Large-scale destress blasting for seismicity control in hard rock mines:A case study

Destress blasting is a rockburst control technique where highly stressed rock is blasted to reduce the local stress and stiffness of the rock,thereby reducing its burst proneness.The technique is commonly practiced in deep hard rock mines in burst prone developments,as well as in sill or crown pillars which become burst-prone as the orebody is extracted.Large-scale destressing is a variant of destress blasting where panels are created parallel to the orebody strike with a longhole,fanning blast pattern from cross cut drifts situated in the host rock.The aim of panel destressing is to reduce the stress concentration in the ore blocks or pillars to be mined.This paper focuses on the large-scale destress blasting program conducted at Vale's Copper Cliff Mine(CCM)in Ontario,Canada.The merits of panel destressing are examined through field measurements of mining induced stress changes in the pillar.The destressing mechanism is simulated with a rock fragmentation factor(a)and stress reduction/dissipation factor(b).A 3D model is built and validated with measured induced stress changes.It is shown that the best correlation between the numerical model and field measurements is obtained when the combination of a and b indicates that the blast causes high fragmentation(a=0.05)and high stress release(b=0.95)in the destress panel.It is demonstrated that the burst proneness of the ore blocks in the panel stress shadow is reduced in terms of the brittle shear ratio(BSR)and the burst potential index(BPI).

Geomechanical effects of stress shadow created by large-scale destress blasting

This study aims to determine if large-scale choked panel destress blasting can provide sufficient beneficial stress reduction in highly-stressed remnant ore pillar that is planned for production. The orebody is divided into 20 stopes over 2 levels, and 2 panels are choke-blasted in the hanging wall to shield the ore pillar by creating a stress shadow around it. A linear-elastic model of the mining system is constructed with finite difference code FLAC3 D. The effect of destress blasting in the panels is simulated by applying a fragmentation factor(α) to the rock mass stiffness and a stress reduction factor(β) to the current state of stress in the region occupied by the destress panels. As an extreme case, the destress panel is also modeled as a void to obtain the maximum possible beneficial effects of destressing and stress shadow.Four stopes are mined in the stress shadow of the panels in 6 lifts and then backfilled. The effect of destress blasting on the remnant ore pillar is quantified based on stress change and brittle shear ratio(BSR) in the stress shadow zone compared to the base case without destress blasting. To establish realistic rock fragmentation and stress reduction factors, model results are compared to measured stress changes reported for case studies at Fraser and Brunswick mines. A 1.5 MPa immediate stress decrease was observed 20 m away from the panel at Fraser Mine, and a 4 MPa immediate stress decrease was observed 25 m away at Brunswick Mine. Comparable results are obtained from the current model with a rock fragmentation factor α of 0.2 and a stress reduction factor α of 0.8. It is shown that a destress blasting with these parameters reduces the major principal stress in the nearest stopes by 10-25 MPa.This yields an immediate reduction of BSR, which is deemed sufficient to reduce volume of ore at risk in the pillar.

Volumetric changes in the focal areas of seismic events corresponding to destress blasting

The typical development of total volumetric change in the focal areas of seismic events,corresponding to destress blasting,is characterized as an explosive phase followed by an implosive phase and with alternating additional phases following on from that.In a few cases,a non-typical development of volumetric change was identified,where the first phase was implosive and the second phase,explosive.This development is mainly typical for induced seismic events recorded during mining,not for destress blasting.Seismic events were recorded during longwall mining in the Czech part of the Upper Silesian Coal Basin,where the destress blasting technique is used as a rockburst prevention active measure.Kinematic source processes in the focal areas of selected seismic events were analyzed by the seismic moment tensor inversion method,as well as by studying geomechanical rock mass conditions at the localities of the seismic events.The main goal of the analysis was to attempt to identify the reasons for non-typical development of volumetric changes in these cases.Volumetric changes were analyzed for seismic events with energy greater than 104 J,recorded in the period of time from 1993 to 2009(1109 events).80%(891)of the recorded seismic events were induced seismic events that were registered during longwall mining and 20%(218)corresponded to destress blasting events.Research shows that the main reason for the non-typical development of volumetric changes in the focal areas of seismic events is an association with destress blasting in the rock mass,which is very close to rock mass overstressing.The detonation of explosives in boreholes,which would dominate the first phase of volumetric changes,probably obscured stress release in the rock mass,as manifested in the first implosion phase of the volumetric changes in this case.

A comparison of seismic response to conventional and face destress blasting during deep tunnel development

A novel design of development face destress blasting was implemented during the construction of an experimental tunnel at great depth.A second tunnel was developed nearby using conventional blasting as a control.The tunnels were developed parallel to one another and perpendicular to a high subhorizontal stress.High resolution seismic monitoring was used to record and compare the seismic response generated by each excavation.Analysis of the seismic data from the conventionally blasted tunnel indicated that the seismogenic zone of stress-driven instability extended up to 3.6 m ahead of the face.Destress blasting within the corresponding zone of the adjacent tunnel had the effect of reducing the rock mass stiffness,primarily due to weakening of the pre-existing natural discontinuities.The reduction in rock mass stiffness was inferred from the spatial broadening of the seismogenic zone and associated reduction in the measured spatial density of events,radiated energy and seismic potency ahead of the face.High strain gradients around the unsupported portion of the conventionally blasted excavation were implied by the rate at which the spatial density of seismicity changed with respect to the tunnel face position.In contrast,the change in the spatial density of seismicity around the destressed development face was much more gradual.This was indicative of lower strain gradients in the rock there.A reduction in rock mass stiffness following destress blasting was also indicated by the much wider variety of seismic source mechanisms recorded adjacent to the destressed tunnel.Seismic source mechanisms associated with destress blasting were also more clearly characteristic of compressive overstressing with fracture closure.The source mechanism data also indicated that destress blasting induced instability on all natural joint sets.When compared to conventional development blasting,destress blasting typically reduced violent strain energy release from the rock mass and the associated seismicity,but not always.

Seismic evaluation of the destress blasting efficiency

In this paper, selected methods of destress blasting efficiency assessment are presented, and novel quantitative methods based on in situ seismic measurements are proposed. The newly formulated solution combines two different approaches. The first, which is useful mostly for the near-field seismic analyses, is based on the analysis of seismic amplitude characteristics, and the second, relevant for farfield evaluation, is extended by the duration and frequency of the seismic wave. Both approaches are based on the seismic analyses of the waveforms generated by blasting recorded by the local seismic network. The proposed solutions are tested and validated in deep underground mines in Poland in which the room-and-pillar mining method is applied. Based on performed analysis, it is shown that both methods may be used as a rockburst hazard control in underground mines. However, developed methods may also be successfully implemented in other engineering practices, including the assessment of seismic vibrations in open pits and quarries.

Numerical modeling of destress blasting for strata separation

Destress blasting(DB)implemented along the perimeter of safety pillars is a special application of destressing in coal longwall mining.The goal is to separate relatively more deformed mined areas from safety pillars,such as shaft pillars or cross-cut pillars,to reduce the transfer of high stresses to the protective pillar.This case study aims to numerically simulate selected destress blasts in the Czech part of the Upper Silesian Coal Basin and examine its impact on stress transfer to the safety pillar area.To separate the area between the protective pillar and the longwall(LW),two fans of five 93-mm blast holes(length of 93e100 m)were drilled from the gate roads into the overburden strata.Each set of blast holes was fired separately in two stages without time delay.The explosive charge(gelatin-type of explosive)of each stage is 3450 kg.The two DB stages were fired when the longwall face was approximately 158 m and 152 m away from the blast.A 3D mine-wide model is built and validated with in situ stress measured with hydrofracturing.Mining and destressing in three 5-m thick coal seams are simulated in the region.Numerical modeling of DB is successfully conducted using a rock fragmentation factor a of 0.05 and a stress reduction/dissipation factor β of 0.95.Buffering of transfer of additional stress from the mining area into the safety pillar is evaluated by comparison of yielding volume before and after DB.It is shown that yielding volume drops after DB by nearly 80%in the area of the destressing panel and near the safety shaft pillar.

Numerical Simulation of the Relationship between the Width of Destressed Zone and Blasthole Depth

Overstress in the surrounding rock of the roadway is a key reason that causes failures of deep roadways. Destressing blasting is one of the promising techniques that could improve the supporting quality. If the depth of the pressure relief blast hole is too shallow, the surrounding rock of the roadway will be broken or even collapsed. If the pressure relief blast hole is too deep, the pressure relief area will be located in the deep part of the surrounding rock of the roadway, which cannot achieve the purpose of releasing the stress in the shallow part of the surrounding rock and cause waste of the blast hole. The width or range of the pressure relief area should just fall in the high stress area of the surrounding rock of the roadway, so the pressure relief blast hole should have a reasonable depth. In order to quantitatively describe the relationship between borehole depth and the width of the stress relief zone, numerical simulations were carried out in ANSYS according to different borehole depths. The results show that the optimal destressing effect is achieved when borehole depth is 4 m. Peak stress of and is significantly reduced by 30.51% and 49.07% after blasting. Meanwhile, the high-stress area shifts about 4.8 m from the roadside to the depth of surrounding rock, thus a 3.8 m wide stress relief zone is formed around the roadside, thus, the aim of quantizing the effects of destress blasting is achieved.

Numerical study on rock failure around a tunnel destressed by a conceptualized notched technique

A destressing method for reducing the strainburst risk in burst-prone grounds is suggested.In this method,the rock is destressed by cutting notches at the excavation boundary.First,the concept of the proposed method is described both analytically and numerically.Then,the method is applied to a tunneling problem.Several numerical models are built to study the destressing process and the failure mechanism around a circular tunnel.Results show that when the notch is added to the model,the rock at the tunnel wall is destressed,and the stress concentration zones shift to a farther distance away from the wall.Also,the analysis of the failure zone around the tunnel and the velocity of the failed elements show that the failure in the notched tunnel is less violent compared to that in the tunnel without the notch.Finally,a parametric study is conducted to investigate the influences of the notch dimensions on the stress distribution,deformation,and failures around the tunnel.

近断层石门揭煤围岩控制技术研究

为研究断层影响下巷道围岩应力的演化规律,以近断层石门揭煤为研究背景,利用数值模拟手段对巷道围岩变形破坏进行分析,并提出了巷帮钻孔卸压技术措施。结果表明:受断层活化的影响,石门在揭煤过程中靠近断层的帮部应力集中程度高,垂直应力达到44 MPa;最佳卸压钻孔布置方案为钻孔直径300 mm、钻孔长度5 m和钻孔间距600 mm,该方案下原应力峰值区应力由44 MPa降低到28.64 MPa,应力释放率可达到35%,巷道围岩控制效果显著。该研究为断层构造下巷道围岩控制提供了一种新思路,可有效防治煤矿断层错动型冲击地压灾害。

特厚煤层掘进巷道两帮卸压钻孔合理参数数值模拟研究

目前钻孔卸压技术为冲击地压矿井巷道掘进期间防冲主要措施之一,钻孔布置和参数决定着卸压效果。目的为了探讨特厚煤层掘进巷道两帮卸压钻孔合理布置参数,方法基于耿村煤矿综放工作面运输巷道地质采矿条件,利用数值计算方法研究掘进巷道两帮在单排布置时孔深、孔径、孔间距,双排布置时排距等不同参数条件下,巷道围岩应力云图和两帮垂直应力分布特征。结果结果表明:(1)单排布置卸压钻孔时,随着孔深、孔径递增,巷道两帮高应力区逐渐向深部围岩转移,而巷道两帮浅部围岩处于低应力区。(2)当钻孔孔深、孔径达到一定值时,巷道两帮支承压力由“单峰型”转化为“双峰型”,浅部围岩处于临界卸压状态;随着孔深、孔径继续增加,巷道两帮处于过度卸压状态。(3)利用卸压后、无卸压时巷道两帮高应力区向深部围岩的转移量ΔL、峰值应力处距巷道表面距离的比值η表示临界卸压和过度卸压状态,确定出卸压钻孔的合理参数。在耿村煤矿13230综放工作面运输平巷进行超前钻孔卸压和锚网索+36U型可缩性金属支架二级支护后,对巷道两帮实施钻孔卸压防冲,效果显著。结论研究结果对冲击地压矿井回采巷道掘进期间的卸压防冲具有指导和借鉴意义。

防治冲击地压的钻孔卸压技术研究进展与展望

钻孔卸压是冲击地压防治中的一种局部防冲措施,但现有研究存在对工程背景和钻孔卸压认识不清等问题,导致一些研究的可信度不高。采用文献调研和理性反思的研究方法,重点对钻孔卸压的界定、我国冲击地压防治中钻孔卸压的早期研究史、钻孔卸压典型工况、钻孔卸压分类、钻孔卸压典型案例等展开研究。结果表明:钻孔卸压是通过钻孔实现卸压目的的一种工程技术,卸压钻孔是为实现卸压目的而施工的钻孔;钻孔卸压并未出现在我国冲击地压防治的早期研究中,进入21世纪以后,我国针对该问题的研究才逐渐增多,钻孔卸压的典型工况包括掘进巷道、回采、修复巷道等;根据不同的分类依据,钻孔卸压可被分为不同类型,钻孔卸压的设计主要涉及卸压钻孔几何参数设计、卸压钻孔组合形式设计和钻孔卸压的施工设计;目前的研究存在钻孔卸压工程与理论不完全对应、过度关注材料特性、对工况区分不清、煤柱侧钻孔卸压研究薄弱等问题,未来应选择符合科学研究范式的方法、手段展开有针对性、有信服力、精细化的研究。

煤矿千米深井巷道围岩支护-改性-卸压协同控制技术

针对煤矿千米深井、软岩、强采动巷道围岩大变形难题,以淮南新集口孜东矿350 m超长工作面运输巷为工程背景,分析了巷道围岩大变形、支护构件失效原因;采用理论分析、实验室试验和井下试验方法,从围岩物性劣化、偏应力诱导围岩扩容、软岩结构性流变及超长工作面采动影响等方面,揭示了高地应力与超长工作面强采动应力叠加作用下巷道围岩大变形机理。以此为基础提出千米深井、软岩、强采动巷道支护-改性-卸压协同控制理念,采用数值模拟对比研究了无支护、锚杆支护、锚杆支护-注浆改性、锚杆支护-注浆改性-水力压裂卸压4种方案巷道围岩应力、变形及破坏规律,阐述了巷道支护-改性-卸压协同控制原理。研发出CRMG700超高强度、高冲击韧性锚杆支护材料,研究揭示了锚杆受拉、剪、扭、弯及冲击复合载荷作用的力学响应特征;开发出微纳米无机有机复合改性材料及配套高压劈裂注浆技术;研发出分段压裂水力压裂卸压技术与设备,形成了巷道支护-改性-卸压协同控制技术。基于上述研究成果,提出口孜东矿示范巷道支护-改性-卸压布置方案与参数,并进行了井下试验与矿压监测。监测结果表明,巷道围岩协同控制技术应用后,巷道变形量降低50%以上,锚杆、锚索破断率降低90%,工作面采动应力明显减小,有效控制了千米深井、软岩、强采动巷道大变形。最后,对下一步的研究工作进行了展望。

卸压开采顶板巷道破坏特征及稳定性分析

通过物理模拟实验显示上行卸压开采顶板岩层运动状况,分析了不同区域顶板巷道的采动破坏特征;利用长距离钻孔跟踪探测桃园煤矿上行开采顶板岩体的裂隙发育过程,得到了顶板不同区域巷道围岩的裂隙分区特征,并推断出该特定条件下上覆岩层采动稳定周期为165 d;采用FLAC软件进一步研究了不同侧压系数和采动应力环境下的顶板巷道的椭圆形断面形状,指出底板必要的加固深度为4.0～6.0 m,明确了卸压区顶板巷道维护的基本原则和控制方法。

煤矿千米深井围岩控制及智能开采技术构想

深地资源开发是我国未来科技发展的重要方向。在分析煤矿千米深井围岩控制及智能开采技术现状和问题的基础上,围绕安全、高效开采这一主题,综合考虑巷道和采煤工作面相互影响,以合理加大工作面长度,实现生产集约化,降低掘进率、提高煤炭回收率为思路,提出要解决的关键科学问题与技术构想。关键科学问题有4个:千米深井巷道围岩大变形机理;巷道围岩支护―改性―卸压协同控制原理;350 m超长工作面应力与覆岩结构演化机理;超长工作面多信息融合智能开采模式,为千米深井围岩控制及智能开采提供理论基础。针对千米深井巷道围岩高应力、强采动的特点,提出巷道支护―改性―卸压"三位一体"协同控制技术,实现高预应力、高强度、高冲击韧性锚杆主动支护,高压劈裂注浆主动改性及水力压裂主动卸压的"三主动"协同作用,解决千米深井巷道围岩控制难题。针对千米深井超长工作面开采过程中覆岩分区破断、矿压动态迁移的特点,以围岩控制为核心,研发液压支架抗冲击技术,开发超长工作面多信息融合的液压支架自适应群组协同控制技术与装备,并系统集成采煤机等其他工作面设备,最终形成千米深井超长工作面智能开采成套技术体系,为深部煤炭资源安全、高效、高回收率开采提供理论与技术保障。

深井煤巷钻孔卸压技术的数值模拟与工业试验

结合工程实例,利用数值模拟方法研究了钻孔卸压的作用机理、巷道围岩动态损伤破坏发展和应力场重新分布的过程.模拟发现:合理布置的卸压孔可以导致巷帮围岩的结构性预裂破坏,从而使围岩高应力向深部转移;在卸压孔和锚网联合支护后改善了围岩附近的应力环境,改变了围岩破坏的时空次序,对控制围岩变形和破坏具有突出的优越性.

深部巷道钻孔卸压机理及关键参数确定方法与应用

基于围岩强度参数衰减规律的室内试验结果,对FLAC3D应变软化模型进行了二次开发,以现场实测矿压显现规律作为已知特征值,反演了岩体力学参数。分析了深部巷道钻孔卸压机理,提出以应力转移效果及围岩变形控制效果作为卸压效果的直接评价指标,初步将卸压程度分为非充分卸压、充分卸压和过度卸压3类,系统研究了卸压钻孔参数(长度、直径和间排距)对深部巷道围岩稳定性的动态作用规律,提出了影响卸压效果的各关键参数的确定方法,完善了钻孔卸压技术体系。工程应用效果表明,采用卸压钻孔参数确定方法设计的卸压方案,围岩控制效果显著,且该方法简单易行。

深部强冲击厚煤层开采上、下解放层卸压效果相似模拟试验研究

为了研究深部强冲击厚煤层开采上、下解放层的卸压效果,采用相似材料模拟试验的方法,通过监测解放层开采过程中及稳定后被解放层应力变化情况,分析应力变化规律,计算下解放层下部合理卸压角,确定了被解放层下平巷的合理位置;比较两次开采上解放层模拟试验的卸压程度,确定了上解放层工作面的合理位置;证明了开采下解放层后,被解放层上平巷附近区域仍是高应力集中区,易发生冲击地压,而开采上解放层后,该区应力明显减小,说明上解放层卸压效果明显;验证了井下发生大能量冲击地压之前,地表沉降出现"反弹"的异常现象。该研究为工作面开采设计提供理论指导,对防治冲击地压具有重要的现实意义。

巷道卸压法应力迁变规律及卸压参数的确定

北洺河铁矿-95 m 水平4#采场进路联巷及回采进路在掘进过程中或成巷后不久就发生片帮冒顶,采用密集的 U 型可缩式金属拱架仍未能控制住巷道围岩的变形与破坏。利用 flac3d 数值模拟的方法研究了不同卸压高度和宽度下采场进路应力分布状态,模拟结果表明,卸压对巷道不同部位的不同应力具有不同程度的卸压效果,卸压可有效降低仰拱处剪应力及巷道仰拱、两帮和底角的最大主应力,巷道顶底板的最大主应力随卸压工程的开挖而增加,卸压宽度对卸压效果影响显著,在北洺河铁矿条件下卸压工程超出巷道边界1～2 m 为宜。根据数值模拟结果和-80 m 分段回采界线的位置,确定卸压工程范围,采用房柱法进行卸压后,采场的应力集中程度得到有效降低,安全回采矿石15万 t,并为高应力矿山开采提供了卸压开采的新思路。

巷道底臌的挠曲效应及卸压效果的分析

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