Physical modeling of railroad ballast using the parallel gradation scaling technique within the cyclical triaxial framework

Due to the difficulties involved with testing large particle granular materials such as rockfill and railroad ballast, several methods of testing scaled down model specimens have been introduced. Of these techniques, the parallel gradation model has been found to be most useful. The parallel gradation model states that a smaller grainsize distribution model granular material, of the same composition as the prototype material, can be used in triaxial testing at a scaled down grainsize, if the model materials grainsize is exactly parallel to the prototype material. Therefore, a model granular material composed of a smaller, but parallel, grainsize distribution can be used to predict shear and compressive properties of a larger rock fill material. Because railroad ballast is loaded in a cyclical fashion, cyclical triaxial testing is considered a reliable method of analysis of strength and deformation characteristics.;The objective of this study is to assess the suitability of the parallel gradation modeling technique for physically modeling permanent axial and volumetric strains, and resilient modulus of scaled down granular materials. Three gradations of railroad ballast will be tested, a prototype ballast material with a top grainsize of 2.5-inches, and two model gradations with top grainsizes of 1.5-inches and 0.75-inches. These three gradations will first be tested monotonically to assess the peak strength of the materials. Based on the monotonic strength, three cyclical stress ratios will be assigned for cyclical triaxial testing for 10,000 cycles. The similarity, or difference in cyclical response will form the basis of assessment for the parallel gradation modeling scheme within a cyclical framework. Cyclical triaxial results will be supplemented with particle shape, attrition, angle of repose, and Los Angeles abrasion analysis.