Experimental Investigation on Hydraulic Properties of Granular Sandstone and Mudstone Mixtures

Abstract

The caved zone during longwall mining has high permeability, resulting in a mass of groundwater storage which causes a threat of groundwater inrush hazard to the safe mining. To investigate the hazard mechanism of granular sandstone and mudstone mixture (SMM) in caved zone, this paper presents an experimental study on the effect of sandstone particle (SP) and mudstone particle (MP) weight ratio on the non-Darcy hydraulic properties evolution. A self-designed granular rock seepage experimental equipment has been applied to conduct the experiments. The variation of particle size distribution was induced by loading and water seepage during the test, which indicated that the particle crushing and erosion properties of mudstone were higher than those of sandstone. Porosity evolution of SMM was strongly influenced by loading (sample height) and SP/MP weight ratio. The sample with higher sample height and higher weight ratio of SP achieved higher porosity value. In particular, a non-Darcy equation, for hydraulic properties (permeability and non-Darcy coefficient ) calculation, was sufficient to fit the relation between the hydraulic gradient and seepage velocity. The test results indicated that, due to the absence and narrowing of fracture and void during loading, the permeability decreases and the non-Darcy coefficient increases. The variation of the hydraulic properties of the sample within the same particle size and SP/MP weight ratio indicated that groundwater inrush hazard showed a higher probability of occurrence in sandstone strata and crushed zone (e.g., faults). Moreover, isolated fractures and voids were able to achieve the changeover from self-extension to interconnection at the last loading stage, which caused the fluctuation tendency of and . Fluctuation ability in mudstone was higher than that in sandstone. The performance of an empirical model was also investigated for the non-Darcy hydraulic properties evolution prediction of crushing and seepage processes. The predictive results indicated that particle crushing and water erosion caused the increase of hydraulic properties, being the main reason that the experimental values are typically higher than those obtained from the predictive model. The empirical model has a high degree of predictive accuracy; however, has a higher predictive accuracy than . Furthermore, the predictive accuracy of increases and decreases with increasing weight ratio of SP.

An Elastoplastic Softening Damage Model for Hydraulic Fracturing in Soft Coal Seams

“…Hence, conventional methods cannot effectively drain gas from those seams [1]. e parameters of permeability and porosity play an important role in the study of accumulation and development of coalbed methane [2][3][4]. To increase the permeability of those coal seams, underground hydraulic fracturing technology to transform the seam structure is an effective way to achieve this goal [5][6][7][8][9][10][11].…”

Bending Moment Characteristics of Hard Roof before First Breaking of Roof Beam Considering Coal Seam Hardening

“…By comparing (13) and (16) The left-hand term of (82) is the counterforce of coal seam in the elastic zone on the left side of = 4 that is applied on the rock beam, while the right-hand term is the counterforce of coal seam in the hardened zone on the right side of = 4 that is applied on the rock beam. Equation (82) clarifies the issue in the calculation that the continuity of the counterforce of the coal seam at the junction between the elastic and hardened zones on the rock beam after the introduction of the hardened zone of coal seam.…”

“…For instance, as the hanging roof distance increases, the overall load above the coal seam increases, and the maximum support pressure increases (Figure 2), but the residual strength of coal seam at coal wall m (i.e., the depth of softened zone remains constant, − 3 =8 m), c3 =8.6, 8.3, 8 m, c4 =2 m, the elastic modulus of roof is =25 GPa, the elastic foundation constant =0.4 GPa, =0.9 MN/m, and = 1.2 . According to (13), (15), and (16) and the method introduced in Section 5, after a couple of trial calculations, the maximum support pressure of coal seam was found to be accurate to the sixth decimal place: Equations (84) and (85) show that as the hanging roof distance of goaf increases, the maximum support pressure of coal seam c3 increases, and the residual strength of coal seam at coal wall 3 ( ) decreases. When =24 m, the residual strength of coal seam 3 ( ) ≈0.…”

“…For a hard roof with advanced fracture, when the working face advances to below the roof fracture line, if the supporting resistance of frame and friction between seams are insufficient, the roof easily subsides in a stepped style, causing collapse accidents under pressure and even rock burst disasters. Many studies have been conducted on the deformation, law of motion, and fracture of a hard roof under the overlying load by field observation, similar material simulation, numerical simulation, and theoretical analysis [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Qian et al [15,16], Miao et al [17], and Li et al [18] analyzed a hard roof by assuming the support pressure relationship between coal seam and immediate roof as an elastic foundation and obtained some basic results such as the deflection solution for a hard roof and the maximum bending moment in the front of a coal wall and for an advanced roof fracture.…”

Research on Seepage Properties and Pore Structure of the Roof and Floor Strata in Confined Water‐Rich Coal Seams: Taking the Xiaojihan Coal Mine as an Example

“…Many scholars have researched from different aspects to study the permeability and pore structure characteristics of the roof and floor strata in coal seams. Based on the nonlinear theory and stochastic differential equation, Miao et al [8][9][10][11][12][13][14] have studied permeability characteristics of cracked rock mass.…”