Designing the main drive motor capacity of Earth Pressure Balanced Tunnel Boring Machines (EPB TBMs) is a crucial task for every EPB tunnelling project. The machine needs to be equipped with sufficient power to master the geotechnical conditions of the respective project. On the other hand, overpowering the machine should be avoided for economic and sustainability reasons. Main drive torque estimation for EPB TBMs is challenging due to a multitude of impact factors and reciprocal mechanisms between the geotechnical conditions and the tunnelling process. In EPB TBM tunnelling active tunnel face support is achieved in soft and mixed ground or weak and unstable rock by generating a pressurized earth paste in the tool gap and excavation chamber of the machine. Complexity arises due to tribological and rheological effects of the active tunnel face support. These elements of uncertainty, the expected main drive torque is frequently overestimated to prevent a jamming of the machine in the ground. Mean main drive torque values often lie below 50 % of the installed nominal main drive torque capacity. In scope of this research machine learning algorithms, such as regressions, decision trees, tree ensembles, support vector machines and gaussian process regressions, have been used to predict the main drive torque. Models have been trained and tested on data collected from 9 different reference projects and validated on the data of 3 additional reference projects to test the transferability of the model. TBM diameters of the reference projects vary between 6,5 and 15,9 m and TBMs have been operating in a wide range of geotechnical boundary conditions. Different feature selection algorithms have been used and prediction results have been compared to models trained on manually selected features. Models using tree ensembles and manually selected features showed best prediction results and model performance. The machine learning approach returned a smaller and more accurate torque estimation range than traditional estimation approaches and prediction accuracy has been improved. Transparent and robust tree ensembles proofed to be suitable tools for TBM torque estimation.

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Distributed fiber optic sensors (DFOSs) possess the capability to measure strain and temperature variations over long distances, demonstrating outstanding potential for monitoring underground infrastructure. This study presents a state-of-the-art review of the DFOS applications for monitoring and assessing the deformation behavior of typical tunnel infrastructure, including bored tunnels, conventional tunnels, as well as immersed and cut-and-cover tunnels. DFOS systems based on Brillouin and Rayleigh scattering principles are both considered. When implementing DFOS monitoring, the fiber optic cable can be primarily installed along transverse and longitudinal directions to (1) measure distributed strains by continuously adhering the fiber to the structure's surface or embedding it in the lining, or (2) measure point displacements by spot-anchoring it on the lining surface. There are four critical aspects of DFOS monitoring, including proper selection of the sensing fiber, selection of the measuring principle for the specific application, design of an effective sensor layout, and establishment of robust field sensor instrumentation. These four issues are comprehensively discussed, and practical suggestions are provided for the implementation of DFOS in tunnel infrastructure monitoring.@en

The short-term deformation behavior of immersed tunnels due to daily or monthly temperature changes and tidal variations is often not monitored but forms important input for a structural health assessment of the tunnel. In this study, distributed optical fiber sensors (DOFSs) are used to monitor the short-term (daily and monthly) deformation behavior of an immersed tunnel. Joint opening and the relative settlement differences between tunnel elements are monitored simultaneously at subhour intervals. Measurements show that the variation in the joint opening is strongly correlated with temperature change, and the joint gap has a tendency to open at low temperatures and to close at increasing temperatures. Simultaneously, the entire immersed section behaves more like a rigid body and moves upwards and downwards periodically due to tidal fluctuations in the river, with an observed vertical movement of slightly less than one millimeter. The tide also causes local tilting of tunnel segments, and this tilting behavior differs between winter and summer, which implies that the (seasonal) temperature-induced joint deformations affect the robustness of the tunnel to tidal loads. A soil-tunnel structure interaction analysis reveals that the cyclic vertical movement of the tunnel is driven by retardation of the tidal wave in deeper soil layers, which can be captured by a coupled flow model. This study provides new insights into the short-term deformation behavior of immersed tunnels.

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Increasing frequency and intensity of extreme weather events in the Netherlands is raising attention on the unsaturated response of geo-infrastructures, promoting research projects to provide an overview of the impact of unsaturated conditions on the response of shallow soil layers and embankments, and to better address maintenance and mitigation measures. As part of this effort, we discuss the results of standard laboratory tests performed on initially unsaturated samples retrieved from the field and tested in natural conditions, as well as after controlled drying and wetting. The variation of the "undrained"(i.e. at constant water content) shear strength with the degree of saturation obtained from the laboratory tests aligns well with CPT measurements performed in the field. An elastic-plastic constitutive model with mixed isotropic-rotational hardening developed for saturated soft soils was extended to unsaturated conditions by following a robust approach previously developed for compacted clayey soils. Coupling between the mechanical and the hydraulic behaviour is provided by the water retention curve. The model nicely captures the response observed in the laboratory, until extreme dry conditions, which possibly alter the structure of the soil, the peak stress, and the brittleness after failure. The model is capable of reproducing the effects of the previous hydraulic history on the stress-strain behaviour observed from the laboratory tests over a wide range of degree of saturation.

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Monitoring the deformations of immersed tunnels is important during the entire tunnel service life to assess the structural integrity of the tunnel. Conventional joint deformation monitoring is based on manual levelling measurements and normally occurs only at multi-year intervals, which does not allow to capture short term deformation behavior. In this study a new joint monitoring system using distributed optical fiber sensors (DOFS) is developed. A special sensor layout is designed that allows simultaneous measurements of both horizontal joint opening and vertical uneven settlement of the immersion and dilation joints. For this sensor scheme the transfer relation from fiber strain to joint deformation is derived and verified by in-lab experiments. The sensor system proves to be able to detect sub-millimeter joint deformations, indicating a more than sufficient accuracy for structural monitoring of immersed tunnel joints. Subsequently, the First Heinenoordtunnel in the Netherlands is instrumented using this distributed optical fiber sensing system, in order to obtain additional data for both long-term and short-term assessment of its structural condition.

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Metro systems have been in use for over 150 years, and new metro lines are still being constructed, either as new metro systems or as expansions of existing metro networks. In many cities the metro system is an essential form of transport to keep the cities functioning. This overview compares the findings of various international studies on metro construction and operation, and the impact that metro systems have on cities. The uncertainties inherent in underground construction, with sometimes uncertain hydro-geological conditions and impacts from nearby existing construction projects, are often apparent during metro construction, and have been widely studied. Similarly, passenger comfort and safety during operation is a topic that has received widespread attention, with the main focus on fire safety, as fire poses the most dangerous risk during operation. More recently, passenger comfort related to indoor air quality and aerodynamic effects has received increased attention. The vulnerability of the running stock and the metro network is a significant factor when determining the safety and efficiency of the metro system. Metro efficiency and reliability have a major impact on the transport, economic, environmental and social aspects of cities. Even though they are designed as separated own-right-of-way transport systems, metro systems strongly influence urban development and drive spatial changes in land use. The combination of metro systems with other urban functions provides great potential for the development of urban underground space and the development of more resilient and efficient urban areas. This in turn has an impact on housing prices and produces wider economic benefits beyond the city. Metro systems have also been shown to affect travel behaviour and have a positive impact on public health and environmental quality, by reducing pollution and emissions, despite the large concentration of passengers present in the metro, which brings its own problems. After an overview of the leading and more recent research topics in these areas, the key research gaps are discussed and recommendations for future research are made.

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Daily and seasonal deformation behavior of immersed tunnels potentially impacts the structural integrity. In this study, distributed optical fiber sensors (DOFS) are used to instrument both dilation and immersion joints of the Heinenoordtunnel, an immersed tunnel in the Netherlands. This DOFS system proves capable of measuring joint opening and uneven settlement at half-hour intervals. The field monitoring shows the Heinenoordtunnel behaves more like a rigid body and exhibits a cyclic vertical movement under daily tide impacts over a period of 12 hours. Moreover, the joints show a cyclic seasonal opening which is negatively correlation with temperature variations, i.e. the tunnel joints are compressed when the outside temperature rises and vice versa. These monitoring results@en

Tunnelling in soft soil conditions, especially with a shallow overburden, faces the risk of face instability due to blowout. Although several blowout models have been proposed to estimate the blowout pressure, mostly based on limit analysis or limit equilibrium, there is a significant gap between the allowable blowout pressures predicted by these models and the values observed in case studies, laboratory experiments and numerical simulations. This paper proposes a compact blowout model, which is more compact compared to a model proposed by Balthaus (1991). This new blowout model is able to predict blowout pressures more closely to the value observed by centrifuge testing, reduced scale experiments and case studies, whilst staying conservative. Its application on the Hochiminh Metroline No. 1 project in this study resulted in a smaller support pressure in the boring stage to avoid the occurrence of a blowout.

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Seasonal joint deformations within an immersed tunnel are important indicators to assess structural behavior and therefore should be monitored in detail. In this study, distributed optical fiber sensors (DOFS) are applied to precisely measure the seasonal joint deformations in an immersed tunnel for the first time. Measurements over a one-year period specifically reveal the impact of seasonal temperature variations on the joint opening and uneven settlement deformation. Field monitoring shows that the variation in joint opening exhibits a cyclic behavior and is strongly correlated with temperature change. The immersion joints generally show a larger range of seasonal opening (with a maximum of about 6 mm) than dilation joints, but at several dilation joints significant opening also occurs. The uneven or differential settlement at most joints stays below 1 mm, except at a few joints where the range is above 1 mm, which are indications of underlying structural defects in the tunnel. The observed joint uneven settlement also shows a seasonal variation, but the correlation with temperature is weak. The impacts of seasonal deformation on the structural integrity and watertightness of the tunnel are assessed, and further suggestions on tunnel maintenance and inspection are made.

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A significant part of the energy consumed during the tunnelling process of Earth Pressure Balanced (EPB) Tunnel Boring Machines (TBMs) is related to the main drive, consisting of a set of motors driving the rotation of the cutting wheel. An energy efficient EPB design requires the optimization of the main drive to avoid over- or under powering of the machine. Key aspect is therefore a precise and reliable estimation of the expected cutting wheel torque. In this paper state-of-the-art torque estimation models are compared to supervised machine learning (ML) approaches, including classification and regression trees (CART), support vector machines (SVM), Gaussian process regression (GPR) and decision tree ensembles (DTE). Feature selection algorithms are compared to models using manually selected input features. ML models are evaluated using accuracy metrics, residual analyses, and model validation. Torque prediction for a real-world validation project shows that utilization rates can be increased distinctively due to the application of ML techniques. @en

In the urbanisation process of the past few decades, the use of urban underground space (UUS) has contributed to new town developments in many cities in the world. This offers an opportunity to map out and systematically describe the potential contributions of UUS for transport, land use, economic, environmental and social development in new towns for planners, designers and policy-makers to consider. This review synthesises key papers on UUS utilisation and new town development. It indicates that the requirements of intensive use of land resources, accessibility reasons, avoidance of urban problems experienced in old cities, cultural and modernity considerations, and the goal of building smart, low-carbon and sustainable cities drive UUS utilisation in new developments. Underground spaces have been developed in new towns in various forms, with subway, underground parking facilities and underground utility infrastructure being the more prevalent forms of UUS utilisation, and UUS utilisation in new towns varying in scale. UUS utilisation for new developments may confront many challenges related to various issues: relationships between new towns and old city centres; compact and high-density urban forms; transit accessibility; awareness of the significance of UUS; balance between market forces and government intervention; coordination of development; full utilisation of UUS resources; and decision-making support. Existing practices used in UUS development provide lessons from which to learn.

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Many cities in the world have developed metro systems. Metro systems affect urban development in many ways, such as enhancing labour force mobility, increasing urban productivity and promoting urban underground space (UUS) utilisation to accommodate urban functions. This paper explores the relationship between metro systems and urban development, with particular focus on the comprehensive impacts of metro development on the economic, environmental and social development of cities. The contribution of metro systems to urban development has been confirmed by numerous studies in many cities in the world. The positive capitalisation of metro systems is reflected in property values in areas surrounding metro systems, although the impacts may vary spatially, temporally and geographically. In addition, metro systems impact on the natural and built environments by reducing air pollution and greenhouse gas emissions, encouraging new development and urban renewal, sharping urban development and land use, facilitating commercial growth and residential development, promoting the utilisation of UUS, and increasing mixed land use and urban density. However, there are mixed effects, both positive and negative, of metro systems on equality of transit opportunity, accessibility and connectivity, public health, travel behaviour, personal identity, travel experience and safety. This study sheds light on the impacts of metro systems on urban development, and provides important information for urban and transport planners and policy-makers wishing to develop metro systems to support sustainable urban development.

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When process disturbances occur, workers on the Tunnel Boring Machine (TBM) will need to operate outside safe zones, reducing or eliminating the safety barrier ‘distance’ between them and potential sources of risk. Consequently, disturbances have a higher risk potential than regular TBM operations. By comparing the risks of registered process disturbances with regular TBM process, we try to predict accident scenarios. The exposure risk is defined by the exposure time and the injury severity. Exposure times have been determined from case histories, where on average 11% of the construction period is attributed to disturbances. The potential number of casualties, including less common incident scenarios, have been determined using an accident scenario building toolkit. We find that factors that contribute most to occupational risk reduction are the (correct) use of available risk prevention measures, correct design of safety barriers and making these barriers available to personnel, as well as detailed planning of procedures such that specific tasks are performed in a uniform and predetermined manner.@en

Distributed optical fiber sensors (DOFS) allow for distributed strain sensing and can be installed to function as extensometers for measuring point-displacements. This paper discusses the metrics of optimal sensing fiber selection for point-displacement measuring. Key metrics include the physical structure, mechanical parameters and light transmission coefficients. Calibration tests for verification of the optical fiber properties are designed and results of four fiber types are presented. Finally, creep and relaxation behavior of optical fibers is discussed based on manual tension test results, and a quantification model is proposed to assess the induced measurement error for sensing fiber. The maximum (absolute) measurement error for two common fiber types used in point displacement measurements is determined to be below 8%, and the study shows that pretensioning of the fiber helps to reduce such measurement errors.

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Determining the scope of (structural) renovation of aging tunnels is a complicated task and modular renovation is becoming increasingly necessary. The COB Tunnel Programme aims at developing the knowledge necessary to renovate these tunnels in a modular fashion, to reduce hindrance as much as possible. Part of this program is the development of a Structural Health Analysis (SHA) for collecting data on the condition of the civil structure of tunnels, which will give tunnel owners insight in the need for research on structural parts of the tunnel, refurbishment and the risks for the tunnel performance in the future. As a lot of knowledge will be gathered and generated, structurally securing of this knowledge is also necessary. To meet this need, the tunnel program also started development of a knowledge management system. This paper describes the aims of the knowledge management system in relation to the overall tunnel program, as well as the tools and methods devised to gather all necessary information for tunnels constructed several decades ago, where not all required information is readily available in digital form, or even in paper archives. @en

Artificial barriers are widely used to prevent leakages. However, due to construction errors during the wall installation, passages with small dimensions may occasionally penetrate through the barrier, undermining its tightness. A three-dimensional discretized algorithm (TDA) is proposed for quantitatively estimating the transient-state discharge rate through defective cutoff walls. By discretizing the wall into a three-dimensional refined mesh grid, the algorithm enables an examination of penetrating passages, an evaluation of defect dimensions, and an estimation of discharge rate through the penetrating passages. A rigorous realization-by-realization comparison between the TDA and the finite element method (FEM) was made, and it was found that the TDA results show strong correlations with the FEM results, but at a remarkably lower (1/10³-1/10⁴) computational cost. The TDA generally gives a discharge rate that is 0.1-1.0 times greater than its FEM counterpart, as the lengthened seepage distance due to random corrugations in the penetrating untreated zone cannot be replicated by the TDA.

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In recent years, numerous models have been developed to predict the torque requirements of Tunnel Boring Machines operating in soft ground. Friction between the TBM cutterhead and surrounding ground is a main contribution to the required torque. As a result, friction coefficients are a major input factor in recent torque estimation models. Conditioned soil samples have been tested in a direct shear box, to assess whether a common lab setup can be used to improve the determination of TBM-soil friction coefficients. Three different soils were each mixed with water, bentonite or foam and sheared against a TBM steel plate. Test results show unconditioned friction coefficients matching with those reported from literature. Compared to the unconditioned friction coefficients, all bentonite-conditioned specimens show reduced friction coefficients, while foam-conditioned sand specimens do not show any reduction in friction. Data from literature and the laboratory tests resulted in two regressions, for sand and clay respectively, relating steel-soil friction coefficients to relative density, grain size, liquid limit and steel roughness.@en