Assessing the range of blasting-induced cracks in the surrounding rock of deeply buried tunnels based on the unified strength theory

Blasting-induced cracks in the rock surrounding deeply buried tunnels can result in water gushing and rock mass collapse,posing significant safety risks.However,previous theoretical studies on the range of blasting-induced cracks often ignore the impact of the in-situ stress,especially that of the intermediate principal stress.The particle displacement−crack radius relationship was established in this paper by utilizing the blasthole cavity expansion equation,and theoretical analytical formulas of the stress−displacement relationship and the crack radius were derived with unified strength theory to accurately assess the range of cracks in deep surrounding rock under a blasting load.Parameter analysis showed that the crushing zone size was positively correlated with in-situ stress,intermediate principal stress,and detonation pressure,whereas negatively correlated with Poisson ratio and decoupling coefficient.The dilatancy angle-crushing zone size relationship exhibited nonmonotonic behavior.The relationships in the crushing zone and the fracture zone exhibited opposite trends under the influence of only in-situ stress or intermediate principal stress.As the in-situ stress increased from 0 to 70 MPa,the rate of change in the crack range and the attenuation rate of the peak vibration velocity gradually slowed.

Borehole stability in naturally fractured rocks with drilling mud intrusion and associated fracture strength weakening:A coupled DFN-DEM approach

Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P w)and pore pressure(P p)during drilling,which may cause wellbore instability.However,the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses,which may yield significant errors and misleading predictions.In addition,only limited factors were analyzed,and the fracture distribution was oversimplified.In this paper,the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM(distinct element method)and DFN(discrete fracture network)method.It provides estimates of the effect of fracture strength weakening,wellbore pressure,in situ stresses,and sealing efficiency on borehole stability.The results show that mud intrusion and weakening of fracture strength can damage the borehole.This is demonstrated by the large displacement around the borehole,shear displacement on natural fractures,and the generation of fracture at shear limit.Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure,which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations.A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore.Moreover,the effect of sealing natural fractures on maintaining borehole stability is verified in this study,and the increase in sealing efficiency reduces the radial invasion distance of drilling mud.This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations,which can consider the discrete fracture network,mud intrusion,and associated weakening of fracture strength.The information provided by the numerical approach(e.g.displacement around the borehole,shear displacement on fracture,and fracture at shear limit)is helpful for managing wellbore stability and designing wellbore-strengthening operations.

Stress evolution of deep surrounding rock under characteristics of bi-modulus and strength drop

Aiming at the circular chamber under uniform stress field in deep energy storage and mining,analytical solutions of stress and plastic zone of the surrounding rock under different far-field stress and internal pressure were derived based on bi-modulus theory and the elastic-brittle-ideal plastic constitutive model.Evolution trend of the elasticplastic stress and plastic region with different elastic constant ratios and residual strength coefficients were analyzed in details.Results revealed that when the internal pressure was small,the three-direction principal stress was compressive stress and the stress field distribution of the surrounding rock was not affected by the moduli difference.The obtained solution was consistent with the solution from the elastic-brittle plastic drop model under the equal modulus theory.On the other hand,when the internal pressure was large,the tangential stress was changed.The surrounding rock can be divided into three zones,i.e.,tensile plastic zone(TPZ),tensile elastic zone(TEZ)and compressive elastic zone(CEZ).The tensile and compressive dual modulus had significant influence on the demarcation point between TEZ and CEZ.In addition,the strength drop and the dual modulus characteristic had a coupling effect on the stress distribution in the surrounding rock.The related achievements further enrich the theory of deep rock mechanics.

Research on the stability analysis and design of soil tunnel surrounding rock

The paper first analyzes the failure mechanism and mode of tunnel according to model experiments and mechanical calculation and then discusses the deficiency of taking the limit value of displacement around the tunnel and the size of the plastic zone of surrounding rock as the criterion of stability. So the writers put forward the idea that the safety factor of surrounding rock calculated through strength reduction FEM(finit element method) should be regarded as the criterion of stability,which has strict mechanical basis and unified standard and would not be influenced by other factors. The paper also studies the safety factors of tunnel surrounding rock (safety factors of shear and tension failure) and lining and some methods of designing and calculating tunnels. At last,the writers take the loess tunnel for instance and show the design and calculation results of two-lane railway tunnel.

Rock burst risk evaluation based on equivalent surrounding rock strength

On-site investigations consistently show that the rock burst inherent to coal seams varies greatly with coal seam thickness.In this study,impact factors related to coal seam thickness and surrounding rock strength were analyzed and a corresponding rock burst risk assessment method was constructed.The model reflects the influence of coal seam thickness on the stress distribution of surrounding rock at the roadway.Based on the roadway excavation range,a stress distribution model of surrounding roadway rock is established and the influence of coal seam thickness on rock burst risk is analyzed accordingly.The proposed rock burst risk assessment method is based on the equivalent surrounding rock strength and coal seam bursting liability.The proposed method was tested in a 3500 mining area to find that it yields rock burst risk assessment results as per coal seam thickness that are in accordance with real-world conditions.The results presented here suggest that coal seam thickness is a crucial factor in effective rock burst risk assessment.

Strength weakening effect of high static pre-stressed granite subjected to low-frequency dynamic disturbance under uniaxial compression

This study aimed to elucidate the strength weakening effect of high static pre-stressed rocks subjected to low-frequency disturbances under uniaxial compression.Based on the uniaxial compressive strength(UCS)of granite under static loading,70%,80%,and 90%of UCS were selected as the initial high static pre-stress(σ_(p)),and then the pre-stressed rock specimens were disturbed by sinusoidal stress with amplitudes of 30%,20%,and 10%of UCS under low-frequency frequencies(f)of 1,2,5,and 10 Hz,respectively.The results show that the rockburst failure of pre-stressed granite is caused by low-frequency disturbance,and the failure strength is much lower than UCS.When theσp or f is constant,the specimen strength gradually decreases as the f or σ_(p) increases.The experimental study illustrates the influence mechanism of the strength weakening effect of high static pre-stress rocks under low-frequency dynamic disturbance,that is,high static pre-stress is the premise and leading factor of rock strength weakening,while low-frequency dynamic disturbance induces rock failure and affects the strength weakening degree.

Experimental Study on Dynamic Mechanical Properties of Sprayed Concrete-Surrounding Rock Combined Body

To investigate the dynamic response problem of the double medium formed by the adherence of sprayed concrete and surrounding rock in the tunnel,a split Hopkinson pressure bar of 75 mm in diameter was adopted at the ages of 3,7 and 10 d.Experimental results showed that dynamic compressive strength and dynamic increase factors(DIF)of the combined bodies increase with the strain rate.With the growth of strain rate,the critical strain of the combined bodies first increases,then deceases.Furthermore,the combined bodies of 3 d reveal the plastic property and brittle property for 7 d and 10 d when the strain rate is over 80/s.The failure characteristic of the sprayed concrete changes from tearing strain damage to crushing damage as the growth of strain rate,and the failure characteristic of rock presents the tensile failure mode as demonstrated by the scanning electron microscope(SEM).

连采连充膏体强度对围岩稳定性的影响分析

文章针对膏体强度对围岩稳定性的影响,分析了不同强度的充填体对连采连充过程中覆岩稳定性的影响。研究结果显示,当充填体强度设定为4MPa时,工作面围岩的应力变化情况及上覆岩层受开采活动的影响最为平稳;二次开采与充填后,工作面顶板及其直接顶区域虽出现了一定程度的弯曲与下沉,但充填体与煤柱之间形成了稳固的承载结构;三次开采与充填后,充填体逐渐成为主要的承载结构,与上覆岩层共同建立起了新的应力平衡状态;采用4MPa强度的充填体能够提升连采连充工作面的覆岩稳定性。

多因素影响下巷道围岩稳定性分析及优化设计研究

针对深部煤层巷道矿压显现剧烈且巷道变形难以控制的问题,基于Kastner公式对巷道高宽比、侧压系数、埋深、断面形状、围岩强度5种影响巷道围岩稳定性的因素展开正交数值模拟实验,通过方差和直观分析图法研究了各因素对巷道变形破坏的敏感性。采用平面弹性复变函数理论分析了不同侧压系数与巷道高宽比下椭圆和矩形巷道围岩的应力分布规律,对椭圆形和矩形巷道宽高比和形状进行了优化设计。研究结果表明:围岩强度与侧压系数是影响巷道围岩稳定性的主要因素,埋深、断面形状、巷道高宽比为次要因素;椭圆形巷道在等轴应力比下围岩应力差值最小;矩形巷道宽度为5 m、高宽比为0.6左右时,应力集中程度较小;针对矩形巷道应力快速增高区,在考虑承压特性和断面利用率的基础上设计了圆角矩形优化巷道,确立了圆角半径的计算公式,通过数值模拟验证了其良好的性能。

巷道蝶形破坏强度准则低敏感性研究及工程应用

巷道围岩的塑性区形态对巷道的破坏形式及破坏程度有重要影响。为探究三向应力场下塑性区形态演化过程,基于弹性力学推导了轴向应力表达式,并依据蝶形塑性区边界方程求解思路,确定了三维强度准则下三向塑性区近似解的求解方法。通过等球应力p、等偏应力q以及不同Lode角θσ来确定围岩应力加载方案,对不同三维强度准则下的围岩塑性区形态演化规律进行深入研究,论证了蝶形破坏的准则低敏感性。基于蝶形破坏理论对羊场湾160206回风巷道的非对称变形破坏机制及控制技术进行深入分析。研究结果表明:(1)在相同p、q及不同θσ的应力加载条件下,5种强度准则下的塑性区形态均呈现圆形、类椭圆及蝶形的演化规律,且每种强度准则在相同θσ的情况下围岩的塑性区形态基本一致。(2)相同应力大小、不同应力方向的加载方案下,围岩的塑性区形态大不相同。圆巷围岩的塑性区形态很大程度上由水平侧压比决定,轴向侧压对围岩的塑性区尺寸影响较大,对塑性区形态影响较小。(3)羊场湾160206回风巷道在叠加采动影响下顶板呈现非对称大变形破坏,基于蝶形塑性区支护思路,应用非对称锚杆索+超前单元支架+钻孔卸压的协同支护技术,取得了良好的支护效果。

深埋综放工作面顺槽围岩动力破坏机理及其支护技术

为探究西部深埋综放工作面围岩动力破坏机理,提出了高强度锚索支护与围岩卸压协同作用的支护方案,完成了5×10^(5)J冲击能量下、不同卸压技术围岩动力破坏计算机仿真分析,确定了工作面顺槽卸压参数,制定了围岩变形规律监测方案并完成了实测工作。数值模拟表明:锚索由原设计φ21.6 mm×8200 mm提高至φ28.6 mm×8200 mm,顶板锚索数量由原7根减少至5根;冲击能量为5×10^(5)J时,顺槽的部分锚杆被拉断,巷道发生失稳破坏,围岩进行卸压后,顶板应力减小15.03%,帮部应力减小6.61%;顶板位移减小31.23%,帮部位移减小38%;地面水平井压裂深度为20~30 m,顺槽围岩应力和围岩变形下降较快,应力下降率降幅明显。实测结果表明:高强度锚索与围岩卸压协同作用,可以有效控制围岩变形。

金沙江上游白格滑坡黏土化蚀变岩的灾变力学行为研究

金沙江构造结合带是典型的板块碰撞作用形成的构造混杂岩带,不仅地质结构复杂,而且发育了工程特性极差的特殊岩土体。本文在金沙江上游白格滑坡成因机理调查与分析过程中,新发现了巨型滑坡体后缘滑壁发育厚度较大、分布较连续的黏土化蚀变岩,是构成滑动带的重要组分。基于相关样品的原位测试和室内系统的物质组成、耐崩解、环形剪切、动三轴等物理力学试验研究,揭示了黏土化蚀变岩的灾变力学特性及其在白格滑坡形成演化中的作用。主要结论如下:1)白格滑坡黏土化蚀变岩是金沙江构造结合带中蛇绿岩蚀变的产物,蚀变程度高,蚀变系数介于0.64~0.86之间,黏土矿物成分以伊利石为主、伊/蒙混层矿物次之,遇水极易崩解,崩解指数达50%以上;2)黏土化蚀变岩中工程性质最差的是蚀变黏土,水和振动荷载作用下剪切强度弱化显著,含水率由10%增至20%后剪切强度降幅达34%,动强度远小于其静强度,并随着振动次数的增加不断降低,蚀变黏土是促进斜坡岩体结构面贯通、破坏的重要因素;3)白格滑坡滑动面的形成受节理结构面与黏土化蚀变岩联合控制,整体上可分为2种模式:黏土化蚀变岩厚层软弱夹层型和充填节理裂隙贯通型;4)金沙江上游发育的不少巨型高位滑坡与黏土化蚀变岩不良工程特性具有密切的关系,岩体结构与特殊岩性的组合是值得关注的区域性易滑地质结构。以上结论对于青藏高原东缘大江大河岸坡稳定性分析和防灾减灾具有较好的启示意义。

反井钻机施工大直径风井工程结构稳定性评价方法研究

针对大直径反井工程在扩孔期间的围岩稳定性问题,提出了一种考虑反井钻井法工艺特点的竖井结构稳定性评价方法,确定了围岩自稳能力是其关键因素,基于强度折减法,提出了以围岩稳定系数为核心的井帮围岩自稳能力定量评估方法,通过数值模拟手段确定了围岩极限自稳状态的强度折减系数k0,并以此作为围岩的稳定系数。为解决工程实际问题,提出了预防空帮失稳的工程处理措施,在扩孔阶段实施随钻临时支护作业,实现围岩的快速封闭。最后以李家壕煤矿回风暗立井工程作为研究对象,提出了局部失稳风险评估意见,并依托工程实践对评估准确性进行了验证,其结果表明评估结论与实际结果一致,验证了该井帮围岩稳定性定量评价方法及工程处理措施的有效性。

卸载状态下非均质圆形寒区隧道围岩弹塑性统一解

以统一强度理论作为卸载状态下寒区隧道冻结围岩屈服准则,综合考虑围岩非均质特性和中间主应力效应对冻结围岩强度影响,建立寒区隧道应力位移弹塑性力学模型,联合各区域边界条件,计算获得冻结围岩均质与非均质状态下,弹性解、塑性统一解以及塑性区半径的隐式方程,分别对其应力位移场讨论分析。研究表明:考虑冻结围岩的非均质特性,塑性区环向应力峰值增大40%,塑性区范围相对减少40.4%,内壁位移减少9.3%,弹性极限承载力提高41%,塑性极限承载力提高14%,影响显著。中间主应力效应能充分发挥非均质冻结围岩承载潜能,计算得到的承载力明显增大,塑性半径明显减小。所得结果可为寒区隧道开挖支护设计以及数值模拟,提供理论指导。

考虑刚度与强度劣化的洞室围岩弹塑性分析

地下洞室锚固围岩进入塑性状态后其刚度与强度均会产生一定程度的劣化。将锚固围岩视为复合岩体并借助参数等效原则求出其刚度与强度参数后,基于D-P屈服准则得到了考虑劣化效应的深埋圆形洞室锚固围岩弹塑性解,借助FLAC-3D有限差分软件验证了该方法的可靠性后对相关影响因素进行了具体分析。研究结果表明:①考虑锚固围岩的刚度与强度同时劣化后得到的围岩变形量更符合实际;②残余黏聚力c"、残余内摩擦角φ"、残余弹性模量E"的大小对锚固塑性残余区半径Rb、洞壁位移u具有控制性作用,当c",φ",E"取值越大时,Rb,u便会越小,反之Rp、Rb、u便会越大;③软化模量Mc、Mφ对塑性残余区半径Rb、洞壁位移u的影响较大,对塑性软化区半径Rp的影响可以忽略;④系统锚杆与支护阻力pi协调作用下可以有效控制塑性残余区范围及围岩变形。

基于收敛——约束法的剪胀——软化型隧道围岩——支护结构相互作用

目前的隧道围岩支护结构计算方法未能很好地同时考虑围岩剪胀及软化特性和中间主应力的共同影响。首先介绍了考虑岩石剪胀及软化效应的隧道围岩力学模型及考虑中间主应力的广义SMP强度准则。而后,根据收敛—约束法中隧道纵向变形曲线、围岩特征曲线、支护特征曲线的相互关系,建立了能够同时考虑隧道围岩剪胀及软化特性和中间主应力影响的隧道围岩—支护结构相互作用力学模型,并验证了其合理性。最后,采用敏感性分析分别探讨了地应力参数、支护结构参数和岩石参数对围岩最终位移、支护结构最终支护力及变形量的影响。结果表明:地应力的增加将导致围岩变形及塑性区增大,所需支护力增加。中间主应力系数对计算结果有较大影响,应予以考虑;支护结构弹性模量的增加可以提高支护力,并大幅度减少围岩和支护结构变形;岩石弹性模量及临界软化系数的增加可以降低围岩变形及支护结构受力。

降雨开挖诱发顺层岩质滑坡稳定性与堆积特征

在自然环境与工程建设中,顺层岩质滑坡因其易发、频发、突发等特征,一直是地质灾害领域研究热点之一。以湖北省建始县大包山滑坡为例,综合现场调查、无人机(UAV)摄影等手段,开展含软弱夹层结构面强度弱化规律试验,考虑强度弱化的降雨-开挖联合作用,进行边坡动态稳定性与堆积特征数值模拟分析,探讨边坡破坏诱因,揭示边坡变形机制。主要结论如下:①自然边坡在天然、暴雨工况下稳定性系数分别为1.35和1.20,开挖边坡在天然、暴雨工况下稳定性系数分别为1.04和0.90;最不利工况下,边坡第四级开挖后,发生失稳破坏,诱因为降雨-开挖联合作用;②大包山滑坡破坏模式为典型的滑移-拉裂破坏,即滑坡体前缘侧向隆起,中后缘牵引式滑动,后缘发生拉裂破坏,滑坡体沿软弱结构面整体性顺层平面滑动;③大包山滑坡的变形破坏和启滑运动堆积过程可划分为5个阶段,即裂缝发展贯通阶段、启滑动态响应阶段、整体高速滑动阶段、平台碰撞制动阶段、堆积侧向挤滑阶段;滑坡前缘于坡脚处堆积,堆积距离约15~25 m,整体滑移水平距离约15 m,滑坡最大位移量高达30 m。研究成果对类似顺层岩质滑坡研究具有一定参考价值。

干热环境下钢纤维-水泥基材料增强增韧效果影响研究

为了研究干热环境下钢纤维对水泥基材料的增强增韧效果,以高地温隧道干热环境为背景,通过调研将隧道的围岩温度分为4个等级(40℃、60℃、80℃和100℃)并设置相应的模拟环境,通过力学性能试验研究不同干热环境对水泥砂浆强度、钢纤维与水泥砂浆界面黏结性和钢纤维拔出功的影响,分析干热环境的劣化机理,最后通过钢纤维混凝土断裂性能试验进行对比验证。主要得到结论:(1)干热环境在早期有利于水泥砂浆抗压强度及钢纤维与水泥砂浆界面黏结强度的增长。7 d龄期以后,干热环境下水泥砂浆抗压强度及钢纤维与水泥砂浆界面黏结强度明显低于标准养护环境,下降的程度与温度呈正相关。(2)干热环境下的钢纤维-水泥砂浆的拔出功大于标准养护条件下的拔出功,表明干热环境可以提高钢纤维的增韧效果,但是温度过高钢纤维的增韧效果下降。(3)随着温度的升高钢纤维-混凝土试件的断裂能均呈现先增大后减小的趋势,且断裂能在60℃时出现最大值。

高强预应力锚杆力学性能实验与支护研究

高强预应力锚杆支护是传统锚杆支护技术的进一步优化发展。由于深部巷道具有围岩破碎、高应力等复杂地质特征,更多的高强预应力锚杆替代传统锚杆应用于其中。本文通过理论分析,预应力转化试验,左右旋锚杆拉拔试验以及锚固力试验,对比分析高强预应力锚杆的力学性能,揭示了新型左旋高强预应力锚杆加固围岩的支护机理。研究结果表明:(1)理论分析锚杆在围岩中的作用机理,阐明了预应力锚杆对围岩的加固作用;(2)滚丝锚杆扭矩转化成预紧力的效率约为等强全螺纹锚杆1.7~1.9倍;不同直径的左旋滚丝锚杆力学性能均优于右旋全螺纹锚杆,左旋螺纹钢锚杆锚固力大于右旋螺纹钢锚杆;(3)分析新型左旋高强预应力锚杆的结构,其具有预应力高、锚固力高、有效断面大等优势。本研究为深部巷道围岩支护技术的优化与创新提供了有益的参考和借鉴。

深井软底沿空留巷围岩变形及控制分析

为解决深井软底沿空巷道在高应力环境下变形严重等问题,以杜儿坪矿43209工作面为例,针对围岩变形特征,提出了“超前弱化钻孔+顶板高强锚注+反底拱让压填充+底板注浆+巷旁柔模充填体”的支护方案,通过施工超前工作面弱化钻孔,结合注浆及底板填充让压材料,实现柔性支护。现场试验结果表明,采用优化后的支护方案后,顶板位移量由327 mm减小至108 mm,底板位移量由614 mm减小至149 mm,煤帮位移量由294 mm减小至62 mm,巷道收敛率明显减小,达到了良好的效果。