ABSTRACT
This chapter introduces the "thickness of granular layer" module, which allows the designer to calculate the required thickness of the granular layers overlying relatively weaker subgrade soils which are prone to bearing capacity-type shear failure and excessive settlement. It presents the steps and procedure to determine the thickness of ballast and capping layers based on two design criteria. The criteria are: preventing excessive subgrade deformation and failures, and limiting the vertical stress on the subgrade soils to less than "threshold stress" in order to protect against subgrade failure by excessive plastic deformation. The threshold stress is calculated from repeated load tests in which the cumulative strain of the subgrade is measured as a function of the number of loading cycles applied. The chapter provides track engineers with an option to predict an equivalent modulus of granular layer once the input parameters have been determined: elastic modulus of ballast; elastic modulus of capping; elastic modulus of structural fill.
