Deformation Behavior and Damage-Induced Permeability Evolution of Sandy Mudstone Under Triaxial Stress

The permeability and mechanical behavior in sandy mudstone are crucial to the hazard prevention and safety mining. In this study, to investigate the evolution and characteristic of permeability and mechanical properties of mudstone during the in-site loading process, triaxial compression-seepage experiments were performed. The increase of permeability and decrease of mechanical strength gradually evaluated to the decrease of permeability and increase of mechanical strength subjected to the increase of confining stress from 5 to 15 MPa, which corresponds to the transformation from brittleness to ductility of mudstone, and the transformation threshold of 10 MPa confining stress was determined. The shear fractures across the sample at brittle regime, while shear fracture does not cross the sample or even be not generated at semibrittle and ductile state. The dynamic decrease, slight decrease, and residual response were determined in axial strain, and the divided zone increases with the increase of confining stress. The relatively higher permeability corresponds to the higher pore pressure as the increase of confining stress. The volumetric strain increases as the increase of confining stress, compared to that decrease correspond to the increase of the pore pressure, and the higher volumetric strain and the lower permeability. In addition, an improved permeability model was developed to describe the loading-based permeability behavior considering the Klinkenberg effect.

Characterization and Modeling Study on Softening and Seepage Behavior of Weakly Cemented Sandy Mudstone after Water Injection

Water injection-induced rock softening and the associated water seepage characteristics are the common and basic problems in underground reservoir construction and the prevention of mine water disaster. In this paper, a series of experimental studies was carried out to investigate these characteristics with the weakly cemented sandy mudstone collected from Shendong Buertai coal mine, China. The characteristics of water softening and the stress-seepage interactions in water-saturated weakly cemented sandy mudstone were directly obtained. Then, a modification method of the constitutive model for rock mass considering the softening effect and a stress-damage-driven model for permeability evolution were established. Research results show that water saturation reduces the tensile strength, compressive strength, and cohesion by 56% and reduces the elastic modulus by 28%. The hydraulic effects on Poisson’s ratio and internal friction angle are negligible. The relationship between the permeability of weakly cemented sandy mudstone with complete compaction deformation is to be divided into three stages of seepage shielding, seepage surge, and seepage recovery. Rock permeability in each stage has a negative exponential relationship with the effective stress. This research provides a theoretical basis for the researches of hydromechanical couplings on weakly cemented sandy mudstone, which is insightful for rock engineering practice.

Characterization and Modeling Study on Softening and Seepage Behavior of Weakly Cemented Sandy Mudstone After Water Injection

Water injection induced rock softening and the associated water seepage characteristics are the common and basic problems in hard roof pressure relief, underground reservoir construction and the prevention of mine water disaster. In this paper, a series of laboratory studies was carried out to investigate these characteristics with the weakly cemented sandy mudstone collected from Shendong Buertai coal mine, China. The characteristics of water softening and the stress-seepage interactions in saturated weekly cemented sandy mudstone were directly obtained. Then a modification method of the constitutive model for rock mass considering the softening effect and a stress-damage-driven model for permeability evolution were established. Research results show that water saturation reduces the tensile strength, compressive strength and cohesion by 56%, and reduces the elastic modulus by 28%. The hydraulic effect on Poisson’s ratio and internal friction angle is negligible. The relationship between the permeability of weakly cemented sandy mudstone with complete compaction deformation is to be divided into three stages of seepage shielding, seepage surge and seepage recovery. Rock permeability in each stage has a negative exponential relationship with the effective stress. This research provides a theoretical basis for the researches of hydro-mechanical couplings on weakly cemented sandy mudstone, which is insightful for rock engineering practice.

Investigation on the Creep Failure Mechanism of Sandy Mudstone Based on Micromesoscopic Mechanics

In this paper, scanning electron microscope (SEM) tests and 3D scanning technologies were adopted to investigate the creep failure mechanism of sandy mudstone from a micromesoscopic view. The SEM test results showed that the fracture surface micromorphology of the specimens that suffered creep loading was more fractured and rougher. It was also found by the fractal analysis of the SEM microscopic images that the fractal dimensions of the creep failure specimens were larger than those of the uniaxial compression failure, indicating that the creep damage increased the irregularity and a larger degree of roughness fluctuation. The 3D scanning technologies combining with the 3D reconstruction methods proved that the crack expansion path of crept specimens was more complicated, showing a more prominent asperity height and slope angle. Finally, a mesostrain dilating microelement model of the sandy mudstone specimen’s fracture surface was proposed to prove that the dilatancy effect would be more pronounced in the creep process.

Stability Analysis and Reasonable Layout of Floor Drainage Roadway above Confined Water and under Mining Influence

The surrounding rock failure range of floor drainage roadway under the mining influence and its effect on the aquiclude are the key to determine the aquiclude thickness of the floor. This paper studied the distribution characteristics of the surrounding rock plastic zone by the numerical simulation when the floor drainage roadway was located at different positions under the working face and determined the rational position. Results show that (1) when the floor drainage roadway is staggered inward, the floor surrounding rock is prone to appear the butterfly plastic zone under single work face mining. And the butterfly plastic zone increases sharply after being affected by secondary mining of adjacent working face. (2) When the floor drainage roadway is staggered outward, the floor surrounding rock plastic zone extends gently affected by a single working face. And the depth of the plastic zone has no obvious change after being affected by secondary mining of adjacent working face. (3) According to the risk of water inrush, the three layout schemes can be ranked as follows: stagger inward 25 m > stagger inward 80 m > stagger outward 15 m . (4) Considering the floor stress environment, gas extraction efficiency, and water prevention and control, the reasonable location of floor drainage roadway below the No. 11060 working face of Zhaogu No. 2 Coal Mine was finally determined. It was arranged in the sandy mudstone layer on the upper part of L9 limestone under the middle part of coal pillar and was drived along the seam floor.

Experimental and Numerical Studies on Crack Initiation and Coalescence in Sandy Mudstone with Prefabricated Cross-Flaws Under Uniaxial Compression

The purpose of this paper is to study the crack initiation, propagation, and coalescence of the sandy mudstone sample with two sets of prefabricated cross-flaws under uniaxial compression. This study is different from previous studies on single or multiple parallel prefabricated flaws. The prefabricated cross-flaws are characterized by the dip of the rock bridge with the direction of the main flaw ( β ) and the angle between the direction of main and minor flaws ( γ ). The effects of these two parameters on crack initiation, propagation, coalescence, crack initiation stress, and coalescence stress are analyzed. Moreover, numerical simulation of the uniaxial compression experiments is performed using PFC2D with a flat-joint model, and the simulation results are in good agreement with those from the experiments. The results demonstrate that the dip angle of the rock bridge with the direction of the main flaw ( β ) has strong effects on the crack initiation and coalescence stresses. The larger the angle between the direction of main and minor flaws γ , the greater the crack initiation and coalescence stresses. The crack initiation stress is reduced for the case with cross-flaws compared with that with non-cross-flaws. Meanwhile, the connection type of main flaws and the width of the crack coalescence zone are difficult to observe through the experiments and are discovered from the numerical simulation.