Judith Wang

Numerical Simulations of Vibration Attenuation of High-Speed Train Foundations With Varied Trackbed Underlayment Materials

Interest in high-speed railway as an alternative means of transportation is steadily increasing around the world. However, high-speed trains come with the concern of track vibration and induced noise and ground vibration. Excessive track vibration can cause damage to trains and tracks and reduce riding comfort for passengers. Ground vibration induced by passing trains can also damage and disturb surrounding infrastructure (especially structures housing precision machines or instruments) and residents. One potential solution toward minimizing these vibrations is the use of rubber-modified asphalt concrete (RMAC) as a material for high-speed train trackbed underlayments. In this paper we present the results of a finite element simulation of a high-speed train foundation. The simulated foundation was subjected to dynamic loading in several test scenarios, with RMAC and other traditional paving materials used as trackbed underlayment materials. The ground accelerations at designated points in these simulations were then monitored and compared with one another to determine the relative effectiveness in vibration attenuation. From these parametric studies, RMAC proves to be more effective than currently used paving materials in damping out vibrations from dynamic loading. Implications for field applications are also discussed.

Three-Dimensional Finite Element Simulations of Ground Vibration Generated by High-Speed Trains and Engineering Countermeasures

In this paper we discuss the benefits of using rubber-modified asphalt concrete in high-speed railway foundations. We present the results from a series of three-dimensional finite element simulations modeling a high-speed train foundation utilizing various trackbed materials. Four trackbed materials were tested for their relative vibration attenuation capacities: ballast, concrete, conventional asphalt concrete, and rubber-modified asphalt concrete. Additionally, studies varying the speed and the weight of the passing train were performed. Parametric studies varying the dimensions of the trackbed underlayment were also examined. From these numerical simulations, it is shown that rubber-modified asphalt concrete outperforms other traditional paving materials in ground vibration attenuation. It is also shown that the speeds and weights of the passing trains and the dimensions of the trackbed have significant effects on the relative performance of the paving materials. Implications for design are discussed.

Dynamic Modulus and Damping Ratio Measurements from Free-Free Resonance and Fixed-Free Resonant Column Procedures

AbstractResonant column procedures may be used to quantify a soil’s shear modulus degradation and damping ratio curves along the small strain (∼10−4 to ∼10−1%) range. However, resonant column procedures often cannot provide measurements of very small strain (<∼10−4%) dynamic mechanical properties. The objective of this study is to determine if free-free resonance (FFR) procedures may be used to provide complementary very small strain, maximum dynamic moduli and minimum damping ratio to small strain shear moduli degradation and damping ratio curves from resonant column testing procedures. A plastic control specimen in six different free-free configurations was used to determine appropriate free-free boundary conditions for axial FFR procedures. Five cohesive soil specimens were then subjected to axial FFR and torsional fixed-free resonant column procedures to evaluate the consistency of the resulting strain-dependent modulus and damping measurements. It can be concluded that: (1) the specific free-free bou...

The effects of variable soil damping on soil-structure dynamics

This study examines the effects of variable dynamic dissipative and elastic soil characteristics on the dynamic response of a soil-structure system. Three equivalent linear modeling techniques (linear hysteretic and Kelvin-Voight constitutive models and weighted modal time history analysis) are used to incorporate the soil and structure’s unique dissipative characteristics. The soil’s critical viscous damping ratio and shear modulus are iteratively determined based upon experimentally observed, upper and lower bound strain-dependent stiffness degradation and damping curves. It is found that the three techniques produce consistent results: as the shear modulus increases, so does the primary natural frequency of the response; as the damping ratio of the soil increases, the magnitude of the response decreases. It is also found that variations in response due to using different values for a soil’s dynamic characteristics are dependent upon the equivalent linear modeling technique used. The impact of soil damping variability is seen to be as influential upon the dynamic response of a structure as shear modulus variability, because dissipative variations have a direct impact on the magnitudes of the displacement responses. The effects of variable soil damping should therefore be considered in addition to stiffness parameters as part of the dynamic characteristics of a system when determining a system’s dynamic performance.

Relationships between Particle Shape Characteristics and Macroscopic Damping in Dry Sands

AbstractThe objective of this study is to investigate the relationships between the scalar particle shape descriptors and macroscopic dissipative properties of dry, clean sands. To define these relationships in a consistent and useful manner, it is important to identify and examine the relevant microscopic particulate shape characteristics and their effects upon the macroscopic strain-dependent damping ratio curves, ξ(γ), as measured by resonant column procedures. The scalar particle shape descriptors are examined to determine the appropriate representative definitions for each of the three scales of particle shape (roughness, roundness, and sphericity). Quantitative mineralogy scanning technology and image analysis procedures are used to quantify these representative particulate descriptors and the specific surface for a significantly larger number of standardized sand samples’ constituent particles than previously examined. Particle descriptor data are projected to represent each of the standardized san...

Gunnison Tunnel: Engineering History of an Early American Reclamation Project

AbstractThis paper describes the surveying, engineering, and construction operations performed in the construction of the Gunnison Tunnel for the Gunnison River Diversion Project. The Gunnison Tunnel, which diverts water from the Gunnison River to the Uncompahgre Valley, was one of the first major engineering projects undertaken by the federal government under the authority of the U.S. Reclamation Service. It has been recognized as a national historic civil engineering landmark for the engineering feats involved in its construction, including the performance of site investigations and survey operations through extremely harsh terrain and its construction crews’ perseverance through several dramatic accidents. It is observed that a potential cause of these multiple accidents was an insufficient knowledge of the geological subsurface properties, leading to significant differences between expected and encountered subsurface conditions. This was particularly problematic with respect to the existence of cohesi...

Finite-Element Analyses of Mechanically Stabilized Earth Walls Subjected to Midlevel Seismic Loads

The objective is to examine the performance of specific detailing components of mechanically stabilized Earth (MSE) walls when subjected to midlevel seismic excitations, such as those expected in the State of Colorado. The motivation for this study is the elevated peak ground accelerations mandated by the 2007 4th Edition of AASHTO LRFD bridge design specifications. According to this revision, highway-related projects must be designed for an elevated 1,000-year return period earthquake, as opposed to the earlier editions’ 500-year return period earthquake. Finite-element analyses are performed using LS-DYNA to examine the displacement-based, dynamic behavior of individual MSE wall components, such as geogrid reinforcement and wall facings. Walls at two heights, 4.57 m (15.0 ft) and 9.14 m (30.0 ft), with two types of facings (modular block and segmental panel walls), reinforced using geogrids, are modeled based on the Colorado Department of Transportation drawings. These walls are subjected to three synthetic earthquake motions generated by the USGS 2002 deaggregation tool for three sites spread across the geographical extent of the State of Colorado. The results show that typical MSE walls perform well with respect to connection details when subjected to midlevel seismic loads.

Modeling the Effects of Intrinsic Damping in Soil-Structure Interaction

The dynamic behavior of a soil-structure infrastructure system is governed by its elastic, inertial, and dissipative characteristics. Finite element procedures for representing the stiffness and mass of an infrastructure systems individual components and their assembly into a global system representation have been well established. However, means for representing intrinsic damping, a materials capacity for mechanical energy dissipation, have lagged in comparison. This is disadvantageous in geotechnical studies of soil-structure interaction, where it is necessary to preserve the distinct dissipative characteristics of both the natural earth and manmade construction materials. In this paper, equivalent linear models for mathematically representing a soils intrinsic damping based upon common geotechnical laboratory procedures are presented. Finite element procedures for using these models in representing nonuniform intrinsic damping in multi-degree-of-freedom soil-structure systems are reviewed. A representative soil-structure system is analyzed to illustrate the applications of the procedures the differences in their predicted responses. It is shown that three independent equivalent linear models based upon three different theoretical premises result in virtually identical dynamic predictions when uniform intrinsic damping is examined. However, when nonuniform intrinsic damping is considered, the three methodologies result in widely divergent responses. Reasons for the disagreements in responses are discussed based upon the analytical simplifications and assumptions made in each model. Recommendations are made to help guide the analyst in using these procedures for modeling nonuniform intrinsic damping in soil-structure systems.