Jayhyun Kwon

Geogrid in Flexible Pavements: Validated Mechanism

Full-scale accelerated testing was used to provide new insight into quantifying the effectiveness of geogrids on low-volume flexible pavement performance. Although several previous studies report that geogrids improve pavement performance by enhancing structural capacity and reducing distress potential, the new study addresses how to maximize the benefits and cost-effectiveness of geogrid. To perform full-scale testing, three cells of flexible pavements, each having three pavement sections, were constructed. The granular base and hot-mix asphalt (HMA) layer thicknesses varied, and each cell had at least one control and one geogrid-reinforced pavement section. Instruments were embedded during construction to measure stress, strain, deflection, moisture, pore-water pressure, and temperature and were used to monitor pavement response to moving load. A moving dual-tire at 8 km/h and 44 kN was used to apply accelerated traffic loading. The performance of the various pavement sections when exposed to accelerated loading is presented. On the basis of pavement measured response as well as visual observation of the pavement cross section after excavation, the study showed that geogrid is very effective in reducing the horizontal shear deformation of the aggregate layer, especially in the traffic direction. Hence, the effectiveness of geogrid is clear for aggregate base layers with thicknesses ranging from 203 to 457 mm, and geogrid is expected to show similar effectiveness for greater base thickness given that thin HMA layer is used. The study also found that the optimal geogrid location in a thin aggregate layer is at the unbound aggregate-subgrade interface. For a thicker base layer, it is optimal to install a single geogrid at the upper third of the layer; the addition of another geogrid at the subgrade-base layer interface may be needed for stability.

Aggregate base residual stresses affecting geogrid reinforced flexible pavement response

This paper presents findings of an analytical study aimed at investigating the effects of unbound aggregate base residual stresses on the resilient response behaviour of geogrid reinforced flexible pavements. A finite element (FE) based mechanistic response model recently developed at the University of Illinois was used to predict critical pavement responses of unreinforced and geogrid reinforced flexible pavements. The nonlinear analyses performed in the base and subgrade also considered different horizontal compressive residual stress distributions introduced as locked-in initial stresses in the base course due to pavement construction and subsequent repeated traffic loading. The primary focus was to study effects of increased confinement and stiffening around geogrid on improved granular layer moduli and reduced critical subgrade vertical strains/stresses by assigning initial residual stresses around the geogrid tensile reinforcement in the FE pavement continuum analysis. An increase in horizontal confinement resulted in significant increases in the moduli of the base and subgrade layers in the vicinity of the geogrid reinforcement. The degree of structural benefit provided by geogrid reinforcement could be successfully quantified in the response analysis to show the commonly observed technical benefit of geogrids in the field.

Geogrid Base Reinforcement with Aggregate Interlock and Modeling of Associated Stiffness Enhancement in Mechanistic Pavement Analysis

The main mechanism by which geogrids reinforce unbound aggregate base and subbase layers of flexible pavements is the geogrid aggregate interlock. Geogrids prevent aggregate material from moving laterally under applied wheel loading; this enhances local strengthening and stiffness in the base layer. This higher stiffness zone essentially benefits the geogrids pavement response by better bridging over the weak subgrade soil and transmitting reduced critical stresses and strains on top of sub-grade. The University of Illinois developed a mechanistic model for the analysis of geogrid reinforced flexible pavements based on the finite element approach. This approach was used to model successfully the development of a stiffer layer associated with aggregate interlock around the geogrid reinforcement by considering compaction-induced residual stresses as the initial condition in the mechanistic analysis. The predicted critical pavement responses matched with the measured values from full-scale pavement test studies. Further, field-observed stiffening and strengthening of the geogrid-reinforced base courses were documented from dynamic cone penetrometer (DCP) test results. Significant reductions in measured pavement responses, especially in base course lateral movements caused by geogrid inclusion, were apparent in the University of Illinois full-scale test sections. Therefore, the longer traffic lives observed established the pavement performance benefits. When the same DCP evaluations were conducted on geogrid base reinforced in-service pavements in California, similar trends were observed in increased base course strength, and stiffness properties were successfully linked to immediate (enhanced compaction) and long-term (retained–improved stiffness) benefits.

Validated Mechanistic Model for Geogrid Base Reinforced Flexible Pavements

A mechanistic response model was recently developed at the University of Illinois to analyze geogrid base reinforced flexible pavements designed for low to moderate traffic volumes with a relatively thin hot-mix asphalt surface layer. This model utilizes the finite element approach and properly considers (1) the nonlinear, stress-dependent behavior of pavement foundation geomaterials, i.e., unbound aggregates and fine-grained soils; (2) anisotropic behavior of the granular base layer; and (3) the compaction and preloading induced unbound aggregate base residual stresses. To validate the developed mechanistic model, field response data were collected from instrumented full-scale pavements constructed with both geogrid reinforced and control test sections. The model predictions using the nonlinear and anisotropic characterizations of the granular base layer moduli were found to better capture the magnitudes and the trends in the measured response data. After trafficking to failure of the pavement test sections, pavement trench studies were conducted to gather constructed layer thicknesses and additional forensic data in an effort to provide refined inputs for the mechanistic response model. An increase in stiffness observed around the geogrid reinforcement in the field was properly modeled by the use of horizontal residual stresses above the geogrid, which resulted in a better match of the predicted with the measured pavement responses of the reinforced sections. As a result, the mechanistic model predictions computed at different locations in the test sections compared reasonably well with a large number of field measured responses under different load levels.

Interface modeling for mechanistic analysis of geogrid reinforced flexible pavements

Recent research at the University of Illinois has focused on the development of a finite element based mechanistic model to provide design tools and solutions for constructing geogrid reinforced flexible pavements. One essential feature of the mechanistic model is the representation of the soil/aggregate-geogrid interface by the use of a simple, realistic interface element having normal and shear spring stiffnesses between the geogrid and soil/aggregate continuum elements. Varying the shear stiffnesses in the interface elements specified the various levels of interface bonding, i.e., perfect bonding and partial bonding. The mechanistic model pavement response predictions including interface shear stresses compared favorably with the ABAQUS contact model results for a set of model parameters. Further, relative displacements computed at the soil/aggregate-geogrid interfaces were small and unlikely to cause any large movement or slip of the geogrid at the interface indicating the use interface elements with spring stiffnesses was adequate.

Characterisation of unbound aggregate materials considering physical and morphological properties

Abstract The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.

Characterization of Mechanically Stabilized Layer by Resilient Modulus and Permanent Deformation Testing

The majority of roadways in the United States are low volume. Pavement structures for low-volume roads (LVRs) are designed according to empirical procedures that are sometimes verified by a mechanistic-based design process. The flexible pavement structure for an LVR consists of a relatively thin asphalt–concrete wearing course and an aggregate base course constructed on subgrade. An asphalt wearing course provides a good riding surface and moisture protection for the base course. Service life of a thin asphalt pavement depends on material quality and thickness of granular layers. A compelling need now exists for reducing the cost of building and maintaining the national transportation infrastructure. The limited lifespan of most construction materials due to continuing wear and tear requires more creative, innovative, economical, and sustainable roadway design techniques. Geogrid reinforcements in LVRs are seen as a particularly promising solution because they can be designed to provide an equivalent service life through use of less material. The combination of the aggregate and geogrid materials creates an improved or mechanically stabilized layer (MSL) with a significantly increased resilient modulus. The objective of this study was to use existing testing protocol to determine and to evaluate material design parameters for an unbound and a geogrid MSL. To determine the effects of an MSL, a test combination that included AASHTO T307 and NCHRP 598 was conducted on both unbound and mechanically stabilized aggregate specimens.

Evaluation of Geosynthetics Use for Pavement Subgrade Restraint and Working Platform Construction

Subgrade restraint design is the use of a geosynthetic placed at the subgrade/subbase or subgrade/base interface to increase the bearing capacity or the support of construction equipment over a weak or soft subgrade. The conventional design criteria of unpaved roads require providing adequate base course or aggregate cover material to prevent bearing capacity type failure. Geogrids and high-strength woven geotextiles can increase bearing capacity of a pavement structure. Increasing the bearing capacity of subgrade soils can reduce required base course or treatment thickness. Several design methodologies exist for the use of geosynthetics in subgrade restraint for pavement construction. This paper evaluates currently available design approaches and design tools including related proprietary software. Comparative analysis results for different design approaches are also presented in this paper.

Full-Scale Evaluation of Geogrid-Reinforced Thin Flexible Pavements

A full-sale test section was constructed and subjected to traffic loading at the U.S. Army Engineer Research and Development Center to evaluate the performance of a geogrid that was used for base reinforcement in a thin flexible pavement. Three test items—a geogrid-reinforced test item and two unreinforced control test items—were constructed under controlled conditions. The test pavements were subjected to accelerated traffic loading to evaluate the relative performance of the pavement structures. Pavement stiffness and permanent surface deformations were measured periodically throughout the testing. The study results showed that the geogrid-reinforced pavement performed better than the unreinforced control pavements did. The results were used to develop traffic benefit ratios and effective base course structural coefficients to enable comparison of the pavement structures.

Determination of Load Equivalency for Unpaved Roads

The load equivalency method is widely used to consider the effect of traffic loading on pavement design, and the equivalent axle load factor (EALF) for paved roads has been studied often. For unpaved roads, however, EALF is not well understood because it is not necessarily the same as it is for paved roads. In this study, cyclic plate load tests were conducted on unpaved road sections (six base-over-subgrade sections and four subgrade-only sections) constructed in a geotechnical box (2 m × 2.2 m × 2 m) to investigate the load equivalency for unpaved roads. The base-over-subgrade sections were constructed as unstabilized, T1 geogrid–stabilized, and T2 geogrid–stabilized base courses of 15% California bearing ratio (CBR) with thicknesses of 0.23 m and 0.30 m over weak subgrade of 2% CBR. The subgrade-only sections were constructed with CBR values of 6.2%, 7.4%, 9.5%, and 11.0%. The intensities of the cyclic loads were increased from 5 kN to 65 kN, at increments of 5 kN. For each load intensity, 100 cycles were applied on one test section. The EALFs were analyzed in terms of permanent deformation. The results showed that the regression powers of the ratios of axle loads for unpaved roads with aggregate bases over weak subgrade ranged from 1.9 to 2.9, which were lower than a power of 4, the typical value used for paved roads. The powers for subgrade-only sections had an even wider range, from 1.1 to 3.4. The increase of base thickness, the presence of geogrid, and the use of a higher-grade geogrid increased the power.