Geotechnical risk assessment based approach for rock TBM selection in difficult ground conditions

There are many potential sources of geotechnical risk in mechanized rock tunnelling. Problems such as encountering fault zones with running and water bearing gouge, tunnel walls instabilities in running or blocky grounds, hard and abrasive rock sections and convergent tunnel sections are principal causes in geotechnical risk occurrence. On the other hand, the performance of each TBM encountering such conditions will be different. Therefore, using different TBMs will have variable risk levels. This paper is to discuss rock TBM selection based on geotechnical risk minimization. So, a new approach was proposed based on decision analysis using decision tree. Based on the newly proposed approach, the most appropriate TBM is one that has the minimum risk level either before or after hazards mitigation measures. To be able to check the performance of this approach in practice, selection of machine for Nosoud water transfer tunnel has been evaluated. A shielded TBM (either single or double shield one) was proposed for the tunnel based on the newly proposed method. However, a double shield TBM was selected because of its more flexibility in difficult ground conditions in comparison with single shield TBM and limitation of project construction duration. The machine performance during tunnelling period verifies the success of excavation using selected TBM.     

Prediction of TBM jamming risk in squeezing grounds using Bayesian and artificial neural networks

This study presents an application of artificial neural network(ANN) and Bayesian network(BN) for evaluation of jamming risk of the shielded tunnel boring machines(TBMs) in adverse ground conditions such as squeezing grounds.The analysis is based on database of tunneling cases by numerical modeling to evaluate the ground convergence and possibility of machine entrapment.The results of initial numerical analysis were verified in comparison with some case studies.A dataset was established by performing additional numerical modeling of various scenarios based on variation of the most critical parameters affecting shield jamming.This includes compressive strength and deformation modulus of rock mass,tunnel radius,shield length,shield thickness,in situ stresses,depth of over-excavation,and skin friction between shield and rock.Using the dataset,an ANN was trained to predict the contact pressures from a series of ground properties and machine parameters.Furthermore,the continuous and discretized BNs were used to analyze the risk of shield jamming.The results of these two different BN methods are compared to the field observations and summarized in this paper.The developed risk models can estimate the required thrust force in both cases.The BN models can also be used in the cases with incomplete geological and geomechanical properties.     

Estimating torque,thrust and other design parameters of different type TBMs with some criticism to TBMs used in Turkish tunneling projects

It is crucially important to select a proper TBM and define its basic specifications such as installed cutterhead torque and TBM thrust capacities for a special job. Underestimation of these parameters would reduce excavation performance. In order to generate a general guidance on determination of some of TBM specifications, a database including 262 TBMs’ design parameters is established. The statistical relationships between the design parameters of 262 TBMs (72 open, 24 single shield, 41 double shield, 86 EPB and 39 slurry TBMs) manufactured after 1985 in the world are investigated and theoretical concepts behind the relationships between TBM diameter and installed thrust capacity, nominal cutterhead torque capacity, total weight, maximum rotational speed of cutterhead, and number of disc cutters are discussed. Some of the correlations between these parameters are found to be strong. The results are summarized in a table given upper and lower limits of predicted values. At the end, some data obtained in different 30 tunnels excavated in different geological conditions with different TBMs in Turkey are discussed to test the validity of predictor equations developed within the frame of this study.     

Case studies of TBM tunneling performance in rock–soil interface mixed ground

This paper investigates the performance of tunnel boring machines (TBMs) in rock–soil mixed-face ground based on TBM tunneling projects in Singapore. Currently several methods are available to estimate TBM tunneling performance in homogenous rock or soil. However, the existing models cannot be effectively applied to predict TBM penetration rate in mixed ground. The tunnels in this study were excavated in adverse mixed-face ground conditions. The geological profiles and the TBM operational parameters are compiled and analyzed. The influence of different geological face compositions on the performance of the TBMs is studied. The statistical analysis shows that there is a possible correlation between the mixed-face ground characteristics and the TBM advancement. Different approaches are used to find a reliable model. Finally, a method is proposed to predict the TBM performance in mixed-face ground for project planning and optimization.     

Probabilistic simulations of TBM tunnelling in highly fractured and faulted rocks

Despite the potential excellent performance of TBMs in favourable ground conditions, the presence of fault zones or heavily jointed rocks represents important geological hazards encountered during tunnel excavation. The effects of these challenging environments on the final tunnel construction time and costs can be investigated through a specific computer code: the Decision Aids for Tunnelling (DAT). In this framework the DAT simulate the tunnel excavation in several geological profiles, where changing ground scenarios are described in terms of different “fault zone” classes (from highly fractured rocks, to faulted and crushed material). For each class a certain reduction of the TBM advance rate is specified based on real data analyses. Although the great uncertainty, the results give a reliable estimation of the effect of degrading rock mass conditions on the tunnelling performance. Finally, a real case-study has been simulated by DAT in order to validate the use of the “fault zone” classes (and the relative advance rate reductions) in the estimation of the final time of tunnel construction. The predicted time values prove to be very close to the ones recorded on the field, confirming the importance of a more detailed and comprehensive characterisation of difficult ground conditions such as fault and highly fractured zones.     

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.     

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.     

不同直径盾构隧道地层损失率的对比研究

地层损失率是引起地面沉降最主要的因素之一。因此,收集了国内盾构隧道地面最大沉降实测数据,利用Peck公式反推得到地层损失率的取值,研究大直径(D10m)与中小直径盾构隧道地层损失率的分布规律及主要影响因素。结果表明:(1)中小直径、大直径盾构隧道施工引起的地层损失率分别有93.19%在0%～2.0%、近70%在0%～0.5%之间,大直径盾构隧道施工引起的地层损失率数值更小,分布更集中;(2)中小直径、大直径盾构隧道引起的地层损失率分别随着地层条件变好、地层渗透性的变小而减小;(3)两种直径盾构隧道的地面最大沉降与地层损失率均具有一定的线性相关性;(4)隧道覆土深度比与地层损失率的相关性较弱;(5)中小直径盾构隧道引起的地层损失率随着地层黏聚力、内摩擦角以及弹性模量的增大而逐渐减小。研究成果可为今后相关地区类似隧道工程施工诱发的地面沉降预测和施工控制提供科学参考。     

地铁双线盾构区间地表横向沉降槽参数分析

城市地铁盾构隧道的横向变形特点是确定工程影响区域和影响范围的重要依据。对我国22个建设城市的58条地铁线路、126个区间、964个地表横向沉降槽资料进行分析,研究了地铁双线盾构区间隧道的地表横向变形特点。根据地层条件的不同,对不同地层区域的沉降槽Peck公式拟合参数进行统计分析,得出了地层损失率和宽度参数的分布形态、相关统计值以及与隧道相对埋深的相关性。研究结果表明:(1)地层损失率和宽度参数的数理统计结果可以很好地指导不同地层区域地铁双线盾构隧道工程的影响区划分和影响范围的确定;(2)建议各地结合地层条件特点,对地表沉降槽进行深入研究,以提出更为适宜的地表横向沉降槽预测参数。     

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.     

地铁盾构隧道下穿建筑基础诱发地层变形研究

城市繁华地区地铁盾构隧道施工常需从建筑基础下穿越，如何确保上部建筑与隧道结构安全是施工中的难题。基于沉降预测理论及FLAC3D进行了地铁施工诱发地层环境损伤评估与控制设计STEAD系统的开发，以广州地铁区间隧道下穿某7层框架结构建筑为例，采用数值模拟研究了地铁盾构隧道穿越建筑基础诱发地层变形的空间效应问题，考虑了不同工况下隧道施工引起地层沉降对该建筑物的影响，采用本研究建议，盾构隧道成功穿越该建筑物，实测证实了变形空间效应研究的科学性与有效性。     

Shield tunneling and environment protection in Shanghai soft ground

Large scale and intensive metro construction through dense urban area increases sharply the impaction on risk control and environment protection. Three typical cases of shield crossing building above ground (SCBA), shield crossing tunnel from above (SCTA) and shield crossing tunnel from below (SCTB) are studied, respectively, based on field measurements and site investigations of actual projects in Shanghai soft ground. The risks of shield crossing sensitive buildings and subways, ground movement prediction and its control regulations, the settings of shield driving parameters such as earth pressure, driving speed, postures and grouting are demonstrated and summarized in detail. It is proposed that stringent stipulations on controlling ground volume loss (GVL) ratio (GVLR) and strengthened monitoring measures are necessary and substantial for eliminating/reducing potential construction risks. It is urgently decisive to improve the performance of shield machine and to make it more flexible for counteracting complications of geology and environment, as refer to the present status of shields in Shanghai, most of them being overused or out of date.     

The “penalty factors” method for the prediction of TBM performances in changing grounds

Tunnel construction by TBMs through hard rock is significantly affected by the geological and geotechnical conditions at tunnel level. Ground parameters such as uniaxial compressive strength, fracturing degree and abrasiveness, and factors such as water inflows and stress level may deeply affect the way a TBM will perform. In addition, different types of TBMs will behave differently in a given condition.This paper presents a method for TBM performance prediction in changing grounds, which has been developed in the framework of the European project “New Technologies for Tunnelling and Underground Works” (NeTTUN). The model starts from an optimum TBM performance in best conditions, i.e. when all ground parameters are in their “best state”. A stepwise reduction of the optimum advance rate is then performed, according to “reduction factors” that quantify the effect of degrading ground conditions on the TBM advance rate. By doing so, the “penalty factors” model is able to take into account a very wide range of ground conditions, from very good to very poor. Two types of TBMs commonly employed in rock tunnelling have been considered, i.e. Gripper and Shielded machines, each of them characterized by its own set of reduction factors.In order to consolidate the factor values and to validate the model, a TBM performance database, also developed in the framework of the project NeTTUN, has been used. The database includes a large number of tunnels excavated in different ground conditions with all standard TBM types. The comparison between the values given by the “penalty factors” model and the actual TBM performances observed during construction shows that the developed tool may provide a reliable estimation of the TBM performance based on simple ground parameters.The “penalty factors” model has also been interfaced with the DAT (“Decision Aids for Tunnelling”). The DAT software, co-developed by MIT and LMR-EPFL, is able to compute the probabilistic distributions of the tunnel construction time and – cost in function of the geology – and construction related uncertainties.The model is conceived to be used in its present form. However, the methodology can be easily adapted to match the expertize of the user, who is free to update the optimal performances, the ground parameters and/or the values of the reduction factors according to his/her own experience. The model can also be extended to other TBM types and to conventional excavation methods.     

Mechanism for buckling of shield tunnel linings under hydrostatic pressure

In this paper, the effects of segmental joints, dimensions of segments, and ground conditions on buckling of the shield tunnel linings under hydrostatic pressure are studied by analytical and numerical analysis. The results show that radial joints have significant impacts on the buckling behavior: the shield tunnel linings with flexible joints buckles in a single wave mode in the vicinity of K joint, while those with rigid joints buckles in a multi-wave mode around the linings. Hydrostatic buckling strength is found to increase with the flexural rigidity of the radial joint and the thickness of segment increasing. This study shows that ground support increases the buckling strength dramatically, while earth pressure reduces the capacity to resist hydrostatic buckling. The tunnel linings during construction are found to be easier to buckle than that during operation. Meanwhile, the buckling of tunnel linings is studied by theoretical analysis of buried tube buckling.     

Correlation of tunnel convergence with TBM operational parameters and chip size in the Ghomroud tunnel, Iran

Evaluating the impact of rock mass properties on a tunneling operation is crucial, especially when using a tunnel boring machine (TBM). It is an integral part of machine selection and performance prediction in the design and bidding stage. Monitoring and analysis of ground conditions during the construction is also essential to allow the operator to take precautionary measures in adverse geological conditions. This involves adjusting TBM operational parameters such as machine thrust and penetration to avoid potential problems caused by face collapse or excessive convergence and subsequent machine seizure that can cause long delays. Tunnel wall convergence is a function of rock mass characteristics, in situ stresses, size of excavation, and rate of penetration (ROP). It is one of the main factors in determining the use of shielded machines in deep rock tunnel projects. The case study of the Ghomroud water conveyance tunnel project, under construction by a double shield TBM, is used to examine the effect of rock mass parameters on tunnel convergence and hence on the need for over excavation and shield lubrication to avoid problems such as shield seizure. Results of a preliminary analysis of field observations show that the amount of the tunnel convergence can have a direct relationship with the percentage of powder and large rock fragments in the muck. In addition, tunnel convergence has shown a strong relationship with the TBM thrust/torque and rate of penetration (ROP). These relationships have been examined and the results of the analysis as well as the resulting formulas will be explained in this paper.     

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.     

考虑盾构隧道埋深影响和岩土特性影响的地表变形计算

盾构施工引起地层变形的众多计算方法中,随机介质理论法和Peck法是我国应用较为广泛的两种实用方法,但这两种方法的计算参数均不太容易确定。根据46例工程实测资料,绘制出地表最大沉降与隧道相对埋深的关系图。结果表明:当盾构隧道相对埋深小于5时,盾构施工引起的地表最大沉降值变化较大;当盾构隧道相对埋深大于5时,其对地表最大沉降的影响较小。对于大部分浅埋城市地铁隧道而言,应该考虑盾构隧道相对埋深对地层变形的影响。基于盾构施工引起地层移动不均匀模型的地表最大沉降计算式,依据随机介质理论法和Peck法,推导出考虑土质软硬、隧道半径和埋深影响的地层变形实用计算方法,并通过对5个工程实例的分析,验证此计算方法的合理性。     

盾构隧道施工过程建模影响因素分析

为研究盾构隧道施工过程建模影响因素及其影响程度,首先,综合隧道开挖过程中盾构机前体与岩土体影响因素耦合作用分析,构建盾构隧道开挖过程仿真模型,给出地表沉降和隧道垂向应力;然后,进一步模拟去除其中一种因素的隧道开挖过程,求出相应的地表沉降和隧道垂向应力;最后,基于傅里叶变换对各种情况下地表沉降量和应力应变状况进行比较分析,找出各因素对建模的影响程度。结果表明:一方面,建模过程中各种因素对地表沉降的影响大于对隧道垂向压力影响;另一方面,盾构机推进给开挖面土体压力、盾尾灌浆延迟管片拼装造成的暂时性土体支撑力不足、盾构机刀盘转动给开挖面土体扭力等因素对模型解算造成的影响最大。本研究对提高盾构隧道施工过程建模的准确性和实效性,指导盾构隧道施工具有重要指导作用。