Abstract
After a gas drainage event causes different degrees of initial porosity in the coal seam, the heterogeneity of the coal mass becomes much more obvious. In this paper, soft coal testing samples with different degrees of heterogeneity were prepared first by a new special experimental research method using hydrogen peroxide in an alkaline medium to generate oxygen. Then, a series of mechanical tests on the soft coal mass samples were carried out under multiple factor coupling conditions of different heterogeneities and confining pressures. The results show that with a low strength, the ductility failure characteristic and a kind of rheology similar to that for soft rock flow were reflected for the soft coal; i.e., the stress-strain curve of the coal mass had no apparent peak strain and residual strength. An interesting phenomenon was found in the test process: there was an upwardly convex critical phase, called the brittle-ductile failure transition critical phase, for the heterogeneous soft coal mass between the initial elastic compression phase and the ductile failure transition phase in the stress-strain curve of the coal mass. An evolution of the brittle-ductile modulus coefficient of the soft coal was developed to analyze the effect of the internal factor (degree of heterogeneity) and external factors (confining pressure) on the transition state of the brittle-ductile failure of soft coal. Further analysis shows that the internal factor (heterogeneity) was also one of the essential factors causing the brittle-ductile transition of soft coal.
Highlights
A soft coal mass with a complex composition and a cementation structure is a typical inhomogeneous medium and has more complicated mechanical properties compared with other rock masses [1,2,3,4,5,6,7]
In high gas and deep mines, the “three fields of surrounding rock,” i.e., the original rock stress field, the mining stress field, and the support stress field, will change after a gas drainage event or roadway excavation, which will result in instability of the surrounding rocks and even lead to some dynamic destructive disasters, such as coal and gas bursts or rock burst, causing an unrecoverable loss of economy and time
The mechanical properties of a heterogeneous coal rock mass are dynamically changing under the influence of different stress fields and gas pressure fields in the deep underground
Summary
A soft coal mass with a complex composition and a cementation structure is a typical inhomogeneous medium and has more complicated mechanical properties compared with other rock masses [1,2,3,4,5,6,7]. Yu et al [41, 42] proved that the fracture expansion of the coal and rock mass caused its permeability to increase, weakening the surrounding rock strength They proposed a core support technology with a prestressed truss anchor cable to support the roadway. Most of the researches on the special characteristics of the brittleductile transition of rock are concentrated on the analysis of mechanical properties of rock by external single factors (e.g., single confining pressure and single temperature), neglecting the influence of mutual coupling with internal factors of rock (e.g., heterogeneity, water, gas, and temperature). The change law of the internal structure of the soft coal mass affected by the gas was inferred
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