Railway track granular layer thickness design based on subgrade performance under repeated loading
The objective of this dissertation is to study the characteristics of subgrade soils under repeated loading and their influence on the design and performance of railway track.
The roles of the subgrade as the track foundation are discussed. The mechanisms for different types of subgrade distress are examined.
A method for the estimation of resilient modulus of subgrade soils is developed. This method takes into account the influence of soil physical state, stress state and soil type. The effect of soil physical state is quantified by combinations of two equations relating resilient modulus to moisture content. The effect of stress state is determined by equations relating resilient modulus at optimum moisture content to deviator stress.
A method for the prediction of cumulative plastic strain of subgrade soils is developed which takes into account the influence of number of load cycles, stress state, soil physical state, and soil type. The stress state is represented by deviator stress. The influence of soil physical state is indirectly represented by the soil static strength. The parameters required for the prediction model are recommended for different soil classifications.
Based on analysis with the GEOTRACK computer model, the effects of superstructure and substructure factors influencing track modulus are illustrated. The relationship between the track modulus and the track performance is analyzed. GEOTRACK is also used for analysis of the stress pulses generated in the subgrade under moving wheel loads.
Two design methods are developed for the track granular layer thickness. The first one is intended to prevent the progressive shear failure of the subgrade and the other one is intended to prevent the excessive plastic deformation in the subgrade. The major considerations for these two methods include the magnitudes of the mix of dynamic wheel loads and the corresponding numbers of repeated loads, the soil type, the soil static strength, the subgrade thickness, the subgrade resilient modulus, and the resilient modulus for the granular layer.
The granular layer thickness design method is evaluated by field test results. The field performance of the test track is reasonably interpreted by the design method developed in this dissertation.