Probabilistic assessment of tunnel convergence considering spatial variability in rock mass properties using interpolated autocorrelation and response surface method

This study aims at the probabilistic assessment of tunnel convergence considering the spatial variability in rock mass properties. The method of interpolated autocorrelation combined with finite difference analysis is adopted to model the spatial variability of rock mass properties. An iterative procedure using the first-order reliability method(FORM) and response surface method(RSM) is employed to compute the reliability index and its corresponding design point. The results indicate that the spatial variability considerably affects the computed reliability index. The probability of failure could be noticeably overestimated in the case where the spatial variability is neglected. The vertical scale of fluctuation has a much higher effect on the probabilistic result with respect to the tunnel convergence than the horizontal scale of fluctuation. And the influence of different spacing of control points on the computational accuracy is investigated.

Zonal disintegration phenomenon in enclosing rock mass surrounding deep tunnels——Elasto-plastic analysis of stress field of enclosing rock mass

The zonal disintegration phenomenon （ZDP） is a typical phenomenon in deep block rock masses. In order to investigate the mechanism of ZDP, an improved non-linear Hock-Brown strength criterion and a bi-linear constitutive model of rock mass were used to analyze the elasto-plastic stress field of the enclosing rock mass around a deep round tunnel. The radius of the plastic region and stress of the enclosing rock mass were obtained by introducing dimensionless parameters of radial distance. The results show that tunneling in deep rock mass causes a maximum stress zone to appear in the vicinity of the boundary of the elastic and the plastic zone in the surrounding rock mass. Under the compression of a large tangential force and a small radial force, the rock mass in the maximum stress zone was in an approximate uniaxial loading state, which could lead to a split failure in the rock mass.

Zonal disintegration phenomenon in rock mass surrounding deep tunnels

Zonal disintegration is a typical static phenomenon of deep rock masses. It has been defined as alternating regions of fractured and relatively intact rock mass that appear around or in front of the working stope during excavation of a deep tunnel. Zonal disintegration phenomenon was successfully demonstrated in the laboratory with 3D tests on analogous gypsum models, two circular cracked zones were observed in the test. The linear Mohr-Coulomb yield criterion was used with a constitutive model that showed linear softening and ideal residual plastic to analyze the elasto-plastic field of the enclosing rock mass around a deep tunnel. The results show that tunneling causes a maximum stress zone to appear between an elastic and plastic zone in the surrounding rock. The zonal disintegration phenomenon is analyzed by considering the stress-strain state of the rock mass in the vicinity of the maximum stress zone. Creep instability failure of the rock due to the development of the plastic zone, and transfer of the maximum stress zone into the rock mass, are the cause of zonal disintegration. An analytical criterion for the critical depth at which zonal disintegration can occur is derived. This depth depends mainly on the character and stress concentration coefficient of the rock mass.

New methods of safety evaluation for rock/soil mass surrounding tunnel under earthquake

The objective of this work is to obtain the seismic safety coefficient and fracture surface and proceed with the seismic safety evaluation for the rock mass or soil mass surrounding a tunnel,and the limitation of evaluating seismic stability is considered using the pseudo-static strength reduction.By using the finite element software ANSYS and the strength reduction method,new methods of seismic safety evaluation for the rock mass or soil mass surrounding a tunnel are put forward,such as the dynamic finite element static shear strength reduction method and dynamic finite element shear strength reduction method.In order to prove the feasibility of the proposed methods,the results of numerical examples are compared with that of the pseudo-static strength reduction method.The results show that 1) the two methods are both feasible,and the plastic zone first appears near the bottom corners; 2) the safety factor of new method Ⅱ is smaller than that of new method I but generally,and the difference is very small.Therefore,in order to ensure the safety of the structure,two new methods are proposed to evaluate the seismic stability of the rock mass or soil mass surrounding a tunnel.A theoretical basis is provided for the seismic stability of the rock mass or soil mass and the lining surrounding a tunnel and also provided for the engineering application.

Model test study of frost heaving pressures in tunnels excavated in fractured rock mass in cold regions

Based on the similarity theory, taking the horseshoe, city-gate and round linings as examples, the value and distribution regularities of normal frost heaving pressures （hereinafter as frost heaving pressures） in tunnels excavated in fractured rock mass in cold regions under different constraints and freezing depths were studied by a test model. It was found that the larger the frozen depth, the larger the frost heaving pressure, and the stronger the top constraint, the larger the frost heaving pressure. For the horseshoe lining and city-gate lining, the top constraint has a greater effect on the frost heaving pressures on the arch and the inverted arch. For the round lining, the influences of the top constraint on the frost heaving pressure in all linings are almost the same. The frost heaving pressure is maximum on the city-gate lining and minimal on the round lining. The largest frost heaving pressure all occur near the foot of the inverted arch for the three kinds of lining. Thus, the test data basically coincide with the observed in situ data.

Back-analysing rock mass modulus from monitoring data of two tunnels in Sydney, Australia

This paper presents two case studies where the rock mass modulus and in situ stress are estimated from the monitoring data obtained during the construction of underground excavations in Sydney,Australia.The case studies comprise the widening of existing twin road tunnels within Hawkesbury sandstone and the excavation of a large cavern within Ashfield shale.While back-analysis from detailed systematic monitoring has been previously published,this paper presents a relatively simple methodology to derive rock mass modulus and in situ stress from the relatively simple displacement data routinely recorded during tunnelling.

Assessment of Rock Mass Quality and Support Estimation along Headrace Tunnel of a Small Hydropower in District Mansehra, Khyber Pakhtunkhwa, Pakistan

The main purpose of this study is to classify the rock mass quality by using rock mass quality (Q) and Rock Mass Rating (RMR) systems along headrace tunnel of small hydropower in Mansehra District, Khyber Pakhtunkhwa. Geological field work was carried out to determine the orientation, spacing, aperture, roughness and alteration of discontinuities of rock mass. The quality of rock mass along the tunnel route is classified as good to very poor quality by Q system, while very good to very poor by RMR classification system. The relatively good rock conditions are acquired via RMR values that are attributed to ground water conditions, joint spacing, RQD and favorable orientation of discontinuities with respect to the tunnel drive. The petrographic studies revealed that study area is mainly comprised of five major geological rock units namely quartz mica schist (QMS), garnet mica schist (GMS), garnet bearing quartz mica schist (G-QMS), calcareous schist (CS), marble (M). The collected samples of quartz mica schist, marble and garnet bearing quartz mica schist are fine to medium grained, compact and are cross cut by few discontinuities having greater spacing. Therefore, these rocks have greater average RQD, Q values, RMR ratings as compared to garnet mica schist and calcareous schist.

Ground reaction curves for circular excavations in non-homogeneous,axisymmetric strain-softening rock masses

Fast methods to solve the unloading problem of a cylindrical cavity or tunnel excavated in elasto-perfectly plastic, elasto-brittle or strain-softening materials under a hydrostatic stress feld can be derived based on the self-similarity of the solution. As a consequence, they only apply when the rock mass is homogeneous and so exclude many cases of practical interest. We describe a robust and fast numerical technique that solves the tunnel unloading problem and estimates the ground reaction curve for a cylindrical cavity excavated in a rock mass with properties depending on the radial coordinate, where the solution is no longer self-similar. The solution is based on a continuation-like approach(associated with the unloading and with the incremental formulation of the elasto-plastic behavior), fnite element spatial discretization and a combination of explicit sub-stepping schemes and implicit techniques to integrate the constitutive law, so as to tackle the diffculties associated with both strong strain-softening and elasto-brittle behaviors. The developed algorithm is used for two practical ground reaction curve computation applications. The frst application refers to a tunnel surrounded by an aureole of material damaged by blasting and the second to a tunnel surrounded by a ring-like zone of reinforced(rock-bolted) material.

Performance characteristics of tunnel boring machine in basalt and pyroclastic rocks of Deccan traps–A case study

A12.24km long tunnel between Maroshi and Ruparel College is being excavated by tunnel boring machine(TBM)to improve the water supply system of Greater Mumbai,India.In this paper,attempt has been made to establish the relationship between various litho-units of Deccan traps,stability of tunnel and TBM performances during the construction of5.83km long tunnel between Maroshi and Vakola.The Maroshi–Vakola tunnel passes under the Mumbai Airport and crosses both runways with an overburden cover of around70m.The tunneling work was carried out without disturbance to the ground.The rock types encountered during excavation arefine compacted basalt,porphyritic basalt,amygdaloidal basalt pyroclastic rocks with layers of red boles and intertrappean beds consisting of various types of shales Relations between rock mass properties,physico-mechanical properties,TBM specifications and the cor responding TBM performance were established.A number of support systems installed in the tunne during excavation were also discussed.The aim of this paper is to establish,with appropriate accuracy the nature of subsurface rock mass condition and to study how it will react to or behave during under ground excavation by TBM.The experiences gained from this project will increase the ability to cope with unexpected ground conditions during tunneling using TBM.

Geotechnical Investigation and Prediction of Rock Burst, Squeezing with Remediation Design by Numerical Analyses along Headrace Tunnel in Swat Valley, Khyber Pakhtunkhwa, Pakistan

This study illustrates the classification of the rock mass and evaluation of rock squeezing, rock burst potential, deformation modulus along the proposed tunnel alignment of small hydropower in Swat Valley, Khyber Pakhtunkhwa (KP), Pakistan. The field and laboratory studies were conducted to classify the rock mass by using geomechanical classification systems i.e. Rock Mass Rating (RMR), tunneling quality index (Q), Rock Mass Index (RMi). The empirical relations classified the ground as non-squeezing and minor to non-squeezing conditions, respectively. Whereas, other methods depict minor to medium bursting potential along chainage 1+000 to 4+000 m, while results along chainage 2+400 - 2+800 m present medium to high bursting potential. Furthermore, numerical analyses were carried out by RS3 for elastic and plastic conditions in order to assess the total displacement of each section in unsupported and supported conditions. The results gave maximum displacement along chainage 2+400 - 2+800 m (19.2 mm in unsupported and 16mm in supported condition) and minimum displacement along chainage 0+876 - 1+000 m (1.4 mm in unsupported and 1.3 mm in supported condition). Hence, the estimated support by empirical methods has been optimized by using numerical analyses for the stability of rock mass along the tunnel.

CSAMT深埋长大隧道破碎岩体靶向勘察研究

铁路深埋长大隧道因其埋深大、里程长,同时伴随复杂多变的地质问题,致使地质勘察难以查明、查细,特别是对深埋长大隧道破碎岩体空间分布的精准定位一直是工程勘察中的难题。基于CSAMT三维正反演理论,研究建立了针对深埋长大隧道中破碎岩体靶向勘察模式,从而实现“点-线-面”的精准勘察。通过开展基于四面体网格的CSAMT三维正演和有限内存拟牛顿法的三维反演研究,以及二、三维模型分析对比,验证三维CSAMT在铁路隧道勘察中的优势。在滇西地区某深埋长大隧道TBM掘进工区开展三维“点-线-面”靶向勘察试验,结果表明该勘察体系可靠且精度高,精准探明了TBM卡机工区段前方的花岗岩蚀变带影响范围及其空间展布形态,确定了断层破碎带的影响范围及其空间展布形态,有效提高深埋长大隧道勘察精度。布置靶向测线从“点-线-面”进行三维CSAMT勘察这一思路,对深埋长大隧道的勘察工作提供一定的应用参考价值。

块裂隧道围岩等效岩体变形参数研究

采用Voronoi离散单元方法建立块裂岩体隧道的随机DEM数值模型,通过大量随机数值计算结果验证采用岩体等效变形参数预测隧道收敛变形的可行性。在规则裂隙岩体等效变形参数计算方法的基础上,针对数值模拟揭示的深埋隧道变形特征提出改进的修正计算公式及相应的特征参数。通过与既有解答以及大量随机数值计算结果的对比和统计分析验证修正解的合理性,修正解较为合理改善既有规则裂隙岩体等效变形参数计算方法在反映随机块裂岩体在隧道卸荷条件下变形估算的较大误差。采用修正计算方法得到的岩体等效变形参数可以很好地预测随机块裂岩体中隧道开挖后收敛变形的数学期望值。提出的修正计算方法对当前深部岩体变形参数研究和隧道洞壁收敛位移的估算具有一定的参考价值。

隧道穿越岩堆体围岩剪切特性及分级

为解决隧道穿越岩堆体围岩分级不准确的问题,通过相似材料直剪试验分析岩堆体的直剪特性,将含水率和填充土含量两个因素带入熵权可拓物元法,并结合地质指标进行分级研究。结果表明岩堆体的抗剪强度随着含水率、填充土含量的增大,先增大后减小,最终趋于平稳,过高或过低的含水率和填充土都会“弱化”岩堆体的抗剪强度。根据岩堆体的特点,选取岩石单轴饱和抗压强度、节理裂隙密度、岩体完整性系数、含水率和填充土含量作为分级指标,结合熵权可拓物元法,将所选指标和熵权可拓物元法结合并在工程实例进行验证,分级结果比原方法准确性更好,证明研究提出的围岩分级方法是合理可行的。研究结果能够为岩堆体隧道的围岩分级提供指导,为施工设计和支护方式提供依据。

基于离散-连续耦合的岩溶隧道防突岩体安全厚度预测

为有效预防岩溶隧道施工中的突涌水灾害,首先,调研近年来国内岩溶隧道突涌水典型案例,总结隧道突涌水形成机理及其影响因素;其次,建立离散-连续耦合模型并采用多元回归理论的方法,明确防突岩体破坏的判定准则,划分岩溶隧道防突岩体破坏模式并提出裂缝发育规律;然后,基于对防突岩体最小安全厚度造成影响的各因素,针对溶洞位于隧道上方、下方及侧方3种情况分别建立防突岩体最小安全厚度预测计算式;最后,依托德庆隧道工程进行验证。结果表明:防突岩体破坏模式主要有弯折破坏、整体剪切破坏和复合破坏3种,可基于裂隙发展速度将其破坏过程分为初始阶段、快速发育阶段、平缓发育阶段3个阶段;各因素按影响显著程度由大到小依次为溶腔内水压、围岩黏结参数、溶腔跨度、溶腔高跨比和隧道埋深;计算得到德庆隧道3处施工段的防突岩体最小安全厚度分别为2.964,5.263和0.961 m,突涌水评估结果准确。防突岩体最小安全厚度预测计算式可用于预测并评估岩溶隧道突涌水风险。

破碎岩体隧道洞口开挖诱导滑坡与古滑坡耦合分析

以大栗树高速公路隧道洞口施工为工程背景,研究了在考虑古滑坡因素下洞口工作面边坡失稳及其加固技术。利用钻孔测斜数据(BCD)和Midas GTS折减强度有限元分析模型,分别推测了最危险滑动面,得知两者滑动面位置相似。并采用不平衡推力传递法,基于BCD推断滑动面计算安全系数。以BCD和有限元法的滑动面为基础,将滑动面近似假设为对数螺旋线,考虑地形因素,建立考虑多台阶边坡上限分析模型,研究了不同加固措施对边坡稳定的影响。结果表明,3种方法计算出的边坡安全系数结果相近。

碎裂岩夹层中大跨度悬索桥隧道锚稳定性分析

隧道锚作为悬索桥的重要受力构件,其稳定性是保证悬索桥安全运行的关键。为探究某大跨度铁路悬索桥隧道锚在碎裂岩夹层中的受荷响应规律,建立精细化数值模型,对隧道锚的稳定性进行综合评估,论证隧道锚在碎裂岩夹层中的适用性和安全性。研究表明:碎裂岩夹层对隧道锚的变形影响显著,隧道锚后锚面及围岩的位移分布曲线呈左高右低的“驼峰状”,锚塞体界面摩阻力在碎裂岩夹层区域产生突变。锚-岩联合体的破坏从碎裂岩夹层中锚塞体拱顶区域开始,逐步向拱腰和拱底扩展,直至锚-岩界面塑性区贯通,其破坏模式为锚-岩接触带的剪切破坏。隧道锚的综合承载力以钢束受拉破坏为控制条件,综合极限承载力为2.3倍设计主缆力。隧道锚在碎裂岩夹层中的稳定性和适应性良好。

基于贯入度指数的TBM围岩可掘性分级研究

隧道掘进机(TBM)前期投入巨大,合理划分TBM可掘性分级并给出施工参数建议值对预测施工工期、控制成本等有重要意义。以深圳地铁6号线6101标段大浪站石岩站区间羊台山隧道TBM施工项目为背景,首先以贯入度指数FPI为掘进性能评价指标,通过Spearman相关性分析FPI作为评价指标的合理性;在此基础上以FPI为输入参数分别建立掘进贯入度、刀盘推力和掘进速度预测公式,并基于羊台山隧道右线掘进数据进行可靠性验证;基于k-均值聚类方法,以FPI为聚类指标,划分出可掘性等级,并根据掘进参数预测公式,给出了对应可掘性等级的掘进参数建议值。结果表明:(1)FPI能够在一定程度合理反映TBM施工中的岩机关系,可作为掘进性能评价指标;(2)建立的TBM掘进参数预测公式,相关性系数R2分别为0.75、0.98和0.97,相关性良好,能够准确为TBM掘进提供预测参数;(3)根据建立的TBM可掘性分级方法将围岩划分为5级,确定了不同等级下掘进参数值。

节理裂隙岩体超大断面隧道开挖工法研究

为研究不同开挖工法对围岩中节理发育的超大断面隧道稳定性影响,以在建大罗山公路隧道为工程背景,运用离散元数值分析方法,研究了双侧壁导坑法、CRD法、CD法和三台阶法四种开挖工法下节理面剪切位移、隧道开挖轮廓位移、地表沉降、围岩位移、支护结构内力的变化规律.研究结果表明:双侧壁导坑法在控制节理裂隙岩体各位移变化及隧道支护结构内力方面效果最好,CRD法和CD法次之,不推荐将三台阶法作为节理发育地层中大断面公路隧道的开挖工法.

基于Weibull分布的非均质隧洞围岩破裂碎胀FDEM数值模拟研究

岩石材料具有非均质特征,矿物颗粒和胶结材料强度在统计学上服从Weibull分布。基于有限元-离散元耦合数值模拟方法(FDEM)提出了三角形单元弹性模量和四边形节理单元强度服从Weibull分布的参数赋值流程,随后研究了非均质岩样在单轴压缩、三轴压缩和巴西劈裂下的力学特征和破坏行为,最后对非均质隧洞围岩在高应力下的破裂碎胀大变形机制进行了模拟研究。结果表明:提出的随机参数赋值方法可建立不同弹性模量三角形单元数目服从Weibull分布、且空间分布具有随机性的FDEM数值模型;非均质岩样单轴抗压强度、三轴抗压强度、巴西劈裂抗拉强度、等效粘聚力和等效内摩擦角随着非均质度m的增大,均呈指数增大,并向均质材料逼近;高应力非均质隧洞围岩随着m的增大,裂隙网络形态变化不大,均以共轭剪切破裂为主,并伴随少量拉伸裂隙,但围岩破坏程度和裂隙扩展范围呈指数降低。

陡倾岩层隧道破坏机理及其稳定性研究

倾斜层状岩体具有明显的各向异性,隧道修建过程中易产生大变形、塌方、初期支护破坏、二衬开裂等一系列问题。为深入研究隧道在层状岩层中的力学特性和破坏机理,以大岗山隧道为项目依托,借助数值模拟研究有、无支护状态下不同节理倾角对隧道稳定性的影响,并针对现有支护方案提出优化。结果表明:节理的存在使得隧道开挖后变形具有非对称性,无支护状态下,当节理倾角在30°~60°变化时,隧道洞周最大变形发生在右拱肩处,塑性区破坏前期以拉伸破坏为主,集中于右拱肩及左拱脚,随着节理角度增大,后期以剪切破坏为主;支护状态下,洞周变形得到有效改善,其中拱底隆起最为明显,但最大隆起量降至30 mm以内,围岩塑性区域减小,当节理角度为45°时,拱顶、拱底产生最大变形,随着节理角度的增大,左右两侧变形非对称性降低;布设局部加长锚杆,增加隧道仰拱厚度的优化方案能增强围岩稳定性,抑制围岩进入塑性状态,减小拱底隆起,增强隧道稳定性,减小节理岩层引起的潜在风险,为现场施工提供理论指导。