The pressure to achieve a higher speed of train operation has increased in the last few years due to the development of high-speed railway lines. In the presence of soft soils, this speed increase is not possible without undertaking soil improvement measures, since the critical speed is lower than the desired speed of train operation. One of the most widely used soil improvement techniques is that of stone columns. In relation to these is the concept of critical length which, up to now, has only been studied for static cases. This research focuses, for the first time, on the enhancement of the critical speed in high-speed railways with stone columns and with special reference to critical length. Moreover, others parameters such as: maximum rail displacement and Dynamic Amplification Factor (DAF) are also studied. Analysing three profiles of ground representative of soft soils, it can be observed how the effect of the stone columns on critical speed is relevant, showing that critical length is different with respect to the three parameters above, the most restrictive one being that which corresponds to the critical speed. The influence of other parameters on the critical length and on the effectiveness of stone columns, such as the area replacement ratio, the dynamic pressure bulb and the existing soil stiffness is analysed and discussed.

Improvement of the critical speed in high-speed ballasted railway tracks with stone columns: A numerical study on critical length

Effect of basal reinforcement on performance of floating geosynthetic encased stone column-supported embankment

Railway track geometry deteriorates under repeated train loading. When linespeed is increased or new rolling-stock is introduced this can alter the future rate of change of differential settlement and track geometry. Therefore this paper presents a novel combined engineering-economic approach to investigate the effect of increasing train speeds, adding additional passenger movements, and adding additional freight movements to an existing line. Firstly a numerical algorithm is presented to compute differential track settlement. An important novelty of the model is its use of the wavenumber finite element method coupled with settlement relationships in a manner that allows for track irregularities to evolve after every load passage (i.e. taking into account the evolution of the track unevenness profile before applying each subsequent train passage). Unlike traditional approaches this allows the model to faithfully simulate mixed traffic conditions, including the coupled interactions between different rolling stock types and track geometry. The engineering model is used to predict tamping intervals, and then coupled with an economic model capable of calculating deterioration elasticities and marginal costs. It is shown that higher speeds result in higher dynamic forces and cause a faster rate of deterioration of track geometry, thus increasing marginal cost. The model is then used to investigate the effect of adding additional train movements to a passenger line. It is shown that additional movements increase the rate of track degradation and marginal costs, particularly if the additional traffic is freight. This is because freight vehicles typically have one only layer of (stiff) suspension, thus generating elevated dynamic forces compared to passenger vehicles. 

The effect of rolling stock characteristics on differential railway track settlement: An engineering-economic model

Development of a Finite Element Model to Study the Settlement of Ballasted Railway Tracks Subjected to Two Adjacent Moving Trains

Calibration of a Finite Element Model to Predict the Dynamic Response of a Railway Track Bed Subjected to Low- and High-Speed Moving Train Loads

In the last 30 years, the scientific community has developed and proposed different models and numerical approaches for the study of vibrations induced by railway traffic. Most of them are formulated in the frequency/wave number domain and with a 2.5D approach. Three-dimensional numerical models formulated in the time/space domain are less frequently used, mainly due to their high computational cost. Notwithstanding, these models present very attractive characteristics, such as the possibility of considering nonlinear behaviors or the modelling of excess pore pressure and non-homogeneous and non-periodic geometries in the longitudinal direction of the track. In this study, two 3D numerical approaches formulated in the time/space domain are compared and experimentally validated. The first one consists of a finite element approach and the second one of a finite difference approach. The experimental validation in an actual case situated in Carregado (Portugal) shows an acceptable fitting between the numerical results and the actual measurements for both models. However, there are some differences among them. This study therefore includes some recommendations for their use in practical soil dynamics and geotechnical engineering.

Benchmarking of two three-dimensional numerical models in time/space domain to predict railway-induced ground vibrations

Critical length of encased stone columns

Influence of non-linear soil properties on railway critical speed

The use of piles as barriers to mitigate vibrations from rail traffic has been increasing in theoretical and practical engineering during the last years. Tyre-derived aggregate (TDA) is a recycled material with some interesting applications in civil engineering, including those related to railway engineering. As a novelty, this paper combines the concept of pile wave barriers and TDA material and investigates the mitigation effect of pile barriers made of TDA on the vibrations transmitted by rail traffic. This solution has a dual purpose: the reduction of railway vibrations and the recycling of a highly polluting material. The mitigation potential of this material when used as backfill for piles is analysed using a numerical scheme based on a 3D finite-difference numerical model formulated in the space/time domain, which is also experimentally validated in this paper in a real case without pile barriers. The numerical results show insertion loss (IL) values of up to 11 dB for a depth closed to the wavelength of Rayleigh wave. Finally, this solution is compared with more common backfills, such as concrete and steel tubular piles, showing that the TDA pile is a less effective measure although from an environmental and engineering point of view it is a very competitive solution.

https://www.mdpi.com/1660-4601/17/24/9191/pdf

Jesús Fernández-Ruiz

Papers

Improvement of the critical speed in high-speed ballasted railway tracks with stone columns: A numerical study on critical length

Benchmarking of two three-dimensional numerical models in time/space domain to predict railway-induced ground vibrations

Critical length of encased stone columns

Influence of non-linear soil properties on railway critical speed

Use of Tyre-Derived Aggregate as Backfill Material for Wave Barriers to Mitigate Railway-Induced Ground Vibrations.