Physical and numerical investigations of target stratum selection for ground hydraulic fracturing of multiple hard roofs

Ground hydraulic fracturing plays a crucial role in controlling the far-field hard roof,making it imperative to identify the most suitable target stratum for effective control.Physical experiments are conducted based on engineering properties to simulate the gradual collapse of the roof during longwall top coal caving(LTCC).A numerical model is established using the material point method(MPM)and the strain-softening damage constitutive model according to the structure of the physical model.Numerical simulations are conducted to analyze the LTCC process under different hard roofs for ground hydraulic fracturing.The results show that ground hydraulic fracturing releases the energy and stress of the target stratum,resulting in a substantial lag in the fracturing of the overburden before collapse occurs in the hydraulic fracturing stratum.Ground hydraulic fracturing of a low hard roof reduces the lag effect of hydraulic fractures,dissipates the energy consumed by the fracture of the hard roof,and reduces the abutment stress.Therefore,it is advisable to prioritize the selection of the lower hard roof as the target stratum.

Combined blasting for protection of gob-side roadway with thick and hard roof

The deformation control of surrounding rock in gobside roadway with thick and hard roof poses a significant challenge to the safety and efficiency of coal mining.To address this issue,a novel approach combining directional and non-directional blasting techniques,known as combined blasting,was proposed.This study focuses on the experimental investigation of the proposed method in the 122108 working face in Caojiatan Coal Mine as the engineering background.The initial phase of the study involves physical model experiments to reveal the underlying mechanisms of combined blasting for protecting gob-side roadway with thick and hard roof.The results demonstrate that this approach effectively accelerates the collapse of thick and hard roofs,enhances the fragmentation and expansion coefficient of gangue,facilitates the filling of the goaf with gangue,and provides support to the overlying strata,thus reducing the subsidence of the overlying strata above the goaf.Additionally,the method involves cutting the main roof into shorter beams to decrease the stress and disrupt stress transmission pathways.Subsequent numerical simulations were conducted to corroborate the findings of the physical model experiments,thus validating the accuracy of the experimental results.Furthermore,field engineering experiments were performed,affirming the efficacy of the combined blasting method in mitigating the deformation of surrounding rock and achieving the desired protection of the gob-side roadway.

Assessment and control of the mine tremor disaster induced by the energy accumulation and dispersion of thick-hard roofs

In order to solve the problem that current theory models cannot accurately describe thick-hard roof(THR)elastic energy and assess the mine tremor disasters,a theoretical method,a Timoshenko beam theory on Winkler foundation was adopted to establish the THR’s periodic breaking model.The superposition principle was used for this complex model to derive the calculation formulas of the elastic energy and impact load on hydraulic supports.Then,the influence of roof thickness h,cantilever length L_(1),and load q on THR’s elastic energy and impact load was analyzed.And,the effect of mine tremor disasters was assessed.Finally,it is revealed that:(1)The THR’s elastic energy U exhibits power-law variations,with the fitted relationships U=0.0096L_(1)^(3.5866^),U=5943.9h^(-1.935),and U=21.049q^(2).(2)The impact load on hydraulic supports F_(ZJ) increases linearly with an increase in the cantilever length,thickness,and applied load.The fitted relationships are F_(ZJ)=1067.3L_(1)+6361.1,F_(ZJ)=125.89h+15100,and F_(ZJ)=10420q+3912.6.(3)Ground hydraulic fracturing and liquid explosive deep-hole blasting techniques effectively reduce the THR’s cantilever length at periodic breakages,thus eliminating mine tremor disasters.

Mechanism and control technology of strong ground pressure behaviour induced by high-position hard roofs in extra-thick coal seam mining

This work aimed at revealing the mechanism of strong ground pressure behaviour(SGPB)induced by high-position hard roof(HHR).Based on the supporting structures model of HHR,a modified voussoir beam mechanical model for HHR was established by considering the gangue support coefficient,through which the modified expressions of limit breaking span and breaking energy of HHR were deduced.Combined with the relationship between the dynamic-static loading stress of supporting body(hydraulic support and coal wall)and its comprehensive supporting strength,the criteria of ground pressure behaviour(GPB)induced by HHR were discussed.The types of Ⅰ_(1),Ⅰ_(2),Ⅱ_(1),andⅡ_(2) of GPB were interpreted.Results showed that types Ⅰ_(1) and Ⅰ_(2) were the main forms of SGPB in extra-thick coal seam mining.The main manifestation of SGPB was static stress,which was mainly derived from the instability of HHR rather than fracture.Accordingly,an innovative control technology was proposed,which can weaken static load by vertical-well separated fracturing HHR.The research results have been successfully applied to the 8101 working face in Tashan coal mine,Shanxi Province,China.The results of a digital borehole camera observation and stress monitoring proved the rationality of the GPB criteria.The control technology was successful,paving the way for new possibilities to HHR control for safety mining.

Field investigation into directional hydraulic fracturing for hard roof in Tashan Coal Mine

t Research and development of safe and effective control technology of hard roof is an inevitable trend at present. Directional hydraulic fracturing technology is expected to become a safe and effective way to control and manage hard roof. In order to make hard roof fracture in a directional way, a hydraulic fracture field test has been conducted in the third panel district of Tashan Coal Mine in Datong. First, two hydraulic fracturing drilling holes and four observing drilling holes were arranged in the roof, followed by a wedge-shaped ring slot in each hydraulic fracturing drilling hole. The hydraulic fracturing holes were then sealed and, hydraulic fracturing was conducted. The results show that the hard roof is fractured directionally by the hydraulic fracturing function of the two fracturing drilling holes; the sudden drop, or the overall downward trend of hydraulic pressure from hydraulic monitoring is the proof that the rock in the hard roof has been fractured. The required hydraulic pressure to fracture the hard roof in Tashan coal mine, consisting of carboniferous sandstone layer, is 50.09 MPa, and the fracturing radius of a single drilling hole is not less than 10.5 m. The wedge-shaped ring slot made in the bottom of the hydraulic fracturing drilling hole plays a guiding role for crack propagation. After the hydraulic fracturing drill hole is cracked, the propagation of the resulting hydraulic crack, affected mainly by the regional stress field, will turn to other directions.

THEORY AND TECHNOLOGY FOR HARD ROOF CONTROL OF LONGWALL FACE IN CHINESE COLLIERIES

This paper introduced systematically the present situation of the research on theory and technology for hard roof control of coal face in Chinese collieries.

STUDY ON HARD ROOF ROCKBURST IN COAL MINE

Based on practical observation in Mentougou Mine, a general law of roof rockburst is put forward. The destabilization theory of roof rockburst has been established. The general laws of microquake premonition and earth sound in roof rockburst is advanced. The relationship between roof rockburst and rockburst of coal body is studied.

Weakening effects of hydraulic fracture in hard roof under the influence of stress arch

For the problem of hydraulic fracture propagation when weakening the hard roof in fully mechanized top-coal caving stope of ultra-thick coal seam, based on the stress arch theory and the fracture mechanics, a two-dimensional model for hydraulic fracture of the roof in the stope was established to investigate the propagation laws of hydraulic fracture. The result shows that, after mining, the principal stress direction of overlaying rock deflects to form the stress arch, whose arrow height and arch thickness increase with the increase of the mining width and the side pressure coefficient. Within the influence range of stress arch, the hydraulic fracture in hard roof deflects towards the stope direction in the course of propagation and forms the ‘‘arch" fracture, which cuts off the roof below the fracture in a laminated way. The deflection angle of hydraulic fracture increases with the increase of the mining width, but decreases with the increase of the side pressure coefficient and the fractured horizon. This research can provide theoretical basis for the application of hydraulic fracturing method in the stope roof weakening.

Pressure relief and structure stability mechanism of hard roof for gob-side entry retaining

In order to explore the pressure relief and structure stability mechanism of lateral cantilever structure in the stope under the direct coverage of thick hard roof and its impact on the gob-side entry retaining, a lateral cantilever fractured structural mechanical model was established on the basis of clarification for the stress environment of gob-side entry retaining, and the equation of roof given deformation and the balance judgment for fracture block were obtained. The optimal cantilever length was proposed based on the comparison of roof structural characteristics and the stress, deformation law of surrounding rocks under six different cantilever lengths by numerical simulation method. Double stress peaks exist on the sides of gob-side entry retaining and the entry located in the low stress area. The pressure of gob-side entry retaining can be relieved by reducing the cantilever length. However, due to the impact of arch structure of rock beam, unduly short cantilever would result in insufficient pressure relief and unduly long cantilever would bring larger roof stress which results in intense deformation. Therefore, there is optimal cantilever length, which was 7-8 m in this project that enables to achieve the minimum deformation and the most stabilized rock structure of entry retaining. An engineering case of gob-side entry retaining with the direct coverage of 10 m thick hard limestone roof was put forward, and the measured data verified the reasonability of conclusion.

Ground response and failure mechanism of gob-side entry by roof cutting with hard main roof

This study is the result of long-term efforts of the authors’team to assess ground response of gob-side entry by roof cutting(GSERC)with hard main roof,aiming at scientific control for GSERC deformation.A comprehensive field measurement program was conducted to determine entry deformation,roof fracture zone,and anchor bolt(cable)loading.The results indicate that GSERC deformation presents asymmetric characteristics.The maximum convergence near roof cutting side is 458 mm during the primary use process and 1120 mm during the secondary reuse process.The entry deformation is closely associated with the primary development stage,primary use stage,and secondary reuse stage.The key block movement of roof cutting structure,a complex stress environment,and a mismatch in the supporting design scheme are the failure mechanism of GSERC.A controlling ideology for mining states,including regional and stage divisions,was proposed.Both dynamic and permanent support schemes have been implemented in the field.Engineering practice results indicate that the new support scheme can efficiently ensure long-term entry safety and could be a reliable approach for other engineering practices.

Influencing factors analysis of hard limestone reformation and strength weakening under acidic effect

Roof disaster has always been an important factor restricting coal mine safety production.Acidic effect can reform the rock mass structure to weaken the macroscopic strength characteristics,which is an effective way to control the hard limestone roof.In this study,the effects of various factors on the reaction characteristics and mechanical properties of limestone were analyzed.The results show that the acid with stronger hydrogen production capacity after ionization(pK_(a)<0)has more prominent damage to the mineral grains of limestone.When pKa increases from−8.00 to 15.70,uniaxial compressive strength and elastic modulus of limestone increase by 117.22%and 75.98%.The influence of acid concentration is manifested in the dissolution behavior of mineral crystals,the crystal defects caused by large-scale acid action will lead to the deterioration of limestone strength,and the strength after 15%concentration reformation can be reduced by 59.42%.The effect of acidification time on limestone has stages and is the most obvious in the initial metathesis reaction stage(within 60 min).The key to the strength damage of acidified limestone is the participation of hydrogen ions in the reaction system.Based on the analytic hierarchy process method,the influence weights of acid type,acid concentration and acidification time on strength are 24.30%,59.54% and 16.16%,respectively.The research results provide theoretical support for the acidification control of hard limestone roofs in coal mines.

巷帮煤体整体滑脱型冲击地压锚杆防冲支护原理及工程实践

冲击地压是目前严重影响煤炭安全有效开采的灾害之一,研究锚杆防冲支护原理和技术对防治巷道冲击地压灾害具有重要意义和价值。通过对巷帮煤体整体冲入型冲击地压发生的地质条件以及破坏特征进行总结分析,认为坚硬顶板与坚硬煤层是此类型冲击地压的重要地质特征,而巷帮煤体整体滑脱是其主要冲击破坏特征。在此基础上,以巷帮滑脱煤体为研究对象,建立了顶板-巷道-底板复合结构体力学模型,建立了巷帮煤体发生水平滑移的极限平衡方程,并对各个参数进行分析。结果表明:由于顶板反弹使巷帮煤体竖直方向压力降低,巷帮煤体被构造应力推入巷道发生冲击地压。基于该发生机制模型,认为目前的锚杆支护设计体系在防治巷帮煤体冲入型冲击地压存在不足,并基于其发生和破坏特征,建立了针对巷帮煤体整体滑脱型冲击地压的锚杆防冲支护设计原则,即将顶和底帮锚杆锚固端分别穿层打入稳定的顶底板内,并使用长锚索取代中部帮锚杆,提供锚杆支护的防冲作用。基于新建立的锚杆防冲支护设计方法,以大屯矿区孔庄煤矿7305工作面防冲支护为工程背景,在宽煤柱段巷帮锚杆支护采取了防冲设计,帮顶、底部锚杆及补强锚索均锚固于顶底板内部,能够有效吸收煤体滑动动能,提高安全性。

冲击地压矿井充填工作面超前采动应力对充填体充实率的反馈机制

高应力是深部矿井冲击地压灾变频率走高的主要原因,充填开采则是控制岩层运动,缓解采动应力集中程度,降低围岩破坏和冲击风险的有效手段。为研究采空区充填体对超前采动应力的控制能力,以山东古城煤矿1123充填工作面为工程背景,采用理论分析、室内试验、现场实测手段探究充填开采工作面采动应力分布规律,揭示超前采动应力对充填体充实率的反馈机制,指导冲击地压矿井充实率确定。结果表明:工作面开采初期充填体充实率低于80%,坚硬顶板下沉量大,超前采动影响范围大于30 m,应力集中系数达到1.5,断层影响区采动应力影响范围和集中系数分别增至60 m和1.65,片帮冒顶等围岩失稳现象增多;实测了采空区充填体承载应力全程动态演化特征,承载应力分布曲线划分为“快速降低—短暂稳定—快速升高—缓慢降低—二次稳定”5个阶段,低充实率条件下采空区上、中、下3个区域承载应力稳定值分别为1.9、5.2、2.8 MPa;将采空区充填体划分为非充分压实区和充分压实区,构建了充填体支撑作用下坚硬顶板连续沉降模型,得到了坚硬顶板“ʅ”型沉降曲线,非充分压实区范围随充实率近似呈线性减小;试验得到充填体弹性模量和单轴抗压强度随凝固时间的演化曲线,结合顶板沉降曲线和推进速度得到工作面前后采动应力全区域分布曲线;建立了超前采动应力集中程度与充填体承载能力的负指数函数关系,揭示了超前采动应力对充填体充实率的负向反馈机制,实现充填开采降载防冲效果的定量评价;提出了充填体充实率“三位一体”协同提升措施,将1123工作面充实率升高至90%,增强了充填体承载能力,超前采动应力集中系数降至1.3,厚顶煤膨胀变形量减少至50 mm,坚硬顶板破断致冲风险显著降低。

基于博弈组合赋权-云模型的坚硬顶板采场来压强度等级评估研究

为了解决坚硬顶板采场来压强度等级评估的不确定性问题,在力学分析的基础上,从顶板条件、地质条件、开采条件和其他条件等4个方面选取14个指标因素,采用序关系法确定主观权重、改进的CRITIC法确定客观权重,运用博弈论组合赋权,得到最佳综合权重,构建坚硬顶板采场来压强度等级评估云模型。通过云相似度测算,提高评估结果的精确性。以淮南张集矿区11129工作面为例,对9-2煤层上覆坚硬顶板来压强度等级进行评估,应用表明:所提方法综合考虑评价对象的主客观因素,权重分配科学合理,评估结果与实际情况一致,能为现场顶板弱化工作提供理论依据。

工作面单双巷顶板爆破应力演化规律研究

坚硬顶板是影响工作面冲击危险的重要因素,实践中常采用爆破技术对工作面顶板进行预裂和卸压。以田陈煤矿3煤层上方存在坚硬顶板的7202工作面为对象,采用理论分析、数值模拟、工业试验等方法,研究了不同爆破高度、工作面单巷和双巷顶板爆破下围岩应力演化及微震规律。结果表明:针对低位亚关键层采用单巷和双巷爆破时,爆破高度越大,巷道围岩峰值应力越低,卸压效果与爆破高度呈正相关关系;但采用双巷爆破时,当爆破高度增加至6倍采高(亚关键层中部位置)以上时,工作面中部区域会出现应力升高现象,且工作面越短应力升高越明显;在7202工作面初采阶段进行高度为3倍采高的单巷顶板爆破时,微震多集中在巷道区域,卸压效果一般,改为双巷顶板爆破并提高爆破高度至6倍采高后,微震多集中到工作面中部区域,无大能量微震事件发生,应力监测预警次数也大幅降低,巷道区域卸压效果较好。

硬岩顶板切顶留巷围岩变形分区补强控制研究

【目的】森达源煤矿传统的宽煤柱开采方式导致采掘接续紧张,并引发严重的围岩变形问题。切顶留巷技术可优化巷道支护条件,提高采煤效率,促进煤矿高效安全生产。【方法】以1号煤层的10101工作面为研究对象,运用FLAC 3D数值模拟软件,深入探究了切顶高度、切顶角度、注浆加固深度对巷道掘进、预裂切顶和切顶留巷过程中煤帮内部、采空区及顶板应力变形特性的影响。确定出最佳切顶留巷方案。在此基础上,进一步模拟分析工作面前后不同区域的围岩受力变形情况。提出回采工作面前方0~30 m内为超前补强区,采取“高预应力锚索+单体配合铰接顶梁”的联合支护。后方0~100 m内为滞后补强区,采用“工字钢+单体支柱”联合支护,同时对矸石帮实施注浆加固。通过工程应用验证了区分补强控制变形措施的有效性。【结果】结果显示:最佳切顶留巷方案为切顶高度6 m,切顶角度15°,矸石帮注浆深度1 m。此方案下,10101工作面的巷道顶底板的最终变形量为155 mm,两帮的为84 mm,均符合矿井使用要求,验证了切顶留巷技术具有良好效果。【结论】结果表明:切顶留巷技术能有效解决传统宽煤柱开采方式中存在的围岩变形问题。然而,该技术的实施受切顶高度、切顶角度和矸石帮注浆深度等因素的影响较大,需结合数值模拟和现场状况进行具体确定。总体上,切顶留巷技术和分区补强控制措施对围岩变形的控制效果显著,为类似条件下的煤矿开采提供了宝贵的经验借鉴,因而具有广阔的应用前景。

倾斜煤层厚硬顶板切顶留巷关键参数优化研究

为解决回采巷道倾斜厚硬顶板应力集中问题,降低顶板覆岩突然垮落的冲击扰动风险及提高倾斜厚硬顶板切顶留巷围岩控制效果,以长城六矿1301N运输巷切顶留巷为工程背景,综合采用理论分析、数值模拟及现场监测的研究方法开展倾斜煤层厚硬顶板切顶留巷关键参数优化研究。首先,基于岩石碎胀系数、砌体梁理论及现场地质资料,推导计算了1301N运输巷切顶高度和切顶角度的理论最小值。其次,为进一步验证并优化理论推导结果,利用PFC^(2D)数值软件建立了1301N工作面切顶留巷模型,采用控制变量法分析了不同切顶高度和切顶角度下留巷顶板应力、位移及组构张量响应特征。分析结果表明:切顶高度与卸压效果之间存在底数大于1的对数增长关系,即随着切顶高度增加,卸压效果逐渐增强但其增长幅度逐渐减小;切顶角度与卸压效果之间存在“S”型增长关系,即随着切顶高度增加,卸压效果先增强后减弱。卸压参数与卸压效果之间的变化规律揭示了切顶高度选择存在最适值,切顶角度选择存在最优解。综合理论分析、经济收益及数值模拟结果,确定长城六矿1301N运输巷倾斜厚硬顶板切顶留巷过程中最适切顶高度为13 m,最优切顶角度为10°。现场监测获得该切顶参数下围岩控制效果较好,留巷围岩变形量较小,可有效满足下一工作面开采需求,验证了优化切顶参数的有效性,为类似地质条件切顶留巷参数的选取提供理论依据和实践参考。

近浅埋多层厚硬顶板大采高综放工作面矿压显现规律研究

针对榆神矿区近浅埋多层厚硬顶板条件下,“大周期”来压支架立柱下缩量大、煤柱应力集中系数高、稳定性差等强矿压问题,采用了数值计算、微震和煤体应力监测等技术手段,对覆岩破断运动及工作面来压规律进行了分析研究,结果表明:覆岩破断运动上,多层厚硬顶板较大的“垂向离层空间”和“自下而上”的顺次交替垮落孕育了顶板“高+低层位组合破断”的时空基础;“高低层位顶板组合破断”是“大周期”来压增强的关键诱因;同时,煤柱应力的时空演化受控于多层厚硬顶板的破断垮落。矿压显现上,工作面“大周期”立柱下缩量平均可达945 mm,持续距离12.10 m,并可由2~5次连续强来压组成;巷道煤柱应力集中系数达到1.80~2.30,影响范围为工作面前方126 m至后方471.3 m,具有明显的“高强度、大范围、长上升期”特点。针对该条件下的强矿压防治,分别提出了“弱化低位顶板、优化破断结构”和“弱化高位顶板、改变破断次序”的技术思路,并取得的了较好的应用效果。

榆神矿区多层厚硬顶板强矿压显现及覆岩破断运动规律研究

针对榆神矿区多层厚硬顶板条件下工作面强矿压和邻空侧巷道变形严重问题,综合采用理论分析、数值计算和微震监测方法,分析了多关键层结构下顶板覆岩破断和邻空侧巷道围岩应力特征。研究结果表明:多层厚硬岩层条件下,顶板破断垂向位移和能量均呈现明显的“低、中、高”分区特征,中高位岩层破断明显滞后于低位岩层,这构成了工作面“大、小周期”的重要结构基础;本工作面开采时,相邻采空区破碎顶板大能量微震事件增多、破断强度较高,并可对邻空回风巷围岩形成强二次采动影响,这是邻空侧回风巷高应力、强变形的主要诱因。同时,高低位多关键层组合破断可导致“大周期”来压发生强度和风险上升,对关键层顶板破断运动规律进行控制是降低“大周期”强度和频率的有效途径之一。

煤矿坚硬顶板灾害水力压裂防治技术监测及评估

水力压裂技术具备安全性好、施工工艺简单的优点,被广泛应用于煤层坚硬顶板灾害治理。但由于岩体的不均质性,压裂孔周围裂隙网络扩展规律复杂,水力压裂裂纹监测精度低,压裂效果评估难度大。针对上述问题,通过对比分析常见水力压裂监测方法及监测方案的差异,揭示微震监测技术的优越性,并对水力压裂效果评估技术进行了讨论。研究表明,微震监测技术具有监测范围广、灵敏度高、连续性好的优点;地面监测、井下监测、联合监测等微震监测方法适用工况不同,监测精度相差较大,应根据现场工程布置与实际岩层条件选取合适的监测方法。坚硬顶板水力压裂效果评估多采用裂缝发育长度、来压步距等间接指标,基于微震参变量直接指标的效果评估相对较少。在此基础上,提出了水力压裂效果评估精细化的发展建议。研究成果对开展坚硬顶板水力压裂现场监测、优化坚硬顶板水力压裂工程设计具有重要指导意义。