Analysis of fracture propagation in a rock mass surrounding a tunnel under high internal pressure by the element-free Galerkin method

Fractures developed around high pressurized gas or air storage tunnels can progressively extend to the ground surface, eventually leading to an uplift failure. A tool reasonably reproducing the failure patterns is necessary for stability assessment. In this study, a numerical method based on the element-free Galerkin (EFG) method with a cohesive crack model is developed to simulate fracture propagation patterns in the rock mass around a tunnel under high internal pressure. A series of physical model tests was also conducted to validate the reliability of the developed method. A qualitative agreement between physical model tests and numerical results can be obtained. The in situ stress ratio, k, has a strong influence on both the position of crack initiation and the propagation direction. The numerical analyses were extended to full-scale problems. Numerical tests were performed to investigate the prime influencing factors on the failure patterns of a high pressurized gas circular tunnel with varying parameters. The results suggest that initial in situ stress conditions with a high k (larger than 1) is favorable for construction of pressurized gas or air storage tunnels.     

不同倾角节理组和锚固效应对岩体特性的影响

首先采用DDARF(discontinuous deformation analysis for rock failure)分析方法对双裂隙岩块进行单轴和双轴压缩模拟试验,研究了裂隙角度和侧向应力大小对岩块特性的影响,得到了裂隙岩块在这两种加载试验中的破坏过程、应力-应变曲线以及岩块中裂隙的起裂应力和岩块的峰值强度。在双轴压缩模拟试验中绘制了裂隙角度为45°的岩块在不同侧向压力下的强度包络线。其次,采用DDARF分析方法模拟劈裂试验中含裂隙试块的锚固效果,得到了4种不同锚固角度试块的轴向荷载–位移变化曲线和裂隙扩展规律。模拟结果与前人的类似条件下的试验结果相符良好。随后又将双裂隙试块双轴压缩模拟试验中得到的参数运用到一个地下洞室的工程实例中,用等效力学特性的方法分析对比了完整岩体和节理岩体洞室开挖完成后的破损状态的差异。最后运用DDARF分析方法研究了随机生成4组节理岩体的地下洞室的稳定性,得到了洞室节理围岩的裂隙扩展过程。同时通过对关键点位移的监测分析了锚杆的锚固效应。     

含倾斜夹层盐岩体变形与破损特征分析

为了研究含倾斜非盐夹层盐岩体的变形与破损特性,开展了含倾斜夹层盐岩试样的单轴、三轴压缩试验,并结合复合岩体破坏的理论分析,探讨了界面黏结应力的变化规律及其影响因素,揭示了倾斜层状盐岩体变形与破坏特征及其机制。研究表明,倾斜夹层对盐岩体的变形与破损具有显著影响。夹层各部破损形式与其所处位置有关;裂纹在界面处最先萌生、进而扩展到夹层和盐岩层;试验观察到盐斑的穿晶断裂现象并揭示了其力学机制;夹层厚度影响着盐岩体整体性能和破损特性,相对于薄夹层、中厚夹层更容易形成宏观裂纹;围压作用减少了夹层不同部位的破裂形态差异及界面黏结应力差异。该结果为含地层倾角盐岩矿区储气库的造腔优化及其稳定性、密闭性分析提供了理论参考,建议造腔前应优选地质层段,以同时满足稳定性和密闭性要求。     

水力作用下岩质斜坡破坏机制和稳定性分析研究现状

基于国内外研究现状和岩质滑坡案例,总结出岩质滑坡的水力致灾机制,归纳考虑水力作用下的岩质斜坡主要失稳破坏模式,评述了岩质斜坡稳定性分析方法。岩质滑坡的水力致灾机制主要由于水对滑体产生的静水压力（岩体侧面的推力、滑面的扬压力和岩体的浮力）和动水压力（向坡外的渗透力）作用。从渗流—应力耦合的角度可较全面评价水渗流对坡体稳定性的影响。斜坡的岩体结构决定了水力作用方式和坡体的失稳破坏形式,考虑水力作用下的岩质斜坡失稳破坏形式主要有:顺层滑动、平推式滑动、楔形体滑移和危岩的崩塌。对于水力作用下岩质斜坡的稳定性分析方法主要有极限平衡法、有限元强度折减法、基于断裂力学的危岩稳定性分析法和渗流—应力耦合模型分析法,其中前两种方法应用较为广泛。      

Upper bound limit analysis of uplift failure in pressurized sealed rock tunnels

The rock around tunnels used for gas storage is subject to high pressures, reaching 30 MPa in the case of compressed air energy storage. Uplift failure of the overlaying rock mass up to the surface represents the main hazard scenario in such cases. The present paper investigates this problem by using the upper bound theorem of limit analysis assuming a continuum rock mass model obeying the Mohr Coulomb failure criterion with tension cut‐off. Tools of the calculus of variations are used to assess the geometry of the failure surface. The effects of geometrical and geotechnical parameters on uplift pressure are analyzed systematically. Charts are then provided, which enable a quick estimation of the upper bound of uplift pressure across a wide range of geotechnical and geometrical conditions.     

Fundamentals of gas containment in unlined rock caverns

Summary The criterion for gas entry into idealized, uniform, water-saturated fractures is examined for one-dimensional, elliptical and cylindrical cavern geometries. This limiting criterion is a necessary condition for gas escape from unlined, pressurized gas storages.Gas-entry phenomena were examined using 450-mm-square glass plates in Hele Shaw configuration with nominal apertures of 50 and 200 microns. The fundamentals of gas entry, as first enunciated by Åberg (1977), are confirmed by the experiments; the critical gas-entry pressure is a function of hydraulic flow regime, fracture-mouth capillarity and fracture-mouth profile. Finite-element simulation based on these fundamentals is substantiated by experimental observations and is therefore used for generalization.The gas-entry criterion is found to be sensitive to details of fracture geometry such as aperture, connection of the fractures to the groundwater regime, geometry of the intersection of the fractures with the gas cavern, disturbance of the rock mass as a consequence of excavation, and cavern shape. In view of the inability to predict or even measure these factors, an alternative criterion which ensures no gas migration away from the gas storage is proposed. This simple criterion insists that gas escape will not occur as long as the water pressure along all possible escape paths increases for some small distance in the direction of potential gas escape. This criterion is a generalization of Åberg's (1977) proposal that the vertical hydraulic gradient should exceed one for no gas escape.     

近置已有盐岩溶腔能源地下储库群特性研究

针对我国须采用密集库群来提高油气地下存储中资源利用率的现状,对近置已有盐岩溶腔库群运营模式下储库稳定性和收缩变形展开了研究。基于声纳测腔成果,建立了符合实际存在的单腔模型及库群模型。对模拟结果进行了对比分析,明确了库群运行模式对储库稳定性的影响规律。发现储库相对位置和深度的差异、夹层地层倾角的存在、相邻储库体积和形状的不等同3个因素会导致某些腔体的收缩变形不对称、腔体形变不规则;不同于单腔的最大位移区域固定出现在腰部,库群模式下受周围腔体的影响,而并无固定的腔壁位移最大点位置;呈现板状存在贯穿整个库群的泥岩刚度大、蠕变性能低,并且其和盐岩交界面的抗剪能力较强,可以提高库群的整体性和稳定性;考虑地层倾角的夹层变形特性成为对库群密闭性不利的影响因素,泥岩-盐岩交接面的拉伸以及泥岩夹层本身的扭转弯曲会使泥岩层产生裂隙,贯通的裂隙可导致各个腔体失去独自运行的能力,影响储库运行效率     

Finite element analysis of a pilot gas storage in rock cavern under high pressure

A three-dimensional finite element analysis was performed to simulate field tests at a pilot gas storage project. The storage was a shallow rock cavern lined with a 6 mm thick steel liner. During the tests, high internal pressure up to 52 MPa was applied and deformation of both rock mass and the steel liner was recorded. The maximum rock displacements of less than 6 mm was numerically predicted and fits the measurement very well. The three-dimensional nonlinear finite element modeling has confirmed the overall stability of the structure and revealed the functioning of each structural component, including the rock mass, concrete lining and the steel liner.     

变权和相对差异函数在盐穴储气库腔体稳定性评价中的应用

盐岩地下储气库腔体以钻井水溶方式开采,溶腔形态可见性差、形态发展可控性难度高,针对畸形溶腔进行科学的稳定性评价是保证其安全运营的重要手段。将变权和相对差异函数理论引入到盐岩溶腔的安全评价中,建立适用于盐穴储气库畸形单腔的稳定性评价模型,首次以溶腔声纳测腔数据为基础,建立三维真实仿真模型并进行数值模拟,将数值模拟结果作为评价指标的评分依据,最后应用该方法对金坛储气库某腔体进行稳定性评价,并针对评价结果提出安全改善措施。研究结果表明,用变权方法对评价指标权重进行处理后得到的结果更加合理,通过将该方法与其他安全评价方法计算结果进行对比,证明了该模型在盐岩储气库畸形溶腔的稳定性评价中具有较好的适用性。     

层间软弱夹层发育的切向边坡失稳模式及稳定性研究

通过系统分析边坡工程地质条件及岩体结构,采用定性分析结合有限元模拟研究了边坡变形破坏模式,并采用块体理论及极限平衡法系统分析边坡局部及整体稳定性。结果表明,切向边坡的变形破坏模式可以分为两类:深层变形破坏为受层间软弱夹层空间展布特征控制的块体失稳,浅表层破坏为局部块体失稳及强风化强卸荷带岩体沿最大剪应力带的滑移-拉裂变形。稳定性分析结果认为边坡浅表层岩体稳定性差,但发生深层破坏的可能性较小。     

Environmental impacts of blast-induced acceleration on slope instability at a limestone quarry

The main objective of this study is to evaluate the influence of blast-induced acceleration on the stability of slopes at Arakli-Tasonu limestone quarry where a planar shear failure has recently been observed. The planar failure occurred within rock layers consisting of limestone and clayey limestone. The triggering mechanism of the planar shear failure was investigated using blast-induced acceleration values obtained from 73 shots, field measurements, laboratory assessment of rock material properties and utilizing limit equilibrium analysis. From the analysis of slope stability, the safety factor at a magnitude of 0.106g was found to be lower than a minimum value of 1.2 in the case of a water-filled tension crack of a slope, 15 m depth. These findings indicate that the planar shear failure was strongly influenced by the acceleration of uncontrolled blasting operations as well as heavy rainfall.     

层状盐岩中储备库油气渗漏风险的故障树分析

油气渗漏是盐岩地下储备库事故的重要类型之一,具有突发性强、损失难以估计的特点。针对国内层状盐岩中储备库运营过程中的油气渗漏风险进行分析,揭示了油气渗漏事故的发生机制并建立了相应的故障树模型。通过对该模型的分析,找出了事故发生的各基本致因事件和故障模式,并得出适用于典型层状盐岩储备库油气渗漏事故的发生概率公式。分析结果表明,国内层状盐岩中储备库发生油气渗漏事故的可能故障模式有28种,且发生条件易于满足而难于防范,因此事故发生的可能性较大;按照各基本事件结构重要度的计算结果,盐岩强度低、盐岩蠕变过量、附近有断层、地震、造腔参数控制不当、非均匀地应力、人为操作失误等是层状盐岩中储备库油气渗漏事故的主要致因事件,并据此提出有效措施防止事故发生;通过专家调查法和故障树法的综合分析得出,金坛盐矿5口老腔储库在近10年的运营期内发生油气渗漏事故的概率为0.703%,属于偶发性事故。     

Displacement Forecasting Method in Brittle Crack Surrounding Rock Under Excavation Unloading Incorporating Opening Deformation

Splitting failure, which is recognized as a special engineering geology phenomenon, occurs continually in the brittle rock mass of caverns during underground excavation. In this paper, a splitting model of linear slippage crack groups is built with fracture mechanics, energy analysis, and crack extension theories. Considering intrinsic cracks in rock mass and change of outer stress, intrinsic cracks propagate into macroscopical splitting cracks that are approximately parallel to the side wall of caverns. The splitting criterion of cavern rock mass and the method for predicting displacement in view of splitting opening displacement are proposed. In the end, the forecasting method is applied to the Jinping-I Hydropower Station, underground caverns engineering in China, the splitting failure zone and forecasting displacement are accordant with the monitoring data. The new forecasting displacement method is proven to contribute to the construction of similar underground caverns.     

Analyses of gas storage capacity in unlined rock caverns

Summary Groundwater control is a means of ensuring that no gas leakage occurs from unlined rock storage caverns. To achieve a better understanding of the principles of gas containment under groundwater and how to obtain maximum gas storage capacity, analyses of groundwater conditions surrounding storage caverns and the effects of different factors on gas storage capacity have been performed and are reported in this paper. The analysis results indicate that the critical gas pressure of a storage facility is considerably less than the natural hydrostatic pressure around the caverns and somewhat less than the water curtain pressure. High gas storage capacity can be obtained by optimizing the design of cavern configuration, water curtain layout, cavern depth, etc. The principles of such optimization are outlined in this paper.     

井喷失控条件下盐岩储库稳定性分析

井喷压力失控是盐岩储气库运营的极端情况。根据一维绝热管流理论解析得到井喷情形下储库卸压规律。在此基础上结合金坛盐矿工程地质条件,模拟对比分析了不同初始内压失控工况下盐岩储库应力状态、变形收敛特征和损伤破坏规律。结果表明,溶腔硬夹层总是先于盐岩体而屈服形成明显的屈服带,失控卸压阶段塑性区逐渐扩展布满整个腔体并向径向延伸6～8 m;运营压力范围内初始内压越低,储库卸压收敛越大,7 MPa下储库体积损失达5.02%;卸压阶段溶腔内壁围岩应变速率跃升至10-2 s-1数量级,属于准静态范畴。研究认为,井喷失控状态下失控初始内压是维持储库腔体稳定性的关键因素,在极端灾害环境中应确保盐岩储库处于较高的运营压力之下     

土岩组合岩体中抗拔桩极限承载力的确定

为全面探究土岩组合岩体中抗拔桩的极限承载力,结合工程岩土参数及试验数据,运用Flac3D数值分析软件对其进行数值模拟分析,即可得到土岩组合岩体中抗拔桩的极限承载力。基于被动状态下的Kotter极限平衡方程式求解土层提供的抗拔力,根据岩石强度,基于Hoek-Brown破坏准则求解抗拔桩嵌岩端岩体的抗拉强度,从而可计算得到嵌岩端岩体的抗拔力;由静力平衡原理,叠加土层及嵌岩端岩体提供的抗拔力及破坏锥体重量,即可得到土岩组合岩体中嵌岩抗拔桩的极限承载力理论解析式。在嵌岩深度较小的情况下,该解析式的理论计算值与数值模拟分析值相接近,但随着嵌岩深度的增加,理论计算值会偏离数值计算值。故结合数值模拟试验值,对提出的极限承载力理论解析式作进一步的修正,得到修正后的极限承载力解析式能反映嵌岩端岩石风化程度、嵌岩深度、土层厚度、桩长对极限承载力的影响。运用修正后的解析式对该地质条件下不同抗拔桩的极限承载力计算表明：数值模拟结果与理论计算结果相吻合,说明所建立的抗拔桩极限承载力解析式的方法是可行的。同时,运用该方法可确定类似工程中嵌岩抗拔桩的极限承载力。     

利用电导率测井与压水试验联合评价岩体渗透性的方法

进行地下水封洞库的水封条件分析评价必须获得准确可靠的建库岩体渗透性参数,而获取岩体渗透系数常用的传统水文地质试验方法存在明显的不足。为了试验数据的精确性,文章基于广义径向流（GRF）理论,依托烟台某地下水封洞工程,以丙烷洞库交通巷道钻孔为例,开展压水试验并采用非稳定流理论的GRF模型优化解析试验数据,结合电导率测井试验确定导水裂隙位置并求出裂隙范围内的渗透系数。试验结果表明:GRF模型比稳定流模型解析结果大1~2倍。原因在于裂隙岩体进行分段压水时,各段水流维度不一,传统稳定流理论假设水流维度只有二维流,而GRF模型为空间n维裂隙流用压水过程全部数据进行拟合,不同时段在相应维度下进行计算,因此其求算的渗透系数K更接近于试验段真值,具有更好的兼容性和实用性。同时利用电导率测井试验计算长度（导水裂隙范围）远远小于压水试验段计算长度的特点,可将GRF模型解析得出的分段渗透系数做进一步细化平均以提高压水试验解析结果的精度,为水封洞库效果评价、洞库涌水量预测提供更加科学可靠的数据基础。     

A semi‐analytical approach for modelling heat transfer during salt‐cavern leaching process

The use of gas‐storage caverns in salt formations is a growing industry that continues to gain momentum because it allows gas to be injected and withdrawn at high rates compared with other underground gas‐storage systems such as porous rock systems. In order to predict cavern production performances, cavern thermodynamics behaviour must be studied by higher accuracy approaches. This behaviour is extremely related to the temperature distribution in the surrounding formations. During the leaching process, the thermal equilibrium of the rock salt surrounding the cavern is extensively disrupted. The purpose of this paper is to study the heat transfer problem during the leaching process and to develop a thermal model that can be easily used in field applications. The results of this work will be the input data for the prediction of the gas temperature and pressure during cavern gas‐storage operation phase. Moreover, the developed model can find its use in the design of salt caverns since it can be coupled with geometrical modelling of salt dissolution codes. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermomechanical effects of a rapid depressurization in a gas cavern

Rapid gas depressurization leads to gas cooling followed by slow gas warming when the cavern is kept idle. Gas temperature drop depends upon withdrawal rate and cavern size. Thermal tensile stresses, resulting from gas cooling, may generate fractures at the wall and roof of a salt cavern. However, in most cases, the depth of penetration of these fractures is small. These fractures are perpendicular to the cavern wall. The distance between two parallel fractures becomes larger when fractures penetrate deeper in the rock mass, as some fractures do not keep growing. These conclusions can be supported by numerical computations based on fracture mechanics. Salt slabs are created. However, these slabs remain strongly bounded to the rock mass and it is believed that in many cases their weight is not large enough to allow them to break off the cavern wall. Depth of penetration of the fractures must be computed to prove that they cannot be a concern from the point of view of cavern tightness.     

非贯通单节理岩体裂纹扩展方向研究

裂纹扩展方向的确定对分析岩桥破坏机制和岩体抗剪强度参数具有重要意义。首先以断裂力学观点推导了复杂应力条件下裂纹尖端应力分布函数的表达式,以节理岩体尖端的扩展裂纹可分为张拉裂纹和剪切裂纹为前提,基于Griffith破坏判据,提出了张拉裂纹扩展方向(张裂角)的计算公式;基于Mohr-Coulomb判据,提出了剪裂纹扩展方向(剪裂角)的计算公式。通过新判据与试验和其他判据的结果对比表明,该判据能准确判断张拉裂纹扩展方向,而剪裂角的扩展方向有待进一步试验验证。分析表明:在单向拉应力作用下,张裂纹扩展方向均有偏向于最大主应力方向的趋势,张裂纹与最大主应力夹角小于15°;双向拉应力作用下,随着微裂纹倾角变大,张裂纹有远离最大主应力方向的趋势;单轴压缩作用下,张裂角随裂纹倾角的增加而减小,而两者的和为先减小后增加。