软煤应变强度硬化冲击灾变

煤与瓦斯突出软煤层不具冲击倾向性,但在深部开采中却发生了软煤层冲击灾变动力现象.为探索软煤冲击灾变的成因,通过文献研究和工程案例实证分析,证明了软煤层冲击灾变现象的客观存在;通过煤样无侧限单轴压缩试验和冲击倾向性测定,得知试验煤样无冲击倾向性,极限载荷后单调应变强度软化;模拟工程背景现场与掘进工作面和采煤工作面中部相同的边界约束和加载条件,开展单自由度边界承压试验,考察灾变全程应力、应变、声发射特征,研究软煤冲击灾变机理.结果表明:单自由度边界条件下加载,3个煤样均出现应变强度软化-硬化-灾变过程,甚至反复软化-硬化;经应变强度硬化,灾变前煤样抗压强度均超过冲击倾向性的阈值条件;定义了应变强度软化、应变强度硬化系数,灾变前应变强度硬化系数分别为1. 26,1. 53,2. 25,应变强度硬化程度比较显著;软煤在单自由度约束条件下承压,应变硬化达到煤样冲击破坏强度条件时,可发生类似硬煤的冲击灾变,合理解释了现场软煤层冲击灾变的成因.深部高应力条件下,煤与瓦斯突出软煤掘进和开采,要对应变强度硬化导致煤层冲击灾变引起重视;工程上,这种冲击灾变的强度一般不高,但其可诱导煤与瓦斯突出或瓦斯异常涌出,危害性很高.

Rock burst disaster in soft coal seams from strain strength hardening

Soft coal seams with coal and gas outburst tendency do not have impulsion tendency, but rock burst disasters occur in soft coal seams under mining in the deep stope, and the existence of this dynamic disaster is confirmed by literature research and cases analysis. The unconfined unidirectional compression test and bursting liability identification indicated that coal samples were not prone to burst, and the monotonic strain strength softened after ultimate load. To study the mechanism of bursting disasters in soft coal seams, the boundary constraints and loading conditions of the excavating face and the mining face under the engineering background were simulated, and the pressure test under the single-degree-of-freedom border was carried out to investigate the stress, strain, and acoustic characteristics of the whole disaster process. The results showed that when coal samples loaded under the boundary condition of single-degree-of-freedom, the strain strength of all the three coal samples appeared as the process of softening-hardening-bursting, and the softening-hardening process even arose repeatedly. The strain strength hardening degree before bursting was remarkable, and the hardening coefficients of the three coal samples were 1.6,1.53, and 2.25, respectively. After strain strength hardening, the compression strength of coal samples before bursting disaster exceeded the threshold value of bursting tendency. Under the pressure with the constraint of single-degree-of-freedom, the bursting disaster, similar to that happens in hard coal seams, occurred when the strain hardening met the strength requirement of bursting, which reasonably explains the cause of rock burst in soft coal seams. When excavating and mining in soft coal seams with coal and gas outburst tendency under the condition of high stress in the deep mines, it is necessary to pay attention to rock burst caused by strain strength hardening. The intensity of such impulsion is generally not high in engineering, but it can induce highly harmful coal and gas outburst or unusual gas emission.