SHEAR STRENGTH CHARACTERISTICS OF DIFFERENT GRADATIONS OF BALLAST USING PARALLEL GRADATION TECHNIQUE

Ballasted rail tracks are most commonly used rail track structure and to be designed to provide a stable, safe and economical foundation. Main components of ballasted rail track structure could be subdivided as track superstructure and track substructure. The loading from the train will be distributed from the superstructure to substructure. The main structural component of substructure is considered as track ballast which is generally crushed hard stones. Railway authorities specify gradation specifications for selection of ballast for rail tracks. Sri Lanka railways also specified a gradation limits in the selection of ballast for rail tracks. Current standard is closely resembled to the Indian rail track ballast specifications. Commonly, the ballast used in Sri Lanka is crushed gneiss rocks which are in abundance. It is well understood that granular materials derive its strength by resistance to shearing. However, there is no examination conducted to evaluate the performance of rail track ballast used in Sri Lanka considering the shear behaviour. The objective of this preliminary study is to investigate the shear strength characteristics of ballast gradation used in Sri Lanka and compare with selected other ballast gradation specifications. Parallel gradation technique was used to model the sample as it is difficult to handle large size ballast in the conventional direct shear box. Direct shear tests were conducted under three normal pressures of 15 kPa, 45 kPa and 90 kPa on different ballast gradations including current Sri Lankan specification. The results showed that the Current Sri Lankan ballast gradation specification which is the same as Indian standard gradation has the highest shear strength compared to other ballast gradations tested