Impact of increasing freight loads on rail substructure from fracking sand transport

•A large scale triaxial testing device is used to examine railroad ballast behavior.•Varying ballasts, frac sands and water contents are used to evaluate performance.•Heavy axle loads are investigated for their impact on maintenance cycles.•For a 30 kip car load increase deformation will increase at a rate of 0.07%/MGT.•Ballast gradation is important for resisting plastic strain.

The increased use of the hydraulic fracturing technique, used extensively in the USA for oil and natural gas extraction, has significantly expanded the frac sand market and will impact existing freight rail corridors. This growth in the freight rail sector is expected to raise the operation and maintenance costs for the railroads due to the increase in heavy axle loads (HAL), traffic in million gross tons per year (MGT), and fouling from surface spillage.The impact of HAL on the ballast layer have been studied in some detail; however, it is unknown how infiltration of frac sand into the ballast layer affects the deformational response of the track structure. The purpose of this study was to determine the deformation response due to surface spillage of frac sand into the ballast layer and quantify the effect of moisture in the frac sand-ballast matrix compared to clean ballast.Three ballast and three frac sand samples were used in this study, of which the following basic properties were characterized: particle-size distribution, mineralogy classification, particle shape, bulk density, void ratio, particle shape, hydraulic conductivity, and soil water characteristic curves. These properties were used to optimize the large-scale cyclic triaxial (LSCT) test method. The results of HAL tests show an increased rate of strain accumulation averaging 0.07%/MGT for every 30-kip car load increase. The results of the surface spillage tests show an average increased rate of strain accumulation averaging 0.05%/MGT, 0.13%/MGT, and 0.31%/MGT for AREMA ballast #24, #4A, and #5 respectively, for a 7% increase in gravimetric water content. These results are crucial for estimating any changes in maintenance cycles and creating life cycle costs estimations. This study will help determine the future of the railway system and how to best engineer solutions for this growing transportation industry.