循环加卸载路径下不同含瓦斯煤渗流及损伤演化特征

循环载荷广泛存在于采矿活动中并对煤岩的强度、损伤及渗透性质产生较大影响,例如煤层群开采多重保护工程中,被保护层煤岩就受到循环加卸载作用,并显著改变了煤岩的力学及渗流特性;瓦斯对煤的力学性质及能量耗散特征也具有显著的影响,不同加卸载路径下煤岩力学及渗流特性与常规加载下的性质存在显著差异,因而有必要研究循环加卸载条件下不同含瓦斯煤的渗流及损伤演化特征。根据煤层群开采条件下被保护层应力状态实时监测的相似模拟实验结果,设计了3种简化的循环加卸载应力路径,即阶梯循环加卸载、逐级增大循环加卸载和交叉循环加卸载,采用重庆大学自主研发的含瓦斯煤流固耦合三轴渗流实验装置对取自平顶山十矿和袁庄煤矿的煤样进行了瓦斯渗流试验。结果表明:在3种循环加卸路径中,2种煤样的渗透率变化与轴向应力应变曲线具有显著的一致性,循环加卸载作用下,煤样渗透率随着应力的增大和循环次数的增加呈减小趋势;应力卸载和加载对渗透率的影响不同;渗透率受到应力和损伤累积的双重影响。相同应力水平下,煤样经过卸载-加载过程后的渗透率有降低趋势,相对恢复率随着循环次数的增加而先降低后增大,只有应力超过煤样的屈服阶段后才能使渗透率增大。主要结论为:①3种循环加卸载路径下煤样在加载阶段的增透率随应力增大和循环次数的增加都可以分为3个阶段且呈增长趋势,单位体积变化引起的渗透率增加在变大,循环荷载的增透效果随着循环次数的增加而增强。②随着峰值应力的增大和煤样中损伤的累积,渗透率对应力的敏感性逐渐降低。随着荷载的施加,应力卸载对渗透率的影响先增强后减弱。③通过计算各循环阶段的加卸载响应比得到了煤样损伤变量的演化规律,通过回归分析可知损伤变量与轴向应力之间的关系可以用Boltzmann函数表征,该函数可以作为损伤的经验公式对实验中煤样的损伤进行预测计算。④循环加卸载对煤样渗透率及损伤的作用受煤种不同的影响不明显。研究结果为深入揭示多重保护下煤层增透机制和基于循环荷载致裂(重复水力压裂等)的煤层强化增透机制及瓦斯抽采工程设计提供理论支撑。

Seepage and damage evolution characteristics of different gas-bearing coal under cyclic loading-unloading conditions

Cyclic loading exists widely in the mining activity and has a strong influence on the strength,damage and penetration property. For example,in the poly-protection engineering of multi-seam mining,the coal in the protected seam are subjected to cyclic loading and unloading,which significantly change the mechanical and seepage characteristics of the coal. The gas has an obvious influence on the mechanical properties and the energy dissipation characteristics of the coal. There are significant difference of the mechanical and seepage properties of the coal under different loading and unloading paths and general methods. Therefore,it is necessary to study the seepage and damage evolution characteristics of different gas-bearing coals under cyclic loading and unloading paths. According to the result of a similarity simulation experiment on the real-time monitoring of stress state of a pro-tected seam on the mining conditions of a coal seam group,three simplified cyclic loading-unloading stress paths were designed. That is stepped-,stepped-increasing-, and crossed-cyclic loading and unloading. The experiment was carried out by using a triaxial seepage experiment device for heat-fluid-solid coupling in gas-containing coal made in Chongqing University,China. The coal samples used for the experiment were taken from Pingdingshan No. 10 coal mine,Henan Province,China and Yuanzhuang Coal Mine,Anhui Province,China. The results showed that under the three cycle loading and unloading stress paths, the curve of the permeability and stress-strain of the two coal samples have a significant consistency,and the permeability of the coal has a decrease trend with the increase of stress and the cycle number. The influence of unloading and loading stresses on the permeability of coal is different. The permeability was subjected to the dual influence of stress and damage accumulation. At the same stress level,after being subjected to stress unloading and loading,the permeability of coal samples gradually decreased and its relative recovery rate declined at first and then increased with increasing axial stress. The per-meability of coal samples did not increase before the stress exceeded the yield stage of coal samples. The main conclusion is:① Under the three cyclic loading-unloading paths,the mining-enhanced permeability of coal samples in loading stage all showed a three-phase increase characteristic with the growth of stress and number of cycles. The mining-enhanced permeability caused by the change per unit volume increased. ② With increasing peak stress and damage accumulation in the coal samples,the sensitivity of the permeability to stress gradually decreased. As loads were applied to the coal samples,the influence of stress unloading on their permeability was first strengthened,then reduced. ③ By calculating the LURRs at various stages of cyclic loading,the evolution of the damage variable of the coal samples was obtained. Through regression analysis,it can be seen that the rela-tionship between the damage variable and number of cycles can be characterized by applying the Boltzmann function. The function can be used as an empirical formula for damage prediction. ④ The effect of cyclic loading and unloading on the permeability and damage of coal samples is not obvious due to the different coal types. The research results provide a theoretical and practical support for further revealing the permeability improvement mechanism of coal seam and gas drainage engineering design under multiple protections. It can provide a theoretical basis for the anti-reflection of coal seam by cyclic loading (pulsating hydraulic fracturing,electric pulse,etc).