Failure Characteristics and Confined Permeability of an Inclined Coal Seam Floor in Fluid-Solid Coupling

Secondary development of FLAC software was carried out based on FISH language, and a 3D fluid-solid coupling numerical calculationmodel was established for an inclined seammining above a confined aquifer in Taoyuan CoalMine. A simulation study was implemented on the mining failure depth of an inclined coal seam floor, conducting height of confined water, and the position of workface floor with easy water inrush during advancement of workface. Results indicated that, during the advancement of the inclined coal seam’s workface, obvious equivalent stress concentration areas existed in the floor strata, and the largest equivalent stress concentration area was located at the low region of workface floor.When the inclined coal seamworkface advanced to about 80m, the depth of floor plastic failure zone reached the maximum at approximately 15.0m, and the maximum failure depth was located at the low region of the workface floor. Before the inclined workface mining, original confined water conducting existed on the top interface of the confined aquifer.)e conducting height of the confined water reached the maximum at about 11.0m when the workface was pushed forward from an open-off cut at about 80m. Owing to the barrier effect of the “soft-hard-soft” compound water-resistant strata of the workface floor, pore water pressure and its seepage velocity in the floor strata were unchanged after the workface advanced to about 80m. After the strata parameters at the workface floor were changed, pore water pressure of the confined water could pass through the lower region of the inclined workface floor strata and break through the barrier of the “softhard-soft” compound water-resistant strata of the workface floor and into the mining workface, resulting in the inclined coal seam floor water inrush. Results of this study can provide a basis for predicting, preventing, and governing the inclined coal seam floor water inrush above confined aquifer.


Introduction
Floor water inrush severely threatens the safety production of coal mines.It is a phenomenon in which confined water floods into an excavation space after deformation and failure of the floor strata under the joint effects of mininginduced stress and confined water pressure [1][2][3].With the increased coal mining depth and magnified mining intensity, the workface floor is increasingly and seriously threatened by Ordovician karst water, highlighting problems related to the prediction, prevention, and treatment of water inrush [4,5].Before coal seam mining, floor confined water along upside preexisting fractures of mudstone or sandstone intrude to a certain height to form an original conducting zone of floor confined water.After coal seam mining, the stress field and seepage field of floor strata change and form a floor mining failure zone.e intruding height of confined water presents upward conducting to form a progressive conducting zone of floor confined water.Water inrush from the seam floor may occur when the progressive conducting zone connects with the floor mining failure zone [6].erefore, water inrush from the seam floor is the product of the joint coupling effect of mining-induced stress and confined water pressure.e mechanism of water inrush from the seam floor can be revealed well only from the angle of fluid-solid coupling to predict water inrush from coal seam floor.
Numerous scholars have conducted relevant studies on water inrush from the coal seam floor based on the fluidsolid coupling mechanism.Fenghua and Yuanjiang [7] used the fluid-solid coupling module in FLAC 3D numerical simulation software to simulate floor plastic zone and floor stress change during coal seam mining as well as water flow vector distribution during dynamic mining.Duoxi and Haifeng [8] conducted a numerical simulation study on the mining-seepage-strain mechanism of workface floor rock mass using the 3D rapid Lagrange fluid-solid coupling analysis module by changing the permeability coefficients.Wei and Dejin [9] implemented the finite element strength reduction method in FLAC 3D and applied it to the simulation of water inrush from a coal seam floor.Wenmin et al. [10] used the powerful fluid-solid coupling function of FLAC 3D to establish the numerical analysis model of the workface above a confined aquifer and conduct a simulation analysis of strata displacement, stress distribution laws, and confined water-conducting height in a floor water-resisting layer.
However, the abovementioned fluid-solid coupling is only realized by assigning a fixed permeability coefficient to each stratum in the model.e permeability of surrounding rock during the coal seam mining process is not changed, and only the pore water pressure varies with mining-induced stress.During actual coal seam mining, the permeability of surrounding rock in the stope continuously changes and the changed fluid seepage force would result in a change of the effective mining stress.
e seepage force and effective mining stress would interact with each other to realize the fluid-solid coupling effect.erefore, Xiaorong et al. [11] conducted a simulation study on floor mining stress and surrounding rock permeability in three combined characteristics of floor strata under a fluid-solid coupling condition.Yanlin et al. [12] established an analysis method combining fluid-solid coupling and strength reduction of water inrush from the front roadway on the basis of fluidsolid coupling theory of water inrush from a confined karst cave and the strength reduction idea of an inrush-preventive rock column.ey then discussed the fluid-solid coupling effect and safety stock of the inrush-preventive rock columns.
China has various coal seam occurrence conditions and a complicated hydrogeology with considerable changes in coal seam dip angle.It has inclined coal seams with large dip angles as well as nearly flat seams with small dip angles [13][14][15][16].Water pressure borne by floor strata from an underlying confined aquifer is no longer uniformly distributed water pressure but has a certain water pressure gradient along the dip direction for an inclined coal seam.In the meantime, owing to the asymmetric characteristics of loadbearing state of the inclined stope surrounding rocks, the failure characteristics of the inclined workface floor strata are different from those of the nearly flat seams with small dip angles [13].However, the above study results are mostly based on the engineering background for flat and nearly flat seams [7][8][9][10][11][12].Research on relevant problems such as fluidsolid coupling failure characteristics and confined permeability characteristics of an inclined coal seam floor above a confined aquifer should be carried out further.
In the current study, the inclined coal seam mining above a confined aquifer in China Taoyuan Coal Mine was taken as the engineering background, and the secondary development of FLAC 3D software was implemented based on FISH language.A 3D fluid-solid coupling numerical model for an inclined coal seam mining above a confined aquifer was established.
e synchronous influence of floor seepage force and mining stress was realized during the mining process of the inclined coal seam.Mining failure depth, confined water-conducting height, and position of workface floor with easy water inrush were studied and analyzed.e zones with water inrush risks were divided at the workface floor to provide a basis for predicting, preventing, and governing the water inrush from the inclined coal seam floor above a confined aquifer.

Fluid-Solid Coupling Model of an Inclined
Coal Seam Floor erefore, the mining failure depth of the 1066 inclined coal seam workface floor, confined water-conducting height, and position of the workface floor with easy water inrush, and partition regions with water inrush dangers from inclined coal seam floor must be determined through a numerical simulation method.Consequently, water inrush from the 1066 inclined coal seam floor can be predicted, prevented, and treated.

Fluid-Solid Coupling Equation.
In current studies on deformation failure and water inrush mechanism from the coal seam floor above a confined aquifer, the influence of seepage field caused by the floor confined water load on stress field of the floor strata is generally neglected.Floor confined water load is expressed in the forms of hydrostatic pressure and pumping pressure.A specific water pressure load distribution form corresponds to a specific seepage field distribution form in any permeable medium.
e change of seepage field distribution would also cause a change of water pressure load.erefore, the influence of seepage field on stress field is realized by changing the volumetric strain of the coal seam floor strata.On the other hand, the change of pore pressure of coal seam floor strata would cause a change of effective stress, which would significantly change the fracture opening, flow velocity, and distribution form of fluid pressure in the fracture.Stress field of the strata affects the permeability coefficient of strata fracture by influencing strata volumetric strain to 2 Advances in Civil Engineering nally a ect seepage eld in the strata fracture.In the meantime, the seepage eld a ects stress eld distribution form by in uencing the strata volumetric strain.is e ect is called the interaction mechanism between stress eld and seepage eld in the strata.
An equivalent continuous medium model is adopted during simulation of the strata uid-solid coupling using FLAC 3D software.Strata are regarded as porous media; uid ow in strata porous media is in accordance with Darcy laws and meets the Biot uid-solid coupling equation [8], as follows: where G and λ are the Lame constants; ε v is the volumetric strain; p is the pore water pressure; x j , u j , and f xj are the coordinate, displacement, and volume forces in j direction, respectively; K is the permeability coe cient; and S is the storage coe cient.e term of zp/zx j re ects the in uence of seepage eld on the solid frame, and its essence is that pore pressure generated during uid ow a ects the e ective stress and deformation of the solid frame.e term of zε v /zt re ects the in uence of volumetric deformation of solid frame on the seepage eld.e above equation can re ect the interaction between pore pressure dissipation and solid frame deformation.
e main reason for coal seam oor water inrush is the in uence of mining on the oor strata, which in turn causes the destroyed fracture in and enhances the permeability of the oor strata.Existing studies indicate that the permeability of strata under stress is not a constant, but instead continuously changes with the development of fracture during the rock mass stress-strain process.However, the medium permeability in (1) is a constant quantity, which does not vary with the medium stress eld.If the permeability coe cient of the rock mass is still considered a xed value during numerical simulation of uid-solid coupling using FLAC, this nding does not accord with the engineering reality.
To re ect the change of medium permeability with the medium stress eld, the relationship between permeability coe cient and strain as proposed by Elsworth and Mao [17] is selected in this paper as the control equation of permeability coe cient during the numerical simulation of uid-solid coupling in rock mass media: where k 0 is the initial permeability coe cient of rock mass media, Δε is the increment of volumetric strain of rock mass media, and n is the porosity of rock mass media.Equation ( 3) is obtained by substituting ( 2) into (1): After ( 3) is embedded into FLAC 3D software through programming procedures using FISH language, a synchronous change of oor strata permeability with rock mass deformation during coal seam mining can be realized to achieve the uid-solid coupling simulation of water inrush from coal seam oor.

Fluid-Solid Coupling Model.
According to the strike longwall mining characteristic of the 1066 inclined coal seam workface in Taoyuan Coal Mine, the secondary development of FLAC 3D software was implemented using FISH language, and a 3D uid-solid coupling numerical calculation model for an inclined coal seam was established with strike longwall mining, as shown in Figure 1.In the model, x is the inclined direction of the workface, y is the advanced direction of the workface, and the advanced direction is shown by a red arrow.e horizontal width of coal columns at two sides of the workface is 40 m, and the strike length of the workface (y direction) is 180 m. e mode is excavated step by step, from y 40 m to y 140 m.Each step is 20 m, the full height is excavated, and the total number of excavated steps is 5. Water pressure acting on the inclined coal seam oor strata presents linear growth along the inclined direction of the coal seam.e water pressure of the oor con ned aquifer at the upper side of the workface is 3.0 MPa, and that at the lower side of the workface is 3.82 MPa.
e model undersurface con ned displacement in the vertical direction, while the front, back, left, and right surfaces con ned displacement in the horizontal direction.e upper surface in the model is free surface, and the overlying strata load, except for the coal seam roof, is loaded to the upper surface of the model in the form of uniformly distributed loads.
With multidrilling geological data and rock mechanics testing of an inclined coal seam floor, the thickness, strength, and location of each of coal seam floor strata, including the water pressure distribution of the confined aquifer, can be determined.Furthermore, the lithology, thickness, density, bulk modulus, shear modulus, tensile strength, cohesion, internal friction angle, permeability coefficient, porosity, and other parameters of each of floor strata of the 1066 inclined coal seam workface can be determined.And Table 1 shows the physical and mechanical parameters of the roof and floor strata of the 1066 inclined coal seam workface in the model (Figure 1).

Fluid-Solid Coupling Failure
Characteristics of an Inclined Coal Seam Floor erefore, pressure relief range of the floor strata was continuously expanded with the advancement of the workface, and it remained unchanged when the workface advanced to about 80 m.
is result is due to the stress concentration generated near the open-off cut and in front of the workface in the initial advancement stage of the workface.e stress concentration degrees at the two positions are approximately thrice those of primary rock stress, and they exerted a joint effect on the workface floor strata, resulting in the continuously expanded pressure relief range of the floor strata.eir joint effect tended to be mild on the workface floor strata until the workface advanced to about 80 m, and then the influence on the floor aquifer remained unchanged.

Equivalent Stress of Floor along the Inclined Direction of
Coal Seam. Figure 3 shows the equivalent stress (Mises stress) of workface floor along the inclined direction of coal seam with the advancement of workface.Under the joint effect of mining stress and pore water pressure on workface floor strata, apparent stress concentration zones existed in the floor strata (two sides of workface floor and goaf floor).With the continuous advancement of workface, stress concentration degree and range were Note.e unit of permeability coefficient in FLAC 3D software is different from that in hydraulics, and their conversion relation is 4 Advances in Civil Engineering continuously expanded.e stress concentration degree and range reached the maximum when the workface advanced to about 80 m. e maximum equivalent stress at the lower side of workface oor was approximately 20.0 MPa. e oor stress concentration degree and range remained unchanged with the continuous advancement of workface.
In addition, when the workface advanced to about 80 m, the stress concentration degree and range reached the maximum at the lower side of the workface oor.e stress concentration degree was even greater than that in the goaf oor, and the action range also a ected the oor aquifer.Hence, the oor region at the lower side of the workface was  Advances in Civil Engineering the region with water inrush risk from the inclined coal seam oor.

Plastic Failure Zone of Floor along the Inclined Direction of
Coal Seam. Figure 4 shows the plastic failure zone of workface oor along the inclined direction of coal seam.Under the joint e ect of mining stress and pore water pressure on the oor strata, plastic failure zones were generated in the workface oor and the top surface of the con ned aquifer.e plastic failure zone in the workface oor continuously expanded with the advancement of the workface, as shown in Figure 5.When the workface advanced to about 80 m, the depth of plastic failure zone reached the maximum, and depth was about 15.0 m at the maximum plastic failure.e depth and range of the plastic failure zone of the workface oor remained unchanged with the continuous advancement of the workface.Moreover, the depth of plastic failure zone in the lower region of the workface oor was the greatest, and the zone was close to the oor aquifer.erefore, the lower region of the workface oor was the region with water inrush risk from the inclined coal seam oor.Advances in Civil Engineering

Confined Permeability Characteristics of an Inclined Coal Seam Floor
Figure 6 shows the pore water pressure and seepage vector distribution of the workface oor along the inclined direction of the coal seam with the advancement of the workface.Before mining of the workface, the original conducting zone existed at the top surface of the con ned aquifer, and the conducting height was about 7 m.e original conducting zone terminated inside the thirteenthlayer siltstone above the aquifer, as shown in number 13 in Table 1.Under the joint e ect of mining pressure and aquifer pressure, the con ned water-conducting height continuously increased with the continuous advancement of the workface, as shown in Figure 7. e con ned waterconducting height reached the maximum value of 11.0 m when the workface advanced to about 80 m.It terminated inside the eleventh-layer sand mudstone (as shown number 0.0000e + 000 to 5.0000e + 005 5.0000e + 005 to 1.0000e + 006 1.0000e + 006 to 1.5000e + 006 1.5000e + 006 to 2.0000e + 006 2.0000e + 006 to 2.5000e + 006 2.5000e + 006 to 3.0000e + 006 3.0000e + 006 to 3.5000e + 006   Advances in Civil Engineering 11 in Table 1) above the aquifer and then remained unchanged with the advancement of the workface.Pressure relief range of the oor also continuously expanded with the continuous advancement of the workface, which resulted in the continuous change of pore water pressure and seepage velocity in the workface oor strata.When the workface advanced to about 60 m, pore water pressure and seepage velocity in the workface oor strata changed.When the workface advanced to about 80 m, pore water pressure and seepage velocity in the workface oor strata clearly changed and then remained unchanged with the continuous advancement of the workface.
e oor mining failure depth and con ned waterconducting height continuously increased with the advancement of the workface and reached the maximum value when the workface advanced to about 80 m.However, the barrier e ect of the "soft-hard-soft" compound water-resistant strata composed by sand mudstone, medium sandstone, and sand mudstone of the workface oor, as shown numbers 9, 10, and 11 in Table 1, which existed between the oor mining failure zone and the con ned water-conducting zone in the 1066 workface oor, could bu er the oor mining failure depth, obstruct the further progressive conducting of the con ned water, and inhibit the occurrence of water inrush from the workface oor [18][19][20].
erefore, when the workface advanced to about 80 m, if the con ned water did not break through the "soft-hard-soft" compound waterresistant strata in the 1066 inclined workface oor, then the compound water-resistant strata would always maintain a favorable water-resisting performance, and it could e ectively obstruct water inrush from the inclined coal seam oor [21,22].

Position of an Inclined Coal Seam Floor with Easy Water Inrush
To determine the position of the 1066 inclined coal seam workface oor with easy water inrush, the physical and mechanical parameters of the 1066 inclined coal seam workface oor strata were changed, especially the strata parameters of the "soft-hard-soft" compound waterresistant strata (composed by sand mudstone, medium sandstone, and sand mudstone) of the workface oor between oor mining failure zone and con ned waterconducting zone.Moreover, the intensity of the oor strata was reduced, and the permeability was strengthened.
Table 2 shows the physical and mechanical parameters of the  10 Advances in Civil Engineering 1066 inclined coal seam workface oor strata obtained by the same method as above after changing from number 6 to number 15.
Figure 8 shows the pore water pressure and seepage vector distribution of workface oor along the inclined direction of the coal seam with the advancement of the workface after the strata parameters of the 1066 workface oor were changed.As shown in Figure 8, the original conducting zone existed at the top surface of the con ned aquifer before workface mining, and the conducting height was about 7 m, which was equivalent to the original con ned water-conducting zone before the strata parameters of the 1066 workface oor were changed.Under the joint e ect of mining pressure and aquifer pressure, the con ned waterconducting height continuously increased with the continuous advancement of the workface, as shown in Figure 9.When the workface advanced to about 80 m, the con ned water-conducting height reached about 16.0 m, which increased by 5.0 m compared with that before the strata parameters of the 1066 workface oor were changed.
Pressure relief range of the inclined workface oor also continuously expanded with the advancement of the workface, which gave rise to the continuous change of pore water pressure and seepage velocity in the workface oor strata.e change was faster and more apparent compared with that before the strata parameters of the workface oor were changed.When the workface advanced to about 40 m, pore water pressure and seepage velocity in the workface oor strata clearly changed.When the workface advanced to about 60 m, seepage velocity at the lower side of the workface oor strata was clearly higher than those in other positions, indicating that the fracture which developed at the lower side of the workface oor strata was higher and had strong permeability.When the workface continuously advanced to about 80 m, seepage velocity at the lower side of the workface oor strata further increased, and the con ned pore water pressure broke through the "soft-hard-soft" compound water-resistant strata of the 1066 inclined workface oor, as shown numbers 9, 10, and 11 in Table 2.As a result, the con ned water-conducting zone and oor mining failure broke through and water inrush might occur at the lower region of the workface oor.When the workface continuously advanced to about 100 m, the con ned waterconducting zone and oor mining failure broke through, and water inrush occurred at the lower region of the workface oor.erefore, the lower region of the workface oor is the position with the highest water inrush risk for inclined workface oor strata without a structural defect.

Conclusion
Secondary development of FLAC 3D software was carried out based on FISH language, and a 3D uid-solid coupling numerical simulation model for inclined coal seam mining above a con ned aquifer in Taoyuan Coal Mine was established.A simulation study was implemented on mining failure depth of the inclined coal seam oor, conducting height of the con ned water, and position of the workface oor with easy water inrush during advancement of the workface.e main conclusions obtained are as follows: (1) Pressure relief range of oor strata continuously expanded with the advancement of the inclined workface, and it remained unchanged until the workface advanced to over 80 m.Obvious, equivalent stress concentration zones existed in oor strata during the advancement process of the workface.e concentration degree reached the maximum when the workface advanced to about 80 m, and the largest equivalent stress concentration zone was located at the lower side of the workface oor, so that the water inrush danger at the lower region of the workface oor was greater.(2) Plastic failure zone of the workface oor continuously expanded with the advancement of the inclined workface.e depth of plastic failure zone reached the maximum value of 15.0 m when the workface advanced to 80 m. e maximum plastic failure depth was at the lower region of the workface oor, which was close to the aquifer.erefore, water inrush danger was greater at the lower region of the workface oor.
(3) Original con ned water conducting existed at the upper part of the con ned aquifer before mining of the inclined coal seam workface, and the conducting height was about 7 m.e con ned waterconducting height continuously increased with the advancement of the workface and reached the maximum conducting height of 11.0 m when the workface advanced to about 80 m. (4) Pore water pressure and seepage velocity at the workface oor strata continuously changed with the advancement of the inclined workface.However, pore water pressure and seepage velocity remained unchanged after the workface advanced to over 80 m owing to the barrier e ect of "soft-hard-soft" Advances in Civil Engineering compound water-resistant strata of the workface floor.After the strata parameters of the workface floor were changed, the confined pore water pressure could pass the lower region of the workface floor strata and rush into the mining workface after breaking the obstruction of the "soft-hard-soft" compound water-resistant strata of the workface floor, which gave rise to water inrush from the inclined coal seam floor.

Figure 1 :
Figure 1: 3D uid-solid coupling numerical calculation model for an inclined coal seam with strike longwall mining.

3. 1 .
Vertical Stress of Floor along the Inclined Direction of Coal Seam. Figure 2 shows the vertical stress of workface floor along the inclined direction of coal seam with the advancement of workface.e pressure relief range of the floor strata continuously expanded with the advancement of the inclined coal seam workface.Pressure relief range of the workface floor started influencing the floor aquifer when the workface advanced to about 40 m. e effect exerted by pressure relief range of the workface floor on the floor aquifer was enhanced when the workface advanced to about 60 m.When the workface advanced to about 80 m, the influence was enhanced further.With continuous advancement, the influence remained unchanged.

FIGURE 2 :
FIGURE 2: Vertical stress of workface oor along the inclined direction of the coal seam with the advancement of the workface.(a) Original vertical stress.(b) Advanced 20 m of workface.(c) Advanced 40 m of workface.(d) Advanced 60 m of workface.(e) Advanced 80 m of workface.(f ) Advanced 100 m of workface.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Equivalent stress of workface oor along the inclined direction of the coal seam with the advancement of the workface.(a) Original equivalent stress.(b) Advanced 20 m of workface.(c) Advanced 40 m of workface.(d) Advanced 60 m of workface.(e) Advanced 80 m of workface.(f ) Advanced 100 m of workface.

Figure 6 :
Figure 6: Pore water pressure and seepage vector distribution of workface oor along the inclined direction of the coal seam with the advancement of the workface (no water inrush).(a) Original pore pressure.(b) Advanced 20 m of workface.(c) Advanced 40 m of workface.(d) Advanced 60 m of workface.(e) Advanced 80 m of workface.(f ) Advanced 100 m of workface.

Figure 7 :
Figure 7: Maximal conducting height of con ned water of workface oor along the inclined direction of coal seam with the advancement of workface (no water inrush).

Figure 8 :
Figure 8: Pore water pressure and seepage vector distribution of workface oor along the inclined direction of the coal seam with the advancement of the workface (water inrush).(a) Original pore pressure.(b) Advanced 20 m of workface.(c) Advanced 40 m of workface.(d) Advanced 60 m of workface.(e) Advanced 80 m of workface.(f ) Advanced 100 m of workface.

Figure 9 :
Figure 9: Maximal conducting height of con ned water of workface oor along the inclined direction of coal seam with the advancement of workface (water inrush).

Table 1 :
Physical and mechanical parameters of the roof and floor strata of the 1066 inclined coal seam workface.

Table 2 :
Physical and mechanical parameters of the 1066 inclined coal seam workface oor strata after changing.