大变幅加卸载下特厚煤层底板断层突水机理模拟研究

A simulation study of mechanisms behind water inrush from fault-bearing floors of ultra-thick coal seams under loading and unloading at significantly variable amplitude

特厚煤层采场空间大、扰动范围广,强烈大变幅荷载易导致底板断层破裂加剧并诱发水害。数值模拟是揭示特厚煤层底板断层活化与突水机理的重要方法,准确反映大变幅加卸载下岩体破裂与裂隙水耦合特征是其合理性的关键。构建损伤变量与塑性应变、应力之间的相关关系,得到完整岩块的拉、加压卸载损伤演化方程;以平方拉剪应力与Benzeggagh-Kenane为初始、完全断裂准则,建立塑性位移与强度劣化关系,建立加卸载韧性断裂本构关系;基于实验数据建立贯通裂隙加卸载剪切本构关系。以基本方程与状态方程为基础,结合浸没边界方法,形成裂隙岩体水力学模拟理论。由此编制流体动力学-有限离散元CFD-FDEM耦合程序,模拟研究特厚煤层底板断层活化突水过程。结果表明:CFD-FDEM耦合程序可数值实现特厚煤层底板断层从(准)连续体到离散体转化,以及断层带裂隙水运移过程。底板断层采动破坏包络线呈W形,最深位于断层及其上盘(48.6m),最浅位于断层下盘(23m)。特厚煤层采场底板断层及其上盘受到较大超前集中应力,而后在采空区内大幅卸载,导致该位置出现显著二次破坏,并形成主要导水通道。研究成果为特厚煤层工作面底板断层水害防治提供理论支撑。

Ultra-thick coal seams feature large stope spaces and extensive disturbance ranges.Significantly variable amplitude loading and unloading are prone to cause the fault rupture of floors of ultra-thick coal seams,inducing water disasters.Numerical simulation serves as a critical method for revealing the fault reactivation and water inrush mechan-isms at the floors of ultra-thick coal seams.The key of this method is to accurately inflect the coupling characteristics of rock mass rupture and fissure water under significantly variable amplitude loading and unloading.This study determ-ined the correlations of the damage variable with plastic strain and stress,deriving the equations for damage evolution of an intact rock mass under tensile/compressive loading and unloading.With the square tensile-shear stress and the Ben-zeggagh-Kenane fracture criteria as the criteria for initial,complete rock mass rupture,this study established the relation-ship between plastic displacement and strength deterioration,along with the constitutive relation of ductile fracturing un-der loading and unloading conditions.Then,using experimental data,this study built the shear constitutive relation of penetrating fractures under loading and unloading conditions.Based on fundamental equations and the equation of state,this study created the hydraulic simulation theory for fractured rock masses using the immersed boundary method.Fi-nally,it developed a computational fluid dynamics-finite discrete element method(CFD-FDEM)coupled program to simulate the fault reactivation and water inrush processes at floors of ultra-thick coal seams.The results show that the CFD-FDEM coupled program can achieve both the numerical transformation of the faults at ultra-thick coal seams’floors from a(quasi)continuous to discrete volume and the migration process of fissure water.The mining-induced dam-age of fault-bearing floors exhibited a W-shaped envelope,with the deepest part of damage located in a fault and its hanging walls(depth:48.6 m)and the shallowest part of damage in the fault’s footwall(depth:23 m).The faults at the stope floors and their hanging walls were subjected to significant advanced concentrated stress,which was then substan-tially unloaded in the goaf.This resulted in significant secondary failure in the goaf,leading to the formation of primary hydraulically conductive channels.The results of this study provide theoretical support for the prevention and control of water disasters at fault-bearing floors of ultra-thick coal seams.