Tunnel Portal Research Articles

<b>Recent Studies on Wayside Environmental Problems</b>

Environmental problems along railway lines caused by passing trains include the following: noise, low-frequency sound, micro-pressure waves radiating from tunnel portals, ground vibrations. To make railways more environment-friendly, it is necessary to mitigate the impact of these phenomena. This paper outlines some recent studies carried out by the Railway Technical Research Institute to address these issues, with a focus on work related to plans to increase Shinkansen running speeds. This paper also introduces specifications and plans for the use of newly constructed experimental facilities by RTRI for research on high-speed railways.

Extended CFD models for numerical simulation of tunnel fire under natural ventilation: Comparative analysis and experimental verification

This paper proposed Air Entrainment Model (AEM) and Mass Transfer-Volumetric Heating Source model (MT-VHS). Then different physical models of tunnel, gas description models and combustion models were used. CFD results were compared with reduced scale experiments and previous theories. The results show that traditional numerical simulation on tunnel model may cause asymmetrical temperature profile, because the pressure of tunnel portal is always set to be constant. Expanded computational domain can avoid this unreasonable setting. Boussinesq approximation underestimates temperature profile under ceiling because it ignores the variation of air property with temperature. Fitting parameters of air properties are more reasonable. In terms of combustion model, Volumetric Heating Source (VHS) neglects the plume generation. Heat release rate by Eddy Dissipation (ED) is not stable because it is affected by turbulent mixing. The mass sink of AEM entrains not only fresh air but also hot smoke. So the temperature profile by AEM deviates from experimental data much. Maximum temperature, temperature profile under ceiling and smoke layer height predicted by MT-VHS is close to experimental data and theoretical predictions, especially when the heat convection between wall and hot smoke is considered.

Peridynamics simulation of structural damage characteristics in rock sheds under rockfall impact

Rock shed structure is widely constructed above a tunnel portal to protect the tunnel from rockfall hazards. The damage mechanism of rock sheds under rockfall impact is crucial, but is yet still unclear. In this study, a dynamic peridynamic model with a relatively simple yet effective failure criterion, which considers the strain rate effect for reinforced concrete shed structures under rockfall impact and is convenient for engineering applications, is developed. After assessing the proposed method by comparing with two experimental tests, varieties of simulation cases are simulated to thoroughly investigate the influence of different factors, including the shape, mass and drop height of the falling rock, on the damage behavior of rock sheds without backfills under rockfall impact. The safety of the tunnel and the damage position for strengthening are evaluated by the proposed method. The structural damage degree of the tunnel increases with the increase of drop height and rockfall mass, but the resulting growth rate differs. The penetration ability of a round falling rock is superior to a squared or an octagon falling rock. A non-circular falling rock could induce a larger damage zone. Under the angular falling rock impact, the structure is more likely to be penetrated with a sharp impact angle. The modes of local crush and flexural failure occur firstly, and then the tunnel is sheared by the falling rock until overall collapse happens. Shear failure is more obvious when the drop height is higher or under the non-circular falling rock impact. The work suggests that all exiting analysis techniques for simplifying the impact force as a static load are not sufficient to understand the resistance of rock sheds under rockfall.

Nonlinear Response of Tunnel Portal under Earthquake Waves with Different Vibration Directions

Tunnel portal sections often suffer serious damage in strong earthquake events. Earthquake waves may propagate in different directions, producing various dynamic responses in the tunnel portal. Based on the Galongla tunnel, which is located in a seismic region of China, three-dimensional seismic analysis is conducted to investigate the dynamic response of a tunnel portal subjected to earthquake waves with different vibration directions. In order to simulate the mechanic behavior of slope rock effectively, an elastoplastic damage model is adopted and applied to ABAQUS software by a self-compiled user material (UMAT) subroutine. Moreover, the seismic wave input method for tunnel portal is established to realize the seismic input under vertically incident earthquake waves with different vibration directions, e.g., S waves with a vibration direction perpendicular or parallel to the tunnel axis and P waves with a vibration direction perpendicular to the tunnel axis. The numerical results indicate that the seismic response and damage mechanisms of the tunnel portal section are related to the vibration direction of the earthquake waves. For vertically incident S waves running perpendicular to the tunnel axis, the hoop tensile strain at the spandrel and arch foot and the hoop shear strain at the vault and arch bottom are the main contributors to the plastic damage of the tunnel. The strain is initially concentrated around the tunnel foot and spandrel, before shifting to the tunnel vault and bottom farther away from the tunnel entrance. For vertically incident S waves running parallel to the tunnel axis, very large hoop shear strain and plastic damage appear at the tunnel haunches. This strain first increases and then decreases with distance from the tunnel entrance. For vertically incident P waves running perpendicular to the tunnel axis, the maximum damage factor of the slope rock and the maximum plastic strain of the tunnel are significantly lower than for S waves. Moreover, with increasing distance from the tunnel entrance, the plastic damage to the tunnel lining rapidly decreases.

Study on Tunnel Air Curtain Insulation System for Cold Areas

With the growing number of tunnels in high‐latitude and high‐altitude areas, freezing damage is all the more obvious. In traditional insulation layer method, insulation layer is laid in the tunnel to prevent invasion of cold air by taking advantage of its low thermal conductivity. However, due to its narrow temperature adaptation range and high construction and maintenance costs, this method cannot completely solve the problem of tunnel freezing damage. To solve this problem, this paper proposes tunnel air curtain insulation system which can generate electricity using solar energy. Based on parameters such as indoor air temperature and wind velocity in the thermal insulation room, on the one hand, it intelligently controls running sets of air curtain, jet flow angle, and velocity, prevents invasion of the external cold temperature into the tunnel, so that the tunnel maintains an interior temperature above 0°C. On the other hand, it intelligently diagnoses air curtain machine to provide system fault warning, so that the system keeps safe and reliable operation. The results show that air curtain technology can effectively solve the problem of tunnel freezing damage in cold areas; multiple air curtains in series connection provide better barrier efficiency. From the measured data and calculation results of Lafashan tunnel, it can be seen that an air curtain is installed at the tunnel portal with jet speed of 22.7 m/s and temperature of 35.24°C, which can meet the tunnel without freezing damage when the external temperature is −10°C, and the external wind speed is 2 m/s, providing a new method for tunnel insulation technology in cold area.

Study on Stability of Slope of Tunnel Portal in Cold Regions Under Multiple Factors

Study on Stability of Slope of Tunnel Portal in Cold Regions Under Multiple Factors

Influences of High-Speed Train Speed on Tunnel Aerodynamic Pressures

To comprehensively investigate the characteristics of aerodynamic pressures on a tunnel caused by the whole tunnel passage of a high-speed train at different speeds, we conduct a series of three-dimensional numerical simulations. Based on the field test results obtained by other researchers, the input parameters of our numerical simulation are determined. The process of a high-speed train travelling through a railway tunnel is divided into three stages according to the spatial relationship between the train and tunnel. Stage I: before train nose enters the entrance; Stage II: while the train body runs inside the tunnel; Stage III: after the train tail leaves the exit. The influences of high-speed train speed on the tunnel aerodynamic pressures of these three stages are systematically investigated. The results show that the maximum peak pressure value decreases with increasing distance from the entrance and increases with increasing train speed in Stage I. There is an approximately linear relationship between the three types of maximum peak pressure (positive peak, negative peak, and peak-to-peak pressures) and the power of the train speed in Stage II. These three types of maximum peak pressure values of the points near tunnel portals increase with increasing train speed in Stage III. Moreover, these three types of maximum peak pressure in the tunnel’s middle section at different train speeds are more complex than those near the tunnel portals, and there is one or more turning points due to the superimposed effects of different pressure waves.

Large eddy simulation of smoke blocking by water sprays in a tunnel fire

This study presents a large eddy simulation (LES) of smoke blocking by water sprays in tunnel fires to investigate the effect of water sprays on the characteristics of the smoke flow. This study uses an LES solver – FireFOAM. In addition, fire experiments using a 1/10 scale model tunnel with two water spray nozzles are conducted to validate the LES results. The length of the model tunnel is 12 m, the height and width are 0.5 m, respectively, and the heat release rate of the fire is 7.38 kW (2.33 MW in full scale). Each spray nozzle operates at a pressure of 0.3 MPa, and the volume median diameter of spray droplets is 226 μm. First, a grid sensitivity analysis on the gas temperatures under free-burn conditions is performed. A 1.25 cm uniform mesh is chosen in the tunnel domain for all subsequent simulations. Then, FireFOAM predictions are validated against experimental data on the distribution of water spray mass flux under conditions without the fire and the gas temperatures measured under conditions with and without the water sprays. The water spray mass flux and gas temperatures predicted by FireFOAM show good agreement with the measurements. Next, the characteristics of the smoke flow are investigated in the presence of the water sprays. It is found that the water sprays block part of the smoke flowing under the ceiling and force it to descend to the ground. The descended smoke flows back in the lower part of the fire zone, and a vitiated air layer is formed. The vitiated air layer is composed of the smoke flowing back and fresh air drawn from the portal. On the other hand, some smoke leaks through the water spray zone and flows along the ceiling on the portal side. It is also found that the spray cooling delays the propagation of the leaked smoke toward the tunnel portal. Furthermore, it is quantitatively shown that the buoyant flow stratification destroyed by the water sprays becomes thermally and chemically stabilized again on the portal side, for the specific case under this study. Last, the smoke blocking performance is quantified based on the mass flow rates of the smoke flowing under the ceiling in the fire zone and the smoke flowing back toward the fire. A comparison of the smoke blocking rates between the predictions (time dependent: 40–50%) and measurements (steady: 39.4%) shows good agreement.

Multistep train nose for reducing micro-pressure waves

The peak value of a micro-pressure wave radiating from tunnel portals of high-speed railways depends on the shape of the train nose. Previous studies have shown that a train nose shape with a small gradient of cross-sectional area distribution, except at the tip and end, is effective at reducing micro-pressure waves. Using acoustic theory, this study showed that multistep train noses are more effective at reducing micro-pressure waves than the previously suggested nose shapes. Moreover, the multistep noses were optimized using the response surface method and computational fluid dynamics techniques for axisymmetric compressible airflow. In addition, model experiments were performed to verify the usefulness of the optimized multistep noses. The optimized multistep noses reduced the peak values of the micro-pressure wave by over 5% compared to the previous noses. Thus, this study theoretically, numerically, and experimentally suggests new optimal nose shapes that can better reduce micro-pressure waves than existing shapes.

Characteristics of pressure waves radiated from tunnel portals in cuttings

The radiation of pressures from the portal of a railway tunnel in a cutting is investigated to determine how consequential disturbances experienced at ground level compare with those from an above-ground portal. As might be expected intuitively, the above-ground amplitudes of disturbances from below-ground portals tend to be smaller than those from above-ground portals. However, this is not true in locations close to the cuttings themselves. The mixed outcomes are explained by analogy with known differences between wave propagation cylindrical (2-D) domains and more open spherical (3-D) domains that are commonly used to model the radiation of pressures from above-ground portals. Four cuttings are considered. It is shown that cylindrical-like behavior is pronounced for deep, narrow cuttings and that its influence can be significant even in shallow wide cuttings. The main body of the paper is focussed on conditions that would be experienced in locations at ground level, but one section is dedicated to an assessment of conditions close to track level where the amplitudes of the disturbances are much greater.

An experimental investigation of burning rate and flame geometric parameters of tunnel fires under canyon cross wind and longitudinal ventilation

In this study, a series of small-scale experiments were carried out to investigate the variations of burning rate and flame shape of highway tunnel fires in alpine gorge areas with canyon cross wind and longitudinal ventilation. The canyon cross wind was set outside one of the tunnel portals, and the longitudinal ventilation was set at the other. The velocities of the canyon cross wind and longitudinal ventilation, and the longitudinal fire position in the tunnel were varied in this study. The experimental results show that the burning rate increases with increasing the velocity of the overall airflow in the tunnel, which is the combination of the canyon cross wind and longitudinal ventilation. For each velocity of the canyon cross wind, there is a corresponding longitudinal ventilation velocity making the flame burn vertically upward, which is proportional to the canyon cross wind velocity. Based on this relationship, the velocity of overall airflow in the tunnel can be obtained under various wind conditions. In the case that the airflow in the tunnel is dominated by the longitudinal ventilation, the prediction model of flame tilt angle with the canyon cross wind and longitudinal ventilation velocities is proposed. The dimensionless flame height can also be fairly well fitted by power function with the velocity of the overall airflow in the tunnel.

Seismic stability assessment of the portal of a hydraulic tunnel under obliquely incident P waves

Tunnel portals are one of the most vulnerable sections that suffer the risk of instability during earthquakes because they are generally located in poor geological conditions. What makes this case studied in this work even more special are the oblique incident seismic waves and the large amounts of water in the hydraulic tunnel, both of which are believed to cause more complex mechanical behaviors and larger seismic response of hydraulic tunnels. In this paper, a full three-dimensional finite element model that takes the oblique incident seismic waves and hydrodynamic pressure into account is established to evaluate the stability of the tunnel portal under different incident angles of seismic waves. The numerical results indicate that: (1) the stress and displacement responses induced by oblique incident P waves are much larger than that by horizontal and vertical incident P waves; (2) the closer to the tunnel portal, the larger seismic response of the tunnel; (3) tunnel portal is where damage occurs first and suffers severe damage; (4) both oblique incident seismic waves and hydrodynamic pressure are likely to cause larger damage zone and more severe damage.

Engineering geological investigations for designs of excavation method and support system of diversion tunnel at Bulango Ulu Dam, Indonesia

This paper presents engineering geological investigations for designs of excavation method and support system of a diversion tunnel at Bulango Ulu Dam, Indonesia. Evaluation of drill cores along the tunnel was carried out to determine the geological engineering conditions. Classifications of rock masses were carried out based on the Central Research Institute of Electric Power Industry (CRIEPI) and Rock Mass Rating (RMR) to analyze the excavation method and support system. The rock masses along the tunnel were classified into D, CL, and CM classes based on the CRIEPI and poor, fair, and good quality based on the RMR. The tunnel portals consisted of poor quality rock masses, while the main tunnel section consisted of good quality rock masses. The excavation method of full-face was proposed for the main tunnel section, while top heading and bench were proposed for the tunnel portals and main tunnel section having fair quality rock masses. The support system recommended for tunnel portals were 4-5 m long, 1-1.5 m spaced systematic bolts, 100-150 mm thick shotcrete, and 1.5 m spaced steel ribs. Meanwhile, the support system proposed for the main tunnel section was 3 m long, 2.5m spaced bolts, and 50mm thick shotcrete in the crown.

Dynamic response characteristics and failure mode of a bias loess tunnel using a shaking table model test

The shallow bias section at the tunnel portal is one of the most seriously damaged areas under earthquake conditions. The seismic responses of the loess slope and the tunnel structure interact in a bias tunnel-loess slope coupling system. In this study, a 1:25 large-scale shaking table model test on the bias loess tunnel is conducted, and the seismic responses and failure modes of the loess slope and bias tunnel are analyzed. Additionally, the interaction between the tunnel structure and loess slope is studied. The experimental results show: (1) Tunnel structure influences on the acceleration distribution characteristic of the slope. The closer the soil at the slope surface is to the tunnel, the higher the peak ground acceleration (PGA) amplification factor. (2) Slope also has an impact on the seismic response of the tunnel structure. The peak accelerations of the tunnel vault and the sidewall at the shallow-buried side are larger than those of the tunnel invert and the sidewall at the deep-buried side. (3) Under earthquake conditions, several tensile cracks appear in the slope top, and shear failure appear along the slope foot to the slope surface above the tunnel vault. (4) Hances and arch feet are easily damaged in an earthquake, and the seismic damage of lining at the deep-buried side is more severe than that at the shallow-buried side.

Pressure fluctuation and a micro-pressure wave in a high-speed railway tunnel with large branch shaft

In the future, long tunnels for high-speed railways will be constructed for efficient urban space utilization. In these tunnels, branch shafts larger than the main tunnel would be installed as passenger evacuation routes. Although the effect of micro-pressure waves (MPWs) radiated from a branch portal are not be negligible, the effect of MPWs in tunnels with large branch shafts have not been investigated. In this study, we investigate the effect of MPWs radiated from a branch shaft and the pressure wave fluctuations for the branch shaft. The train speed is set to 260–360 km/h, and the ratio of the cross-sectional area of the branch to that of the tunnel Rb is set to 0.5–3.6, which is larger than those of branch shafts installed in conventional Japanese high-speed railway tunnels. The experimental results show that the pressure rise in the main tunnel due to the train nose passing through the branch junction (passing wave) increases with Rb, and that value converges to the pressure rise of entry wave (the pressure wave generated when the train nose enters the tunnel) in the infinite case Rb→∞. Furthermore, the MPW generated by the passing wave would be the largest MPW radiated from the tunnel portal when Rb > 1.0.

Experimental study on smoke movement characteristics in tunnel fires with different canyon cross wind yaw angles

High-speed canyon cross wind with random directions is common in mountain areas and induces complex three-dimensional flow fields inside the tunnel while flowing across the tunnel portal, which changes smoke temperature distribution and movement characteristics in tunnel fires. The effect of canyon cross wind yaw angle on maximum ceiling gas temperature, position of maximum ceiling gas temperature, smoke back-layering length and downstream smoke spread length of tunnel fires was investigated by experiments. Results showed that the maximum ceiling gas temperature increased with the increasing wind yaw angle in the range of 45°-90°, but remained steady when the angle was in 90°-135°. The maximum ceiling gas temperature was higher than other fire locations when the fire was close to the upstream portal because of the airflow barrier. Additionally, a normalized maximum ceiling gas temperature linear correlation was established based on dimensional analysis. The position of maximum ceiling gas temperature was different in both horizontal and longitudinal directions due to the canyon wind. The transverse smoke distribution in the tunnel was uneven, which resulted in a double peak value phenomenon of ceiling gas temperature. Besides, when the wind yaw angle was in 45°-90°, the smoke back-layering was resisted and the downstream smoke spread was promoted by the canyon wind. However, the smoke back-layering was promoted and the downstream smoke spread was resisted due to the negative pressure induction when the angle was in 90°-135°.

Numerical investigation on the smoke behaviour and longitudinal temperature decay in tilted tunnel fire with portal sealing

Blocking the tunnel portal is one strategy in railway tunnel firefighting. In order to evaluate the effect of tunnel portal sealing ratio on fire behaviour, Fire Dynamics Simulator (FDS) was used to simulate tilted tunnel fire with different slope angles varying from 0% to 5%, heat release rate varying from 10 to 50 MW and sealing ratios varying from 0% to 75%. Results show that the experimental data of the temperature distribution inside the tilted tunnel were in good agreement with the simulation results. Moreover, the ceiling temperature rise decreases along the tunnel with the increase of the tunnel portal sealing ratio at initial stage and then tends to stabilize because of less oxygen supply when the heat release rate is relatively large. The maximum temperature rise decays exponentially along the tunnel ceiling with distance. The current model for temperature decay beneath the tunnel ceiling was proposed to be modified by taking the tunnel entrance sealing ratio into account. The predictions by the modified model agree well with the experimental measurement. The results could provide practical information and knowledge in ventilation system design and emergency evacuation for inclined railway tunnels.

Acceleration and Deceleration Parameter Calibration of Tunnel Entrance Based on the Naturalistic Driving Test of Passenger Car

The precise calibration of acceleration and deceleration parameters is crucial for improving the accuracy of operating speed prediction and analysis tools at tunnel entrances. Therefore, acceleration and deceleration data of passenger car captured from 20 drivers at 30 tunnel entrances were collected from 200 m outside to 200 m inside the tunnel portal and averaged across four study zones. The results show that, first, the distribution of deceleration rates based on speed differs from that of acceleration rates based on speed in all zones. Second, significant differences in the probability density distribution of deceleration were found between each zone ( p < 0.001 ), but differences in acceleration could not be found between any zones ( p > 0.05 ). Third, the feature values (breakpoints) of the acceleration/deceleration cumulative frequency curves were located near the 95th percentile, differing from the traditional 85th percentile found with the extant model. The feature values of acceleration in the four zones coincided at 0.5 m/s2 and those of deceleration were 0.93, 0.85, 0.70, and 0.47 m/s2 under zones 1–4, respectively. This study provides accurate feature values of acceleration and deceleration for modelling an updated tunnel entrance operating speed prediction model.

Study on Rationality of Modified Construction Methodof Urban Tunnel Portal under Loose Deposits

Study on Rationality of Modified Construction Methodof Urban Tunnel Portal under Loose Deposits

Numerical study of the influence of tunnel wall properties on ceiling jet temperature in tunnel fires

In tunnel fires, hot gas, called a ceiling jet, propagates beneath the ceiling from the fire source to the tunnel portals. The properties of the ceiling jet must be understood for conducting a fire safety assessment. Heat loss to the tunnel walls (ceiling and side walls) significantly affects the property of the ceiling jet movement in the tunnel. This study aims to estimate the influence of the tunnel wall property on the gas temperature using numerical simulation. Moreover, the correlation between the gas temperature and thermal parameter of the tunnel walls is derived. The simulation results showed that the gas temperature beneath the ceiling is approximately same when the thermal inertias of the tunnel walls are equal. The influence of heat penetration on the gas temperature was significant, and therefore it was important to confirm thermal penetration while conducting the model experiment. Moreover, it was revealed that a linear relationship exists between the gas temperature rise and a dimensionless parameter, which is a function of the Biot number and the Fourier number. The equation expressing this relationship can be used to estimate the influence of the tunnel wall property on the gas temperature.