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.     

高地应力作用下大理岩岩体的TBM掘进试验研究

滚刀破岩效率的研究主要集中在室内线性试验机破岩试验和数值分析2个方面,在工地开展TBM掘进试验尚不普遍。锦屏二级水电站采用3台TBM开挖隧道群,3台TBM在不同洞深(不同地应力)条件对大理岩岩体进行TBM掘进试验、岩石渣片筛分试验及大渣片统计分析,研究岩体条件、TBM机器参数、TBM运行参数对TBM掘进速度的影响及高地应力作用下岩体可掘性指数的变化。研究结果表明:在高地应力条件下,尽管TBM掘进速度随推力增加而增大,但推力超过一定值后,TBM并不在优化状态下运行,TBM的运行需与岩体条件及地应力条件相匹配。     

Hard rock tunnel boring machine penetration test as an indicator of chipping process efficiency

The transition from grinding to chipping can be observed in tunnel boring machine(TBM) penetration test data by plotting the penetration rate(distance/revolution) against the net cutter thrust(force per cutter) over the full range of penetration rates in the test.Correlating penetration test data to the geological and geomechanical characteristics of rock masses through which a penetration test is conducted provides the ability to reveal the efficiency of the chipping process in response to changing geological conditions.Penetration test data can also be used to identify stress-induced tunnel face instability.This research shows that the strength of the rock is an important parameter for controlling how much net cutter thrust is required to transition from grinding to chipping.It also shows that the geological characteristics of a rock will determine how efficient chipping occurs once it has begun.In particular,geological characteristics that lead to efficient fracture propagation,such as fabric and mica contents,will lead to efficient chipping.These findings will enable a better correlation between TBM performance and geological conditions for use in TBM design,as a basis for contractual payments where penetration rate dominates the excavation cycle and in further academic investigations into the TBM excavation process.     

双护盾TBM施工隧洞围岩分类方法及应用

针对引水隧洞围岩,在现有规范关于隧洞围岩分类基础上,提出了围岩分类新方法,综合考虑TBM掘进机掘进参数变化、围岩与护盾(管片)之间的间隙、出渣量、弃渣的块径、渣块形状、百分量、渣中刀具破岩新鲜面与围岩中结构面的比例、渣中软硬岩比例等因素,应用于达坂引水隧洞的围岩分类,所提出的方法具有重要的工程应用价值。     

Tunneling in fault zones, Tuzla tunnel, Turkey

The Tuzla tunnel was excavated mainly in fault zones, shale and limestones using the conventional and shielded tunnel boring machine (TBM) methods. Fault zones in shales are brecciated and clayey, while those in limestones are of blocky structure. The rock mass rating, rock mass classification and support systems proposed for fault zones in Tuzla tunnel are insufficient for explaining the deformation and failure mechanisms encountered in the tunnel. In addition, dyke exposures, the fault-collapsed karstic system and groundwater also caused some problems during the excavation of the tunnel. The most important event relevant to fault zones in the Tuzla tunnel was the selection of a TBM. Before the excavation of the tunnel, the rock was determined to be of poor to fair quality. Therefore, tunneling with a TBM in rock of poor to fair quality was thought to be economic. However, during the excavation, fault zones with poor to very poor rock characteristics were encountered along an area comprising 70% of the tunnel length. The fault zones caused jamming of the TBM cutter and deviation from the tunnel alignment. In this respect, tunneling with the TBM method was quite problematic. Geotechnical problems encountered in the fault zones required special measures to be taken in the tunnel. With these measures, excavation and supporting of the tunnel were completed successfully by transforming heterogeneous conditions in the fault zones to homogeneous conditions in the tunnel impact area.     

In situ TBM penetration tests and rock mass boreability analysis in hard rock tunnels

Boreability is popularly adopted to express the ease or difficulty with which a rock mass can be penetrated by a tunnel boring machine. Because the boreability is related to the rock mass properties, TBM specifications and TBM operation parameters, an accurately definable quantity has not been obtained so far. In order to analyze and compare rock mass boreability, a series of TBM shield friction tests were conducted in a TBM tunneling site. Two sets of TBM penetration tests were performed in different rock mass conditions during tunneling in rock. In each step of the penetration test, the rock muck was collected to perform the muck sieve analyses and the shape of large chips was surveyed in order to analyze the TBM chipping efficiency under different cutter thrusts. The results showed that a critical point exists in the penetration curves. The penetration per revolution increases rapidly with increasing thrust per cutter when it is higher than the critical value. The muck sieve analysis results verified that with increasing thrust force, the muck size increases and the rock breakage efficiency also increases. When the thrust is greater than the critical value, the muck becomes well-graded. The muck shape analysis results also showed with the increase of the thrust, the chip shape changes from flat to elongated and flat. The boreability index at the critical point of penetration of 1 mm/rev. defined as the specific rock mass boreability index is proposed to evaluate rock mass boreability.     

深部高应力隧道TBM护盾长度和推力计算方法研究

岩石隧道掘进机(TBM)法开挖长隧道是一种安全、快速、有效的隧道开挖方法,但TBM复杂高应力隧道掘进时易发生卡机事故,因此,TBM在设计之初应尽可能考虑地质环境的影响,降低TBM的卡机风险。通过分析高应力常规地层和高应力软弱破碎地层对TBM的影响,提出了高应力常规地层和高应力软弱破碎地层TBM卡机的两个判据。根据两个判据提出了考虑围岩力学参数的高应力隧道TBM护盾长度设计和推力设计理论计算方法,并给出了参数选取依据。最后依据西南地区某高应力隧道的实际围岩地质参数,计算分析了现有TBM设计的合理性。本研究可为TBM的盾体长度和推力设计计算提供围岩力学参数依据。     

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.     

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.     

Hybrid ensemble soft computing approach for predicting penetration rate of tunnel boring machine in a rock environment

This study implements a hybrid ensemble machine learning method for forecasting the rate of penetration (ROP) of tunnel boring machine (TBM), which is becoming a prerequisite for reliable cost assessment and project scheduling in tunnelling and underground projects in a rock environment. For this purpose, a sum of 185 datasets was collected from the literature and used to predict the ROP of TBM. Initially, the main dataset was utilised to construct and validate four conventional soft computing (CSC) models, i.e. minimax probability machine regression, relevance vector machine, extreme learning machine, and functional network. Consequently, the estimated outputs of CSC models were united and trained using an artificial neural network (ANN) to construct a hybrid ensemble model (HENSM). The outcomes of the proposed HENSM are superior to other CSC models employed in this study. Based on the experimental results (training RMSE = 0.0283 and testing RMSE = 0.0418), the newly proposed HENSM is potential to assist engineers in predicting ROP of TBM in the design phase of tunnelling and underground projects.     

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.     

双护盾TBM施工隧洞岩爆特征微震监测研究

为了研究深埋隧洞在双护盾TBM施工环境下的岩爆特征,以某公路隧道为例对微震监测成果进行了统计整理,并在此基础上分析了微震事件与掌子面位置、岩体完整性以及隧洞埋深等之间的相关关系,获取了该工程岩爆事件发生的规律。成果表明,在隧洞采用双护盾TBM开挖过程中岩爆事件并非主要集中在掌子面附近,岩爆存在一定的延迟性和超前性,岩爆发生与开挖过程的时间效应及距离范围存在一定规律。同时,岩爆的能量等级和发生频次与岩体结构及隧道埋深也存在关联性。深埋隧洞在双护盾TBM施工环境下的岩爆规律的研究对类似工程具有一定的借鉴意义。     

双护盾TBM开挖膨胀岩洞段施工应用技术研究

某膨胀岩洞段,围岩强度低,抗风化能力弱,水理性质不良,易发生吸水膨胀失水收缩现象。基岩洞段褶皱、褶曲发育,地层产状变化大,围岩呈层状～碎裂结构。双护盾TBM开挖后,隧洞发生不同程度的坍塌与塌方。现采取“三低一连续”（低推力、低转速、低贯人度;快速连续掘进）快速封闭围岩,严格控制施工用水;使用化学材料灌浆固结围岩等施工应用技术,成功通过了泥质软岩类隧洞长地段,为今后双护盾掘进机施工提供了宝贵经验,社会和经济效益巨大。     

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.     

Causes of reduction in shield TBM performance - A case study in Seoul

This study investigates the fundamental reasons of reduction for excavation performance on shield TBM in the Han River riverbed tunnel by means of analysis of field measurement data. For the study: (1) measurement data were analyzed by both the thrust and penetration rate data in a given section with rock types and direction types, (2) a number of disk cutter consumption and types of abrasion were analyzed, (3) an exhaust ratio of screw conveyor and design problem of screw conveyor were analyzed by earth mud which becomes the rock powder. As a result of this study, it was found that the lack of penetration depth caused by lower thrust, disk changes according to rapidly changing rock types, and installed angle of screw conveyor, play an important role in the excavation performance.     

高强度基岩爆破预处理泥水盾构掘进特征研究

泥水盾构在高强度基岩地层中掘进,盾构自身刀具无法有效破岩,采用基岩爆破预处理的控制钻爆法能严格控制破碎碴块体量,提高刀具破岩能力,但盾构施工过程中,掘进参数与岩石之间关系非常复杂。以台山核电站取水隧道为背景,对现场获得的工作数据进行挖掘,分析基岩爆破前后地层中盾构刀盘推力、刀盘扭矩及掘进速度的参数的选用特点。在此基础上,引入参数转换量FPI、TPI指数及比能进行研究,研究表明,未经处理基岩地层,岩石强度过高,刀盘推力先达到最大值,但切深很浅,扭矩很难发挥最大能力;经爆破处理后,基岩岩体受到不同程度破碎,盾构机推力及扭矩能有效发挥其功能,掘进效能提高。因此,在盾构机的工作负荷范围内,应依据基岩岩体破碎效果,调整刀盘推力和扭矩,使之更好地适应地层,力争达到高效安全的掘进目的。     

Excavation of underground research laboratory ramp in granite using tunnel boring machine: Feasibility study

Underground research laboratory (URL) plays an important role in safe disposal of high-level radioactive waste (HLW). At present, the Xinchang site, located in Gansu Province of China, has been selected as the final site for China’s first URL, named Beishan URL. For this, a preliminary design of the Beishan URL has been proposed, including one spiral ramp, three shafts and two experimental levels. With advantages of fast advancing and limited disturbance to surrounding rock mass, the tunnel boring machine (TBM) method could be one of the excavation methods considered for the URL ramp. This paper introduces the feasibility study on using TBM to excavation of the Beishan URL ramp. The technical challenges for using TBM in Beishan URL are identified on the base of geological condition and specific layout of the spiral ramp. Then, the technical feasibility study on the specific issues, i.e. extremely hard rock mass, high abrasiveness, TBM operation, muck transportation, water drainage and material transportation, is investigated. This study demonstrates that TBM technology is a feasible method for the Beishan URL excavation. The results can also provide a reference for the design and construction of HLW disposal engineering in similar geological conditions. © 2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).     

Geomechanical model test for analysis of surrounding rock behaviours in composite strata

Due to the large differences in physico-mechanical pro perties of composite strata,jamming,head sinking and other serious consequences occur frequently during tunnel boring machine(TBM) excavation.To analyse the stability of surrounding rocks in composite strata under the disturbance of TBM excavation,a geomechanical model test was carried out based on the Lanzhou water supply project.The evolution patterns and distribution characteristics of the strain,stress,and tunnel deformation and fracturing were analysed.The results showed that during TBM excavation in the horizontal composite formations(with upper soft and lower hard layers and with upper hard and lower soft layers),a significant difference in response to the surrounding rocks can be observed.As the strength ratio of the surrounding rocks decreases,the ratio of the maximum strain of the hard rock mass to that of the relatively soft rock mass gradually decreases.The radial stress of the relatively soft rock mass is smaller than that of the hard rock mass in both types of composite strata,indicating that the weak rock mass in the composite formation results in the difference in the mechanical behaviours of the surrounding rocks.The displacement field of the surrounding rocks obtained by the digital speckle correlation method(DSCM) and the macro-fracture morphology after tunnel excavation visually reflected the deformation difference of the composite rock mass.Finally,some suggestions and measures were provided for TBM excavation in composite strata,such as advance geological forecasting and effective monitoring of weak rock masses.     

Predicting penetration rate of hard rock tunnel boring machine using fuzzy logic

Predicting the penetration rate of a tunnel boring machine (TBM) plays an important role in the economic and time planning of tunneling projects. In the past years, various empirical methods have been developed for the prediction of TBM penetration rates using traditional statistical analysis techniques. Soft computing techniques are now being used as an alternative statistical tool. In this study, a fuzzy logic model was developed to predict the penetration rate based on collected data from one hard rock TBM tunnel (the Queens Water Tunnel # 3, Stage 2) in New York City, USA. The model predicts the penetration rate of the TBM using rock properties such as uniaxial compressive strength, rock brittleness, distance between planes of weakness and the orientation of discontinuities in the rock mass. The results indicated that the fuzzy model can be used as a reliable predictor of TBM penetration rate for the studied tunneling project. The determination coefficient (R ²), the variance account for and the root mean square error indices of the proposed fuzzy model are 0.8930, 89.06 and 0.13, respectively.     

An application of three-dimensional analysis around a tunnel portal under construction

Deformation of a tunnel portal during tunnel excavation is very complex, and it can affect adjacent portal structures. During the construction of the Seoul Metro Line 7, a temporary vertical shaft was constructed by cut and cover method to take out a shield machine and to excavate subsequent tunnel by New Austrian Tunneling Method. Due to the complex structure of tunnel portals, the temporary support system of the vertical shaft was expected to be subjected to an additional load during the tunnel excavation. Thus, three-dimensional analysis of the vertical shaft, including tunnel excavation process, was performed in order to ensure the safety of overall system. This paper presents behavior of the vertical shaft based on the three-dimensional numerical analysis results during the construction sequence.