Xiaochao Tang

Evaluation of geogrids for stabilising weak pavement subgrade

This study attempts to identify mechanical and physical properties of geogrids that are critical to their effectiveness in the stabilisation of pavement subgrade. Geogrid properties, including aperture size, wide-width tensile strength and junction strength, for four geogrid products are correlated with bench-scale interface test results, including direct shear and pull-out, and accelerated pavement testing (APT) results. APT is conducted through the use of a one-third scale APT device, the model mobile load simulator (MMLS3) on geogrid-reinforced pavement sections. The pavement sections are constructed on a subgrade soil with a low California bearing ratio. The performance of each pavement section is evaluated by measuring surface rutting at various trafficking stages. The analysis reveals a strong relationship between performance and junction and tensile strength of geogrids at small strains, whereas aperture size has a positive correlation with pull-out test results.

Reinforcement Tensile Behavior Under Cyclic Moving Wheel Loads

Numerous studies have revealed the benefits of using geogrids in a flexible pavement, especially for reducing permanent deformation. One of the questions that remain about the effectiveness of a geogrid in reinforcing of pavement is the extent to which the geogrid is engaged and mobilized throughout its service. This paper presents results of a laboratory study on various geogrid products embedded in flexible-pavement sections. The laboratory-scale pavement sections were subjected to cyclic moving wheel loads by using reduced-scale accelerated pavement testing (APT). During the APT, strains that developed in the geogrids were measured at intervals of loading applications by strain gauges installed in pairs on the upper and lower surfaces of the geogrid ribs. Permanent deformation of the subgrade was also measured at the same intervals of loading applications. The measurements of geogrid strains throughout the construction process indicated that the construction resulted in a considerable prestressing effect on the geogrids. Measurements from the individual strain gauges in pairs showed that the gauges installed on the upper surfaces of the ribs were in compression while those on the lower surfaces were in tension; the situation suggested a significant effect on the flexural deflection of the ribs on the tensile strain measurements from the strain gauges. Furthermore, it was observed that geogrid ribs in the longitudinal direction of traffic loading were not mobilized, while considerable strains were developed in geogrid ribs in the direction transverse to traffic loading. A clear correlation was found between the reinforcing forces developed in the geogrids and the performance of the reinforced subgrade in relation to resisting permanent deformation.

Permanent deformation behaviour of reinforced flexible pavements built on soft soil subgrade

This study focuses on evaluating the effectiveness of using various geogrid products to improve permanent deformation resistance in soft subgrade soils commonly encountered during roadway construction in Pennsylvania. Permanent deformation behaviours of the soft soils both with and without the inclusion of geogrids were investigated. Cyclic moving wheel loads were applied through a reduced-scale accelerated pavement testing (APT) device, a one-third-scale model mobile load simulator (MMLS3). Tests were conducted on two soil types, each modified with three different biaxial geogrids placed at the base–subgrade interface. The total permanent deformation/surface rutting of the pavement and the permanent deformation of the subgrade were measured at selected intervals of the wheel loading applications. The pavement sections were trenched upon completion of the accelerated testing to measure the deformed profiles of the cross sections from which the permanent deformations in the asphalt layer and the base layer were determined. Sections modified with geogrids were found to have similar performance with the control section in terms of the total permanent deformation. While the geogrids did not show significant effects on the asphalt layer permanent deformation, sections with geogrids consistently showed a significantly higher base layer permanent deformation as compared to the control sections. Measurements of the subgrade permanent deformation showed that two of the geogrids consistently reduced the permanent deformation of subgrade built with the two types of soft soil. The relative layer contribution to the total permanent deformation suggested a base layer failure in both sets of the accelerated tests, most likely due to the inadequate compaction of the base layer during construction. Sections modified with geogrids exhibited a significantly higher base layer contribution, along with a significantly lower subgrade contribution, to the total permanent deformation, whereas the control section showed the opposite of the layer contributions.

Performance of Reinforced–Stabilized Unpaved Test Sections Built over Native Soft Soil Under Full-Scale Moving Wheel Loads

This paper presents the findings from an ongoing research study that evaluated two recently developed geosynthetic products: a triaxial geogrid and a high-strength woven geotextile for reinforcing or stabilizing roads constructed over native soft soil in the state of Louisiana. Six full-scale test lane sections were constructed: two were reinforced by one and two layers of triaxial geogrids, respectively, while high-strength geotextile was used to reinforce two of the other sections with aggregate layers of different thicknesses. The remaining two sections were left as controls: one was constructed over a sand embankment 30 cm thick representing the common practice in southern Louisiana. The unpaved test sections were subjected to a full-scale moving wheel load applied by the accelerated loading facility. A variety of instrumentation was used to measure the load-associated and the environment-associated pavement responses and performance. Results of the full-scale testing on the unpaved test sections demonstrated the benefits of geosynthetic reinforcement–stabilization in reducing the permanent deformation in the pavement structure. The test sections’ resilient behavior did not seem to be influenced by the presence of geosynthetics. In addition, instead of the soft soil subgrade, the aggregate layer was the primary contributor to the total permanent deformation–surface rutting of the unpaved sections under the testing conditions in this study. Geosynthetics were mobilized and generally exhibited a strain around 0.2%.

Resilient and Permanent Deformation Characteristics of Unbound Pavement Layers Modified by Geogrids

The benefits of geogrids in reducing permanent deformation of flexible pavements are generally recognized and have been shown in numerous studies. In contrast, although resilient moduli of unbound materials are important property inputs in mechanistic–empirical pavement design, the impact of geogrids on the resilient behavior of unbound layers remains unclear from previous studies. This paper focuses on resilient behavior of unbound layers and permanent deformation of subgrades modified by geogrids at the base–subgrade interface. Two sets of tests were conducted on scaled pavements built over two types of soil. Each set of tests consisted of four sections. Three were modified with different geogrids and one was left unmodified as a control. Instruments were installed to measure the subgrades’ resilient and permanent deformations. The pavement layers were tested with a lightweight deflectometer (LWD). Surface deflections and subgrade resilient deformation under impulsive LWD loads were recorded. The test sections were then subjected to repetitive reduced-scale moving-wheel loading. Subgrade resilient and permanent deformations were measured at select intervals of the wheel passes. The measurements of surface deflection and subgrade resilient deformation under the LWD load indicate that the geogrids did not have an appreciable impact on the resilient behavior of the base layer and subgrade. Subgrade resilient deformations, along with the repetitive wheel loads, also suggest that the effects of the geogrids on the resilient responses of the subgrade were not evident. However, two of the geogrids consistently exhibited benefits in reducing permanent deformation in the subgrade.

The use of a multi-set-up, reduced-scale accelerated trafficking simulator for evaluating roadway systems and products

This paper describes the use of an accelerated trafficking device, the one-third scale model mobile load simulator (MMLS3), for evaluating roadway systems and products. While the majority of accelerated load testers have focused on investigating rutting behaviours and moisture damage susceptibility of bituminous materials, this paper sheds light on broader applications using the MMLS3, including accelerated tests on field sections, scaled pavement structures, roadway reflective markings, roadway slip-resistant plates, in addition to performance evaluation tests on hot-mix asphalt mixtures. Results of experiments for the various applications indicate that the MMLS3, when equipped with ancillary instrumentation and devices, is a valuable tool for investigating the structural responses of a roadway system and for evaluating the effectiveness and durability of roadway pavement products. This study shows that the results of accelerated trafficking tests using the MMLS3 are comparable with field full-scale accelerated tests due to the nature of similitude in the MMLS3 design. Using the MMLS3 for accelerated traffic testing for practical and research purposes in the area of pavement engineering is a reliable and economical alternative to full-scale accelerated testing, given the savings in the required time and resources.

Accelerated Testing of Geogrid-Reinforced Subgrade in Flexible Pavements

This paper describes a pit-scale experimental study aimed at quantifying and evaluating geogrid reinforcement in flexible pavements. Two sets of pavement sections are constructed on two different subgrades and trafficked using a Model Mobile Load Simulator (MMLS3). Each set of test sections consists of the same pavement materials and structure except for the geogrid type used for stabilizing the subgrade. Rutting of all sections are measured using a profilometer at various trafficking stages. Geogrid reinforcement effectiveness is found to be related to the difference in geogrids properties. Test results show that the geogrid reinforcement enhances the pavement performance with respect to rutting resistance compared to a non-reinforced system.

Exploratory Analysis of Accelerated Wear Testing to Evaluate Performance of Pavement Markings

Wear testing of pavement markings in the United States is commonly performed by AASHTOs National Transportation Product Evaluation Program. The pavement markings evaluated in this program are applied in a transverse manner along existing roadways, and a variety of objective measures are collected over a 2-year period. This paper reports the findings from an exploratory study on using the Model Mobile Load Simulator, third scale (MMLS3), to evaluate pavement marking performance along transverse test sections. Accelerated trafficking on wet and dry pavement surfaces was applied on sections of markings consisting of various waterborne paint products, marking thicknesses, and bead types and densities. Pavement marking retroreflectivity measurements were recorded at various stages of MMLS3 trafficking and compared with measurements of a test deck consisting of the same pavement markings subjected to live traffic. The findings suggest that the MMLS3 can consistently replicate the degradation pattern of the various pavement markings with regard to loss of retroreflectivity as a function of traffic passages. Performance varied among the various types of markings, with higher retroreflectivity observed for thicker markings with higher bead densities. Trafficking on wet pavement surfaces accelerated the degradation of the markings’ retroreflectivity.

Evaluation of Geosynthetics in Unpaved Roads Built over Natural Soft Subgrade Using Full-Scale Accelerated Pavement Testing

In the state of Louisiana, roadways often have to be built over a weak subgrade due to the soft nature of Louisiana soil, which generates many design and construction difficulties. As an alternative to the traditional method of treating the soft soil with lime or cement, two geosynthetic products, a triaxial geogrid and a high-strength woven geotextile were evaluated for reinforcing unpaved roads constructed over natural soft subgrade. A total of six full-scale unpaved test sections were constructed, among which two sections were reinforced by one and two layers of triaxial geogrids respectively while high-strength geotextiles were used to reinforce two of the other sections with different base thicknesses. The remaining two sections were left as control, of which one was constructed over sand embankments as is common practice in southern Louisiana. The test sections were subjected to a full-scale moving wheel load applied by the accelerated loading facility (ALF). A variety of instrumentation was used to measure the load-associated and environment-associated pavement responses and performance. Results of the full-scale testing demonstrate the benefits of geosynthetics in reducing the permanent deformation in the pavement structure.

Inverse Analysis of Pavement Structural Properties Based on Dynamic Finite Element Modeling and Genetic Algorithm

With the movement towards the implementation of mechanistic-empirical pavement design guide (MEPDG), an accurate determination of pavement layer moduli is vital for predicting pavement critical mechanistic responses. A backcalculation procedure is commonly used to estimate the pavement layer moduli based on the non-destructive falling weight deflectometer (FWD) tests. Backcalculation of flexible pavement layer properties is an inverse problem with known input and output signals based upon which unknown parameters of the pavement system are evaluated. In this study, an inverse analysis procedure that combines the finite element analysis and a population-based optimization technique, Genetic Algorithm (GA) has been developed to determine the pavement layer structural properties. A lightweight deflectometer (LWD) was used to infer the moduli of instrumented three-layer scaled flexible pavement models. While the common practice in backcalculating pavement layer properties still assumes a static FWD load and uses only peak values of the load and deflections, dynamic analysis was conducted to simulate the impulse LWD load. The recorded time histories of the LWD load were used as the known inputs into the pavement system while the measured time-histories of surface central deflections and subgrade deflections measured with a linear variable differential transformers (LVDT) were considered as the outputs. As a result, consistent pavement layer moduli can be obtained through this inverse analysis procedure.