Quantifying the Influence of Geosynthetics on Pavement Performance (2017)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

145 CHAPTER 6. SUMMARY AND SUGGESTED RESEARCH Summary A well-designed LST testing protocol with extensive instrumentations was conducted on flexible and rigid pavements using an 8-ft-diameter by 6-ft-high circular steel tank. The implemented testing program differed from previous studies, which traditionally assessed the influence of the base reinforcement on pavement performance under repetitive surface loading until failure. Instead, the experimental program focused on the following distinctive characteristics of the geosynthetic, which were essential for proper modeling of the geosynthetic material for base reinforcement, and ultimately for enhanced predictions of the performance of pavements with geosynthetic-reinforced base courses:  The stress distributions across the geosynthetic under dynamic loading in both the AC and PCC pavements.  The strain measurements in the geosynthetic and at the bottom of the surface layer (AC or PCC) under dynamic loading.  The deformed shape of the geosynthetic and the potential slippage at the geosynthetic and the unbound aggregate material interface under dynamic loading.  The potential slippage between the bottom of the PCC slab and the supporting CAB layer at the edge of the loaded slab. Upon completion of the testing program, a database of pertinent pavement responses with and without reinforcement of the base layer collected under realistic pavement loading conditions was assembled. The established database was then used in the numerical investigation (model and input parameters, etc.) of the LST data to assess the validity and applicability of the finite element numerical modeling of geosynthetic-reinforced pavement structures. Building upon the results of the finite element numerical validation, the Composite Geosynthetic–Base Course Model was developed as a subroutine for the AASHTOWare Pavement ME Design software. Both types of geosynthetics were included in the model: geogrids and geotextiles. The subroutine supplemented the Pavement ME Design software by making it possible to predict the performance of pavements when they were supported by unbound base/subbase courses that were reinforced with geosynthetics. The model required the input of the material properties of the unreinforced, unbound base course and the selected geosynthetics. It also required the input of the location of the geosynthetics within or beneath the base course. The input geosynthetic material property was its sheet stiffness measured at low strain levels (approximately 1 percent). The model also required the input of the shear interaction coefficient, which was determined by laboratory testing and indicated the degree to which the embedded geosynthetic restrained the surrounding unbound base course material. Internal to the subroutine, the model converted the two separate sets of material properties into a composite reinforced base course material property. This composite material property was used in a series of ANN models that were created in this project to predict the critical strains and stresses used in the current version of the Pavement ME Design software to predict several measures of pavement performance. The asphalt pavement performance measures are roughness (IRI), rutting, and fatigue cracking.

146 The ANN models were generated from large computed databases of the critical strains and stresses in asphalt pavements. No ANN models were developed for concrete pavements because of the insensitivity of critical stresses in concrete pavements to either type or location of geosynthetics. The databases incorporated the calculated values of the strains and stresses as they varied within a wide range of layer thickness, layer modulus, base course anisotropic ratio, embedded geosynthetic sheet stiffness, and geosynthetic location within or beneath the base course. The finite element program that was used to create these databases was capable of representing anisotropy, stress-dependent material properties, and small plastic deformation zones. The finite element program also had interface elements to represent the interaction, including the slippage, of the base course with the geosynthetic. The use of this finite element program was undertaken only after it had demonstrated the ability to match closely the measured displacements, strains, and stresses in the LST tests of typical asphalt and concrete pavements. Triaxial laboratory test methods were used to determine the resilient modulus and repeated load permanent deformation properties of the base course both with and without embedded geosynthetics. The test protocol was arranged to extract the anisotropic and stress- dependent properties of a base course as well as of a base course that was altered by geosynthetic reinforcing. The resilient modulus and repeated load permanent deformation test protocols have been prepared in the standard AASHTO format and are presented in Attachments A and B. A method was developed to use these same test results to determine analytically the shear interaction coefficient. It was also determined that a commonly used geosynthetic test, the pullout resistance test, may be used to determine the shear interaction coefficient. Suggested Research Based on the findings of this project, the following topics are suggested for future research:  Exploring the massive data generated by the LST tests: Much useful information and data were generated by the LST tests, only a fraction of which was used directly in this project. The experience that was gained in the instrumentation, measurement, and data interpretation has been documented and can be found in Appendices E through K. The data have extensive implications for material characterization, modeling, and improved performance prediction. As an example, models of faulting and erosion in concrete pavements can be substantially improved with the data observed and recorded and the permanent deformation models developed in this project. Inclusion of these results in the Pavement ME Design software will require a major revision of the concrete pavement structural subsystem within that software.  Expanding the range of material properties for the developed subroutine: The input to the Composite Geosynthetic–Base Course Model requires unbound base course material properties and geosynthetic properties. These properties can be provided at any of three levels of data refinement. There is a need to have a wider range of these properties covering all of the more commonly used base courses and geosynthetics

147 available to pavement designers in an electronically accessible catalog. This catalog would make this subroutine more extensively used and useful.  Extending the capabilities of the developed subroutine: Extending the ability of the Composite Geosynthetic–Base Course Model to include modified or stabilized materials is the next step to make this development more extensively applicable. These modified or stabilized models will be able to use the same three levels of input properties. The calibration of these models is expected to be considerably different from that of the unbound models. The research team has located a number of in-service pavements with stabilized base courses reinforced with geosynthetics in the LTPP database, which can be used for the calibration process.  Developing relations between soil suction and water content of the base course: The effects of the ambient moisture in the base course on the resulting performance of the pavement it supports suggest that a highly productive area for future research is to develop the soil-water characteristic curves (SWCCs; relation between the soil suction and the water content of the base course) of the more commonly used base courses. The SWCCs are critical for the following applications: o Allowing the Pavement ME Design software to consider the effect of this important variable in the design of both asphalt and concrete pavements as it varies with climatic and ground water conditions. o Being used in quality control (QC) and quality assurance (QA) processes during construction. The research focus on the moisture effects on the base courses will require the development of the soil-dielectric characteristic curve (SDCC) of base courses in addition to the SWCC. Having the SDCC available during construction permits the use of ground penetrating radar (GPR) as a QC/QA measurement technique. The advantage of using GPR for compaction QC/QA is that measurements can be made at highway speeds with approximately 1-ft spacing between individual measurements. Verifying compaction water content and density with a GPR-created strip map provides a better overview of the construction production quality than randomly selected sampling, as is done at present. Also, it can be done much more quickly with no loss of accuracy.