Improvement of granular media and formation soil using geosynthetics with special reference to rail track environment
Rail track substructure comprises of graded layers of granular media of ballast and subballast (capping) placed above compacted subgrade (formation soil). Ballast particles breakdown and deteriorate progressively under heavy cyclic loading, and subgrade which consists of soft soils fails due to repetitive stress. Consequently, the major part of rail track maintenance cost is associated with ballast related problems and subgrade failure. The necessity of keeping a competitive edge against other means of transportation has increased the pressure on the railway, industry to improve its efficiency and decrease maintenance and infrastructure costs. Using geosynthetics, which are safe and economical alternatives to conventional practice, track conditions can be improved and maintenance cost can be reduced. This paper addresses the important factors affecting the geotechnical behaviour of ballast as well as the effect of using geosynthetics for improving the deformation characteristics of rail track ballast and formation soil. The factors studied includes; shear strength, ballast breakage, ballast particle size distribution and confining pressure. The prospective use of different types of geosynthetics in enhancing the performance of ballast deformation and degradation under cyclic loading was investigated using a large-scale prismoidal triaxial rig. A plane strain finite element analysis (PLAXIS) of the prismoidal triaxial rig was also carried out and used to obtain the optimum location of geosynthetics in rail track substructure. Furthermore, a large-scale consolidometer was employed to determine the effect of prefabricated vertical drains (PVDs) and to optimise the accelerated primary consolidation in soft formation of the track. The main factors affecting the efficiency of PVDs when employed in rail tracks with soft formation were also discussed. The paper also includes a section where recommendations are made how to prepare the stability of rail tracks on surface formation soils considerably disturbedlremoulded by the Asian tsunami, based on the first author field experience in Sri Lanka. The research findings reveal that track substructure stabilised with geosynthetics has a good potential for resilient track construction and for reducing the cost of track maintenance.