Improvement Of Backcalculation Method For Asphalt Pavement Based On The Deflection Basins

Falling Weight Deflectometer(FWD)can apply a load similar to the traffic load to the pavement structure,which can simulate the mechanical response of the pavement in use.The several displacement sensors can be used to collect the deflections.Then,the layer moduli can be backcalculated with structural information and the load level.Therefore,FWD is widely used to evaluate the bearing capacity of pavement structures throught their life cycle.With the development of computer technology and pavement mechanics theory,a variety of modulus backcalculation methods have been established.However,in order to futher improve the backcalculation method accuracy and applicability to the pavement structures,there are still several peoblems.The accuracy of the rigid layer depth setting method needs to be improved.When the asphalt pavement surface layer thickness is thin,this kind of pavement structure accounts for a large proportion,the accuracy of the backcalculated surface layer moduli is very low and the variability is very large.Although the inertial point method and the characteristic point method proposed in recent years can solve the uniqueness of the backcalculation results in the traditional backcalculation methods,the range of pavement parameters selected is small,the combination of modulus and thickness is unreasonable,and the range of applicable pavement structures is limited.In view of the above problems,this paper proposes a method for setting the rigid layer depth based on the vertical compressive strain index of the subgrade layer,using the pavement deflections detected layer by layer.For the asphalt pavement structure of thin surface layer,the reasons for the large errors in the backcalculation results were found,and a new method that can significantly improve the backcalculation accuracy was proposed.A new method of virtual inertial point parameter calculation was proposed,which improved the backcalculation accuracy.The main research results are as follows:Firstly,in terms of the rigid layer depth setting model,based on the layer-by-layer detection deflection basins of different types of pavement structures,a setting method named the controlled strain method with the control of the compressive strain of the subgrade layer as an indicator was established with the objective of minimizing the backcalculated subgrade moduli variability.It was found that the rigid layer should be set at the position where the vertical compressive strain is 7.64με.Through the comparative analysis with other methods of setting the rigid layer depth,it was shown that the rigid layer setting method was more reasonable.Secondly,based on theoretical and measured deflections,it was verified that when the thickness of asphalt surface layer was thin(less than 10 cm),the backcalculated surface layer modulus were highly variable.In this study,it was found that the actual strain values of the continuous system assumed in the elastic layer theory were not exactly the same at the interface bewteen the asphalt surface layer and the base layer,and there was a situation of uncoordinated deformation,which was the main reason for the large difference of the backcalculation results.Therefore,based on the elastic layered theory,a method to consider this deformation incoordination in the backcalculation analysis was proposed.Through the analysis of a large number of measured deflections,it was found that the interlayer coordination coefficient increased with the increase of the asphalt surface thickness,and decreased with the increase of temperature.Accordingly,a function of the interlayer coordination coefficients was established.The function of considering the coordination coefficient between layers was added in the inversion software,and the improved software was verified.Thirdly,in the improvement research of the virtual inertial point method,the current range of pavement structure thickness and modulus parameters and the structure combination modes were formulated.Through extensive calculation and analysis,it was found that the inertial point position was closer to the load center with the subgrade modulus increased,while it was farther from the load center with the structural layer thickness and the rigid layer depth increased.On the other hand,the inertial point deflection gradually decreased with the increase of the subgrade modulus and the rigid layer depth,but gradually increased with the increase of the structural layer depth.Accordingly,the inertial point parameter equation of the flexible base pavement structure was established and improved for the semi-rigid base pavement structure.It was shown that the improved virtual inertial point method could effectively enhance the accuracy of the subgrade backcalculation modulus according to the validation analysis of theoretical and measured deflections.Finally,a modulus backcalculation method was established,which used the virtual inertial point method to backcalculate the subgrade modulus and the deflection basin matching method to backcalculate the surface and base layer moduli.The backcalculation method was programmed into software called In-Simu.The accuracy and applicability of the modulus backcalculation method were verified by comparing the measured deflections of different asphalt surface layer thicknesses with several other types of backcalculation software.