THE INFLUENCE OF BALLAST BED THICKNESS AND TIE SIZE AND SPACING ON SUB-STRUCTURE LOADING

The steady increase in axle loading of railway vehicles and in train speeds not only cause greater stressing of the rails, but also higher loading of the sub-structure. Test results and theoretical considerations show the influence that the size, shape and spacing of the ties, the thickness of the ballast bed and depth of the formation, have on the loading of the sub-structure. Conclusions included the following: a ballast bed over a bad substructure that will carry 20 tons axle load, when over a good substructure, cannot carry more than 25 tons axle load. For axle loads of 30 tons, a protective layer over the substructure and a deepening of the ballast bed are necessary. With regard to the stresses on the ballast bed from an axle load of 25 tons to 30 tons, the tie spacing must be made smaller and with longer ties, the depth of the ballast bed and the thickness of the protective layer over the road bed can be kept to lesser limits.