Tunnel Portal Research Articles

Numerical simulations of the separated flow around a freight train passing through a tunnel using the sliding mesh technique

The main aim of this investigation is to analyse the flow around a freight train as it passes through a tunnel. The separated flow around the train nose is related to energy losses, lateral vibration, noise and streamline deviation, and it also influences the velocity magnitudes around the train. Such effects are expected to become more important with the prospect of increasing freight train speeds. The numerical simulations performed in this study use a Class 66 locomotive connected to eight container wagons, scaled to 1/25th, moving at a train speed of 33.5 m/s through a tunnel with a blockage ratio of 0.202. The k–ω model combined with a high advection scheme solves the governing equations on a structured hexahedral mesh using the sliding mesh technique. The pressure histories at the tunnel walls and train surface as well as the velocity field around the train were validated with experimental data obtained using a moving model. The longest separation bubble is found at the middle-height and middle-width of the locomotive due to extended corners at these regions. When the train enters the tunnel, the separation length is reduced by 32% at the roof and 31% at the sides, compared to open air. The maximum separation length is found at the sides of the train where it reattaches at 19% of the locomotive length, influencing the velocity peak at a short distance from the train surface. The larger the separation length, the higher the length/duration of this peak. When the train head is halfway through the tunnel, the nose velocity peak reduces by 30% compared to open air. The position of the nose inside the tunnel affects not only the slipstream velocity but also the velocity field at the tunnel portal and exit. These novel findings can be used as a benchmark for designing new freight train and tunnel shapes.

Influence of ballast quantity on compression wavefront steepening in railway tunnels

The impulsive pressure wave (micro-pressure wave) emitted from a tunnel portal is one of the environmental problems associated with high-speed railways. The principle underlying the countermeasures for this micro-pressure wave is to reduce the pressure gradient of the compression wavefront at the tunnel portal. This study investigated the influence of the ballast layer on the rate at which compression waves steepen in railway tunnels. Field measurements in an actual Shinkansen tunnel with slab tracks were recorded before and after installing ballast along most of the length of the tunnel. Previously published results on the spread of the compression wavefront in a ballasted track tunnel are also presented for comparison. A rapid method using an empirical equation is shown to accurately estimate the variation in the maximum pressure gradient of the compression wavefront in certain conditions. A more widely applicable estimation method using a numerical analysis is also demonstrated, which describes the distortion of the compression waveform. The results show that both of these methods can estimate the influence of the ballast quantity on mitigating the compression wavefront steepening in Shinkansen tunnels.

Seismic responses and damage mechanisms of the structure in the portal section of a hydraulic tunnel in rock

Tunnel portals, being shallow underground structures, are well known to be more vulnerable to damage during earthquakes because they are generally located in areas of poor geological conditions that likely lead to amplified ground motions; this is especially true for hydraulic tunnel portals, which are additionally subjected to considerable hydrodynamic pressure. In this work, a full three-dimensional (3D) seismic analysis of the portal structure of a hydraulic tunnel is presented. Additionally, the hydrodynamic pressure and the rock-structure interaction (RSI) are both considered in this analysis to approximate actual field conditions, and their impacts on the seismic response and the damage characteristics of the tunnel portal are studied in detail. The numerical results indicate that the hydrodynamic pressure and the RSI contribute to a greater seismic response of the tunnel structure, with the peak displacements likely increasing by 48% when the hydrodynamic pressure is considered. The tunnel sections as far as 40 m from the entrance may suffer severe damage, and the constraint effects of the surrounding rock can efficiently reduce the damage of the tunnel structure. Two typical damage patterns are deduced from the simulation results and can be reasonably interpreted by the corresponding damage mechanisms.

Effects of In-Vehicle Navigation on Perceptual Responses and Driving Behaviours of Drivers at Tunnel Entrances: A Naturalistic Driving Study

The perceptual responses and driving behaviours of drivers at tunnel entrances vary, which could cause interference and accidents. This study investigated the effects of in-vehicle navigation on the perceptual responses and driving behaviours and whether these effects are actually valid for safety improvement. For this purpose, a series of naturalistic driving experiments was conducted and a comparative analysis was performed considering two different experiment conditions, control condition and in-vehicle navigation condition. Under each condition, the performances of twenty drivers at seven tunnels were evaluated. The area from 200 m outside the tunnel portal to 200 m inside the tunnel portal was averagely divided into four zones. In each zone, two types of perceptual responses (visual responses and psychological responses) and driving behaviours were analysed using six indicators: number of fixations, average duration of fixations, time interval between continuous R-waves, skin conductance response, speed difference in zones, and maximum deceleration. The results showed that in-vehicle navigation significantly affects the perceptual responses and driving behaviours of drivers, and these effects varied in different zones of the tunnel entrance. Furthermore, in-vehicle navigation was found to be valid for safety improvement because beneficial changes in four of the six indicators proved to be effective at appropriate zones. The remaining two indicators, average duration of fixations and maximum deceleration, were not valid, implying that the difficulty of driving information cognition and driving comfort could not be improved by in-vehicle navigation. Moreover, a negative correlation was discovered between the number of fixations and speed difference in zones. This study provides engineers a new knowledge by extending the quantifiable approaches to the analyses of the effectiveness of the effects of in-vehicle navigation.

Numerical modeling of umbrella arch technique to reduce tunnelling induced ground movements

One of the most important aspects of tunnelling in soft grounds is to reduce any possible adverse environmental impact. That is, pre-supporting techniques are found to boost tunnelling advance speed whilst, at the same time, keeping the environmental consequences due to tunnelling process appreciably low. The umbrella arch method is one of the most common ground pre-supporting techniques used in tunnelling. The interactions between the ground and umbrella arches are very complex. There are also several parameters affecting the performance of the umbrella arch technique. Sophisticated numerical modeling is, therefore, required to gain a proper understanding of the mechanism of ground movement based on the umbrella arch method. In this paper, the role of the pipe roofing and face bolting on the stabilization of the northern portal of Sabzkooh tunnel is assessed using three-dimensional (3D) numerical method. Furthermore, the effects of design parameters such as the pipes installation angle, the pipes diameter and the transverse spacing of pipes on the performance of umbrella arch system are investigated through numerical modeling. The vertical displacements of the tunnel crown and the internal forces induced in the initial support system are extracted from the numerical modeling and are carefully assessed to identify the role of the pre-supporting methods. The outcomes of the numerical modeling demonstrate that when pipe roofing and face bolts are utilized, the tunnel convergence decreases and the induced axial forces in the initial support system tend to increase. These clearly highlight the role of pre-supporting technique on stabilizing the tunnel and transferring ground loads to the support measure. Results also show that reduction in the installation angle and transverse spacing of pipes and increasing the diameter of pipes can all considerably reduce the tunnel crown displacement while they increase the maximum induced axial forces in the initial support system. Stability analysis using bearing capacity diagrams also indicates that the initial support system with different configurations of pipe roofing and face bolts at different cross sections along the tunnel alignment is fairly sturdy and robust against the applied loads.

Evaluation of the NO<SUB align="right">2 burden within a joint street canyon and tunnel portal micro environment

Substantial breaches in NO2 limits in the vicinity of a tunnel portal and street canyon micro environment provided the motivation for this paper. The impact of portal and surface road emissions was assessed and mitigation scenarios computed, in order to aid the authorities responsible. Emphasis was placed on detailed emission processing. A unique data set from nine NO2 monitors located near the portal and within the street canyon was used to evaluate the model approach using a Lagrangian particle model. Good agreement with measurements was obtained using simple NOx/NO2 conversion approaches. The NOx/NO2 concentration pattern is dominated by surface road emissions. A kerbside area of ~30 m in the driving direction is clearly dominated by the portal emissions indicating effective diffusion at the portal. Major uncertainties are related to the interaction of flow and turbulent mixing from ambient atmospheric conditions and traffic, particularly at low wind speed conditions.

Influence of Water Inrush from Excavation Surface on the Stress and Deformation of Tunnel‐Forming Structure at the Launching‐Arrival Stage of Subway Shield

The launching‐arrival stage of the shield is the most dangerous construction stage in subway construction. During the conversion process of the soil and air medium in the shield machine, water inrush at the excavation surface often occurs because of the effect of groundwater. Previous research has focused on the overall stress and deformation of existing tunnels caused by water inrush from the excavation face of the shield machine excavation stage. However, the stress and deformation states of the segments and anchors at different assembly locations of the tunnel, as well as the interaction between the soil reinforcement region and the segments and anchors in the launching‐arrival stage have not been considered in previous studies. In this study, the inrush model of the launching‐arrival stage of the subway shield was established by utilizing the equivalent refinement modeling technology and ABAQUS simulation analysis with consideration of the fluid‐solid coupling effect of water and soil to study the influences of different water head differences on the mechanical and deformation properties of segments and anchors in shield construction under the conditions of water inrush on the excavation surface. The results showed that the water inflow from the tunnel excavation surface caused significant surface subsidence at the tunnel portal, vertical convergence at the cross section of the shield tunnel, and significant increases in the axial and shear forces on the bolt. In addition, based on the existing subway regulation, combined with the simulation results of soil reinforcement measures at different depths, the emergency control criterion for controlling water inrush on the excavation surface was established by using the depth of soil reinforcement. The minimum depth of the reinforced soil from the ground surface at 15 m is recommended to ensure construction safety of the subway shield at the launching‐arrival stage.

Evaluation of the NO<SUB align="right">2 burden within a joint street canyon and tunnel portal micro environment

Substantial breaches in NO2 limits in the vicinity of a tunnel portal and street canyon micro environment provided the motivation for this paper. The impact of portal and surface road emissions was assessed and mitigation scenarios computed, in order to aid the authorities responsible. Emphasis was placed on detailed emission processing. A unique data set from nine NO2 monitors located near the portal and within the street canyon was used to evaluate the model approach using a Lagrangian particle model. Good agreement with measurements was obtained using simple NOx/NO2 conversion approaches. The NOx/NO2 concentration pattern is dominated by surface road emissions. A kerbside area of ~30 m in the driving direction is clearly dominated by the portal emissions indicating effective diffusion at the portal. Major uncertainties are related to the interaction of flow and turbulent mixing from ambient atmospheric conditions and traffic, particularly at low wind speed conditions.

Optimizing cross-sectional area of tunnel entrance hood for high speed rail

A micro-pressure wave (MPW) is radiated from the tunnel exit when a train enters a tunnel at high speed. The MPW causes infrasound and audible noise near the tunnel portal; hence, it must be reduced for environmental reasons. A hood with openings is a typical countermeasure. The openings must be suitably distributed to effectively reduce the MPW. The optimum distribution depends on the shape of the train nose and its velocity; therefore, it needs to be readjusted, and the hood sometimes needs to be extended when the train velocity increases or a new type train is put into operation. This study reports on numerical studies and model experiments to investigate the effective composition of an unvented hood because such a hood would eliminate the need to adjust the distribution of the openings according to the train characteristics. The results show that the optimum cross-sectional area of the hood is 2.6 times the cross-sectional area of the tunnel, and its optimum length is 1.8 times the tunnel diameter when the train velocity is 350 km/h, and the train nose length is three to six times the train radius. Finally, the unvented hood is economically advantageous compared with the currently used hood.

Risk Assessment of Tunnel Portals in the Construction Stage Based on Fuzzy Analytic Hierarchy Process

AbstractThis study presents a fuzzy analytic hierarchy process (FAHP) by integrating analytic hierarchy process (AHP) and fuzzy cognition to evaluate the construction risks of tunnel portals. Wuguanyi Tunnel is taken as the research objective to validate the performance of the proposed method. The result shows that the proposed decision making method can effectively identify risk factors and determine the risk level during the construction of tunnel portals. Finally, the corresponding control measures during the construction of the Wuguanyi Tunnel portal are proposed according to the risk assessment results.

3D modelling of the ancient underground quarries of the famous Parian marble in the Aegean Sea, Greece and assessment of their stability using LiDAR scanning

Laser scanning has proven useful in the stability assessment of underground openings. High accuracy points are used to generate 3D surface models to evaluate their stability by assessing rockmass structural features. A series of scans obtained from underground ancient quarries in Paros Island were processed. The structural analysis of the openings was complemented by field observations and measurements, serving as input parameters for numerical software used to evaluate the potential failure mechanisms and overall stability. Blocks with a higher risk of detaching are encountered mostly at tunnel portals where support is required to secure unstable blocks to preserve this geotope.

Full-Scale Measurement of Micropressure Waves in High-Speed Railway Tunnels

We aim to perform a series of field measurements for high-speed railway tunnels in China to obtain the micropressure wave (MPW) at the tunnel exit and the transient pressures near the tunnel portals. The relationship between the MPW and the nose-entry wave and the effects of train speed and the tunnel exit hole on the MPW are analyzed. The results show that the MPW decreases with increasing distance from the tunnel exit, but increases rapidly with increasing train speeds. Additionally, holes in the hoods near the tunnel exit could decrease the MPW near the tunnel exit by 10–20%.

Risk Evaluation Model of Highway Tunnel Portal Construction Based on BP Fuzzy Neural Network.

Risk assessment for tunnel portals in the construction stage has been widely recognized as one of the most critical phases in tunnel construction as it easily causes accident than the overall length of a tunnel. However, the risk in tunnel portal construction is complicated and uncertain which has made such a neural network very attractive to the construction projects. This paper presents a risk evaluation model, which is obtained from historical data of 50 tunnels, by combining the fuzzy method and BP neural network. The proposed model is used for the risk assessment of the Tiefodian tunnel. The results show that the risk evaluation level is IV, slope instability is the greatest impact index among four risk events, and the major risk factors are confirmed. According to the evaluation results, corresponding risk control measures are suggested and implemented. Finally, numerical simulation is carried out before and after the implementation of risk measures, respectively. The rationality of the proposed risk evaluation model is proved by comparing the numerical simulation results.

Slope stability analysis and protection measures in bridge and tunnel engineering: a practical case study from Southwestern China

This study addresses the instability problem of typical slopes in the engineering constructions of bridges and tunnels in Southwestern China by combining the results of detailed field monitoring, geological and geometrical investigation, and numerical simulations and then proposes comprehensive protection measures. The study area is a vital part of the Chengdu–Lanzhou high-speed railway in Sichuan Province, located in the 5·12 Wenchuan’s meizoseismic area. The slope stability was analysed based on the data collected by field monitoring. The results indicated that the slope toe was stable under a natural state, while a large deformation was observed occasionally owing to tunnel blasting and excavation. Subsequently, a digital binocular camera installed on a fixed-wing unmanned aerial vehicle was used to identify unstable rockmass. By analysing the slope and the dip direction angles, three potential paths of rockfall hazard were selected for further studies. A retaining wall and a soft protecting net were installed along the slope to intercept potential debris flow and possible falling rocks above the tunnel portal, respectively. A Mohr–Coulomb model was built using universal distinct element code to identify the rockfall initiation position. Overall, the field monitoring results were in good agreement with those obtained by numerical simulations. Moreover, the Rocscience Rocfall program was used to calculate the bounce height and the total kinetic energy of falling rocks. Finally, mitigation measures for the potential landslide, rockfall, and mudslide hazard were proposed and adopted in practical engineering applications, and aimed to provide a useful reference for similar engineering practices.

Numerical Research on the Mixture Mechanism of Polluted and Fresh Air at the Staggered Tunnel Portals

In longitudinal ventilation, circulating air is formed in portals for closely spaced twin tunnels, which causes mixing between the polluted air exhausted from one tunnel and the fresh air flow of another tunnel, thus leading to the rising costs of ventilation system construction and operation. In this study, for the closely spaced tunnel with staggered inlet and outlet, the computational fluid dynamics (CFD) numerical simulation method was adopted to reveal flow characteristics of the circulating air as well as variation rules of the circulating air mixing ratio φc with tunnel structure and operation parameters. Results show that both reducing inlet air velocity and increasing outlet air velocity and lateral distance can reduce the impact of the negative-pressure zone at the tunnel entrance on the jet flow structure at the tunnel exit, thus weakening the circulating air. When the inlet is placed behind or aligned with the outlet (staggered distance ∆l ≤ 0), φc will increase linearly along with the increase of staggered distance; when the inlet is placed before the outlet (∆l > 0), φc will first increase and then decrease with the increase of staggered distance. An expression to predict circulating air mixing ratio was created by sections. The predictions show a good correlation with the measurements and indicate that the front slope gradient of the tunnel portal is also one of the factors affecting the circulating air mixing ratio.

Study on Cause Analysis and Treatment Measures of Lining Cracking and Ballast Bed Freezing at Tunnel Portal in Seasonally-Frozen Regions

The purpose of this study is taking the ZEST tunnel of Xilin hot-Ulanhot Railway in Inner Mongolia province of China as an example to investigate the cause and treatment measures of lining cracking and ballast bed freezing at tunnel portal in seasonally-frozen regions. Field investigation method was used to investigate the characteristics of lining cracking and ballast bed freezing in winter. The results show that the longitudinal cracks in the side wall are symmetrical and located in the middle of the wall. The longitudinal cracks have obvious seasonal characteristics, which mainly occur in winter, and the width of cracking was alternatively varied and broadened year by year, of which the process proved to be irreversible. The number of cracks and the width and length of the cracks all have the trend of further development in the last year. Numerical calculation method was used to study the internal force change law of the lining structure of the ZEST tunnel under seasonal frost heaving force. The results show that the lining cracking is mainly caused by the frost heaving force load. Based on the above analysis results, the treatment measures for the frost heave cracking, ballast bed freezing and the leakage of water leakage during the spring thawing period are proposed, the remediation measure proposed is quite effective and the problems such as cracking, ballast bed freezing and water leaking of tunnel lining are completely solved.

Prediction of pollutants dispersion patterns around two adjacent urban road tunnels

Road tunnels have recently become prevalent in urban areas of China. In areas adjacent to these tunnels, air quality and human health are important public health concerns. As such, assessing pollutant dispersion and decay rates around tunnel portals in the surrounding environment is an important field of study. This study established a three-dimensional computational fluid dynamics (CFD) model containing two adjacent urban road tunnels, to estimate pollution dispersion characteristics outside the tunnels. The results indicate that, when pollutants were exhausted from the portals only, crossflow between the two adjacent tunnels was quite severe. The highest carbon monoxide (CO) concentration was nearly 50 ppm, exceeding the 10 ppm limit National Standard in China for 1 h exposure when exhaust shafts are absent. When shafts exhausted 80% of the pollutants from the tunnel portal, crossflow between the adjacent tunnels decreased to 10-50% of that without shafts. The simulation results demonstrate that the combination of shafts and portals was an effective and energy-saving strategy to exhaust pollution from adjacent super-long urban road tunnels. These findings increase our fundamental understanding and provide guidelines for evaluating pollutant exposure risk around tunnel portals, the design of tunnel exhaust shafts, and environmental planning purposes in urban environments.

Transient Analyses and Energy Balance of Air Flow in Road Tunnels

The issue of airflow dynamic in a road tunnel is considered in this study and its impact on smoke management and people safety is highlighted. It was an attempt to estimate the time needed to reach a final steady state of airflow when the operation mode of jet fans was switched. The numerical model of the tunnel was solved with the use of Ansys Fluent. To reproduce the decrease of atmospheric pressure with height, relative static pressure was applied using UDF (User Defined Function). The ambient weather conditions were taken into account as well. The wind influence was introduced by the additional component of dynamic pressure applied against one of the tunnel portals also using UDF. There are some theoretical foundations of airflow in a tunnel presented in this paper. The obtained results were compared with the measurements carried out in a real road tunnel and the results obtained while applying the above-mentioned physical model. The main contribution of the presented work is the indication of a relatively high relaxation time of airflow in a tunnel, which could be important when designing the emergency pattern of a ventilation system. Additionally, some considerations of kinetic energy exchange between fan jet and air volume would be helpful when choosing fans for ventilation systems being designed.

Modeling of carbon monoxide dispersion around the urban tunnel portals

The aim of this study is to investigate the problems caused by discharge of polluted air from tunnels into the environment with a specific focus on residential areas. In city tunnels, portal or stacks, pollutant management is a big challenge. Nowadays, air quality management, particularly in urban tunnels, is considered as a part of the ventilation system design. The goal is to see the environmental impacts beforehand. From environmental aspects, preventive measures are required either inside or outside the tunnel in some cases. Niayesh tunnel in Tehran is taken as a case for proving the objectives presented in this study. Concentration of carbon monoxide at the vicinity of the portals is calculated using the proper dispersion simulation. The results of dispersion modeling for the assumed worst case of ventilation can help to understand the environmental impact of ventilation. The worst traffic emissions for a congested traffic scenario,are selected as an emission source for dispersion modeling. According to the traffic condition and fleet composition, the crucial emission extracted from the tunnel is carbon monoxide. Therefore, the performed simulation only focuses on carbon monoxide dispersion modeling. From the other side, carbon monoxide is taken as a demonstration pollutant, because it is inert and chemical reactions can be neglected in short-term considerations. A lagrangian model composed of Graz Lagrangian Model and Graz Mesoscale Model is used for flow-field and dispersion calculations.

An Investigation of Driver Behavior on Urban General Road and in Tunnel Areas

The objective of this study is to examine experience-related differences in microscope driving behavior as drivers performed six separate maneuvers, namely 1) driving on general urban roads, 2) approaching a tunnel portal, 3) driving through a tunnel’s threshold zone, 4) driving in the interior tunnel zone, 5) driving in the zone ahead the tunnel exit and 6) driving after the tunnel exit. An on-road experiment was conducted with 20 drivers in two groups. The first group was made up of new licensed drivers, and the second group contained the more experienced drivers. The study consisted of one between-subject (experience) and five within-subject variables (drive environment type). The drivers’ behavior was measured through Mean Glance Duration, AOI Attention Ration, Horizontal Eye Activity, Vertical Eye Activity, Percentage of Eyelid Closure, and Heart Rate Variability. With respect to the relevant psychological measures, the results show that in general more attention is focused on the far left-hand side of the road and the near front road when driving through tunnel areas when compared with driving on general roads. In addition, the psychological measurements indicate that tunnel’s dark narrow environment causes anxiety on driving for lower heart rate variation coefficient (RRCV). New licensed drivers were more severely affected by the tunnel environment than the experienced drivers.