Urban Subway Research Articles

Optimal Control Algorithm for Subway Train Operation by Proximal Policy Optimization

With the increasing scale of the urban subway, the total energy consumption of the subway has increased dramatically and poses a great challenge to the comfort of passengers and the punctuality of train operation. In order to ensure on-time train operation and passenger comfort, and at the same time reduce the energy consumption of subway operation, this paper proposes a Proximal Policy Optimization (PPO)-based optimization algorithm for the optimal control of subway train operation. Firstly, a reinforcement learning architecture for optimal control of subway train operation is constructed with the position and speed of train operation as the reinforcement learning state, energy consumption and comfort as the optimization objectives, and train operation time as the constraint. The proposed reinforcement learning model is trained by the PPO algorithm, and the reward scaling is added to the training process to accelerate the training speed and improve the efficiency of the algorithm. The experimental results show that the proposed PPO with reward scaling algorithm can effectively reduce train energy consumption and improve passenger comfort while ensuring on-time train operation.

Prediction of GHG emissions from Chengdu Metro in the construction stage based on WOA-DELM

With the mass construction of urban subways, the global greenhouse gas (GHG) emissions have been on the rise. This paper provides statistical evidence to support the infrastructure of subway emissions reduction through a study of GHG emissions during the construction stage of 6 stations and 7 sections of Chengdu Metro Line 18. Using the emission coefficient method, the GHG emissions from building material production, transportation and site construction in subway stations and shield sections were calculated, and a subway GHG emissions prediction model dependent on deep extreme learning machine (DELM) with whale optimization algorithm (WOA) was established(i.e., WOA-DELM). Compared with some optimized DELMs, namely wind driven optimizer (WDO) -DELM, grey wolf optimizer (GWO) -DELM, particle swarm optimizer (PSO) -DELM, artificial bee colony (ABC) -DELM, multi verse optimizer (MVO) -DELM, and atom search optimizer (ASO) -DELM, and some non-optimized algorithm models, namely back propagation neural network (BPNN), kernel extreme learning machine (KELM) and DELM, the correlation consistency of WOA-DELM algorithm prediction results (0.757) was found to be slightly higher. Through sensitivity analysis of the main input variables of subway GHG emissions with the WOA-DELM algorithm model, it was determined that the key influencing factors of station GHG emissions prediction were the station length and the depth of track surface, with relative change rates of corresponding variables of GHG emissions at 30.1% and 23.1% respectively. Finally, a rough prediction formula of GHG emissions from Chengdu Metro stations and shield sections were fitted based on the key influencing factors of GHG emissions. This study provides a practical and effective reference for reducing GHG emissions in subway construction and operation.

A Bayesian spatial–temporal model for predicting passengers occupancy at Beijing Metro

The growing population density in cities requires urban transportation to meet the travel needs of citizens fast and accurately. Therefore, the correct prediction of daily passenger flow in urban subway transportation is of great practical importance for rationalizing the traffic arrangement and safely responding to unexpected passenger flow. This work builds a Bayesian spatial–temporal model for predicting station occupancy. The proposed one provides point estimations of daily passenger flow, a reliable assessment of their uncertainty, and the possibility of understanding traffic features. It also provides a prediction accuracy that meets the standards of the Beijing Metro enterprise. The model discussed in this paper is in force at Beijing Metro Group Ltd to programme daily train schedules.

The Influence of Pile-Beam-Arch Construction on the Stratum and Station Support Structure

The PBA construction method is suitable for urban subway construction in situations with dense traffic and many surrounding buildings and can effectively reduce surface disturbance and control surface subsidence. Based on field-measured data, this study creates numerical finite element models of 3D stratum-structure, studies the influence of each process conversion on the subsidence of the ground surface and the force of the station supporting structure during the construction of the PBA station, and clarifies the optimal construction parameters. The research results show that the consistency between the field monitoring data and the simulation data is more than 80%, which verifies the correctness of the model. It is concluded that the formation of the pile-beam-arch is the main part of the supporting structure and that digging the pilot tunnel and the formation of the beam arch are the control factors of surface settlement. Based on this, the best sequence of pilot tunnel excavation and arch buckle of the two-story two-span station is obtained, and the optimal pilot tunnel excavation scheme of first the middle and then the side, first the upper and then the lower, and the best arch buckle scheme of the synchronous arch buckle on both sides is determined. The results of the research can serve as a guideline when selection the construction parameters for similar PBA stations.

Analysis of the Influence of Deep Foundation Excavation on Adjacent Viaduct Pile Foundation Considering Train Dynamic Loads

The development and utilization of underground space is an effective way to make intensive use of resources, solve "big city disease" and achieve high-quality development. The expansion and renovation of underground space in a central urban area is likely to cause serious damage to surrounding structures. In this study, a deep foundation excavation for the reconstruction of an urban subway station in the Greater Bay Area was chosen for analysis using the finite element method. Different from common excavation engineering, the interaction between the three coupling factors of train dynamic load, foundation excavation, and viaduct pile foundation were analyzed. Six different cases were calculated considering different working conditions of excavation depth and train dynamic load. Soil was evaluated using modified Cam-Clay model. The physical parameters of the soil were determined through on-site and laboratory tests. The results were compared with monitoring data, and the accuracy of the finite element model was verified. The settlement and influence range of the soil, and displacement and internal forces of viaduct piles were analyzed. The maximum settlement of the soil occurred in the direction of the short side of the foundation pit. The maximum value was approximately 0.53 times the excavation depth. The settlement increased by approximately 49% when applying the train load. The dynamic load had an aggravating influence on the horizontal displacement of the top of the pile, with a maximum increase of 51%. Moreover, the dynamic load increased the negative bending moment of the viaduct piles. This study provides a reference for the design and construction of geotechnical engineering projects.

Travel-Energy-Based Timetable Optimization in Urban Subway Systems

Timetable optimization for urban subways is aimed at improving the transportation service. In congested subway systems, the effects of crowding at stations and inside the vehicles have not been properly addressed in timetabling. Moreover, it is difficult to show the time of values in different riding conditions. In this paper, we consider the passenger-travel process as a physical activity expending energy and formulate a travel energy expenditure function for a heavily congested urban subway corridor. A timetable optimization model is proposed to minimize the total energy expenditure, including waiting on the platform and travelling in the vehicle. We develop a heuristic generic algorithm to solve the optimization problem through a special binary coding method. The model is applied to the Yi-zhuang line in the Beijing subway system to obtain a passenger-oriented energy-minimizing timetable. Compared with using the existing timetable, we find a 20% reduction in average energy expenditure per passenger and a RMB 47,500 increase in social profits as the result of the timetable optimization.

Study on the Disturbance of Existing Subway Tunnels by Foundation Sloping Excavation

Urban subway tunnels are located in a very complex operating environment, and the surrounding construction disturbances can lead to the deformation of existing tunnels or even produce tunnel disease issues. The disturbance of adjacent tunnels by pit excavation is an important geotechnical issue among scholars. In our study, the two-stage method is used to derive the deformation of the existing tunnel caused by the use of sloping excavation in the foundation pit. Subsequently, the correctness of the theoretical calculation formulae and the results are verified by constructing numerical calculation models. The effects of different slope footings θ, different slope widths b and different excavation depths H on the deformation of the existing tunnel are analyzed. The results show that: the excavation of the foundation pit can effectively reduce the tunnel deformation by using the sloping excavation method. The soil directly above the tunnel is the main factor causing the deformation of that tunnel. When the foundation pit spans a single-lane existing tunnel, the sloping excavation method of excavating soil on both sides and reserving the middle core soil can be used. When the foundation pit spans a double-lane existing tunnel, the sloping excavation method of excavating the middle soil and reserving core soil on both sides can be adopted. The error between the theoretical calculation results and the numerical simulation results is small, which verifies the correctness of the theoretical calculation results. The load distribution width of the soil after sloping excavation can be taken as the median line width. The deformation of the existing tunnel is influenced, to a greater extent, by changes in the one-time excavation depth H and slope width b, and to a lesser extent, by changes in the slope angle θ.

Modeling and Complex Characteristics of Urban Subway Co-Opetition Network: A Case Study of Wuhan

In this study, to explore the co-opetition (competition–cooperation) relationship between urban subway lines, we constructed three directed weighted subway models of a competition network, cooperation network, and co-opetition network based on the Space R model of a complex network. Taking Wuhan, China, as the research area, we established a Wuhan subway co-opetition network and analyzed the network’s complex characteristics. Through the analysis, we found that the competition network, cooperation network, and co-opetition network are all scale-free networks that present the characteristics of a high-clustering and short-distance small-world network. The co-opetition relationship between subway lines was mostly of a conventional type (55.56%) and unrelated type (41.67%), with only cooperative-dominant types found among the conventional types. The co-opetition effect between lines in the long-term network increased from 7.616 to 15.17, and the relationship strength of the competition and cooperation between lines increased significantly. The competition effect deflection angle of all subway lines in the network was found to be smaller than the cooperation effect declination angle. Additionally, all lines had a significant role in cooperation within the network, and cooperation played a dominant role in the co-opetition relationship.

Forward prediction for tunnel geology and classification of surrounding rock based on seismic wave velocity layered tomography

Excavation under complex geological conditions requires effective and accurate geological forward-prospecting to detect the unfavorable geological structure and estimate the classification of surrounding rock in front of the tunnel face. In this work, a forward-prediction method for tunnel geology and classification of surrounding rock is developed based on seismic wave velocity layered tomography. In particular, for the problem of strong multi-solution of wave velocity inversion caused by few ray paths in the narrow space of the tunnel, a layered inversion based on regularization is proposed. By reducing the inversion area of each iteration step and applying straight-line interface assumption, the convergence and accuracy of wave velocity inversion are effectively improved. Furthermore, a surrounding rock classification network based on autoencoder is constructed. The mapping relationship between wave velocity and classification of surrounding rock is established with density, Poisson's ratio and elastic modulus as links. Two numerical examples with geological conditions similar to that in the field tunnel and a field case study in an urban subway tunnel verify the potential of the proposed method for practical application.

Expert Intelligent Systems Integrating BIM and VR for Urban Subway Microenvironmental Health Risks Management

Expert Intelligent Systems Integrating BIM and VR for Urban Subway Microenvironmental Health Risks Management

Research on the Influence of Urban Subway on Neutral Current of High-Voltage Substations Along the Line Under Complex Environment

During the operation of the urban subway, there will be stray current leaking into the ground, which may invade the neutral point of the transformer along the line, causing the working magnetization curve of the transformer core to shift, thereby causing the direct-current (DC) bias effect of the transformer. Since there are unknown parameters in the equivalent circuit model of the stray current of the subway, in order to simplify the process of the influence of the subway operation on the neutral current, based on the analysis of the formation mechanism of stray current, a real subway line has been selected and a monitoring platform in the nearby high-voltage (HV) substation has been built to record the neutral current. Meanwhile, through the on-site recording of the entry and exit time of subway trains at four consecutive stations, the running trajectories of the subway trains can be reversed. According to the trajectory of the subway train, the recorded neutral current was re-divided into time periods, and the synergistic analysis of the neutral current and the train trajectory in the corresponding time period was carried out. The neutral current measurement results for 4 consecutive days indicate that the influence of the subway train on the neutral current of the substation is mainly the influence of the unbalanced load, and the degree of the unbalance mainly comes from the distance and the acceleration and braking of the subway train. The trajectory of the subway train is related to the amplitude of the neutral current. The closer the subway train is to the substation, the greater the impact on the neutral current.

Retracted: Intelligent Performance Evaluation of Urban Subway PPP Project Based on Deep Neural Network Model.

[This retracts the article DOI: 10.1155/2022/1536881.].

Research on the Complex Characteristics of Urban Subway Network and the Identification Method of Key Lines

Based on the complex network theory, we established a topological network of the Beijing subway under Space L, Space P, and Space C. Then, we analyzed the complex characteristics of the subway network under each topological network, proposed the global impact indexes (including aggregation impact coefficient, path length impact coefficient, network efficiency impact coefficient, and connectivity impact coefficient), and interline impact indexes (including degree centrality impact coefficient, near-centrality impact coefficient and intermediate centrality impact coefficient, the higher the value, the more obvious the effect on other lines; degree centrality sensitivity coefficient, near-centrality sensitivity coefficient and intermediate centrality sensitivity coefficient, the higher the value, the more vulnerable to the impact of other lines). At the global and local levels, it is possible to analyze the effect of different lines on the global situation and other lines. The concept of the “line importance index” is proposed to identify the key lines in the Beijing subway network. The network is characterized by scale-free and small-world characteristics under Space P, and scale-free network characteristics but no small-world characteristics under Space L and Space C. Subway Line 10, Line 9, Line 1, Line 2, and Line 5 are the five subway lines with the highest importance. Subway Line S1, Changping Line, Xijiao Line, Capital Airport Line, and Daxing International Airport Line are the five subway lines with the lowest importance.

Reducing Virus Transmission from Heating, Ventilation, and Air Conditioning Systems of Urban Subways.

Aerosols carrying the virus inside enclosed spaces is an important mode of transmission for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as supported by growing evidence. Urban subways are one of the most frequented enclosed spaces. The subway is a utilitarian and low-cost transit system in modern society. However, studies are yet to demonstrate patterns of viral transmission in subway heating, ventilation, and air conditioning (HVAC) systems. To fill this gap, we performed a computational investigation of the airflow (and associated aerosol transmission) in an urban subway cabin equipped with an HVAC system. We employed a transport equation for aerosol concentration, which was added to the basic buoyant solver to resolve the aerosol transmission inside the subway cabin. This was achieved by considering the thermal, turbulent, and induced ventilation flow effects. Using the probability of encountering aerosols on sampling surfaces crossing the passenger breathing zones, we detected the highest infection risk zones inside the urban subway under different settings. We proposed a novel HVAC system that can impede aerosol spread, both vertically and horizontally, inside the cabin. In the conventional model, the maximum probability of encountering aerosols from the breathing of infected individuals near the fresh-air ducts was equal to 51.2%. This decreased to 3.5% in the proposed HVAC model. Overall, using the proposed HVAC system for urban subways led to a decrease in the mean value of the probability of encountering the aerosol by approximately 84% compared with that of the conventional system.

Research on Environmental Suitability Evaluation of the Transfer Spaces in Urban Subway Stations

The transfer space realizes the connectivity of subway intersections. Passengers generally express that they have a poor experience in the use of this space, so improving the environmental suitability of transfer spaces at subway stations is a top priority. Based on a literature review and field research, this study established an environmental suitability evaluation system for transfer spaces and used the fuzzy comprehensive evaluation method to evaluate the environmental suitability of eight samples in Shanghai. The results showed that the evaluation results of the eight samples were ranked as follows: Hanzhong Road Station > People’s Square Station > East Nanjing Road Station > Century Avenue Station > Xujiahui Station > Laoximen Station > Jiangsu Road Station > Shanghai Railway Station. Through the analysis of the relationship between the indicators, it was found that the environmental suitability of a transfer space is greatly affected by safety and convenience, while practicality, comfort, and aesthetics were found to have a weak influence on the suitability of transfer spaces. These evaluation methods and results provide a reference for the improvement of the environmental quality of subway transfer spaces in other cities.

Effect of Using Plastic Waste Bottles on Soil Response above Buried Pipes under Static Loads

Development and population growth have made using shallow buried pipes in urban areas, highways, and subways inevitable. In this study, the performance and behavior of shallow unplasticized polyvinyl chloride (uPVC) pipes buried in sand reinforced with PET (polyethylene terephthalate) bottles produced by the end consumer filled with soil under static loads were investigated. The bottle reinforcement mattress filled with soil was vertically installed above the buried uPVC pipe inside the soil bed at the required depth; after that, backfilling was performed. The effects of the relative density of soil, placement depth, and the width of the soil-filled bottle-reinforced block were examined. The increase in relative density has shown a noticeable decrease in footing surface settlement and load transferred to the buried pipe. The test results have shown significant improvement in the ultimate bearing capacity of bottle-reinforced soil with a reduction in surface settlement. The backfill reinforced with soil-filled bottle block has shown a 71% improvement in ultimate bearing capacity. Additionally, the improvement in bearing capacity increases as the placement depth decreases and width increases. The optimum depth of placement and width of the bottle-reinforced mattress were 0.50 and 2.08 B. The proposed soil reinforcement method may be a good and relatively inexpensive alternative to traditional geosynthetic reinforcement while providing geotechnical and environmental benefits.

The Monitoring and Analysis of Land Subsidence in Kunming (China) Supported by Time Series InSAR

As urban construction has been leaping forward recently, large-scale land subsidence has been caused in Kunming due to the special hydrogeological conditions of the city; the subsidence scope has stretched out, and the subsidence rate has been rising year by year. As a consequence, Kunming’s sustainable development has seriously hindered. The PS-InSAR (Persistent Scatterer Interferometric Synthetic Aperture Radar) and the SBAS-InSAR (Small Baseline Subsets Interferometric Synthetic Aperture Radar) technologies were adopted to process the descending Sentinel-1A data stacks from July 2018 to November 2020 to monitor the land subsidence of Kunming, so as to ensure the sustainable development of the city. Moreover, the causes were analyzed. As revealed by the results, (1) the overall subsidence trend of Kunming was large in the south (Dian lakeside), whereas it was relatively small in the north. The significant subsidence areas showed major distributions in Xishan, Guandu and Jining district. The maximal average subsidence rates of PS-InSAR and SBAS-InSAR were −78 mm/a and −88 mm/a, respectively. (2) The ground Subsidence field of Kunming was analyzed, and the correlation coefficient R2 of the two methods was reported as 0.997. In comparison with the leveling data of the identical period, the root mean square error (RMSE) is 6.5 mm/a and 8.5 mm/a, respectively. (3) Based on the urban subway construction data, geological structure, groundwater extraction data and precipitation, the causes of subsidence were examined. As revealed by the results, under considerable urban subways construction, special geological structures and excessive groundwater extraction, the consolidation and compression of the ground surface could cause the regional large-area subsidence. Accordingly, the monthly average precipitation in Kunming in the identical period was collected for time series analysis, thereby indicating that the land subsidence showed obvious seasonal variations with the precipitation. The results of this study can provide data support and facilitate the decision-making for land subsidence assessment, forecasting and construction planning in Kunming.

Analysis and Research on the Thermal Environment of Subway Stations in Wuhan

With the rapid advancement of urban construction, urban subway construction in China has entered an advanced development stage and the thermal environment of subway stations has become a key factor affecting passengers' thermal comfort. To further understand the actual situation of the thermal environment of subway stations, field measurement and CFD simulation were carried out on a subway station hall in Wuhan from July to August 2021, and the thermal environment changes and influencing factors during subway operation were compared and analyzed. The present situation of the thermal environment in subway stations was analyzed, and some suggestions and measures for improving the thermal environment were put forward.

Analysis of the Degree of Convenience of Xi'an Metro Station

Urban subways have developed rapidly in recent years and has gradually become the primary choice of public transportation in big cities. Subways can effectively alleviate urban traffic congestion and improve passengers’ green travel experience, especially in the old urban areas where urban space is limited, community facilities are aging and historical features are gathered. Subways play a huge role in guiding the expansion of urban space form and improving urban landscape environment. As the only channel for passengers to get in and out of the station, the subway entrance and exit act as a bridge. It is not only the transition space between the subway station space and the urban above ground space, but also an important node for the exchange and circulation of various elements in the urban space. In this paper, the influencing factors of subway entrance and exit space environments in the old city were investigated, and relevant basis for future planning, design, and construction of subway entrances and exits were provided. Firstly, the research results and shortcomings of subway entrance and exit channels were outlined. Secondly, based on the perspective of residents and passengers, this paper uses the analytic hierarchy process to determine six first-class evaluation indexes, including safety, convenience, practicability and aesthetics. This paper constructs the evaluation system of spatial environment index of subway station entrances and exits in the old urban area of Xi’an. Lastly, according to the evaluation results, the paper puts forward the targeted optimization strategies for the current situation of the entrance and exit space environment of the subway stations in the Xi’an.

Seismic Response of a Two-Story Three-Span Subway Structural Model under High-Accelerated Geotechnical Centrifuge Shaking Table Test

The seismic response of underground structures such as subway stations is critical. However, the complex underground structure experiments under a dynamic centrifuge shaking table are significantly limited. This study conducts a shaking table test of a two-story three-span underground structure under 50 g gravitational centrifuge acceleration to investigate soil–structure interaction effects (SSI). The test is performed on a sand soil-structural model using a laminar shear box with depths of 2.5 m, and the input motion is a Parkfield wave. The experimental results indicate that the central column of the two-story three-span underground structure is the weak component during the earthquake. In addition, the numerical simulations of the soil–structure system are carried out to study the effect of buried depth and foundation soil type on the seismic response of the underground structures. The experimental and numerical results proved that the performed centrifuge test can reproduce the key seismic response characteristics of the SSI in the prototype underground structure and provided guidelines to design a similar underground structure in the future development of the urban subway systems.