Influence of principal stress effect on deformation and permeability of coal containing beddings under true triaxial stress conditions

Abstract

In situ stress is generally an anisotropic/true triaxial stress ( σ 1 > σ 2 > σ 3 ). Bedding weakens the continuity and integrity of coal. It is critical to understand the mechanical behaviour and gas migration of coal under true triaxial stress conditions. We performed experiments of cubic coal samples to investigate the permeability evolution and mechanical behaviour of coal under true triaxial stress conditions by using newly developed true triaxial geophysical apparatus. We analysed the effect of principal stresses on deformation and permeability characteristics of coal containing bedding planes. The results show that volumetric strain, stress states and bedding directions determine the permeability comprehensively. The variable quantity of strain was the largest in the direction normal to the bedding plane. The expansion or compression degree was characterized by the difference between the major and minor principal strain ( ɛ 1 − ɛ 3 ). Essentially, this represents the difficulty degree with regard to coal being compressed at the initial stress state and the deformation degree in ɛ 1 and ɛ 3 direction. The variation of ( ɛ 1 − ɛ 3 ) was consistent with that of permeability. Under an identical true triaxial stress condition, permeability was smaller when larger stress was applied in the direction normal to the bedding plane. Additionally, stress level in the direction parallel to the bedding planes and the directions between stresses in the direction parallel to the bedding planes and the flow direction also affect the permeability and strain. By solving lateral expansion coefficient, coal also exhibited anisotropic properties.

The permeability properties of bedded coal and rock: Review and new insights

“…Currently, domestic and foreign scholars have studied the permeability properties of bedded coal and rock and obtained abundant research results. On the one hand, many scholars have studied on the permeability properties of bedded coal and rock under true triaxial loading or unloading conditions 16‐30 . Yin's team developed the multi‐functional true triaxial fluid–solid coupling test system.…”

“…Yin's team developed the multi‐functional true triaxial fluid–solid coupling test system. Moreover, they analyzed the stress path effect, unloading effect, lithology effect, coal–rock combination effect, anisotropy, intermediate principal stress effect, and other factors on the permeability properties of bedded coal, shale, sandstone, and coal–rock combination 16‐30 . Additionally, the anisotropic permeability evolution characteristics of bedded coal, shale, sandstone, and coal–rock combination were better characterized.…”

“…On the one hand, many scholars have studied on the permeability properties of bedded coal and rock under true triaxial loading or unloading conditions. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Yin's team developed the multi-functional true triaxial fluid-solid coupling test system. Moreover, they analyzed the stress path effect, unloading effect, lithology effect, coal-rock combination effect, anisotropy, intermediate principal stress effect, and other factors on the permeability properties of bedded coal, shale, sandstone, and coal-rock combination.…”

“…Moreover, they analyzed the stress path effect, unloading effect, lithology effect, coal-rock combination effect, anisotropy, intermediate principal stress effect, and other factors on the permeability properties of bedded coal, shale, sandstone, and coal-rock combination. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Additionally, the anisotropic permeability evolution characteristics of bedded coal, shale, sandstone, and coal-rock combination were better characterized. Meanwhile, the permeability evolution models of bedded coal and rock under X, Y, and Z direction were both constructed, which revealed the mechanical mechanism of deformation and permeability of bedded coal and rock under true triaxial condition.…”

Experimental Study on Vacuum Grouting in Silty Sand Stratum

“…Traditional penetration grouting is to press the slurry with a certain pressure into the soil stratum, which needs to be pretreated. However, because the pressure is relatively low, it is difficult to inject into the fine-grained strata, such as sand and silt strata [2][3][4][5]. e principle of vacuum grouting is to generate a vacuum field through the vacuum well in the soil.…”