Cyclic triaxial test of dynamic soil properties for wide strain range.

There is a great need to develop rail networks over long distances and within cities as more sustainable transport options. However, noise and vibration are seen as a negative environmental consequ...

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Modelling, simulation and evaluation of ground vibration caused by rail vehicles

When high speed trains travel close to the wave propagation velocity of the supporting track-ground system, large amplitude track deflections are generated. This has safety implications, and also results in a significant increase in track maintenance due to subgrade deterioration. Thus, this paper presents a method to rapidly predict the speed at which these ‘critical velocity’ effects occur. The method is based upon a dispersion analysis of both the track (either ballast or non-ballasted/slab track) and the underlying ground, which are treated as uncoupled systems. Unlike previous approaches, the new calculation approach is fully automated thus not requiring any post-processing to extract the soil dispersion curve. It also works for soil layers of arbitrary depth, uses minimal computing power and can calculate critical speeds associated with higher soil modes. The dispersion based method can be deployed on new/existing lines via a drop-weight test, or using existing geotechnical data. Its accuracy is tested by comparing the results against an alternative semi-analytical, quasi-static railtrack model, and found to be 97% accurate. The code is useful for railway track infrastructure design and its short run times mean it can be used as a scoping tool for newly proposed high speed railroad lines. To obtain new insights into the key variables effecting critical velocity, a sensitivity analysis is undertaken using 1000 random soil profiles. It is found that on average, for the same track height, slab tracks are less likely to encounter critical velocity issues than ballasted tracks because their critical speed is typically 11% higher. It is also shown that track height plays an important role with increases in slab track thickness and reductions in ballasted track thickness both causing increases in critical velocity. Furthermore, it is found that soil saturation affects critical speed considerably (by up to 12–17% depending on track type) because changes to Poisson’s ratio alter the dispersion characteristics of layered soils in the mid-frequency range, where critical velocity effects occur. Finally, it is shown that railpad stiffness has a low influence, and that increasing the rail bending stiffness on ballasted tracks increases critical speed.

Railway critical velocity – Analytical prediction and analysis

The effect of boundary conditions, model size and damping models in the finite element modelling of a moving load on a track/ground system

The deflections of the track under a moving train depend on the stiffness of the underlying soil as well as the properties of the track and the train. In many situations, small-strain linear properties can be assumed for the soil. However, particularly for soft soil, as the load speed approaches the speed of Rayleigh waves in the ground, the deflections increase considerably. In such situations the use of the small-strain soil stiffness may lead to inaccuracies in the estimates of track deflections or of the critical speed. A finite element model of the track and ground has been developed to study the deflections induced by trains running on soft ground. Soil nonlinearity is introduced through a user-defined subroutine. The nonlinearity is specified in terms of the shear modulus reduction as a function of octahedral shear strain, which can be based on data obtained from laboratory tests on soil samples. The model is applied to the soft soil site at Ledsgard in Sweden, from which extensive measurements are available from the late 1990s. It is shown that the use of a linear model based on the small-strain soil parameters leads to an underestimation of the track displacements when the train speed approaches the critical speed, whereas the nonlinear model gives improved agreement with measurements. In addition, an equivalent linear model is considered, in which the equivalent soil modulus is derived from the laboratory curve of shear modulus reduction using an ‘effective’ shear strain. For this approach it is shown that the predictions in this specific case are improved by using a value of 20% of the maximum strain as the effective strain rather than the value of 65% commonly used in earthquake studies.

The influence of soil nonlinear properties on the track/ground vibration induced by trains running on soft ground

As high-speed trains operate in more and more countries and their maximum speed increases,
the vibration induced by the train has become an important issue. The dynamic coupling between
the vehicle, track and ground has to be assessed for its effects on safety, stability and
maintenance costs. A particular issue, especially for soft soils, is the critical speed at which
the train reaches the wave speed in the ground leading to large deflections of the track and
ground. The aim of this paper is to develop a three-dimensional time-domain approach that
describes how the moving dynamic loads of a high-speed train are distributed through the
track components. This model should allow the critical speed to be determined and the vibration
of the whole system to be analysed. To represent the complex vehicle/track/ground system
a finite element approach is used based on the ABAQUS software. Rather than using a
user-defined subroutine to apply the moving loads, the multi-body vehicle model and
track/ground system are coupled as deformable bodies using a contact model that allows for
large scale motion of the vehicle relative to the track

/pdf/assessment-of-track-ground-coupled-vibration-induced-by-high-3kvkqlr2jk.pdf

Assessment of track-ground coupled vibration induced by high-speed trains

Accurate modelling of railway ballasted track dynamics is an important issue for a variety of
applications such as the assessment of wheel/rail contact force and critical speed of the vehicle.
Track design and assessment against safety and stability criteria can now rely on a number of
advanced and validated dynamic models. However, there is a large range of different models that
can be used to predict ballasted track dynamics. They vary from fast and simple rigid multi-body
models as used in commercial Multibody System approach (MBS) vehicle dynamics calculations,
to more complex and expensive three-dimensional (3D) Finite Element (FE) models. This paper
investigatesthe influence of different modelling options up to 2000 Hz for characterising ballasted
track dynamics with the aim of providing guidelines for simplifying the model and summarising the advantages and limitations of each option. Five different models, a two-degrees-of-freedom (2 dof) multi-body track model, 2D FE model, 3D FE models with/without consideration of
sleeper flexibility, and a 3D FE track model with homogeneous ballast layer are used to represent
the ballasted track as a two-layer support and compared against an analytical solution. Consideration is given to the flexibility of the sleepers, inclusion of ballast density and geometry, element discretization level and FE model length. Equivalent parameters to convert input data from one model to another are summarized

Modelling options for ballast track dynamics

Uneven track settlement inevitably occurs for ballasted track and eventually results in poor track geometry and support stiffness leading to considerably high maintenance cost. Considerable in situ and laboratory experiments have been carried out and empirical formulas have been proposed in order to predict track settlement. Nevertheless, laboratory tests are usually restricted in size for financial reasons and the site characteristics vary significantly to fully understand influential parameters. Therefore, the main aim of the present work is to develop an efficient model capable of replicating localised track settlement for different circumstances. A generic ballasted track simulation package BaTrack is introduced combining the Finite Element (FE) software Abaqus, Python, and Fortran. The three-dimensional (3D) FE model includes rail, sleepers, rail-pads, under sleeper pads (USPs), ballast and foundation layers. An advanced non-linear ballast material model is introduced using porous material properties and extended Drucker-Prager model with hardening and is able to account for different confining pressure values. The model is firstly used for comparison against a series of monotonic triaxial tests and has shown good agreement. It is then validated against a series of full size tests carried out at the Southampton Railway Testing Facility (SRTF). A number of settlement analyses are carried out and characteristics of the stress, contact pressure distribution and void evolution from different track configurations are discussed in detail.

https://research.birmingham.ac.uk/portal/files/67795430/Shih_et_al_Settlement_analysis_using_a_Transportation_Geotechnics_2019.pdf

Jou-Yi Shih

Papers

The effect of boundary conditions, model size and damping models in the finite element modelling of a moving load on a track/ground system

The influence of soil nonlinear properties on the track/ground vibration induced by trains running on soft ground

Assessment of track-ground coupled vibration induced by high-speed trains

Modelling options for ballast track dynamics

Settlement analysis using a generic ballasted track simulation package