Coal Strength Development with the Increase of Lateral Confinement

Abstract

The high stress environment brings many challenges in underground coal mining. In order to address the strength behavior of coal under various confining stresses and hence shed light on coal pillar design optimization, compressive tests were conducted under the lateral confinement of 0–8.0 MPa, and the strength enhancement mechanism was studied from the grain scale using PFC modeling. The results show that the coal strength and cumulative axial strain at failure increased with the confinement, while the Young’s modulus of coal is independent of confinement. However, this confinement-dependent strength property can be significantly weakened by existing cracks. Compared to the significant increase in peak compressive stress, the crack initiation stress slightly increased with the confinement. The strength component mobilized with the confinement enhancement. In the early stage of loading, the high confinement restrained the development of microcracks, while in the later stage, it enhanced the frictional resistance strength component. The two mechanism shifted the compressive strength of coal together and the latter one contributed to the strength component mobilization. The coal showed three failure modes sequentially with the increase of confinement, namely axial splitting, mixed failure and shear failure mode. With regard to failure envelope, the Mohr-Coulomb, Hoek-Brown and S-shaped failure criteria can generally represent the confinement-dependent coal strength with R-square larger than 0.9. The confinement of rapid strength promotion section of S-shape failure envelope falls in a range of 1.5–3.0 MPa. This leads to the difficulty of S-shaped failure envelope justification due to the soft nature and heterogeneity of coal.

Significantly enhances caking index of long‐flame coal under tetralin‐glycerol/H2 system

“…As the pressure increases, the content of S gradually increases, and M 2 has almost twice the S content as the SF coal. In this case, S is added as a cocatalyst and generates a catalytically active phase Fe 1‐x S. The sulfur‐rich pyrrhotite surface has a large number of iron vacancies that can adsorb active hydrogen, stabilize free radicals, and promote the decomposition of some organic functional groups, such as ether bonds, resulting in an increase in the oil yield and coal conversion …”

Experimental study on the evolution law of mining‐induced fractures and deformation rock mass structure of gently inclined layered slope

“…Tang et al 21 established a fracture failure prediction model of a bedding rock slope to analyze the stress and a bending moment of the bedding rock slope caused by underground mining, and predict the location of the fracture failure of the side slope. Zhang and Li 22 studied the progressive failure process and failure mode of the mined‐out slope by centrifugal model test and numerical simulation. It was pointed out that the deformation and failure mode of the simplified model of mined‐out slope consists of four stages: shear breakage and collapse of roof strata caused by lateral compressive stress of coal pillar → connecting sliding surface through slope toe, slope shoulder, and mined‐out area formed by tensile and compressive stress → overall shear of slope in sliding surface → secondary tensile deformation of slope surface.…”

Mechanical Behavior Evolution and Damage Characterization of Coal under Different Cyclic Engineering Loading

“…Different from monotonic load, cyclic loading is time dependent in a repeated manner and usually expressed by superposition of a mean stress σ mean and a periodically time-dependent cyclic amplitude Δσ in mathematics [3]. The cyclic loading at different loading rates and frequencies can degrade the strength properties of rocks [4][5][6][7]. Thus, it is unfavorable to the stability of rock structures in underground coal mining.…”