Influence of principal stress rotation on the behaviour of soft railway upgrade

Of the various modes of transport available in Australia, railways are the fastest and most economic and therefore railway traffic has expanded in recent years to become a specific and irreplaceable means of transporting people and goods. For this reason improvements in rail track embankments are the direct result of trying to reduce maintenance costs and increasing the volume of traffic. This rapid development of railways means they must inevitably be constructed on subgrade consisting of soft estuarine clays, and long term repeated loading induced by rail traffic results in irrecoverable or permanent deformation, as well as inducing the generation of excess pore water pressure; all of which undermines the design life of railway substructure and exacerbates the cost of maintenance. It is therefore necessary to understand the behaviour of soft subgrade soil subjected to repeated loadings when a new rail track is designed or when an existing track undergoes maintenance. A series of undrained cyclic triaxial tests were carried out on reconstituted mixtures of sand and clay in isotropically consolidated and fully saturated conditions. The soft subgrade soil was studied under continuous cyclic loading with three cyclic stress ratios (CSR = 0.2, 0.3 and 0.4) and four frequency values (from 0.01 to 1 Hz), and then the accumulation of axial strain, the generation of excess pore water pressure and degradation of the resilient modulus throughout the number of cycles were analysed. Fundamentally, a higher cyclic stress ratio and lower frequency loadings had a greater detrimental effect on soft soil. The effect of the cyclic stress ratio and frequency on the initial resilient modulus was also evaluated