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中文题目:回村二矿 3.0Mt/a新井设计 外文题目:3.00MT/A NEW MINE DESIGNING FOR HUI CUNER MINE 附录A冲击地压的预防和预测摘要:冲击地压,煤层一种极端活动行为的显示,严重威胁着矿工的生命安全。冲击地压同样影响着矿工的工作效率和生产力。在我们的研究中,借助煤岩弹塑性脆性变形破坏模型,通过理论分析,实验室实验,现场测试、模拟和其他手段,很好地预测了瞬间和延迟冲击地压。基于电磁辐射、声发射、微震等先进技术,从煤岩样品的变形到挤压破坏,一个多参数的预测技术随之形成。在很大程度上取决于这三种形式的共同作用。这样,在时间和空间上预测岩石脉冲的分类系统就建立起来了。我们已经提出了岩石强度弱化理论和“强-弱-强”脉冲结构模型来控制冲击地压对岩石巷道的影响。奠定了坚实的理论基础,达到了弱化、控制冲击地压的作用。为达到预防的目的,定向水力压裂关键技术参数得到了显示。基于以上的结论,以及在煤层、岩层中打的深层爆破钻孔,综合防治技术得到了建立;在有冲击地压危险的巷道中,抗冲击液压支柱也得到了开发。这些技术在中国的大部分煤矿得到了应用,有效控制了灾害,并取得了很好的经济和社会效益。关键词:冲击地压;岩弹塑性脆性变形破坏模型;多参数预测;强度弱化;“强-弱-强”结构;定向水力压裂;抗冲击液压支柱1 介绍煤炭资源是我国主要的能源资源,95%的煤炭产量来源于井工煤矿。随着开采深度的增加(大约20m/年),和地质条件的恶化,深部开采和前部开采在开采环境和基本理论方面呈现出显著的不同,并且呈现出明显的非线性动态不稳定特征。从而容易导致动力灾害数量的增加,例如:冲击地压、顶板大面积离层垮落等一系列严重问题,严重威胁着煤矿的安全生产。冲击地压就是煤矿生产的典型危险源之一,源于围岩弹性能量的突然发出,煤岩质量的迅速、激烈增加甚至会增加其他动力事故的可能性,比如:煤与瓦斯突出,瓦斯爆炸等。在我国,100多座煤矿有冲击地压危险。特别是在抚顺、阜新、兖州、开滦、大同、徐州和华亭。例如,2005年2月14日,在辽宁省阜新市孙家湾煤矿发生了一次巨大的冲击地压和瓦斯爆炸事故,在强度为2.5ML的冲击地压发生之后,大量的瓦斯气体被激发了,诱导一次严重的瓦斯爆炸,造成大量的人员伤亡。因此,煤矿井下的安全、高效生产受到冲击地压的严重威胁。冲击地压发生机理是一个相当复杂的问题。尽管在世界范围内,从冲击地压作用机理的研究到灾害的预测和控制进行了大量的探讨,但仍存在许多关键性的问题需要进一步研究。本文介绍了中国矿业大学近年来在冲击地压的预防和控制上进行的探索和研究。2 煤岩化合物样品冲击地压倾向通过对以往发生冲击地压的巷道顶底板结构的分析,呈现出大量的冲击地压发生于坚硬顶底板结构下的趋势。特别的,煤层上部厚而坚硬的砂岩顶板是影响冲击地压的一个主要因素。在坚硬顶底板的条件下,煤层的强度和厚度同样对煤岩二次开挖后产生的压力有所影响。因此,在“煤层顶底板”系统中,通过对煤岩混合物样品冲击地压的倾向性的分析调查,同时,调查煤层的强度和厚度影响冲击地压的结果将有利于对冲击地压危害的预防和控制。通过在实验室里对煤岩混合物的研究,我们的结论表明,煤岩混合物中顶板的比重越高,样品的弹性模量,损坏度越大,含水层样品抗压强度和冲击地压的趋势也更大。如图1所示,另外,随着煤岩混合物中煤的组分含量的降低,爆破能量指数逐渐降低。相反地,随着煤岩混合物中煤的组分含量的升高,爆破能量指数逐渐升高。煤岩混合物中煤的含量越高,爆破能量的指数也越高。图1 冲击地压指标和煤岩高度比之间的关系顶板的坚硬程度越高,工作面所受的集中应力和垂直应力也越大。图片2表示了开采深度达到700m时,工作面所受到垂直应力的模拟分布。顶板的厚度为6m、20m和10m时,体积变化模量一般为16GPa、27GPa和16 GPa ; 4GPa、 3GPa和 4 GPa。如图所示,煤岩体中,坚硬顶板的条件下所受的垂直应力大于松软顶板条件。两种条件下,垂直应力的最大差值在44兆帕左右。松软顶板的条件下,最大水平应力减少近45%。在坚硬顶板条件下,局部集中应力出现在采空区顶板下的煤附近。另外,在坚硬顶底板的条件下,煤层的坚硬度和厚度的变化同样对作用在煤壁上的垂直应力分布起着重要的作用。如图3所示,随着煤体集中应力的减小,煤层的坚硬程度随之减小,两者最大情况下相差44.8兆帕。最大水平应力随着煤层厚度的增加而增加。我们从数值模拟中得到的数据进行分析,我们得知最大水平应力F与煤层厚度h的关系可用一个二次函数表示F =ah2 bh+c,因此,在煤层顶底板系统中,煤层的厚度越小,煤体所承受的最大水平应力越大。换句话说,在坚硬顶底板的条件下,煤层厚度的比例越小,冲击地压发生的几率越大。图2 不同顶板强度条件下工作面前方煤体垂直压力分布图3 煤体边缘垂直压力分布和煤层刚度之间的关系3 多参数分类预测冲击地压技术3.1 电磁发射、声发射、微震等技术在煤岩样品的破坏、变形中的应用在中国矿业大学的煤炭资源和安全开采重点实验室,电磁发射、声发射、微震等技术在煤岩混合物的实验样品在发生冲击压作用下发生破坏的过程中进行了实验。对于煤岩混合物样品来说,声发射或电磁发射技术在岩层顶板或煤层中会产生数字频率(或脉冲数字)。当加载的负荷达到煤样的最大承载能力的时候,顶板开始弯曲并逐渐脱落。对于顶板部分,这不会因为冲击地压而造成破坏,原因在于它的高的抗压和抗弯强度。煤壁的破坏会给顶板带来弹性应力。在破坏变形的期间,声发射或电磁发射的数字频率信号会有所减弱。期间,在煤壁发生变形破坏之前,由于顶板开始垮落并产生弹性恢复变形,很多能量得到了释放,这会加速煤壁的变形破坏,声发射或电磁发射的数字频率信号会达到最大强度,因此,冲击地压的预警信息之前,声发射或电磁发射的关系可以描述为: nt=nu1.u1+nu2.u2 (1)nt=nu2.u2 (2)其中,nt是电磁发射、声发射信号的计数率在时间上的积累。一般地,u1, u2是顶板和煤体变形速度的变化,而nu1, nu2是与顶板和煤体脆性与抗压强度密切相关的参数。图片4展示了电磁信号和声发射信号在化合物样品破坏变形中计数率的变化情况。图4 电磁发射、声发射信号随时间变化趋势如图片4所示,电磁信号的计数率和声发射信号的脉冲数在试件样品重复的加载和卸载直到冲击地压发生的变化过程中,两个阶段的破坏特征是有所区别的。在试件发生破坏特征之前,电磁信号和声发射信号的计数率和脉冲数会达到极限点,之后,信号的强度迅速地减弱。第二阶段的信号主要是破坏变形达到峰值之后的所反映出的微弱和平稳的信号。由于这些结果,发生在采煤过程中和工作面掘进过程中发生岩石破坏的风险,可以根据煤岩化合物在出现冲击特征的过程中所反映出的征兆和多参数电磁信号和声发射信号,在正确的时间被检测和预测到。3.2 冲击地压的预测和分类技术基于理论分析、实验室实验和广泛的现场实践,零、微弱、中等、强冲击地压可以用冲击地压风险指数定量分类出来。根据冲击地压危险等级的变化,如表1所示,可采用相应的控制措施。表1 冲击地压的危险等级危险等级风险阶段风险指数控制措施A无风险0.75停止作业,从危险地点撤离工作人员。采取冲击地压强度减弱措施。强度监测之后,强度再次弱化之后,直到冲击地压危险完全消除之后,采煤工作可以继续通过煤矿不同采煤方法产生冲击地压危险的区分,地质和煤层赋存条件是首要因素,通过综合指数方法的分析之后,冲击地压的危险区域和关键监测区域被标注出来,这样,冲击地压的早期预测工作就完成了。基于早期预测,区域性预测中,在合适的时间微震技术得到了采用。微震活动异常区域,电磁发射技术在进一步监测工作中得到了采用。另外,可采用钻孔方法来预测和确认危险区域。因此,冲击地压的危险等级可以被这些不同的预测技术所广泛地确定,对于危险的区域和危险点,可以通过强度弱化技术来控制。图5展示了现场实践过程中,冲击地压的分类预测方法以及在现场加以落实的措施。图5 测冲击地压的分类方法和应用步骤对于有冲击地压危险的区域,分类预测方法和控制技术如下所示:早期综合预测(使用综合指数方法来决定主要的监测区域)实时监测区域预测(之后使用连续的微震技术进行区域性预测)局部地点的预测(验证区域性预测的准确性,使用钻孔方法、推进点的预测等)逐步确认和减弱等级危险减轻风险过程(弱化控制煤岩体的强度,消除冲击地压的危险)测试控制效果(使用声发射、微震技术、钻孔方法来验证弱化结果)4 验证冲击地压的削弱理论当考虑到采掘震动受到煤岩体的动态载荷影响时,工作面前方、公路旁积累变化的弹性应变能,可以通过以下的功能来表示。其中,U是煤岩体在任意时候积累的弹性应变能,Uf是受采动影响的余震能量,U0是煤岩体所受的最初的和散失的弹性应变能量的区别。Uj是煤岩体有效的存储能量,Ut是煤岩体在任意时刻弹性应变能量增量的积累,而Ue则通过散射减轻、释放弹性能量。图6使冲击地压的削弱模式具体化,如图所示,通过散射,整个顶底板的强度和厚度得到降低,煤岩体的强度得到降低,降低了煤岩体混合物的能量积聚率。整个拉伸力得到了降低,进而,冲击地压的强度降低得到了实现。这就是冲击地压强度降低的理论基础。图6 煤岩体强度减弱前后能量积聚和减少具体的是,我们应考虑两方面两方面的事情。(1)在一个有冲击地压危险的区域,通过松动煤岩体来减小顶板的强度、厚度、完整性,可以减小冲击地压的倾向性。(2)在煤岩体强度减小之后,压力的最高区域移向深部直到煤岩体的中心处,可使煤岩体煤岩体的能量积聚率减小。(3)通过采取能量削弱措施,煤岩体的能量得到释放,冲击地压的强度得到降低。5 强-弱-强结构对巷道周围岩石的影响巷道周围岩体内部的压力将根据开掘的巷道进行重新分布。当叠加的围岩应力场和从外部震源产生的压力冲击波超过拱形围岩的巷道顶板的有限承载力时,岩体的稳定结构遭到破坏,围岩将会被瞬间或累积的损伤作用所产生的压力波而破坏。另一方面,尽管有时外部震源产生的冲击波不够强大,如果叠加应力场超过拱形巷道顶板的有限承载能力时,冲击地压灾害危险将会继续存在。因此,冲击波的传播和扰动可能是引起井下巷道开采或即将到来的冲击地压灾害的关键因素。也就是说,当叠加应力强度超出巷道支护体系最大承载力时,巷道周围会发生冲击地压。基于外部冲击波的扰动。控制巷道周围岩体的“强-弱-强”结构模型将会被建立。在图片7所示的“强-弱-强”结构模型中,相对的,外部强烈结构的完好状态,引起的地震波衰减指数是相对小的。当地震波在这个结构上传播的时候,地震能量没有明显的衰减而只有一小部分被吸收。因此,在其能力范围内,作为一种弱势特征的,能削减能量的强烈的外部结构得到了提出。由于其拙劣的完整性、连续性和高孔隙率,在中间松软的结构上,地震波在很大程度上得到了分散和吸收。因此,在其能力范围内,作为一种强势特征的,能消耗能量的中间松软结构得到了提出。特征越强烈,对周围岩石的保护能力越强。由于其紧密的和完整的强势内部结构,整个结构随着中间松软结构的失真而移动,而它自身的变形是相对较小的。因其有限的消耗能力,作为一种弱势特征的,能削减能量的强烈的内部结构同样得到了提出。因此,从能量消耗冲击波这个角度来说,“强-弱-强”结构可以同样作为“弱-强-弱”结构而得到提出。图7 控制巷道围岩的“强-弱-强”结构模型因此,控制巷道中冲击地压可以通过减少地震源的外部载荷、通过合理设置一个软的结构、通过提高支撑强度和其他方法来实现。6 控制冲击地压的关键技术(1) 深孔方法减少煤体的散射作为削弱强度的最主要方法,在井下采煤冲击地压的控制中,深孔减少煤体散射起着重要的作用。到目前为止,这项技术已经被广泛应用到10多个矿区。例如:徐州的孙河间煤矿,济宁的济三煤矿,甘肃的华亭煤矿和雁北煤矿等等。图8展示了济三煤矿6303工作面巷道顶板减少煤体散射的深爆破孔布置方案。图8 巷道顶板减少煤体散射的深爆破孔布置方案、(2) 坚硬顶板的定向水力压裂技术坚硬顶板的定向水力压裂技术目的在于减少有冲击地压危险巷道的顶板的坚硬度。特别地,一个危险工作面的厚顶板是很难垮落的。目前,这项技术已经在大同煤业集团的忻州窑煤矿和煤峪口煤矿得到了很好的实践。图9展示了初始裂纹尖端周围控制开采应力分布的数值模拟。如图所示,注水之后,拉伸应力集中产生在坚固的岩石上面,最大的拉伸力能达到52兆帕。在拉应力的作用下,初始裂纹尖端开始断裂,在30兆帕高压水的作用下,裂缝沿着水平方向延伸。根据在煤峪口煤矿410区段81008工作面,定向水力压裂技术在巷道中的成功应用,当泵站的压力从35兆帕降到30兆帕时,压裂半径可以达到10m。图9 初始裂纹尖端周围控制开采应力分布的数值模拟(3) 抗冲击单体液压支柱在巷道和危险工作面超前支护部分,为了减轻冲击地压的影响,单体液压支柱的保护装置已经被设定,该装置由一个铰接顶梁,一个销钉,一个缓冲罐,一个缓冲弹簧和防护垫构成。铰接顶梁通过销钉与缓冲罐连接,这个缓冲罐被直接设置到液压支柱上,缓冲弹簧被设置到缓冲罐的里边。当冲击地压突然发生的时候,保护装置被激活的同时,销钉被自动切割。这样,单体液压支柱就得到了保护。目前,该装置已经获得国家实用新型专利,并且在山东省临沂采煤组的古城煤矿取得了非常显著的效果。7 结论我们已经调查了煤岩混合物样品冲击地压的倾向性,并掌握了由电磁发射、声发射、微震等为主要构成的多参数识别预兆性技术。这样,一个在时间上和空间上分类预测冲击地压的技术得到了建立。同时,我们建立了冲击地压强度削弱理论和控制巷道围岩的“强-弱-强”结构碰撞模型,为弱化控制冲击地压奠定了坚实的理论基础。另外,一个在煤层和岩石中基于定向水力压裂和深孔散射的积极控制方法得到了形成。适合煤岩巷道,防止冲击危害的单体液压支柱同样得到了发展。到目前为止,根据调查结果,这些先进技术已经被广泛应用到10多个有冲击倾向性的区域,特别是像徐州的孙河间煤矿、兖州的济三煤矿、北京的木城涧煤矿、甘肃雁北的华亭煤矿等,在实践中均取得了非常好的效果。冲击地压产的冲击波在岩墙中的传播已经通过与岩石相似的模拟材料进行了真实的测试。8 附加随着开采深度的增加,冲击地压的强度也持续性地增加,严重威胁着煤矿的安全生产。冲击地压的控制机理已经得到了也来越多科学家的关注。科学家们相继提出了一系列冲击地压的理论起源的机理和组织冲击地压发生的对策。经典理论包括:强度理论、能量理论、影响趋势理论等。然而,至今还没有一个能被广泛接受的冲击地压形成理论由于它相当复杂的动态不稳定性。从目前的情况看,研究如冲击地压等隐藏动力危险具有一定的参考价值。众所周知,冲击地压的主要影响因素归因于坚硬岩石的变形能和被公认为静止的岩石的爆发的力学过程。然而,静态负荷理论不能完全解释冲击地压的作用机理。应变能量是积累的一个必要条件,但不是一个对冲击地压有参考价值的充分和必要条件。因此,对于冲击地压来说,外部干扰是一个必备条件。更多的是,冲击波可能由驾驶、爆破、顶板破碎、来压步距的地方、工作面推进的过程中在冲击地压发生之前,往往都会产生外界扰动。所以,研究冲击地压的冲击波产生机理很重要。同时,在冲击地压的研究过程中,仿真研究是一种重要手段,因为它可以冲击地压过程进行数值模拟,并对其机理、故障点和失败的方法提供一些重要资讯。岩石的破坏由波的正弦叠加引起,包括应力波的干扰,减小了岩石的强度。扰动和冲击地压之间的关系需要进一步的调查。习通过在岩石中使用一个测试仪,已知裂缝和不稳定故障点,模拟静态载荷在传播时受到的影响,由此,得到了它传播速度的分布。然而,动态载荷作用下岩石的裂纹实验还没有进行过。在我们的调查中,冲击地压的发展和事故及其作用机理我们已经通过相似模拟材料进行了研究。在岩土工程中,选择和使用相似模拟材料进行实验,相对弹性阶段的校正问题较为成熟。然而,针对这种材料,一些调查很好反映了它的特点,特别是超出弹性阶段的动态不稳定的破坏。在我们的试验中,根据列表1的要求和这种材料的特征,沙子、石膏、水泥、水等被采用,用来制作相似材料。为了测量机械性能,直径50mm、高度100mm的标准试件被准备好,接下来晒干,然后在仿真实验中应用,测试系统的测试材料根据标号为1-8的材料进行准备。模具的尺寸为520 mm 400 mm 70 mm,与物理模型尺寸相同。一个尺寸为50mm40mm70mm的模块在制作物理模型时,放在模具中作为巷道,再用一块带有15mm直径圆孔的模块作为爆破钻孔。在我们最初的模拟资料中,巷道模块和爆破钻孔被抽出去生成物理模型中的巷道和爆破钻孔。模型中的爆破点被设置在巷道的正上方。物理模型被放在测试架上,上覆岩层载荷的应用与上覆地层的载荷重量成比例。加载装置如图2所示,同时扰动载荷被设置在爆破点上。在巷道结构的位置上,分析材料的性质和效果,按照比例,1-3号3个装备被采用,测量点的布置如图4所示。测量点的垂直距离为40mm,在模型中,通过一个系在模块上的铁块的加速计来检测信号的加速度。自从加速度传感器附属于那个铁块,那块测量加速度的铁块与物理模型大体上可以看作是相同的。利用集成放大器,加速传感器的输出信号得到放大。具有记录和分析功能的动态虚拟网络分析仪和相关的软件在重庆大学得到了开发。波形在电脑屏幕得到显示。表2显示了每个测量点上的峰值位置、峰值速度和峰值加速度。另外,从这三个拟合圆弧上看,与该材料的性能相似,最大速度和最大加速度得到衰减。在1号和3号材料之间存在一个很大的区别。在围岩上的距离、功能上的差别、垂直加速度上的变化可以解释为围岩的应力、应变行为。相对于柔软的材料,从1号材料的波信号来看,因其材料较为坚硬,所受的极限应力相对较高。因此,能量的吸收和耗散相对较小,而在坚硬材料的波幅则相对较大。图8显示了1号材料围岩破坏下的爆炸应力波。从图8可以看出,裂缝出现的上边界是围岩应力集中的地方,并且平行于边界地区进行延伸。基于原始压力分布,平行于边界形成了很多裂隙。这些裂纹的传播,导致沿主应力的方向在压力波之下。尽管在我们的相似材料和真实的岩石之间存在很大差异,但当适合的比例和材料被选择之后,岩石的失稳现象和过程可以被准确地模拟。应力波的衰减特征基于相关材料的特征。对于松软材料,应力波以更快的速度衰减。当爆破载荷赋存在巷道的顶部时,裂隙的数量和长度随着爆破载荷和巷道之间距离的减小而增大。在距离巷道顶板280mm的距离时,巷道周围没有明显的裂隙。然而,当距离巷道210mm时,巷道边缘附近出现许多小裂隙,当距离为140mm时,更大的裂隙出现,在一个给定的压力之下,裂隙的出现于压力的强度有关。为了减少巷道围岩裂隙的出现,要控制压力波的强度,并且要防止形成分层裂纹结构。附录B Prevention and forecasting of rock burst hazards in coal minesAbstract:Rock bursts signify extreme behavior in coal mine strata and severely threaten the safety of the lives of miners, as well as the effectiveness and productivity of miners. In our study, an elastic-plastic-brittle model for the deformation and failure of coalrock was established through theoretical analyses, laboratory experiments and field testing, simulation and other means, which perfectly predict sudden and delayed rock bursts. Based on electromagnetic emission(EME), acoustic emission (AE) and microseism effects (MS) in the process from deformation until impact rupture of coal-rock combination samples, a multi-parameter identification of premonitory technology was formed, largely depending on these three forms of emission. Thus a system of classification for forecasting rock bursts in space and time was established. We have presented the intensity weakening theory for rock bursts and a strong-soft-strong (3S) structural model for controlling the impact on rock surrounding roadways, with the objective of laying a theoretical foundation and establishing references for parameters for the weakening control of rock bursts. For the purpose of prevention, key technical parameters of directional hydraulic fracturing are revealed. Based on these results, as well as those from deep-hole controlled blasting in coal seams and rock, integrated control techniques were established and anti-impact hydraulic props, suitable for roadways subject to hazards from rock bursts have also been developed. These technologies have been widely used in most coal mines in China, subject to these hazards and have achieved remarkable economic and social benefits.Keywords:rock bursts; elastic-plastic-brittle model; multi-parameter premonitory; intensity weakening; strong-soft-strong structure; directional hydraulic fracturing; anti-impact hydraulic prop1 IntroductionCoal resources are the main source of energy in China and 95 of the coal produced comes from underground mines. As the mining depth increases (about 20m per year)and geologica1 conditions deteriorate, the mechanical environment and basic behavior in deep-leve1 mining is significantly different from that in shallow mining and shows obvious characteristics of nonlinear dynamic instability, which may easily lead to an increase in dynamic disasters, such as rock burst, roofs collapsing over large areas and other problems which pose serious threats to the safety of coal production in mines. The rock burst is one of the typical dynamic hazards in coal mining, which is caused by elastic energy emitted in a sudden, rapid and violent way in a coal-rock mass and even can increase the possibility of other dynamic accidents such as coal and gas outburst, explosions.etc. Rock burst hazards exist in over 100 coal mines in China, especially in Fushun, Fuxin, Xinwen, Yanzhou, Kailuan, Datong, Xuzhou and Huating. For example, a serious rock burst and gas explosion accident occurred in the Sunjiawan coal mine in Fuxin Liaoning province on Feb, 14, 2005. After the ML=2.5 rock burst occurred, a large amount of gas was emitted, which then induced a serious gas explosion and resulted in many injuries and loss of life. Thus, the safety and highly efficient production of the underground mine had been severely impacted by the rock burst.The rock burst mechanism is a quite complicated problem. Although much significant research has been carried out around the world, from rock burst mechanism studies to rock burst forecasting and hazard contro1, there are still many key issues requiring further research. Our study mainly presents recent progress in research on the prevention and control of rock bursts conducted at the China University of Mining Technology.2 Tendencies in rock burst of compound coal-rock samplesFrom the analysis of roof and floor structures in previous rock bursts, it appears that a considerable number of rock bursts occur under conditions of hard roof and floor structures. Especially hard thick sandstone roofs overlying coal seams is the one of major factors affecting rock bursts. Under conditions of “two-hard” (hard roof and floor), the strength and thickness of the coa1 seam also has certain effect on the distribution of secondary stress after the excavation of a coal-rock mass. Therefore, the research on the tendency of rock bursts in compound coal-rock samples in the system of “roof-coal seam-floor”, as well as the effects of the strength and thickness of coal seams on rock burst occurrences, will greatly benefit the prevention and control of rock burst hazards. From the laboratory research on compound coal-rock samples, our results indicate that the higher the proportion of the roof component in the compound coal-rock, the higher the modulus of elasticity and degree of breakage and the greater the tendency of rock bursts and unconfined compressive strength (UCS) in the samples. as seen in Fig.1. Additionally, the index of bursting energy decreases gradually, fo1lowed by an increase in the proportion of coal components in compound coal-rock. In contrast, the index of elastic energy increases gradually as the proportion of coal components increases, i.e. the higher the coal height in the compound coa1-rock, the larger the index of elastic energy.Fig.1 Relationships between indices of rock burstand ratio of coal and roof heightThe 1arger the hardness of the roof, the higher the degree of stress concentration and vertical stress around the working face. Fig.2 shows the simulated distribution of vertical stress around a workface when the mining depth is 700m, the thickness of the roof 6, 20 and 10m. the bulk modulus l6, 27 and 16GPa and 4, 3 and 4GPa, respectively. As shown in the figure, the vertical stress in the coal-rock mass is much higher under conditions of hard roof than under soft roof conditions. The difference in maximum vertical stress under the two conditions is nearly 44 MPa.The maximum vertical stress in soft roofs decreases approximately 45. The concentration of local stress appears in the goaf roof near the coal mass side under hard roof conditions. In addition, the variations of hardness and thickness of coal seams also have a large effect on vertical stress distribution of a coal-rock mass under the “two-hard” condition, as seen in Fig.3. The vertical stress in the coal body decreases, with a decrease in the hardness of coal seams, with the difference between hard and soft coal conditions approaching 44.8 MPa. The maximum vertical stress decreases as the thickness of the coal seam increases. From the data of our numerical simulation, a regression analysis shows that the relationship between the maximum vertical stress F and the thickness of coal seam h can be expressed as a quadratic function, i.e. F =ah2 bh+c. Therefore, in the system of “roof-coal seam-floor”, the smaller the thickness of the coal seam, the larger the maximum vertical stress of the coal body. In other words, the smaller the proportion of the coal seam thickness,the easier a rock burst can be induced under the “two-hard” condition.Fig.2 Vertical stress distribution of coal mass ahead of the working faceunder different hardness conditions of the roofFig.3 Relationship between vertical stress distributionon the coal mass side and stiffness of the coal seam3 Technique of multi-parameter classification forecasting of rock bursts3.1 Effects of AE, EME & MS in the deformation and failure of a coal-rock massThe electromagnetic emission (EME), acoustic emission (AE) and microseism (MS) effects in the process of rock burst failures of a compound coal-rock mass were tested in the State Key Laboratory of Coal Resource and Mine Safety, China University of Mining& Technology.For the compound coal-rock samples, the count rates(or pulse numbers)of the AE EME include both the deformation and failure of roof and coal. When the applied load approaches the ultimate strength of the coal samples, the roof begins to yield and unload. For the roof part, this will not cause a burst failure in the roof because of its high compressive strength and the deformation and failure of the coal mass will result in an elastic resilience of the roof. In the period of deformation resilience, the count rates of the AE EME signals decrease.Meanwhile, before the burst failure of the coal mass, much energy is released because the roof starts to unload and springs back,which will accelerate the deformation and failure of the coal mass and the count rates of the AE & EME signals will reach their maximum valuesTherefore, the rules for the relations of AE EME in the premonitory information of rock burst can be written as:nt=nu1.u1+nu2.u2 (1)nt=nu2.u2 (2)where nt is the time series of the accumulated count rates of the AE & EME signals, u1, u2 are the variations in velocity of the deformation of roof and coal mass, respectively, nu1, nu2 are the parameters which are closely correlated with the brittleness and UCS of roof and coal body, respectively.Fig.4 shows the distribution of the count rates (or pulse numbers) of the AE & EME signals by the deformation and failure of the compound sampleFig. 4 Time series of AE & EME signalsAs shown in Fig.4, the count rates of AE and pulse numbers of EME have distinct, two-stage failure features in the process of repeated loading and unloading until impact rock burst failure occurs. The count rates or pulse numbers of AE & EME reach their extreme points before the burst failure of the samples, after which the intensity of the signals decreases suddenly. The signals in the second stage are mainly the results of the deformation and failure in the post-peak phase, with poor and steady signals.As a consequence of these results, the risk of rock bursts in mining and extraction processes of working faces can be monitored and predicted in real time according to the multi-parameter premonitory features of AE & EME in the process of burst failures in the compound coal-rock.3.2 Technique of classification forecasting of rock bursts Based on a theoretical analysis, laboratory testing and extensive field trials, the zero, weak, middle and strong risk of rock bursts can be classified quantitatively by a risk index of rock bursts. According to the various danger levels of rock bursts, correspondingly controlling measures can be taken, as shown in Table 1.Table 1 Danger levels of rock burstsDanger levelRisk stateRisk indexControlling measureANon-risk0.75Stop mining work and withdraw workers from dangerous locations. Take rock burst intensity weakening measures. After the checking of intensity, weakening effort by monitoring again, further mining work can continue until the risk of rock bursts has been eliminated. For mines and mining districts with a danger of rock bursts, the geology and mining conditions are in first instance, analyzed by a comprehensive index method and then the danger zone of rock bursts and key monitoring regions are marked of. Thus early rock burst forecasting can be achieved. Based on early forecasting, micro seismic monitoring is used for regional monitoring in real-time. For regions with abnormal micro seismicity,an electromagnetic emission (EME) method is adopted for further local monitoring. Again, a drilling method can be used for forecasting and effort verification in dangerous local
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