Urban Utility Tunnel Research Articles

Self-extinction characteristics of fire extinguishing induced by nitrogen injection rescue in an enclosed urban utility tunnel

Extensive studies have been conducted on effect of sealing ratio on combustion process and temperature field of pool fire, while still lacking emergency rescue research after the failure of the original fire extinguishing system. Therefore, a series of tests were carried out in a 1/3 reduced-scale tunnel to explore the nitrogen injection into the closed tunnel on the effectiveness of fire-fighting. The results show that, the presence of nitrogen not only significantly lowers the oxygen concentration and mass loss rate of the air drawn into the combustion area, but the flame continues to entrap the polluted low-oxygen air, resulting in the continuous inadequate combustion of the fire and a large amount of asphyxiating smoke, thus shortening the self-extinguishing time. The concentration range of oxygen concentration of flame extinguishing during nitrogen injection is 11.13 %–14.98 %. Simultaneously, an analytically derived model to estimate the extinction time of nitrogen injection into the closed utility tunnel was developed, which is found that the deviation between the improved classical model and the experimental data within 10 %. The present work positively provides a tool for the assessment of thermal environment and estimation of fire detection time in engineering practice.

Experimental and Theoretical Investigation of Longitudinal Temperature Attenuation and Smoke Movement in Urban Utility Tunnel Fires

The urban utility tunnel is an indispensable part of modern engineering construction. However, the fire risk cannot be ignored due to the narrow space and limited ventilation of the utility tunnel. A study of smoke filling is performed in a 1/8-scaled utility tunnel (25 m × 0.5 m × 0.45 m). Five heat release rates (5, 10, 15, 20 and 25 kW) and four positions of fire sources are used for tests. The initial position of the one-dimensional smoke movement of strong plume is determined. Based on the traditional model, the longitudinal temperature attenuation model of tunnel smoke is established with consideration of radiation and convection heat losses. The theoretical value of the longitudinal temperature rise of smoke is in good agreement with the experimental value. A one-dimensional spreading velocity model is established that coincides well with the experimental value, and the relative error is less than 20%. The spreading velocity of smoke is increased by the heat release rate. The velocity of the smoke spreading at the near end is smaller than that at the center, due to the long spreading route. The current conclusions disclosed in this study provide important guidance for the ventilation design of utility tunnels for fire smoke scenarios.

Image-based automatic multiple-defect detection of urban utility tunnel using UUTNet

In-service urban utility tunnels (UUT) suffer from cracks, corrosion, and leakage defects, which rises the chance of major accidents. However, prevailing detection methods for UUT remain reliant on manual inspection and subjective judgment, or traditional image processing technologies, such methods may not be able to obtain accurate defect information. This study proposes a novel and effective network called UUTNet based on the constructed UUT dataset for defects detection. Considering that the UUT defects has a certain distribution correlation, the attention module is introduced to the Pyramid Scene Parsing Network to capture the relation. By adding the hybrid dilated convolution after the feature extraction layer, the receptive field is expanded to further extract global and local features. The performance of UUTNet was evaluated based on the metrics MIoU, F1-score, Accuracy, and robustness. Comparative experiments were conducted, and the results showed the UUTNet achieved the best detection performance, achieving 0.7615 MIoU, 0.9806 Accuracy and 0.8012 F1-score. The MIoU was further improved to 0.7847 by utilizing the Bayesian optimization. Three extreme inspection scenes, including uneven illumination, high brightness, and obstacle interference, were applied to validate model robustness. The proposed method offers robust technical assistance for detecting defects in the UUT and precisely assessing the distribution and extent of these defects.

An Investment Decision Model for Underground Urban Utility Tunnel Based on MIVES and Real Option Theory from a Sustainable Perspective

Although the importance of urbanization, urban renewal and sustainable development have been increasingly recognized, with the accelerating process of urbanization, the urban above-ground space is no longer sufficient for the process of urbanization, and downward development of the city has become inevitable. Underground Urban Utility Tunnel (UUUT) is an effective measure to promote the sustainable development of urban underground space (UUS). However, decision makers still cannot fully consider the economic, social, environmental and technological factors, as well as the future risks of the project and the value of flexibility in management. In this paper, an investment decision model for UUUT is proposed that combines the Integrated Value Model for Sustainable Assessment (MIVES) and the real option theory, which comprehensively considers the social, economic, environmental and technological impacts, and assists the government in carrying out the investment decision analysis of UUUT from a sustainability perspective by applying the real option theory to the economic evaluation process. The primary process of this study can be divided into four steps. (1) establishment of the investment decision index system for UUUT; (2) determination of the quantitative criteria for each indicator; (3) calculation of the feasibility of UUUT; and (4) a case study to demonstrate the feasibility of the proposed model, as well as the achieved results. The proposed investment decision model can be used as an auxiliary tool in the early planning stage of UUUT, and also for the comparison and selection of different options for UUUT.

Combustion characteristics and thermal decomposition mechanism of the flame-retardant cable in urban utility tunnel

Although urban utility tunnel act as innovative technique for sustainable development, the cable fire of tunnel should be carefully treated. Extensive studies have usually focused on the combustion or pyrolysis stage of cables by a single test, but comprehensive tests still need to further investigation. Therefore, the cable ignition, expansion charring, pyrolysis and combustion from the macro and micro perspectives were comprehensively explored with cone calorimeter and simultaneous thermal analysis infrared gas chromatography (STA-FTIR-GC/MS). It was found that in addition to the outermost sheath material of the cable, its insulation material and flame retardant material play a non-negligible role in the fire. To more accurately reflect the actual pyrolysis and combustion characteristics of the material, a simplified cable ignition model was established, the combustion laws of typical cables in the urban utility tunnel under different radiation intensities were explored, the quantitative characterization and risk assessment of the toxicity of the gas products were studied.What's more, the pyrolysis characteristics and reaction mechanism of cables under different thermal environments were revealed. The work provides the necessary support for the establishment of database and design standard of fire engineering in urban utility tunnel.

Experimental study on the flame merging and ceiling impingement behavior of transversely located double fire sources in an urban utility tunnel

The investigation of the fire characteristics and propagation behaviour in the urban utility tunnel is of great practical importance, especially for the conditions with double fire sources. In this work, a 1/8th scaled urban utility tunnel model was built to conduct the experiments to characterize the flame merging and ceiling impingement behaviour of transversely located double fire sources. Two rectangular fire sources with the same dimension were used, and their heat release rate (HRR) and fire source spacings were varied to consider the typical scenarios. Results show that for the smaller HRR, the tunnel ceiling and sidewalls have little influence on the flame merging of double fire sources. With the increase of HRR, the flame gradually impinges on the tunnel ceiling and forms a stable ceiling jet flame extension, and the ceiling and sidewalls of the tunnel can promote the merging of the flames of the double fire sources. Besides, with the increase of the fire source spacings, the flame merging probability P m can be divided into three stages, that is, (i) complete merging stage, (ii) intermittent merging stage and (iii) complete separation stage. On this basis, the predicting relation of P m was obtained by using the piecewise function.

Model test study on the influence of the collapsibility of loess stratum on an urban utility tunnel

Model test study on the influence of the collapsibility of loess stratum on an urban utility tunnel

A CNN Based Anomaly Detection Network for Utility Tunnel Fire Protection

Fire accident is one of the significant threats to the urban utility tunnel (UUT) during operation, and the emergency response is challenging due to the compact tunnel structure and potential hazard sources involved. Traditional fire detection techniques are reviewed in this study, and it has been determined that their performance cannot satisfy the requirements for early fire incident detection. Integrating advanced sensing technologies and data-driven anomaly detection has recently been regarded as a feasible solution for intelligent safety system implementation. This article proposed an approach that utilized a fiber-optic distributed temperature sensing (FO-DTS) system and deep anomaly detection models to monitor the fire exotherm during the early stages of accidents. The variable fire exotherm is simulated with an embedded-system controlled electrical heating platform. Moreover, autoencoder (AE) based and convolutional neural network (CNN) based methods have been designed for anomaly detection. The temperature data collected from the FO-DTS in the experiment was employed as the training set for the data-driven models. Furthermore, the anomaly detection models were tested, and the results showed that the proposed CNN model can achieve a higher accuracy rate in detecting the simulated fire exotherm.

Quasi-Static Test Study on Seismic Behavior of Large-Section Fabricated Utility Tunnel

Based on the urban utility tunnel project in Xiong’an New Area, to clarify the seismic behavior of the utility tunnel structure, the overall structure model of the four-cabin large-section utility tunnel and the quasi-static test of the joint specimens have been completed. The results show the following. (1) The hysteresis curve of the overall structure is relatively full, showing good ductility and energy consumption. The middle joint has better bending performance. The ductility of the side joint is worse than that of the middle one. (2) The skeleton curve of the joint specimen is divided into three stages: obvious cracking, yielding, and ultimate load. After the overall structure reaches the ultimate load, the load curve basically tends to be horizontal. (3) The stiffness degradation of the overall structure and the joint specimens continues to be uniform during the entire loading process. (4) The structural design of the fabricated utility tunnel can ensure the seismic behavior under normal condition.

Measurement of indoor environmental parameters and analysis of the condensation phenomenon in urban utility tunnels

Condensation is a major issue in the safe operation of utility tunnels. To address the condensation problem, the indoor air temperature, relative humidity (RH) and surface temperature in an urban utility tunnel in Jining were continuously measured, and the condensation conditions were surveyed and analysed. The results indicated that under natural ventilation conditions, the air temperature in the comprehensive cabin varied from 23.4°C to 24.5°C, the RH fluctuated between 86.4% and 95.3%, and the corresponding air dew point temperature (DPT) remained in the range of 22.2°C–22.9°C. The surface temperature of the water supply pipeline ranged from 17.8°C to 18.5°C, which was far lower than the DPT in the tunnel, resulting in serious condensation. A water supply pipeline with an anti-condensation design was developed based on environmental test data. A 25-mm-thick rubber plastic sponge insulation layer was used to thermally insulate the water supply pipeline, preventing further dew condensation. Furthermore, mechanical ventilation had little effect on reducing the RH in the tunnel and may actually cause dew condensation; therefore, a ventilation control mode was proposed in this study. These results are expected to provide basic data for further research and reference for the safe management of utility tunnels.

Ventilation and heat transfer modeling in urban utility tunnel

Ventilation and heat transfer modeling in urban utility tunnel

The Influence of Construction Scheme of Asymmetric Three-Cabin Utility Tunnelling on the Surface Settlement Behaviour

The asymmetric three-cabin structure is one of the favourite choices in urban utility tunnel engineering, as it can provide a higher space utilization and clearer classification for pipelines. However, few work has been done on utility tunnel in such special cross-section during undercutting construction until now. Besides, as the loose soil layers such as sand and silt are often encountered during the utility tunnelling, its influence on surface settlement become one of the main concerns. This study focuses on the influence of the construction scheme on the surface settlement details for an asymmetric three-cabin utility tunnel. A 3D finite element method (FEM) model was established including both the stratum and a utility tunnel based on the Beijing Daxing International Airport (BDIA) Expressway Urban Utility Tunnel project. With this model, the behaviour of the surface settlement with the undercutting construction of the utility tunnel was investigated, and the causes and values of surface settlement in five construction stages were analyzed. By changing the construction spacing and sequence of mid and side cabins, the surface settlement and structural deformation were compared. The numerical results demonstrated a strong correlation between the cross-section shape and surface settlement characteristics. And the symmetry line of the surface settlement trough was offset by 1 m to the side with the larger cross-section. The second (excavation of upper bench in mid-cabin) and fourth (excavation of upper bench in side-cabin) stages caused the largest surface settlement, which were 41.9% and 18.07% of the total settlement, respectively. In addition, after optimizing the field tunnelling scheme, the surface settlement was reduced by 31.9% using the side-tunnel first construction sequence. The proposed numerical model is able to predict the settlement characteristics in each construction stage, which is important and provides a basis for further studies on the surface settlement mechanism and optimal design of the asymmetric three-cabin utility tunnelling scheme.

An Integrated Quantitative Risk Assessment Method for Urban Underground Utility Tunnels

With the rapid urbanization, urban underground utility tunnels have seen fast growth in China in the past few years. Urban utility tunnels can house various kinds of city ‘lifelines’ such as natural gas pipeline, heat pipeline, water supply system, sewer pipeline, electricity and telecommunication cables, which are of great significance to guarantee essential flows of energy, information and logistics for urban life. If a utility tunnel accident occurs, the consequences could be catastrophic. Risk assessment has been an important tool to examine the safety performance of industrial facilities and the effectiveness of safety measures. In this study, an integrated model based on dynamic hazard scenario identification (DHSI), Bayesian network (BN) modeling and risk analysis is proposed for risk assessment of urban utility tunnels. The worst-case scenario of urban utility tunnel accidents is identified by DHSI and modelled by BN. Meanwhile, risk analysis is conducted based on the results of BN considering casualties and economic losses. Finally, the integrated method is applied to evaluate the risk level of a real-world utility tunnel. The results indicate that the integrated quantitative risk assessment framework is an alternative and effective tool for safety assessment and land-use planning of urban utility tunnels.

Experimental investigation on extinguishing characteristics of liquid nitrogen in underground long and narrow space

As an efficient and environmentally friendly cryogenic extinguishing agent, liquid nitrogen (LN2) is highly promising for fire extinguishing in narrow and long underground confined spaces. It is difficult to tackle the urban utility tunnel fire due to its complex and narrow structural characteristics. Thus work aims to study the feasibility and effectiveness of LN2 fire suppression in urban utility tunnel. An experimental study on the fire extinguishing characteristics of LN2 in urban underground utility tunnel was carried out in a 1/3.6 reduced-scale model tunnel with different injection parameters, such as the direction of the LN2 nozzle and the distance between the LN2 nozzle and the fire source. The results show that LN2 can quickly extinguish a utility tunnel fire. The rapid extinguishing mechanism of LN2 involves the combined effect of cooling and inerting. When the LN2 release distance is 2 m and 4 m, the fire extinction time increases by 8.7% and 27.5% respectively for the horizontal injection. On the contrary, the vertical injection is more effective when the LN2 nozzle is far from the fire source. By injecting LN2 into the utility tunnel, an abrupt evaporation occurs, followed by a thermal expansion. The vaporizing cryogen forms a low temperature nitrogen cloud, which rapidly cools the gas. As the cloud spreads, the pyrolysis gas is rendered inert, and the fire is isolated from oxygen and finally extinguished.

Blast responses of polyurea retrofitted utility tunnel reinforced with basalt fibre reinforced polymer bars

Polyurea coating and carbon fibre reinforced polymer reinforcing techniques were applied to retrofit a severely damaged urban utility tunnel (UUT). The blast responses of the retrofitted UUT were investigated through in-filed explosion experimental tests, and the displacements, strains, accelerations, and damage were compared. The retrofitted UUT exhibited comparable or even better blast resistance than the intact UUT. Although the polyurea coating was flexible, it exhibited excellent performance in improving the blast resistance of the damaged UUT. As the UUT reinforced with basalt fibre reinforce polymer (BFRP) bars has smaller damage compared with the UUT reinforced with steel bars, its retrofitting is simple, and the cost is low. The roof was simplified as an elastically simple support one-way slab. Euler beam theory was adopted to analyse the dynamic responses of UUT roof considering the interaction between soil and structure, which agree well with the experiments in the first three cases.

Estimating the two-dimensional thermal environment generated by strong fire plumes in an urban utility tunnel

Although utility tunnels act as an innovative technique for the sustainable development of city’s utilities and limited underground space today, the fire issues in this type of tunnel should be carefully treated as they are different from traffic tunnel fire scenarios. In order to propose an easy-to-use two-dimensional (2D) temperature estimation methodology that is helpful to the designers, operators and firefighters, the temperature distributions and ceiling jet thickness (the vertical distance from the ceiling to where the temperature within the fire-induced smoke flow drops to half of its maximum at a certain location) were investigated with full-scale fire tests in a utility tunnel. The vertical temperature distributions were represented as a normalized single convex-concave profile. Self-similarity was confirmed and three types of similarity profiles were found. In order to correlate the inner boundary layer region and outer region of these ceiling jet flow profiles, a series of mathematical functions combining a power function and Gortler’s error function were employed. Furthermore, the ceiling jet thickness was found to vary as a power function with the longitudinal distance, rather than being a constant as previous research suggested. The vertical distribution model, the ceiling jet thickness model and the longitudinal temperature decay model were combined into a framework to empirically estimate the 2D thermal environment generated by strong fire plumes in utility tunnel’s fire protection engineering practice.

Prefabricated Urban Underground Utility Tunnels: A Case Study on Mechanical Behaviour with Strain Monitoring and Numerical Simulation

The prefabricated urban utility tunnels (UUTs) have many advantages such as short construction period, low cost, high quality, and small land occupation. However, there is still a lack of in‐depth analysis of the mechanical performance of the prefabricated urban utility tunnel (UUT) structure with bolted connections under different working conditions. In this paper, the force performance of a prefabricated UUT in Tongzhou District, Beijing, was studied under different working conditions using two methods: field monitoring and numerical simulation. The multichannel strain monitor was used for monitoring, and the internal wall concrete and bolt strain change data under the two conditions of installation and backfill were obtained. Combined with the construction process of the UUTs, a three‐dimensional numerical model was established by COMSOL, where the build‐in bolt assembly was used to simulate the longitudinal connection of the tunnel. The simulation results were compared with the measured data to verify the rationality of the computational model. The simulation results showed that the concrete and bolts on the inner wall of the tunnel work well under the two conditions of installation and backfilling; The deformation of the top plate of the prefabricated tunnel was approximately parabolic, with the largest vertical displacement (0.37 mm) in the middle and the most sensitive to the vertical load in the central part of the roof. The central portion of the side wall had the largest displacement (0.17 mm) in the inner concave. The tensile stress of bolt 3 increased the most (30.75 MPa) but was still much smaller than the yield strength of the bolt. The concrete and bolts of the UUT were found to work well through force analysis under operating conditions. In conclusion, analysis of structural forces and deformation failure modes will help design engineers understand the basic mechanisms and select the appropriate UUT structure.

Simulation of the dynamic response of an urban utility tunnel under a natural gas explosion

Based on the coupling of the arbitrary-Lagrangian-Eulerian (ALE) and finite element (FE) algorithms, the paper presents a numerical model that takes into account the fluid properties of an explosive gas and the mechanical characteristics of a structure to simulate the dynamic process of a natural gas explosion in an urban utility tunnel. The dynamic response of the tunnel is clarified by analysing the pressure, displacement, velocity, and effective stress of the structure. Furthermore, the mechanical behaviour of the tunnel in which the explosion occurs, corresponding to different gas leakages and cube sizes, is investigated. It is determined that an increase in gas leakage and decrease in cube size increase the blast pressure, propagation velocity of the explosion cloud, and effective stress acting on the tunnel.

Radio Wave Propagation and WSN Deployment in Complex Utility Tunnel Environments.

The significant growth of wireless communications systems in the last years has led to the adoption of a wide range of applications not only for the general public but, also, including utilities and administrative authorities. In this context, the notable expansion of new services for smart cities requires, in some specific cases, the construction of underground tunnels in order to enable the maintenance and operation works of utilities, as well as to reduce the visual impact within the city center. One of the main challenges is that, inherently, underground service tunnels lack coverage from exterior wireless communication systems, which can be potentially dangerous for maintenance personnel working within the tunnels. Accordingly, wireless coverage should be deployed within the underground installation in order to guarantee real-time connectivity for safety maintenance, remote surveillance or monitoring operations. In this work, wireless channel characterization for complex urban tunnel environments was analyzed based on the assessment of LoRaWAN and ZigBee technologies operating at 868 MHz. For that purpose, a real urban utility tunnel was modeled and simulated by means of an in-house three-dimensional ray-launching (3D-RL) code. The utility tunnel scenario is a complex and singular environment in terms of radio wave propagation due to the limited dimensions and metallic elements within it, such as service trays, user pathways or handrails, which were considered in the simulations. The simulated 3D-RL algorithm was calibrated and verified with experimental measurements, after which, the simulation and measurement results showed good agreement. Besides, a complete wireless sensor network (WSN) deployment within the tunnels was presented, providing remote cloud data access applications and services, allowing infrastructure security and safety work conditions. The obtained results provided an adequate radio planning approach for the deployment of wireless systems in complex urban utility scenarios, with optimal coverage and enhanced quality of service.

Evaluation of blast-resistant ability of shallow-buried reinforced concrete urban utility tunnel

To check the protective performance of shallow-buried steel bars reinforced concrete urban utility tunnel (UUT) under accidental explosion, a scaled shallow-buried UUT was designed, constructed and tested. In-field explosion experiments were carried out on this UUT. Dynamic responses, damage evolutions and failure styles of the UUT were revealed by the experiments. The tunnel roof is the most vulnerable component with longitudinal main cracks developing along the tunnel. When the scaled distance was larger than 1.10 m/kg1/3, no cracks appeared and the UUT was intact. When the scaled distance was reduced, the damage degree became more and more serious with longitudinal crack number developing from one at 1.000 m/kg1/3, two at 0.881 m/kg1/3, to three at 0.800 m/kg1/3. At 0.557 m/kg1/3 the width of the central crack abruptly increased to 11.2 mm and the UUT was seriously damaged. Damage level can be classified by the mid-span displacements of the roof, the width and number of the roof cracks, the accelerations and the maximum strains. The roof was simplified as an elastically restrained one-way slab. The Euler beam theory was adopted to analyze the dynamic responses, which are in agreement with the experiments in the first three cases.