The Research On Structural Modulus Of Unbound Granular Layer And Subgrade In Flexible Asphalt Pavement

Resilient modulus is the most important parameter of unbound granular layer and soil subgrade in flexible asphalt pavement structure design.Because of the nonlinear characteristic of granular and soil and the repetitive load-unload cycle from traffic,the resilient modulus of unbound granular layer and subgrade become stress dependent.Based on the nonlinear characteristic of granular and soil,this dissertation conducted a systematic study,targeting the problem of choosing proper design moduli for unbound granular layer and subgrade.A two-dimensional rotational symmetrical finite element model based on Uzan universal model was developed and verified.The moduli distribution in the unbound granular layer and subgrade of flexible asphalt pavement was studied.It is found that moduli of granular layer and subgrade changes with stress state within the pavement structure.Popular linear elastic moduli of unbound granular layer and subgrade were compared and evaluated,and the differences of pavement responses between different methodologies are significant.The differences of tensile strain at bottom of asphalt layer are-22%?-8%,and differences of vertical displacement at top of subgrade can be as much are-32%?6%.Flexible pavement structure behaviors under moisture-thermal influence were studied.Field pavement structure,material and continuous environment monitoring data for a year were collected,with 95 sets of temperature-moisture conditions were identified.Effects of overload and thin asphalt layer were also investigated.Nonlinear elastic constitutive model,linear elastic modulus of granular layer and subgrade selected by Mechanistic Empirical Pavement Design Guide(MEPDG-2012)and Specifications for Design of Highway Asphalt Pavement(JTG D50-2017)were employed for asphalt pavement structural analysis,with a total of 855 cases analyzed.It is found that structure response and long-term performance are significantly different among the three methodologies.In accordance to different combinations of structure and load,the relative differences of maximum tensile strain at bottom of asphalt layer can be as high as 80%,and relative differences of maximum permanent deformation at top of subgrade can reach 40%.The fatigue life of asphalt layer varies between-47%and 30%,while service life of subgrade ranges from-33%to 50%.The conflicts between "nonlinear elastic material model" and "linear elastic design method" was spotlighted.Two methodologies to solve structural modulus were proposed:one is "based on equivalent surface deflection" and the other "based on equivalent structural response".The former is derived from the layer modulus backcalculation principle,while the latter is calculated by an iterative algorithm developed by the author.Both methods are able to get a converged structural modulus from a variety of structure,environment and load conditions quickly and robustly,but the values of these two structural modulus are vastly different.The pros and cons of the two methods are evaluated:the former methodology is in line with the current back-calculation principle,while the latter methodology is able to produce a structural modulus value that can be used directly in pavement structure design,making it a promising future.The relation between the two structural modulus were developed,and can be well fitted by second-order polynomial,indicating that the two structure modulus can be converted and predicted from each other.The stress differential constitutive equations of granular material were developed,based on which the load sequence of Repeated Load Triaxial Test was modified.An advanced method to test dynamic resilient properties of granular material was proposed,which can solve the stress dependency and anisotropy of resilient modulus as well as Poisson's ratio simultaneously.The influences of different factors were also studied.The laboratory study for dynamic resilient properties of granular material is improved and expanded.In addition,asphalt pavement structural analysis and structural modulus research were conducted with consideration of anisotropy.Influence of anisotropy on pavement structural response and structural modulus was investigated.When anisotropy of unbound granular layer and subgrade was considered,pavement response are increased,while the structural modulus is decreased.What's more,the stress state at the bottom of unbound granular base is improved.The current hypothesis of isotropic material carries the risk of underestimating pavement damage.This paper proposed the methodology for structural modulus of unbound granular layer and subgrade,based on nonlinear mechanical behavior analysis of flexible asphalt pavement structure.A study of dynamic resilient property of granular material was further conducted.The results are believed to provide basis for research on sustainable asphalt pavement and are of significance for improving asphalt pavement design.