Railway Ballast Permeability and Cleaning Considerations

Abstract

The unbound aggregate ballast layer is a major structural and drainage component of railroad track that is known to degrade over time. Progressive degradation increases the fine-grained content of the ballast layer through particle breakage and abrasion or from external sources, such as subgrade or foreign material. The point at which ballast should be cleaned of these materials to avoid significant problems for drainage, track geometry, or ride quality is not well known. This paper attempts to ascertain the current state of the art on ballast permeability by reviewing previous studies, to fill any gaps by generating new laboratory test data, and to begin developing ballast cleaning considerations. A new and relatively simple test apparatus, the University of Illinois Constant Head Aggregate Permeameter, was used to study railroad ballast permeability as a function of degradation. Results of tests performed indicate that the cleaner the ballast, the more nonlinear the relationship between discharge velocity and hydraulic gradient, contrary to the findings of previous studies. In addition, flow decreased greatly after small increases in ballast degradation. Detailed findings related to the characteristics of flow—other than whether flow is impeded by in-service ballast condition—may not be extremely useful for rail practitioners because the amount of ballast degradation is difficult to determine in the field. However, the emerging ballast imaging technology described may be able to provide railroad personnel with a threshold for when ballast should be cleaned.

Influência da degradação do agregado na permeabilidade de lastros ferroviários

“…O ensaio de avaliação da permeabilidade foi feito a partir de uma adaptação do experimento desenvolvido por SCHMIDT et al [7]. Utilizou-se um permeâmetro com 9,96cm de diâmetro e altura útil de 16,8cm.…”

“…Assim, foram obtidas as vazões do fluxo através das amostras 1, 2, 3 e 4. De acordo com WILKINS (1956 apud SCHMIDT et al [7]), a velocidade de descarga para materiais granulares de grandes dimensões é não linear e segue a Equação 6.…”

“…Para um lastro altamente colmatado, amostra 4, a velocidade sofreu uma redução de 99,9% quando comparada ao lastro limpo representado pela amostra 1. Tais resultados condizem com aqueles apresentados por SCHMIDT et al [7] dado que a velocidade da amostra com FI ≥ 40 atingiu valores próximos aqueles verificados para areia. Observa-se ainda, que para a amostra 4 a curva da Figura 14, apresenta-se praticamente linear, outra característica similar àquelas verificadas em ensaios com areia.…”

“…WENTY [6] realizou um estudo relacionando o ensaio de abrasão Los Angeles com a qualidade da geometria da via. Já SCHMIDT et al [7] estudaram a permeabilidade de lastros ferroviários por meio de um aparato construído pela Universidade de Illinois denominado de UICHAP (University of Illinois Constant Head Aggregate Permeameter). Neste contexto, o propósito deste trabalho foi avaliar os efeitos da degradação do lastro ferroviário na drenabilidade da estrutura.…”

Evaluation of Railway Ballast Permeability Using Machine Vision–Based Degradation Analysis

“…where n is the discharge velocity, C is an empirical constant depending on the fluid viscosity and the mean particle hydraulic radius defined by Taylor (13), i is the hydraulic gradient, and n is an empirical exponent typically found to be 0.54. Schmidt et al (15) recently conducted constant head permeability tests and validated the power-curve nature of the discharge velocity-hydraulic gradient relationship to correctly represent the permeability of uniformly graded ballast coarse aggregates.…”