Geosynthetic-stabilized flexible pavements: Solution derivation and mechanistic-empirical analysis

Geosynthetics have been widely applied in flexible pavements for decades. However, the mechanistic-empirical analytical approach for geosynthetic-stabilized flexible pavements based on the elastic solution derived from the layered elastic theory has not been established. In this study, the solution for a typical three-layer geosynthetic-stabilized flexible pavement was derived according to the layered elastic theory. In the derivation, lateral restraint and tensioned membrane effect of geosynthetics quantified in terms of layer permanent deformations were considered at the interface as a continuity condition. The derived solution was then incorporated into the mechanistic-empirical approach for the calculation of pavement rutting and fatigue cracking. The result indicates that the solution derived in this study is capable of analyzing the geosynthetic-stabilized three-layer flexible pavement. The pavement elastic responses calculated using the solution obtained in this study are in line with those by the previously established solutions in the literature. The rut depths estimated using the proposed solution reasonably match those measured in the previous study. For rut reduction, the geosynthetic placed underneath the base layer is more effective. For the tensile strain relief at the bottom of the asphalt layer, the geosynthetic placed at the bottom of the asphalt shows more benefit.     

Evaluating long-term benefits of geosynthetics in flexible pavements built over weak subgrades by finite element and Mechanistic-Empirical analyses

Finite element (FE) models were developed to evaluate the benefits of geosynthetic reinforcement in flexible pavements built over weak subgrades. The parametric study was conducted to evaluate the effect of different variables such as base thickness, geosynthetic type, geosynthetic stiffness, and double-geogrid layers. FE analyses were performed for 100 load cycles, and the permanent deformation (PD) was used to calibrate the empirical parameters in MEPDG equations for each layer, which were used to extrapolate PD data for the service life of pavements. The PD curves for unreinforced and similar reinforced sections were used to evaluate the Traffic Benefit Ratios (TBR) at different rut depths. The results showed that the inclusion of one geogrid/geotextile layer at the base-subgrade interface could significantly reduce pavement rutting. The use of geogrid is more effective than geotextile in reducing pavement rutting. The derived TBR values range from 1.91 to 8.9 for one geogrid layer and from 1.71 to 5.92 for one geotextile layer. The TBR values increase with increasing the rutting depth and geosynthetic stiffness. The TBR value demonstrates an optimum at a base thickness of 10 in. The results demonstrated the superior benefits of using double geogrid layers compared to single-layer cases.     

Reduction of subgrade fines migration into subbase of flexible pavement using geotextile

Pumping in pavement is defined as traffic-induced migration of saturated subgrade fines into overlying granular layers or onto the surface of the pavement, negatively impacting the performance and service life of the pavement. The objective of this study was to assess the capability of geotextile as a separation and filtration layer in reducing subgrade fines migration. A one-third scale Model Mobile Load Simulator, an accelerated pavement testing device, was used to simulate the cyclic traffic loading on a scaled model of a flexible pavement. The results from three scaled pavement tests were compared to evaluate the effectiveness of geotextile separation and filtration in reducing subgrade fines migration. The three tests had identical configurations, except that a geotextile layer was placed at the interface of subgrade and subbase in one of the tests. The lab testing revealed that, under cyclic traffic conditions, the migration of subgrade fines into subbase was significant. However, using a geotextile at the subgrade-subbase interface significantly reduced the subgrade pumping. At the end of the test, the fines that migrated to the subbase, based on % mass of subbase, were 6.39% in the tests without geotextile and 1.81% in the test with geotextile. An approximately 30% reduction was observed in the amount of pavement rutting when using geotextile at the top of the subgrade. The subgrade soil migration in mass percentage increased with the traffic loading cycles, and more migration occurred in the bottom half than in the top half of the subbase. The study concludes that geotextile can be used as an effective means to reduce pumping of subgrade fines in pavement by providing both separation and filtration.     

Evaluation of geosynthetic reinforcement in unpaved road using moving wheel load test

The common cause of failure of the unpaved road is associated with undesirable ruts and deformations. Use of geosynthetic reinforcement is a solution to this pavement distress problem as experienced in limited research works, especially in the laboratory studies. This study presents the performance of geosynthetic-reinforced unpaved roads subjected to moving wheel load tests to investigate the effect of geosynthetic reinforcement on the pavement surface deformation of the unpaved roads. Unreinforced and geosynthetic-reinforced unpaved road test sections consisting of varied reinforcements were constructed in a test pit, 9 m long and 2.7 m wide. Geogrid and geotextile were used for reinforcing the unpaved road test sections. The rut depth was measured in the transverse direction of the wheel path after certain number of wheel passes. Traffic Benefit Ratio (TBR) and Performance Index (PI) were employed in the study for the evaluation of the effectiveness of geosynthetic reinforcement in unpaved roads. After 350 vehicle passes, the geotextile-reinforced and geogrid-reinforced test sections get rutting reduced by 44.89% and 28.57%, respectively. The test results indicate that inclusion of geosynthetic reinforcement significantly improves the rutting resistance and stability of reinforced test sections compared to the unreinforced test sections.     

Influence of asphalt thickness on performance of geosynthetic-reinforced asphalt: Full-scale field study

In this study, a series of controlled traffic loadings was conducted on unreinforced and geosynthetic-reinforced full-scale asphalt overlays. Unlike the common objective of using paving interlayers to mitigate the development of reflective cracks, the main purpose of adopting geosynthetics for this study was to render an increased roadway structural capacity. The project involved instrumented test sections constructed during the rehabilitation of an in-service roadway in Texas, USA. The rehabilitation involved repairing the pre-existing pavement, placing tack coat, installing a geosynthetic interlayer (except in the unreinforced section), and finally constructing a 75 mm-thick asphalt overlay. This overlay comprised a 50 mm-thick, dense-graded (TY-D) layer overlain by a 25 mm-thick, thin-overlay mixture (TOM) layer. Controlled traffic loadings were conducted, which involved driving standard and light axle loads directly above asphalt strain gauges that had been installed at mid-depth of the pre-existing asphalt layer. Comparison of tensile strains among the different test sections revealed significantly smaller tensile strains in the geosynthetic-reinforced sections compared to those obtained in the unreinforced section. Consequently, and even though geosynthetic interlayers have often been adopted to minimize reflective cracking in asphalt overlays, the field monitoring results generated in this study demonstrate that they also provide added roadway structural capacity.     

Performance of geosynthetic-reinforced soil foundations across a normal fault

This paper presents a series of model tests on geosynthetic-reinforced soil (GRS) foundations across a normal fault. The aim was to evaluate the performance of reinforced foundations as a mitigation measure for surface faulting hazards. Experimental tests modeled a 3-m thick foundation in prototype subjected to a fault displacement up to 90 cm. Test variables included the number of reinforcement layers, reinforcement stiffness and location, and foundation height. Digital image analysis techniques were applied to determine the ground settlement profile, angular distortion, shear rupture propagation, and mobilized reinforcement tensile strain at various magnitudes of fault offset. Test results revealed that compared with the unreinforced foundation, reinforcement inclusion could effectively prevent the shear rupture propagating from the bedrock fault to the ground surface. It also spread the differential settlement to a wider influential zone, resulting in an average reduction of 60% in the fault-induced angular distortion at the ground surface. The maximum angular distortion decreased as the foundation height, number of reinforcement layers, and reinforcement stiffness increased. Relationships between the maximum angular distortion and maximum mobilized reinforcement tensile strain with fault displacement were therefore established. Based on the findings from this study, design suggestions and implications are discussed.     

Stiffness characterisation of full-scale airfield test pavements using computational intelligence techniques

The falling weight deflectometer (FWD) is a non-destructive test equipment used to assess the structural condition of highway and airfield pavement systems and to determine the moduli of pavement layers. The backcalculated moduli are not only good pavement layer condition indicators but are also necessary inputs for conducting mechanistic based pavement structural analysis. In this study, artificial neural networks (ANNs)-based backcalculation models were employed to rapidly and accurately predict flexible airport pavement layer moduli from realistic FWD deflection basins acquired at the U.S. Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF). The uniformity characteristics of NAPTF flexible pavements were successfully mapped using the ANN predictions.     

Field behaviors of a geogrid reinforced MSW slope in a high-food-waste-content MSW landfill: A case study

A one-year field monitoring of a geogrid reinforced municipal solid waste (MSW) slope was conducted in the Xingfeng Landfill. Settlement tubes, strain gauges and earth pressure cells were used to measure the vertical settlement, the reinforcement strains and the vertical earth pressures in the reinforced MSW slope, respectively. During the monitoring period, the waste sliding occurred and the fresh MSW was dumped at the top of the reinforced slope. The vertical settlement along the reinforcement was nonlinear and the peak settlement occurred at the central part of the reinforcement. The reinforcement strains and the vertical earth pressures at various positions were affected by the sliding and the waste dumping to differing extents. Along the lengths of the geogrid reinforcements, the reinforcement strains showed single-peak distributions. The peak strains were attained in the central part of the reinforcements and the minimum strains were attained at the tail ends. The vertical earth pressures mainly depend on the overlying loads; however, the distributions of them along the reinforcement were nonlinear. Based on the monitoring results, the slope stability evaluation was conducted. It shows that the internal stability of the reinforced MSW slope might be sufficient, while the external stability was insufficient, meaning that this reinforced project was unsuccessful. Finally, various lessons and design suggestions learned from this unsuccessful project were discussed, which could provide valuable references for the future practice of geosynthetic reinforced MSW.     

Numerical analysis of fatigue cracks growth in flexible pavements with the elastoplastic fracture mechanics method

In this paper the elastoplastic fracture mechanics method to analyse the fatigue damage of the flexible pavement has been applied. This approach is an appropriate alternative for analysing the fatigue cracking damage of a road pavement, with respect to traditional methodologies based on the use of the experimental fatigue laws of the bituminous mixtures. In this regard, a numerical method to evaluate the stress intensity factor at the edge of the crack has been used and the Paris' law to analyse the crack propagation into bituminous layer has been applied. Then, a comparison with the linear elastic fracture mechanic method and with the traditional approach based on the use of the fatigue laws has been carried out. A numerical example has shown the influence of the elastoplastic properties of the bituminous materials in the phenomenon of propagation of the fracture.     

Performance of two-tiered reinforced-soil retaining walls under strip footing load

In the current study, an attempt was made to investigate the performance of two-tiered mechanically stabilized earth walls (T-TMSEWs) under static footing loading using reduced-scale model tests. For this purpose, twenty-four T-TMSEW models were constructed with three different types of reinforcement (metal strips, geogrid and geostraps) and were loaded using the rotatable and non-rotatable strip footings in different distances to the wall crest. Findings indicated that, although decreasing the reinforcement stiffness and the soil-reinforcement interaction reduces the ultimate bearing capacity of footings, the use of extensible reinforcements with low pull-out capacity and allowing the footing to tilt can be two effective solutions in T-TMSEWs to minimize deformations of backfill surface and connection loads as well as lateral pressures. It was observed that the use of a two-tiered configuration in MSE walls and also reducing tensile stiffness and soil-reinforcement interaction simultaneously, not only lead to change in the slip surface geometry but also prevent the development of deep slip surfaces in the lower tier. On the other hand, increasing the footing distance to the wall crest in the range of reinforced zone was found to be another influential solution to improve the bearing capacity, reduce wall deformations and also minimize lateral pressures.     

Pullout resistance of geogrid and steel reinforcement embedded in lightweight cellular concrete backfill

Lightweight Cellular Concrete (LCC) has been increasingly used as backfill material for retaining walls, ground improvement, and pavements due to its low self-weight, quick installation, and high compressive strength as compared with soils. This paper presents a series of pullout tests performed in the laboratory to investigate the pullout resistance of geogrid (extensible reinforcement) and steel strip (inextensible reinforcement) embedded in LCC. Pullout displacements and pullout forces were monitored using displacement transducers (DT) and a load cell during the pullout process. This study investigated the effects of age, normal stress, fly ash, the presence of a cold joint, and re-pullout on the pullout resistance and calculated the pullout resistance factors F* for geogrid and steel strip embedded in LCC. Test results show that for the geogrid embedded in LCC, the maximum pullout force increased as the normal stress increased. For the steel strip embedded in LCC, the maximum pullout force was independent of the normal stress and increased as the age and the cement to fly ash ratio increased. Test results also show that the presence of a cold joint did not reduce the pullout resistance, while the re-pullout test had lower pullout resistance compared to the original pullout test for the same specimen. The pullout resistance factors F* for steel strips were greater than those for geogrids and these factors decreased as the normal stress increased.     

Application of innovative meandrically arranged geotextiles for the protection of drainage ditches in the clay ground

The geotextiles produced from meandrically arranged Kemafil ropes were prepared. The ropes were produced from textile waste materials: woollen nonwoven and nonwoven from the blend of recycled fibres. The ropes were arranged into segments which were used for the protection of the bank of the deep drainage ditch and reinforcement of shallow roadside ditch in the clay ground. The geotextiles were installed in the ground and their behaviour during one vegetation season was observed. It was stated that during heavy rains the meandrically arranged ropes form a system of micro-dams which slow down the stream of water flowing down on the surface of the ditch bank as well as the stream flowing along the ditch. The geotextiles installed on the ditch banks eliminate the formation of erosive channels and protect the banks against sliding. The geotextiles absorb water what ensures retention of water flowing along the ditch. Due to enhanced soil and water holding capacity geotextiles protect grass seeds from being washed out and facilitate the development of protective vegetation. Materials used for the production of the ropes reveal sufficient resistance to biological degradation. Slow biodegradation of the materials enable keeping the protective potential of geotextiles for at least one vegetation season.     

Design of geosynthetic-reinforced slopes in cohesive backfills

Currently, geosynthetic reinforcements for slopes are calculated assuming the ground strength to be purely frictional, i.e. without any cohesion. However, accounting for the presence of even a modest amount of cohesion could allow using locally available cohesive soils as backfills to a greater extent and less overall reinforcement. But cohesive soils are subject to the formation of cracks that tend to reduce slope stability so their presence has to be accounted for in the design of the slope reinforcement. In the paper, limit analysis was employed to derive a semi-analytical method for uniform $c??$ slopes that provides the amount of reinforcement needed as a function of ground cohesion, tensile strength, angle of shearing resistance and of the slope inclination. Both climate induced cracks as well as cracks that form as part of the slope collapse mechanism are accounted for. Design charts providing the value of the required reinforcement strength and embedment length are plotted for both uniform and linearly increasing reinforcement distributions.From the results, it emerges that accounting for the presence of cohesion allows significant savings on the reinforcement to be made, and that cracks are often significantly detrimental to slope stability so they cannot be overlooked in the design calculations.     

Performance of a 33m high geogrid reinforced soil embankment without concrete panel

Geosynthetic reinforced soil embankment are extensively applied in the construction of high-speed railway and highway in mountainous regions but limited field monitoring is conducted on high and steep cases. Aiming to acquire better understanding, a 33-m-high single-tiered wrapped-facing geogrid reinforced soil embankment with the slope of 1 V:0.5H in China was monitored for 2 years during and after construction. Vertical earth pressure, strain of geogrids, horizontal displacement and settlement per layer were recorded and analysed. The results show that the geogrid tensile strains gradually increased during construction. And they were still developing after completion due to creep and subsequent vehicle surcharge load. The predictions of reinforcement loads by the FHWA methods were much higher than the estimated ones from measured strains. The vertical earth pressures linearly grew during construction and then stabilized fast. The horizontal displacement increases with height and the largest value achieved around the top of the slope two years after the construction is 0.14% the total height approximately. The settlement per layer is larger in the lower and middle portion of the embankment and no obvious change is observed over time. This study hopes to serve as a case reference for design and construction of similar reinforcement projects in the future.     

Effects of transverse members on geogrid pullout behavior considering rigid and flexible top boundaries

Geogrid pullout tests have been regarded as the most direct and effective way to describe the interfacial behavior between geogrid and soil. To investigate the coupled effects of geogrid transverse members and top-loading boundaries on the geogrid-soil interaction, numerical simulations of geogrid pullout tests using the Discrete Element Method (DEM) were carried out in this study. The rigid top boundary was simulated by a rigid wall, while the flexible top boundary was modeled with a string of bonded particles that could rotate and move up and down freely. The coupled effects of geogrid transverse members and top boundary conditions on the geogrid-soil interaction under pullout loads were visualized not only by the force distributions along the geogrids and in the specimens but also by the displacements of soil particles and geogrids. Additionally, the quantitative geogrid force and strain distributions along the geogrids, the lateral force distributions on the front walls, and the vertical displacements of top boundaries also showed the influence of transverse members on the geogrid pullout behavior considering the rigid and flexible top boundaries. The DEM investigation results of this study may provide helpful guidelines for regulating the geogrid pullout test apparatus and methods.     

CFD-DEM modeling of geotextile clogging in tunnel drainage systems

In this study, a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) method we used to investigate the hydraulic deterioration of a geotextile due to clogging in tunnel drainage systems. Initially, a framework was developed to generate and test a numerical representation of a typical non-woven geotextile. Following model validation, we carried out parametric analysis to examine the effect of fine content, crack angle, and groundwater inflow. The results showed a general trend of pressure increase associated with increasing both the crack angle and fine content. This increase was found to decay at larger crack angles and high content of fines. Interestingly, increasing groundwater inflow was found to had minimal effect on the final deposition of the clogging particles. Finally, an approximate semi-analytical model was developed to describe the pressure increase due to clogging. The model was able to provide a good match with the data obtained from the numerical analysis.     

Laboratory and field investigation of the effect of geogrid-reinforced ballast on railway track lateral resistance

Continuous welded rail (CWR) tracks have particular advantages over common tracks with jointed rails such as increased ride comfort, reduced noise and vibration and decreased maintenance costs due to the removal of joints in rail connections. Alternatively, some complications associated with CWR tracks, for instance increased lateral forces, are the main reason of track buckling and its subsequent lateral deformation. These problems are usually more severe in curved tracks. In order to overcome the large lateral forces caused by temperature deviations of CWR tracks which results in railway vehicle instability, the ballasted track lateral resistance should be improved. Among the various methods proposed in this area, no specific study has been carried out on the effect of geogrid reinforcement on ballasted track lateral resistance. Thus, the present research was allocated to investigating the effect of geogrid on the lateral resistances of both single tie and track panel via laboratory and field tests. In this regard, at the first stage, the ballast layer was reinforced with various number of geogrid layers, the effect of which was investigated by conducting the single tie push test (STPT) in the lab environment to assess the optimum number of geogrid layers and their installation levels along the ballast layer thickness. Afterwards, a test track was executed in the field including various sections which were reinforced in the same way as the lab tests. Consequently, many STPTs and track panel displacement tests (TPDTs) were accomplished. As a result, the STPTs in the lab and field confirmed more than 31% and 42% increase in single tie lateral resistance for ballast layers reinforced respectively with one and two geogrid layers, while these values were reached to 29% and 40% in the case of TPDT.     

Monitoring the structural integrity of a flexible riser during a full-scale fatigue test

Flexible risers are presently seen as an attractive alternative to their rigid counterparts in the offshore oil exploration industry, mainly because of their relatively simpler mounting and transportation. However, the complex multi-layered structure which guarantees high flexural compliance is not favorable for inspection with most non-destructive testing (NDT) techniques, which makes structural integrity evaluation difficult. Initial efforts concentrated on the search for a single technique which would ensure reliable detection of armor failure, but recent studies have shown that the safest approach may be to take advantage of the redundancy obtained when different instruments are combined. This work presents results of a full-scale dynamic loading test of a 6 m-long section of a flexible pipe with end-fittings which was instrumented with a range of sensors. Of these, measurements of torsional angle variation, axial displacement and Acoustic Emission were selected for a direct comparison. A detection pattern is observed during forced rupture of the wires of one of the outer armor layers, and reliable identification of events is possible when information from the three techniques is combined.     

Life cycle cost of flexible pavements and climate variability: case studies from Virginia

Abstract

Climate factors have not become a typical metric to consider for pavement life cycle cost analysis (LCCA). Changes in climate may affect pavement rutting, roughness, and cracking and lead to consequent changes in maintenance decision-making and life cycle costs. This study develops a methodology to incorporate the effects of climate variability into flexible pavement LCCA and to derive the additional life cycle costs incurred due to changes in climate. Case studies were performed for three road sections in Virginia (US) to demonstrate the methodology, using approximate mean climate change trends predicted for the investigated regions. It is estimated that climate change will incur additional vehicle operating costs ranging between US$2.30 and $4.40 on average per vehicle/annum if roads are used under a 2050 high greenhouse gas emission scenario and without being maintained. Assuming responsive maintenance, the budget demand for maintenance will arrive much earlier in the pavements’ life cycles (7–11?years earlier under the 2050 high-emission scenario). This is found to add up to 64% of agency costs (net present value) to repair each kilometre of the investigated roads in a 40-year design life. Agencies need to be aware of earlier or more frequent demands on their maintenance budgets.     

Optimal placement of reinforcement in piggyback landfill liners

The optimal placement of geogrid reinforcement in clay liners subject to differential settlement was investigated both numerically and with centrifuge modelling. Two unreinforced liners, a liner reinforced at the top-quarter depth, a liner reinforced at the bottom-quarter depth and a double reinforced liner were modelled in the centrifuge. Differential settlement was induced on the model liners by lowering a trapdoor overlain with sand. By considering: 1) the magnitude of differential settlement required to induce micro-cracks in the liners, 2) the strain fields across the liners during differential settlement and 3) the distribution of these strain fields, it was found that dividing the available reinforcement equally between the top-quarter and bottom-quarter of the liner, i.e. double reinforcement, represents the optimal reinforcement strategy.