Indian experiences with Q and RMR systems

The Q-systemir of Barton et. al. (1974, 1976) and the RMR-system of Bieniawski (1973) have been evaluated on the basis of measured tunnel support pressures from 26 tunnel sections, 2 to 14 m wide, covering both squeezing and non-squeezing ground conditions. The comparison shows that the Q-system is unsafe for large tunnels under squeezing ground conditiona. A new correlation has been developed considering tunnel depth, tunnel radius, tunnel closure, and Rock Mass Number—i.e., “stress free Q”—to obtain reliable estimates of tunnel support pressures. Changes suggested by Sheorey (1991) for satisfactory application of the Q system to coal-mine roadways on the basis of 44 case histories are presented. Unal's (1983) correlation for coal-mine roadways is shown as overly safe for large tunnels under non-squeezing ground conditions, and unsafe for all sizes of tunnels under squeezing ground conditions. Correlations between tunnel support pressure, tunnel depth, tunnel closure, and Bieniawski's RMR have been developed to provide reliable tunnel support pressures for all sizes of rock tunnels under varying ground conditions. The correlations between RMR and Q proposed by Bieniawski (1976) and by Rutledge and Preston (1978) are not reliable, because RMR and Q are not truly equivalent. Therefore, an acceptable correlation between rock mass number N and RMR_mod, i.e., RMR without joint orientation and intact rock strength, has been presented for a better interrelation.     

挤压性围岩隧道大变形机理及分级标准研究

结合高地应力条件下挤压性大变形隧道--乌鞘岭隧道工程实例,通过变形的现场量测结果,分析了挤压性围岩隧道大变形的基本特征。采用室内试验及现场量测等手段,综合分析确定了围岩物理力学参数,定量分析了洞室周边产生塑性区的条件、塑性区及洞壁位移的影响因素、塑性区半径与洞壁位移的关系。以现场量测数据为依托,结合理论计算,参考以往类似隧道经验,分别考虑了围岩的相对变形、强度应力比、原始地应力、弹性模量作为分级指标,并提出了综合系数α指标,采用综合指标判定法给出了大变形的分级标准,以及相应的防治措施。     

Uncertainty analysis of tunnel squeezing for two tunnel cases from Nepal Himalaya

Weak rocks such as shale, slate, phyllite and schist, and the rock mass of weakness/fault zones are incapable of sustaining high tangential stress. Severe tunnel squeezing is therefore common in the tectonically active Himalayan rock mass and is one of the major areas of concern regarding stability. A reliable prediction of the extent of squeezing is essential so that a strategy can be established regarding stabilizing measures and for optimizing the support well in advance (during planning and design). In this paper, a probabilistic approach to uncertainty analysis that focuses on the effect of the variations in each input parameter of squeezing is used for analyzing and predicting the extent of tunnel squeezing for two tunnel cases in Nepal; Kaligandaki “A” (completed) and Middle Marsyangdi (under construction). A semi-analytical method suggested by Hoek and Marinos [Predicting tunnel squeezing problems in weak heterogeneous rock masses. Tunnels Tunnell Int, 2000; 32(11 and 12): 45–51 and 34–36] for predicting squeezing, an empirical formula proposed by Panthi [Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD thesis, Department of Geology and Mineral Resources Engineering. Norwegian University of Science and Technology (NTNU), Norway, 2006] and the Palisade's 2002 version of @Risk statistical software program have been used for the uncertainty analysis. The analysis results for Kaligandaki indicate fairly good correlation between predicted and actually measured squeezing. The same approach has been used for predicting the degree of tunnel squeezing at Middle Marsyangdi tunnel. It is concluded that the methodology proposed in this paper has potential for predicting the squeezing of future tunnel projects in weak rock mass conditions.     

Physical model simulation of rock-support interaction for the tunnel in squeezing ground

The existence of squeezing ground conditions can lead to significant challenges in designing an adequate support system for tunnels.Numerous empirical,observational and analytical methods have been suggested over the years to design support systems in squeezing ground conditions,but all of them have some limitations.In this study,a novel experimental setup having physical model for simulating the tunnel boring machine(TBM)excavation and support installation process in squeezing clay-rich rocks is developed.The observations are made to understand better the interaction between the support and the squeezing ground.The physical model included a large true-triaxial cell,a miniature TBM,laboratoryprepared synthetic test specimen with properties similar to natural mudstone,and an instrumented cylindrical aluminum support system.Experiments were conducted at realistic in situ stress levels to study the time-dependent three-dimensional tunnel support convergence.The tunnel was excavated using the miniature TBM in the cubical rock specimen loaded in the true-triaxial cell,after which the support was installed.The confining stress was then increased in stages to values greater than the rock’s unconfined compressive strength.A model for the time-dependent longitudinal displacement profile(LDP)for the supported tunnel was proposed using the tunnel convergence measurements at different times and stress levels.The LDP formulation was then compared with the unsupported model to calculate the squeezing amount carried by the support.The increase in thrust in the support was backcalculated from an analytical solution with the assumption of linear elastic support.Based on the test results and case studies,a recommendation to optimize the support requirement for tunnels in squeezing ground is proposed.     

成兰铁路千枚岩隧道初期支护形式试验研究

在高地应力软岩地层中开挖隧道,易发生大变形,合理地选用初期支护的钢架形式,对软岩大变形的控制是非常重要的。本文依托成兰铁路-茂县隧道,通过现场试验和数值计算研究挤压性软岩大变形隧道的初支钢架选型,主要得出以下结论：（1）高地应力软岩隧道不宜选用强度较弱的钢架作为第一层支护,第一层支护应能保证支护封闭时的合理洞形|（2）挤压性大变形隧道宜采用多层、多次的支护方法,适当释放围岩应力,保证隧道的长期稳定|（3）大变形隧道多采用分部工法开挖,易导致支护结构受力不均,应提早支护封闭时机,改善结构受力。     

Determination of support reaction curve for steel-supported tunnels

Based on the analysis of instrumentation data obtained from several tunnels in India, an empirical approach has been proposed for determination of the support reaction curve for tunnels supported with the steel rib-backfill support system. Expressions have been obtained for determining the stiffness of this support system with three types of backfills—concrete, tunnel muck, and gravel. These expressions show that the steel rib-backfill support system exhibits a non-linear behaviour under pressure, unlike the normally assumed linear elastic behavlour, due to the continuously changing backfill stiffness. The backfill, though not the main load carrying element, significantly influences the behaviour of the support system under pressure. The behaviour of three types of backfills—concrete, gravel and tunnel muck—under pressure, has been studied. The concrete backfill provides a stiffer support than the other two types backfills and is, therefore, preferable for the elastic ground condition. The tunnel-muck and the gravel backfills may be more suited to the moderately squeezing and the highly squeezing ground conditions, respectively, as the latter is relatively more flexible.     

Linearization of the Hoek and Brown rock failure criterion for tunnelling in elasto-plastic rock masses

We present a novel methodology for estimation of equivalent Mohr–Coulomb strength parameters that can be used for design of supported tunnels in elasto-plastic rock masses satisfying the non-linear empirical Hoek–Brown failure criterion. We work with a general adimensional formulation of the Hoek–Brown failure criterion in the space of normalized Lambe's variables for plane stress, and we perform linearization considering the stress field in the plastic region around the tunnel. The procedure is validated using analytical solutions to a series of benchmark test cases. Numerical solutions are also employed to validate the procedure in cases for which analytical solutions are not available. Results indicate that the stress field in the plastic region around the tunnel, as well as the linearization method employed and the quality of the rock mass, has a significant impact on computed estimates of equivalent Mohr–Coulomb strength parameters. Results of numerical analyses also show that our proposed linearization method can be employed to estimate loads and moments on the tunnel support system. We recommend the equating model responses (EMR) method to compute equivalent Mohr–Coulomb strength parameters when the tunnel support pressure is accurately known, and we further show that our newly introduced linearization method can be employed as an alternative to the best fitting in the existing stress range (BFe) and best fitting in an artificial stress range (BFa) methods, providing performance estimates that are generally better than estimates of the BFe and BFa methods when differences with the response of the Hoek–Brown rock mass are of engineering significance (say more than 10%).     

Equivalent Mohr–Coulomb and generalized Hoek–Brown strength parameters for supported axisymmetric tunnels in plastic or brittle rock

The evaluation of equivalent Mohr–Coulomb (M–C) strength parameters to the prototype Hoek–Brown (H–B) ones for tunnels has been tackled in different ways for many years. The extension of the H–B criterion to the generalized one has made the challenge even greater. Most of the latest methods did not account for the effect of the support pressure and none gave formulae for equivalent parameters of supported or brittle rock. Here, an almost exact explicit solution for the evaluation of the critical pressure, of a tunnel in a rock mass satisfying the generalized H–B criterion, is initially investigated. Then, formulae are derived for the evaluation of equivalent parameters, of either elastoplastic or elastic–brittle plastic rock. They are based either on a best fitting procedure of the two envelopes or on the equation of selected responses of the models. Supported tunnels in equivalent M–C rock masses are then validated against those excavated in the prototype H–B rock masses.     

Critical strain and squeezing of rock mass in tunnels

The squeezing of tunnels is a common phenomenon in poor rock masses under high in situ stress conditions. The critical strain parameter is an indicator that allows the degree of squeezing potential to be quantified. It is defined as the strain level on the tunnel periphery beyond which instability and squeezing problems are likely to occur. Presently, in the literature, the value of critical strain is generally taken as 1%. It is shown in this study that the critical strain is an anisotropic property and that it depends on the properties of the intact rock and the joints in the rock mass. A correlation of critical strain with the uniaxial compressive strength, tangent modulus of intact rock and the field modulus of the jointed mass is suggested in this paper. It is also suggested that the modulus of deformation being anisotropic in nature should be obtained from field tests. In absence of field tests, use of a classification approach is recommended, and, expressions are suggested for critical strain in terms of rock mass quality Q. A rational classification based on squeezing index (SI) is proposed to identify and quantify the squeezing potential in tunnels. Applicability of the approach is demonstrated through application to 30 case histories from the field.     

Future tunnelling projects in Iceland

More than 300 km of hydro tunnels and 80–90 km of road tunnels could be excavated in Iceland before the year 2050. In order to complete this task, an average of 6–7 km of tunnel per year would have to driven. This volume of tunnelling is estimated to cost more than $US1 billion, which could be divided as follows: a) about 100 km of 3.5-m-wide diversion hydro tunnels (unsupported), for a total of $90 million; b) approx. 100 km of 5-m-wide hydro tunnels (supported), for a total of $210 million; c) about 100 km of 7.6-m-wide hydro tunnels (supported), for a total of $380 million; and d) approx. 85 km of road tunnels with 25 m² cross-section, for a total of $435 million.     

A theoretical solution for analysis of tunnels below groundwater considering the hydraulic–mechanical coupling

An analytical solution for the analysis of tunnels below groundwater table in plane strain axisymmetric condition is presented. Seepage body force and secondary permeability of the rock mass due to the mechanical–hydraulic coupling are taken into account. The strain-softening behavior model and Hoek–Brown empirical strength criterion for the rock mass are used in the analysis. As the derived analytical equations do not have closed form solutions, a computer program has been prepared for solving the corresponding equations numerically and examining the analysis. It is shown that the tunnel stability depends on the seepage and the pore water pressure particularly in the case of high pore pressure gradient.     

Tunnel behaviour and support associated with the weak rock masses of flysch

Flysch formations are generally characterised by evident heterogeneity in the presence of low strength and tectonically disturbed structures. The complexity of these geological materials demands a more specialized geoengineering characterisation. In this regard, the paper tries to discuss the standardization of the engineering geological characteristics, the assessment of the behaviour in underground excava- tions, and the instructions-guidelines for the primary support measures for flysch layer qualitatively. In order to investigate the properties of flysch rock mass, 12 tunnels of Egnatia Highway, constructed in Northern Greece, were examined considering the data obtained from the design and construction records. Flysch formations are classified thereafter in 11 rock mass types （I-XI）, according to the siltstone -sandstone proportion and their tectonic disturbance. A special geological strength index （GSI） chart for heterogeneous rock masses is used and a range of geotechnical parameters for every flysch type is presented. Standardization tunnel behaviour for every rock mass type of flysch is also presented, based on its site-specific geotechnical characteristics such as structure, intact rock strength, persistence and complexity of discontinuities. Flysch, depending on its types, can be stable even under noticeable overburden depth, and exhibit wedge sliding and wider chimney type failures or cause serious deformation even under thin cover. Squeezing can be observed under high overburden depth. The magnitude of squeezing and tunnel support requirements are also discussed for various flysch rock mass types under different overburdens. Detailed principles and guidelines for selecting immediate support mea- sures are proposed based on the principal tunnel behaviour mode and the experiences obtained from these 12 tunnels. Finally, the cost for tunnel support from these experiences is also presented.     

Squeezing rock conditions at an igneous contact zone in the Taloun tunnels, Tehran-Shomal freeway, Iran: a case study

Squeezing rock conditions at the contact zone of an andesitic-basaltic body and tuff country rocks in the Taloun tunnels were investigated and analyzed. Evaluation of the rock mass properties illustrates the fact that they were significantly reduced at the contact zone, especially when wet. Detailed monitoring and measurements of tunnel-wall convergence at the contact zone in the Taloun service tunnel, during the 10 months following excavation and installation of initial support, prior to installation of heavy support, showed greater than 3% of the normalized tunnel closure. This confirms moderate squeezing conditions at the contact zone. The measured displacement was even higher than that of the fault zone in which deformation was decreased during the first month and eventually stabilized. Similarly, numerical modeling of the deformation at the contact zone not only confirmed a higher value of the tunnel convergence but also demonstrated the reduction of in situ stress and development of plastic zones across the contact zone. These data are also believed to account for the squeezing condition at the contact zone. It is expected that this condition will be significantly increased in the main road tunnels due to the fact that these tunnels are twice as wide as the service tunnel. Therefore, proper and timely support must be applied. Numerical analysis of the support at the contact zone showed that the stress due to bending moment is greater than that of the axial forces on the lining. This calls for certain support measures in the form of permanent lining and two layers of steel bars to compensate for the tensile stress exertion on the lining.     

Back-analysis of viscoelastic displacements in a soft rock road tunnel

The rheological behavior of the rock proximate to excavations is of importance and practical interest for engineers in the design of soft rock tunnels. In situ rheology tests may be conducted to determine the appropriate parameters of the rock masses and the ground stress component; however, this can cost a great deal in both expense and time. Hence, it is useful to study back-analysis of viscoelastic displacements induced by the excavation of tunnels. Taking four rheological models, namely, Kelvin body, Maxwell body, Poynting–Thomson body and Kelvin–Voigt body as examples, this paper presents the basic finite element equations for such back-analysis, and the associated calculation methods for practical applications. A trial calculation method for the different models is adopted so as to obtain the most suitable rheological model and corresponding parameters. Furthermore, the authors have successfully applied the back-analysis method in a test section of a road tunnel excavated in soft rock. Compared with the direct in situ rheology tests, the back-analysis method presented by this paper is more concise and involves less money and time.     

Finite element analysis of tunnel–soil–pile interaction using displacement controlled model

Significant additional loads could be induced in pile foundations adjacent to new tunnels. Accurate prediction of magnitude and shape of the ground displacements, which define curvature changes, is crucial for the computation of tunnelling induced bending and axial stresses in pile foundations. The finite element simulation of tunnelling by removing forces corresponding to initial stress-state, tend to predict incorrect shape of ground displacement profiles, hence incorrect forces in pile foundations adjacent to tunnels. To overcome this difficulty, this paper describes the development and application of a simple and useful displacement controlled model (DCM) to predict the effects of tunnel excavation on adjacent pile foundations. The DCM simulates tunnelling by applying displacements to the tunnel boundary. A method to determine magnitude and direction of tunnel boundary displacements, based on convergence patterns observed in field and centrifuge test results, is proposed. Back analyses of numerous greenfield tunnel case histories using the DCM indicate good agreement between computed displacement profiles and field/test data. The suitability of the DCM in modelling tunnel–soil–pile interaction problems is demonstrated through back analysis of a centrifuge test and a field case study.     

阿公店水庫越域引水路工程泥岩隧道之設計

阿公店水庫位於高雄縣燕巢鄉境,係以防洪為主並兼具灌溉及公共給水之多目標水庫,惟因庫區淤積情況嚴重,為恢復水庫原有功能並達水資源永續利用之目的,經核定辦理"阿公店水庫更新工程計畫",採台灣首創排潤蓄清之空庫防淤操作,輔以越域引水路導引旗山溪潔淨水源蓄存運用,以同時解決水庫防洪、淤積及給水問题於一舉。隧道基於水理及施工性考量採4.0m内徑2R-3R-3R之正馬蹄型斷面,隧道開挖主要可能遭遇問題包括擠壓破壞及砂岩夾層蕴藏地下水狀況,由於泥岩隧道岩體分類及支撐設計不盡適用CSIR-RMR及NGI-Q值之經驗設計法則,遂採近似Terzaghi之岩壓估算方式,輔以數值分析法計算隧道變形與岩體-支撐之應力,俾檢核支撐設計之適用性,施工期間並彙整監測资料進行反算分析回饑設計,以期達兼顧安全與經濟之合理設計。考量儘早有效抑制隧道岩體擠壓及潛變變形,確保隧道長期之穩定性,並可維護洞内施工動綜提升施工效率,隧道仰拱設計以預鑄混凝土版進行斷面支撐閉合。     

深埋高地应力TBM隧道挤压大变形及其控制技术研究

深部岩体构造复杂、地应力高,对于挤压性地层开挖后容易产生挤压大变形,大量实践经验表明,若不及时合理处置应对将造成巨大的经济损失。关注TBM隧道,介绍围岩挤压大变形的机制、关于挤压性地层的提前预报方法和监测辨识指标,讨论预测围岩收敛变形的方法并详细介绍人工智能和非确定性分析方法,最后总结归纳常用的应对挤压性地层的处置手段。各种关于TBM隧道挤压大变形的辨识公式、预测方法以及处置手段都是依托相应的工程案例而建立,盲目的套用并不一定能起到预期的效果,建议具体工程问题应具体分析,综合比选,各取所长。     

Effect of large excavation on deformation of adjacent MRT tunnels

A large excavation of approximately 140 m wide, 200 m long and 15 m deep was made close to two Mass Rapid Transit (MRT) tunnels of 6 m diameter with invert depth of 15–27 m. In view of the scale and distance of excavation, significant effects on the MRT tunnels were expected. The paper presents the monitoring of the tunnel deformations during the excavation. A sophisticated monitoring system using a motorised total station was installed in the MRT tunnels to monitor their displacements and to ensure that the stringent requirements for safeguarding the tunnels were not violated during any part of the excavation works. The paper also presents the modelling of the excavation using a finite element program. The results obtained were reasonably close to the monitoring results. It was found that the stiffness of the tunnel lining has significant influence on the displacement and distortion of tunnels caused by an adjacent excavation. A stiffer lining undergoes less displacement and distortion but is likely to experience significantly greater bending moments.     

Tunnelling through an embankment using all ground fasten method

The Tohoku Shinkansen Railway between Morioka and Hachinoe was opened on December 1, 2002. In this section, there are 20 tunnels. Second Itsukaichi tunnel is one of them, and it is a relatively short tunnel of 1175 m. At the Morioka portal, this tunnel crosses under National Highway No. 4 with a small depth of overburden 2–5 m where approximately 15,000 vehicles pass a day. The tunnel was excavated by NATM method. All Ground Fasten method was used as an auxiliary method. As a result, the face was prevented from collapsing and the tunnel was excavated while keeping the ground surface settlement to a minimum without interfering with road traffic.     

Effect of intermediate principal stress on strength of anisotropic rock mass

The Mohr-Coulomb criterion needs to be modified for highly anisotropic rock material and jointed rock masses. Taking σ₂ into account, a new strength criterion is suggested because both σ₂ and σ₃ would contribute to the normal stress on the existing plane of weakness. This criterion explains the enhancement of strength (σ₂ – σ3) in the underground openings because σ₂ along the tunnel axis is not relaxed significantly. Another cause of strength enhancement is less reduction in the mass modulus in tunnels due to constrained dilatancy. Empirical correlations obtained from data from block shear tests and uniaxial jacking tests have been suggested to estimate new strength parameters. A correlation for the tensile strength of the rock mass is presented. Finally, Hoek and Brown theory is extended to account for σ₂. A common strength criterion for both supported underground openings and rock slopes is suggested.