Multilayer Elastic Theory Research Articles

Prediction of airfield pavement responses from surface deflections: comparison between the traditional backcalculation approach and the ANN model

This study investigated traditional and new approaches for predicting airfield pavement responses from surface deflections measured under Heavy Weight Deflectometer (HWD) testing conducted at National Airport Pavement Testing Facility (NAPTF). In the traditional approach, pavement layer moduli were backcalculated and then pavement responses were predicted based on the multilayer elastic (MLE) theory and the finite element (FE) method. In the new approach, an Artificial Neural Network (ANN) model was developed to predict the pavement response directly from surface deflections without backcalculation. The ANN model was trained using the synthetic database that was built based on the FE simulation results using different combinations of material property, layer thickness, HWD loading magnitude, and pavement temperature. It was found that the backcalculated moduli of the asphalt surface layer were similar between MLE and FEM methods; however, discrepancies were observed for the backcalculated moduli of unbound materials. In general, the traditional approach of backcalculation and forward calculation overestimated tensile strain in asphalt layers, especially for the pavement section with a thin asphalt layer. On the other hand, the prediction accuracy of the ANN model was found better than the traditional method regarding field measurements. Further analysis of the ANN model showed that the Area Under Pavement Profile (AUPP) and Surface Curvature Index (SCI) had good correlations with critical tensile strain and shear strain in the asphalt layer, respectively.

Numerical analyses of rigid and flexible pavements responses under heavy vehicles’ loading

In an attempt to better understand the interaction between heavily loaded trucks and the pavement structure; rigid and flexible pavements’ responses (stresses, strains and deflections) were evaluated under simulated loading of 11-axle trucks (MI-20, MI-18, MI-14, MI-13) at gross vehicle weights (GVW) of 164,000 lb with different axle configurations, which are allowed in the state of Michigan. These responses were compared against results of a standard 5-axle semi-trailer at GVW nearing the common 80,000 lb standard in most states. Using these heavy truck types, the combined effects of truck loading, pavement thickness, joint spacing, material properties and thermal conditions on the pavement damages were evaluated. The major fatigue and faulting damages for rigid pavements as well as fatigue damage and subgrade rutting for flexible (asphalt) pavements are analysed. The finite element method (FEM) based program ISLAB2000 and multilayer elastic theory (MLET) based program JULEA were used to compute rigid and flexible pavements responses, respectively. Results show that the standard 5-axle truck yields up to 50% more fatigue damage potential under positive temperature gradient across slab (during daytime) even at a much lower gross vehicle weight than that of Michigan (MI) trucks. However, the 11-axle MI trucks provide up to 45% higher fatigue damage potential under negative temperature gradient (during nighttime). In comparison with the standard truck, the 11-axle MI trucks yield up to 17% less faulting damage for the typical rigid pavement with a thickness of 8 mm and up to 18% more faulting for larger pavement thickness. In flexible pavements, the standard truck exhibits up to 39% higher asphalt concrete fatigue damage than MI trucks. However, up to 80% more subgrade rutting damage for typical flexible pavements is observed for MI trucks with the multi-axles group configuration and larger GVW than the standard truck.

Comparative study on using static and dynamic finite element models to develop FWD measurement on flexible pavement structures

The deflection basin obtained through backcalculation analysis is compared with the measured deflection basin to determine the moduli of each pavement layer. Most computer programs use multi-layered elastic theory (MET) to perform backcalculation for determining deflection basin. Other structural analysis techniques, such as finite element method (FEM) and finite difference method (FDM), can be used to model flexible pavement structures when conducting FWD tests. Unlike FEM, MET analysis does not take into account nonlinear materials and dynamic loading. This study aims to develop a better finite element (FE) model by using the static and dynamic analyses in the ANSYS computer program. A comparative study was conducted by using varying sizes of model geometry and different types of elements and sizes to determine how they affect the developed FE model. The results of the analyses show that transient dynamic analysis is the best method for simulating FWD test. The percentage of errors between FE deflection basin and measured deflection basin range between 0.94 and 5.01%. Model geometry of 5000 × 5000 mm is sufficient to produce a good deflection basin which approximates the measured deflection. To ensure the accuracy of the developed model, the information on material properties must be valid. Additionally, finer and higher order elements should be used close to the loading region, for instance four or eight-node quadrilateral element (CAX4 or CAX8) with quadratic interpolation function.

Implementation of stress-dependent resilient modulus of asphalt-treated base for flexible pavement design

Asphalt-treated base (ATB) is a widely used base course material in flexible pavements. Due to its lower binder content and lower quality granular material, resilient modulus (MR) of ATB exhibits a stress dependent property. While most of the pavement analysis software used in the current routine design tasks is based on Burmister’s multi-layer elastic theory, in which the MR is the primary fundamental material property, the software cannot truly deal with the stress-dependent MR of paving material. In this study, MR model in the Mechanistic Empirical Pavement Design Guide was adopted to accommodate the stress-dependent MR of ATBs and the model was implemented in pavement mechanistic analyses through Abaqus finite element software and its user programmed subroutine. The comparison of critical pavement responses was made among pavements with three types of ATBs. Pavement responses obtained using the multi-layered elastic theory and based on resilient moduli recommended by Alaska Department of Transportation & Public Facilities, were also included for comparison.

The Effect of Flexible Pavement Mechanics on the Accuracy of Axle Load Sensors in Vehicle Weigh-in-Motion Systems.

Weigh-in-Motion systems are tools to prevent road pavements from the adverse phenomena of vehicle overloading. However, the effectiveness of these systems can be significantly increased by improving weighing accuracy, which is now insufficient for direct enforcement of overloaded vehicles. Field tests show that the accuracy of Weigh-in-Motion axle load sensors installed in the flexible (asphalt) pavements depends on pavement temperature and vehicle speeds. Although this is a known phenomenon, it has not been explained yet. The aim of our study is to fill this gap in the knowledge. The explanation of this phenomena which is presented in the paper is based on pavement/sensors mechanics and the application of the multilayer elastic half-space theory. We show that differences in the distribution of vertical and horizontal stresses in the pavement structure are the cause of vehicle weight measurement errors. These studies are important in terms of Weigh-in-Motion systems for direct enforcement and will help to improve the weighing results accuracy.

Assessing the Effects of Rising Groundwater from Sea Level Rise on the Service Life of Pavements in Coastal Road Infrastructure

Coastal communities with road infrastructure close to the shoreline are vulnerable to the effects of sea level rise caused by climate change. The sea level in coastal New Hampshire is projected to rise by 3.9 to 6.6 ft (1.2 to 2.0 m) by 2100. Climate change vulnerability and adaptation studies have focused on surface water flooding caused by sea level rise; however, little attention has been given to the effects of climate change on groundwater. Groundwater is expected to rise with sea level rise and will intersect the unbound layers of coastal road infrastructure, thus reducing the service life of pavement. Vulnerability studies are an essential part of adaptation planning, and pavement engineers are looking for methods to identify roads that may experience premature failure. In this study, a regional groundwater flow model of coastal New Hampshire was used to identify road infrastructure for which rising groundwater will move into the unbound materials during the design life of the pavement. Multilayer elastic theory was used to analyze typical pavement profiles in several functional classifications of roadway to determine the magnitude of fatigue and rutting life reduction expected from four scenarios of sea level rise. All the evaluation sites experienced service life reduction, the magnitude and timing of which depended on the current depth to groundwater, the pavement structure, and the subgrade. The use of this methodology will enable pavement engineers to target coastal road adaptation projects effectively and will result in significant cost savings compared with implementation of broad adaptation projects or the costs of no action.

Influence of asphalt visco-elastic properties on flexible pavement performance

Even though every layer of pavement structure is important and affects pavement performance, the asphalt layers visco-elasticity plays significant role. Bitumen properties, as well as asphalt mixture properties, vary depending on temperature and loading conditions. These variations influence entire pavement bearing capacity and has to be evaluated in pavement design. The main challenge is material behaviour description through simple models to incorporate them to pavement design. Generally, pavements are designed using Multilayer Elastic Theory assuming that all materials are elastic, isotropic, and homogenous. This paper presents analysis of two pavement structures response calculated according to three pavement design approaches. The dynamic modulus and phase angle of asphalt mixtures was estimated using Hirsch model after binder complex shear modulus tests. The visco-elastic behaviour was described with rheological Huet-Sayegh model and pavement responses estimation was done using MnLayer and ViscoRoute2 software. The analysis reviled static and dynamic load influence on pavement structure based on elastic and visco-elastic properties of asphalt layers. This allowed optimisation of layer thicknesses and determination of more cost beneficial pavement structure with appropriate performance.

Deformation Modelling of Instrumented Flexible Pavement Structure

A flexible test road structure was built and tested in an Accelerated Pavement Test (APT) using a Heavy Vehicle Simulator (HVS) to monitor its performance behaviour. In the tests more than one million load cycles were applied, but mid-way the water table was raised, allowing the structure to be analysed in “moist” and “wet” state. The raised water level had a significant effect, decreasing the resilient modulus and increasing the rate of accumulation of permanent deformation (PD). The structure was instrumented to measure stress, strain and deflection responses as a function of load repetitions as well as PD manifested on the surface as rutting. The structure has been modelled in an axisymmetric analysis using a 2D multi layer elastic theory (MLET) as well as a 3D finite element method (FEM). The methods generally both agreed well with the measurements. The observed accumulation of PD of the unbound layers was modelled using three different material models, both stress and strain based with responses gained from both MLET and FEM. The modelled deformation, is compared to the measured deformation in both “moist” and “wet” state, with some difference observed between models and methods but generally reasonably agreement was established.

Impact of Dual Gauge Railway Tracks on Traffic Load Induced Permanent Deformation of Low Embankments

There is a growing interest in recent years of many African countries to revamp their neglected railways in order to promote regional trade and transportation integration. Investors are faced with problems of railway track gauge conversions to promote railway inter operability. The objective of the work documented here was to numerically evaluate the impact of track gauge conversions on traffic load induced permanent deformation (PD) of low embankment on soft sub-grade. A method to predict the traffic load induced settlement of low embankment on soft sub-grade is proposed. Using the user-defined material subroutines (UMAT) in ABAQUS, a 2-D finite element (FE) model was formulated. These models are converted into a numerical formulation for implementation in FE analysis and the traffic load induced dynamic stress in the sub grade are calculated by using the multi-layer elastic theory. Then the plastic vertical strain in the sub-grade is calculated by an empirical equation, whose constants are related to the physical and mechanical properties of the sub-grade soil. The method was applied to analyze a 700m long section of a low embankment on the soft black cotton soil of Nakuru plains in Kenya. Corresponding results showed that the application of traffic loads on alternate rail tracks due to gauge conversions have a significant effect on the permanent deformation of the sub grade soil. The depth significantly influenced by traffic loading was found to be close to 6 m below the base of the embankment. The analysis also shows that increasing the thickness and stiffness of the sub grade is a very effective way of reducing the traffic load induced permanent deformation of soft sub grade soil. The proposed method can be used for settlement analysis on low embankments as well as a useful tool for making decisions on railway track gauge conversions.

Viscoelastic Response Modelling of a Pavement under Moving Load

This paper demonstrates the application of a generalized layered linear viscoelastic (LVE) analysis for estimating flexible pavements’ structural response. The procedure is based on the Multi-Layered Elastic Theory (MLET) and the elastic-viscoelastic correspondence principle using a numerical inverse Laplace transform. A comparison of the direct layered viscoelastic responses with approximate solutions based on the elastic collocation method was also carried out. Furthermore, it is proposed to use the collocation method using LVE solutions at selected time durations in order to improve the accuracy of the elastic collocation method. The LVE collocation method was further extended for analysis of moving loads. The method was illustrated by analysing a pavement structure subjected to moving wheel loads of 30, 50, 60 and 80 kN using a Heavy Vehicle Simulator (HVS). The various responses (stresses and strains) in the pavement, at different pavement temperatures, were measured using various types of sensors installed in the structure. The LVE calculations agreed very well with the measurements.

A mixed elastohydrodynamic lubrication model for simulation of chemical mechanical polishing with double-layer structure of polishing pad

Mixed elastohydrodynamic lubrication (EHL) model has been used to study slurry flow and contact pressure distribution in the chemical mechanical polishing (CMP) process. However, in various mixed EHL simulation frameworks, only a single-layer polishing pad is considered. Consequently, the double-layer polishing pad, which is widely used in semiconductor industry, cannot be designed and optimized from the tribology perspective with the assistance of theoretical simulation. In this paper, multi-layered elastic theory (MLET), which is frequently used to calculate flexible pavement response to truck loading, is introduced into the mixed EHL model. Pad deformation calculated by the MLET has a similar accuracy to that by the 3D finite element method, but its computational cost is much lower. It is very important for accurate and efficient simulation of the tribological behavior and material removal rate (MRR) in CMP. Based on the new simulation method, advantages of a different double-layer structure of polishing pad are discussed. It is found that the upper-hard lower-soft pad has better performance for improving the uniformity of MRR when the wafer surface is uneven, but the boundary exclusion region is bigger than single-layer pad and upper-soft lower-hard pad. The upper-soft lower-hard pad has advantage to reducing the edge exclusion, but the effect is not remarkable when the thickness of the upper layer is not extremely thinner than the lower layer.

The Different Pavement Structure Combination Shear Stress Analyze

Based on the multi-layer elastic system theory, large general used finite element software is used to analysis in the paper. The pavement structure layer bottom three-direction maximal shear stress SXY, SYZ and SXZ are computed under the different axis load for four kind combinations. Through contrasting the three direction shear stress, the paper obtained some curves and laws about the pavement structure layer bottom three-direction shear stress along with the axis load change.

Maximum Shearing Stress Analysis of Asphalt Pavement Structure

Based on the multi-layer elastic system theory, large general used finite element software is used to analysis in the paper. The maximal shear stress SXY is analysed entirely by computing the structure layer bottom maximal shear stress SXY of the different combinations under the different axis load. The results show: the more the axis load, the more the shear stress SXY; the shear stress SXY decreases quickly along with the structure layer deep increasing; the relations between the basic layer rigidity and the shear stress are curve change laws.

Laboratory Test Method for Dynamic Rebound Modulus of Subgrade Red Clay in Moist-Heat Area

Ten representative expressway pavement structures in the southern humid area were investigated, and then the stress state of subgrade soil under the pavement structure was calculated and analyzed by multi-layer elastic system theory. Based on the latest research results at home and abroad, combined with the stress and physical properties of subgrade soils in south moist and heat area, the laboratory test method for dynamic rebound modulus of subgrade soil in this area was determined.

Stress Distribution Characteristics of Runway Pavement under New Generation Large Aircrafts

New Generation Large Aircrafts became a new member of China large airports. However, its heavy weight and high contact pressure would have great impact on the flexible runway pavement. in this paper, based on the multilayered elastic theory, the tensile stress distribution under different airplane was studied to obtain the critical load positions. Furthermore, parameter sensitivity analysis was conducted, including thickness and modulus of Asphalt Stabilized Base (ATB) layer. Results indicated that increasing ATB thickness can only decrease the tensile stress a little, and increasing the modulus of the ATB layer will be more efficient than increasing its thickness. Finally pavement design suggestions composing of both the structural and material considerations were proposed.

The Different Pavement Combination Deflection Change Law Discuss

Based on the multi-layer elastic system theory, large general used finite element software is used to analysis in the paper. Among the three kind pavements, the flexible pavement maximal deflection and wheel gap deflection difference is most, and the rigidity pavement is least, and the semirigid pavement is centered. Whatsoever, under the same axis load, the surface layer bottom deflection diminish speed is quicker than the basic layer bottom deflection diminish speed after a sort along with the structure layer rigidity increase. Showing the pavement structure layer upside deflection is sensitive to the structure layer intensity and rigidity increase.

Effects of New Generation Large Aircrafts on Flexible Runway Pavement

In this paper, influences of New Generation Large Aircrafts (NGLA) on the runway rutting were analyzed and the importance of combining laboratory performance tests and suitable analytically-based methodology was emphasized. Based on the multilayered elastic theory, the vertical compression stress distributions under different airplanes including NGLA were studied. Then the dynamic creep test was conducted to obtain the visco-plastic characteristics of pavement including three mixture types. Furthermore, the rutting depth under different airplane was predicted by employing the Shells method and the dynamic creep test results. The results showed rutting depth would be more significant by loading cycles of NGLA, compared with that by ordinary airplane. Suggestions composed of both the structural and material considerations were proposed for runway pavement design.

The Different Road Surface Structure Combination Stress Contrast Analyse under the Axis Load

Based on the multi-layer elastic system theory , a large general used finite element software is used to analysis in the paper. To the asphaltum concrete surface layer pavement structure, the basic layer shares more load for surface layer along with the basic layer rigidity increasing. To the cement concrete surface layer pavement structure, the basic layer shares less load for surface layer along with the surface layer rigidity increasing. Whichsoever, the soil ground equivalent stress below subgrade 2m deep is almost not affected from the pavement structure layer rigidity change, but it will increase with the load increasing.

Cracks Resisting Mechanism Analysis of Large Stone Mixture Base Based on Mechanics Calculations

In order to study the cracks resisting mechanism of large stone asphalt mixture base, a multi-layer elastic theory program was used to calculate the loading stress in different pavement structures. Then a Finite Element model was established based on a twinkling heat conduct hypothesis to calculate temperature stress and strain of pavement structure when temperature dropped. Finally, the stress and strain of all the structural layers was calculated considering the coupling effect of loading and temperature. It is found that temperature stress which is caused by temperate quick dropped is far more lager than loading stress cause by standard loading, while considering the co-effect of vehicle loading and temperature quickly dropped. Thus it is revealed that cracking in pavement is mainly caused by temperature quickly dropped. By contrast, it is found that pavement stress and strain caused by loading and temperature of the structure with asphalt macadam mixture (ATB30) base are less than that of the conventional semi-rigid pavement. Finally, an asphalt macadam mixture base applied in asphalt pavement structure is believed to be an efficient way in reducing asphalt pavement cracking.

Fast layered elastic response program for the analysis of flexible pavement structures

One of the key components in analysing pavement structural behaviour is the response model which is used to estimate the stresses, strains and displacements of the pavement structure subjected to the existing traffic, taking into account the material properties and prevailing environmental conditions. Multilayer elastic theory (MLET) is often preferred over other methods such as the finite element method, due to its computational performance for repeated applications. A new elastic response analysis program has been developed based on the Burmister MLET theory to calculate the response of flexible pavement structures. In the development of the program, the time-consuming part of MLET processes was optimised. To improve the convergence and accuracy of responses in the vicinity of the surface of the top layer, an approach based on Richardson's extrapolation was employed. Moreover, an iterative approach to model stress dependency of unbound granular materials was incorporated. A comprehensive comparison of the program with two frequently used programs demonstrated an excellent agreement and improved performance.