Debakanta Mishra

Simulating Ballast Shear Strength from Large-Scale Triaxial Tests

The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.

Quantifying Effects of Particle Shape and Type and Amount of Fines on Unbound Aggregate Performance Through Controlled Gradation

Construction of a pavement working platform often is needed on soft, unstable soils to provide sufficient stability and adequate, immediate support for equipment mobility and paving operations without excessive rutting. Standard specifications may allow the use of a wide range of aggregate materials for subgrade applications regardless of aggregate properties. The aggregate type and quality are important factors in determining the required treatment and replacement thicknesses. This paper describes laboratory findings from an ongoing research study at the University of Illinois at Urbana–Champaign to evaluate aggregate cover thickness requirements on soft subgrade. Evaluation was accomplished by characterizing strength and deformation behavior of crushed limestone and dolomite and uncrushed gravel, commonly used in Illinois for subgrade replacement and subbase. The initial laboratory phase consisted of moisture–density, unsoaked California bearing ratio, imaging-based aggregate shape characterization, and shear strength tests. These were based on a comprehensive experimental test matrix, which considered both plastic and nonplastic fines (passing No. 200 sieve or 0.075 mm) blended in the engineered gradations at 4%, 8%, 12%, and 16% target fines contents. From the test results, the most important property at low fines contents (less than 8%) was the aggregate type governed by the angularity (i.e., crushed or uncrushed) and the amount of voids in the aggregate matrix. The uncrushed gravel more quickly filled the voids at lower fines percentages, thus making gravel less tolerable to negative effects of increasing fines. When plastic fines (with a plasticity index of 10 or higher) were included, the amount of fines had a drastic effect on aggregate performance.

Characterization of Railroad Ballast Behavior Under Repeated Loading

Characterizing railroad ballast behavior under repeated train loading is of significant importance for evaluating field settlement or permanent deformation potentials of unbound aggregate ballast layers. For the proper characterization of ballast behavior under dynamic loading, a new triaxial test setup was recently developed at the University of Illinois at Urbana–Champaign. Capable of accommodating cylindrical specimens with a diameter of 305 mm (12 in.) and a height of 610 mm (24 in.), this closed-loop servohydraulic test setup used a load cell and four displacement transducers mounted on the specimen to quantify deformation behavior under loading. Preliminary test results evaluating effects of different applied stress states as well as geogrid reinforcement on ballast behavior established the consistency and repeatability of this new test equipment. Laboratory findings are presented from an ongoing research study aimed at investigating the effects of different ballast types and field degradation trends on permanent deformation accumulation. The ballast type with the highest mill abrasion value was found to accumulate the highest permanent deformation under repeated load triaxial testing. Permanent deformation trends observed for four other ballast types showed direct correlations to the degrees of particle degradation observed in track sections constructed with these ballast materials and trafficked for approximately 18 months with a total track usage of 320 million gross tons.

Railroad Track Transitions with Multidepth Deflectometers and Strain Gauges

Railway transitions such as bridge approaches experience differential movements related to differences in track system stiffness, track damping characteristics, foundation type, ballast settlement from fouling or degradation, as well as fill and subgrade settlement. Identification of factors contributing to this differential movement and developing design and maintenance strategies to mitigate the problem are imperative for the safe and economical operation of both freight and passenger rail networks. Findings are presented from an ongoing research study at the University of Illinois that focuses on the instrumentation and performance monitoring of railroad bridge approaches with multidepth deflectometers. Sensors installed at the selected approaches are introduced, and details of the instrumentation activity are explained. Track settlement data acquired over time are presented to compare the contributions of different substructure layers with the permanent deformation accumulation. Similarly, transient track deformation data gathered under dynamic train loading are analyzed to quantify the contribution of individual track substructure layers to the total transient deformations. Finally, a new approach is presented; it quantifies the support conditions under instrumented ties and assesses the percentage of the wheel load carried by the instrumented tie. Instrumentation of track transitions with multidepth deflectometers has been shown to quantify the contributions of substructure layers to track settlement adequately. In the bridge approaches instrumented with multidepth deflectometer technology, the ballast layers appear to be the primary source of accumulation for both permanent and transient deformations.

Framework for Development of an Improved Unbound Aggregate Base Rutting Model for Mechanistic-Empirical Pavement Design

This paper presents findings from an ongoing research study at the University of Illinois that aims to develop and calibrate improved models for unbound aggregate rutting through laboratory characterization of aggregate materials used for unbound base and subbase applications in the state of North Carolina. Extensive triaxial laboratory testing was performed to establish a robust link between the number of load applications, stress levels, shear stress and sheer strength ratios, and permanent deformation responses. A framework was established for considering the strong correlation that commonly exists between permanent deformation and shear strength characteristics, as opposed to resilient modulus properties, in the laboratory characterization of the permanent deformation behavior of various types of aggregate materials. Trends of permanent strain accumulations from repeated load triaxial tests were adequately captured in a new rutting model whose development took into account the shear stresses applied at 25%, 50%, and 75% of the shear strength properties of these materials under similar field loading confinement conditions. The research shows that this model is an improvement on the rutting damage model for unbound aggregate currently used in AASHTOs mechanistic–empirical pavement design approach because it offers better material characterization and rutting prediction of the unbound base or subbase layer.

Investigation of Geogrid-Reinforced Railroad Ballast Behavior Using Large-Scale Triaxial Testing and Discrete Element Modeling

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.

INVESTIGATION AND MITIGATION OF DIFFERENTIAL MOVEMENT AT RAILWAY TRANSITIONS FOR US HIGH SPEED PASSENGER RAIL AND JOINT PASSENGER/FREIGHT CORRIDORS

As with most highway bridges, railway transitions experience differential movements due to differences in track system stiffness, track damping characteristics, foundation type, ballast settlement from fouling and/or degradation, as well as fill and subgrade settlement. This differential movement is especially problematic for high speed rail infrastructure as the “bump” at the transition is accentuated at high speeds. Identification of different factors contributing towards this differential movement, as well as development of design and maintenance strategies to mitigate the problem is imperative for the safe and economical operation of both freight and passenger rail networks. This paper presents the research framework and preliminary findings from a recently initiated research effort at the University of Illinois at Urbana-Champaign. Aimed at developing design and repair techniques to mitigate differential movement at railway transitions, this research project involves instrumentation, performance monitoring and numerical modeling of new and existing track transitions.

Performance Evaluation of Uncrushed Aggregates in Unsurfaced Road Applications Through Accelerated Pavement Testing

The design of low-volume roads and unsurfaced pavements traditionally involves covering prepared subgrade with an aggregate layer of sufficient thickness such that traffic-induced loads are adequately distributed and stresses on the subgrade can be tolerated. Aggregate gradation and field density requirements are commonly the only considerations for constructing acceptable aggregate layers. Aggregate quality aspects and properties often are not considered in detail when aggregate sources with the lowest material hauling and transportation costs are selected. An approach based solely on economic considerations may result in the selection of locally available material for routine use as the primary load-bearing pavement layer. Full-scale test sections were constructed at the University of Illinois at Urbana–Champaign with aggregate materials of different types and qualities over a weak subgrade of controlled strength and tested to failure with the use of an accelerated transportation loading assembly. Pavement performance under near-optimum and flooded aggregate moisture conditions was monitored by measuring the surface profile. In addition, transverse trench sections were excavated to determine the mechanisms contributing to failure. Accelerated testing of a pavement test cell constructed with an uncrushed gravel with high amounts of nonplastic fines showed excessive rut accumulation through internal shear failure of the aggregate layer under near-optimum aggregate moisture conditions. Excavated trench sections clearly indicated the development of a shear surface within the gravel layer that caused a lateral offset in the subgrade deformation with respect to the wheel path. However, under flooded conditions, excessive subgrade movement was found to be the primary mechanism contributing to pavement failure.

Performance Evaluations of Unbound Aggregate Permanent Deformation Models for Various Aggregate Physical Properties

Permanent deformation or rutting is the main performance indicator of unbound aggregate layers used in flexible pavements. This paper evaluates the prediction abilities of unbound aggregate base or subbase permanent deformation models in use or proposed for use in the Mechanistic–Empirical Pavement Design Guide (MEPDG) approach. Repeated load triaxial-type permanent deformation tests were conducted on three unbound aggregate materials—limestone, dolomite, and uncrushed gravel—commonly used for pavement base and subbase and subgrade replacement applications in Illinois. The test matrix was designed to evaluate effects of aggregate physical properties, including moisture content, gradation, types and amounts of fines, aggregate mineralogy, and particle shape, texture, and angularity. The laboratory-measured permanent deformations were compared with those predicted by four rutting models evaluated in this study. The permanent deformations predicted by the original 1989 Tseng–Lytton model and the 2006 El-Badawy model were generally in good agreement with the measured values. The current MEPDG rutting model and its enhanced version proposed in 2013 by Hashem and Zapata tended to overpredict permanent deformations and have a low sensitivity to changes in aggregate physical properties. In addition to enhancements recommended for the four evaluated models, a unified rutting model was developed; it used a shear stress ratio concept and imaging-based aggregate morphological indexes. With a single set of calibrated model parameters, the unified rutting model produced reasonably accurate permanent strain predictions for all unbound aggregate materials used in this study.

Overlay Thickness Design for Low-Volume Roads: Mechanistic–Empirical Approach with Nondestructive Deflection Testing and Pavement Damage Models

Identifying the appropriate overlay thicknesses is critical to a local transportation agencys ability to maintain its pavement network. Local agencies often use empirical approaches for designing the overlay thickness for low-volume pavements. For example, overlay design for low-volume roads in Illinois is currently carried out using assumed layer coefficients for a limited number of material types. Although such empirical approaches are fairly simple to use, they are often not suitable for considering the effects of recycled and nontraditional construction materials that are more commonly considered in current-day sustainable pavement applications. The lack of mechanical testing for evaluating the pavement structural condition often leads to uneconomical practices in the rehabilitation of low-volume roads. This paper presents a mechanistic–empirical approach for overlay thickness designs of low-volume pavements through a combination of nondestructive deflection testing and preestablished pavement damage models. Five pavement sections, with varying structural and traffic characteristics, were selected from two counties in Illinois. Three sets of falling weight deflectometer tests were conducted over a period of one year to monitor changes in pavement deflection responses. Structural conditions of the pavement sections in their original configuration were evaluated first. Then, the corresponding required overlay thicknesses were determined by using two methods currently used by local agencies. The inability of the currently available methods to properly account for current pavement structural conditions was highlighted. A new mechanistic–empirical overlay thickness design method introduced in this study successfully identified structural deficiencies in the original pavement configurations.