Advances in Civil Engineering

Volume 2018, Article ID 2356390, 12 pages

https://doi.org/10.1155/2018/2356390

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

^{1}School of Energy and Safety, Anhui University of Science and Technology, Huainan 232001, China^{2}State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China

Correspondence should be addressed to Jian Sun; nc.ude.tmuc@323js

Received 4 May 2018; Accepted 10 July 2018; Published 1 August 2018

Academic Editor: Hugo C. Biscaia

Copyright © 2018 Jian Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Secondary development of FLAC^{3D} software was carried out based on FISH language, and a 3D fluid-solid coupling numerical calculation model was established for an inclined seam mining above a confined aquifer in Taoyuan Coal Mine. 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 seam workface advanced to about 80 m, the depth of floor plastic failure zone reached the maximum at approximately 15.0 m, 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. The conducting height of the confined water reached the maximum at about 11.0 m when the workface was pushed forward from an open-off cut at about 80 m. 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 80 m. 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 “soft-hard-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.

#### 1. 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 mining-induced stress and confined water pressure [1–3]. 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. The 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]. Therefore, water inrush from the seam floor is the product of the joint coupling effect of mining-induced stress and confined water pressure. The 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 fluid-solid 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. The 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. The seepage force and effective mining stress would interact with each other to realize the fluid-solid coupling effect. Therefore, 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 fluid-solid coupling theory of water inrush from a confined karst cave and the strength reduction idea of an inrush-preventive rock column. They 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 [13–16]. 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 load-bearing 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 [7–12]. Research on relevant problems such as fluid-solid 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. The 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. The 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.

#### 2. Fluid-Solid Coupling Model of an Inclined Coal Seam Floor

##### 2.1. Engineering Background

Strike length and inclined length of the 1066 inclined coal seam workface in Taoyuan Coal Mine of Huaibei Mining Group Co. Ltd. in China are 790 and 112 m, respectively. Burial depth of coal seam at the track trough is 500 m, average thickness of the coal seam is 3.4 m, and dip angle is 28°. The roof strata of the coal seam are mainly fine sandstone, siltstone, and medium sandstone. Taiyuan Formation limestone aquifer is approximately 53 m away below the 1066 inclined coal seam workface floor. Its water pressure reaches as high as 3.0 MPa, and floor water inrush danger exists during the mining process of the 1066 workface. Therefore, 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.

##### 2.2. 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. The change of seepage field distribution would also cause a change of water pressure load. Therefore, 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 finally affect seepage field in the strata fracture. In the meantime, the seepage field affects stress field distribution form by influencing the strata volumetric strain. This effect is called the interaction mechanism between stress field and seepage field in the strata.

An equivalent continuous medium model is adopted during simulation of the strata fluid-solid coupling using FLAC^{3D} software. Strata are regarded as porous media; fluid flow in strata porous media is in accordance with Darcy laws and meets the Biot fluid-solid coupling equation [8], as follows:where and are the Lame constants; is the volumetric strain; is the pore water pressure; , , and are the coordinate, displacement, and volume forces in direction, respectively; is the permeability coefficient; and is the storage coefficient. The term of reflects the influence of seepage field on the solid frame, and its essence is that pore pressure generated during fluid flow affects the effective stress and deformation of the solid frame. The term of reflects the influence of volumetric deformation of solid frame on the seepage field. The above equation can reflect the interaction between pore pressure dissipation and solid frame deformation.

The main reason for coal seam floor water inrush is the influence of mining on the floor strata, which in turn causes the destroyed fracture in and enhances the permeability of the floor 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 field. If the permeability coefficient of the rock mass is still considered a fixed value during numerical simulation of fluid-solid coupling using FLAC, this finding does not accord with the engineering reality.

To reflect the change of medium permeability with the medium stress field, the relationship between permeability coefficient and strain as proposed by Elsworth and Mao [17] is selected in this paper as the control equation of permeability coefficient during the numerical simulation of fluid-solid coupling in rock mass media:where is the initial permeability coefficient of rock mass media, is the increment of volumetric strain of rock mass media, and 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 floor strata permeability with rock mass deformation during coal seam mining can be realized to achieve the fluid-solid coupling simulation of water inrush from coal seam floor.

##### 2.3. 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 fluid-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. The 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. The 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 floor strata presents linear growth along the inclined direction of the coal seam. The water pressure of the floor confined 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. The model undersurface confined displacement in the vertical direction, while the front, back, left, and right surfaces confined displacement in the horizontal direction. The 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.