DEM–FDM coupled analysis of composite element test for geogrid-reinforced ballast under cyclic loading

Geogrids have been widely used in the roadway construction as reinforcement in pavement systems. Geogrids have been effective in practice for reducing rutting damage, distributing traffic loads within the pavement foundation layers, increasing the resilient modulus of base course, and stabilization effects on the subgrade layer. Evaluating structural benefits of geogrids in the pavement structure depends on factors such as geogrid stiffness, geogrid rib shape, the geogrid location, hot mix asphalt thicknesses, base aggregate stiffness, thicknesses, and subgrade stiffness. This research project is a comprehensive study of geogrid reinforcement of flexible pavements and includes laboratory tests, field tests, and finite element simulations to evaluate geogrid reinforcement advantages and GE gain factors in reinforced pavements. The overall research methodology is proposed by a highly-qualified and multi-disciplinary team with expertise in pavement foundations, geogrids, field, and laboratory testing, construction, and cost analysis at Iowa State University (ISU), Ingios Geotechnics incorporation, and technical advisory panel in MnDOT. This research fills the gaps in research performed by previous researchers, and includes a strong literature review in the field of research. The granular equivalent (G.E.) factor was determined based on the results of the geosynthetic reinforced, and the unreinforced section was compared to obtain the G.E. factors based on the mentioned factors of reinforcements. Then, the table of G.E. factors based on different parameters is presented in this dissertation, according to the performed comparison of the results of performed laboratory and field experimental tests plus numerical studies. The results of this study can be used by the designers to evaluate the geosynthetic reinforcement of flexible pavements in their designs as well. In this research, an Integrated Mobile Accelerated Test System (IMAS) and an Automated Plate Load Test (APLT) experimental device, and finite element simulation approaches were used to evaluate the reinforcement effects of geogrid reinforcement. Test configurations were constructed by varying geogrid types (i.e., light-duty biaxial, heavy-duty biaxial, light-duty triaxial, and heavy-duty triaxial geogrids), geogrid locations in base course (i.e., at the interface between base and subgrade or in the base course), and base aggregate thicknesses assumed in the laboratory and field experimental tests. Eight IMAS and APLT test sections were performed to evaluate the reinforced base course behavior using different types of geogrids in different locations. These sections named as GE0, GE1, GE2, GE4, GE5, GE7, GE12 and GE15. Where GE0 is the control section, so no geogrid was installed in this section. Biaxial geogrid was used in GE1, GE2, and GE5, and Triaxial geogrid was used in GE4, GE7, GE12, and GE15. For GE1, GE2, GE4, and GE12, the geogrid products were placed at the interface between the base course layer and the subgrade layer; for GE5 and GE7, the geogrid was installed at the middle of the base course layer. The finite element method (FEM) models will be calibrated based on the results of these eight sections. Then the calibrated FEM models will be used to determine G.E. values of remaining sections. The results of cyclic deformation, permanent deformation, elastic modulus, stiffness, resilient modulus, cyclic stresses, and the number of cycles calculated in real-time presented. Testing results included resilient modulus, deflection, and permanent deformation of the pavement foundation for evaluating the structural benefits of geogrids as a function mentioned before. The results of this research reveal that improvement of the pavement performance due to the geosynthetic reinforcement relates to various factors and variables, including geogrid stiffness and geometry, geogrid location/depth, asphalt surface, and aggregate thicknesses, and subgrade stiffness. A new formulation is proposed to predict the Granular Equivalent (G.E.) factor of geogrid reinforcement of flexible pavements. The benefit of this study is realizing and understanding the structural benefits of geogrids and applying G.E. factors to a pavement design to reduce the thickness of gravel and/or asphalt and consequently extend the service life and reduce maintenance costs. It is expected that the appropriate use of geogrids can be a significant cost saving per project.

The geometry of the railway track is degraded under traffic load, and must be maintained periodically. This degradation is due to the arrangement of ballast particles under loads and vibrations, which results into irreversible plastic settlements. Tamping is a procedure of maintenance, used to restore the correct geometrical position of the ballasted tracks. However, the penetration of the vibrating tines into the ballast causes an increase in the fine particles content by rupture of the edges of the grains and leads to a progressive degradation of the ballast and thus to a loss of its mechanical properties. The main consequence is that the interval between two tamping operations decreases, along with the long-term effectiveness of the process. This vicious circle can be broken only by ballast renewal. Ballast and geometry maintenance represent 40 to 50% of the total railway maintenance budget during lifetime. The goal of this research is a better understanding of ballast degradation and the development of a tool to evaluate the effectiveness of the track tamping, according to the ballast condition and the infrastructure stiffness. This knowledge will make it possible to decrease the destroying effects of tamping over ballast and, consequently, increase the durability of track geometry and ballast. A full-scale laboratory study has been done on ballasts and infrastructures of different qualities. The dynamic loading of freight trains is simulated with a big hydraulic actuator, while the deformations of the track and his infrastructure are continuously measured. The accelerated loading corresponds to a period of 20 years of traffic operation on a high loaded line. Empirical settlement laws are written, based on the traffic results. Tamping process is performed with a special machine, which was built by purpose. The parameters to regulate tamping process can be controlled. The whole process and the equipment are completely instrumented, in order to control and measure several parameters in real time. The evolution of the ballast behaviour is analysed with the measurement of the bearing modulus. Plate load tests are performed under specially equipped sleepers, after each traffic cycle and tamping. The parameters of a new tamping are tested. The short and long-term effects on ballast are analysed and discussed. A two-dimensional finite element model has been applied to simulate the response of the ballast layer under traffic loading, and the development of settlements. Ballast at different degradation levels is studied. The influence of the subgrade stiffness is also taken in account. The elastoplastic Hujeux behaviour law is applied, in order to study the cyclic plasticity. Results are presented and the critical zones, which show the most plastic behaviour, are identified. A phenomenological model is also developed to analyse separately the effects of traffic loading and track tamping on ballast behaviour and degradation. This analysis is based on a backcalculation approach, and is performed on a 3-D finite elements model of the track. The Young modulus of the ballast and the infrastructure layers in situ is calculated by iteration. Along with the traffic simulation, the evolution of the ballast elastic modulus is measured. A comparison between two sections with different infrastructures is done.

Of primary concern in design of 1aterally and axially loaded piles for offshore platforms is their response under dynamic (cyclic and transient) loadings developed by storm or earthquake conditions. This paper addresses four primary areas:Factors controlling the dynamic response of idealized pile-soil systems.Stress-strain' characteristics of clays subjected to dynamic and cyclic loadings.Influence of dynamic and cyclic loadings on the capacity, deformability, and energy dissipation characteristics of laterally loaded piles driven into clays.Influence of dynamic and cyclic loadings on the capacity, deformability, and energy dissipation characteristics of axially loaded piles driven into clays. Available laboratory test data are summarized to generalize the influence of dynamic and cyclic loadings on the stress-strain properties of clays. In excess of 300 field pile load tests have been reviewed to define the ranges of influence of dynamic loadings on the load-deformation characteristics at the pile head (mudline). Results from analytical models and field experiments are used to define the energy dissipation characteristics of the pile-soil system. INTRODUCTION This paper addresses the response of piles to dynamic loadings. The scope of the paper includes individual, tubular steel piles driven into clay soil s. Dynamic loadings with rise times of 1 to 2 sec and periods of 1 to 20 sec are considered. Pile-head characteristics are developed primarily from considerations of field pile load tests. The response of simple single degree of freedom systems and laboratory tests on clays are summarized to provide a basis for interpreting the pile load test results into design applications. SUMMARY The following points are developed in this paper:The response of a pile to dynamic loading .is determined primarily by ratio of the duration of the force pulse to the natural period of the pile-soil system (the strain rate properties of the soils are reflected in the natural period of the system). Also of significance are the degree of periodicity of the input force (number of cycles), the details of the input force (rise time and shape), the amount of inelastic action in the system (plastic straining, slip), and the amount of damping (hysteretic, radiation).For typical lateral dynamic loadings due to waves, the dynamic lateral force resistance of the pile is 1.2 to 1.8 (average of 1.4) times the static capacity. Pile capacity is only moderately decreased by load cycling at loads 1ess than 0.7 to 0.8 times the static capacity. At greater levels of .loading pile displacements can be very large (depending on number of cycles). The magnitude of damping is strongly dependent on level of loading, pile stiffness and diameter, and soil stiffness.For typical axial dynamic loadings due to waves, the dynamic axial force resistance of the pile is 1.3 to 1.8 (average of 1.5) times the static capacity. Pile capacity is only moderately decreased by cycling at loads less than 0.7 to 0.8 times the static capacity. At greater levels of loading, pile displacements can be very large (depending on number of cycles).

The main objective of the paper is the investigation of localized fatigue fracture phenomena in thermo-viscoplastic flow processes under cyclic dynamic loadings. Recent experimental observations for cycle fatigue damage mechanics at high temperature and dynamic loadings of metals suggest that the intrinsic microdamage process does very much depend on the strain rate and the wave shape effects and is mostly developed in the regions where the plastic deformation is localized. The microdamage kinetics interacts with thermal and load changes to make failure of solids a highly rate, temperature and history dependent, nonlinear process. A general constitutive model of elasto-viscoplastic damaged polycrystalline solids developed within the thermodynamic framework of the rate type covariance structure with a finite set of the internal state variables is used (cf. Dornowski and Perzyna [16], [17], [18]). A set of the internal state variables is assumed and interpreted such that the theory developed takes account of the effects as follows: (i) plastic nonnormality; (ii) plastic strain induced anisotropy (kinematic hardening); (iii) softening generated by microdamage mechanisms (nucleation, growth and coalescence of microcracks); (iv) thermomechanical coupling (thermal plastic softening and thermal expansion); (v) rate sensitivity; (vi) plastic spin. To describe suitably the time and temperature dependent effects observed experimentally and the accumulation of the plastic deformation and damage during a dynamic cyclic loading process the kinetics of microdamage and the kinematic hardening law have been modified. The relaxation time is used as a regularization parameter. By assuming that the relaxation time tends to zero, the rate independent elasticplastic response can be obtained. The viscoplastic regularization procedure assures the stable integration algorithm by using the finite difference method. Particular attention is focussed on the well-posedness of the evolution problem (the initial-boundary value problem) as well as on its numerical solutions. The Lax-Richtmyer equivalence theorem is formulated, and conditions under which this theory is valid are examined. Utilizing the finite difference method for a regularized elasto-viscoplastic model, the numerical investigation of the three-dimensional dynamic adiabatic deformation in a particular body under cyclic loading condition is presented. Particular examples have been considered, namely a dynamic adiabatic cyclic loading process for a thin plate with sharp notch. To the upper edge of the plate is applied a cyclic constraint realized by rigid rotation of the edge of the plate while the lower edge is supported rigidly. A small localized region, distributed asymmetrically near the tip of the notch, which undergoes significant deformation and temperature rise, has been determined. Its evolution until occurrence of fatigue fracture has been simulated. The propagation of the macroscopic fatigue damage crack within the material of the plate is investigated. It has been found that the length of the macroscopic fatigue damage crack distinctly depends on the wave shape of the assumed loading cycle.

Discrete element modelling of cyclic loads of geogrid-reinforced ballast under confined and unconfined conditions

Behavior of sand-contaminated ballast reinforced with geogrid under cyclic loading

Behavior of geogrid-reinforced ballast under various levels of fouling

Effectiveness of geogrid reinforcement in improvement of mechanical behavior of sand-contaminated ballast

Reliable and durable operation of the railway track under the dynamic load of the rolling stock depends considerably on the ability of the ballast layer to get the load from the sleepers and distribute it to the subgrade. In this paper, the experimental study of the distribution properties of the ballast layer under the impact of dynamic loading depending on the density of the ballast layer is carried out. The ballast behaviour during load cycles is estimated by pressure measurements at the ballast prism base along the axis of a sleeper with simultaneous video observation of the ballast particles movement through transparent sidewalls of the box with crushed stone. Measurements of pressure distribution are carried out with the developed microcontroller system of measurements and developed load cells. The system allows performing multi-point measurements of stress in combination with measurements of acceleration and photogrammetry. The results of measurements showed a significant effect of the ballast layer consolidation on the distribution of stresses under the sleeper. The performed research opens up opportunities for practical improvement of the existing types of track structures and the technology of the ballast layer tamping in terms to provide the optimal conditions for the ballast layer operation.

Geogrids have been found to improve the performance of unbound aggregate layers in transportation applications by providing confinement and arresting movement through interlock between individual aggregate particles and their apertures. Geogrid reinforcement offers an effective remedial measure when railway structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength and permanent deformation behaviour of geogrid-stabilised railroad ballast. Geogrids with triangular, rectangular and square apertures were tested in the laboratory experiments. Cylindrical ballast specimens were prepared and tested with geogrids placed at different heights within the specimen using a large-scale triaxial apparatus. An imaging-based discrete-element-method approach was developed to model triaxial test results and investigate geogrid-reinforcement mechanisms. With the capability to create actual ballast aggregate particles as three-dimensional polyhedron elements having the same particle-size distributions and imaging quantified average shapes and angularities, the modelling was able to capture the ballast behaviour with and without geogrid reinforcement reasonably accurately.

Geogrids are well known for improving the performance of unbound aggregate layers in transportation applications by providing confinement and restraining movement through interlock between individual aggregate particles and geogrid apertures. Geogrid reinforcement offers an effective remedial measure when railroad track structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents findings from an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength behavior of railroad ballast. The effects of two geogrid types on ballast shear strength were evaluated through laboratory testing and numerical modeling. An imaging-based discrete element method (DEM) modeling approach was used to identify the optimal position for geogrid reinforcement to achieve the maximum shear strength gain in cylindrical triaxial specimens. Geogrids were installed at five depths within the cylindrical specimen and tested for shear strength properties with a large-scale triaxial test setup to evaluate the effectiveness of both geogrid aperture shape and reinforcement depth. Placing two layers of geogrids in the middle of the specimen was found to result in the maximum shear strength gain. Such placement of the geogrid ensured the intersection of the shear failure plane with the reinforcement layer, ultimately leading to significant shear strength gains. The DEM simulations were observed to capture accurately the ballast shear strength behavior with and without geogrid reinforcement.

This paper presents an experimental study on reduced scale geosynthetic reinforced soil (GRS) abutment models subjected to cyclic traffic loading, aimed at investigating the influences of cyclic load amplitude, self-weight of bridge superstructure, and reinforcement vertical spacing on the cumulative deformations. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. A static load was first applied to account for the self-weight of bridge superstructure, and then the cyclic loads were applied in several phases with increasing amplitude. The results indicate that significant cumulative footing settlement under cyclic loading mainly occurs within the first few hundred loading cycles, and the settlement increases with increasing cyclic load amplitude. The cyclic load amplitude and reinforcement vertical spacing have significant impacts on the cumulative deformations of GRS abutments under cyclic loading. The maximum facing displacement under cyclic loading occurs near the top of the wall. The cyclic load has a greater impact on the reinforcement strains near the upper middle reinforcement layers, while it has a smaller impact on the lower reinforcement layers.

The deformation and degradation behavior of unstabilized ballast and that stabilized with elastomer and geogrid was evaluated using process simulation test (PST) apparatus at various loading frequencies (f). The results indicated that elastomer has significantly reduced the extent of both vertical and lateral deformations of ballast. For example, elastomer reduced the extent of vertical settlement (Sv) in ballast from 22.76 to 9.83 mm, and the lateral deformation (ld) from 8.14 to 1.95 mm (f=15 Hz). It was also seen that the deformation and degradation of both stabilized and unstabilized ballast increased nonlinearly with the increase in the applied loading frequency (f). Further, the beneficial effect of elastomer in restraining the lateral flow of ballast was seen over the entire depth of treated ballast, unlike the geogrid, whose efficiency was maximum at its placement location and then reduced at farther locations. Moreover, elastomer has significantly enhanced the resilient modulus (Mr) and damping ratio (D) of ballast when compared with geogrid-reinforced ballast. For example, the elastomer increased the Mr and D of ballast by 25.9% and 66.6%, respectively, in comparison with an increment of only 15.1% and 25.3% in the case of geogrids (f:15 Hz). The Mr and D of ballast were found to be influenced by the effectiveness of the ballast stabilization technique in reducing lateral displacements. Additionally, elastomer has significantly reduced the vertical stress (σv) at the ballast–subballast interface by 34%. Further, elastomer was found to be more efficient than the geogrid in reducing the dynamic amplification factor (DAF) at any loading frequency. Similarly, elastomer reduced the breakage of ballast by 71% compared with the 40% of geogrid reinforcement (f=15 Hz).

Evaluation of dynamic soil stress distribution in GRS bridge abutments subjected to cyclic loading

For the ballasted railway, under the action of dynamic load caused by the train traffic, the higher loading frequency and amplitude may eventually result in excessive settlement in the ballast, thereby reducing the passengers’ comfort and even affecting the safety of train operation. To study the influence of train traffic load with higher frequency and amplitude imposed on the ballast and the employment of geogrid, ballasted railway model test on the cumulative settlement of geogrid-reinforced ballast under different train loads such as high-speed and heavy axle load and different types of geogrid reinforcement conditions, and triaxial test results of ballast specimens with and without geogrid under different confining pressures are discussed in this paper. The cumulative settlement and stress distribution of the ballast layer, the sleeper vibration, and the strain of geogrid of ballasted subgrade in the model tests under different train loads and geogrid reinforcement conditions are analyzed throughout the test, to explore the effect of geogrid and its working mechanism. Multiple groups of experimental results of axial strain, circumferential strain, and volumetric strain with various confining pressure in the triaxial tests are also compared and studied in this paper. These tests indicated that the installation of geogrid depresses the development of volumetric deformation and effectively increases the peak stress values in the ballast layer. The loading frequency and amplitude also play a vital role in the settlement development and degradation of ballasted subgrade at the same time.