Stress-strain and degradation behaviour of railway ballast under static and dynamic loading, based on large-scale triaxial testing
Mainly due to the Sydney Olympics, the State Railway Authority of New South Wales has been under considerable pressure to improve the current track conditions to meet the dramatically increased demand for rail transport. Rail track upgrading should cater for high speed trains with heavier carriages, and increased traffic frequency. Consequently, increased degradation and settlement of ballast foundations are anticipated, together with higher maintenance costs. In order to minimise maintenance costs, a thorough understanding of the strength-deformation properties of ballast under static and dynamic loading is essential. Due to the relatively larger size of ballast, conventional laboratory tests often give misleading stress-strain results and failure loads (Indraratna et al., 1998). To overcome the size effects, it is imperative to characterise coarse aggregates by conducting tests using proportionately larger geotechnical equipment. This paper describes the strength-deformation and degradation aspects of ballast (latite basalt) in large-scale triaxial equipment. The large cylindrical triaxial apparatus (specimen size 300mm diameter x 600mm high) was used to test specimens under fully drained compression (static loading) at low confining pressures (10-200 kPa) that are typical of a conventional ballast bed. The influence of confining pressure and initial porosity (compaction) on the ultimate shear strength, angle of internal friction, dilation rate and degree of particle degradation is studied in detail. The results are also compared with the behaviour of rockfill materials, which are usually subjected to much higher confining pressures (up to 1,000 kPa).