Subgrade instability and fluidisation under cyclic railway loading

Over the last few decades, the railways have become an integral part of transport infrastructure offering an economical and viable alternative to the roadways. Of the money invested by the organisations on the development of railways, a significant share is allocated to track maintenance. The rail structure consists of three main geotechnical components; the ballast, the subballast (or the capping layer, as it is commonly known in Australia) and the subgrade. The subgrade layer forms the foundation of the railway and plays a crucial role in controlling track stability. A longstanding challenge faced by the rail geotechnical engineers is why and how the subgrade fines form a slurry under the action of moving traffic loads? The phenomenon involving the upward migration of the fine subgrade particles into the coarser ballast layers is termed as mud pumping. This thesis is broadly divided into three segments: (i) a critical review of the existing literature (ii) Experimental approach and analysis of the test results to study the mechanism and (iii) Numerical simulations to propose remediation measures. The thorough review of the reported sightings of mud pumping across the world indicated that soils having low-to-medium plasticity are more vulnerable to pump when subjected to traffic loads. A series of laboratory experiments were carried out to investigate the cyclic response of remoulded soil specimens collected from a problematic track site near Wollongong city, NSW, Australia. The specimens were tested at loading frequency ranging from 1.0 to 5.0 Hz and a cyclic stress ratio (CSR) from 0.2 to 1.0. The laboratory test results showed that when the CSR exceeds the critical cyclic stress ratio (CSRc), there is an internal redistribution of moisture within the specimen which causes the top portion of the specimen to soften and fluidise. To rationalise on the mechanism governing mud pumping, the experimental results were analysed by evaluating the stiffness degradation of the soil specimens during cyclic loading. The stiffness degradation index was calculated by the ratio of the axial dynamic stiffness of the specimen at a given loading cycle to that of first cycle. The analysis revealed that there is a significant drop in the stiffness of the fluidised specimen irrespective of the loading frequency. The mean excess pore pressure plots combined with the degradation index plot provided an estimate of the critical number of cycles required for the onset of fluidisation. The main factor contributing to mud pumping is the generation of high excess pore pressure in the railway subsoil. A numerical study was carried out to model the vertical drain inclusions in the railway substructure. The predictions indicated the efficiency of vertical drains in regulating and dissipating the excess pore pressure build-up and thereby delaying the onset of fluidisation of the railway subsoil