Year
2020
Degree Name
Doctor of Philosophy
Department
School of Civil, Mining and Environmental Engineering
Abstract
Mud pumping induced by the fluidisation of saturated railway subgrade due to heavy haul rail loads is a huge problem in the coastal areas of Australia and in other countries around the world. There are many studies on the cyclic loading of soft clay subgrade but less attention has been given to mitigating subgrade fluidisation. Existing reports of mud pumping incidents indicates that it occurs mainly in saturated subgrades with a low to medium plasticity. In response, isotropically consolidated undrained cyclic triaxial tests were carried out on reconstituted samples of low plastic clay soil to investigate the mechanism of mud pumping. Development of excess pore water pressure and axial deformation due to cyclic loading under different cyclic stress ratios (CSR) and frequencies were analysed. The specimens failed due to fluidisation (mud pumping) when the CSR is greater than a critical value (CSR > CSRc) under a particular loading condition. When the specimens were subjected to higher cyclic stresses, excess pore pressure and axial strains development was high. In the fluidised specimens, fine particles and a higher water content accumulated at the top of the specimens due to the internal redistribution of fines and moisture. The rate at which excess pore pressure and axial strain developed increased rapidly after a critical number of cycles (Nc); this critical number of cycles varies with the CSR and the frequency. Due to the higher deformation and generation of excess pore pressure under undrained conditions, the soil rapidly loses it stiffness under higher cyclic loads. A theoretical model was developed to capture the stiffness degradation in the initial cycles.
Being able to understand the factors that trigger subgrade fluidisation, investigating a measure that would mitigate mud pumping was crucial. A series of laboratory experiments were carried out using prefabricated vertical drains (PVD) to determine how effectively they can mitigate subgrade fluidisation. Specimens with a scaled down PVD were subjected to cyclic loading under partially drained conditions and then the results were compared with the undrained cyclic triaxial tests. It was clear that PVD enabled the generated excess pore pressure to quickly dissipate radially and vertically through the drains. Moreover, the volumetric strains also developed, thus reducing the void ratio, and as the void ratio decreased, so did the development of excess pore pressure due to further train passages, thus mitigating the susceptibility to fluidisation. The specimens subjected to cyclic stresses that were higher than the CSRc in undrained conditions did not fluidise under partially drained conditions. Therefore, there was no slurry formation or increase in the moisture content at the top of those specimens subjected cyclic loading under partial drainage with PVD.
A finite element (FE) analysis was carried out to investigate the practical application of PVD under cyclic loading in mitigating subgrade fluidisation. The magnitude of the cyclic stress was varied to simulate the different axle loads of typical freight trains. The numerical model further confirmed that PVD can be used to prevent subgrade fluidisation by dissipating the generated higher excess pore pressures under cyclic loading.
Recommended Citation
Abeywickrama Bamunusing Kankanamge, Aruni Lakmali, Investigation of Subgrade Fluidisation under Cyclic Train Loads and Mitigation Measures, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2020. https://ro.uow.edu.au/theses1/974
FoR codes (2008)
0905 CIVIL ENGINEERING
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.