Advances in the Use of Geosynthetics in Pavement Projects

Geosynthetics have been used as reinforcement inclusions to improve pavement performance. While there are clear field evidences of the benefit of using geosynthetic reinforcements, the specific conditions or mechanisms that govern the reinforcement of pavements are, at best, unclear and have remained largely unmeasured. Significant research has been recently conducted with the objectives of: (i) determining the relevant properties of geosynthetics that contribute to the enhanced performance of pavement systems, (ii) developing appropriate analytical, laboratory and field methods capable of quantifying the pavement performance, and (iii) enabling the prediction of pavement performance as a function of the properties of the various types of geosynthetics. Zornberg, J.G. (2015). “Advances in the Use of Geosynthetics in Pavement Projects. ” Keynote Lecture, Proceedings of the 2nd Iberic Conference on Geosynthetics, Geosintec Iberia 2, 07-08 October, Madrid, Spain (CD-ROM). eral drainage within its plane. In addition, geosynthetics have been used to mitigate the propagation of cracks by sealing the asphalt layer when used in pavement overlays. Finally, geosynthetics have been used in flexible pavements for reinforcement, which is the main focus of this paper. While the reinforcement function has often been accomplished using geogrids, geotextiles have also been used as reinforcement inclusions in transportation applications (Bueno et al. 2005, Benjamin et al. 2007). The geosynthetic reinforcement is often placed at the interface between the base and sub-base layers or the interface between the sub-base and subgrade layers or within the base course layer of the flexible pavement. This leads to lower stresses over the subgrade than in unreinforced flexible pavements (Figure 2). Figure 2: Relative load magnitudes at subgrade layer level for (a) unreinforced flexible pavement and (b) geosyntheticreinforced flexible pavement The improved performance of the pavement due to geosynthetic reinforcement has been attributed to three mechanisms: (1) lateral restraint, (2) increased bearing capacity, and (3) tensioned membrane effect (Giroud and Noiray 1981, Giroud et al. 1984, Perkins and Ismeik 1997, Holtz et al. 1998). These three mechanisms are illustrated in Figure 3. The primary mechanism associated with the reinforcement function for flexible pavements (Figure 3a) is lateral restraint or confinement (Bender and Barenberg 1978). The name of this mechanism may be misleading as lateral restraint develops through interfacial friction between the geosynthetic and the aggregate, thus the mechanism is one of a shearresisting interface (Perkins 1999). When an aggregate layer is subjected to traffic loading, the aggregate tends to move laterally unless it is restrained by the subgrade or by geosynthetic reinforcement. Interaction between the base aggregate and the geosynthetic allows transfer of the shearing load from the base layer to a tensile load in the geosynthetic. The tensile stiffness of the geosynthetic limits the lateral strains in the base layer. Furthermore, a geosynthetic layer confines the base course layer thereby increasing its mean stress and leading to an increase in shear strength. Both frictional and interlocking characteristics at the interface between the soil and the geosynthetic contribute to this mechanism. Consequently the geogrid apertures and base soil particles must be properly sized. A geotextile with good frictional capabilities can also provide tensile resistance to lateral aggregate movement. Figure 3: Reinforcement mechanisms induced by geosynthetics (Holtz et al. 1998): (a) Lateral restraint; (b) Increased bearing capacity; and (c) Membrane support As illustrated in Figure 3b, the increased bearing capacity mechanism leads to soil reinforcement when the presence of a geosynthetic imposes the development of an alternate failure surface. This new alternate plane provides a higher bearing capacity. The geosynthetic reinforcement can decrease the shear stresses transferred to the subgrade and provide vertical confinement outside the loaded area. The bearing failure mode of the subgrade is expected to change from punching failure without reinforcement to general failure with reinforcement. The geosynthetic can also be assumed to act as a tensioned membrane, which supports the wheel loads (Figure 3c). In this case, the reinforcement provides a vertical reaction component to the applied wheel load. This tensioned membrane effect is induced by vertical deformations, leading to a concave shape in the geosynthetic. The tension developed in the geosynthetic contributes to support the wheel load and reduces the vertical stress on the subgrade. High deformations (i.e. high rutting depth) are required to mobilize this mechanism. This reinforcement mechanism has been reported to develop only in cases with subgrade CBR values below 3 (Barksdale et al. 1989). The aforementioned mechanisms require different magnitudes of deformation in the pavement system to be mobilized. In the case of unpaved roads, significant rutting depths (in excess of 25 mm) may be tolerable. The increased bearing capacity and tensioned membrane support mechanisms have been considered for paved roads. However, the deformation needed to mobilize these mechanisms generally exceeds the serviceability requirements of flexible pavements. Thus, for the case of flexible pavements, lateral restraint is considered to contribute the most for their improved performance. 3 DESIGN METHODOLOGIES FOR GEOSYNTHETIC-REINFORCED FLEXIBLE PAVEMENTS The design philosophy of flexible pavement systems was initiated by the Romans, evolving into the current design approaches. The design approach involves providing a protective layer over the subgrade that improves the serviceability under traffic and environmental loads. The Cover Based Design Method was developed after the great depression in the 1930s. It required a single input in terms of the California Bearing Ratio (CBR), but it also required use of significant engineering judgment. Subsequently, and after completion of the American Association of State Highway Officials (AASHO) Road Test in the 1960s, a series of design methods were proposed. They were more sophisticated than the Cover Based Method, requiring a greater number of design parameters as input. In the 1970s, the linear mechanistic-empirical (M-E) design method was proposed by researchers from South Africa. Since the early 1990s, the focus in the US has shifted to M-E design methods that incorporate features from purely empirical methods to sophisticated non-linear finite element methods. Attempts have been made to incorporate the use of geosynthetic reinforcements into AASHO and M-E design methods.