Quantifying the Influence of Geosynthetics on Pavement Performance (2017)

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5 CHAPTER 2. SYNTHESIS OF CURRENT KNOWLEDGE Geogrids and geotextiles have been the most commonly used geosynthetic products for enhancing pavement performance (1–5). Beneficial effects of the geosynthetic layer have been identified on the responses of pavements under traffic loading through two major mechanisms (6–13):  Lateral confinement, which is produced by the interface frictional interaction and interlocking between base course aggregates and the geosynthetic layer. Significant tensile stress is generated in the geosynthetic layer when a spread motion is created by traffic loading, which in turn reduces the vertical stress and shear stress dramatically due to the increased base course stiffness.  Vertical membrane effect. The inward shear stress caused by membrane deformation reduces the outward shear stress generated by repetitive wheel loading. As a result, the vertical stress is then reduced and distributed widely around the geosynthetic layer. In addition to the above major reinforcement mechanisms, the layer separation provided by geotextiles is another important function that prolongs pavement service life. Layer separation reduces the base course contamination, which significantly increases the resilient modulus of the base course and then increases the pavement service life. To extend the use of geosynthetics in pavements, there is a need to incorporate geosynthetic material into pavement design. Accurate prediction of geosynthetic-reinforced pavement performance is a key to pavement design in this respect. The Pavement ME Design software is usually used to predict the pavement performance by taking into account a variety of factors, such as pavement structure, material property, traffic, and climate. However, it does not include geosynthetic material for pavement design. Thus, it is desirable to develop a methodology to incorporate geosynthetic material into the Pavement ME Design software so that the performance of geosynthetic-reinforced pavements can be accurately predicted. Generally, there are three critical steps involved to achieve this target: (a) laboratory characterization of geosynthetic-reinforced unbound granular material, (b) numerical modeling of geosynthetic- reinforced pavement, and (c) prediction of geosynthetic-reinforced pavement performance using the computed pavement responses. Laboratory Characterization of Granular Materials with Geosynthetics Many studies have been conducted to characterize the effect of geosynthetic reinforcement on the vertical resilient modulus of the unbound granular materials (UGMs). It was found that the geosynthetic did not have a significant effect on enhancing the vertical resilient modulus of the reinforced UGMs when the specimen was fabricated as a 15-cm- diameter and 30-cm-high cylinder (14) or a 20-cm-diameter and 40-cm-high cylinder (15). In contrast, Rahman et al. (16) reported that the geosynthetic was effective at improving the resilient modulus of the reinforced UGMs when the specimen size was reduced to a dimension of 15-cm diameter and 20-cm height. Therefore, it was inferred that the effect of the geosynthetic reinforcement on the resilient modulus of the UGMs depended on the dimensions of the UGM specimen. Yang and Han (17) developed an analytical model to predict the resilient modulus of the geosynthetic-reinforced UGMs at any given dimensions. According to this analytical model,

6 the geosynthetic was more effective in increasing the resilient modulus of the UGMs with a larger diameter and a smaller height. McDowell et al. (18) and Schuettpelz et al. (19) showed that the geosynthetic provided the reinforcing effect in an area that is typically approximately 3 cm to 7.5 cm in thickness on both sides of the geosynthetic. Since the geosynthetic reinforcement influence zone had such a small range, quantifying the influence of geosynthetics on the vertical resilient modulus of the UGMs with a 30-cm height or more would be inappropriate. Recent studies revealed that the UGMs exhibit cross-anisotropic resilient behavior (i.e., the resilient moduli in the vertical plane were different from the horizontal resilient moduli, while the resilient moduli in the horizontal plane were the same in all directions) (20, 21). The cross-anisotropic nature of the UGMs was demonstrated to be a major factor that influences pavement performance (22). Therefore, quantifying the influence of geosynthetics on the resilient properties of UGMs should focus on evaluating the effect of geosynthetics on the cross- anisotropic properties of the base course—an effort that was not identified in any of the literature that was reviewed in this study. Compared to the increase of the resilient modulus, the reduction of the permanent deformation of UGM is a more important benefit of the geosynthetic reinforcement. Perkins et al. (23), Wayne et al. (24), and Nazzal et al. (14) found that the geosynthetic considerably reduced the permanent deformation of the UGMs using the repeated load triaxial tests. Moghaddas-Nejad and Small (15) and Abu-Farsakh et al. (25) showed that for a particular confining stress, the reduction of the permanent deformation by the geosynthetic increased rapidly with the increase of the deviatoric stress, until a peak was reached, and then it decreased gradually. This finding indicated that the stress level significantly influenced the effects of the geosynthetic on the reduction of the permanent deformation of the UGMs. It was known that the stress induced by the traffic load was non-uniformly distributed in the base course of pavements. Therefore, quantifying the effect of stress level on the permanent deformation characteristics of the geosynthetic-reinforced UGMs was critical to accurately predicting the pavement performance. The permanent deformation of the base layer was directly related to the rutting of flexible pavements and the faulting of the joints in rigid pavements. Since the present Pavement ME Design does not permit the use of permanent deformation of the base layer to predict either the erosion or the faulting of the joints in rigid pavements, a major revision of the structural subsystem of the rigid part of the Pavement ME Design is required. Modeling of Pavements with Geosynthetics The influence of geosynthetics on pavement structures has been evaluated using finite element models. Specifically, the finite element models were constructed to compute pavement responses (stresses, strains, and deformations) of pavements (with/without a geosynthetic layer) under different loading configurations. These pavement responses were used to evaluate the influence of using the geosynthetic layer as base reinforcement (2, 3, 9, 26–29). The elements addressed in the finite element models included geosynthetic geometric characteristics, traffic loading, constitutive models of materials, and interface condition. Table 2.1 summarizes the features of the finite element models constructed for geosynthetic-reinforced pavements and the

7 corresponding modeling techniques. All the pavements represented in Table 2.1 were flexible pavements; no models were found for rigid pavements. Table 2.1. Summary of Finite Element Model Studies on Geosynthetic-Reinforced Pavements Developer Geometry Surface Constitutive Model Base Constitutive Model Geosynthetic Constitutive Model Interface Model Subgrade Constitutive Model Barksdale and Brown (30) Axial symmetric Isotropic nonlinear elastic Anisotropic linear elastic Isotropic linear elastic membrane Linear elastic- plastic Isotropic Dondi (31) Three dimension Isotropic linear elastic Isotropic elastoplastic D-P Isotropic linear elastic membrane Elastic- plastic Mohr-C Isotropic elastoplastic Cam-Clay Wathugala et al. (32) Two dimension Isotropic elastoplastic D-P Isotropic elastoplastic D-P Isotropic, elastoplastic membrane None Isotropic elastoplastic HiSS Perkins (3) Three dimension Anisotropic elastic- perfectly plastic Isotropic plastic Anisotropic elastic-plastic membrane Mohr-C Isotropic plastic Saad et al. (27) Three dimension Isotropic linear elastic Isotropic elastic- plastic D-P Isotropic linear elastic membrane Perfect bonding Isotropic elastoplastic Cam-Clay Luo (33) Two dimension Isotropic linear elastic Isotropic linear elastic Isotropic linear elastic truss element Perfect bonding Isotropic linear elastic Kwon (4) Two dimension Isotropic linear elastic Anisotropic nonlinear elastic Isotropic elastic membrane Linear elastic element Isotropic linear elastic Design Methods for Pavements with Geosynthetics Empirical and mechanistic-empirical design methods have been developed for pavements with geosynthetics (1, 34). The empirical design approaches for geosynthetic-reinforced pavements were based on relating the laboratory testing results to the field conditions. The limitation of the empirical design method was that it could be applied only to the limited field conditions from which the data were taken. Compared to the empirical design methods, the mechanistic-empirical design methods were based on finite element models and were more reliable for geosynthetic-reinforced pavement design (3, 4, 29, 35, 36). Table 2.2 presents a

8 summary of the design methods for pavements with geosynthetics. Design methods were not found in the literature for rigid pavements with geosynthetics. Table 2.2. Summary of Design Methods for Pavements with Geosynthetics Developer Design Method Geosynthetic Reinforcement Modification Distress Mode Practice Support Mechanics Support Barksdale and Brown (30) Mechanistic- empirical Isotropic, linear elastic model using membrane element Surface deformation Field results Finite element model Webster (35) Empirical Direct extrapolation from field test results Rut depth Field results None Perkins et al. (34) Mechanistic- empirical Isotropic, linear elastic model using membrane element Surface deformation Field results Finite element model Giroud and Han (11) Empirical Bearing capacity factor, bearing capacity mobilization coefficient, stress distribution angle Stresses at the base course/subgrade interface, rut depth Field wheel load test, laboratory cyclic plate loading test None Kwon (4) Mechanistic-empirical Anisotropic, nonlinear elastic model using membrane element Vertical strain on the top of subgrade, vertical deflection Full-scale test results Finite element model