Subgrade Structure Research Articles

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.

A Study on Development of New Type Rubber Boot for Sleeper Floating Track System (STEDEF): Materials and Shapes

Urban railway sleeper floating track (STEDEF) reduces the block vibration transmitted to the subgrade structure by structurally separating the sleeper and the concrete bed, using a rubber boot and a resilient pad. Recently, the replacement of rubber boot material (SBR) after long-term wear and tear has become of utmost importance because of durability problems such as deformation, tearing, and abrasion. This study investigates rubber boots—a component of the urban railway sleeper floating track—to resolve these concerns and proposes the material and shape of a novel rubber boot. The proposed rubber boot reduces the maximum displacement and strain by more than 83% and 90%, respectively, compared with the existing rubber boots. In addition, the results of numerical analysis and indoor tests show that type 3 rubber boots can prevent displacement and stress generation in rubber boots.

Using Foamed Concrete Layer to Optimize the Design of Pavement and Subgrade Structures: from the Perspectives Economy and Durability

Using Foamed Concrete Layer to Optimize the Design of Pavement and Subgrade Structures: from the Perspectives Economy and Durability

Freeze-thaw erosion mechanism and preventive actions of highway subgrade soil in an alpine meadow on the Qinghai-Tibet Plateau

Freeze-thaw erosion is widely distributed in China, where the difference of natural conditions brings a series of characteristics for different regions. The alpine meadow area of the Qinghai-Tibet Plateau (QTP) has a special climate, geography, and hydrogeology environment, with multiple road frost heaves and frost boiling failures, which seriously threaten the highway constructions in the QTP. This study summarizes the freeze–thaw geology of the southeastern QTP. For the highway constructions in the frozen soil areas, multiple problems such as frost heave, thawing settlement, freeze–thaw weathering, and pavement cracks and highway operation and management issues may rise as a result of the activity of the freeze–thaw geology, which damages the subgrade structure. To prevent the occurrence of possible hazards and reduce hazards and risks, this study discusses a series of countermeasures and suggestions. A case history is referred to, specifically the Budongquan highway in the QTP, where the study section is conducted, to evaluate the effectiveness of the adopted treatment measures. The results show that these treatment measures effectively prevent the surface water from infiltrating into the subgrade in the rainy season, block the migration of groundwater to the upper subgrade during the freezing season, and discharge the water accumulated in the subgrade as soon as possible after thawing. Thus, this study further understanding of the long-term durability and stability of subgrade, and provide useful information for the structure design of the highway subgrade in cold regions.

Long-term monitoring and analysis of the longitudinal differential settlement of an expressway bridge–subgrade transition section in a loess area

To solve the problem of "bridgehead bumping" in the transition section between the road and bridge of an expressway in a collapsible loess area, a lime–soil compaction pile composite foundation is used for the first time in the transition section between the road and bridge of an expressway in China; the loess subgrade is improved by adding lime, and the subgrade is arranged in a multilayer geogrid for the joint treatment of various engineering measures. At the same time, a new type of precision differential pressure settlement meter is used to monitor the long-term settlement of a bridge–subgrade transition section with a small settlement magnitude after the joint treatment, and the distribution characteristics and variation laws of the settlement along the longitudinal direction of the line are obtained. The results show that the effect is better and the differential settlement is smaller when using a lime–soil compaction pile composite foundation; lime improves the loess subgrade backfill, and the multilayer geogrid addresses the bridge–subgrade transition in the collapsible loess area. The differential settlement and settlement rate of the subgrade and abutment increase with increased monitoring time, and the differential settlement increases gradually, while the growth rate decreases gradually and finally tends to be stable. The differential settlement of the transition section is predicted and analysed by using a hyperbolic curve, exponential curve and their combination in a prediction model, and the prediction analysis shows that the combined prediction model has the best prediction effect. These research results can provide guidance and reference for the design and construction of subgrade structures similar to the wet transition section between roads and bridges.

Field Measurements and Numerical Simulation of Snowdrift on Railway Subgrade

Abstract Snowdrift is a major natural hazard, endangering basic installation and human life in cold regions. Drifting snow can cause snow grains to accumulate on the road, blocking transportation, reducing visibility, and even leading to fatal traffic accidents. Field measurements and numerical simulations of roadbed snow characteristics were carried out along the railway in snowy weather to study the effect of snowdrift on railway subgrade. The influence of a typical roadbed structure is considered, and the basic model of a typical subgrade structure under the action of snowdrift is proposed. Snow particles that accumulate on the embankment present a U-shaped distribution along the inflow direction. Within the increase of embankment height, wind velocity on the roadbed surface is accelerated, and snow cover is reduced, thus mitigating the snow drift disaster of the embankment. Snow covers at embankment heights of 1 and 2 m are considerably greater than those at 3 and 5 m, and identical with those at roadbed heights of 3 and 5 m. Along the inflow direction, the snow cover on the cutting presents an M-shaped distribution. As cutting depth is reduced, wind velocity inside the cutting is decelerated, and snow cover is increased, which aggravates the snow drift disaster of the cutting. The snow depths of roadbed parts at 1-, 2-, and 3-m depths of the cutting are nearly the same, and that at a 5-m depth of the cutting is relatively small but still larger than that on the embankment. Numerically simulating the snow-driving wind via computational fluid dynamics, which can consider the motion characteristics of snow particles and their influence on flow field, is feasible. Simulation results agree with the measured results. Thus, this research provides a path for the further prediction of snow cover pattern.

Editorial: Special Issue on Smart and Green Infrastructures with Optimum Life Cycle Solutions

Editorial: Special Issue on Smart and Green Infrastructures with Optimum Life Cycle Solutions

Study on salt expansion mechanism of subgrade-culvert transition section in saline soils and cold regions

The theory of salt expansion in saline soils is a multi-physics coupling problem, the deformation process of the subgrade-culvert transition section is more complex. This paper proposed a mathematical model to describe the coupled water-thermal-salt-mechanical process in saline soils. Then, experimental studies of water-salt migration and salt expansion under one-way cooling condition were done to verify the correctness of the model. Finally, the numerical model of the subgrade-culvert transition section of the Golmud-Qarhan Highway was established. The results of the calculations show that the water-salt migration laws and salt expansion characteristics of saline soils can be correctly described by the mathematical model, under one-way cooling condition, with a deformation rate of about 3.47% for saline soils with a sulphate salt content of 1.49%. Then, because of the dramatic temperature change around the culvert, a salt accumulation area is created around the culvert during the cold season, which is the main reason for the salt expansion of the subgrade structure. The maximum deformation in the subgrade-culvert transition section occurs close to the road border above the culvert, and the maximum deformation in a freeze-thaw period lags the coldest time of the cold season.

Research on Mechanical Characteristics of Asphalt Graded Gravel Layer with Ballast Bed under Train Load

This article carries out laboratory tests, driving tests, and discrete element method simulations to study the mechanical properties of ballasted tracks including asphalt graded gravel and the influence of factors such as the thickness of asphalt graded gravel layer and train speed on the dynamic characteristics of the closed structure of asphalt graded gravel. It was discovered that the asphalt graded gravel can improve the bearing capacity of the ballasted track subgrade structure and improve the stability of the ballast bed. Comparative analysis shows that as the depth of the ballast bed increases, the amplitude and frequency of the velocity fitting function of each layer gradually decrease. Appropriately increasing the depth is beneficial to the stability of the ballast bed; the vertical dynamic deformation and dynamic stress decrease with the increase of the thickness of asphalt graded gravel layer; with the increase of train speed, the peak frequency of the corresponding spectrum of asphalt graded gravel layer acceleration increases. There are currently few research studies on the asphalt graded crushed stone structure laid in the ballast bed, especially the mechanical properties of the asphalt graded crushed stone structure laid by the discrete element simulation are still blank. The acceleration of the asphalt graded gravel layer increases corresponding to the peak frequency of the spectrum. The peak frequency of the acceleration response of the asphalt graded gravel layer increases. The research rules can provide a reference for the research and development of shock absorption equipment. The research results of this article can provide technical support for the application of asphalt graded gravel in railway engineering.

Numerical Analysis of the Influence of Foundation Replacement Materials on the Hydrothermal Variation and Deformation Process of Highway Subgrades in Permafrost Regions

Affected by global warming, permafrost thawing in Northeast China promotes issues including highway subgrade instability and settlement. The traditional design concept based on protecting permafrost is unsuitable for regional highway construction. Based on the design concept of allowing permafrost thawing and the thermodynamic characteristics of a block–stone layer structure, a new subgrade structure using a large block–stone layer to replace the permafrost layer in a foundation is proposed and has successfully been practiced in the Walagan–Xilinji section of the Beijing–Mohe Highway to reduce subgrade settlement. To compare and study the improvement in the new structure on the subgrade stability, a coupling model of liquid water, vapor, heat and deformation is proposed to simulate the hydrothermal variation and deformation mechanism of different structures within 20 years of highway completion. The results show that the proposed block–stone structure can effectively reduce the permafrost degradation rate and liquid water content in the active layer to improve subgrade deformation. During the freezing period, when the water in the active layer under the subgrade slope and natural ground surface refreezes, two types of freezing forms, scattered ice crystals and continuous ice lenses, will form, which have different retardation coefficients for hydrothermal migration. These forms are discussed separately, and the subgrade deformation is corrected. From 2019 to 2039, the maximum cumulative settlement and the maximum transverse deformation of the replacement block–stone, breccia and gravel subgrades are –0.211 cm and +0.111 cm, –23.467 cm and –1.209 cm, and –33.793 cm and –2.207 cm, respectively. The replacement block–stone subgrade structure can not only reduce the cumulative settlement and frost heave but also reduce the transverse deformation and longitudinal cracks to effectively improve subgrade stability. However, both the vertical deformation and transverse deformation of the other two subgrades are too large, and the embankment fill layer will undergo transverse deformation in the opposite direction, which will cause sliding failure to the subgrades. Therefore, these two subgrade structures cannot be used in permafrost regions. The research results provide a reference for solving the settlement and deformation problems of subgrades in degraded permafrost regions and contribute to the development and application of complex numerical models related to water, heat and deformation in cold regions.

Protection of under ballast mats against damage Due to loads from railroad tracks

Abstract: The paper presents the application of geogrids with uniform radial stiffness in subgrade structures in the function of protecting under ballast mats from damage caused by the railroad superstructure. The importance of the stabilizing function as a key mechanism of cooperation between geogrid and aggregate was pointed out, affecting the increase of subgrade bearing capacity, extension of ballast life and ensuring the required operating parameters of the ballast placed on anti-vibration mats. Keywords: Mechanical stabilization; Hexagonal geogrids; Under ballast mats; Railway trackbed

Dynamic response of a multi-layered pavement structure with subgrade modulus varying with depth subjected to a moving load

The dynamic response of a three-dimensional multi-layered pavement system subjected to a moving load is calculated by an analytical solution method. In this method, the pavement structure is assumed to be a transverse isotropic viscoelastic multi-layered structure with partially bonded interface, and the subgrade is considered to be a transverse isotropic half-space medium with its modulus varying with depth. The load is considered as a three-dimensional elliptical moving load with a constant speed. This study is divided into three key steps. Firstly, the ordinary differential equations of pavement and subgrade structures are obtained via the double Fourier transform, and the general solutions of dynamic response are obtained according to the theory of ordinary differential equations and the Frobenius method. Secondly, combined with the boundary conditions, the method of wave vector matrix, and the inverse Fourier integral transform, the dynamic responses of pavement structure in time domain are calculated. The accuracy of the analytical solution is verified by comparing against the solution from a finite element method. Lastly, the sensitivity analysis is conducted to assess the influence of four parameters on the dynamic response, namely, the horizontal load, the condition of interlayer contact, the transverse isotropy, and the non-uniform distribution of subgrade moduli. The calculation methods and the corresponding results can further develop a theory of the multi-layered system.

A constitutive model for cyclic densification of coarse-grained soil filler for the high-speed railway subgrade considering particle breakage

PurposeThis study aims to propose a semiempirical and semitheoretical cyclic compaction constitutive model of coarse-grained soil filler for the high-speed railway (HSR) subgrade under cyclic load.Design/methodology/approachAccording to the basic framework of critical state soil mechanics and in view of the characteristics of the coarse-grained soil filler for the HSR subgrade to bear the train vibration load repeatedly for a long time, the hyperbolic empirical relationship between particle breakage and plastic work was derived. Considering the influence of cyclic vibration time and stress ratio, the particle breakage correction function of coarse-grained soil filler for the HSR subgrade under cyclic load was proposed. According to the classical theory of plastic mechanics, the shearing dilatation equation of the coarse-grained soil filler for the HSR subgrade considering particle breakage was modified and obtained. A semiempirical and semitheoretical cyclic compaction constitutive model of coarse-grained soil filler for the HSR subgrade under cyclic load was further established. The backward Euler method was used to discretize the constitutive equation, build a numerical algorithm of “elastic prediction and plastic modification” and make a secondary development of the program to solve the cyclic compaction model.FindingsThrough the comparison with the result of laboratory triaxial test under the cyclic loading of coarse-grained soil filler for the HSR subgrade, the accuracy and applicability of the cyclic compaction model were verified. Results show that the model can accurately predict the cumulative deformation characteristics of coarse-grained soil filler for the HSR subgrade under the train vibration loading repeatedly for a long time. It considers the effects of particle breakage and stress ratio, which can be used to calculate and analyze the stress and deformation evolution law of the subgrade structure for HSR.Originality/valueThe research can provide a simple and practical method for calculating deformation of railway under cyclic loading.

Numerical Analysis of the Influence of Block-Stone Embankment Filling Height on the Water, Temperature, and Deformation Distributions of Subgrade in Permafrost Regions

The hydrologic and thermal states of foundation soils have an important influence on subgrade stability in degrading permafrost regions. However, thawing settlement remains a problem in the permafrost regions of Northeast China, because there are few relevant research results in this area, and the foundation deformation mechanism caused by the hydrologic and thermal changes of foundation soils is not clear. Therefore, a subgrade structure with large block stones as embankment filler was proposed to improve the foundation thawing settlement, and it has been successfully implemented in the road project (Yiershi–Chaiqiao section of 308 Provincial Highway) in Inner Mongolia Autonomous Region. To study the action mechanism of this structure on the subgrade deformation, a numerical model of the hydro–thermal–mechanical interaction process in unsaturated frozen soil was established, and the calculation of the water content, temperature, and deformation conditions of the subgrade within 20 years of the highway were carried out. The results show that the embankment block-stone layer can reduce the total heat of the deep foundation, and reduce the migration of unfrozen water from the deep foundation to the active layer. It can also increase the hydrologic and thermal convective flux inside and outside the block-stone layer, thus raising the permafrost table and reducing the subgrade deformation. The block-stone layer thickness has an important influence on the subgrade stability. By comparing the hydrologic, thermal change, and deformation of the block-stone structures with different thicknesses (5.0 m, 4.0 m, 3.5 m, 3.0 m, and 2.0 m), we found that when the thickness of the block-stone layer is 4.0 m, the subgrade stability is the best. In this case, the maximum uneven settlement and the maximum transverse deformation difference of the top surface of the subgrade are +0.521 cm and +0.462 cm, respectively. The subgrade stability is less optimal when the block-stone layer thickness is greater or less than 4.0 m. Thus, there exists an optimal embankment filling height related to the hydrologic and thermal conditions of the foundation soils. This study helps to elucidate the effect of unfrozen water content change on subgrade deformation during permafrost degradation and provides an important reference for solving the problem of thawing settlement of subgrade in permafrost regions.

Road Pavement Thickness and Construction Depth Optimization Using Treated and Untreated Artificially-Synthesized Expansive Road Subgrade Materials with Varying Plasticity Index.

Road pavement thickness and their depth of construction take a chunk of the overall cost of road construction. This has called for a need for reduced road pavement thickness by improving the engineering properties of subgrade such as the California bearing ratio (CBR). The CBR of road subgrade has been a major determining factor for road pavement thickness, and expansive subgrades generally have a low CBR, resulting in major road defects. In this study, road pavement thickness and construction depth optimization were conducted using the CBR values achieved in this study. Additives proportions of 8% lime and 20% cement were used in expansive subgrade to improve their engineering properties, making them suitable for use in road construction. The study investigated the characteristics, mineral structure, Atterberg limit, compaction, CBR, swell and microstructural properties of expansive subgrade. The results show a reduction in road pavement thickness and a construction depth with an increase in CBR value. All CBR values for treated samples were above 2%, making them usable in road construction. A reduction in swell potential up to 0.04% was observed for treated expansive subgrade. The study concluded that pavement thickness and construction depth can be reduced by enhancing subgrade materials and using cement and lime as binders.

Research on Cumulative Deformation Characteristics of Subgrade Filling under Cyclic Train Loading

The smoothness of high-speed railway is extremely strict. As an important part of the high-speed railway foundation, the subgrade structure has strict deformation requirements. It is necessary to ensure that the subgrade structure does not have plastic cumulative deformation under the long-term cyclic load of the train, which is the most critical geodynamic problem in high-speed railway subgrade. This paper studied the cumulative deformation characteristics of subgrade filling under cyclic train loading through the cyclic triaxial test, three types of accumulative plastic deformation trends, and the value of critical volume shear strain was discussed, which would provide the theoretical support for the dynamic design of high-speed railway.

Analysis of the extension behavior of reflective cracks in asphalt pavements based on dry shrinkage property

In order to study the extension behavior of dry shrinkage type reflective cracks in asphalt pavements, this paper uses the differential equation theory to derive the calculation formula for the inhomogeneous gradient distribution of dry shrinkage stress in the subgrade. Meanwhile, based on the traction separation rule and damage behavior equation, the extended finite element method is applied to simulate the extension path and tip stress response of dry shrinkage type reflection crack, and then analyze the influence of Semi-rigid subgrade structure and material parameters on the extension behavior of dry shrinkage type cracks. The results show that the dry shrinkage stress of Semi-rigid type base is related to the elastic modulus of base course material, dry shrinkage coefficient, uniform variation of moisture content, horizontal deformation resistance coefficient, and structural dimensions of Semi-rigid type base. Micro cracks in the Semi-rigid type base under the action of dry shrinkage stress will occur two-way extension behavior, one is to continue cracking below the base course, and the other one is the reflection extension into into the surface layer, in addition, the extension speed of the reflected cracks in the surface layer is much larger than the crack cracking speed in the base course. Reducing the modulus of the base course, increasing the thickness of the base course and controlling the variation of the moisture content of the base course within the limit value can effectively retard the emergence and extension rate of reflection cracks.

The construction of a street never opened to traffic. The extraordinary discovery of pavement engineering in vicolo dei Balconi of Pompeii

A significant portion not yet investigated in the Regio V of the ancient city of Pompeii is currently the subject of the most remarkable excavation in the post-World War II period. These activities relate to a triangular area, the so-called “wedge”, which is placed between the houses of Nozze d'Argento and Marco Lucrezio Frontone. Besides the astonishment and scientific emotions resulting from the finding of magnificent frescoed and well preserved thermopolium, dwellings and balconies, also a feeling of extreme interest in the pavement engineering arose due to a further one of a kind discovery. The Somma-Vesuvium volcanic eruption of 79 AD buried a narrow alley, i.e. vicolo dei Balconi, during the construction of its stone pavement, in which the laying of compact lava stone flags was interrupted about halfway from north to south. The eruption and the following superimposition of lapilli, ash and pumice blankets has therefore maintained in a state of perfect documental conservation the whole area, preserving the new laid flagstones still showing the original surface finishing and the preparatory subgrade. Thus, this historical and archeological source becomes a priceless engineering evidence describing some of the design and construction techniques used by the Romans at Pompeii. The excavated area has been analyzed following a cross-disciplinary approach, adopting several minimally intrusive or non-destructive techniques and instruments belonging to different branches of science to capture some specific aspects. Specifically, the alley has been initially surveyed with a terrestrial laser scanner (TLS) device to build an accurate layout model of the worksite. Although the TLS survey provided an invaluable source of information for the street structures documentation, specific stone paving elements were also analyzed using close range photogrammetry techniques and their friction properties have been measured exploiting the potential of a modern road engineering device, i.e. a skid resistance tester, also known as the British pendulum tester (BPT). The consistency analysis of the subgrade structure and homogeneity in the unpaved section involved the use of two dynamic procedures, i.e. a penetrometric (dynamic cone penetrometer - DCP) and a deflectometric (light weight deflectometer - LWD) test. Besides, some design and urban planning considerations have been drawn from the analysis of the relationships between the finished structures and the pavement construction site according to the lines of the existing curbs, the thresholds of access to the dwellings and the placement of enigmatic prismatic blocks arranged in trenches, probably intended as a useful target points for the stonemasons or temporary pedestrian passage. In a wider reading a succession of paving works (and not repaving) using stone elements in this part of the city starting from the Cardo via del Vesuvio (from west to east) is recognizable.

Analytical calculation method for passive tensile damage of asphalt concrete waterproof sealing structure of ballastless track subgrade

The theoretical development of the temperature effect of the full-section asphalt concrete waterproof sealing structure (ACWSS) of the ballastless track subgrade lags the engineering practice. In order to reveal the passive tensile damage behavior of ACWSS from the perspective of theoretical analysis, in this paper, the base plate of the ballastless track and the ACWSS is simplified into an elastic–viscoelastic double layer composite beam model. The analytical calculation method for passive tensile damage of ACWSS under temperature load is derived theoretically. The change of surface stress and strain of ACWSS with time is calculated and analyzed. The theoretical analysis and calculation results show that the variation of the surface strain, the maximum strain, and the time corresponding to the strain peak of ACWSS are consistent with the measured results. The model of the double-layer composite beam has high reliability, and the analytical calculation method of passive tensile damage is reasonable and practical. The temperature stress of ACWSS at the expansion joint of the base plate is synchronized with the temperature change, indicating that the asphalt concrete at this location is mainly subjected to axial tension (or compression). It is suggested that the structural design should be judged by comparing the number of periodic temperature loads and the allowable fatigue number of asphalt concrete, which further improves the design method and technical system of the ACWSS in China's high-speed railway. It has important theoretical and practical significance.

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.