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.     

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.     

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.     

TBM tunnelling under adverse geological conditions: An overview

The adverse geological conditions frequently encountered during TBM tunnelling present great challenges, and may trigger potential hazards if no precaution and treatment measures are taken. Comprehensive studies on adverse conditions are essential and critical to successful TBM tunnelling. In this overview paper, attempts are made to define the adverse geological conditions for TBM tunnelling. A simple classification and the influencing factors related to the adverse geological conditions are presented for better understanding of the topic. The main problems involved and the corresponding mitigation measures for TBM tunnelling under adverse geological conditions are discussed. Finally, further research needs for better coping with these problems are emphasized.     

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.     

Modelling of the EPB TBM shield tunnelling advance as a tool for geological characterization

One of the main causes of problems during the construction of tunnels with tunnel boring machines (TBMs) is a lack of characterization of the soil. Both geological and hydrogeological characterizations are essential to avoid unexpected events. The advance of TBMs produces groundwater oscillations due to hydraulic and mechanical effects. The magnitude of these oscillations depends on the characteristics of the soil and on the “parameters” of the TBM (e.g., earth pressures, penetration). Given that the impact caused in the groundwater could be estimated numerically, this paper proposes to use hydrogeological models based on the parameters of the TBM to validate or improve the previous geological characterization. This procedure was tested by modelling the advance of a TBM-type earth pressure balance (EPB) at a real site. This study arose during the construction of the tunnel for the high speed train in Barcelona. The previous geological characterization revealed a vertical fault whose exact position was unknown. The advance of the EPB was modelled to validate the previous characterization and to locate the fault. The numerical model included a detailed geology and hydrogeology of the study site and the parameters of the EPB. Note that the parameters of the EPB used in the model were more related to the groundwater response. These were determined statistically from all of the measures taken by the machine. Given the results obtained, hydrogeological modelling of EPBs was revealed to be a useful tool to validate previous characterizations, both the geological and the hydrogeological, and to determine the position of some geological structures, such as faults.     

Soft ground tunnelling in Egypt: Geotechnical challenges and expectations

Several large projects of tunnels and underground structures were executed in Egypt during the past 15 years. It is expected that these specialized construction activities will continue at similar rate during the next ten years to complete the existing plans of new infrastructures. Furthermore, the growing solidarity to improve, or at least maintain, the quality of urban environment will require moving more services below ground surface in the near future. For instance, more active utilization of underground space is badly needed for solving traffic problems and for expanding the limited available space for car parking within the congested urban areas. The majority of recently completed underground developments in Egypt were built through water bearing soft ground. For underground works, these subsurface conditions are considered problematic. Design guidelines and construction specifications of most of these tunnels and underground structures also demand preserving the surrounding buildings and other structures. Hence, several new construction technologies had to be utilized, for the first time in Egypt, to deal with these difficult tasks and constrains. Extensive studies have been conducted before and during the implementation of these subsurface projects. The methodology of coupling in-situ monitoring programs with sophisticated numerical modelling proved to be the most realistic scientific approach to document the geotechnical performance of these projects. The compiled results from these studies have provided a reliable base for detailed quantitative assessment of various construction techniques under the existing subsurface conditions. In this contribution, some of the main findings of the studies performed on recent soft ground tunnelling projects in Egypt are presented. The discussion is focused on some of the challenges facing geotechnical engineers during the implementation of these specialized projects in Egypt. Recommendations for the needed improvements in the currently used design tools and construction techniques are also offered.     

Pressurised TBM tunnelling in mixed face conditions resulting from tropical weathering of igneous rock

Weathering is a process that turns rock into soil. Deep weathering is prevalent in tropical and sub-tropical areas. The resulting sub-surface conditions can be very onerous for tunnelling, with tunnel drives commonly encountering a significant proportion of mixed face conditions, comprising partly rock and partly soil. Problems that have been encountered have included: inability to maintain the face pressure, ground loss, sinkholes, slow rates of tunnelling, rapid tool wear, damage to tools, mixing arms and other parts of the TBM, very frequent and long interventions, clogging and blow-outs. The nature and extent of the problems on any particular tunnel have depended on the type and design of the TBM, the nature of the rock and the proportion of the tunnel in mixed ground. In Singapore this has resulted in a change from mainly EPB to mainly slurry tunnelling in weathered igneous rock; however, predominantly EPB TBMs have been used in weathered sedimentary rock. Information from EPB and slurry TBM drives is used to illustrate the issues involved.     

Three-dimensional numerical modelling for TBM tunnelling in consolidated clay

This paper presents a new field to analyze three-dimensional (3-D) coupled linear flow for Tunnel Boring Machine (TBM) tunnelling in saturated porous medium. This is important to control ground deformation and excess pore water pressure due to the process of shield tunnelling in three-dimension and time-dependent. A numerical model to simulate explicitly the behaviour of excess pore water pressure mobilization and its dissipation in time is presented. For the TBM tunnelling techniques, the positive pressure is applied to support the tunnel face and the grouting material is injected to decrease the deformation into the tail void gap behind the shield. Hence, this study is employed on 3-D model to investigate the impact of the most important parameters, which are slurry pressure and grouting pressure. The governing equations are derived in the light of the generalized Biot theory where displacement and excess pore pressure are the primary unknowns. The excavation stages during the advance of the machine in 3-D consolidation analysis is simulated. An isoparametric quadratic solid consolidation elastic soil model is used for this analysis. Results of this study indicate that a realistic modelling of soil behaviour, especially the distribution shape of the excess pore water pressure around the TBM tunnels during the construction stages and its dissipation during the consolidation time can be assessed. Thus, short-term as well as long-term effects of the TBM tunnelling are predicted. The practical importance of this analysis is the optimization of values and quantities of the slurry pressure and grouting pressure required for TBM technology. A design criterion based on this study can be suggested to tunnelling procedure in consolidated clay.     

A new model for TBM performance prediction in blocky rock conditions

The term “blocky rock conditions” is generally associated with face instabilities in blocky/jointed rock masses. These events are generally promoted by unfavorable rock mass structural conditions, in terms of joint frequency and orientation, and acting stresses. As a result, rock blocks are formed and then detach from the excavation face which becomes “blocky”, with a markedly irregular and uneven profile. This condition may have a paramount effect on TBM tunneling, leading to a high maintenance frequency and a low TBM advancement rate. Based on the TBM performance data recorded during excavation of tunnels in blocky rock conditions, a TBM performance prediction model has been developed. The model is based on the Field Penetration Index for blocky rock conditions, FPI_blocky, which was previously introduced to analyze the TBM performance in blocky grounds at the Lötschberg Base Tunnel. Through a multivariate regression analysis, a new expression has been introduced to predict the FPI_blocky based on the volumetric joint count (J_v) and the intact rock uniaxial compressive strength (UCS). An attempt has also been made to quantify the downtimes that may occur in blocky rock conditions and to estimate a reliable value of TBM daily advance.     

Mechanical performance of TBM cutterhead in mixed rock ground conditions

The mechanical performance of TBM cutterhead including thrust, torque, eccentric force and overturning moment was calculated and analysed in different mixed rock ground conditions. The calculation model was built by identifying the rock type under each cutter using a ray intersection algorithm and calculating the cutting forces of each cutter using the CSM model. The mixed rock ground conditions were simplified as rock type distribution and rock strength classification. In the present paper, the rock distributions of the Layer-Banded Rock (LBR) and Random-Distributed Rock (RDR) types were considered. The influences of rock strength (Uniaxial Compressive Strength: UCS, Brazilian Tensile Strength: BTS), rock locations and number of rock layers were studied. For verification, a boring experiment was designed and conducted using an experimental cutterhead with 14 disc cutters. The rock box was poured with concrete C20, C40 and C60 layer by layer to prepare the LBR condition. The average torque and thrust of the calculation model and experiment were in good agreement. Some conclusions were drawn from the study on the rock strength, rock locations, area percentages of different rock layers and number of rock layers. And hence, some suggestions were proposed to enhance the tunnelling efficiency and reduce damage to the cutterhead.     

Theoretical analysis and seismic investigation for TBM jamming in squeezing fissile slate

The S tunnel is a 4.2 km-long headrace tunnel. In the tunnelling project, the ground was assumed to be hard slate and suitable for TBM excavation based on the primary site investigation. However, TBM jamming frequently occurred with the increase of the tunnel cover, and the TBM excavation was cancelled. In order to investigate the TBM jamming, theoretical analyses and seismic investigations were conducted. It was found that analytical model proposed in this paper well explained the influence of the cover on the possibility of TBM jamming. It was also found that the depth of the loosened zone was expanded 6–8 m at the location where TBM jamming occurred.     

Three-dimensional numerical simulation of a mechanized twin tunnels in soft ground

The increase in transportation in large cities makes it necessary to construct of twin tunnels at shallow depths. Thus, the prediction of the influence of a new tunnel construction on an already existing one plays a key role in the optimal design and construction of close parallel shield tunnels in order to avoid any damage to the existing tunnel during and after excavation of the new tunnel.Most of the reported cases in the literature on parallel mechanized excavation of twin tunnels have focused on the effects of the ground condition, tunnel size, tunnel depth, surface loads, and relative position between the two tunnels on tunnel behaviour. The numerical investigation performed in this study, using the FLAC^3D finite difference element programme, has made it possible to include the influence of the construction process between the two tunnels. The structural forces induced in both tunnels and the development of the displacement field in the surrounding ground have been highlighted.The results of the numerical analysis have indicated a great impact of a new tunnel construction on an existing tunnel. The influence of the lagged distance between the two tunnels faces has also been highlighted. Generally, the simultaneous excavation of twin tunnels causes smaller structural forces and lining displacements than those induced in the case of twin tunnels excavated at a large lagged distance. However, the simultaneous excavation of twin tunnels could result in a higher settlement above the two tunnels.     

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.     

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.     

Multifactorial fuzzy approach to the penetrability classification of TBM in hard rock conditions

Rate of penetration of a tunnel boring machine in a hard rock environment is generally a key parameter which expresses the ease or difficulty with which the rock mass can be excavated. In this paper, the penetrability of TBM in hard rock conditions was investigated with the developed fuzzy classification system. TBM penetration rate and rock properties (such as Uniaxial Compressive Strength (UCS), Brazilian Tensile Strength (BTS), rock brittleness/toughness, Average Distance between Planes of Weakness (DPW) and orientation of discontinuities in rock mass) were evaluated by using the multifactorial fuzzy approach which is a special case of multiple objective multifactorial decision making for the penetrability classification of TBM in hard rock conditions. Using the decision function, the penetrating performance of TBM was classified into three categories; Good, Medium and Poor. Eventually, it is possible to evaluate the penetrability and determine the advance rate for new conditions by carrying out the proposed rock properties tests and using the developed fuzzy classification system.     

Effect of replacing disc cutters with chisel tools on performance of a TBM in difficult ground conditions

A summary of a research program covering a period of two years on the performance of a TBM in a very complex and difficult geology is presented in this study. The formations in the study area varied from alluvium, sludge, mudstone, shale and limestone to quartzite with strengths from soft to very hard. The dykes frequently intruded the sedimentary rocks resulting in different degrees of weathering and fracturing in the country rock causing tremendous delays in progress rate of the TBM. The disc cutters started cutting inefficiently in clayey medium strength ground with extreme water income, at where also excessive disc consumptions started due to insufficient friction between the disc cutters and very soft (sludgy) formation, and it was decided to replace all disc cutters with chisel tools (ripper, scraper). Before making this important decision that could affect totally the excavation efficiency and production rate, some theoretical estimations were performed using the Evans’ cutting theory after some modifications based on the previous experimental studies for relieved cutting mode and wear flat, front ridge and vee-bottom angles found in complex shapes of chisel tools to estimate deterministically the torque and thrust requirements of the TBM.Field measurements of the torque and thrust requirements of the TBM equipped with the chisel tools validated the theoretical considerations and the deterministic model used for predicting the performance. Statistical analysis indicated that the model could be used reliably for performance prediction. This study also gave a unique opportunity to compare the performance of disc cutters and chisel tools used on the same TBM at variety of grounds and to analyze the effect of replacing disc cutters with chisel tools on the performance of the TBM. The field measurements indicated that the chisel tools were superior to the disc cutters in especially soft to medium strength rocks.     

Assessment of ground conditions with respect to mechanised tunnelling for the construction of the extension of the Athens Metro to the city of Piraeus

This paper refers to the proposed western extension of the Athens Metro to Piraeus city. The railway consists of a 9.45 m diameter, 8.2 km long tunnel and seven stations. Tunnelling works are expected to be undertaken within a variety of lithological formations ranging from very strong alpine limestones to recent soft littoral deposits. The tunnel alignment was divided into 12 areas with respect to the geological and geotechnical conditions that may be encountered during construction. In many of these zones, the geotechnical conditions together with the presence of sensitive surface and/or subsurface structures led to the selection of a closed-face TBM (Earth Pressure Balance Machine or Slurry Tunnel Boring Machine) as the appropriate tunnelling technique. The Piraeus extension is a project where in places both types of commonly used closed-face tunnelling machines will encounter ‘text book’ application areas, but not always. The applicability of each type of TBM is discussed using the available data obtained from an extensive site investigation.      

Hard rock TBM tunneling in challenging ground: Developments and lessons learned from the field

TBM tunneling is an ever-increasing challenge for underground construction, and with each new tunnel bored there are unknown elements when boring through the earth. The most extensive Geotechnical Baseline Reports can miss fault lines, water inflows, squeezing ground, rock bursting, and other types of extreme conditions. This paper will draw on the considerable experience within Robbins to analyze successful methods of dealing with the most challenging conditions encountered, with a particular focus on fractured and faulted ground, mixed face tunneling, and tunneling in karst or water-bearing conditions. It will discuss new methods, including Dual Mode or “Crossover” type machines, which can increase the efficiency of excavation in such conditions.     

Semi-analytical model for umbrella arch systems employed in squeezing ground conditions

Methods of approximation that predict the mechanical responses of the tunnel support systems in conjunction with ground behaviour are invaluable to the tunnel design engineer. Analytical models are often used in order to predict and/or validate ground-support behaviour. Conventionally, these analytical models do not account for the complex loading and reacting conditions of umbrella arch support systems throughout the tunnel excavation and support sequence. As such, a semi-analytical model is proposed within this paper for umbrella-arch systems that employ an umbrella ache with forepoles, in squeezing-ground conditions. The semi-analytical model is based on an assortment of applicable methods and theories depending on the relevant loading. Beam theory, elastic foundation theory, and the Convergence-Confinement Method (CCM) are all incorporated within the proposed analytical method. After a review of the literature it became apparent that a limited amount of models existed for squeezing-ground conditions. Previous models were based on gravity-driven (Silo Theory) loading conditions rather than the more applicable stress-driven (squeezing) loading conditions. The results of the semi-analytical approach included herein were able to reasonably capture the displacement profiles associated with captured field data. This semi-analytical approach can be considered for use by tunnel design engineers in order to aid them with tunnel support design.