Investigation of aggregate properties influencing railroad ballast performance

railroad;ballast;aggregate;geotechnical;Geology;petrology;shear strength testing;los angeles abrasion;permeability;single particle crush testing

Wnek, Michael ; Tutumluer ; Erol

关键词: railroad;    ballast;    aggregate;    geotechnical;    Geology;    petrology;    shear strength testing;    los angeles abrasion;    permeability;    single particle crush testing;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46748/Michael_Wnek.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

A large portion of the annual budget to sustain the railway track system goes toward the maintenance and renewal of ballast. Railroad ballast is a uniformly-graded coarse aggregate placed between and immediately underneath the crossties. Large variations in the source properties of new ballast aggregate materials provided by suppliers resulted in the need to understand how these variations affect ballast performance and quality. The ballast aggregate materials featured eight trap rocks, four granitoid rocks, one aggregate composed of approximately half trap and half granitoid rock, and one quartzite rock.First, the physical properties of all ballast aggregate materials were determined. First, gradation, specific gravity, absorption, density, and void ratio properties were determined. Next, the image analysis-based particle shape properties, Angularity Index (AI), Flat and Elongated Ratio (FER), Surface Texture Index (STI), surface area, and volume were measured using the Enhanced University of Illinois Aggregate Image Analyzer. The last two properties were combined to estimate Specific Surface Area (SSA). Finally, an outside company contracted by the project sponsor determined the petrography of the rocks. Next, a series of mechanical and hydraulic behavior tests were performed on the fourteen ballast aggregates. First, direct shear tests conducted provided the shear strength of the ballast aggregates at three confining pressures followed by single particle crush tests to determine the statistical particle strength of each ballast aggregate material. Next, Los Angeles Abrasion (LAA) tests were performed to 400 and 1,000 revolutions to measure the relative degradation of the ballast aggregates. Finally, permeability tests of the ballast aggregates were conducted using the University of Illinois Constant Head Aggregate Permeameter. The final portion of the project was the detailed petrographic investigation of two rhyolite aggregates: Ballast II and Ballast XIV. Relationships between the test results and physical properties of the ballast aggregates were then investigated. Ballast aggregates with lower AIs had higher densities due to the more cubical nature. Trap rocks had higher specific gravities, fractured (at the quarry) more uniformly, and were generally smoother than granitoid rocks due to their smaller grain size. High values of absorption affected shear strength properties detrimentally. Void ratio formed distinct groupings when plotted against shear strength, which may indicate the existence of an optimum void ratio within each grouping resulting in maximum shear strength. Particle strength was found to increase with decreasing particle size. Trap rocks were also found to have stronger particles than granitoid rocks. Increasing specific gravity was linked to increased long-term ballast aggregate durability. Trap rocks were also found to be more durable than granitoid rocks. Very low SSA was linked to significantly increased permeability for ballast aggregates. Void ratio again formed distinct groupings when plotted against permeability, which again may indicated an optimum void ratio resulting in maximum permeability within each grouping. The Kozeny-Carman equation for predicting permeability was found to be ineffective at predicting the permeability of uniformly-graded ballast aggregates.The American Railway Engineering and Maintenance-of-way Association’s (AREMA) recommended specifications for ballast aggregate quality were investigated and found to be effective at identifying poor quality ballast aggregates, but ineffective at identifying higher quality ballast aggregates. Single particle crush tests hold promise for use as an evaluation test for ballast aggregate quality to identify higher quality ballast aggregates. Finally, after performing a petrographic examination on the two rhyolite aggregates, Ballast XIV’s larger grain size was identified as the main (though not the only) factor that negatively affected its performance. Overall, ballast aggregates do not necessarily differ significantly in properties, which results in difficulty in establishing correlations between individual properties. Thus, properties must be used in combination with one another. Future research is recommended to explore these interdependencies.