Undrained instability and fluidization of soft subgrade soils under cyclic rail loading
- Publisher:
- Taylor & Francis
- Publication Type:
- Chapter
- Citation:
- Smart Geotechnics for Smart Societies, 2023, pp. 10-24
- Issue Date:
- 2023-01-01
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The rapidly increasing demand for rail infrastructure to transport passengers and freight has inevitably led to a need for more robust and sustainable track foundations. However, heavy haul rail tracks often run across unfavourable soil conditions (e.g., saturated soft estuarine soils), resulting in frequent track failures accompanied with high maintenance cost. Recent site investigations along the South Coast rail line (NSW, Australia) showed considerable track degradation induced by subgrade mud pumping, requiring urgent attention and studies. This paper thus presents extensive laboratory and numerical investigations on the mechanism and possible solutions for mud pumping. While the effects of mud pumping on track performance are investigated through large-scale permeability and shear tests using the collected fouled ballast, the subgrade soil is subjected to cyclic triaxial and large-scale cyclic testing to understand the soil fluidization and fine particle migration. Transducers are installed at different layers to observe the depth-dependent response of soil foundation under various conditions. The results show that when specimens are subjected to unfavourable cyclic stresses, the fine particles migrate from the lower toward the upper soil layers along with the redistribution of pore water. When the water content of the upper soil layer reaches the liquid limit, the top of the specimen transforms into a slurry. Micro-analysis based on CFD-DEM coupling indicates a serious degradation in soil fabric and strength, thus the formation of mud under rising excess pore water pressure (EPWP). The study indicates that adding reasonable mass of cohesive fines can enhance the resistance of subgrade soils to fluidization, while geotextiles and geocomposites can also be used to mitigate mud pumping. Prefabricated vertical drains (PVDs) can continually reduce the buildup of EPWPs over the time due to the substantial reduction in drainage path, suggesting the efficiency of using the combined PVD-geocomposite system to mitigate mud pumping.
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