Geotechnical risk management approach for TBM tunnelling in squeezing ground conditions

Historically, attempts to use tunnel boring machines (TBMs) in Himalayan geology have been unsuccessful, particularly where weak rocks exist at the significant depths often required for hydroelectric hydraulic tunnels resulting in squeezing ground conditions. The use of segmental tunnel linings erected by shielded TBMs presents additional risk, such that the advantages of potentially high rates of advance using this form of construction have not previously been realised. Programme demands for the 330 MW Kishanganga Hydroelectric Project in India required that 15 km of the 23 km headrace tunnel be constructed using a double-shield TBM erecting a segmental lining. Preliminary studies suggested difficult ground due to squeezing conditions along the 1400 m deep tunnel through weak meta-sedimentary rocks. To allow planning and construction to commence, a risk management approach to design and construction was formulated with contingency procedures and criteria developed to allow the risks to the TBM and the lining to be managed effectively. Advanced numerical modelling included analysis of the tunnel with the ground represented by a Stress Hardening Elastic Viscous Plastic (SHELVIP) model to take account of time dependent loading. The Kishanganga tunnel represents the first segmentally lined TBM tunnel to be successfully constructed in the Himalaya. This paper describes the risk-mitigation approach, the special measures developed to address the risks, the numerical modelling and laboratory testing undertaken, and includes results from the segmental lining monitoring. Recognition of the risks, the development of an innovative methodology and the provision of the means by which geotechnical risk could be managed effectively during construction, gave confidence to all stakeholders to proceed with a method of construction that had not previously been implemented successfully in the Himalaya.     

TBM隧道豆砾石-地层抗力系数计算方法研究

豆砾石回填与灌浆技术在护盾式TBM施工的隧道中获得了广泛应用,但是豆砾石填充层对地层抗力系数以及管片结构受力的影响有还有待研究。本文利用厚壁圆筒弹性应变理论和温克勒假设,首先建立轴对称条件下的豆砾石-地层抗力系数公式并对影响该公式的因素进行了分析。在此基础上,考虑了豆砾石填充层的非均匀分布,建立了非轴对称条件下的豆砾石-地层抗力系数公式。研究表明:(1)是否考虑豆砾石填充层的存在以及豆砾石填充层是否均匀分布对地层抗力系数具有重要影响;(2)不同岩性条件下,豆砾石填充层弹性模量均存在一临界值使豆砾石-地层抗力系数的性质发生突变;(3)豆砾石填充层厚度的增加则具有强化豆砾石-地层抗力系数性质的作用。     

Evaluation of ground convergence and squeezing potential in the TBM driven Ghomroud tunnel project

One of the new components of water conveyance system in central Iran is the Ghomroud water conveyance tunnel that is being excavated by a double shield TBM. The 36 km long tunnel mainly passes through the metamorphic weak rocks of Jurassic age. Key geotechnical design issues for the tunnel, which has up to 650 m of overburden, include the potential for high ground pressure due to high in situ stress. In order to prevent the shield jamming in these weak rocks, it was necessary to evaluate the amount of ground pressure on the outer surface of TBM shield in the vicinity of the tunnel face. The stress and strain condition in the vicinity of the tunnel face has a 3D nature and it is not realistic to assume a two-dimensional stress state at the tunnel face area. In the convergence-confinement method, it is possible to simulate the tunnel face effect with an internal fictitious pressure that is imposed on the tunnel perimeter. In this study, based on the convergence-confinement method, a new method was introduced to calculate the tunnel face effect on ground pressure distribution around the tunnel face region. Then by using this method, critical areas with potential for shield jamming was predicted along the Ghomroud water conveyance tunnel. The obtained results by this method are in good agreement with the current TBM jamming situations along the Ghomroud tunnel.     

盾构隧道管片及纵向接头力学性能数值模拟研究

管片接头作为盾构隧道衬砌结构的薄弱部位,其力学行为直接反映隧道结构整体受力性能。考虑到接头由相邻管片连接而成,本文首先选用非线性有限元法对单个管片的静力响应开展研究,并与文献试验结果对比以验证数值模型。借此模型分析了混凝土剪胀角、拉压损伤行为等对管片结构力学行为影响。在此基础上,依托文献中管片纵向接头静载试验,详细地考虑了接头构造及接头处的相互作用,建立相应模型模拟试验加载过程,同时将数值分析结果与实测试验结果的破坏模式对比分析。基于提出的有限元模型,进一步研究了管片纵向接头力学特性随各关键因素变化规律,包括接头外侧间隙、螺栓倾斜角度及伸入管片长度、接缝两侧密封垫弹性模量和厚度,以及螺帽与手孔混凝土间初始接触状态。获得不同工况下接头挠度、接缝变形及接头转角等变化规律。     

On the influence of face pressure, grouting pressure and TBM design in soft ground tunnelling

A three-dimensional finite element simulation model, which includes all relevant shield tunnelling components and allows for the modelling of the step-by-step construction process of the tunnel advance is used to analyse the influence of TBM operation parameters and design parameters for a shallow tunnel advance in homogeneous, soft, cohesive soil below the ground water table. The numerical sensitivity studies presented in this paper focus on the face support pressure, the grouting pressure, the trailer weight and the length, weight and taper of the shield machine. The simulation results are evaluated with respect to the settlements of the ground surface, the shield movement and the loading of the tunnel lining. The evaluation of the sensitivity analyses helps to obtain a more detailed insight into the influence of selected parameters relevant for the design and steering of TBM tunnel advances.     

Observed Behaviour of a Tunnel in Sand Subjected to Shear Deformation in a Centrifuge

The paper describes observed behaviour of a model tunnel embedded in dry sand subjected to cyclic ground shear deformation in a centrifuge, as well as the behaviour of the model ground during shear deformation. Detailed data on earth pressures acting on the tunnel lining and the sectional forces of the lining are presented during ground shear deformation. The data suggest that the earth pressure at tunnel crown before ground shear deformation is smaller than the full overburden pressure probably due to the formation of arch action and the arch action may deteriorate with the cyclic ground shear deformation, resulting in an increase in the earth pressure at crown and changing the distribution of the sectional forces, which are largely influenced by conditions between tunnel lining and invert.     

An appraisal of TBM performances in Turkey in difficult ground conditions and some recommendations

The geology of Turkey is very complex and major Northern and Eastern Faults including minor faults associated to these faults create tremendous problems, like squeezing of the TBM, excessive water ingress, TBM face collapses, as encountered in the Kargi power tunnel, the Dogancay energy tunnel, the Gerede water tunnel, and the Nur Dagi railway tunnel. Mixed ground conditions with ophiolites, graphitic schists and melanges with boulders are other fundamental difficulties leading to squeezing and blocking of the TBMs or even causing complete failures of the segments and abandoning of the tunnel. A typical example for tunnel abandoning is the Kosekoy high speed tunnel and an example for excessive TBM squeezing is the Uluabat energy tunnel. The affects of dykes in the Istanbul region is known well by practicing tunnel engineers. These andesitic rocks, make fractures in the country rock and cause several problems during TBM excavation like blocking the cutterhead and excessive disc cutter consumption. Typical examples are the Goztepe-Kadıkoy Metro tunnels, and the Melen water tunnel. The Beykoz utility tunnel is one of the most difficult tunnelling projects in Istanbul. Presence of clay minerals existing within the geologic formations is also one of the main reasons clogging the cutterhead of TBM as encountered in the Suruc water project. The effects of complex geology on the excavation efficiencies of different type of TBM’s used in the ten projects mentioned above are explained in this paper and some recommendations with a ground classification system for proper use of TBMs in faultyzones are given.     

Improved analytical model for circumferential behavior of jointed shield tunnels considering the longitudinal differential settlement

Differential settlements of the shield tunnel along the longitudinal direction can significantly affect the circumferential behavior of segmental lining, degrading both the structure safety and serviceability. In this paper, the authors introduce both the shearing effect and flattening effect on the tunnel cross section, which was caused by the longitudinal differential settlement, into an existing analytical model of jointed shield tunnels. An example is then presented to illustrate the variation of the circumferential behavior of segmental lining, including both the structure safety and serviceability, along the longitudinal position. Finally, a series of parametric studies are performed to investigate how factors such as tunnel longitudinal settlement, property of surrounding ground, and the design parameters of segmental lining affect the circumferential behavior of the segmental lining.     

Numerical simulation of the creation and performance of extruded concrete linings in microtunnelling

Microtunnelling traditionally utilises pipe-jacked or segmental lining systems, however when employed over long, continuous distances, which may include changes in depth and direction, both lining systems have associated limitations that would reduce the drive rate of the TBM and increase the costs of the project. The use of an extruded lining system could circumvent a number of these limitations and make such an undertaking economically viable. A numerical model has been developed to simulate the extrusion of a tunnel lining from a microtunnelling TBM. The model was created in Plaxis (3-D Tunnel), a commercially available finite element modelling package. A methodology was developed and tested to approximate the movement of a TBM and the curing of the extruded lining, both of which are time-based phenomena. The extruded materials were subsequently investigated, using published strength-curing time for a normal strength concrete and a polymer concrete, for two drive rates. The outcome of the modelling suggests that the relationship between the developing stiffness and time in the early phases of curing is critical if an extruded lining system is to be deployed behind a TBM without slowing its drive rate.     

Effect of adverse geological condition on TBM operation in Ghomroud tunnel conveyance project

With the planned length of 36 km, Ghomroud tunnel is one of the longest tunnels under construction in central Iran. About half or 18 km of this tunnel was excavated by a double shield TBM. Several adverse geological conditions encountered, consisting of ground squeezing and face collapse, hindering TBM performance, and caused several TBM stoppages and jamming. This paper presents the impact of ground conditions on machine performance based on the information obtained from field observations and geotechnical site investigations. As built geological conditions are described while the method and results of tunnel convergence measurements and their impacts on tunneling operation is examined. Based on the detail study of the available geological information and tunnel convergence measurements, it was evident that the existence of weak structures in rock mass resulted in high rate of the convergence, which was the dominant factor in the TBM jamming. Since it was not possible to make observation and measurements of geological parameters when working in a lined tunnel built by a shielded machine, an attempt was made to correlate TBM operational parameters and ground convergence. The preliminary result of the analysis has indicated a good correlation among machine’s operational parameters and tunnel convergence. If the system is fully developed, these parameters can be used as an indicator of the potential for high rates of convergence. An early warning on ground convergence is essential for taking precautionary measures to avoid TBM from getting jammed by squeezing ground.     

FE analysis of time-dependent behavior of tunneling in squeezing ground using two different creep models

Ground movement and contact pressure on the lining of Stillwater Tunnel (Utah, USA) were investigated. Axisymmetirc finite element analysis was used in the analysis. Power law and hyperbolic creep models were used to model ground squeezing and to show the differences in the results between the two models. Creep parameters for the two models were evaluated based on the experimental creep and strength tests that were performed by other investigators on the gouge materials encountered along the tunnel axis through the heavily sheared and fault zones of the Red Pine shale. The results of the analysis which include normalized inward movement at the tunnel crown, normalized radial ground convergence with depth, and lining-ground contact pressure were compared with the results that were measured along the tunnel axis by other investigators. The results of the analysis show that lining pressure and deformation can be predicted well from the use of power law creep model if the delay time before lining erection is considered.     

An experimental study of the effect of local contact loss on the earth pressure distribution on existing tunnel linings

This study presents the results of the experimental investigation that has been conducted to examine the effect of local contact loss between a tunnel lining and the surrounding ground on the earth pressure distribution acting on the tunnel liner. An experimental setup has been designed using a mechanically adjustable tunnel model to simulate the initial lining pressure that results from shield tunnelling. A local separation between the lining and the surrounding soil was introduced at different locations around the tunnel and the changes in contact pressure were measured. Results indicated significant changes in earth pressure in the close vicinity of the area that has experienced the contact loss. The changes in earth pressure differed greatly depending on the location of the induced separation. When located at the invert and haunches results showed an increase in pressure by about 28%, whereas a pressure decrease of about 75% was measured immediately above the separated section when located at the springline. The above results suggest that the presence of a small lining area that is not in direct contact with the surrounding ground can have a significant impact on the performance of the tunnel lining.     

Examining the effect of adverse geological conditions on jamming of a single shielded TBM in Uluabat tunnel using numerical modeling

Severe shield jamming events have been reported during excavation of Uluabat tunnel through adverse geological conditions, which resulted in several stoppages at advancing a single shielded tunnel boring machine(TBM). To study the jamming mechanism, three-dimensional(3D) simulation of the machine and surrounding ground was implemented using the finite difference code FLAC3D. Numerical analyses were performed for three sections along the tunnel with a higher risk for entrapment due to the combination of overburden and geological conditions. The computational results including longitudinal displacement contours and ground pressure profiles around the shield allow a better understanding of ground behavior within the excavation. Furthermore, they allow realistically assessing the impact of adverse geological conditions on shield jamming. The calculated thrust forces, which are required to move the machine forward, are in good agreement with field observations and measurements. It also proves that the numerical analysis can effectively be used for evaluating the effect of adverse geological environment on TBM entrapments and can be applied to prediction of loads on the shield and preestimating of the required thrust force during excavation through adverse ground conditions.     

Dimensionierung von maschinellen Tunnelvortrieben mit Tübbingausbau im Fels

In diesem Aufsatz werden auf theoretischer Grundlage abgeleitete Berechnungsansätze vorgestellt, durch die es möglich ist, das Risiko der Verklemmung des Schildmantels bei einer Tunnelbohrmaschine mit Schild (TBM‐S) beurteilen und die Größe des erforderlichen planmäßigen Überschnitts bemessen zu können. Ebenso werden analytische Berechnungsansätze für den Gebirgsdruck aufgeführt, die zu einer wirtschaftlicheren Dimensionierung der Tübbingsicherung für die zuvor genannte Vortriebsart beitragen können. Dimensioning of shield tunnelling with segment lining in hard rock. In the available essay formulations based on theoretical elaborations are introduced, by which it is possible to estimate the risk of blocking of the shield skin of tunnel boring machine with shield (TBM‐S) and the size of the necessary regular overcut can calculated. Likewise analytic formulations for the ground pressure are specified, with which it is possible to design an economic segment lining for the kind of driving specified before.     

达坂引水隧洞TBM选型

结合大坂隧洞工程地质特征,开展长大引水隧洞掘进机选型研究,确定达坂引水隧洞采用双护盾隧洞掘进机。介绍了掘进机遇到高地应力软岩、高外水压力以及膨胀岩地段的通过技术,同时介绍了出现涌水、涌泥、涌砂等灾害时的应对措施与方法。研究表明,长大隧洞掘进机选型关系着工程建设的成败,其选型应在满足成本、效率和安全性要求基础上,与工程的具体地质条件相适应,尤其是不良地质地段的通过技术和相应的设备改造与支护技术,对于选型成功具有重要意义。     

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.     

Analytical computation of support characteristic curve for circumferential yielding lining in tunnel design

Circumferential yielding lining is able to tolerate controlled displacements without failure,which has been proven to be an effective solution to large deformation problem in squeezing tunnels.However,up to now,there has not been a well-established design method for it.This paper aims to present a detailed analytical computation of support characteristic curve(SCC)for circumferential yielding lining,which is a significant aspect of the implementation of convergence-confinement method(CCM)in tunnel support design.Circumferential yielding lining consists of segmental shotcrete linings and highly deformable elements,and its superior performance mainly depends on the mechanical characteristic of highly deformable element.The deformation behavior of highly deformable element is firstly investigated.Its whole deforming process can be divided into three stages including elastic,yielding and compaction stages.Especially in the compaction stage of highly deformable element,a nonlinear stress-strain relationship can be observed.For mathematical convenience,the stress-strain curve in this period is processed as several linear sub-curves.Then,the reasons for closure of circumferential yielding lining in different stages are explained,and the corresponding accurate equations required for constructing the SCC are provided.Furthermore,this paper carries out two case studies illustrating the application of all equations needed to construct the SCC for circumferential yielding lining,where the reliability and feasibility of theoretical derivation are also well verified.Finally,this paper discusses the sensitivity of sub-division in element compaction stage and the influence of element length on SCC.The outcome of this paper could be used in the design of proper circumferential yielding lining.     

Numerical investigation into the composite behaviour of over-deformed segmental tunnel linings strengthened by bonding steel plates

Bonding steel plate has been used as a strengthening approach to repair disrupted segmental lining of operational tunnels. This paper introduces numerical investigation into the composite behaviour of the initially deformed segmental tunnel linings strengthened by bonding steel plates using finite element modelling. Cohesive zone modelling was used to simulate the interface bonding behaviour between the segmental linings and steel plates. The full history of the tunnel behaviour before and after strengthening were simulated, where the segmental tunnel lining is initially loaded to create some deformation, then after bonding steel plate, the strengthened tunnel is reloaded until failure occurs. By comparing the results with experimental data from the literature, the proposed model was proved to be capable of simulating the strengthened lining behaviour and able to capture the strengthening failure process in terms of the interface debonding. Subsequently, the segmental lining response and interface shear stress distribution and propagation were analysed to interpret the interaction and failure mechanism of the steel plate strengthened segmental linings. The influence of the initial deformation and the steel plate thickness were investigated and discussed in terms of the strengthened stiffness and capacity. It has been found that the interface shear stress concentration occurred at the positions of the segment joints, where bond damage first initiated. The ultimate failure of the steel plate strengthening happened suddenly once a local debonding zone close to the segmental joint was formed. In addition, the predicted results indicate that a delay in strengthening would result in an increase in strengthened capacity but a decrease in strengthened stiffness. By using thicker steel plates, the strengthened stiffness was improved, while the strengthened capacity could be improved only if the thickness was relatively thin.     

Thrust force requirements for TBMs in squeezing ground

Rapidly converging ground may exert such a high pressure on the shield that the thrust force is no longer sufficient to overcome shield skin friction and the TBM becomes jammed. This paper advances a number of theory-based decision aids, which will support rapid, initial assessments to be made of thrust force requirements. A comprehensive parametric study has been carried out using the finite element method and, based on the numerical results, dimensionless design nomograms have been worked out that cover the relevant range of material constants, in situ stress and TBM characteristics. The nomograms make it possible to assess the feasibility of a TBM drive in a given geotechnical situation and to evaluate potential design measures or operational measures such as reductions in shield length, the installation of a higher thrust force, increases in the overcut or the lubrication of the shield surface, thus making a valuable contribution to the decision-making process.     

管片衬砌承担高内水压力的可行性分析

 双护盾掘进机掘进与管片衬砌几乎同时进行,具有快速施工的特点,但管片衬砌能否承受电站引水隧洞高内水压力作用,是决定引水隧洞能否采用管片衬砌及TBM施工的关键之一。结合青松电站引水隧洞,对管片衬砌在承受高内水压力作用下的应力与变形特性进行三维有限元计算,对影响管片接缝张开的主要因素进行分析,获得在高内水压力作用下的管片应力、变形分布,深化对高内水压力作用下的管片衬砌特性的认识。计算发现在内水压力作用下,管片衬砌具有柔性变形特征,就青松电站而言,在现有的接缝止水技术条件下,管片衬砌可承受高水头内水压力作用,这对推动管片衬砌及TBM在引水隧洞中的应用具有重要意义。