Quantifying the Influence of Geosynthetics on Pavement Performance (2017)

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1 CHAPTER 1. INTRODUCTION Introduction Geosynthetics are available in a wide range of forms and materials and are used in many applications. Geosynthetics are often used by highway agencies in conjunction with unbound base layers (i.e., within the layer or as a subgrade/base interface layer) to enhance the performance of flexible and rigid pavements. Although much research has been performed on the properties of these materials and their use in pavement structures, limited research has dealt with the methodologies of quantifying their influence on pavement performance in a manner that would allow incorporation into the mechanistic-empirical pavement design and analysis procedures. The AASHTOWare Pavement ME Design software provides a methodology for the analysis and performance prediction of pavements. However, use of geosynthetics in pavement layers and their influence on distress models have not been included in Pavement ME Design. Procedures that quantify the influence of geosynthetics on pavement performance will help in determining the payoff of using these materials and selecting the appropriate material for a specific application. However, such information is not readily available. As a result, the research in this project was initiated to (a) evaluate those tests currently used for characterizing geosynthetics and identify how tests relate to performance, and (b) develop a methodology for quantifying the influence of geosynthetics on performance for use in pavement design and analysis. This information can be incorporated into the Pavement ME Design software, thus allowing a rational analysis and design procedure of flexible and rigid pavements in which geosynthetics are used in conjunction with unbound bases and subbases. Objective The objective of this research project was to develop a methodology for quantifying the influence of geosynthetics on pavement performance for use in pavement design and analysis. The methodology should be consistent with the Pavement ME Design framework to facilitate incorporation into the AASHTOWare Pavement ME Design software. This project focused on the use of geosynthetics in unbound base/subbase layers or as a base/subgrade interface layer for flexible and rigid pavements. Research Scope and Approach The project was divided into six components: (a) full-scale laboratory testing of typical asphalt and concrete pavement sections in an instrumented large-scale tank (LST); (b) laboratory triaxial testing of different base courses with geosynthetics at different locations within the test samples; (c) finite element computations to match the results of the full-scale tests; (d) use of the same finite element program to develop full factorial sets of pavement data to construct the Artificial Neural Network (ANN) models of the critical strains and stresses in pavements; (e) generation of a new model of permanent deformation to predict pavement performance; and (f) comparison of the predicted performance of pavements with and without geosynthetics embedded in the unbound base courses.

2 The deliverables of this project included a computer subroutine written for incorporation into the Pavement ME Design software to predict the performance of pavements with geosynthetics. This computer program was named “Composite Geosynthetic–Base Course Model” and was supported by other major deliverables, including testing protocols that produced the geosynthetic property and modified base course property inputs for the Pavement ME Design software that were needed to accurately predict their influence on pavement performance. These predictive relations were based on full-scale measurements made in an LST of typical flexible and rigid pavements under static and dynamic loading. The complete set of measured data in electronic form was another major deliverable of this project. Summaries of these data are presented in Appendices E through K. The measurements and observations were matched closely with computations made with a finite element computer program equipped with interface elements. Multiple runs with the finite element program over a wide range of pavement variables produced the data for constructing ANN models of the critical strains and stresses in pavements used to predict the performance of those pavements. The permanent deformation models of the base course and the subgrade were replaced with other models that fit the data more reliably and incorporated the stress state levels. These are the subroutines referred to above. These new models of the critical stresses and strains and permanent deformation were used to compute the roughness and the principal distresses of pavements. The computation of the critical stresses in rigid pavements for predicting transverse cracking showed that these stresses are insensitive to the type or location of geosynthetics (see discussions in Chapter 4). This result indicates that geosynthetics will have negligible influence on this type of rigid pavement distress. However, there is a strong likelihood that the ability of geogrids to reduce the permanent deformation of base courses, as shown in Chapter 4, will be able to improve pavement performance by reducing the joint faulting and roughness in these rigid pavements. The calibration of these critical stresses and strains and permanent deformations to these different measures of flexible pavement performance were left unchanged, relying on the validity of the calibration that was done in the existing version of the Pavement ME Design software. Verification of these relations was accomplished to the extent possible with existing in-service pavement sections with embedded geosynthetics. The relationship between the existing model in the Pavement ME Design software and the Composite Geosynthetic–Base Course Model developed in this project is illustrated in Figure 1.1. Examples of using the Composite Geosynthetic–Base Course Model to analyze pavement structures with/without a geosynthetic layer are provided in Appendix Q.

3 Figure 1.1. Compatibility of Proposed Program with AASHTOWare Pavement ME Design Traffic Material Properties Climate EICM Pavement Structure Pavement Response (σ, ε) Flexible Pavement Distress Models INPUT Pavement Response AASHTOWare Model Composite Geosynthetic–Base Course Model Geosynthetic Geogrid Geotextile Property Location Rigid Critical Strains, Stresses, Displacement, Permanent Deformation Property Location Modified Material Properties ANN Models Flexible Rigid Critical Strains, Stresses, Displacement, Permanent Deformation Pavement Performance Prediction

4 Organization of the Report This report is organized into six chapters:  The first chapter presents the introduction and research approach used in this research project.  The second chapter presents a synthesis of current knowledge of quantification of influence of geosynthetics on pavement performance.  The third chapter presents a research plan of this project.  The fourth chapter presents the experiments, models, and major findings. These include the tests that were conducted to determine the effects of both geogrids and geotextiles on the anisotropic properties of unreinforced, unbound base courses; the results of the LST tests on both flexible and rigid pavements; the identification of the conditions under which slippage occurred between the base course and the geosynthetic material; the close correspondence between the stresses, strains, and displacements that were observed in the LST tests and modeled in the finite element program; the important properties of geosynthetics that affected the performance of a pavement; the method of converting the properties of an unreinforced base course and geosynthetics into a composite input value to the ANN models of reinforced base/subbase courses; the use of the pullout resistance test to obtain design values of the interaction coefficient between the base course and geosynthetics; the improved prediction of permanent deformation of base courses under repeated loading by replacing the current Pavement ME Design model with the new, recently developed model; and the comparison of the critical strains, stresses, and displacements in a pavement with a reinforced base versus one without a reinforced base. The commentary in this chapter provides an overview, while many of the corresponding details are contained in a number of appendices introduced in the chapter.  The fifth chapter presents interpretation, appraisal, and application of the findings discussed in the second chapter. It gives examples of the application of the new models to specific pavement conditions, traffic levels, and geosynthetic placement in different locations within and beneath the base course.  The sixth chapter presents conclusions and suggested further research. The main body of the report is written to give an accurate overview of the approach taken and the results of this project. More detailed discussions of the topics in the report are contained in Appendices A through Q.