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    <managingEditor>tris-trb@nas.edu (Bill McLeod)</managingEditor>
    <webMaster>tris-trb@nas.edu (Bill McLeod)</webMaster>
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      <title>Wave Propagation Analysis for Flexible Pavements and Its Application to Backcalculation Analysis</title>
      <link>https://trid.trb.org/View/1092778</link>
      <description><![CDATA[The objective of this research is to present derivation of closed form solutions using wave propagation technique as well as backcalculation analysis of time series Falling Weight Deflectometer (FWD) data. FWD is an impact load testing device resulting in waveform displacements that propagate on the pavement surface. It is under this premise that wave propagation solutions forms the basis for the development of backcalculation algorithm. In order to derive solutions based on wave propagation technique, mechanical properties of the layers constituting the multilayered flexible pavement system are represented using either Voigt or complex modulus models. Considering the FWD loading plate is circular in shape, the governing equations for wave propagation problems in a multilayered system are represented using cylindrical coordinates system. Further, analytical solutions are derived by expressing the displacements, in the wave propagation equations, in terms of two harmonic potentials. Thereafter, closed form solutions are derived using Hankel Transform in space domain together with Fast Fourier Transform in time domain. Software developed is called Wave Propagation Analysis for Layered System (Wave-PALS). Results obtained using Wave-PALS are compared with the ones obtained using a commercial finite element package, ADINA. Good agreement of the results from the two software packages confirms the validity and accuracy of the derived solutions as well as Wave-PALS. Furthermore, in order to carry out backcalculation analysis, analytical displacements in frequency domain have to be transformed to time domain so that unknown parameters, which result in good agreement between computed and measured displacements at discrete times, may be determined. Modified Gauss-Newton method was used in the backcalculation algorithm giving stable and well converged results for FWD time series data. Evaluation of the results have shown that static backcalculation tend to overestimate elastic moduli of the upper layers. Further, mechanical characterization of the pavement materials using Voigt model give overall better fit of computed deflections to measured deflection than complex modulus model. Finally, static and dynamic backcalculation analyses are compared and the results show that layer moduli for the asphalt concrete layer differ between the two approaches while layer moduli for the underlying layers are relatively close between the two methods.]]></description>
      <pubDate>Mon, 21 Mar 2011 14:13:38 GMT</pubDate>
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      <title>MODIFIED NEWTON ALGORITHM FOR BACKCALCULATION OF PAVEMENT LAYER PROPERTIES</title>
      <link>https://trid.trb.org/View/378422</link>
      <description><![CDATA[An efficient algorithm for the backcalculation of pavement layer moduli from measured surface deflections is presented.  The algorithm is an iterative one and can use any mechanistic analysis program for forward calculations (presently an extended precision CHEVRON program is used for this purpose).  Most mechanistic-based backcalculation methods attempt to find the layer moduli that minimize the weighted sum of the relative or absolute errors between measured and predicted surface deflections.  Using a search technique to achieve such a minimization sometimes requires hundreds of calls to a mechanistic analysis program, and some programs try to speed this up by using a previously created data base.  The algorithm presented here is different in that it uses a modified Newton method to obtain the least-squares solution of an overdetermined set of equations.  This gives the proposed algorithm a robustness that some other approaches appear to lack.  For example, the predicted moduli are not too sensitive to the initially assumed seed moduli or the location of the stiff layers (e.g., CRAM section, composite pavements, shallow or deep bedrock, etc.).  Further, a set of auxiliary equations that are totally independent of those used in the modified Newton method and that relate surface deflections to the compressions in each pavement layer are used to improve the speed of convergence. The algorithm is also extended to improve incorrectly specified layer thicknesses.  The algorithm is being implemented in a new backcalculation program named MICHBACK.]]></description>
      <pubDate>Wed, 18 Aug 1993 00:00:00 GMT</pubDate>
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