Influence of preload and nonlinearity of railpads on vibration of railway tracks under stationary and moving harmonic loads

In railway track dynamics, the stiffness and damping properties of railpads have a significant effect on track vibration, decay rates as well forces transmitted to the track supporting structure. Many studies have shown that railpads exhibit pronounced nonlinear behaviour, with preload and frequency dependent properties. This paper presents a three parameter railpad model, together with its differential equation of motion and the required model parameters obtained from experimental data. A time domain model of a rail discretely supported on these railpads is then formulated using the finite element method. The model is subjected to static and dynamic loading in order to study the effects of preload and frequency on its dynamic behaviour. Results are shown as time histories and frequency spectra for the track displacements and reaction forces for various preload levels. They emphasise the necessity of accounting for nonlinear behaviour based on the large disparities (up to 20 dB) observed between the linear and nonlinear cases for the parameters used in this study

A mixed space-time and wavenumber domain model for predicting ground vibration from railway tracks

In this paper, a mixed model for studying ground vibration generated from surface railway tracks is presented. A ballasted track with nonlinear resilient components is modelled in the time domain using the Finite Element method. The ground is modelled as a linear homogeneous half-space in the wavenumber domain for faster computation. The interaction between the track and the ground is incorporated into the track model through a lumped parameter model representing the vertical dynamic stiffness of the ground. The coefficients of the components of the lumped parameter model are obtained by curve fitting of the transfer function of the half-space for a load applied at its origin. The coupled equation of motion for the track/ground system is formulated with excitation from a stationary point load-consisting of static and dynamic partsacting at the centre of the rail. The coupled equation is solved by numerical integration. The calculated interaction forces at the ballast/ground interface from the space-time domain track model are Fourier transformed to the wavenumber domain and used as excitation to the ground model in order to calculate free-field surface vibration of the ground. Results are presented for the vertical dynamic stiffness for the ground, and for the track and ground displacement in the vicinity of the track and in the free-field. A comparative study between the mixed formulation with the lumped parameter model for the ground, and a fully coupled wavenumber domain model is conducted for linear parameters. Using the fully coupled model as a benchmark, it is observed that the inclusion of the lumped parameter ground model in the track model gives good estimation of the transmitted forces, and hence ground vibration, both in the near and far fields. Finally, the effect of nonlinear track components is briefly investigated for different levels of static preload

Persistent 'hotspots' of lymphatic filariasis microfilaraemia despite 14 years of mass drug administration in Ghana.

Among the 216 districts in Ghana, 98 were declared endemic for lymphatic filariasis in 1999 after mapping. Pursuing the goal of elimination, WHO recommends annual treatment using mass drugs administration (MDA) for at least 5 years. MDA was started in the country in 2001 and reached national coverage in 2006. By 2014, 69 districts had 'stopped-MDA' (after passing the transmission assessment survey) while 29 others remained with persistent microfilaraemia (mf) prevalence (≥1%) despite more than 11 years of MDA and were classified as 'hotspots'. An ecological study was carried out to compare baseline mf prevalence and anti-microfilaria interventions between hotspot and stopped-MDA districts. Baseline mf prevalence was significantly higher in hotspots than stopped-MDA districts (p<0.001). After three years of MDA, there was a significant decrease in mf prevalence in hotspot districts, but it was still higher than in stopped-MDA districts. The number of MDA rounds was slightly higher in hotspot districts (p<0.001), but there were no differences in coverage of MDA or long-lasting-insecticide-treated nets. The main difference in hotspots and stopped-MDA districts was a high baseline mf prevalence. This finding indicates that the recommended 5-6 rounds annual treatment may not achieve interruption of transmission