Xuecheng Bian

Cumulative settlement of track subgrade in high-speed railway under varying water levels

Water-level variation in the subgrade of high-speed railway influences the cumulative settlement of the subgrade. Full-scale model test on high-speed railway under varying water levels within the subgrade was conducted in this research. Dynamic soil pressures in different depths within the subgrade and cumulative settlements of subgrade were measured under different train speeds. The results demonstrated that the dynamic soil pressure increases significantly after the first wetting–drying cycles and then remains stable. The measured dynamic soil pressure was compared to the requirements in the current design method. Increasing train speed leads to higher cumulative settlement. Furthermore, a modified model is proposed in the article for the determination of cumulative settlement considering the influence of initial stress state, material properties of subgrade, and dynamic stresses of train loadings.

A full-scale physical model test apparatus for investigating the dynamic performance of the slab track system of a high-speed railway

This study involves the development of a full-scale physical model test apparatus to investigate the dynamic performance of the slab track system of a high-speed railway. A portion of a railway ballastless track with embankment and underlying soil strata was built inside a steel box (15 m long, 5 m wide and 6 m high). A sequential-load-generating system consisting of eight high-performance actuators was developed to apply a load that was equivalent to that generated by a train moving at speeds up to 360 km/h. Two loading conditions can be implemented in the tests, a stationary cyclic loading (using one actuator) or a simulated moving train loading (using all eight actuators). Three test cases were performed to demonstrate the capacity of the proposed test facility. The first case was to test the load-sharing ratio of fasteners beneath the rails to determine the load distribution on the track slab. A formula, based on the test results, was proposed to determine the transfer process of the loads between the train wheel and the rail fasteners. The second case was a stationary cyclic loading test, which applied loads with varying frequencies to the rails using a single actuator. The resonance frequency of the track/embankment/soil system was determined in these tests to be about 16 Hz, which coincided with theoretically computed results and field measurements. The final case was to simulate the dynamic excitation resulting from the passage of a train on the track at various speeds (up to 360 km/h). The test results on track vibrations in the physical model test showed reasonably good agreement with field measurements. These three test cases fully demonstrate the ability of the newly developed full-scale test facility to simulate dynamic excitation from trains and its ability to explore the dynamic performance of railway structures under train loadings.

Impact of water level rise on the behaviors of railway track structure and substructure: full-scale experimental investigation

Extreme weather events, such as floods and heavy rainfall, have become increasingly severe worldwide. Railways can be exposed to high water levels, which may decrease their performance and jeopardize the safety of running trains. This paper evaluates the performance of high-speed railways under various water levels. A full-scale physical model of a ballastless railway was constructed in a test box measuring 15 m wide, 5 m long, and 6 m deep. Water levels in the physical model were raised and lowered with a water level control system. Stationary cyclic loading and train moving loading tests were carried out at four typical water levels: at the subsoil bottom, at the subsoil surface, at the subgrade surface, and falling back to the subsoil surface. Variations in the water level of the subsoil had a minor influence on the dynamic performance of the railway infrastructure. When the water level rose in the subgrade, the track modulus degraded significantly. Both displacement and tensile strain of the track structure increased rapidly with the submerged sub-grade. The transverse distribution of the contact pressure under the track structure changed significantly with variations in the water level. Fine particles were observed to migrate out from the subgrade; this condition became more serious when traffic loading was applied. Appropriate measures should be taken to prevent water level rise in the subgrade.

A practical approach for calculating the settlement and storage capacity of landfills based on the space and time discretization of the landfilling process

The settlement of any position of the municipal solid waste (MSW) body during the landfilling process and after its closure has effects on the integrity of the internal structure and storage capacity of the landfill. This paper proposes a practical approach for calculating the settlement and storage capacity of landfills based on the space and time discretization of the landfilling process. The MSW body in the landfill was divided into independent column units, and the filling process of each column unit was determined by a simplified complete landfilling process. The settlement of a position in the landfill was calculated with the compression of each MSW layer in every column unit. Then, the simultaneous settlement of all the column units was integrated to obtain the settlement of the landfill and storage capacity of all the column units; this allowed to obtain the storage capacity of the landfill based on the layer-wise summation method. When the compression of each MSW layer was calculated, the effects of the fluctuation of the main leachate level and variation in the unit weight of the MSW on the overburdened effective stress were taken into consideration by introducing the main leachate levels proportion and the unit weight and buried depth curve. This approach is especially significant for MSW with a high kitchen waste content and landfills in developing countries. The stress-biodegradation compression model was used to calculate the compression of each MSW layer. A software program, Settlement and Storage Capacity Calculation System for Landfills, was developed by integrating the space and time discretization of the landfilling process and the settlement and storage capacity algorithms. The landfilling process of the phase IV of Shanghai Laogang Landfill was simulated using this software. The maximum geometric volume of the landfill error between the calculated and measured values is only 2.02%, and the accumulated filling weight error between the calculated value and measured value is less than 5%. These results show that this approach is practical for satisfactorily and reliably calculating the settlement and storage capacity. In addition, the development of the elevation lines in the landfill sections created with the software demonstrates that the optimization of the design of the structures should be based on the settlement of the landfill. Since this practical approach can reasonably calculate the storage capacity of landfills and efficiently provide the development of the settlement of each landfilling stage, it can be used for the optimizations of landfilling schemes and structural designs.

Probabilistic Analytical Model for Settlement Risk Assessment of High-Speed Railway Subgrade

AbstractIt is crucial for the design and operation of a high-speed railway to estimate and control the accumulative settlement of the subgrade induced by cyclic train loading. In this study, an analytical model, considering the effect of the initial stress state, is proposed to predict the accumulative settlement of high-speed railway subgrade. Dynamic load triaxial tests are conducted to determine the parameters involved in the computational model. Full-scale model experiments are carried out to verify the effectiveness of the proposed computational model in predicting the accumulative settlement of high-speed railway subgrade. A probabilistic analytical model is developed for a reliability-based settlement risk assessment of the subgrade by considering the uncertainties and randomness of the relevant parameters. The coefficient of variation (COV) of the dynamic stress on the subgrade surface caused by train loading is derived from field data measured on the Wuhan-Guangzhou High-Speed Railway, China. A M...

Cyclic and Postcyclic Triaxial Testing of Ballast and Subballast

AbstractBallast and subballast are two layers of noncohesive, granular materials with different grain sizes filled in the track substructure to bear traffic loadings from train passages. The performance and sustainability of a track substructure depend on both of these two granular layers. This paper presents the experimental results of cyclic triaxial testing and postcyclic triaxial testing on individual ballast and subballast materials. Also, to investigate mechanical behavior at the ballast/subballast interface, cyclic tests were performed on composite specimens consisting of the upper half ballast layer and underlying half subballast layer. These cyclic tests indicated that the existence of the ballast/subballast interface depresses the development of permanent axial or volumetric deformation at the interface zone, and this effect vanishes as the initial compaction and confining pressure increase. Postcyclic failure tests show that the ballast/subballast interface zone’s strength is close to that of t...

An equivalent-time-lines model for municipal solid waste based on its compression characteristics

Municipal solid waste (MSW) demonstrates a noticeable time-dependent stress-strain behavior, which contributes greatly to the settlement of landfills and therefore influences both the storage capacity of landfills and the integrity of internal structures. The long-term compression tests for MSW under different biodegradation conditions were analyzed. It showed that the primary compression can affect the secondary compression due to the biodegradation and mechanical creep. Based on the time-lines model for clays and the compression characteristics of MSW, relationships between MSWs viscous strain rate and equivalent time were established, and then the viscous strain functions of MSW under different biodegradation conditions were deduced, and an equivalent-time-lines model for MSW settlement for two biodegradation conditions was developed, including the Type I model for the enhanced biodegradation condition and the Type II model for the normal biodegradation condition. The simulated compression results of laboratory and field compression tests under different biodegradation conditions were consistent with the measured data, which showed the reliability of both types of the equivalent-time-lines model for MSW. In addition, investigations of the long-term settlement of landfills from the literature indicated that the Type I model is suitable for predicting settlement in MSW landfills with a distinct biodegradation progress of MSW, a high content of organics in MSW, a short fill age or under an enhanced biodegradation environment; while the Type II model is good at predicting settlement in MSW landfills with a distinct progress of mechanical creep compression, a low content of organics in MSW, a long fill age or under a normal biodegradation condition. Furthermore, relationships between model parameters and the fill age of landfills were summarized. Finally, the similarities and differences between the equivalent-time-lines model for MSW and the stress-biodegradation model for MSW were discussed.

Full-Scale Model Testing on Ballasted High-Speed Railway: Dynamic Responses and Accumulated Settlements

High-speed trains generate much higher vibrations in track structures than conventional trains and intensive train passages (e.g., on the Beijing–Shanghai high-speed railway line where the train passage interval is less than 5 minutes) cause accumulated permanent settlement in the railway track substructures, which will decrease track performance and jeopardize the safety of trains. Since very few field measurements on ballasted high-speed railways are available in literature, this paper presents experimental results of vibration velocity, dynamic soil stress, and the accumulated settlement of a ballasted high-speed railway from a full-scale model testing facility with simulated trains moving loads at various speeds. A portion of a realistic ballasted railway consisting of track structure, ballast layer, subballast, embankment, and piled foundation was constructed in a larger box. An eight-actuator sequential loading system was used to generate equivalent vertical loadings on the track structure for simulating the dynamic excitations due to train movements. Dynamic stresses measured in the track substructure layers (ballast, subballast, and embankment) were found to be strongly dependent on train speeds especially for speeds higher than 144 km/h. It was found that both the vibration velocity and the dynamic soil stress were greatly amplified as the train speed increased to 300 km/h, and the ballast layer effectively reduced the vibrations transmitted from the track structure to underlying soil. The accumulated settlement of the substructure did not reach a stable state even after 100,000 moving train loads at a speed of 300 km/h.

Field Test and Analysis on Subway Train Induced Vibration

With the development of society and the high-speed growing of urban population, traffic jam is becoming an increasing seriousness to people’s normal life. Subway construction as a result can be an efficient means to solve this problem. However, vibration induced by subway’s operation can seriously affect the structure of subway tunnel and the environment around it. So, getting into a thorough research of the environmental vibration response becomes an important issue. This paper introduces a field vibration test on Ningbo subway tunnel at different spots of track slab and tunnel lining, compares the vibrating performance at different spots. Then the propagating and damping properties of vibration are analyzed.

Effects of One Weak Interlayer on Seismic Response of Municipal Solid Waste Landfill

Weak interlayers often exist in the municipal solid waste (MSW) landfills due to the dumping of urban sewage sludge with MSW and the use of soil covers, which have significant influences on the seismic response of landfills. Forty one dimensional (1D) landfill profile models subjected to different ground motion inputs and one weak interlayer with various depths and thicknesses in the MSW body were developed, and 1D frequency independent equivalent-linear method was used to explore the influences of the depth and thickness of one weak interlayer on the dynamic behavior of landfills. Considering the variation of unit weight and shear wave velocity of MSW with the buried depth, and selecting the rational dynamic parameters of MSW and soils, the horizontal acceleration and equivalent shear strain were calculated by the direct stiffness method of EDT (Elastodynamics Toolbox for MATLAB) software. The results showed the maximum value of horizontal acceleration happens about 10 m below of the weak interlayer. And the weak interlayer’s influence on the maximum horizontal acceleration of MSW body declines after it reaches a critical thickness. In addition to that, the equivalent shear strain at the weak interlayer decreases with the increase of weak interlayer’s thickness, and hence thin weak interlayers in the MSW body should be avoided.