Carlton L. Ho

Numerical modeling of subglacial sediment deformation: Implications for the behavior of the Lake Michigan Lobe, Laurentide Ice Sheet

We apply a numerical model of the late Wisconsin (circa 20,000 years B.P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate how fine-grained subglacial sediment might influence lobe behavior, particularly rapid millennial-scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (“hard bedded”) basal boundary condition. In areas overlain by fine-grained sediment (“soft bedded”), the base of the ice is coupled to a deformable sediment layer, using a rate-dependent stress-strain law. Geotechnical tests of clay-rich till deposited by the LML provide control for sediment rheologic parameters. Simulated cross-sectional profiles are consistent with reconstructions from geologic evidence. Time-dependent simulations suggest that a soft-bedded lobe could have reached steady state in about 20,000 years or less, in contrast to 50,000 to 60,000 years for an otherwise identical hard-bedded lobe. A soft-bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial-scale shifts in accumulation or ablation as a nonsliding hard-bedded lobe, but in both cases the response is slower than that indicated by the geologic record. Results suggest that while strong millennial-scale changes in accumulation and ablation can produce responses in hard-bedded or soft-bedded ice that are consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for millennial- and submillennial-scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior.

Use of Recycled Tire Rubber to Modify Track-Substructure Interaction

Resiliently bound ballast (RBB) is a new engineered material being developed as an alternative or supplement to conventional ballast for use in transit, passenger, and freight railways to improve mechanical behavior and control the modulus and damping when recycled tire waste material is used. RBB is a stable mixture of standard ballast stone and tire-derived aggregate (TDA) bound together with a purpose-designed resilient epoxy binder. Initial laboratory tests were conducted on specimens 6 in. (151 mm) and 10 in. (254 mm) in diameter of two mixes of RBB. Tests were conducted on fully bound (cemented) samples of ballast, TDA, and epoxy as well as individual particles with TDA bound to the individual ballast particles with the resilient epoxy binder. The tests included static triaxial compression tests and dynamic cyclic triaxial tests. Static tests indicated that the addition of the TDA and epoxy resulted in an increase in cohesive strength. The dynamic tests indicated changes in modulus and damping depending on the mixture of rock, TDA, and epoxy. One proposed use of RBB is to affix the RBB to the bottom of concrete ties to modify the interaction between the tie and the ballast material and improve ballast durability and modify resilience and damping. Box tests were conducted on a section of concrete tie with and without RBB attached. The tie with a section of rail attached was vertically loaded with a sinusoidal load to model repetitive axle loading. Observations indicated that the box test that used a concrete tie without RBB produced more ballast breakage compared with the test that used a concrete tie with RBB; however, more abrasion between particles occurred with the RBB-bound tie.

Field validation of a three-dimensional dynamic track-subgrade interaction model

An efficient three-dimensional dynamic track-subgrade interaction model has been formulated and then validated by field investigations at various field and traffic conditions including the effect of different train speeds and types of trains. The model contains a two-dimensional discrete support track model and three-dimensional computation-efficient finite element soil subgrade model. In the two-dimensional track model, the rail beam is modelled as an Euler-Bernoulli beam. The two-dimensional track model discretizes the tie and ballast as rigid bodies with designated spacing. The three-dimensional finite element subgrade model is simulated by plane-stress quadrilateral finite elements. The longitudinal direction of the subgrade model is expanded in the frequency domain and is assumed to be homogeneous. Therefore, the computing time could be largely reduced. A moving dynamic loading is applied on top of the rail. The model is capable of taking train speed variations and the profile change of the cross section into consideration. Multiple field instrumentation tests covering the two train types and different train speeds at the test site were then conducted to verify the accuracy of the dynamic track-subgrade interaction model. Testing site is located on the Amtraks highest speed line (Northeast Corridor: 250 km/h) near Kingston, Rhode Island in the United States. A method to obtain the tie deflection from accelerometer data at Kingston was proposed and then validated at another site on the Northeast Corridor. Tie deflections measured in the field were compared with those predicted by the three-dimensional dynamic track-subgrade interaction model. It is concluded that this model can predict track performance accurately for the Kingston site.

Evaluating the Correlation Between the Geotechnical Index and the Electromagnetic Properties of Fouled Ballasted Track by a Full-Scale Laboratory Model

To determine the correlation between water content or fouling of a railroad track and ground-penetrating radar (GPR) signals, a full-scale railway track model was designed and constructed at the University of Massachusetts Amherst. A track section was simulated by creating a model consisting of rails and ties placed on a ballast layer on a subballast layer. The model was constructed three times with three fouling percentages. Each model was tested with water-content conditions of dry, field capacity, and two points in between these extremes. Frequency antennae of 450 MHz and 2 GHz were used to evaluate the conditions. The design and construction of the full-scale track in addition to the GPR data analysis and interpretation are presented in this paper. The results show that the dielectric permittivity and frequency spectrum can be used as an indicator of percentage of fouling and water content of a track. In addition, a linear correlation was observed between the percentage of fouling and the water content under field capacity conditions.

MODEL TRACK STUDIES BY GROUND PENETRATING RADAR (GPR) ON BALLAST WITH DIFFERENT FOULING AND GEOTECHNICAL PROPERTIES

In recent years there has been an increase in the knowledge of, and need for, non-invasive monitoring of ballast in order to identify the problematic sections of track and decrease the maintenance cost. Various technologies such as Ground Penetrating Radar (GPR) are becoming accepted for investigating the condition of ballast. However since these techniques were not originally developed for engineering applications, their applicability in ballast evaluations can be sometimes uncertain. Continued empirical studies and condition specific calibrations are needed to demonstrate repeatable and quantifiable results. In this study large-scale track models with trapezoidal section area were constructed at the University of Massachusetts to investigate the effects of breakdown fouling, and the effects of changing geotechnical properties on GPR traces. This paper presents the design and construction of large scale track models, and methods used for GPR data collection. GPR data are presented in this paper that demonstrate sensitivity to the track model properties and variables. In particular, the experiments are being used to evaluate changes in GPR data with changing geotechnical properties of the ballast such as density, water content, grain size distribution (GSD), and fouling percentage.

Ground Acceleration Based Characterization of High-Speed Rail Track Bed

A new ground acceleration based method of evaluating High Speed Rail (HSR) track bed integrity has been developed. The purpose of the implementation of the system is to provide an efficient means for monitoring HSR track to predict settlement and deformation conditions that would require maintenance. The method relies upon the use of an accelerometer array that can be installed in the substructure of the track, i.e., within the ballast, subballast and subgrade layers. The array can be installed on a permanent basis and monitored periodically to identify changes in the small strain elastic properties of the substructure layers. The methodology utilizes conventional downhole seismic shear wave testing. The accelerometers are used in concert to simultaneously evaluate the arrival of seismic shear waves produced by surface initiated impact methods. The methodology was successful in measuring initial shear wave velocities. The shear wave velocities were used to calculate shear moduli and elastic moduli for the ballast, subballast and subgrade layers. Shear moduli were in the range of 14×10 3 kPa for the fouled ballast to 118×10 3 kPa for the subgrade. Elastic moduli were in the range of 39×10 3 kPa for the fouled ballast to 320×10 3 kPa for the subgrade. These values were compared with values that were measured separately at the site on similar soils and were found to be consistent. The monitoring can be done on a regular schedule requiring no interruption of track operations. The methodology has other applications including highway pavements, airport pavements and tunnels/underground facilities.

Verification of Box Test Model and Calibration of Finite Element Model: Evaluation of Railroad Ballast Performance

The development of innovative models to evaluate railroad ballast performance can reduce railroad maintenance costs and improve train operation and safety. A significant portion of the maintenance costs associated with track transportation systems can be attributed to ballast degradation (fouling). Understanding the mechanisms and the effects of ballast fouling is becoming increasingly important with the increased demand on track transportation systems and as trains become faster and carry more weight. This paper presents the results and findings of a finite element (FE) model to evaluate and validate a railroad ballast box test apparatus developed at the University of Massachusetts, Amherst. FE modeling of the box test was conducted to provide insight for subsequent laboratory box test experiments. Boundary and scale effects of the box test were evaluated through the FE model, and the stress–strain behavior and distribution were explored. The predicted relationship between the elastic modulus and the percentage of fouling of the ballast by wet clay was found through calibration of the FE model. The FE model showed that the boundary conditions of the box governed the behavior of the model, specifically, that the addition of smooth and flexible walls reduced strength capacity and increased plastic deformation of the ballast. The FE model also showed that by increasing the size of the box by approximately 50%, stress concentrations below the tie, which would have led to a failure mechanism resembling punching shear, were avoided.

Spectral Analysis of Ground Acceleration-Based Testing

Spectral analysis of downhole acceleration data is used to identify changes in elastic properties of track substructure. The spectral analysis is based on the power spectral density (PSD) as an indicator of frequency content. This method was developed and tested at the Transportation Technology Center, Inc., in Pueblo, Colorado. The system provides an efficient means for monitoring track bed to predict settlement and deformation conditions that would require maintenance. The method relies on a triaxial piezoelectric accelerometer inserted into a permanently installed inclinometer casing installed into the subgrade. Periodic measurement of the dynamic ground response allows for the identification of changes in the small strain elastic properties of the substructure layers. The methodology uses conventional downhole seismic shear wave velocity testing. For the purpose of comparison, test sections were constructed with clean and fouled ballast. Shear wave velocities indicated the differences in material properties. PSDs of the signals were analyzed to determine variation resulting from differences in soil conditions. PSDs were calculated at different intervals of loading. With increased loading, settlement of the subtrack soils occurs. The densifi-cation can be seen as an increase in the predominant frequency of the PSD. The PSD from the section with the clean ballast had a higher predominant frequency than the PSD from the section with the fouled ballast. A decrease in predominant frequency over time would indicate the decrease in stiffness and the potential of increased fouling of the ballast with fines. Changes in the frequency content would indicate changes in elastic response.

Evaluating fouled ballast using seismic surface waves

This paper presents the equipment and Spectral Analysis of Surface Wave (SASW) approach for non-invasively characterizing railroad track ballast and foundation layers. Surface wave testing on a railroad track is more complicated than that on soil sites or pavements because of the presence of ballast, crossties, and rails as well as the complexity of ballast-soil foundation structure in terms of the variation of shear-wave velocity with depth. Using portable SASW equipment, the Young’s Modulus of the ballast was calculated for both clean and fouled ballast in wet and dry conditions. In addition, the local modulus is determined at different locations under the tie, e.g. tie center or edge, to investigate modulus variation and tie support under a single tie. Expansion of the system to measure the modulus under two adjacent ties is also discussed and may be suitable for evaluating ballast performance under §213.103, which requires ballast to perform the following serviceability functions: (1) transmit and distribute the load of the track and railroad rolling equipment to the subgrade; (2) restrain the track laterally, longitudinally, and vertically under dynamic loads imposed by railroad rolling equipment and thermal stresses exerted by the rail; (3) provide adequate drainage for the track; and (4) maintain proper track crosslevel, surface, and alignment”.Copyright

SEISMIC SURFACE WAVE TESTING FOR TRACK SUBSTRUCTURE ASSESSMENT

This paper presents some results of a Federal Railroad Administration (FRA) sponsored research project on using seismic surface waves to evaluate track substructure (ballast and subgrade) condition and performance. The main objective of this project is to develop a system for rapidly, nondestructively, and quantitatively assessing the engineering properties of the track substructure (ballast and subgrade). These engineering properties — shear modulus, Young’s modulus, and shear strength — are derived from measurement of the shear wave velocity profile and can be used to evaluate track safety and to predict inspection and maintenance intervals. This paper describes the seismic testing, results of field measurements, and numerical modeling of the seismic wave propagation in the track substructure. Procedural issues addressed by the numerical models and presented herein include the size and location of the excitation source and orientation and spacing of the receiving accelerometers.Copyright