q1 is the load on the upper part of the floor in the OA section. a, b, f, and h are x axis coordinates of points A, B, C, D, respectively. •

Vertical displacement of floor surface

The vertical displacement of the floor surface caused by the triangularly distributed strip load in the stress concentration zone of the solid coal side (OA section) is expressed as follows: q

(1 + µ)(1 − µ)(K − 1)γH w1 = aπE

Z a 0

ξ ln( q

( x − ξ )2 + L2 + L ( x − ξ )2 + L2 − L

)dξ

(11)

The vertical displacement of the floor surface caused by the triangularly distributed strip load in the stress concentration zone of the solid coal side (AB section) is expressed as follows: q w2 =

Z (1 + µ)(1 − µ)KγH b

(b − a)πE

a

(b − ξ ) ln( q

( x − ξ )2 + L2 + L ( x − ξ )2 + L2 − L

)dξ

(12)

The vertical displacement of the floor surface caused by the evenly-distributed load in the RSB (CD section) is expressed as follows: q w3 =

Z (1 + µ)(1 − µ)(q1 − γH ) h

πE

f

ln( q

( x − ξ )2 + L2 + L ( x − ξ )2 + L2 − L

)dξ

(13)

The vertical displacement of the floor surface caused by the evenly distributed load in the original stress zone (AC section) is expressed as follows: q w0 =

Z (1 + µ)(1 − µ)γH f

πE

a

ln( q

( x − ξ )2 + L2 + L 2

(x − ξ ) +

L2

)dξ

(14)

−L

The vertical displacement of the floor surface after entry retaining is as follows: w = w1 + w2 + w3 − w0

(15)

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where µ is the Poisson’s ratio of the floor, K is the stress concentration factor of the solid coal side, E is the elastic modulus of the floor rock, and L is the mining influence range in the y direction. γ is the rock volume weight, H is the coal seam depth, q1 is the load on the upper part of the floor in the OA section. a, b, f, and h are x axis coordinates of points A, B, C, D, respectively. 4. Floor Heave Mechanism of Gob-Backfilled GER 4.1. Analysis of Influencing Factors for Floor Heave of Gob-Backfilled GER It can be seen from Equations (10) and (15) that the stress field and the displacement field of the backfilled GER floor are mainly affected by the following three factors: mining geological conditions, load distribution of solid coal side, and gob-side load distribution. The mining geological conditions include the coal seam depth, elastic modulus of floor rock, internal friction angle, cohesion, etc. The load distribution of the solid coal side is mainly determined by the stress concentration factor of the solid coal side. The load distribution of gob side is mainly affected by the support force of RSB and BFA. The influence of each factor on the stress field and displacement field of floor can be directly analyzed by means of theoretical analysis, providing a basis for designing the control technology for floor heave in backfilled GER. 4.1.1. Influence of Geological Conditions on the Floor To analyze the Energies 2017, 10, 2085   

influence of each factor on the floor heave characteristics, the elastic modulus, 11 of 20  coal seam depth, internal friction angle, and cohesion were analyzed as single factors, under the condition that other conditions were not changed. As shown in Figure 8, under the condition that the  condition that other conditions were not changed. As shown in Figure 8, under the condition that the burial depth, load, cohesion, and internal friction angle are not changed, the maximum floor heave  burial depth, load, cohesion, and internal friction angle are not changed, the maximum floor heave of of  GER  is  inversely  proportional  the  elastic  modulus  of  the  floor  rock,  reflecting actually  reflecting  the  GER is inversely proportional to theto elastic modulus of the floor rock, actually the resistance resistance of the floor rock to the deformation under the influence of gob. The maximum floor heave  of the floor rock to the deformation under the influence of gob. The maximum floor heave of the of the roadway increases linearly with the increase of the coal seam depth (Figure 9), which can be  roadway increases linearly with the increase of the coal seam depth (Figure 9), which can be seen from seen from Equations (11)–(15). However, there is not a regularly linear relationship between them in  Equations (11)–(15). However, there is not a regularly linear relationship between them in practical practical  engineering,  mainly  due  to deformation the  plastic  of deformation  of  the  (which  has  not in been  engineering, mainly due to the plastic the rock (which hasrock  not been considered the considered  in  but the the elastic  model),  but  the  internal  relationship  between  them  can  be qualitative obtained  elastic model), internal relationship between them can be obtained according to the according to the qualitative analysis in this paper.  analysis in this paper.

Maximum floor heave (mm)

500

400

300

200

100

1

2

3

E (GPa)

4

5

 

Figure 8. Influence of the elastic modulus on the maximum floor heave. Figure 8. Influence of the elastic modulus on the maximum floor heave. 

m floor heave (mm)

400

300

200

100

1

2

3

4

5

E (GPa) Energies 2017, 10, 2085

 

Figure 8. Influence of the elastic modulus on the maximum floor heave. 

11 of 19

Maximum floor heave (mm)

400

300

200

100

400

500

600

700

800

900

1000

H (m)

 

Figure 9. Influence of the coal seam depth on the maximum floor heave. Figure 9. Influence of the coal seam depth on the maximum floor heave. 

In addition, the approximate distribution range of the plastic zone in the floor can be obtained  In addition, the approximate distribution range of the plastic zone in the floor can be obtained by by introducing the yield criterion of floor strata, based on the analytical solution of the stress field of  introducing the yield criterion of floor strata, based on the analytical solution of the stress field of the the  overall  where  the  yield  condition  of  the  floor  rock  meets  the  Mohr–Coulomb  strength  overall floor,floor,  where the yield condition of the floor rock meets the Mohr–Coulomb strength criterion: criterion:  σ1 − σ3 sin φ = - 2c3 cot φ σ1 + σ31 + Energies 2017, 10, 2085    12 of 20  sinφ        3  2as: ccotφ 1 Thus, the plastic zone of the floor rock isexpressed  1 -  3  -  1   3  sin  ≥ 2 c cos    (16) Thus, the plastic zone of the floor rock is expressed as:  (16) (σ1 − σ3 ) − (σ1 + σ3 ) sin ϕ ≥ 2c cos ϕ From Equation (16), it can be seen that the cohesion and the internal friction angle of the floor    rock  have  Equation an  important  the  development  of  the  zone  in  the  backfilled  GER  in  From (16), effect  it can on  be seen that the cohesion andplastic  the internal friction angle of the floor addition to the stress distribution law of the floor. It can be seen that the depth of the plastic zone in  rock have an important effect on the development of the plastic zone in the backfilled GER in addition the  floor  decreases  with  the  of the  internal friction angle of  rock  through  to the stress distribution law increase  of the floor. It can be seen that the depth the  of the plastic zonecalculation,  in the floor showing  nonlinear  relationship  between  them angle (Figure  10). rock When  the  internal  friction  angle  decreasesa with the increase of the internal friction of the through calculation, showing a increases from 10° to 30°, the depth of the plastic zone in the floor is reduced by 73%. In addition, the  nonlinear relationship between them (Figure 10). When the internal friction angle increases from 10◦ depth  plastic  zone  decreases  the  increase  cohesion  a  to 30◦ , of  thethe  depth of the plastic zone inlinearly  the floorwith  is reduced by 73%.of  Inthe  addition, the (Figure  depth of11)  thewith  plastic gradient of −1 m/MPa.  zone decreases linearly with the increase of the cohesion (Figure 11) with a gradient of −1 m/MPa. 6

Plastic zone depth (m)

5

4

3

2

1

0 10

15

20

φ (dgree)

25

30

 

Figure 10. Influence of the internal friction angle on the depth of the plastic zone. Figure 10. Influence of the internal friction angle on the depth of the plastic zone.  5

h (m)

4

3

1

0 10

15

20

25

30

φ (dgree)

 

Energies 2017, 10, 2085

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Figure 10. Influence of the internal friction angle on the depth of the plastic zone.  5

Plastic zone depth (m)

4

3

2

1

0 1

2

3

c (MPa)

4

5

 

Figure 11. Influence of the cohesion on the depth of the plastic zone. Figure 11. Influence of the cohesion on the depth of the plastic zone. 

4.1.2. Influence of Load of the Solid Coal Side on the Development Depth of the Plastic Zone in the  4.1.2. Influence of Load of the Solid Coal Side on the Development Depth of the Plastic Zone in the Floor Floor  In addition to the rock characteristic parameters of the floor, the stress field distribution in the two In addition to the rock characteristic parameters of the floor, the stress field distribution in the  sides of the roadway also has an important effect on the deformation of the floor. When the two  sides  of  the  roadway  also  has the an granular important  effect  on  the BFA deformation  of  the  floor. rock When  the  gob-backfilled GER is completed, gangues in the restore their original stress gob‐backfilled GER is completed, the granular gangues in the BFA restore their original rock stress  20 m to the rear of the working face and enter the compaction stable zone under the combined action 20 m to the rear of the working face and enter the compaction stable zone under the combined action  of the initial pushing of the backfill hydraulic support and the roof subsidence at the late stage, playing of  the  initial  of  the  support  and  subsidence  at the the condition late  stage,  a major role inpushing  supporting the backfill  roof. Thehydraulic  supportive effect of thethe  BFAroof  on the roof acts in of playing a major role in supporting the roof. The supportive effect of the BFA on the roof acts in the  roof subsidence, so it is a passive support. Therefore, due to the overall structure of the roof, the given  deformation of the roof at the solid coal side will inevitably occur after coal mining, resulting in the concentration of stresses at the solid coal side. The main reason causing the floor heave of the backfilled GER is the redistribution of floor stress after mining and retaining. Therefore, the stress concentration factor of the solid coal side is one of the key factors to determine the stability of the roadway floor. From Figure 12, it can be seen that the depth of the plastic zone in the floor increases with the increase of the stress concentration factor of the solid coal side. When K is less than 2.0, the increase rate is low; when K is greater than 2.0, the increase rate is high. To control the floor heave, the stress concentration factor of the solid coal side should be minimized. The results [31] of the unconfined compression test of granular gangue show that the axial displacement under axial stress of 2 MPa reaches 80% of that of 6 MPa. Therefore, when the geological conditions and the roadway section remain unchanged during excavation, the initial compaction of granules in the BFA needs to be properly improved to reduce the early deformation of the BFA to limit the rotational deformation of the main roof, to reduce the given deformation of the roof of the solid coal side, and achieve the purpose of reducing the stress concentration factor K. 4.1.3. Influence of Gob-Side Load on the Developmental Depth of the Plastic Zone in the Floor The load of the gob-side backfilled GER includes the working resistance of the RSB and the vertical stress of the BFA. In Figure 13, q1 /γH indicates the stress level of q1 . It can be seen that the depth of the plastic zone in the floor decreases with the increase of the support reverse force of the RSB. The depth of the plastic zone decreases rapidly when q1 /γH is within 0.5–1.5, but decreases slowly when q1 /γH is within 1.5–2.3. Therefore, it can be assumed that the increase of the support force of the RSB is favorable to the control of floor heave when q1 /γH is less than 1.5. In addition, the proper reduction of backfilling pressure near the side of the roadway in the BFA can transfer the pressure from the plastic area in the floor to the side of the BFA, thus providing a new channel for floor deformation and pressure release, which is favorable for the control of floor heave in the roadway.

reaches 80% of that of 6 MPa. Therefore, when the geological conditions and the roadway section  remain  unchanged  during  excavation,  the  initial  compaction  of  granules  in  the  BFA  needs  to  be  properly improved to reduce the early deformation of the BFA to limit the rotational deformation of  the  main  roof,  to  reduce  the  given  deformation  of  the  roof  of  the  solid  coal  side,  and  achieve  the  Energies 2017, 10, 2085 13 of 19 purpose of reducing the stress concentration factor K. 

3.3

Plastic zone depth (m)

3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

K

 

Energies 2017, 10, 2085   

14 of 20  Figure 12. Influence of the stress concentration factor of solid coal on the depth of the floor plastic zone. Figure 12. Influence of the stress concentration factor of solid coal on the depth of the floor plastic  zone.  3.4

4.1.3. Influence of Gob‐Side Load on the Developmental Depth of the Plastic Zone in the Floor  3.3 Plastic zone depth (m)

The  load  of  the  gob‐side  backfilled  GER  includes  the  working  resistance  of  the  RSB  and  the  3.2 vertical stress of the BFA. In Figure 13,  q1 γH   indicates the stress level of  q1 . It can be seen that the  3.1

depth of the plastic zone in the floor decreases with the increase of the support reverse force of the  q1 γH   is within 0.5–1.5, but decreases  RSB. The depth of the plastic zone decreases rapidly when  3.0 slowly when  q1 γH   is within 1.5–2.3. Therefore, it can be assumed that the increase of the support  2.9 force of the RSB is favorable to the control of floor heave when  q1 γH   is less than 1.5. In addition,  2.8

the proper reduction of backfilling pressure near the side of the roadway in the BFA can transfer the  2.7 pressure from the plastic area in the floor to the side of the BFA, thus providing  a new channel for  floor  deformation  and 2.6 pressure  release,  which  is  favorable  for  the  control  of  floor  heave  in  the  roadway.  0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

q1/ γH

 

Figure 13. Influence of resistance of RSB on the depth of the plastic zone in the floor. Figure 13. Influence of resistance of RSB on the depth of the plastic zone in the floor. 

4.2. Floor Heave Mechanism of Working Face in Gob-Backfilled GER of the Test Mine 4.2. Floor Heave Mechanism of Working Face in Gob‐Backfilled GER of the Test Mine  The distributions of stresses in the rock floor can be obtained from Equation (9), including the The distributions of stresses in the rock floor can be obtained from Equation (9), including the   vertical stress (Figure 14a), the horizontal stress (Figure 14b), and the shear stress in the xz direction  vertical stress (Figure 14a), the horizontal stress (Figure 14b), and the shear stress in the xz direction

of the  the floor  floor (Figure Then, the  the minimum  minimum principal  principal stress  stress (Figure  (Figure 14d),  14d), the  the distribution  distribution of  of the  the of  (Figure  14c). 14c).  Then,  maximum principal stress, and the plastic zone of the floor strata (Figure 14e) can be obtained from maximum principal stress, and the plastic zone of the floor strata (Figure 14e) can be obtained from  Equation (10). From Figure 8, it can be seen that the relief area of vertical stress of the floor is mainly Equation (10). From Figure 8, it can be seen that the relief area of vertical stress of the floor is mainly  distributed in the range from the floor surface of the roadway to a depth of 22.8 m, deflecting to the distributed in the range from the floor surface of the roadway to a depth of 22.8 m, deflecting to the  solid coal side at the shallow, and the gob side at the deep. The stress distribution of the floor surface solid coal side at the shallow, and the gob side at the deep. The stress distribution of the floor surface  is given by the load. Through calculation, the distribution range of the vertical stress-increasing zone is given by the load. Through calculation, the distribution range of the vertical stress‐increasing zone  of the floor significantly increases with the increase of the stress concentration factor of the solid coal of the floor significantly increases with the increase of the stress concentration factor of the solid coal  side, support load of RSB, and their action range. The stress concentration factor of the test mine side, support load of RSB, and their action range. The stress concentration factor of the test mine is  is 1.53, and the support load of RSB is 1.3γH. Through calculation, the influence depth of the vertical 1.53, and the support load of RSB is 1.3γH. Through calculation, the influence depth of the vertical  stress-increasing zone coal is is  7.37.3  m,m,  andand  thatthat  of the RSBRSB  floorfloor  is 2.3is  m.2.3  The range stress‐increasing  zone below below solid solid  coal  of  the  m. influence The  influence  of horizontal stress mainly extended to bothto  sides insides  the direction of the base angle of the roadway. range  of  horizontal  stress  mainly  extended  both  in  the  direction  of  the  base  angle  of  the  The influence area of shearing stress of the floor is mainly distributed in the abrupt change region of roadway. The influence area of shearing stress of the floor is mainly distributed in the abrupt change  the load on the floor surface, including the stress concentration zone of the solid coal side and the floor region of the load on the floor surface, including the stress concentration zone of the solid coal side  area below the RSB. and the floor area below the RSB. 

Anchor cable Bolt

σz/γH 0.1000 0.2375

1.53, and the support load of RSB is 1.3γH. Through calculation, the influence depth of the vertical  stress‐increasing  zone  below  solid  coal  is  7.3  m,  and  that  of  the  RSB  floor  is  2.3  m.  The  influence  range  of  horizontal  stress  mainly  extended  to  both  sides  in  the  direction  of  the  base  angle  of  the  roadway. The influence area of shearing stress of the floor is mainly distributed in the abrupt change  region of the load on the floor surface, including the stress concentration zone of the solid coal side  Energies 2017, 10, 2085 14 of 19 and the floor area below the RSB. 

σz/γH

Anchor cable

0.1000

Bolt

0.2375 0.4750

Coal seam 1.425

z (m)

1.188

0.9500

1.188

0.2375

0.4750

5

0.7125

BFA RBB

1.188

0.7125

1.425

10

1.662

15

1.900

0.9500

20

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15 of 20 

(a) 4

1.120

8

12

16

1.258

20

24

28

σx / γH

0.8450

0.7075

0.2950 0.4325 0.5700 0.7075 0.8450 0.9825 1.120 1.258 1.395

 

z (m)

5 10 15 20

0

4

8

(b)

12

16

20

24

28 τxz / γH

-0.2630

z (m)

5 10

-0.3500 -0.2630 -0.1760 -0.0890 -0.0020 0.0850 0.1720 0.2590 0.3460

-0.1760

0.2590 0.1720

-0.0890

-0.0020 0.0850

15

-0.0020

20

0

4 1.046

8

1.221

0.5231 0.3488

5 z (m)

(c)

12

0.8719

16

20

24

28

σ3 / γH

0.1744

0.000 0.1744 0.3488 0.5231 0.6975 0.8719 1.046 1.221 1.395

0.6975

10 15 20

(d) 0

4 1.304

z (m)

5

8 1.472

12

16

20

24

28

σ1 / γH

0.9675 1.136

Plastic zone

10 15 20

0.2950 0.4631 0.6312 0.7994 0.9675 1.136 1.304 1.472 1.640

(e) Figure  14.  Stress  distribution  in  the  floor  of  the  gob‐backfilled  GER  in  the  test  mine.  (a)  σ

γ H ;   

z Figure 14. Stress distribution in the floor of the gob-backfilled GER in the test mine. (a) σz /γH; (b)  σ x γ H ; (c)  τxz γ H ; (d)  σ 3 γ H ; and (e)  σ 1 γ H   and the plastic zone.  (b) σx /γH; (c) τxz /γH; (d) σ3 /γH; and (e) σ1 /γH and the plastic zone.

The change of minimum principal stress of the floor mainly lies in the bottom of the roadway,  The change of minimum principal stress of the floor mainly lies in the bottom of the roadway, and its depth is much larger than the floor on both sides of the roadway. The maximum principal  and its depth is much larger than the floor on both sides of the roadway. The maximum principal stress  mainly  increases  below  the  stress  concentration  zone  of  the  solid  coal  side  and  decreases  stress mainly increases below the stress concentration zone of the solid coal side and decreases below below the roadway with a small distribution range and a depth of 2 m. The values of the maximum  theand minimum principal stresses of the floor are positive, indicating that the principal stress in the  roadway with a small distribution range and a depth of 2 m. The values of the maximum and minimum principal stresses of the floor are positive, indicating that the principal stress in the relief relief zone of the roadway floor is compressive stress. The distribution of the plastic zone in the floor  zone of the roadway floor is compressive stress. The distribution of the plastic zone in the floor can be can be obtained as shown in Figure 14e by submitting the measured parameters of the test mine in  obtained asinto  shown in Figure submittingdepth  the measured parameters the testis  mine Table  1  Equation  (16). 14e The by development  of  the  plastic  zone  in of the  floor  2.68 in m. Table In  1 addition, the angle  α1   between the maximum principal stress  σ1   and the x direction and the angle 

α3   between  the  minimum  principal  stress  σ3   and  the  x  direction  can  be  calculated  with  the  following equation: 

 - 1  arctan( 1 x )  xz

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into Equation (16). The development depth of the plastic zone in the floor is 2.68 m. In addition, the angle α1 between the maximum principal stress σ1 and the x direction and the angle α3 between the minimum principal stress σ3 and the x direction can be calculated with the following equation: α1 = arctan( σ1 τ−xz σx ) α3 = arctan( σ τ−xz σx )

(17)

1

The results show that the direction of the maximum principal stress in the stress relief zone beneath the roadway is almost parallel to the x-axis. The horizontal stress of the floor in the roadway is the external factor directly causing floor heave in the roadway. Other external factors affect the failure characteristics of the roadway floor by changing the horizontal stress of the floor in the roadway. Compared with the general roadway where the roof is managed using the caving method, the difference in the load of the floor at the gob side is the main reason leading to the difference in the deformation characteristics of roadway floor. Under normal conditions, the gob area is in an uncompacted state, the plastic zone and the stress relief zone of the floor are shifted to the gob side. As the floor in the gob is not limited in the vertical direction, the floor stress and deformation are released along the gob floor, reducing horizontal stresses of the roadway floor to a certain degree. In the backfilled GER engineering, the horizontal stress of the roadway floor is large due to the constraints in the vertical direction of the BFA. Therefore, slots with a certain depth can be excavated in the roadway floor to reduce the horizontal stress of the floor by absorbing the horizontal deformation of the floor via the stress relief slots to control the floor heave. 5. Field Test and Monitoring 5.1. Control Principle of Floor Heave Based on the above analysis results, the following suggestions can be proposed for the design of process parameters and floor heave control scheme of backfilled GER in practice engineering: (1)

(2)

(3)

With respect to improving the mechanical properties of floor strata: the deformation of the floor decreases with the increase of the elastic modulus of the floor rock, and the development depth of the plastic zone decreases with the increase of c and φ value. Based on the above results, the broken soft rock in the plastic zone of the floor can be performed with grouting reinforcement and repair to improve their elastic modulus and c, φ values, reducing the plastic zone of the floor and controlling the floor deformation (Figure 15a). In addition, the length, angle, and other parameters of floor grouting pipe can be rationally designed according to the developing depth and distribution characteristics of the plastic zone. With respect to increasing the minimum principal stress of the roadway floor: the principal stress of the roadway floor is compressive stress, and squeezing action is exerted in the roadway. Therefore, the floor can be poured with a concrete-forming counter-arched slab to optimize the load-bearing structure of the floor and improve its flexural rigidity. Meanwhile, according to the calculation results of the distribution of the plastic zone in the floor under different conditions, anchor bolts are set from the counter-arched floor to the deep surrounding rock and fixed in the elastic zone of the floor; that is, the length of the bolts should be greater than the calculated value of the depth of the plastic zone in the floor (Figure 15b). Therefore, the concrete-poured floor can be integrated with the deep intact rock through the tension of the bolts, increasing the minimum principal stress of the floor, optimizing the stress environment of the floor and reducing the floor heave in the roadway. With respect to reducing the maximum principal stress of the roadway floor: the horizontal stress of the floor on the roadway is the external factor causing floor heave in the roadway, and thus stress relief slots can be arranged on the floor to reduce the principal stress in the horizontal direction of the roadway (Figure 15c). The depth of the stress relief zone can refer to the calculated distribution

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range of the relief zone of the maximum horizontal stress. In addition, the stress relief slots should be arranged near the solid coal side. The reason is that the stress relief slots provide two free surfaces for the floor rock to release deformation in the horizontal direction. The horizontal displacement of the floor near the stress relief slot is the superposition of the deformation in the horizontal direction of each position. The maximum horizontal displacement of the roadway floor should occur near the stress relief slot under the action of horizontal stress. Therefore, when the distance between the stress relief slot and the RSB is small, the horizontal displacement of the lower part of the RSB increases, which is not conducive to the stability of the RSB. (4) With respect to optimizing the upper load of the floor: the horizontal pushing force of the Energies 2017, 10, 2085    17 of 20  backfill hydraulic support on the rear BFA is improved to increase the initial compaction of the granular gangue in the BFA and reduce the initial subsidence of the roof at the gob side, granular gangue in the BFA and reduce the initial subsidence of the roof at the gob side, and  and thus, to decrease the given deformation of the roof at the solid coal side and reduce the stress  thus, to decrease the given deformation of the roof at the solid coal side and reduce the stress concentration factor K of the solid coal side. In addition, the working resistance of the RSB at concentration factor K of the solid coal side. In addition, the working resistance of the RSB at the  the gob side should the horizontal  horizontal pushing  pushing force  force of  of the  the gob  side  should be be appropriately appropriately improved. improved.  Meanwhile, Meanwhile,  the  hydraulic support is adjusted, and the support pressure of the BFA close to the roadway side is hydraulic support is adjusted, and the support pressure of the BFA close to the roadway side is  reduced to move the plastic zone of the floor to the gob side, and provide new channels for the reduced to move the plastic zone of the floor to the gob side, and provide new channels for the  release of deformation and stress of the floor in the gob, which is favorable for maintaining the release of deformation and stress of the floor in the gob, which is favorable for maintaining the  stability of the floor in the roadway.  stability of the floor in the roadway.

Invert arch  Stress relief groove  Grouting area 

(a) 

 

Bolt 

Plastic zone 

(b)

(c) 

Figure  15.  Floor  control  technology  of  gob‐backfilled  grouting;  (b)  Inverted  Figure 15. Floor heaveheave  control technology of gob-backfilled GER.GER. (a)  (a) FloorFloor  grouting; (b) Inverted arch arch pouring; and (c) Stress relief groove in the floor.  pouring; and (c) Stress relief groove in the floor.

5.2. Design of the Floor Heave Control Scheme  5.2. Design of the Floor Heave Control Scheme C30 concrete was used to pour into a hardened floor with a thickness of 300 mm in the working  C30 concrete was used to pour into a hardened floor with a thickness of 300 mm in the working face  of  the  test  mine  under  the  original  support  scheme,  resulting  in  a  large  floor  heave  after  the  face of the test mine under the original support scheme, resulting in a large floor heave after the completion of entry retaining (Figure 5b). Based on the above design principles and the geological  completion of entry retaining (Figure 5b). Based on the above design principles and the geological conditions of the mine, a floor heave control scheme for the working face of the gob‐backfilled GER  conditions of the mine, a floor heave control scheme for the working face of the gob-backfilled GER in in  the  test  mine  was  proposed.  The  field  application  effect  was  tracked  and  monitored.  The  floor  the test mine was proposed. The field application effect was tracked and monitored. The floor heave heave control scheme (Figure 16a) is designed as follows:  control scheme (Figure 16a) is designed as follows: (1) The  horizontal  pushing  force  of  three  end  backfill  hydraulic  supports  at  the  working  surface  (1) The close  horizontal pushing of three end backfill hydraulic at the working surface close to  the  side  of force the  roadway  is  eliminated,  which supports is  favorable  for  the  movement  and  to the side of the roadway is eliminated, which is favorable for the movement and deformation deformation release of the plastic zone in the floor of the roadway.    release of the plastic zone in the floor of the roadway. (2) Holes are drilled in the roadway floor 500 mm away from the two sides for grouting. Grouting  holes are set outside of the roadway by 20° with a depth of 2.6 m and a grouting pressure of 2  (2) Holes are drilled in the roadway floor 500 mm away from the two sides for grouting. Grouting MPa.  grout  with  cement  P.O.  42.5  and ma and water‐cement  of  1:0.8.  holes are Cement  set outside of is  theused  roadway by 20◦ label  with of  a depth of 2.6 a groutingratio  pressure of Through the diffusion of the grout, the deep plastic zone in the floor is reinforced to carry the  2 MPa. Cement grout is used with cement label of P.O. 42.5 and a water-cement ratio of 1:0.8. horizontal pressure of the floor of the roadway.  Through the diffusion of the grout, the deep plastic zone in the floor is reinforced to carry the (3) Stress  relief  slots  on roadway. the  floor  100  mm  away  from  the  solid  coal  side  of  the  horizontal pressure ofare  the arranged  floor of the roadway. The stress relief slots are 300 mm wide and 700 mm deep. As the surface of roadway  (3) Stress relief slots are arranged on the floor 100 mm away from the solid coal side of the roadway. floor is a free surface and the maximum principal stress of the shallow floor is distributed in the  The stress relief slots are 300 mm wide and 700 mm deep. As the surface of roadway floor is a horizontal direction, the bearing structure determines that the shallow surrounding rock of the  free surface and the maximum principal stress of the shallow floor is distributed in the horizontal floor  does  not  have  high  carrying  capacity.  Therefore,  stress  relief  slots  are  arranged  in  the  direction, the bearing structure determines that the shallow surrounding rock of the floor does shallow rock to provide space for the deformation release of the shallow floor in the horizontal  not have high carrying capacity. Therefore, stress relief slots are arranged in the shallow rock to direction,  which  can  effectively  reduce  its  maximum  principal  stress  and  ensure  the  provide space for the deformation release of the shallow floor in the horizontal direction, which completeness of the floor surface of the roadway.  (4) A  300‐mm  thick  hardening  layer  is  formed  on  the  floor  surface  by  concrete  pouring.  The  concrete strength of the hardening layer is C30. Steel plates are laid at the location of stress relief  slot on the concrete surface.  5.3. Field Monitoring 

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can effectively reduce its maximum principal stress and ensure the completeness of the floor surface of the roadway. A 300-mm thick hardening layer is formed on the floor surface by concrete pouring. The concrete strength of the hardening layer is C30. Steel plates are laid at the location of stress relief slot on the concrete surface.

5.3. Field Monitoring To analyze the effect of the new floor heave control scheme of gob-backfilled GER, measuring stations were arranged on the roadway to record the convergence deformation during advancement of the2017,  working face. The roof-floor displacement of the roadway was larger than the two-side Energies  10, 2085    18 of 20  displacement during advancement of the working face. The maximum roof-floor displacement was 427 mm, of which the floor heave was 320 mm by the original support scheme. After adopting the new 427 mm, of which the floor heave was 320 mm by the original support scheme. After adopting the  scheme, the maximum roof-floor displacement was 112 mm, which was 73.8% lower than that with the new scheme, the maximum roof‐floor displacement was 112 mm, which was 73.8% lower than that  original scheme. Thescheme.  roadwayThe  section meets the designmeets  and utilization requirements when with  the support original  support  roadway  section  the  design  and  utilization  the deformation stabilizes (Figure 16b).stabilizes  The field(Figure  test results show the control principles of floor requirements  when  the  deformation  16b).  The that field  test  results  show  that  the  heave of gob-side backfilled GER based on the theoretical model in this paper have been successfully control principles of floor heave of gob‐side backfilled GER based on the theoretical model in this  applied in the field construction. paper have been successfully applied in the field construction. 

Φ17.8×8300mm anchor cable

Φ22×2400mm bolt

°

800

Φ22×2200mm counter-pulled bolt

Grouting hole

1000

400

Stress relief groove

300

900

500

20 °

20

20 °

Grouting area

300

 

(a) 

(b)

Figure 16.  16. Design heave control scheme of gob-backfilled GER and field control (a)effect.  Floor   Figure  Design ofof floor floor  heave  control  scheme  of  gob‐backfilled  GER  and  field  effect. control  heave control scheme; and (b) Field control effect of floor heave. (a) Floor heave control scheme; and (b) Field control effect of floor heave. 

6. Conclusions 6. Conclusions  To counter serious floor heave of GER backfilling mining, amining,  theoretical To  counter the the  serious  floor  heave  of  with GER fully-mechanized with  fully‐mechanized  backfilling  a  calculationcalculation  model for the gob-backfilled GER was established. The influence the mechanical theoretical  model  for  the  gob‐backfilled  GER  was  established.  The ofinfluence  of  the  properties of floor strata, the granular compaction of BFA, the vertical support of RSB, and the stress mechanical properties of floor strata, the granular compaction of BFA, the vertical support of RSB,  concentration of solid coal on floor heave in the gob-backfilled GER was studied. The following and the stress concentration of solid coal on floor heave in the gob‐backfilled GER was studied. The  conclusions are drawn: following conclusions are drawn:  (1) The theoretical model established herein can be used to obtain the analytical expressions of the  The theoretical model established herein can be used to obtain the analytical expressions of the (1) stress field and displacement field in the vertical and horizontal directions of the floor strata, stress field and displacement field in the vertical and horizontal directions of the floor strata, 

and thus to obtain the distribution law of the principal stress field and plastic zone of the floor,  providing an effective means for directly analyzing the floor heave mechanism of GER.  (2) The parametric analysis results show that the maximum floor heave of the roadway increases  with the increase of the coal seam depth, and decreases with the increase of the elastic modulus  of the floor rock. The development depth of the plastic zone in the roadway floor decreases with 

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(2)

(3)

(4)

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and thus to obtain the distribution law of the principal stress field and plastic zone of the floor, providing an effective means for directly analyzing the floor heave mechanism of GER. The parametric analysis results show that the maximum floor heave of the roadway increases with the increase of the coal seam depth, and decreases with the increase of the elastic modulus of the floor rock. The development depth of the plastic zone in the roadway floor decreases with the increase of the c and φ value of the floor rock and the support resistance of the RSB, and increases with the increase of the stress concentration factor of the solid coal side. The distribution results of the stress field and the plastic zone of the floor in the gob-backfilled GER of the test mine show that the development depth of the plastic zone in the test mine is 2.68 m. The horizontal stress of the floor in the roadway is the external factor directly causing floor heave in the roadway. Other external factors affect the failure characteristics of roadway floor by changing the horizontal stress of the floor in the roadway. Based on the theoretical analysis results, floor heave can be effectively controlled by improving the mechanical properties of floor strata, adjusting the principal stress of the floor and optimizing the upper load of the floor. The field test and monitoring results show that the comprehensive control scheme of adjusting backfilling pressure, deep grouting reinforcement, shallow opening stress relief slots, and surface pouring can effectively control the floor heave of the gob-backfilled GER. The roadway section meets the design and application requirements when the deformation stabilizes. The research results overcome the technical bottleneck of floor heave control in gob-backfilled GER, providing a reliable basis for the design of the floor heave control scheme.

Acknowledgments: This work is supported by the National Natural Science Foundation of China (Nos. 51323004, 51674250, 51074163, 51574228); Major Program of National Natural Science Foundation of China (No. 50834005, 51734009); the Graduate Innovation Fund Project of Jiangsu Province (No. CXZZ13_0924); Open Fund of State Key Laboratory for Geomechanics and Deep Underground Engineering (SKLGDUEK1409). Author Contributions: All the authors contributed to this work. Peng Gong and Zhanguo Ma conceived and established the theoretical model; Xiaoyan Ni and Peng Gong deduced the formula; and Ray Ruichong Zhang and Xiaoyan Ni analyzed the data; Peng Gong wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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