STUDY ON THE PRECURSOR CHARACTERISTICS OF ULTRA-LOW FRICTION-TYPED ROCKBURST AND THE INFLUENCE OF COAL-ROCK INTERFACE PROPERTIES
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摘要: 超低摩擦效应易使煤岩界面处发生侧向滑移, 诱发严重的冲击地压事故. 为了揭示超低摩擦效应诱发冲击地压机理, 以煤岩组合结构为研究对象, 采用自行研制的超低摩擦试验装置, 应用高速相机及声发射等监测设备, 基于实验室角度复现了静、动载组合扰动下煤层断裂及超低摩擦滑移失稳诱发冲击地压过程. 并以煤岩界面粗糙度、夹层含水率和夹层厚度为变量, 应力、位移和声发射信号为分析指标, 研究了煤岩及巷道动态破坏特征. 结果表明: (1) 超低摩擦型冲击地压发生过程呈现初始、预滑移、动滑移和滑移终止4个阶段性变化特征, 其中, 预滑移阶段是煤层临近断裂与滑移前的微小滑动阶段, 预示着煤岩体已进入不稳定阶段; 动滑移阶段是煤层发生断裂后的剧烈滑动, 伴随着较大能量释放和明显的超低摩擦现象; (2) 煤岩界面属性对超低摩擦型冲击地压具有重要影响, 煤岩界面粗糙度越小, 或夹层含水率越大, 或夹层厚度越大, 煤层滑移启动水平应力临界值越小, 煤层滑移量越大, 巷道挤压破坏越严重, 超低摩擦型冲击地压强度越大; (3) 不同煤岩界面属性下, 预滑移时间具有明显规律性变化特征, 可作为超低摩擦型冲击地压事故滑移预警时间指标; (4) 根据声发射前兆信号, 给出了超低摩擦型冲击地压风险预测方法, 并将超低摩擦型冲击地压风险划分为: 无、低和高风险区. 研究结果对冲击地压的预测和防治具有重要意义.Abstract: The ultra-low friction effect tends to cause lateral slip at the coal-rock surface, which induces serious rockburst accidents. To reveal the occurrence mechanism of rockburst induced by ultra-low friction effect, a coal-rock composite structure was investigated as the research object, a self-developed ultra-low friction test device was used, and high-speed cameras, acoustic emission monitoring equipment were applied, the whole process of rockburst induced by coal seam fracture and ultra-low friction slip instability under the combined disturbance of static and dynamic loads was reproduced in laboratory tests. With the coal-rock interface roughness, the moisture content of interlayer, and the thickness of interlayer as variables, and the stress, displacement, velocity, and acoustic emission energy as analytical indicators, the dynamic failure characteristics of coal-rock and roadway were studied. The results show that: (1) The occurrence process of ultra-low friction-typed rockburst shows four stages characterized by initial stage, pre-slip stage, dynamic slip stage, and slip termination stage. Among these, the pre-slip stage is a small sliding stage before the coal seam approaches the fracture and slip, indicating that the coal-rock system has entered an unstable stage; The dynamic slip stage is the violent sliding that occurs when the fracture of coal seams, accompanied by large energy release and obvious ultra-low friction phenomena. (2) The coal-rock interface properties have an important influence on the ultra-low friction rockburst. The smaller the roughness of the coal-rock interface, or the greater the water content of the interlayer, or the greater the thickness of the interlayer, the smaller the critical value of the horizontal stress of the coal seam slip start, the greater the amount of coal seam slip, the more serious the roadway extrusion damage, and the greater the strength of the ultra-low friction-typed rockburst. (3) Under different coal-rock interface properties, the pre-slip time exhibits obvious regular variation characteristics, which can be used as the slip warning time index of ultra-low friction-typed rockburst accident. (4) Based on the acoustic emission precursor signals, a risk prediction method for ultra-low friction-typed rockburst is obtained, and the risk of ultra-low friction-typed rockburst is divided into no, low and high risk areas. The research results are of great significance to the prediction and prevention of rockburst.
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Keywords:
- ultra-low friction effect /
- rockburst /
- pre-slip /
- dynamic slip /
- slip warning /
- coal-rock interface properties
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引 言
随浅部资源枯竭, 煤矿开采深度日益加深, 煤岩动力灾害现象频增, 出现了“超低摩擦”等新的特征科学现象[1-6]. 超低摩擦效应是在动载扰动下发生的, 即动载扰动下相互作用的块系煤岩体压紧与分离交替变化, 块体间出现短暂的摩擦力减弱甚至消失的现象. 我国钱七虎院士证实, 大多冲击地压及工程性地震的发生都与超低摩擦效应的呈现及其产生条件紧密相连[7]. 如2017年沈阳红阳三矿冲击地压事故, 开采扰动使煤层断裂, 大量能量释放, 巨量煤层整体向巷道滑移约3 m, 顶、底板破坏较轻. 用超低摩擦效应理论可以合理解释这一异常现象. 因此, 李利萍等[8-9]提出了超低摩擦型冲击地压的新概念. 目前, 对超低摩擦型冲击地压发生过程缺乏系统研究, 其发生机理与预测、防治已成为亟待解决的重要工程问题[7]. 此外, 超低摩擦型冲击地压与煤岩界面属性密切相关. 对于顶、底板均为岩石的深部煤岩体, 动力扰动使煤岩界面产生划痕和碎屑, 改变了煤、岩表面粗糙度和夹层材料属性. 煤岩表面粗糙度及夹层碎屑属性不同, 煤层滑移程度必然存在差别.
目前, 国内外学者在超低摩擦方面进行了一些研究工作. 超低摩擦效应最早由俄罗斯学者Kurlenya等[10-11]提出, 并通过块系岩体冲击扰动试验验证了其存在. 王明洋等[12]认为超低摩擦现象影响块体间接触面的稳定性. Tarasov等[13]认为超低摩擦剪切作用是导致深部岩体破裂和高能释放的主要原因. 王洪亮等[14]对超低摩擦现象进行了实验和数值计算, 得到了与Kurlenya等[10-11]一致的结论, 指出产生超低摩擦现象的根本原因在于法向力重分布以及动摩擦因数变化. 吴昊等[15-16]对超低摩擦进行实验研究, 建立并求解数学模型, 得到了工作块体水平位移表达式. 许琼萍等[17]将仅有水平静力作用时的最大静摩擦力与有竖直冲击作用时的最大静摩擦力的比值定义为摩擦减弱系数, 根据该系数给出了超低摩擦现象的发生条件. Boneh等[18]研究了滑动速度和法向应力对岩块间摩擦力的影响. Rutter等[19]通过实验研究了软弱夹层对摩擦效应的影响, 发现软弱夹层对超低摩擦滑动起重要作用. 潘一山等[20]对块系岩体建立数学模型并进行求解, 给出了以块体间相对位移为指标的超低摩擦效应发生判据. Rashed等[21]研究了煤样破坏、煤样宽高比和界面摩擦之间的关系, 发现界面摩擦会影响煤样破坏程度. 王来贵等[22]对地震作用下含单一结构面边坡进行理论与数值模拟研究, 发现结构面法向正应力减小时出现超低摩擦效应. 李新平等[23]发现爆破开挖扰动会在块体与围岩接触面上产生超低摩擦效应, 引发块状结构岩体产生变形突变. 何满潮等[24]和Liu等[25]用数字图像相关方法和声发射技术观察在正弦波激励下的超低摩擦现象. Heimisson等[26]研究了超低摩擦引起滑动失稳的过程. 李利萍等[8-9, 27]考虑冲击倾向性等对超低摩擦特性的影响, 以及能量转化特征, 并提出应用声发射能量峰值预测超低摩擦效应强度.
国内外学者对超低摩擦现象进行了深入研究, 但研究仅限于超低摩擦现象本身, 如: 证实超低摩擦效应的存在、产生超低摩擦效应的原因, 以及影响因素等, 而目前对超低摩擦效应诱发冲击地压的过程缺乏系统研究, 鲜有学者从试验角度复现其过程. 因此, 本文基于实验室角度模拟静、动载组合扰动下超低摩擦型冲击地压发生过程, 分析煤岩及巷道动态破坏特征, 揭示其发生机理, 给出影响规律与前兆特征, 以期为冲击地压的预测和防治提供更准确的科学方法.
1. 超低摩擦试验
1.1 试验装置
(1) 超低摩擦试验系统
试验系统如图1所示, 包括加载系统和数据采集系统. 试验加载系统为自主研制的超低摩擦试验装置, 具有轴压、垂直扰动和水平冲击等加载装置, 实现动、静组合加载, 复现超低摩擦现象, 较好地模拟超低摩擦效应诱发冲击地压时煤岩系统的真实应力状况. 其中轴压调节范围为0 ~ 25 MPa, 垂直扰动幅值和频率调节范围分别为0 ~ 5 MPa和0 ~ 50 Hz. 水平冲击头直径为ϕ30 mm, 水平冲击速率恒定为8 mm/s. 数据采集系统包括应力采集、水平位移采集、声发射(acoustic emission, AE) 信号采集和高速相机图像采集. 声发射探头布置在煤块侧面, 声发射监测系统放大器增益40 dB, 门槛值45 dB, 采样频率1 MHz. 高速相机采集速率为1000 fps. 位移传感器最大测距为50 mm, 精度为0.001 mm.
(2) 三维扫描仪
使用手持式三维扫描仪扫描煤块表面三维形貌, 获取三维数据, 用于定量表征煤岩界面粗糙度. 三维扫描仪如图2所示, 扫描精度为0.020 mm, 最高测量扫描速率1.25 × 106 s−1.
1.2 试件制备
试验所用煤、岩试样取自鄂尔多斯地区, 煤岩取样后用保鲜膜封存, 保持煤岩自然含水状态. 试验模型如图1(d)所示, 煤岩块体为自然堆叠方式组合, 老顶用于传递静、动载荷, 避免动载直接加载于与煤层直接接触的顶板而影响试验结果的准确性. 顶、底板为细砂岩, 尺寸为150 mm × 100 mm × 100 mm, 单轴抗压强度为57.38 MPa, 弹性模量为9.53 GPa; 煤块尺寸为120 mm × 100 mm × 100 mm, 单轴抗压强度为30.67 MPa, 弹性模量为2.03 GPa, 抗拉强度为2.56 MPa.
试验分别以煤岩界面粗糙度(试件编号: CRLF-Ⅲ-1-1 ~ 6)、夹层含水率(试件编号: CRLF-Ⅲ-2-1 ~ 6)、夹层厚度(试件编号: CRLF-Ⅲ-3-1 ~ 6)为变量, 进行3组试验, 试验材料如图3所示. 夹层材料的制备, 是将煤岩界面处的试样破碎后筛成60 ~ 80目煤粉, 烘干后配置成不同含水率的夹层材料. 煤岩界面粗糙度的制备, 是以煤块上表面粗糙度为变量, 即仅改变煤块上表面粗糙程度, 减少其他变量影响. 使用三维扫描仪扫描煤块上表面投影面积(xoy)为120 mm × 100 mm的三维粗糙表面, 其中以粗糙度为变量的6组煤块(CRLF-Ⅲ-1-1 ~ 6) 表面三维形貌如图4所示.
为了对煤块表面粗糙程度进行量化表征, 沿y方向等间距5 mm 提取20条二维(xoz)曲线数据, 采用式(1)和式(2)计算粗糙度系数值(JRC), 取平均JRC值为煤块三维粗糙表面粗糙度系数[28-29]
$$ {Z_2} = {\left[ {\frac{1}{{{N_0} - 1}}\sum\limits_{i = 1}^{{N_0} - 1} {{{\left( {\frac{{{z_{i + 1}} - {z_i}}}{{{x_{i + 1}} - {x_i}}}} \right)}^2}} } \right]^{\tfrac{1}{2}}} $$ (1) $$ JRC = 32.2 + 32.47\;{\text{lg}}{Z_2} $$ (2) 式中, Z2为剖面线均方根, N0为计算JRC值时沿二维(xoz)曲线选取的坐标点个数, (xi, zi)为曲线上样本点坐标.
经计算, CRLF-Ⅲ-1-1 ~ 6煤块上表面JRC值分别为10.12, 11.90, 13.27, 14.86, 17.05和19.33, 下表面JRC值为15 ~ 16. CRLF-Ⅲ-2-1 ~ 6和CRLF-Ⅲ-3-1 ~ 6煤块上、下表面JRC值均为15 ~ 16, 作为顶、底板的砂岩JRC值均为12 ~ 13. 必要时采用砂纸打磨块体表面, 以确保JRC值在试验要求范围内.
1.3 试验方案
巷道开挖后, 巷道周围煤岩体表现为二向应力状态, 即仅受上覆岩层垂直地应力与水平地应力共同作用. 当动力扰动作用于煤岩体, 可将扰动分解为垂直方向和水平方向. 因此将深部煤岩体所处复杂应力状态简化为轴压、垂直扰动与水平冲击共同作用. 试验模拟深度为地下1000 m, 煤岩体所受垂直地应力约为24.7 MPa. 因深部煤层受到围压作用, 其抗压强度可提升近4倍[27], 所以考虑到围压影响, 轴压取垂直地应力的1/4, 即轴压设置为6 MPa. 将垂直扰动简化为正弦波形式: Pv(t) = Psin(2πft) (其中P为扰动幅值, MPa; f为扰动频率, Hz), 水平扰动为恒定速率冲击, 数值为8 mm/s. 为了避免煤岩块体因垂直扰动幅值较大而出现明显损伤, 扰动幅值设置为1 MPa, 扰动频率设置为25 Hz[30].
共设置3组试验, 试验方案如表1所示. 第1组试验变量为JRC, 第2组试验变量为煤岩界面夹层含水率(w), 第3组试验变量为煤岩界面夹层厚度(h). 试验目的: 复现静、动载组合作用下超低摩擦效应诱发冲击地压过程, 分析受载煤体断裂和滑移特性, 揭示超低摩擦型冲击地压前兆特征, 以及定量分析煤岩界面属性影响规律. 试验方法: 超低摩擦效应是在动载扰动下发生的, 在超低摩擦效应下煤层易发生滑移, 因此首先对煤岩组合试件施加轴压, 待轴压稳定加载于煤岩组合试件后在顶部施加垂直扰动, 垂直扰动持续时间不少于400 ms, 然后施加水平冲击. 声发射仪、高速相机、应力和位移传感器监测发生超低摩擦型冲击地压时煤岩及巷道的动态破坏过程. 每组试验重复进行3次, 以确保试验结果的准确性.
表 1 试验方案Table 1. Test schemeTest conditions Specimen number JRC w/% h/mm Axial
compression/MPaVertical disturbance
amplitude P/MPaVertical disturbance
frequency f/HzHorizontal impact
rate v/(mm·s−1)test 1: change JRC CRLF-Ⅲ-1-1 10.12 25 2.0 6 1 25 8 CRLF-Ⅲ-1-2 11.90 25 2.0 CRLF-Ⅲ-1-3 13.27 25 2.0 CRLF-Ⅲ-1-4 14.86 25 2.0 CRLF-Ⅲ-1-5 17.05 25 2.0 CRLF-Ⅲ-1-6 19.33 25 2.0 test 2: change w CRLF-Ⅲ-2-1 15 ~ 16 15 2.0 6 1 25 8 CRLF-Ⅲ-2-2 15 ~ 16 20 2.0 CRLF-Ⅲ-2-3 15 ~ 16 25 2.0 CRLF-Ⅲ-2-4 15 ~ 16 30 2.0 CRLF-Ⅲ-2-5 15 ~ 16 35 2.0 CRLF-Ⅲ-2-6 15 ~ 16 40 2.0 test 3: change h CRLF-Ⅲ-3-1 15 ~ 16 25 1.0 6 1 25 8 CRLF-Ⅲ-3-2 15 ~ 16 25 1.5 CRLF-Ⅲ-3-3 15 ~ 16 25 2.0 CRLF-Ⅲ-3-4 15 ~ 16 25 2.5 CRLF-Ⅲ-3-5 15 ~ 16 25 3.0 CRLF-Ⅲ-3-6 15 ~ 16 25 3.5 2. 受载煤体断裂及滑移过程分析
2.1 试验结果
超低摩擦型冲击地压发生过程(以试件CRLF-Ⅲ-1-1试验结果为例) 如图5所示. 当t = 400 ms时煤层受到水平冲击作用, t = 680 ms时煤层表面出现纵向宏观裂隙, t = 680 ~ 1082 ms期间纵向裂隙逐渐发展贯通, t = 1082 ms左侧断裂煤层瞬间向巷道滑移抛出并伴有煤屑崩落, t = 1150 ms 滑移终止, 趋于稳定. 顶、底板岩块未发生明显破坏, 与红阳三矿冲击地压事故现场观测现象相符.
冲击地压的诱发因素较多, 将这种以超低摩擦效应为诱因之一, 并伴随着超低摩擦效应向巷道瞬间滑移的冲击地压称为超低摩擦型冲击地压, 属于滑移型冲击地压的一种新类型.
2.2 应变云图
基于数字散斑技术, 试验中采用高速相机记录煤层及巷道动态破坏过程, 结合Matlab软件处理图像, 煤层断裂及超低摩擦滑移失稳过程中应变云图(以试件CRLF-Ⅲ-1-1试验结果为例) 如图6所示.
由图6中法向应变云图可知, 煤层右侧受到水平冲击后受到挤压, 煤层整体具有卸载滑脱趋势. 由水平应变云图可以看出, 煤岩界面局部出现应力集中, 并开始出现微小张拉裂隙. 随着水平冲击继续加载, 裂隙两侧应变逐渐明显, 裂隙逐渐发展贯通, 形成贯通型张拉裂隙, 并出现明显滑移.
2.3 应力和位移时程曲线
超低摩擦型冲击地压发生过程中应力、位移时程曲线(以试件CRLF-Ⅲ-1-1试验结果为例) 如图7所示.
由图7可知, 根据法向应力、水平应力和水平位移变化过程, 受载煤体断裂及超低摩擦滑移失稳诱发冲击地压过程可以划分为 Ⅰ, Ⅱ, Ⅲ和Ⅳ 阶段.
Ⅰ 阶段: 初始阶段. 煤岩系统受到轴压与垂直扰动共同作用, 顶、底板和煤层间法向应力规律性增、减交替改变, 水平应力与水平位移发生微小波动.
Ⅱ 阶段: 预滑移阶段. 煤层右侧受到水平冲击作用, 煤岩界面局部应力集中, 水平应力近似线性增长, 煤层向左侧出现微小滑移, 是煤层临近断裂前的微小滑动阶段, 能量迅速积累过程中伴随能量微小释放. 预滑移的出现预示煤岩系统已进入不稳定阶段, 煤层即将出现断裂、滑移.
Ⅲ 阶段: 动滑移阶段. 随着能量积聚到一定程度, 煤层塑性区出现纵向裂隙, 并迅速发展贯通, 煤层断裂时积累的能量瞬间释放, 法向应力和水平应力产生动态应力降, 在超低摩擦效应影响下, 断裂的煤层瞬间向左侧巷道滑移抛出, 挤压巷道, 发生冲击地压. 动滑移阶段是煤层发生断裂后的剧烈滑动, 伴随着较大能量释放和明显的超低摩擦现象.
Ⅳ 阶段: 滑移终止阶段. 水平应力和水平位移逐渐趋于平缓, 在煤岩界面摩擦阻力下断裂煤层滑移终止.
2.4 滑移速度时程曲线
对煤层水平滑动位移时程曲线进行一阶求导, 得到水平滑移速度时程曲线(以试件CRLF-Ⅲ-1-1试验结果为例), 如图8所示.
由图8可知, 根据煤层断裂及超低摩擦滑移失稳过程速度变化规律, 同样可以划分为 Ⅰ, Ⅱ, Ⅲ和Ⅳ 阶段.
Ⅰ 阶段: 初始阶段. 煤岩系统仅受垂直方向力的作用, 水平速度约为0.
Ⅱ 阶段: 预滑移阶段. 煤层右侧受到水平冲击作用, 临近出现纵向裂隙时刻水平速度出现微小波动.
Ⅲ 阶段: 动滑移阶段. 当预滑移达到临界值, 即煤层出现贯通裂隙时, 能量释放, 断裂煤层获得最大初速度, 以动能形式克服煤岩界面摩擦力做功, 向巷道滑移过程中速度逐渐降低.
Ⅳ 阶段: 滑移终止阶段. 滑移终止时刻速度惯性波动, 煤岩系统逐渐稳定, 水平速度趋于0.
2.5 声发射能量演化
超低摩擦型冲击地压发生过程中声发射能量和累计能量演化(以试件CRLF-Ⅲ-1-1试验结果为例)如图9所示.
由图9可知, 根据受载煤体断裂及超低摩擦滑移失稳过程声发射能量演化特征, 同样可以划分为 Ⅰ, Ⅱ, Ⅲ和Ⅳ 阶段.
Ⅰ 阶段: 初始阶段. 声发射能量均匀分布, 累计声发射能量曲线近似线性增长, 斜率较小.
Ⅱ 阶段: 预滑移阶段. 声发射活动明显增加, 累计声发射能量曲线近似线性增长, 斜率相对增大, 预示着煤岩系统已进入不稳定阶段, 煤层即将出现断裂、滑移, 可视为煤层断裂及滑移诱发冲击地压的前兆信号.
Ⅲ 阶段: 动滑移阶段. 煤层出现纵向裂隙时声发射信号显著增加, 裂隙逐渐发展贯通过程中声发射能量较大, 裂隙完全贯通时刻声发射能量出现最大值, 声发射累计能量曲线显著增长, 斜率迅速增大, 局部近似垂直.
Ⅳ 阶段: 滑移终止阶段. 煤岩系统逐渐稳定, 声发射能量均匀分布, 类似于初始阶段, 累计声发射能量曲线斜率与初始阶段相近.
应力、位移、速度和声发射能量变化的 Ⅰ, Ⅱ, Ⅲ和Ⅳ 阶段 (初始阶段、预滑移阶段、动滑移阶段和滑移终止阶段) 划分相同, 变化趋势一致.
3. 煤岩界面属性影响分析
以煤岩界面粗糙度、夹层含水率和夹层厚度为变量, 以水平应力、位移为指标, 分析煤岩界面属性对超低摩擦型冲击地压的影响.
3.1 不同粗糙度
根据试验1结果, 煤岩界面粗糙度不同时, 煤层滑移水平应力临界值和断裂煤层滑移量变化如图10所示. 其中, $ \sigma $为煤层滑移水平应力临界值; S为断裂煤层滑移量.
试验表明, 当JRC值为10.12, 11.90, 13.27, 14.86, 17.05和19.33, w = 25%, h = 2.0 mm时, 煤层断裂、滑移应力条件及位移变化规律为:
(1) 使断裂煤层发生滑移的水平应力临界值$ \sigma $分别为0.82, 0.78, 0.96, 1.02, 1.23和1.35 MPa, 随着煤岩界面粗糙度减小, 断裂煤层滑移启动水平应力临界值呈指数性减小. 断裂煤层滑移水平应力临界值与煤岩界面粗糙度系数拟合关系为
$$ \sigma(J R C)=0.32 \exp (J R C / 14.16)+0.12, \quad R^2=0.923 $$ (3) (2) 断裂煤层向巷道滑移量S分别为17.24, 15.18, 13.76, 14.11, 12.27和11.55 mm, 随着煤岩界面粗糙度减小, 煤层滑移量呈指数性增大, 煤层滑移量与煤岩界面粗糙度系数拟合关系为
$$ S(J R C)=36.32 \exp (-J R C / 6.14)+10.11,\quad R^2=0.925 $$ (4) 从煤层滑移应力条件和位移变化规律可知, 煤岩界面粗糙度越小, 煤层滑移启动所需应力条件越小, 煤层滑移量越大, 巷道挤压破坏越严重, 超低摩擦型冲击地压强度越大.
3.2 不同夹层含水率
根据试验2结果, 煤岩界面夹层含水率不同时, 煤层滑移水平应力临界值和断裂煤层滑移量变化如图11所示.
试验表明, 当w分别为15%, 20%, 25%, 30%, 35%和40%, JRC值为15 ~ 16, h = 2.0 mm时, 煤层断裂、滑移应力条件及位移变化规律为:
(1) 使断裂煤层发生滑移的水平应力临界值$ \sigma $分别为1.27, 1.21, 0.92, 0.74, 0.66和0.63 MPa, 随着夹层含水率增大, 断裂煤层滑移启动水平应力临界值呈指数性减小. 断裂煤层滑移水平应力临界值与夹层含水率拟合关系为
$$ \sigma(w)=2.09 \exp (-w / 24.86)+0.17,\quad R^2=0.921 $$ (5) (2) 断裂煤层向巷道滑移量S分别为12.81, 13.53, 13.66, 15.63, 15.84和17.88 mm, 随着夹层含水率增大, 煤层滑移量呈指数性增大. 煤层滑移量与夹层含水率拟合关系为
$$ S(w)=0.85 \exp (w / 19.20)+10.98, \quad R^2=0.932 $$ (6) 从煤层滑移应力条件和位移变化规律可知, 煤岩界面夹层含水率越大, 煤层滑移启动所需应力条件越小, 煤层滑移量越大, 巷道挤压破坏越严重, 超低摩擦型冲击地压强度越大.
3.3 不同夹层厚度
根据试验3结果, 煤岩界面夹层厚度不同时, 煤层滑移水平应力临界值和断裂煤层滑移量变化如图12所示.
试验表明, 当h分别为1.0, 1.5, 2.0, 2.5, 3.0和3.5 mm, JRC值为15 ~ 16, w = 25%时, 煤层断裂、滑移应力条件及位移变化规律为:
(1) 使断裂煤层发生滑移的水平应力临界值$ \sigma $分别为1.24, 1.22, 0.95, 0.86, 0.71和0.68 MPa, 随着夹层厚度增大, 断裂煤层滑移启动水平应力临界值呈指数性减小, 断裂煤层滑移水平应力临界值与夹层厚度拟合关系为
$$ \sigma(h)=1.95 \exp (-h / 4.93)-0.31, \quad R^2=0.919 $$ (7) (2) 断裂煤层向巷道滑移量S分别为12.36, 12.73, 14.27, 15.86, 15.92和16.83 mm, 随着夹层厚度增大, 煤层滑移量呈指数性增大, 煤层滑移量与夹层厚度拟合关系为
$$ S(h)=14.55 \exp (h / 4.80)+23.91, \quad R^2=0.926 $$ (8) 从煤层滑移应力条件和位移变化规律可知, 煤岩界面夹层越厚, 煤层滑移启动所需应力条件越小, 煤层滑移量越大, 巷道挤压破坏越严重, 超低摩擦型冲击地压强度越大.
3.4 煤岩界面属性综合分析
以上分别以煤岩界面粗糙度、夹层含水率和夹层厚度为变量进行了单因素分析, 为了给出煤岩界面属性综合特征, 将单因素拟合公式整合为多因素函数式. 由于3组试验中均同时含有粗糙度、夹层含水率和夹层厚度变量, 为了消除另外两个变量的影响, 需要对不同粗糙度、夹层含水率和夹层厚度下的拟合公式进行归一化处理.
对于断裂煤层滑移水平应力临界值, 多因素函数式为
$$ \sigma (JRC,w,h) = \frac{{\sigma (JRC)}}{{{\sigma _1}}}\frac{{\sigma (w)}}{{{\sigma _2}}}\frac{{\sigma (h)}}{{{\sigma _3}}}{\sigma _4} $$ (9) 式中, $ {\sigma _1} $, $ {\sigma _2} $和$ {\sigma _3} $分别为归一化参数; $ {\sigma _4} $取$ {\sigma _1} $, $ {\sigma _2} $和$ {\sigma _3} $的平均值. 试验交叉组为CRLF-Ⅲ-1-4, CRLF-Ⅲ-2-3和CRLF-Ⅲ-3-3, 所以$ {\sigma _1} $, $ {\sigma _2} $和$ {\sigma _3} $分别取1.02, 0.92和0.95 MPa, $ {\sigma _4} $计算结果为0.96 MPa. 式(9)可表示为
$$ \sigma (JRC,w,h) = 1.077\;\sigma (JRC)\;\sigma (w)\;\sigma (h) $$ (10) 对于煤层滑移量, 多因素函数式为
$$ S(JRC,w,h) = \frac{{S(JRC)}}{{{S_1}}}\;\frac{{S(w)}}{{{S_2}}}\;\frac{{S(h)}}{{{S_3}}}\;{S_4} $$ (11) 式中, S1, S2和S3分别为归一化参数; S4取S1, S2和S3平均值. 该S1, S2和S3分别取14.11, 13.66和14.27 mm, S4计算结果14.52 mm. 式(11)可表示为
$$ S(JRC,w,h) = 5.28 \times {10^{ - 3}}S(JRC)\;S(w)\;S(h) $$ (12) 将试验结果与多因素函数计算结果对比, 结果如图13所示. 不同煤岩界面属性下试验结果与计算结果误差范围为0.71% ~ 5.88%, 误差较小, 采用式(10)和式(12)可以较好地预测不同煤岩界面粗糙度、夹层含水率和夹层厚度下煤层滑移水平应力临界值与滑移量.
4. 超低摩擦型冲击地压前兆特征分析
分析煤层断裂及滑移的前兆特征, 是预测和防治超低摩擦型冲击地压事故的关键问题之一. 预滑移阶段是煤层临近断裂、滑移前的微小滑动阶段, 预示着煤岩系统已进入不稳定阶段, 煤层即将出现断裂、滑移, 可视为发生超低摩擦型冲击地压的前兆信息.
冲击地压前兆信息特征值如表2所示. 其中, Tw为预滑移时间; k1和k2分别为 Ⅰ和Ⅱ 阶段声发射累计能量曲线斜率, 由 Ⅰ和Ⅱ 阶段声发射累计能量曲线线性拟合而得.
表 2 超低摩擦型冲击地压前兆信息特征值Table 2. Characteristic value of ultra-low friction-typed rockburst precursor informationSpecimen number JRC w/% h/mm Tw/ms k1 k2 k2/k1 CRLF-Ⅲ-1-1 10.12 25 2.0 680 167.28 573.35 3.43 CRLF-Ⅲ-1-2 11.90 25 2.0 675 182.83 625.03 3.42 CRLF-Ⅲ-1-3 13.27 25 2.0 640 164.81 716.77 4.35 CRLF-Ⅲ-1-4 14.86 25 2.0 585 168.50 802.48 4.76 CRLF-Ⅲ-1-5 17.05 25 2.0 495 173.44 835.02 4.81 CRLF-Ⅲ-1-6 19.33 25 2.0 410 175.54 877.36 5.00 CRLF-Ⅲ-2-1 15 ~ 16 15 2.0 465 168.46 854.86 5.07 CRLF-Ⅲ-2-2 15 ~ 16 20 2.0 480 166.25 847.31 5.10 CRLF-Ⅲ-2-3 15 ~ 16 25 2.0 510 198.17 823.57 4.16 CRLF-Ⅲ-2-4 15 ~ 16 30 2.0 575 162.55 778.39 4.79 CRLF-Ⅲ-2-5 15 ~ 16 35 2.0 685 172.64 735.25 4.26 CRLF-Ⅲ-2-6 15 ~ 16 40 2.0 710 178.73 657.92 3.68 CRLF-Ⅲ-3-1 15 ~ 16 25 1.0 455 169.52 847.48 5.00 CRLF-Ⅲ-3-2 15 ~ 16 25 1.5 490 162.88 820.37 5.04 CRLF-Ⅲ-3-3 15 ~ 16 25 2.0 535 176.24 814.65 4.62 CRLF-Ⅲ-3-4 15 ~ 16 25 2.5 670 192.17 765.66 3.98 CRLF-Ⅲ-3-5 15 ~ 16 25 3.0 695 183.48 750.21 4.09 CRLF-Ⅲ-3-6 15 ~ 16 25 3.5 705 178.51 714.23 4.00 4.1 滑移预警时间
由表2可知, 不同煤岩界面属性下, 预滑移时间具有明显规律性变化特征. 因此, 定义预滑移时间Tw为滑移预警时间指标, 用于预测发生超低摩擦型冲击地压事故的时间, 进行时间预警. 不同煤岩界面属性下, 预滑移时间变化规律如图14所示.
试验表明, 不同煤岩界面属性时滑移预警时间变化规律为:
(1)当JRC值为10.12, 11.90, 13.27, 14.86, 17.05和19.33, w = 25%, h = 2.0 mm时, 滑移预警时间Tw分别为680, 675, 640, 585, 495和410 ms, 随煤岩界面粗糙度增大, 滑移预警时间呈指数性减小, 即发生滑移的时间越快. 推测其原因为界面粗糙度越大, 煤岩系统积聚能量越快, 煤层发生断裂时间越快. 滑移预警时间与煤岩界面粗糙度拟合关系为
$$ \mathit{T} _{ \mathrm{w}} \mathrm{(} \mathit{JRC} \mathrm{) = 35.55exp(-} \mathit{JRC} \mathrm{/7.83) + 823.43\text{, }}\quad \mathit{R} ^{ \mathrm{2}} \mathrm{ = 0.979} $$ (13) (2)当w分别为15%, 20%, 25%, 30%, 35%和40%, JRC = 15 ~ 16, h = 2.0 mm时, 滑移预警时间Tw分别为465, 480, 510, 575, 685和710 ms, 随煤岩界面夹层含水率减小, 滑移预警时间呈指数性减小, 即发生滑移的时间越快. 推测其原因为夹层含水率越小, 煤岩系统积聚能量越快, 煤层发生断裂时间越快. 滑移预警时间与煤岩界面夹层含水率拟合关系为
$$ \mathit{T} _{ \mathrm{w}} \mathrm{(} \mathit{w} \mathrm{) = 86.31exp(} \mathit{w} \mathrm{/24.80) + 293.50\text{, }}\quad \mathit{R} ^{ \mathrm{2}} \mathrm{ = 0.929} $$ (14) (3)当h分别为1.0, 1.5, 2.0, 2.5, 3.0和3.5 mm, JRC = 15 ~ 16, w = 25%时, 滑移预警时间Tw分别为455, 490, 535, 670, 695和705 ms, 随煤岩界面夹层厚度减小, 滑移预警时间呈指数性减小, 即发生滑移的时间越快. 推测其原因为夹层厚度越小, 煤岩系统积聚能量越快, 煤层发生断裂时间越快. 滑移预警时间与煤岩界面夹层厚度拟合关系为
$$ \mathit{T} _{ \mathrm{w}} \mathrm{(} \mathit{h} \mathrm{) = -886.49exp(} -\mathit{h} \mathrm{/4.97) + 1163.82\text{, }}\quad \mathit{R} ^{ \mathrm{2}} \mathrm{ = 0.930} $$ (15) (4)对于滑移预警时间, 多因素函数式为
$$ {T_{\text{w}}}(JRC,w,h) = \frac{{{T_{\text{w}}}(JRC)}}{{{T_{{\text{w1}}}}}}\;\frac{{{T_{\text{w}}}(w)}}{{{T_{{\text{w2}}}}}}\;\frac{{{T_{\text{w}}}(h)}}{{{T_{{\text{w3}}}}}}\;{T_{{\text{w4}}}} $$ (16) 式中, Tw1, Tw2和Tw3分别为归一化参数, Tw4取Tw1, Tw2和Tw3平均值. 该Tw1, Tw2和Tw3分别取585, 510和535 ms, Tw4计算结果为543 ms. 式(16)可表示为
$$ {T_{\text{w}}}(JRC,w,h) = 3.4 \times {10^{ - 6}}{T_{\text{w}}}(JRC)\;{T_{\text{w}}}(w)\;{T_{\text{w}}}(h) $$ (17) 将试验结果与多因素函数计算结果对比, 误差范围为1.50% ~ 7.18%, 式(17)可以较好地预测煤层发生滑移时间.
由以上分析可知, 滑移预警时间是预滑移阶段重要参数, 是受载煤体断裂及超低摩擦滑移失稳诱发冲击地压的前兆信息, 可作为预测和防治冲击地压事故的重要预警指标.
4.2 声发射风险指标
相对于初始阶段(Ⅰ 阶段), 预滑移阶段(Ⅱ 阶段)声发射活动明显增加, 累计声发射能量曲线斜率相对增大, 预示着煤岩系统已进入不稳定阶段, 煤层即将出现断裂和滑移, 可视为发生超低摩擦型冲击地压的声发射前兆信号.
因此, 根据声发射能量阶段性变化特征, 对超低摩擦型冲击地压危险性进行分析. 定义Rw为超低摩擦型冲击地压风险指标, 对发生超低摩擦型冲击地压事故风险进行预测. Rw计算公式为
$$ {R_{\text{w}}} = \frac{{{k_2}}}{{{k_1}}}\; $$ (18) 式中, k1和k2分别为 Ⅰ和Ⅱ 阶段声发射累计能量曲线斜率近似值.
由表2可知, 3组试验的k1值在162.55 ~ 198.17范围内随机分布, Ⅰ 阶段斜率比值变化范围为0.82 ~ 1.22. Ⅱ 阶段k2值相对于k1值显著增长, 范围为573.55 ~ 877.36, 声发射预警指标Rw = 3.43 ~ 5.10, 其分布如图15所示. 根据Rw分布, 界面粗糙度系数为10.12 ~ 19.33、夹层含水率为15% ~ 40%、夹层厚度为1.0 ~ 3.5 mm时, 超低摩擦型冲击地压风险可划分为: ① 无风险区, 0 < Rw < 1.22; ② 低风险区, 1.22 < Rw < 3.43; ③ 高风险区, Rw > 3.43. Rw值越大, 发生超低摩擦型冲击地压事故风险越大.
5. 结论
通过开展不同煤岩界面属性下超低摩擦型冲击地压模拟试验, 得到以下结论.
(1) 超低摩擦型冲击地压发生过程呈现出: 初始阶段、预滑移阶段、动滑移阶段和滑移终止阶段变化特征. 其中, 预滑移阶段是煤层临近断裂与滑移前的微小滑动阶段, 预示着煤岩体已进入不稳定阶段; 动滑移阶段是煤层发生断裂后的剧烈滑动, 伴随着较大能量释放和明显的超低摩擦现象.
(2) 采用单因素与多因素分析法, 定量给出了煤岩界面属性与滑移启动应力临界值、煤层滑移量之间的函数关系. 煤岩界面粗糙度越小, 或夹层含水率越大, 或夹层厚度越大, 滑移启动水平应力临界值越小, 煤层滑移量越大, 巷道挤压破坏越严重, 超低摩擦型冲击地压强度越大.
(3) 定义了滑移预警时间指标Tw, 并给出了含有煤岩界面粗糙度、夹层含水率和夹层厚度的滑移预警时间函数式. 滑移预警时间是受载煤体断裂及超低摩擦滑移失稳诱发冲击地压的前兆信息, 可作为预测和防治冲击地压事故的重要预警指标.
(4) 根据声发射前兆信号, 给出了超低摩擦型冲击地压风险指标Rw, 可对发生超低摩擦型冲击地压事故风险进行预测. 煤岩界面粗糙度为10.12 ~ 19.33、夹层含水率为15% ~ 40%、夹层厚度为1.0 ~ 3.5 mm时, 超低摩擦型冲击地压风险可划分为: ① 无风险区, 0 < Rw < 1.22; ② 低风险区, 1.22 < Rw < 3.43; ③ 高风险区, Rw > 3.43. Rw值越大, 发生超低摩擦型冲击地压事故风险越大.
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表 1 试验方案
Table 1 Test scheme
Test conditions Specimen number JRC w/% h/mm Axial
compression/MPaVertical disturbance
amplitude P/MPaVertical disturbance
frequency f/HzHorizontal impact
rate v/(mm·s−1)test 1: change JRC CRLF-Ⅲ-1-1 10.12 25 2.0 6 1 25 8 CRLF-Ⅲ-1-2 11.90 25 2.0 CRLF-Ⅲ-1-3 13.27 25 2.0 CRLF-Ⅲ-1-4 14.86 25 2.0 CRLF-Ⅲ-1-5 17.05 25 2.0 CRLF-Ⅲ-1-6 19.33 25 2.0 test 2: change w CRLF-Ⅲ-2-1 15 ~ 16 15 2.0 6 1 25 8 CRLF-Ⅲ-2-2 15 ~ 16 20 2.0 CRLF-Ⅲ-2-3 15 ~ 16 25 2.0 CRLF-Ⅲ-2-4 15 ~ 16 30 2.0 CRLF-Ⅲ-2-5 15 ~ 16 35 2.0 CRLF-Ⅲ-2-6 15 ~ 16 40 2.0 test 3: change h CRLF-Ⅲ-3-1 15 ~ 16 25 1.0 6 1 25 8 CRLF-Ⅲ-3-2 15 ~ 16 25 1.5 CRLF-Ⅲ-3-3 15 ~ 16 25 2.0 CRLF-Ⅲ-3-4 15 ~ 16 25 2.5 CRLF-Ⅲ-3-5 15 ~ 16 25 3.0 CRLF-Ⅲ-3-6 15 ~ 16 25 3.5 表 2 超低摩擦型冲击地压前兆信息特征值
Table 2 Characteristic value of ultra-low friction-typed rockburst precursor information
Specimen number JRC w/% h/mm Tw/ms k1 k2 k2/k1 CRLF-Ⅲ-1-1 10.12 25 2.0 680 167.28 573.35 3.43 CRLF-Ⅲ-1-2 11.90 25 2.0 675 182.83 625.03 3.42 CRLF-Ⅲ-1-3 13.27 25 2.0 640 164.81 716.77 4.35 CRLF-Ⅲ-1-4 14.86 25 2.0 585 168.50 802.48 4.76 CRLF-Ⅲ-1-5 17.05 25 2.0 495 173.44 835.02 4.81 CRLF-Ⅲ-1-6 19.33 25 2.0 410 175.54 877.36 5.00 CRLF-Ⅲ-2-1 15 ~ 16 15 2.0 465 168.46 854.86 5.07 CRLF-Ⅲ-2-2 15 ~ 16 20 2.0 480 166.25 847.31 5.10 CRLF-Ⅲ-2-3 15 ~ 16 25 2.0 510 198.17 823.57 4.16 CRLF-Ⅲ-2-4 15 ~ 16 30 2.0 575 162.55 778.39 4.79 CRLF-Ⅲ-2-5 15 ~ 16 35 2.0 685 172.64 735.25 4.26 CRLF-Ⅲ-2-6 15 ~ 16 40 2.0 710 178.73 657.92 3.68 CRLF-Ⅲ-3-1 15 ~ 16 25 1.0 455 169.52 847.48 5.00 CRLF-Ⅲ-3-2 15 ~ 16 25 1.5 490 162.88 820.37 5.04 CRLF-Ⅲ-3-3 15 ~ 16 25 2.0 535 176.24 814.65 4.62 CRLF-Ⅲ-3-4 15 ~ 16 25 2.5 670 192.17 765.66 3.98 CRLF-Ⅲ-3-5 15 ~ 16 25 3.0 695 183.48 750.21 4.09 CRLF-Ⅲ-3-6 15 ~ 16 25 3.5 705 178.51 714.23 4.00 -
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