考虑渗流特性的岩体结构面分形特性研究

 裂隙的连通性和密度是影响岩体渗流特性的重要因素。从岩体渗流研究的需要出发,对计算机模拟的岩体裂隙网络,应用分形几何理论,提出考虑裂隙连通性和密度影响的岩体结构面信息维数的计算方法,建立信息维数与岩体渗透系数的关系,进而可以用信息维数和岩体结构面几何参数来直接推求各向异性裂隙岩体的渗透系数张量。工程算例表明：(1) 考虑渗流应力耦合作用时,用容量维数计算的渗透系数比用信息维数计算的值高出2倍多,说明用容量维数计算岩体渗透系数会夸大裂隙岩体的渗透能力;(2) 信息维数能较好地反映裂隙密度对渗流的影响。信息维数越大,表明岩体内连通裂隙数量越多,因而岩体的渗透性就更大一些。     

埋深对软岩圆形巷道变形破裂规律的影响分析

为研究埋深对深部岩体稳定性的影响,采用离散颗粒元软件PFC^3D和透明岩体实验技术对不同埋深下圆形巷道周边岩体的变形破裂时空演变规律展开研究。研究结果表明:(1)不同埋深条件下,圆形巷道顶底板岩体与帮部岩体都会约在模型两个对角线方向上发生水平交错分离现象;(2)巷道周边岩体的径向位移都与其距巷道表面的距离呈指数衰减式分布,且随着埋深的增大,巷道周边岩体的连续性变形现象将越来越不明显;(3)随着巷道埋深的增加,巷道两帮岩体破裂逐渐往深处及拱顶底方向扩展,导致巷道顶底部岩体会因在拱腰处“立足不稳”,形成“X”型破裂;(4)巷道岩体裂纹总数和横断面岩体裂隙分形盒维数分别与巷道埋深呈指数增长和线性增长关系。     

含孔多裂隙岩石力学特性与破裂分形维数相关性研究

为研究含孔多裂隙岩石破裂及分形特征,制备含孔多裂隙岩石试件并对其进行单轴压缩试验,运用RFPA2D软件数值研究破裂过程,构建悬臂梁和结构面力学模型解释拉伸裂纹的形成机制以及裂隙倾角对试件抗压强度的影响效应。此外,引入盒维数法计算分形维数,以此定量表征最终破坏后的物理、数值模型试件裂纹的几何分布特征及其与破裂特性的相关性。研究结果表明:(1)试件破坏模式可划分为穿切裂隙剪切破坏(I型)和沿裂隙剪切破坏(II型)2类,试验与数值模拟结果吻合;(2)孔洞两侧的岩桥可简化为悬臂梁模型,由于非几何对称部位作用的弯矩较大,导致孔洞周边出现较多的拉伸裂纹;(3)预制多裂隙显著弱化了试件的抗压强度,随着裂隙倾角的增大,抗压强度呈先减小后增大的趋势,弱化系数W的变化规律与之相反,与结构面力学模型的解释一致;(4)最终破坏后的含孔多裂隙岩石新生裂纹几何分布的分形维数D与破坏特性密切相关;相比于II型破坏,发生I型破坏的试件抗压强度更大,受力过程中出现的宏观裂纹更多且扩展更充分,导致分形维数更大,数据拟合进一步证明抗压强度与分形维数近似服从正相关关系。     

扰动岩体裂隙网络分形性质分析

应用相似材料试验模型,研究扰动岩体裂隙网络的演化特征和规律,借助分形几何方法计算分析了扰动岩体裂隙网络的分形维数和分形演化特征,展望了研究覆岩力学性质与裂隙网络演化的依赖关系,对评价采动覆岩变形过程、力学行为和渗流性质具有重要意义.     

地下开挖影响下岩体分形性质演化问题探讨

提出了天然岩体的分形性质;研究并总结了地下开挖影响下岩体初始裂隙分布分维、裂纹扩展或断裂表面生成分维、采动岩体再生裂隙分维等一系列能用分形几何来描述的现象随时间的动态演化规律;揭示了岩体受力变形过程中的应力状态、力学性能、物理和化学性质的演化与其相关分维值的关系;展望了岩体分形性质及其受力演化规律的研究意义,为从更深层次认识岩体变形破坏的非线性、复杂性架起了桥梁,包括岩体动力学演化过程的混沌特征、岩体分形演化力学行为中的突变现象和岩体裂纹演化的协同效应;最后展望了岩体分形性质及其受力演化规律的应用领域及其发展前景。     

隧道开挖过程中复杂裂隙围岩的固流耦合分析

隧道通过裂隙岩体的含水区段时,人为扰动了裂隙岩体、地下水等构成的复杂地质系统,是造成各种涌水、突水、突泥事故的重要原因。为了研究复杂地质条件下隧道开挖过程中岩体变形、流体运移相互作用过程,探讨其对隧道涌、突水的影响,在上述复杂过程进行理论分析的基础上,根据深埋隧道围岩裂隙发育规模与工程尺度的关系,建立可以同时考虑不同级别裂隙网络的复杂裂隙岩体水力学模型,采用有限元法对复杂裂隙岩体中开挖隧道的固流耦合过程进行了数值模拟,模拟结果体现了主干裂隙在渗流中的强导水作用和网络状裂隙的贮水功能与渗流滞后效应,开挖过程中复杂裂隙岩体渗流场与应力场的耦合作用显著的增加了隧道围岩屈服区。     

岩体裂缝面数量三维分形分布规律研究

采用计算机仿真的数值试验方法,首先证明了岩体裂隙面数量服从三维分形分布规律这一自然现象。然后根据大量的计算以及理论推演,得到了裂隙面的2个重要的分形参数(分形维数和分形分布初值),以及分形维数DS和分形分布初值NS与二维剖面裂隙迹线分形参数DL,NL的相关关系。即二维分形维数与三维分形维数遵循:DL = DS-1,二维分形维数DL与裂隙面的其他参数无关;二维分形分布初值NL与三维裂隙面的倾角ST,方位角SP遵循投影关系;二维分形分布初值NL与三维分形分布初值NS遵循正比关系:NL = kNS,k值决定与岩体剖面和三维裂隙面的投影关系。实测结果证实了这些关系的正确性。这些结论为研究裂隙岩体的其他物理力学性质奠定了基础。     

锦屏二级水电站深埋引水隧洞稳定性研究

裂隙岩体的渗流应力耦合机理是岩体水力学的研究重点。基于多孔介质有效应力原理,以体积应变为切入点,建立了考虑岩体变形特性的渗透系数动态演化方程和弹塑性耦合损伤本构模型,并将所建立的本构模型编制程序导入到大型商用有限元程序ABAQUS,研究锦屏二级水电站深埋引水隧洞运营期围岩和衬砌的受力和变形特征。研究成果表明:考虑渗透系数动态演化的力学模型可以更好地反映隧洞运营期衬砌的外水压力特征,比不考虑渗透系数动态演化的计算结果更加合理,由于锦屏4个引水隧洞的间距较大,隧洞开挖和运行放水所产生的裂隙损伤演化区没有形成贯通区域。     

分形及损伤力学在矿山开采沉陷中的应用研究

 博士学位论文摘要　岩体是经过漫长的地质演化过程而形成的复杂结构体, 由于地质构造运动的影响, 其内部存在大量的断层、节理、层理和地质弱面。这些地质结构面(即损伤) 的存在, 破坏了岩体的整体性, 影响着岩体的变形性质和强度特性, 从而导致岩体开采沉陷更加复杂, 即: 在规律上的非规范性、在程度上的剧烈性和在损害上的严重性。因此, 仅采用常规的开采沉陷理论来分析和解决矿山开采沉陷工程问题已暴露出诸多的缺陷和不足。基于上述问题和认识, 提出了矿山开采沉陷理论研究和实践应用的新思路: 将损伤力学及分形几何等现代非线性科学应用于开采沉陷学科领域, 在现有的开采沉陷理论基础上, 进一步揭示岩体(特别是地质构造复杂的岩体) 开采沉陷的更深层次的机理、特征和规律, 进一步发展矿山开采沉陷学科, 使其在理论上更完备、实践上更符合实际、工程预测上更准确、应用上切实可行。在此研究思路的指导下, 就节理对岩体采动沉陷规律的影响、采动岩体裂隙分形分布及演化规律、采动断层活化的分形界面效应等3 个问题进行了一系列的理论研究和实验研究:(1) 初始节理(初始损伤) 对采动岩体沉陷规律的影响研究①为了研究岩体内初始损伤(初始节理) 对开采沉陷的影响规律, 分别制作无节理和考虑节理倾角单因素的相似材料模型, 进行了系统的实验研究;②为了进一步得到岩体内初始损伤(初始节理) 对开采沉陷的影响的定量规律, 应用损伤力学原理统计研究了初始节理(初始损伤) 在开采沉陷中的控制作用, 建立了开采沉陷量值与损伤量值的数量关系;③应用损伤力学原理定性分析非贯通节理岩体采动沉陷的损伤岩梁弯曲效应, 进一步验证实验研究成果。(2) 采动岩体分形裂隙网络研究①制作一套考虑不同开采宽度的相似材料模型, 在实验观测的基础上统计分析了采动岩体裂隙分布随采宽增加(工作面推进) 的变化规律和分形性质;②制作一套考虑不同岩性的相似材料模型, 分析了岩性对采动岩体裂隙分布的影响和分形性质;③研究初始节理对采动岩体裂隙网络的影响规律, 检验其分形性质。(3) 采动断层活化的分形界面效应研究①进行野外实地勘查、勘测和研究断层面的结构形态与分形性质;②构造了自仿射分形曲线来模拟实际的断层面, 制作一套含有这种分形断层面的相似材料模型, 系统研究分形断层面对断层活化的影响规律;③对采动断层活化的分形界面效应进行数值模拟。通过上述实验和研究工作获得了如下的成果:(1) 岩体内存在的初始节理打破了岩体采动破坏和地表移动的正常规律, 与无节理岩体采动沉陷规律相比较(地表移动稳定后) , 初始节理在岩体采动沉陷过程中起催化作用, 打破了采动岩体裂隙的分布规律和扩展方向, 使覆岩破坏范围和地表移动范围增大, 地表移动量值也随之增加。(2) 通过实验现场观测发现: 不同展布的初始节理, 使岩体表现出不同的破坏机制。水平初始节理使岩体出现层裂、层滑现象, 使岩体在采动过程中出现离层数量增多、离层高度增大、地表移动矢量增大; 倾斜初始节理使采动覆岩中出现沿倾斜节理滑移的现象, 从而导致岩体破坏区域和地表移动曲线出现偏态, 最大下沉点和最大水平移动点均偏向节理的倾斜方向; 高角度初始节理使岩体破坏出现尖角突变现象, 使覆岩破坏更加剧烈。(3) 考虑初始节理倾角单因素的影响, 随着节理倾角的增大, 采动岩体裂隙扩展角随之增大, 覆岩破坏范围和地表移动范围随之增大, 地表下沉值和水平移动值也随之增大, 且地表下沉值的增加幅度较水平移动增加幅度大。　(4) 初始损伤对岩体采动沉陷具有严重的影响,损伤岩体采动沉陷量值较无损伤岩体采动沉陷量值要大。其中的垂直移动量主要以决于损伤岩体中初始孔隙在竖直平面内的总占位,其占位越大,下沉值越大;水平移动量主要取决于损伤岩体中初始孔隙在水平面内的总占位,其占位越大,水平移动值越大。(5) 在给定的研究条件下,地表移动特征值(最大下沉值Wmax 、下沉系数q、背离节理倾斜方向的最大水平移动值U+max和水平移动系数b + 、顺着节理倾斜方向的最大水平移动值U2max和水平移动系数b2) 与岩体损伤变量Ωy 存在如下明确的定量对应关系:Wmax = - 416. 667Ω2y - 568. 333Ωy + 1 399. 4q = - 0. 115 7Ω2y - 0. 4277 8Ωy + 0. 881U+max = 358. 796 0Ω2y - 176. 389Ωy + 386U-max = - 1 562. 5Ω2y + 771. 677Ωy + 379. 3b + = 0. 694 44Ω2y - 0. 15Ωy + 0. 286b - = - 0. 864 17Ω2y - 586. 333Ωy + 1 399. 4(6) 采动岩体裂隙形成、扩展、分布极其复杂,处于明显的混沌状态,具有分形特征,用分形维数可以综合描述采动岩体裂隙化程度(裂隙条数、迹线长度、张开度等) 。(7) 随着工作面的推进,采宽的逐渐增加(或重复采动) ,使采动岩体裂隙分布越趋复杂:破坏范围增大、破坏程度增大、不规则性增加,导致采动岩体分形裂隙网络出现升维现象,在给定的研究条件下的打动岩体分形裂隙网络的分形维数D 与采宽L 具有很好的定量关系:D = 0. 000 172 432L2 - 0. 010 265 4L + 1. 259 6(8) 岩体特性对采动岩体裂隙的分布规律有一定影响。抗压强度较大的岩体,采动后形成的裂隙空间占位较大,其形成的分形裂隙网络的分形维数值也较大;而地表下沉值则较小,地表下沉值与采动后岩体中形成的裂隙空间占位的分维值成反比关系。(9) 含有规则展布初始节理的岩体采动后形成的裂孙网络具有分形性质,且随着初始节理倾角的增大,采动岩体裂隙网络的分形维数值越趋减小。(10) 在给定的研究条件下,地表最大下沉值Wmax与采动岩体裂隙网络分形维数值D 具有明确的定量关系:Wmax = - 16. 872 7D3 + 64. 117D2 - 81. 060 2D + 75. 370 1(11) 地质断裂面粗糙不平,具有统计自相似分形性质,且表现为各向异性分形特征。(12) 地质断裂面具有多层次性,分形维数反映了它的粗糙起伏程度,很好地反映了断层面的逐级包络的形态特征,分形维数是描述断裂面全貌起伏特征的有效数量指标。(13) 地质断裂面在采动影响下的活化现象严重受控于断裂面的分形几何形态,具有明显的分形界面效应,表现为断层面分形维数越大,断层面两侧岩体互相约束性越大。(14) 通过对含断层面的模型实验发现:含有分形维数较小断层面的模型中,覆岩破坏比较迅速,移动比较充分,弯曲带内的离层最终表现为不发育;而含分形维数较大断层面的模型中,覆岩破坏和移动表现为较前者缓慢和不充分,弯曲带内的离层最终表现为较发育。(15) 分形断层面打破了采动断层的正常活化规律,导致地表移动随着断层面分维的减小而愈加剧烈(下沉速度愈趋增大) 断层两侧的台阶落差值和地表最大下沉值均随着断层面分维的减小而增大。在给定的研究条件下,台阶落差值H、地表最大下沉值Wmax和某一时间段的平均下沉速度V 与断面分形维数D 的经验数量关系为H = 1 528. 5D2 - 4 654. 29D + 3 602. 1Wmax = 1 200D2 - 5 832D + 4 094V = - 15. 00D2 + 30. 9D + 20. 58(16) 采动断层活化分形界面效应的数值模拟,不仅有效地验证了实验成果,而且获得了断层面分形性质对采动岩体内部应力分布的影响规律。断层面的分形性质对X 方向的拉应力影响不大;各模型的X 方向压应力和Y 方向拉应力在分形断层面的下部集中程度大体一致,而靠近断层露头附近则随分形维数的增大,集中程度越来越弱,说明断层活化量越来越小,从而造成断层面活化后其两侧岩体的接触程度越来越弱. Y 方向的压应力随断层面分形维数的变化规律是:随着断层面分形维数的增加,整个采场上覆岩层的压应力值逐渐减小,说明随着断层面分形维数增大,断层两侧岩体互相约束性越趋增大,导致采场上覆岩体向下的移动越趋减小,其中的离层越趋增多。上述研究成果表明,将分形几何和损伤力学等先进的理论应用于矿山开采沉陷学科领域中,有助于深入认识和理解复杂的矿山开采沉陷现象,有助于研究矿山开采沉陷更深层次的机理、特征和规律以及此为出发点的控制开采损害的技术措施,将推动矿山开采沉陷理论的进一步发和工程实用。     

隧洞开挖过程中渗透破坏细观机制研究

渗透破坏是隧洞施工过程中面临的一类重要的工程灾害问题,对隧洞渗透破坏机制的研究可应用于隧道施工安全预报.利用颗粒流法在模拟散体及破碎介质方面的优势,结合流体动力学数值模拟的有限体积法,建立由黏结颗粒材料组成岩块的二维数值模型,模拟储水断层中不同的水压力时,隧洞开挖引起的渗透破坏现象,模拟过程可以实时反映开挖过程中微裂隙的扩展及渗透张量的变化.模拟结果表明,颗粒流法模拟岩体渗透破坏过程是可行的,对于隧洞工程开挖具有一定的指导意义.     

Geological and geotechnical observations from the Niagara Tunnel Project

The Niagara Tunnel Project is a water diversion tunnel that was constructed in Niagara Falls, ON, Canada. The tunnel was excavated by a 14.4-m-diameter tunnel boring machine that passed through 11 geological formations. The rock types included limestone, sandstone, siltstone, shale, and mudstone. The overbreak was divided into four zones, evaluated by observations as the tunnel excavation progressed. Overbreak within the Rochester, Neagha, and Grimsby Formations was <0.6 m deep typically, measured perpendicular to the designed excavation perimeter. The Power Glen Formation had overbreak as deep as 2.7 m. The deepest overbreak was measured in the Queenston Formation, where 3–4 m was typical and associated with the tunnel alignment. Maximum overbreak occurred when the tunnel was aligned perpendicular to the maximum horizontal stress. The overbreak was focused in the crown and inverted, with only minor volumes of sidewall overbreak occurring. The deep overbreak within the Queenston was overcome by utilizing spiles and adjusting the tunnel alignment to minimize the duration of excavation in the Queenston Formation. The tunnel went into operation in March, 2013.     

Evaluating extent and causes of overbreak in tunnels

Damage or overbreak not only endangers safety of structure but also increases cost of construction and time of completion. Drilling and blasting being cost-effective for excavation of any underground structure should strictly adhere into specialized controlled blasting pattern to minimize the unacceptable impact on peripheral in situ rock mass. The paper reveals that in addition to geo-technical properties of rock mass, in situ stress condition plays an important role in enhancing the magnitude of overbreak. Implementation of same blast pattern throughout the length of tunnel results into different magnitudes of overbreak and the magnitude increases in highly stressed zone. Furthermore, implementation of same controlled perimeter blast pattern along the tunnel cross-section may result into different magnitudes of overbreak. Different magnitude of overbreak along the tunnel cross-section i.e., in left and right wall and crown has been observed even with implementation of same controlled blast pattern throughout the tunnel cross-section. Feasibility and compatibility of drilling equipment with respect to tunnel cross-section also adds to the quality and magnitude of overbreak. Undersize drilling equipment leads to angular drilling on either walls or crown and enhances the magnitude of overbreak. The paper with the help of statistical and graphical analysis revealed that blast pattern for peripheral rock mass should consider geo-technical properties and in situ stress condition of rock mass to minimize the magnitude of overbreak. The authors also emphasized that to contain magnitude of overbreak within allowable limit; the implemented blast pattern should be different for different sections viz., right wall, left wall and crown of tunnel. Furthermore, in poor rock mass condition or in highly stressed zone, drivage of tunnel should be carried out in small sections and in different phases until the excavation reaches the required excavation profile in that area. Excavation in small sections and in different phases would lead to proper excavated profile and minimize overbreak and damage of peripheral rock mass.     

Causes, impact and control of overbreak in underground excavations

Drill and blast system is used in hard rock excavation due to its economics and adaptability to changing rock mass conditions. Common question during mining and tunneling operations is ‘whether overbreak has been caused by blasting practice or poor rock mass quality’. Critical evaluation of the factors influencing blast damage is required to address such questions.In order to understand the mysterious nature of blast damage prediction and control, the field work involved the small scale blasting of physical models and the assessment of blast damage during drifting operations. The damage was measured by the Half cast factor, percentage overbreak and the Blast damage index. The influence of rock mass features, explosive characteristics and blast design parameters on overbreak has been examined in this study. A new approach for the judicious design of perimeter hole pattern and charge concentration has been proposed. Implications of blast damage have also been outlined in this paper.     

基于损伤能量耗散的岩体块度分布预测

提出一整套岩体块度分布预测方法。首先，根据岩石分形断裂切割岩块的块度形成机制，利用能量守恒关系，建立损伤–能量–碎块尺寸理论关系式。为方便应用，将该式简化。然后，根据块度分布的自相似性，将岩石块度分布特性应用于岩体块度分布预测。提出适合工程应用的损伤模型建立方法，并针对岩体块度分布的特点，定义岩体裂隙损伤参数。作为实例，计算金川节理岩体的块度分布。该研究对于自然崩落法采矿岩体块度分布预测具有重要意义。     

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

     

Determination of geological strength index of jointed rock mass based on image processing

The geological strength index(GSI) system,widely used for the design and practice of mining process,is a unique rock mass classification system related to the rock mass strength and deformation parameters based on the generalized Hoek-Brown and Mohr-Coulomb failure criteria.The GSI can be estimated using standard chart and field observations of rock mass blockiness and discontinuity surface conditions.The GSI value gives a numerical representation of the overall geotechnical quality of the rock mass.In this study,we propose a method to determine the GSI quantitatively using photographic images of in situ jointed rock mass with image processing technology,fractal theory and artificial neural network(ANN).We employ the GSI system to characterize the jointed rock mass around the working in a coal mine.The relative error between the proposed value and the given value in the GSI chart is less than 3.6%.     

三维节理岩体分形维数与RQD相关性研究

 应用三维随机不连续面网络模拟技术对岩体结构统计均质区内的不连续面进行计算机模拟,再现岩体结构,并应用分形理论计算三维节理岩体的分形维数。平行于三维节理岩体网络模型的x,y,z轴布置测线,计算不同阈值下三维岩体的岩石质量指标(RQD)。对获取的分形维数与RQD分析表明,三维岩体RQD随分形维数增大而减小,二者具有良好的线性关系。最后,推导三维节理岩体的分形维数与若干个阈值下RQD的关系式,并根据不同阈值下RQD与分形维数关系曲线的斜率确定合理阈值。     

Some engineering properties of limestone: Tunnel Stra?ina case study (Croatia)

The paper presents geological and engineering geological characteristics of the Stra?ina Tunnel along the Bisko-Šestanovac section of the Zagreb-Split-Dubrovnik highway in Croatia. This paper compares the actual conditions of the rock mass during the excavation with a prediction model that preceded the excavation. From the engineering-geological viewpoint the rock mass in the tunnel was of a significantly higher quality than the prediction model. The specific geological feature of the Stra?ina Tunnel, with its right and left tunnel tube, is the passage of the right tunnel tube through a transgressive contact between Upper Cretaceous rudist limestones and Eocene foraminiferal limestones. Since this is the only tunnel in Croatia excavated through this particular transgressive contact, the geological and engineering properties of the transgression zone were up to now only assumed. Therefore, additional mineralogical, petrographical and engineering geological observations were carried out in order to determine and describe the transgression zone. The results are presented in this paper. In the left tunnel tube the contact between the mentioned litostratigraphical Units is of the fault type. This paper also briefly deals with the significance and cause of the overprofile excavation during tunneling through strongly karstified carbonate rocks. Consequently, special attention was paid to the overprofile during excavation since it can significantly affect tunneling costs.     

Engineering geological evaluation and preliminary support design for the metro extension tunnel,Ankara, Turkey

The paper reports an assessment of the engineering geological characteristics of the rock mass to be encountered between Mecidiye and Gazino stations on the new extension of the Ankara metro and the determination of appropriate support and excavation methods. The rock mass quality was estimated using the rock mass rating (RMR), geological strength index (GSI) and rock mass quality (Q) systems and the tunnel divided into sections. The RMR, Q and NATM systems were used to determine the support and excavation methods in these areas. The deformations and stress concentrations around each tunnel section were investigated and the interaction of the support systems with the rock mass was analyzed using finite element software. It is concluded that rock mass classification systems should be used in tandem with numerical tools, although it is emphasized that the estimation of rock mass properties is not an exact science and both rock properties and numerical models should be refined based on observations and the results of instrumentation installed during the construction of a tunnel.      

Analysis and prediction of TBM performance in blocky rock conditions at the Lötschberg Base Tunnel

This paper focuses on the analysis of the TBM performance recorded during the excavation of the Lötschberg Base Tunnel. The southern part of the tunnel was excavated by two gripper TBMs, partly through blocky rock masses at great depth. The jointed nature of the blocky rock mass posed serious problems concerning the stability of the excavation face. A detailed analysis has been carried out to obtain a relationship between the rock mass conditions and the TBM performance, using the Field Penetration Index (FPI). In blocky rock conditions, the FPI is defined as the ratio between the applied thrust force and the actual penetration rate. A database of the TBM parameters and the geological/geotechnical conditions for 160 sections along the tunnel has been established. The analysis reveals a relationship between the FPI and two rock mass parameters: the volumetric joint count (J_v) and the intact rock uniaxial compressive strength (UCS). Through a multivariate regression analysis, a prediction model for FPI in blocky rock conditions (FPI_blocky) is then introduced. Finally, other TBM performance parameters such as the penetration rate, the net advance rate and the total advance rate are evaluated using FPI_blocky.