Subgrade Bed Research Articles

Experimental study on the dynamic behavior of a new medium–low-speed maglev subgrade structure

Constructing medium–low-speed magnetic levitation (MLS maglev) subgrade by borrowing the construction standards of high-speed railway (HSR) subgrade can result in enormous engineering waste because the amplitude, distribution width, and dynamic coefficient of the dynamic load of MLS maglev trains are much smaller than those of HSR trains. To address this problem, an innovative subgrade structure was proposed to match the dynamic loads of MLS maglev trains. Two experimental models are constructed to study the proposed MLS maglev subgrade at train speeds of 100 km/h, 120 km/h, 160 km/h, and 200 km/h. The two models are labeled as Models I and II, and the only difference between these two models is the type of surface layer material of the subgrade bed, whereas the other structural features are the same. Specifically, the Model Ⅰ employs graded gravel, whereas the Model Ⅱ uses A group filling. The experimental results indicate that for both models, the dynamic earth pressure amplitudes are all less than 20 % of the subgrade’s self-weight at the bottom surface of the subgrade bed. In addition, the cumulative settlement of the two models is below the cumulative settlement limit. Although both models with the proposed MLS maglev subgrade exhibit excellent long-term performance, the Model Ⅰ has superior attenuation characteristics of dynamic earth pressure, and smaller acceleration and dynamic deformation than that the Model Ⅰ. This is attributed to the graded gravel used in the Model Ⅰ, which possesses a greater dynamic modulus and damping ratio than the A group filling used in the Model Ⅱ. The results presented in this study could contribute to the comprehension of the dynamic behavior of the subgrade under the dynamic loads of MLS maglev trains. More importantly, compared to the existing MLS maglev subgrades, the proposed MLS maglev subgrade has lower construction standards and is thus suitable for practical engineering applications.

Investigation on Dynamic Stability of Cement-Stabilized Expansive Soil Subgrades Subjected to Repeated Heavy-Haul Train Loads

The dynamic characteristics of the filler are intricately linked to the stability of the subgrade. In this investigation, relying on Haoji (Haolebaoji-Ji’an, China) heavy-haul railway engineering, cyclic triaxial tests were executed to scrutinize the dynamic attributes exhibited by the 3%–5% cement-stabilized expansive soil (CSES) across a series of diverse cyclic stress, confining pressures, and frequencies. Concurrently, in situ vibration trials were undertaken to dissect the dynamic characteristics inherent to the CSES subgrade. The outcomes of cyclic triaxial tests indicate that the augmentation in both the dynamic shear strength and modulus of CSES by a factor of 2–3, coupled with an escalation of the critical dynamic stress threshold by five tosix times, is attributed to the heightened internal structural density within the CSES compared to virgin expansive soil. In identical settings, it is noteworthy that the mean critical dynamic stress threshold observed for CSES surpasses that of Group A filling by a factor of 1.5–1.7. Furthermore, the maximum critical dynamic stress exhibited by CSES achieves a 1.2-fold superiority over its lime-stabilized expansive soil (LSES). The outcomes gleaned from the in situ vibration tests elucidate that, when subjected to the passage of a high-velocity train traveling at 120 km/hr, bearing the load of 25–30 tons per axle, the subgrade surface exhibits dynamic stress ranging from 98.57 to 116.07 kPa. Meanwhile, the dynamic stress undergoes a notable escalation due to rainfall infiltration, intensifying by a factor of 1.02–1.28 times its original magnitude. The influence depth of dynamic stress extends 1.4–1.6 times beyond the designed subgrade bed thickness of 2.5 m. Notably, the critical dynamic stress of the filler surpasses the dynamic stress at the same position, underscoreing the capacity of 3%–5% CSES filling for heavy-haul railways to ensure long-term dynamic stability.

Calculation method for composite modulus of flexible pile composite foundation for high-speed railway under post-work settlement source

The flexible pile composite foundation (FPCF) in deep soft soil regions often experiences post-work settlement, leading to rail irregularities in the high-speed railway (HSR). However, few studies have identified the characteristics of post-work settlement sources (PWSS) for FPCF. Hence, a calculation method for the composite modulus under the PWSS has yet to be established. Based on a full-scale field test of the FPCF for a high-speed railway (HSR), this study analysed the time-varying characteristics of the settlement and location of the PWSS. By analysing the properties of soft soil materials and the additional stress on FPCF, the formation mechanisms of PWSS were revealed. A 3D finite element model (FEM) of the FPCF-subgrade-ballastless track system was established, and the settlement curve was input at the location of the PWSS. The rationality of the numerical results was first verified through a scale-model experiment. Subsequently, based on the computational results of typical settlement transfer processes, it was found that settlement undergoes fast settlement, transitional following settlement, and following settlement during its transfer from the underlying layer to the rail. During the settlement transfer process, the reinforcement zone and embankment significantly reduced the settlement irregularities, which made the settlement of the ballastless track follow that of the subgrade bed. The pile compression modulus primarily regulated the rail amplitude, whereas the area replacement ratio primarily regulated the rail wavelength. Finally, based on the settlement transfer results, a method for determining the composite modulus was proposed, and a formula for calculating the composite modulus was derived. This formula contributes to a faster analysis of the influence of the PWSS of the FPCF on rail smoothness.

Assessment of the Simultaneous Use of Asphalt Concrete and Foam Concrete as a Full Cross-Section Layer in the Substructure of the China Railway Track System (CRTS) III Slab Ballastless Tracks

This paper proposes the simultaneous application of asphalt concrete (AC) and foam concrete (FC) as a full cross-section layer in the substructure of the high-speed railway ballastless track. AC is designed as a waterproof layer and is placed as part of the top surface layer of the subgrade bed where the mixture is made from three types of performance grade (PG) binders (normal, high, and modified grade binders) named PG64-22, PG70-22, and PG76-22, respectively. FC is designed as a subgrade material to replace the traditional material in the bottom layer of the subgrade bed. It has a target density of 500–700 kg/m3 and polypropylene (PP) fiber is added at a ratio of 0.25% and 0.40% by volume. The mechanical properties of both materials are reviewed from the authors’ previous research, in which they were assessed through laboratory testing in accordance with ASTM standards. In addition, the numerical model analyzed the dynamic response of the whole structure when AC and FC were applied as full cross-section layers under different thicknesses of FC and then compared with traditional track structures. Laboratory test results indicate that PG76-22, or modified asphalt binder and FC at a density of 600 kg/m3 with the addition of 0.25% PP fiber, is suitable for use in slab ballastless tracks. In addition, the prediction of the model shows that AC at a thickness of 0.12 m and FC at a thickness of 1.00 m can reduce the stress and vibration of the track structure better than the traditional structure, resulting in stabilization and long-term service life.

Research on micro-influence mechanism of fine particles on dynamic characteristics of graded macadam materials in heavy-haul railway subgrade beds

Fine particle content is an important physical state parameter of graded macadam materials used in heavy-haul railway subgrade beds. Understanding the micro-influence mechanism of the fine particle content on the filler’s dynamical characteristics is of great significance. In this study, the granular system generation logic in PFC3D software was modified to enhance efficiency and quality, and the three-dimensional discrete element numerical model of graded macadam materials dynamic triaxial specimen was established. The servo program was used to simulate the dynamic triaxial numerical test confining pressure and dynamic loading conditions. A series of laboratory tests were performed using a GCTS large-scale cyclic triaxial test system to calibrate and verify the numerical model. Finally, dynamic triaxial numerical tests were conducted for graded macadam materials with different fine particle contents. These tests had the aim of determining the variation law for the contact force chain distribution, mechanical coordination number, average rotational angular velocity, and energy dissipation of the specimens under cyclic loading with the increase of fine particles and then revealing the influence mechanism of fine particles on the dynamic characteristics of graded macadam materials used in heavy-haul railway subgrade beds from a microscopic perspective. The study’s findings serve as a framework for describing the mechanism of subgrade bed disease occurrence and proposing effective remediation measures.

Field Comprehensive Testing and Study on New Subgrade Structure of High-Speed Railway in Diatomaceous Earth Region

Part of the Hang-Tai High-speed Railway (Hangzhou via Shaoxing to Taizhou in Zhejiang Provence) in China passes through the diatomaceous earth area, which is the first time in the history of Chinese high-speed railway construction. This type of soil has significant compressibility, swelling and disintegration. Diatomaceous earth also shows a sharp reduction in strength when exposed to water, which severely impacts the safety of the project. However, no studies have been carried out on the engineering practice of building a high-speed railway in the diatomaceous earth area in China. Moreover, there is limited experience in the construction of ballastless track through the diatomaceous earth area. In order to ensure the stability of the high-speed railway subgrade in diatomaceous earth area, and considering the high level of precipitation in the location of this railway, a kind of waterproof and drainage subgrade (WDS) is proposed to reduce the influence of precipitation on the strength of the diatomaceous earth foundation. The subgrade has a flexible waterproof and drainage layer (WDL) inside, which consists of capillary waterproof and drainage plates and medium-coarse sand. In the present study, field tests including immersion tests and excitation test are carried out on a subgrade test section to verify the subgrade structure. The tests mainly focus on construction technology, waterproof performance and dynamic characteristics. The studies show that the subgrade bed with the WDL can effectively avoid the diatomaceous earth foundation from rainfall interference and maintain the long-term stability of the subgrade. The flexible WDL in WDS has a significant energy dissipation effect in comparison with the traditional subgrade (TS) filler and can play a key role in vibration damping, promoting the attenuation of dynamic response in the downward and cross-sectional directions within the subgrade. The dynamic response of the WDS attenuates along the depth. In comparison with the existing high-speed railway subgrade measured data, its dynamic response attenuation coefficient is within acceptable limits. The laying of the WDL does not change the subgrade dynamic characteristics transfer law. The proposed structure meets the requirements of ballastless track construction for high-speed railways, and the WDL can be used in the design of high-speed railways for enhanced drainage protection in diatomaceous soil areas or other special soil areas.

Physical modeling of long-term dynamic characteristics of the subgrade for medium–low-speed maglevs

To investigate the dynamic characteristics and long-term dynamic stability of the new subgrade structure of medium–low-speed (MLS) maglevs, cyclic vibration tests were performed under natural and rainfall conditions, and the dynamic response of the subgrade structure was monitored. The dynamic response attenuation characteristics along the depth direction of the subgrade were compared, and the distribution characteristics of the dynamic stress on the surface of the subgrade along the longitudinal direction of the line were analyzed. The critical dynamic stress and cumulative deformation were used as indicators to evaluate the long-term dynamic stability of the subgrade. Results show that water has a certain effect on the dynamic characteristics of the subgrade, and the dynamic stress and acceleration increase with the water content. With the dowel steel structure set between the rail-bearing beams, stress concentration at the end of the loaded beam can be prevented, and the diffusion distance of the dynamic stress along the longitudinal direction increases. The dynamic stress measured in the subgrade bed range is less than 1/5 of the critical dynamic stress. The postconstruction settlement of the subgrade after similarity ratio conversion is 3.94 mm and 7.72 mm under natural and rainfall conditions, respectively, and both values are less than the 30 mm limit, indicating that the MLS maglev subgrade structure has good long-term dynamic stability.

Dynamic Strength Characteristics of Cement-Improved Silty Clay under the Effect of Freeze-Thaw Cycles

In the seasonally frozen soil regions of northern China, silty clay is widely used as a subgrade bed filler in heavy-haul railway construction. In this paper, the influence of freeze-thaw cycles on the dynamic strength properties (strength parameters and dynamic critical stress) of silty clay fillers before and after cement improvement was investigated by a series of dynamic triaxial tests under different confining pressure conditions, and the test results were quantified to analyze the improvement effects of cement improvement. The results show that cement modification can significantly improve the dynamic strength parameters (dynamic strength, dynamic strength index, and critical dynamic stress) of silty clay before and after freezing and thawing. The dynamic strength of cement-improved silty clay (CSC) was improved by 2.8 to 5.2 times compared to silty clay, and a high level of dynamic strength can be maintained after multiple freeze-thaw cycles. The dynamic cohesion was increased by 1.5 to 3 times and the dynamic internal friction angle was increased by 1.5 to 4 times. The attenuation rate of the critical dynamic stress of CSC with the number of freeze-thaw cycles was greater than that of the plain filler, while the relative lifting effect of the critical dynamic stress of the cement improvement was significant after three freeze-thaw cycles, and the maximum value was reached at a cycle number of three, with a relative increase of 2.5 times. A new index of critical dynamic stress attenuation of CSC for freeze-thaw cycles was introduced, which provides a useful reference for subgrade improvement and reinforcement along the silty clay railway in northern China.

Multi-objective optimisation methodology for stiffness combination design of bridge-embankment transition zones in high-speed railways

Bridge-embankment transition zones, in which unexpected amplifications of vehicle-track interactions and infrastructure degradation are concentrated, are critical areas of high-speed railways. To mitigate the dynamic issues induced by uncertain filler stiffness, an available stiffness combination of transition zone components is required. Combining with robust designs, a multi-objective optimisation methodology is proposed to determine the compromise design of transition zones, which considers trade-offs among dynamic stress, dynamic displacement and system robustness. An optimal stiffness combination of subgrade bed surface layer (E1) and graded broken stone area (E2) in a typical bridge-embankment transition zone is presented by employing this methodology. The results indicate that the Pareto fronts visually show the dynamic response contradiction. Based on the conflicting evaluation criteria, i.e., minimizing the displacement and stress of the subgrade and maximizing the system robustness, the knee points objectively render the best compromise design with E1 and E2 values of 1200 and 1000 MPa, respectively. Not only the dynamic response of the vehicle-track-subgrade system but also the negative effect brought by filler uncertainties are well controlled. Such a methodology is available for configuration design of projects in which uncertainties exist and design criteria are hard to weigh.

Line Shape Analysis and Dynamic Response of Ballastless Track during Jacking Rectification Fixing.

In order to study the railway line deformation and dynamic response of ballastless track structure under train load during jacking rectification fixing, a three-dimensional numerical model of the CRTS II slab ballastless track on subgrade is established by using the finite element method. The line deformation rule and local damage rule of ballastless track under jacking force are analyzed. The dynamic response laws of track structure and subgrade bed are compared considering four different connection modes between the base plate and subgrade bed under different train speeds in the process of jacking rectification fixing. The results show that jacking force and dissociation length have a small influence on the deviation value and the critical jacking force should be smaller than 375 kN in single point jacking. Under the condition of multi-point jacking, when the jacking loading length equals to 5 slabs, the critical jacking force should be smaller than 275 kN and the maximum lateral deviation value is about 22.11 mm. It is necessary to restrict the speed of passing trains to no more than 150 km/h during the jacking rectification fixing for dissociation condition without temporary restraint. When temporary restraint is applied, the speed of the train can be increased appropriately according to the actual situation. The above study results could be used as a theoretical reference for the ballastless track deviation correction.

Study on prevention and treatment of capillary water rise for construction waste expansive soil subgrade

This paper solves the technical difficulties of the capillary water rising height of the construction waste improved expansive soil subgrade (CWS), analyses the capillary water rising mechanism of the construction waste improved expansive soil subgrade cushion and evaluates the improvement effect of the improved cushion by comparing different working conditions with simulation software. The analysis results show that: (1) a reasonable ratio of the improved subgrade bed can effectively improve the porosity of the cushion layer, thus effectively inhibiting capillary water rising height. (2) the practical improvement of the thickness of the cushion layer can slow the matrix suction effect of expansive soil, (3) the construction waste had a more significant influence on the particle gradation for the capillary water rise, poor grading cushion layer has a significant influence on the capillary water rise, (4) the construction waste particles in the improved graded cushion of construction waste have strong water absorption, which improves the matrix suction effect of expansive soil. (5) Through experiments and simulation tests, this paper concludes that the optimal grade subgrade bedding of construction waste expansive soil is of specific theoretical value and practical significance for developing the construction waste material transformation subgrade. Keyword: Construction Waste, the capillary water rise mechanism, well-graded CWS.

Use of COVID-19 personal protective equipment pollutants to improve physical properties of silty sand in Yellow River flooded area

Silty sand in the Yellow River flood area (YRFA) of China exhibits a low cohesive force and water-holding capacity. Its direct use for subgrade filling leads to phenomena such as subgrade bed depression, slurry, and mud. Therefore, from the perspective of waste utilization, this study investigates disposable protective clothing (DPC) and disposable nitrile gloves (DNG) produced due to COVID-19 in combination with silty sand as subgrade filling for the YRFA. Through an unconfined compressive strength and permeability test, we studied the influence of personal protective equipment (PPE) with different moisture content, concentration, and size on the strength and permeability of mixed samples. Further, we discuss its impact on the improvement of subgrade filling in YRFA. Results show that adding DPC improves both the compressive strength and ductility of the sample. However, the impermeability of the sample remains unchanged. In contrast, the addition of DNG does not increase the compressive strength and ductility of the sample at the same time, whereas it significantly improves its impermeability. The size of PPE has a considerable influence on the compressive strength of the mixed sample, whereas it does not affect the sample permeability. This method makes use of the PPE waste produced due to COVID-19, reduces environmental pollution, and provides a novel concept for improving silty sand in the YRFA.

Frost resistance and mechanical degradation characteristics of cement-stabilized macadam under the long-term freezing–thawing cycles

• Three different cement contents and removal of two different fine particle sizes of cement-stabilized macadam (CSM) were tested. • The mechanical strength characteristics were degraded by long-term freezing−thawing cycles. • The removal size of larger fine particles had adverse effects on the strength and degradation rate of CSM. • The increase in the cement content led to an increase in the frost resistance and strain energy density of CSM. • The initial elastic modulus determined using unconfined compressive strength tests could be applied to characterize the stiffness of CSM for high-speed railway lines. The high-speed railway subgrade in seasonal frozen soil regions should be able to resist long-term freezing and thawing cycles. In consideration of cement-stabilized macadam (CSM) as the subgrade surface material needs to have resistant to freeze–thaw strength deterioration. This study aimed to evaluate the mechanical degradation characteristics of CSM under the long-term freezing-thawing cycles by the unconfined compressive strength tests. Three different cement contents (3 %, 5 %, and 7 %) and removal of two different fine particle sizes (0.1 and 0.25 mm) of CSM were studied and analyzed. The number of freezing–thawing cycles was set to 0, 1, 3, 5, 7, 10, 15, 20, 30, 40, and 50, respectively. Based on the unified calculation model of strength deterioration, evaluation index of frost resistance, and analysis of strain energy density failure, C 5 P 0.1 could be used in seasonal frozen soil regions. If the subgrade bed had requirements for permeability, fine particles with larger sizes could be removed. The results indicated that, first, the strength of fine particles with larger size removal was smaller and the strength deterioration was faster under the same cement content. Second, when the cement content was higher, CSM showed better resistance to freezing–thawing strength degradation. Further, when the cement content increased, the increase in the amplitude of strain energy density varied significantly; the increasing effect was the best when the cement content increased from 3 % to 5 %. Finally, the initial elastic modulus of the Duncan-Chang hyperbolic model was used to characterize the stiffness of CSM in the high-speed railway subgrade bed. This study revealed the mechanical degradation characteristics of CSM under the long-term freezing–thawing cycles with different cement contents and removal sizes of fine particles. The findings were useful for understanding the mechanical behavior of CSM under the long-term freezing–thawing cycles.

Accumulative Plastic Deformation of the Improved Completely Weathered Granite Subgrade in High-Speed Railway

The improvement and reuse of completely weathered granite (CWG) with poor engineering properties for backfilling embankments can help protect the environment and bring great economic benefits. The embankment of high-speed railways demands an extremely low additional subgrade settlement under long-term dynamic loads. To analyze the applicability of the improved CWG used for subgrade filling, a series of laboratory dynamic triaxial and field cyclic loading tests were carried out. On the basis of the test results, a calculation model of accumulative plastic strain considering the dynamic stress ratio and the cement content was established. Using the proposed model and the numerically calculated dynamic stress, the accumulative plastic deformation of an embankment filled with improved CWG under various driving conditions was calculated and discussed. The variation of accumulative plastic deformation of the improved CWG with loading cycles could be described by a power function. The dynamic stress level had a significant influence on the accumulative plastic strain, and the strain in the bottom layer of the subgrade bed was greatly attenuated. Accordingly, most of the accumulative plastic deformation attenuation occurred within the depth range of the subgrade bed. The final total accumulative plastic deformations of the embankment were less than 2 mm (5 mm for the limit value) after 4 million times of high-speed train loadings, proving that the improved CWG can be used for subgrade filling. The influence of the axle load on accumulative plastic deformation was more significant than that of the train speed. The proposed calculation model of the accumulative plastic strain could provide valuable reference for similar railway engineering.

A full-section asphalt concrete waterproof sealing structure for the high-speed railway subgrade

PurposeThis study aims to investigate the service performances of a new full-section asphalt concrete waterproof sealing structure (FSACWSS) for the high-speed railway subgrade through on-site tracking, monitoring and post-construction investigation.Design/methodology/approachBased on the working state of the waterproof sealing structure, the main functional characteristics were analyzed, and a kind of roller-compacted high elastic modulus asphalt concrete (HEMAC) was designed and evaluated by several groups of laboratory tests. It is applied to an engineering test section, and the long-term performance monitoring and subgrade dynamic performance testing system were installed to track and monitor working performances of the test section and the adjacent contrast section with fiber-reinforced concrete.FindingsResults show that both the dynamic performance of the track structure and the subgrade in the test section meet the requirements of the specification limits. The water content in the subgrade of the test section is maintained at 8–18%, which is less affected by the weather. However, the water content in the subgrade bed of the contrast section is 10–35%, which fluctuates significantly with the weather. The heat absorption effect of asphalt concrete in the test section makes the temperature of the subgrade at the shoulder larger than that in the contrastive section. The monitoring value of the subgrade vertical deformation in the test section is slightly larger than that in the contrastive section, but all of them meet the limit requirements. The asphalt concrete in the test section is in good contact with the base, and there are no diseases such as looseness or spalling. Only a number of cracks are found at the joints of the base plates. However, there are more longitudinal and lateral cracks in the contrastive section, which seriously affects the waterproof and sealing effects. Besides, the asphalt concrete is easier to repair, featuring good maintainability.Originality/valueThis research can provide a basis for popularization and application of the asphalt concrete waterproof sealing structure in high-speed railways.

Field investigation of dynamic responses of a culvert-embankment transition zone in high-speed railway

With the rapid development of high-speed railway, special areas such as transition zones have attracted increasing attention. To analyze dynamic responses of subgrade bed in the culvert-embankment transition zone, field tests were carried out on the Beijing-Guangzhou high-speed railway in China. The field instrumentation data undergone a series of signal processing operations were classified by train running conditions. The amplitude and frequency characteristics of dynamic responses were then analyzed comprehensively. The synergistic influences of train speed, culvert approaching/departing, and axle load were discussed. It was found that the abrupt change in track supporting condition caused longitudinal fluctuation of dynamic displacement responses, and the dynamic stress responses in the longitudinal direction resembled the “V” shape. The culvert approaching/departing exerted a significant influence on the longitudinal distribution of dynamic displacement responses, which exhibited a linearly ascending trend with increasing train speed. The influence of increasing train speed on dynamic stress responses attenuated rapidly with increasing depth. The dominant frequencies of subgrade bed were more significantly related to train geometry rather than track and substructure geometry, and the dominant frequencies were particularly affected by the periodic excitation frequency associated with the railcar length. The proportion of frequency components within 80 Hz became greater as the longitudinal distance away from the culvert increased. The dynamic stress responses were more sensitive to axle load than train speed. The dynamic displacement responses were more significantly affected by train speed than axle load for train speed exceeding beyond about 260 km/h. The findings of this study could provide technical reference for long-term operation and maintenance of high-speed railway track in transition zones.

Polyurethane grouting materials with different compositions for the treatment of mud pumping in ballastless track subgrade beds: properties and application effect

Mud pumping in subgrade beds under ballastless tracks will deteriorate the dynamic performance of infrastructure under railway lines, reduce the smoothness of the railway lines, and seriously affect the comfort and safety of the trains. Due to their good mechanical properties, two-component polyurethane materials can be used for grouting to treat the fouling problems caused by ballastless track mud pumping. To develop a polyurethane formula suitable for the treatment of ballastless track mud pumping, we first performed indoor experiments to investigate the mechanical properties and gelation time of polyurethane elastomers synthesized with different raw material composition ratios, to determine an optimal composition ratio of the raw materials. Then, we conducted a dynamic field test to verify the remediation effect of the polyurethane material fabricated according to the design ratio. The results showed that polyurethane grouting material with the selected design ratios improved the contact characteristics between the surface layer of the subgrade bed and the base plate in the area, coordinating the dynamic response between the track structure and the subgrade bed. Thus, the obtained polyurethane grouting material could be used to renovate mud pumping areas of ballastless tracks with a good treatment effect.

Analysis of Behaviors of the Railway Subgrade with a New Waterproof Seal Layer.

This study proposes a new waterproof sealing layer to reduce the impact of water on subgrade beds. The proposed waterproof sealing layer is composed of a polyurethane adhesive (PA) mixture, which aims to control interlaminar deformation and prevent seepage. A variety of laboratory tests were first performed to analyze the attenuation characteristics and mechanical properties of various polyurethane polymer (PP)-improved gravel mixtures under thermohydraulic coupling effects. In addition, a waterproof performance model test of the PP-improved gravel layer was conducted to investigate its waterproof and drainage performance and hydraulic damage mechanism. Finally, the feasibility and effectiveness of the surface structure of the waterproof drainage subgrade bed containing the PA mixture was tested in combination with the treatment project of the Ciyaowan station of the Baoshen heavy-haul railway. According to the experimental and model results, (1) the waterproof layer containing the polyurethane mixture exhibited satisfactory stiffness, elasticity and flexibility. The waterproof layer containing the polyurethane mixture also controlled the deformation between layers, and its mechanical properties remained stable. (2) The waterproof layer with the dense polyurethane mixture performed well in terms of the waterproof aspect, and no infiltration occurred under cyclic load (3). Long-term field monitoring revealed that the effect of the implementation of a PP-improved gravel layer to treat mud pumping was remarkable. The settlement of the PP-improved gravel layer only reached 13.21 mm, and the settlement remained stable in the later stage.

Experimental Study of the Dynamic Characteristics of a New Antidrainage Subgrade Structure for High-Speed Railways in Diatomaceous Earth Areas.

The experience needed to carry out engineering and construction in diatomaceous earth areas is currently lacking. This project studies the new Hang Shaotai high-speed railway passing through a diatomaceous earth area in Shengzhou, Zhejiang Province, and analyzes the hydrological and mechanical properties of diatomaceous earth on the basis of a field survey and laboratory. Moreover, a new antidrainage subgrade structure was proposed to address the rainy local environment, and field excitation tests were performed to verify the antidrainage performance and stability of the new subgrade structure. Finally, the dynamic characteristics and deformation of the diatomaceous earth roadbed were examined. The hydrophysical properties of diatomaceous earth in the area are extremely poor, and the disintegration resistance index ranges from 3.1% to 9.0%. The antidrainage subgrade structure has good water resistance and stability under dynamic loading while submerged in water. After 700,000 loading cycles, the dynamic stress and vibration acceleration of the surface of the subgrade bed stabilized at approximately 6.37 kPa and 0.94 m/s2, respectively. When the number of excitations reached 2 million, the settlement of the diatomaceous earth foundation was 0.08 mm, and there was basically negligible postwork settlement of the diatomaceous earth foundation. These results provide new insights for engineering construction in diatomaceous earth areas.

Effect of lime-stabilised copper slag cushion on swelling behaviour of highly plastic clay

Clay soil is prone to seasonal volume change due to variation in water content. The use of waste materials in civil engineering, especially in road construction, has been in vogue all over the world due to the development of road infrastructure. Copper slag is one such waste material available abundantly from the copper industry in India. This paper discusses the heave and swelling of copper slag–cushioned clayey subgrade prepared in a model test tank. The results revealed that the methodology adopted is effective in reducing the swelling of clay. The thickness ratios of the stabilised copper slag cushion and clayey subgrade bed adopted are 0.25, 0.5, 0.75 and 1.0. The copper slag cushion is stabilised with lime content varying from 2 to 10%. A reduction in heave of clayey soil subgrade bed is observed with an increase in the percentage of lime and the thickness of the copper slag cushion. For a cushion–clay soil subgrade thickness ratio h c/h s = 1, the reduction in the heave of a clayey subgrade bed is 84.4%. The swell potential observed from the present study follows well the trend obtained from the rectangular hyperbolic model.