Mechanics of railway ballast behaviour
It is important to have consistent ballast testing methods that provide results reflecting the performance of different ballast materials in the railway trackbed. In this research, extensive laboratory tests were conducted to investigate the correlation between simple ballast index tests, and box tests simulating ballast field loading conditions in a simplified and controlled manner. In the box test, a sleeper load of 40kN was applied to a simulated sleeper on the top of a sample of ballast in a box of dimensions 700x300x450mm. The ballast was tamped using a Kango hammer which caused particles to rearrange as the level of the sleeper was raised. The ballast tests investigated in this project are those ballast tests specified in the Railtrack Line Specification (RT/CE/S/006 Issue 3, 2000), in addition to single particle crushing tests, oedometer tests, petrographic analysis, and box tests. It was found that there was some correlation between the single particle crushing tests, oedometer tests, box tests and petrographic analysis. One of the current ballast tests, namely the Aggregate Crushing Value (ACV) test, which is analogous to the oedometer test, is not appropriate because the ACV test uses 10-14mm ballast particles, and there is a size effect on the strength of ballast and different ballasts have different size effects. However, if an oedometer test is used on track ballast, the results correlate better with ballast field performance as simulated in the box tests. Six ballasts were tested: A, B, C, D, E and F (mineralogy of these ballasts can be found in the appendix). The aim was to examine the relative performance of these ballasts and to establish which index tests were most indicative of performance in the box test. Simple index tests were performed on each of the ballasts, whilst box tests were only performed on ballasts A, B, C and D. The box tests were generally performed wet by adding a known volume of water at each tamp. For ballast A, controlled tests were also performed on dry ballast, and tests involving traffic loading only and tamping only were also conducted. A box test on 10-14mm ballast A was also conducted to investigate the size effect on ballast behaviour in the box. The Wet Attrition Value (WAV), Los Angeles Abrasion (LAA), and Micro-Deval Attrition (MDA) seem to be suitable parameters to indicate ballast performance in the box test. However, this is considered to be due to the rearrangement of particles in the box test caused by the simulated tamping. In addition to the laboratory tests, the application of discrete element program PFC3D (Itasca Consulting Group, Inc., 1999) in simulating ballast behaviour was also investigated. Single particle crushing test was simulated to produce crushable agglomerates with a distribution of strengths of ballast A. These agglomerates were then used to simulate the oedometer test. The resulting normal compression line was compared with that for real oedometer tests: discrepancies can be attributed to the simplified geometry of the agglomerates. Due to the high computational time in simulating a box test with crushable agglomerates, uncrushable spherical balls and uncrushable angular agglomerates were used to represent individual ballast particles in the box. Important aspects of ballast behaviour under repeated loading, namely resilient and permanent deformation, were studied. It was found that the box test on uncrushable angular agglomerates give less permanent deformation compared with the test on spherical balls, because of the additional resistance provided by the irregular shape of the agglomerates.