Unsaturated Subgrade Research Articles

Modelling the dynamics of ballastless railway tracks on unsaturated subgrade

Concrete slab tracks help shield the supporting railway earth structure from external water ingress. However, the inevitable cracks that arise during its lifespan provide a pathway for water penetration, leading to changes in the degree of saturation of the underlying support. This can affect the dynamic response of the structure, however is challenging to model due to the computational requirements of three-phase unsaturated soil simulation. To address this, this paper presents two main novelties: 1) an efficient moving frame of reference approach for railway ballastless tracks on unsaturated earthworks subject to train loading, 2) new findings into the effect of degree of subgrade bed saturation on ballastless track dynamics. First the model is presented, including formulations for vehicle-track interaction and unsaturated subgrade dynamics. Considerations for numerical stability are then discussed and the model is validated, before investigating the role of subgrade bed saturation on pore water pressure and displacements. It is shown to have a high impact on pore water pressure generation, but a limited impact on deflections. The effect of train speed is then investigated and it is found that higher train speeds induce higher pore water pressures. Track irregularities are also investigated and it is found that they play an important role in pore water pressures.

Comparative study on in-situ resilient modulus of subgrade estimated using in-situ modulus detector

This study presents a comprehensive evaluation of the resilient modulus of unsaturated subgrade soils using a novel in-situ modulus detector (IMD) along with various laboratory and field-testing methods. The current study employs several testing devices, including bender elements, standard dynamic cone penetrometer, and crosshole-type dynamic cone penetrometer (CDP), to assess the reliability of the IMD in estimating the resilient modulus profiles of small-scale homogeneous subgrade models in unsaturated conditions. The index properties, compaction and strength characteristics of the compacted soils, are evaluated. The resilient modulus values obtained from the IMD are compared with those obtained from repeated load triaxial tests under various stress states. Experimental results show a decrease in the penetration depth per blow with increased dry unit weight. In addition, elastic modulus and shear wave velocity increase with soil density. The resilient modulus estimated from the IMD increase with both depth and soil density. A robust correlation has been established between the resilient modulus values estimated from the IMD and those estimated from the CDP or bender element tests. Furthermore, the resilient moduli estimated from the universal model and the IMD demonstrate linear relationships with a coefficient of determination greater than 0.8. Thus, the IMD may be useful for the in-situ evaluation of resilient modulus in unsaturated subgrade soils, considering influencing factors such as soil density and confining pressure.

Research on the Coupled Dynamic Response of Vibratory Roller-Unsaturated Subgrade under Different Compaction and Moisture Content States

The dynamic response between the vibratory roller and subgrade under vibratory compaction is essential to improve the compaction quality and optimize the construction technology. Considering the nonlinear hysteresis characteristics of the subgrade in vibratory compaction, a two-degree-of-freedom viscoelastic-plastic, vibratory roller-unsaturated subgrade coupling model, is established to evaluate the effect of compaction stage and moisture content on the vibratory roller-unsaturated subgrade coupling dynamic response. The dynamic resilient modulus semi-empirical model is established based on the dynamic resilient modulus experiment by considering the matrix suction of the unsaturated subgrade and it is the basis of the vibratory roller-unsaturated subgrade coupling model to consider the moisture content influence of compaction. In addition, a plasticity index was proposed to divide the selection of coupling models to consider the influence of compaction stages. The results show that the semi-empirical model takes into account the effect of moisture content by soil–water characteristic curve and it performs well in responding to the experiment results and other experimental data in the published literature. Based on this, the vibratory roller-unsaturated subgrade coupling dynamic response varied for different compaction stages and the moisture content of the unsaturated subgrade. The compaction quality gradually improved as the increase of compaction passes and the decrease of the moisture content. However, the compaction quality of the unsaturated subgrade is weakened by over compaction and excessive moisture content and should balance the compaction operation and quality assurance of the unsaturated subgrade. The results of the research will indicate the compaction mechanism of unsaturated subgrade and provide a theoretical basis for the development of intelligent compaction technology.

Permanent deformation characteristics of unsaturated subgrade soils under cyclic loading

Excess permanent deformation of subgrade may cause critical rutting in pavements, and subsurface water significantly affects the accumulation of permanent deformation. This study aims to reveal the permanent deformation behaviour of unsaturated subgrade soils. Subgrade box tests were performed at varying moisture content, stress levels and loading frequency, respectively. The test results show that the permanent deformation significantly increased with moisture content. The concept of a permitted moisture content-applied dynamic load envelope was proposed. Based on curve fitting, a prediction model of permanent deformation of subgrade soils was developed, and the effects of moisture content and loading cycles were included. The validation results show that the model exhibited reasonably good performance in modelling the deformation. This study may provide an alternative way for predicting the permanent deformation of unsaturated subgrade soils.

Stresses in Saturated and Unsaturated Subgrade Layer Induced by Railway Track Vibration

The theoretical and practical studies of the cyclic loads resulting from the movement and passage of trains on the unsaturated subgrade to determine the effect of the degree of saturation and moisture content on the foundations and infrastructure of the railway lines, especially the settlement in the railway lines as a result of the development of the train loads. Thirty-six laboratory experiments were carried out using models that simulate a railway with nearly half the scale of the real one, using an iron box of (1.5 × 1.0 × 1.0) meters and a layer of clay soil with a thickness of 0.5 m representing the base layer, were constructed inside it. Above it, there is a layer of crushed stone representing a 0.2 m thick ballast, topped by a rail line of 0.8 m long installed on three sleeper beams with dimensions of 0.9 m (0.1 × 0.1 m). The subgrade layer has been constructed at different saturation degrees as follows: 100, 80, 70, and 60%. The tests were carried out using different load amplitudes and frequencies. These experiments investigated the effect of the subgrade degree of saturation on the value of the stresses generated on the surface and the middle (vertical and lateral stresses) and the settlement of the subgrade. In the case of unsaturated subgrade soil, an increase in load frequency has a clear effect on increasing the generated stresses in the subgrade layer, especially with lower saturation levels. However, the results and measurements of these experiments found that the load frequency almost had no effect on the values of the stresses generated on the surface and inside the subgrade layer with a 100% degree of saturation. The results of the investigation demonstrated that, while load frequency had a minimal effect on track-panel settlement, it increased with the load amplitude and subgrade soil saturation degree. The change of settlement of the track panel with the number of cycles has a high rate at the beginning; after a while from that, it decreases gradually until, after some value of the number of cycles, the settlement changes at a very low rate and gradually.

Effect of complex climatic and wheel load conditions on resilient modulus of unsaturated subgrade soil

Resilient modulus (Mr) is a fundamental mechanical property vital for assessing the resistance of pavement structures to cyclic vertical loads. It has played a pivotal role in pavement design and has been instrumental in predicting pavement responses and fatigue life. The Mr of subgrade soil is affected by a multitude of factors, including stress, moisture, and temperature conditions, all of which interact to define the response of the soil. This research investigated the effect of complex climatic conditions on Mr with a particular focus on areas experiencing significant seasonal changes in snowy cold regions like Hokkaido, Japan. Previous studies have proposed predictive models for Mr, incorporating the concept of matric suction, to account for moisture conditions. However, these models have rarely considered suction hysteresis in the soil–water characteristic curve (SWCC) or the effects of wheel loading on frost-susceptible subgrade soil during different seasons. In this study, a series of Mr tests were conducted on two types of subgrade soil under various climatic and wheel loading conditions. The test results promise to enhance our understanding of the complex interplay of climatic and stress conditions on Mr of standard sand and frost susceptible subgrade soil along different drying and wetting paths, particularly in regions with significant seasonal variations.

Humidity field for unsaturated subgrade accounting for the hydro-mechanical coupling and the difference of compactness: A simulation method

The safety and stability of road structures heavily rely on the precise assessment of subgrade humidity. However, existing methods often fall short in fully considering the coupling effect between humidity and stress states, as well as variations in compactness across different subgrade areas. In this study, a new model is proposed for simulating water migration in soil subgrade, incorporating hydro-mechanical coupling and the degree of compaction. The model is implemented using a newly added physical field in the COMSOL Multiphysics platform, and a numerical method is developed for calculating subgrade humidity. Moisture migration behavior in subgrades is investigated by selecting two atmospheric environments, namely dry and wet conditions. The results demonstrate that the developed humidity calculation method, accounting for hydro-mechanical coupling and compactness variations, yields reasonable and accurate predictions. Substantial differences in subgrade humidity are observed between wet and dry conditions. Compared to the dry conditions, pronounced fluctuations in subgrade humidity are observed in areas near slopes, with the equilibrium moisture content increasing by approximately 50% compared to the initial content. Additionally, the equilibrium time for moisture migration is significantly shortened under wet conditions. Based on these findings, it is recommended that waterproof should be considered in subgrades exposed to wet atmospheric environments to avoid excessive moisture content in subgrade.

Vibration analysis of a beam on a layered transversely isotropic unsaturated subgrade subjected to a moving load

Performance evaluation of the beam subject to the variation of saturation change of the subgrade is of significance to the optimal design of the high speed railway (HSR). This study proposes a mathematical modelling to describe the dynamic interaction between a beam and a layered transversely isotropic unsaturated subgrade. Based on the Euler-Bernoulli theory and the dynamic unsaturated soil theory, governing equations of this mathematical model are established. With the aid of the precise integration method, Fourier transform and compatibility conditions, the equivalent stiffness of the subgrade is obtained in the transformed domain. Introducing the equivalent stiffness into the beam motion equation, the dynamic solution of the beam-subgrade system is acquired by further introducing the inverse Fourier transform. After verifying the correctness of this theory, a series of numerical examples are provided to discuss the parametric sensitivity.

A numerical model for hydrothermal transport in unsaturated freezing soils considering thermodynamics equilibrium

AbstractThe analysis of frost heave in unsaturated subgrade soils should consider the coupled movement of liquid water, vapor, and heat, accompanied by phase transition of water, vapor, and ice. In this paper, the equilibrium relationship of the three phases of water in soils was comprehensively considered, and the water head and vapor density were determined under frozen and unfrozen conditions from the perspective of thermodynamics. Based on that, the governing equations of mass and energy conservation were then derived and a new transient finite element model was developed, considering the behavior of the phase change and coupled hydrothermal transport under frozen as well as unfrozen conditions. Since the discontinuities between the frozen and the unfrozen zones may bring difficulties to the numerical solution, two kinds of smoothing techniques were proposed to solve the discontinuities at the phase transition interface, which guaranteed robust convergence and enhanced computational efficiency. The model proposed was verified by comparing with some available test results and applied to analyze the frost heave process of unsaturated subgrade under impermeable road pavements, demonstrating the model's accuracy and applicative prospect. It is revealed that the movement of liquid water and vapor in the frozen zones may still exist, and its mechanism and performance are different from that in unfrozen zones. The significance of vapor migration on the subgrade frost heave is also emphasized.

Forward calculation of displacement fields with multilayered unsaturated highway system induced by falling weight deflectometer using dynamic response method

A new dynamic forward model of a multilayered highway structure, which considers the unsaturated characteristics of subgrade soil, is established in this paper to assess the dynamic response induced by the falling weight deflectometer (FWD) load. The Laplace-Hankel transform and dynamic stiffness matrix method are employed to solve the governing equations of the unsaturated soil layer. Combined with the global stiffness matrix and boundary conditions, analytical solutions of the entire system can be derived in the transform domain. A numerical integration method is utilized to perform the Laplace-Hankel inverse transform to obtain time-domain solutions. An analysis of the effects of saturation on the stress, displacement, and pore water pressure is presented. It is revealed that the effect of saturation of unsaturated subgrade soil on vertical stress is negligible, whereas vertical displacement and pore water pressure increase significantly with increasing saturation. Additionally, the vertical displacement response on the pavement surface is sensitive to subgrade deterioration and variations in material properties. Moreover, subgrade deterioration can be reflected in the variation in vertical displacement at approximately 1.2 m from the load center for the studied road structure. The proposed model, which is more realistic and reasonable, can provide a theoretical basis for the back-calculation of the elastic modulus of multilayered highway systems.

Implementación del espectro de curvas de diseño ramcodes de suelos no saturados de subrasantes en el diseño de pavimentos flexibles

The design process for a pavement structure assumes that the subgrade will saturate at some point during the service life, however, the saturation of the subgrade depends on the conditions depending on the region in which it is located. Therefore, it is necessary to study design conditions for partially saturated subgrade. The present research aims to implement the spectrum of RAMCODES design curves for unsaturated subgrade soils, applied to the design of flexible pavements under environmental and traffic conditions associated with the Oyon-Ambo highway. To construct the RAMCODES design curves, CBR tests were performed by varying compaction energies and moisture contents. Likewise, using the OriginPro 2019 program, the curves were obtained and a mathematical model was used to predict the variation of the degree of saturation in the subgrade over time. Subsequently, the seasonal CBR were obtained and correlated to obtain the resilient moduli. Finally, two flexible pavement structure designs were carried out, one using the traditional subgrade characterization methodology and the other according to the proposed methodology. From the results obtained, it is interpreted that the consideration of the unsaturated behavior of the subgrade optimizes the pavement design since the bearing capacity of the subgrade is increased by 28.8%, which means that the thickness of the subbase layer is reduced by 25%.

Long-Term Performance of the Water Infiltration and Stability of Fill Side Slope against Wetting in Expressways

Different settlements and instabilities of unsaturated subgrade subjected to wetting have been paid increasing attention in the southeast coastal areas of China. However, the treatments are costly when they are used in engineering. In addition, the long-term performances of the treatments are unclear. Based on seepage theory for unsaturated soils, a novel subgrade using a capillary barrier was proposed in this study to reduce the different settlements and stabilities. Compared with previous studies, a capillary barrier was merely applied in the landfill. The long-term performance and feasibility of a capillary barrier applied in a tilted subgrade slope is worthy of study, particularly in humid climates. Using Geo-Studio, the feasibility was verified by comparing a conventional subgrade with a subgrade using a capillary barrier in southeast coastal areas in terms of pore-water pressure, water content, settlement, and the safety factor. The numerical results showed that the subgrade using a capillary barrier could provide significant improvements in the performance of reducing the impact of pore-water pressure distribution it suffered from, so as to lead to smaller different settlements. The vertical settlement of the pavement using a capillary barrier over a 1 year period was 1 cm. Compared with a conventional subgrade, the settlement fell by 94%, and the safely factor increased by 15% for the subgrade using the capillary barrier.

Elastodynamic analyses of transversely isotropic unsaturated subgrade–pavement system under moving loads

AbstractIn this paper, elastodynamic responses of the layered transversely isotropic unsaturated subgrade–pavement system subjected to the moving load are investigated. The unsaturated subgrade soil is described via a triphasic Biot‐type model introducing the effect of matric suction, immiscible flows of water and air, and transverse isotropy. The rigid pavement is modeled by the Kirchhoff plate theory, while the flexible pavement is modeled by the stratification medium theory. First, the governing equations of the unsaturated subgrade soil are obtained after algebraical manipulations. The precise integration method (PIM) is further combined with the double Fourier transform to solve the governing equations. The subgrade soil solution is utilized as the flexible coefficient to describe the plate–soil interaction, and the solution of the pavement is further acquired via the double inverse Fourier transform. Four verification examples regarding literatures and COMSOL are presented to verify the correctness of the theory. Finally, numerical examples are conducted to investigate the effect of various parameters on the dynamic behavior of the unsaturated subgrade–pavement system.

Hydraulic characteristics of stabilised expansive subgrade soils in road pavements

ABSTRACT Civil engineers face significant challenges in the safe design and construction of durable road infrastructure in the presence of expansive soils. These problematic soils exacerbate undesirable serviceability concerns induced by pavement distress. Soil stabilisation has been recognised as a sustainable approach to alleviate the problematic nature of expansive subgrades. Road pavements constructed on top of expansive subgrade soils are generally under unsaturated conditions, where the moisture variations can significantly impact the pavement response. However, current pavement design and modelling frameworks overlook unsaturated soil behaviour by adopting simplified approaches. This study examines the hydraulic behaviour of expansive clayey soils stabilised with non-traditional and traditional chemical based additive. Tests were conducted to determine the state variation and stabilisation influence on the soil water characteristic curve using the dewpoint potentiometer for an expansive subgrade commonly found in Melbourne geology. Results show that the stabilisation has strong influence on soil hydraulic characteristics at various initial state conditions tested. Experimental data have been applied to illustrate the significance of incorporating realistic hydraulic response using a simulated practical application in road pavements. The research highlights the significance of incorporating accurate hydraulic characteristics for simulating and assessing the response of pavement constructed with stabilised unsaturated subgrade soils.

Semianalytical Solution for Dynamic Responses of Railway Track System on Unsaturated Poroelastic Half-Space Subjected to Moving Trainload

Abstract A track-unsaturated ground model was proposed to study the dynamic responses of the unsaturated subgrade induced by moving trainloads. The subgrade was modeled by an unsaturated poroelasti...

Comparison of numerical modeling results from laboratory and field obtained unsaturated flow parameters

Moisture and suction variation beneath pavement contribute significantly to the volumetric deformation of expansive subgrade in response to climatic loading. In order to quantify the damage due to climatic loading, estimation of moisture, and suction variation in pavement subgrade is of paramount importance. The objective of the current study is to investigate the moisture and suction variation in response to precipitation events with the aid of numerical modeling in the unsaturated pavement subgrade. In unsaturated soils where the voids are filled with both water and air, the SWCC describes the volume of the voids that remain filled with water as the soil drains pore water. The SWCC has been identified as the vital soil information required to analyze seepage, stability, and volume change problems involved in unsaturated soils. However, the selection of unsaturated flow parameters is typically laboratory-based which represents specific conditions rather than a dynamic scenario of the field. In this study, an attempt was undertaken to conduct numerical modeling using field generated unsaturated flow parameters along with five other predictive models. Results indicated the distinct variation of moisture and suction distribution in subgrade from the same rainfall event while using laboratory versus field generated SWCC unsaturated flow parameters. In addition, predicted model yielded output was also varying with the field generated parameters’ output. Instead of using laboratory-generated static parameters, it was found that field generated dynamic unsaturated flow parameters were able to capture better suction variation at the pavement subgrade.

Estimating the Contribution of Suction on the Resilient Modulus of Subgrade Soils Using Capillary Saturation

AbstractThe matric suction is well known to significantly affect the resilient modulus of unsaturated subgrade soils. However, the contribution of matric suction on the resilient modulus varies wit...

Generalized Plastic Mechanics–Based Constitutive Model for Estimation of Dynamic Stresses in Unsaturated Subgrade Soils

Abstract This paper presents a generalized plastic mechanics–based constitutive model for the estimation of dynamic stresses in unsaturated subgrade soils under repeated vehicle loading conditions....

Field evaluation of subgrade soils under dynamic loads using orthogonal earth pressure transducers

Subgrade soils experience repeated traffic loads, such that dynamic stress and displacement are accumulated to jeopardize the stability of pavement foundations. Laboratory element testing is often conducted to apply cyclic loads to the soil. In fact, the stress state of subgrade soil is complex due to the constant rotation of principal stress induced by moving vehicles. It is hard to select constitutive relations for unsaturated subgrade soils in numerical or analytical calculation, since most models are developed without proper consideration of principal stress rotation. Furthermore, most available earth pressure sensors cannot provide stress measurements in three orthogonal directions for a soil element at the same time. A novel orthogonal earth pressure transducer is developed to measure dynamic stresses of subgrade soils in three directions under a roadway at Chencun, China. The cumulative effect of dynamic stress with load cycles is observed explicitly. Results indicate that the dynamic stress becomes negligible at 2 m from the roadway, and stabilization is suggested to be limited within this width. A series of constitutive relations is proposed to correlate the dynamic stress with the resulted displacement in different directions, which can be used to guide the design of unsaturated subgrade soils in South China.

Behaviour of Unsaturated Subgrade Soil Under Highway Load

This paper studied the application of 2-D Plaxis (v8.6, 2011) software on a pavement layer structure set on unsaturated subgrade soil. An axisymmetric finite element (FE) model was used to analyze the behavior of pavement layers subjected to dynamic loadings. The model was loaded with an incremental contact pressure from 50 to 550 kPa with different variable such as water table level (1,2 and 3m), suction of soil and degree of saturation (100, 90, 80, 70 and 20%). The results indicated that during loading on pavement layer with increases water table level and different degree of saturation the vertical settlement was decreased by about (11, 15, and 18%) for water table level= 1m, (9, 13, 16%) for water table level= 2m and (28%) for water table level= 3m (dry soil) respectively. The effect of degree of saturation on the vertical settlement is apparent at the lower value for water table level (1 and 2m) and the vertical settlement is decreased with increasing soil suction. The results also show the negative pore water pressure decreased with decreased of degree of saturation and development increases with depth and beginning of dynamic load. The effect of unsaturation greater at the center line of pavement layer and limited far away the center line.