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.

Railway ballast degrades progressively as a result of accumulated traffic primarily through abrasion and particle breakage. Degraded ballast may cause reduced lateral and longitudinal stability, ineffective drainage, and excessive settlement of track structures, all of which would adversely affect the performance of ballasted track. Traditional methods of ballast degradation assessment involve time-consuming field sampling and laboratory sieve analysis; moreover, determining the level of track performance deterioration at which ballast maintenance is best considered still remains challenging. This paper investigates the permeability of railway ballast through laboratory testing and provides insight into its field drainage capacity under degraded condition using an innovative approach of field imaging. Constant head permeability tests were conducted on clean and degraded ballast samples which indicated nonlinear power-curve trends, especially for clean ballast, of unit flow amount with its hydraulic gradient. Imaging-based degradation analysis using machine vision technology was also performed on clean and degraded in-service ballast to correlate Fouling Index (FI) from laboratory sieving with Percent Degraded Segments (PDS) obtained from the recently developed image segmentation algorithm. Accordingly, a new Permeability Index (PI) is introduced in this paper to define ballast permeability in the form of a bilinear model developed from the machine vision–based ballast degradation analysis. Based on the findings of this study, a two-stage ballast cleaning process for determining the timeframe of ballasted track maintenance considering its drainage capacity is proposed.

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A laboratory study was conducted to demonstrate that sustainable cement-treated base courses can be achieved through the application of waste quarry by-products (QB) and fractionated reclaimed asphalt pavement (FRAP). Aggregate packing tests were performed on blends of QB and FRAP to determine an optimal blend that would minimize the void content of the aggregate structure while achieving acceptable strengths. The optimal aggregate packing proportions were found to be 70% QB with 30% FRAP. Modified Proctor samples were prepared to determine the moisture–density relationship with several cement contents (2% to 4% by total volume) on dolomite or FRAP coarse aggregate mixed with QB and 0.4% synthetic macrofibers. Mixture design performances were evaluated through strength (compression and split tension) and modulus tests. As expected, higher cement content increased both the strength and elastic modulus for all mixes tested. Mixtures containing virgin aggregates with QB yielded statistically greater elastic moduli than mixtures with FRAP and QB. Fibers did not have a statistical effect on strength or elastic modulus but did provide residual shear capacity across cracks. The QB and FRAP or virgin mixtures with 3% to 4% cement content exceeded the strengths for typical cement-stabilized base materials in the literature. The measured strength and elastic modulus properties show that QB, together with FRAP or virgin aggregates, can be successfully applied as a cement-treated foundation layer.