Influence of Kaolin content on the cyclic loading response of railway subgrade
Rail tracks passing through saturated subgrade soil often face a serious deterioration of bearing capacity and excessive deformation. One major reason is the excessive cyclic pore pressure that accumulates under the track that leads to soil softening and infiltration over the surface, i.e., subgrade mud pumping (fluidization). Although this issue has received considerable attention in recent decades, how the cyclic stress ratio and soil properties such as plasticity and void ratio influence the cyclic loading response of soft subgrade soil is still not properly understood. In this study, Kaolin - an artificial cohesive fines soil is used to modify a low plasticity subgrade soil to examine how the Kaolin content (cK) can affect its cyclic response. Soil specimens including the original soil and its mixture with 10 and 30% of Kaolin have been subjected to undrained cyclic testing. A cyclic stress ratio (CSR) varying from 0.2 to 1.2 is used and a low initial confining pressure of 20 kPa is applied. The results show 3 different responses of soil, i.e., (i) stable, (ii) cyclic undrained failure, and (iii) fluidization, depending on the magnitude of CSR. Where fluidization becomes imminent, the shear stress rapidly decreases at early stages. Adding cohesive fines, i.e., Kaolin reduces the static undrained shear strength and increases the plasticity index. This enables the test specimen to undergo a larger number of cycles (N) before failure, thus enhancing its resistance to fluidization. Specimens with a smaller initial void ratio, i.e., greater level of compaction, are less susceptible to fluidization because they can withstand larger CSR and N. Moreover, this study shows where there is potential fluidization upon cyclic loading, a significant redistribution of the water content seems to occur over the height of the test specimens.