Unreinforced Pavement Research Articles

Effects of 3D Geocells on Flexible Pavement Foundations

Abstract: The current below study examines the effects of adding a Cubic Three-dimensional Geo-synthetics material known as Geo-cells to the foundation layer of flexible Bitumen pavements. In this study, the test of California bearing ratio (CBR) of 5% is used to compare the steel reinforced and unreinforced pavement made sections built on that subgrade. To comprehend the impact of Geo-cell reinforcement upon the pavement section's load-carryings mechanism under static and repetitive loading circumstances, a number of model tests were conducted. The following characteristics were examined: surface deformation profile the test sections, real rut subgrade level, load-settlement response of the pavement sections, and pressure is transmitted into the subgrade soil beneath into the Geo-cell reinforced foundation layer.

Evaluation of commercial and scrap tyre cellular reinforcement infilled with demolition waste for granular sub-base of flexible pavements: A sustainable approach

Nowadays, use of geocell is gaining more importance in pavement industry as it offers improved performance through confinement and tensioned membrane actions. In the recent years, solid waste management seeks greater attention as it poses serious issues related to their disposal, health and environmental hazards. Effective recycling and reuse of these wastes could be a viable and economical approach towards sustainable construction. In this regard, two such wastes (scrap tyres and construction & demolition (C&D) waste) are considered for investigation. The recycled concrete aggregates (RCA) obtained from the C&D waste is used as granular sub-base material and its performance is compared with conventional crushed aggregates (CCA). Scrap tyre is used as cellular reinforcement (recycled tyre cell (RTC)) and its effectiveness as a reinforcing material is compared with the commercial cellular reinforcement (commercially available geocell (CGC)). The study involves construction of laboratory pavement sections under unreinforced and reinforced conditions and testing under repeated loading. The pavement sections supplemented with CGC and RTC reinforcement shows reduced permanent deformation by 27 & 33% and 30 & 35% respectively for CCA and RCA infill materials. The comparable performance of RCA with CCA is attributed to the self-cementation effect offered by the adhered residual cement mortar. With CGC and RTC reinforcement, residual pressure at the subgrade level reduces by 30 and 37% respectively. Provision of cellular reinforcement improves the stress distribution angle than the respective unreinforced pavements and highest improvement is observed in case RTC. Improved behaviour with RTC is due to the increased strip thickness and tensile strength of recycled tyre strip than the ones used in the preparation of CGC. Irrespective of the infill material, higher strains are experienced under the loading zone than the neighbouring cells. Overall, the study promotes use of scrap tyres and RCA in bulk volume for pavement construction offering an eco-friendly, cost-effective, sustainable and long-lasting pavements.

Performance of a triaxial geogrid-reinforced crushed rock base underlain by a soft clay subgrade

This large-scale test investigated the behavior of a pavement structure composed of a crushed rock base underlain by a soft clay subgrade reinforced with a triaxial geogrid under static loading. The fully instrumented pavement model included a 0.8-m-thick soft clay layer and a 0.2-m-thick crushed rock layer. A triaxial geogrid, Triax TX150, was installed at the interface between the subgrade and base layers and the mid-depth of the crushed rock base. Both unreinforced and reinforced pavement models were prepared in a square steel mold of 1.5 m width and 1.2 m height. Plate load tests were conducted according to ASTM standards. The results revealed that the number of geogrid layers and their locations influenced the reinforced pavement’s ultimate bearing capacity and failure mechanisms. Two-layer reinforcement (between the crushed rock and soft clay layers and in the middle of the crushed rock layer) provided the maximum bearing capacity, with an improvement factor of 1.63 compared to that of the unreinforced pavement, because the triaxial geogrids changed the failure mode of the unreinforced pavement from punching failure in the crushed rock and local shear failure in the soft clay to only punching failure in the crushed rock. Thus, triaxial geogrids are suggested for enhancing the performance of pavements on soft clay subgrades.

Numerical Study of Geotextile-Reinforced Flexible Pavement Overlying Low-Strength Subgrade

Construction of low-volume flexible pavements on low-strength subgrade poses design, construction, and maintenance challenges. While researchers have generally acknowledged the potential for geosynthetics as reinforcement material, they mainly focused on permanent deformation. Therefore, this paper presents a numerical study of low-volume flexible pavement reinforced with geotextile material under static loading to determine the improvement due to reinforcement based on three criteria: rutting performance, geosynthetic placement location, and base course thickness reduction. Based on the Finite Element Method (FEM), three-dimensional modeling using Abaqus/CAE software was performed. From the study, a significant decrease in rutting of up to 25.2% for the unreinforced pavement system was attained with geotextile reinforcement at base–subgrade and AC–base interfaces. The deflection response behavior of the pavement system is affected by the elastic modulus of the geosynthetic material, placement location, and the number of reinforcement layers. As a result of reinforcement, a base course thickness reduction of up to 30% was achieved without sacrificing the pavement’s structural integrity.

Influence of Geocomposite Properties on the Crack Propagation and Interlayer Bonding of Asphalt Pavements.

The application of geocomposites as reinforcement in asphalt pavements is a promising solution for the maintenance/rehabilitation of existing pavements and for the construction of new pavements, whose effectiveness strongly depends on the physical and mechanical properties of the geocomposite. This study aims at assessing the influence of four different geocomposites, obtained by combining a reinforcing geosynthetic with a bituminous membrane, on the crack propagation and interlayer bonding of asphalt pavements. First, a laboratory investigation was carried out on double-layered asphalt specimens. The crack propagation resistance under static and dynamic loads was investigated through three-point bending tests (carried out on specimens with and without notch) and reflective cracking tests respectively, whereas the interlayer shear strength was evaluated through Leutner tests. Then, a trial section was constructed along an Italian motorway and a Falling Weight Deflectometer (FWD) testing campaign was carried out. The laboratory investigation highlighted that—as compared to the unreinforced system—the geocomposites increased the crack propagation energy in the layer above the reinforcement from five to ten times, indicating that they can significantly extend the service life of the pavement by delaying bottom-up and reflective cracking. However, they also worsened the interlayer bonding between the asphalt layers (de-bonding effect). The field investigation indicated that all geocomposites decreased the stiffness of the asphalt layers with respect to the unreinforced pavement as a consequence of the de-bonding effect, thus corroborating the laboratory results. Based on the results obtained, it is desirable that the geocomposite possess a high energy dissipation capability and an upper coating ensuring good adhesion between the asphalt layers. The monitoring of the existing trial section in the future will provide useful data on the long-term field performance of reinforced pavements subjected to actual motorway traffic.

Investigation of Mechanical and Hydrologic Characteristics of Porous Asphalt Pavement with a Geocell Composite.

Porous asphalt pavement is a part of the permeable pavement system, which can be used to mitigate the negative impacts of urbanisation on the water hydrological cycle and environment. This study aims to assess the mechanical and hydrologic characteristics of porous asphalt pavements, with and without geocell composites, using a plate load test, falling weight deflectometer test, and rainfall simulation test. The corresponding results indicate that the elastic modulus of the unreinforced pavement is lower than that of the reinforced pavement. The analysis demonstrates that the use of geocell composites effectively increases the load-bearing capacity of the pavement. When the base layer is reinforced with geocells, its load-bearing capacity increases. Observation of the rainfall simulation tests on the reinforced pavement indicates that the reinforced pavement effectively handles the surface runoff.

Full-Scale Field Investigation of Asphalt Pavements Reinforced with Geocells

Based on full-scale field trials, this study investigated the mechanical properties of geocell-reinforced asphalt pavements. The field site is a four-lane highway in Hubei province in China. The dynamic load tests were performed for the geocell-reinforced and unreinforced pavement under three different conditions of axle loads and running speeds. The static load tests were carried out under three different axle loads which were the same as the dynamic load tests. The dynamic and static responses were studied in relation to the peak of the longitudinal strain, creep strain, and the improvement factors. Field trial results revealed that the longitudinal strain peak value of the pavement structure reinforced by Polypropylene geocell was significantly reduced in comparison with the pavement response without geocell. In addition, the reinforcement system had a certain inhibiting effect on the creep strain and the improvement factor increased with increasing axle load and decreased with increasing speed. Under 100 kN axle load, the improvement factors were 0.78, 0.68, 0.54, and 0.39 at speeds of 0, 20, 40, and 60 km/h. Under 150 kN axle load, the improvement factors were 0.96, 0.89, 0.75, and 0.61 at speeds of 0, 20, 40, and 60 km/h. Under 200 kN axle load, the improvement factors are 1.21, 1.1, 0.97, and 0.79 at speeds of 0, 20, 40, and 60 km/h. This research helps to provide a new and effective method of asphalt pavement reinforcement.

Performance of coir geotextile reinforced subgrade for low volume roads

Reinforcing flexible pavements using different types of geosynthetics is a technique that is widely used to increase the service life, reduce the maintenance costs and, guarantee high performance throughout the service life. This paper presents the insights from combined experimental and numerical analysis conducted on organic soil reinforced with coir geotextiles. A series of small-scale in-box plate load tests were performed to investigate the behavior of coir geotextile reinforced high plastic organic soil under circular loading. Tests were conducted on both homogeneous (sub grade alone) and layered configurations. Two types of commercially available woven coir geotextiles viz. H2M5 and H2M6, were utilized to study the contribution of coir geotextiles when used as reinforcement at the interface of sub-base and subgrade layers in enhancing the strength and stiffness of the low volume road. The primary objective of this exploratory study is to investigate the improvement in the performance of the low volume pavement as a result of the added geotextile reinforcement. The key performance parameters, namely, displacement, stress and strain responses of both reinforced and unreinforced pavement sections are obtained by analyzing the pavement using ABAQUS, which is a software suite for Finite Element Method (FEM) analysis. Test results indicate that the coir geotextile reinforcement substantially improved the performance of high plastic organic subgrade. A maximum reduction of 38% and 24% in vertical strain was observed on top of the subgrade in the case of H2M5 and H2M6 coir geotextile reinforced sections, respectively. Also, a maximum reduction of 30% and 18% in vertical displacement was observed in the case of H2M5 and H2M6 coir reinforced sections, respectively. At an average radial distance of about 1 m from the center of simulated static wheel load, very small displacement and strain value were observed for reinforced sections, relative to the unreinforced sections. Hence, the conclusion is drawn that the H2M5 type of coir geotextile contributes more than the H2M6 type to the structural performance improvement of pavements when used as reinforcement.

Design and Sustainability Aspects of Geogrid-Reinforced Flexible Pavements—An Indian Perspective

Design of flexible pavements is not straightforward when reinforcement materials, such as geogrid, geocell, and other types of geosynthetic materials, are used in pavement construction. Presently in India, elasticity theory is used to analyze strains due to wheel load applied on multi-layered soil system through a pavement analysis program, IITPAVE, to design the unreinforced pavement section as per Indian Roads Congress guidelines (IRC-37: 2012). The improvement in the performance of geogrid- reinforced pavement with respect to unreinforced pavement can be quantified in terms of Layer Coefficient Ratio (LCR) or Traffic Benefit Ratio (TBR). In the present study, both LCR and TBR based approaches are proposed to design geogrid-reinforced base courses of pavements with specific goals. These specified goals included designs based on (a) reduction in consumption of aggregates, and (b) reduction in the overall cost of construction of pavement reinforced with geogrids. Design charts are provided based on LCR and TBR values corresponding to selected traffic and California Bearing Ratio (CBR) of subgrades. The benefits of reinforcement in the pavement structure are found to be high when used over weak subgrades (CBR < 5%). For example, a reduction in thickness of aggregate layer is found to be in the range of 28% to 45%. Additionally, the sustainability of geogrid-reinforced pavement is quantified by comparing the embodied carbon (EC) generated from construction of geogrid-reinforced and unreinforced pavements. EC of reinforced pavements is found to have reduced by as much as 58 to 85 tCO2e/km in comparison with unreinforced pavement.

Effect of geocomposite reinforcement on the performance of thin asphalt pavements: Accelerated pavement testing and laboratory analysis

The objective of this study is to assess the effect of geocomposite reinforcement on fatigue cracking, reflective cracking and permanent deformation accumulation of thin asphalt pavements. For this purpose, a full-scale trial section was constructed with different interfaces: unreinforced (reference) and reinforced with three types of geocomposites, formed by the combination of a bituminous membrane with a fabric or grid. The experimental program included accelerated pavement testing (APT) carried out by means of Fast Falling Weight Deflectometer (FastFWD) and laboratory tests (three point bending tests) on samples taken from the trial section. After APT, significant permanent deflections were observed, likely due to the plastic yielding of the unbound layers. Nevertheless, all the geocomposites improved the permanent deformation resistance as compared to the unreinforced pavement by reducing the vertical strain at the top of the subgrade. Moreover, the geocomposites increased the energy necessary for the crack propagation by three to eight times with respect to the unreinforced pavement. Overall, these findings indicate that the use of geocomposites can extend the service life of thin asphalt pavements in terms of both cracking and permanent deformation accumulation.

Effectiveness of geogrid and its position on the performance of unpaved roads under repetitive loading

Road network provides access to communities and plays a vital role in the economic development of a country. These days, the transportation sector is given the utmost importance as premature failures are very common owing to rapid industrialization and urbanization. Also, subgrade failure is a very common scenario in pavement failures. To prevent these failures, geosynthetics have been used widely. In this study, model pavement sections were built in a steel tank with black cotton soil as subgrade forming a weak subgrade layer followed by granular sub-base (GSB) and sacrificial layer at the top. Two biaxial geogrids were considered as reinforcing material, and their position was varied within the GSB layer. The pavement thickness was varied by varying the thickness of GSB. The GSB of thickness 0.25 m, 0.30 m, and 0.35 m were considered. The effectiveness of geogrid on the pavement performance was evaluated by varying the position (one-third from the top, middle, two-third from the top and interface of subgrade and GSB) of geogrid within GSB. The pavement was subjected to repeated load test to simulate traffic loading. The loading frequency was kept at 0.01 Hz. The settlement of the pavement was recorded in terms of total and plastic settlements along the surface below the load point and away from the load point. Based on the test results, the geogrid placed at a depth of two-third thickness of GSB from the top showed improved performance and is effective compared with other positions. With geogrid reinforcement at optimal position, the reduction in plastic settlement is in the order of 34% to 52% depending on the geogrid stiffness and GSB thickness thereby improving the rutting performance of the pavement. Also, with geogrid reinforcement, it is possible to reduce the pavement thickness with equivalent and/or better performance of the pavement when compared to unreinforced pavement depending on the tensile strength of the geogrid. Thus, geogrid can be used to design pavements that are durable and with low maintenance cost making sustainable pavements.

Cyclic plate testing of geosynthetic-reinforced airfield pavements

Numerous cyclic plate load tests have been performed over the past several years to investigate the potential performance benefits of including geosynthetics in paved and unpaved applications. A majority of these studies have focused on relatively thin pavement structures subjected to highway loads. Airfield pavements can be substantially thicker and include multiple aggregate layers, and data are needed to quantify geosynthetic contributions under high contact pressures (CPs). Eleven representative airfield pavement structures (four unreinforced sections, four containing geosynthetics at the subbase−subgrade interface and three containing geosynthetics at the base−subbase interface) were constructed in a laboratory containment facility and subjected to cyclic loading under 1750 kPa simulated aircraft CP. Permanent surface deformation and vertical pressure response data were collected to determine relative improvement when compared with an unreinforced pavement structure and to evaluate the influence of geosynthetics. Some geosynthetics increased the number of cycles to failure, and it was found that some level of permanent deformation (e.g. 25 mm) may be required to engage reinforcing benefits when placement occurs at the subgrade−subbase interface. Changing subbase material from a California bearing ratio of 15–18 to 55 resulted in 1·5 orders of magnitude more cycles to 25 mm of permanent deformation than any of the geosynthetic-reinforced sections evaluated.

Development of full-scale laboratory accelerated pavement testing facility: a step toward performance assessment of geosynthetics reinforced pavement

The road network in India is growing at an exponential rate. The biggest challenge in this scenario is an availability of good-quality pavement materials at a reasonable rate. The demands of these pavement materials are increasing at an accelerated rate due to which good-quality material available in nature is getting depleted rapidly. This may create a threat to the environment in the near future. The rate of depletion may be reduced to a certain extent by adopting measures, viz. use of recycled pavement materials, stabilization of locally available poor-quality materials, and use of reinforced pavement. Assessment of such measures in a limited time is the biggest challenge. This challenge may be tackled by using full-scale accelerated pavement testing setup. However, the full-scale accelerated pavement test is not readily available. Over the globe, accelerated pavement testing setups are developed at only a few universities and research institutes. This paper presents the development details of a full-scale laboratory accelerated pavement testing setup. The calibration of the applied axle load is also presented. The developed full-scale laboratory accelerated pavement testing setup will prove opportunity to test new material/technique which will build up the confidence due to gain of knowledge. This paper also aims to investigate the variation of vertical stress and deformation at the base–subgrade interface of geotextile-reinforced and unreinforced pavement using the developed pavement testing facility.

Short- and long-term properties of glass fiber reinforced asphalt mixtures

ABSTRACT This study is aimed at investigating short- and long-term aged properties of glass fibre (GF)-reinforced asphalt mixtures using indirect tensile strength (ITS) and associated fracture energy, moisture susceptibility, creep compliance, resilient modulus, and indirect tension continuous and discontinuous (IDT) fatigue tests. Also, to portray the influence of fibre reinforcement at micro level, a micromechanical finite element model (MFEM) was built utilising representative volume element (RVE) to consider the effect of fibre dose, length, and dispersion (fully aligned or random) on the mechanical characteristics (or stress-strain curve) of the composite under unidirectional strain perturbation. Based on the study results, for the same binder, compared to shorter fibre (i.e. 6-mm length), the application of 12-mm GF to the mixtures showed unnoticeable change in the short- and long-term properties of the reinforced mixtures. On the whole, the GF can impart positive characteristics to the asphalt mixture such as increase strength, resistance to rutting, less susceptibility to moisture damage, retard fatigue cracking, enhance healing capability, and combat adverse aging changes. Unlike the effect of fibre length, the numerical model proved that the orientation and concentration of GF showed noticeable influence on the mechanical response of the composite. Overall, using the GF in asphalt mixtures can help produce durable bituminous pavements. Although the present study investigated the GF-reinforced bituminous materials from performance stand point, one can speculate that building GF-reinforced pavements is more costly initially compared to unreinforced pavements however the benefits may be received over time by extending service life and saving the maintenance cost of the bituminous pavement.

A mechanistic-empirical approach to quantify the influence of geogrid on the performance of flexible pavement structures

The objective of this study was to develop a methodology for quantifying the influence of geogrid on the performance of flexible pavement structures in a manner that would allow incorporation into Pavement Mechanistic-Empirical (ME) Design. The finite element technique was used to develop the geogrid-reinforced flexible pavement structure models, which focused on the characterization of the lateral confinement and vertical membrane effect of geogrid. A full-scale Soil Tank test was conducted to assess the validity of the developed geogrid-reinforced models by comparing the model predicted pavement responses (i.e., surface deflections, tensile strain at the bottom of asphalt concrete, and vertical stress distributions) to those tank test measurements. In general, the developed finite element models were capable of accurately predicting the responses of geogrid-reinforced and unreinforced pavement structures. It was found that the placement of geogrid increases the stiffness of base course and significantly reduces the vertical stresses around the geogrid layer, but cannot effectively reduce the tensile strain at the bottom of asphalt concrete. This indicated that the geogrid reinforcement is beneficial for reducing the rutting damage in base course and subgrade, but not effective in prolonging the fatigue life of flexible pavement. To quantify the influence of geogrid, the geogrid-reinforced flexible pavement structure was equivalent to an unreinforced flexible pavement structure with the modified material properties, which was based on a pavement response equivalency approach. The determined modified material properties was then input into Pavement ME Design software to predict the performance of geogrid-reinforced flexible pavement structures. Two case studies were conducted to predict the performance of two geogrid-reinforced pavement sections that were identified from the Long-Term Pavement Performance (LTPP) database. The predicted performance results (i.e., rutting depth and fatigue cracking area) were coincident with those field measurements, which validated the prediction accuracy of the proposed approach.

Performance evaluation of geocell-reinforced pavements

Weak subgrades are the common problem in road construction and these weak subgrades lead to large deformation resulting in rutting failure of the pavement structure. These days, Geocells have been widely used to reinforce/stabilise the structures with unbound materials. This paper presents the results of repeated load tests on the unreinforced and geocell-reinforced model pavement sections. The model pavement sections were built in a steel tank of size 2 m × 2 m × 2 m. Stage construction was adopted in the construction of model pavement sections. The geocell layer of varying aspect ratio was considered and in all the geocell-reinforced pavement sections, geocell layer was placed at the interface of subgrade and granular sub-base course layers. Haversine loading was adopted to simulate the vehicular loading. A maximum pressure of 760 kPa (pressure exerted by a truck with single axle and single wheel) was applied at the top of the test section using a hydraulic jack of capacity 100kN. The tests were terminated at 500 loading cycles. The pressure – settlement behaviour of unreinforced and geocell-reinforced pavement sections is reported. The plastic settlement is found to be the predominant mode of settlement which ultimately leads to rutting failure of the pavement. The provision of geocells reduces rutting to an extent of 13–71% and increases the rut life of the pavement sections by a factor of 1.6 to 3.5 with varying aspect ratio and pavement thickness when compared with the respective unreinforced pavement sections. The reduction in plastic settlement (RPS) increases with increase in aspect ratio of geocells and higher aspect ratio (i.e. 0.67) shows maximum reduction in rutting. Geocell reinforcement is also found to reduce surface heaving and the reduction in plastic settlement is quantified in terms of a non-dimensional improvement factor varying in the order of 2.19–2.64 with varying pavement thickness and aspect ratio. The improvement increases with increase in aspect ratio of geocells and higher aspect ratio geocell (i.e. 0.67) corresponds to maximum reduction in rutting, RPS, reduced heaving and maximum improvement factor. The introduction of geocell at the interface of subgrade and sub-base results in more durable, environment-friendly and long-lasting pavement.

Prediction of geogrid-reinforced flexible pavement performance using artificial neural network approach

This study aimed to develop a methodology to incorporate geogrid material into the Pavement ME Design software for predicting the geogrid-reinforced flexible pavement performance. A large database of pavement responses and corresponding material and structure properties were generated based on numerous runs of the developed geogrid-reinforced and unreinforced pavement models. The artificial neural network (ANN) models were developed from the generated database to predict the geogrid-reinforced pavement responses. The developed ANN models were sensitive to the change of base and subgrade moduli, and the variation of geogrid sheet stiffness and geogrid location. The ANN model-predicted geogrid-reinforced pavement responses were then used to determine the modified material properties due to geogrid reinforcement. The modified material properties were finally input into the Pavement ME Design software to predict geogrid-reinforced pavement performance. The ANN approach was rapid and efficient to predict geogrid-reinforced pavement performance, which was compatible with the Pavement ME Design software.

Numerical modeling of geogrid-reinforced flexible pavement and corresponding validation using large-scale tank test

This study aimed to develop a finite element model to simulate the geogrid-reinforced flexible pavement structure by taking into account the lateral confinement effect of geogrid layer, the interaction between geogrid and aggregate/soil, and the nonlinear cross-anisotropy of geogrid-reinforced unbound granular material (UGM). First, an analytical model was proposed to quantify the effect of the lateral confinement of geogrid layer on the resilient modulus of UGM. By comparing to the laboratory triaxial test results, the developed analytical model was proven to accurately predict the resilient modulus of geogrid-reinforced UGM. Second, the Goodman interface element model was used to characterize the contact behavior of geogrid-aggregate/soil interface. In order to simulate the nonlinear cross-anisotropic behavior of geogrid-reinforced UGM, a user-defined material (UMAT) subroutine was programmed using the secant modulus approach. The accuracy of the developed UMAT was verified by comparing the numerical simulation results to the analytical solutions in a virtual triaxial test.Two pairs of geogrid-reinforced and unreinforced pavement models were analyzed in this study. It was found that the geogrid reinforcement is effective in mitigating the rutting damage of base course and subgrade, but cannot significantly extend the fatigue life of flexible pavement. The geogrid reinforced in the middle of the base course is better at reducing the rutting damage of base course than that placed at the base/subgrade interface. However, the geogrid reinforcement is much more effective in reducing the rutting damage of the subgrade when it is placed at the bottom of the base course. A comprehensive large-scale tank (LST) testing program was designed to record the critical pavement responses, including the surface deflection, the tensile strain at the bottom of the asphalt concrete, and the vertical stresses in base course and subgrade. The developed geogrid-reinforced and unreinforced finite element models were finally validated by comparing the model predictions with those measurements from the LST test.

Finite Element and Distress Models for Geosynthetic-reinforced Pavements

A finite element (FE) response model has been developed to describe stress and strain response parameters for geosynthetic-reinforced flexible pavement systems where the geosynthetic is placed at the bottom of the unbound aggregate layer. The model contains membrane elements and an anisotropic linear-elastic material model for the geosynthetic inclusion. Elasto-plastic models are used for the asphalt concrete, base aggregate and subgrade layers. Principal response parameters extracted from the FE model include vertical strain in the top of the subgrade and bulk stress in the unbound base aggregate layer. These response parameters are used in empirical damage models for the prediction of long-term pavement performance and the definition of reinforcement benefit. Reinforcement benefit is defined in terms of an extension of service life of the pavement, a reduction in aggregate thickness for equivalent service life, or a combination of the two. The damage models are calibrated from reinforced and unreinforced pavement test sections. The model is shown to provide general descriptions of reinforcement mechanisms that are consistent with those previously observed in instrumented pavement test sections. A companion paper (S.W., Perkins and M.Q., Edens (2003) A Design Model for Geosynthetic-Reinforced Pavements, International Journal of Pavement Engineering) describes the use of the response and damage models in a parametric study from which a design model for geosynthetic-reinforced pavements is developed.

Mitigation of short term rutting by interlocking layer developed around a geogrid-sensitivity analysis

This paper presents a numerical investigation of the mitigation of the rutting process developed in the early stages of the utilization of a flexible pavement structure. The analysis is supported by the results of experimental research available in the technical literature. The mitigation of rutting achieved by application of a geogrid is tackled in the framework of sensitivity theory. The fact that the installation of a geogrid results in a decrease of permanent deformations is associated with the interlocking action that develops around a geogrid. It creates the formation of a new, stiffer layer of composite material. The experimental results on the rutting process provide information in terms of accumulated permanent deformations caused by repetitive load of constant magnitude. The constitutive model adopted for analysis is a type of rigid, perfectly plastic material. It is described by a modified Hooke's model that incorporates a modulus of permanent deformation. The functional of permanent displacement is formulated with respect to an unreinforced pavement system with the aid of a suitable adjoint system. The changes of permanent displacements caused by the insertion of a geogrid at the midst of the base layer are described as changes (first variation) of modulus of permanent deformation of the composite layer developed around the known location of the geogrid. Sensitivity theory allows for an extension of the constitutive equation due to the postulate that material characteristics are also considered as variables. The simplifications of the constitutive relationships expressed in variational form result from the fact that variations of permanent displacement are imposed on primary structure in the presence of constant load. The determined final form of the sensitivity equation of permanent displacement due to the changes of modulus of permanent deformation of the composite layer forms the basis for the numerical investigations. They are performed by means of the finite element method (FEM) for each (N)-th load repetition. The results in terms of permanent modulus of deformation of the composite layer surrounding the geogrid for some discrete values of its thickness demonstrate a considerable increase in stiffness of the newly developed layer as compared with an unreinforced pavement.