Excess pore water pressure generation in crushed and fine granular materials under cyclic traffic loads

Abstract: Excess pore water pressure can develop in subgrades of railway and pavement substructures due to cyclic loading from heavy traffic, leading to the migration of fine particles into upper layers. This migration can clog pores and diminish the drainage capacity of upper layers,negatively impacting the long-term performance of sub-structures and service life, ultimately risking failure. Therefore, understanding the mechanisms behind the accumulation of excess pore water pressures and the migration of fine particles under cyclic loading is essential for efficient and cost-effective maintenance methods. The main objectives of this research include (1) investigating excess pore water pressure generation in crushed and fine granular materials under cyclic loading, (2) evaluating the migration of these materials into upper layers under cyclic loading, and (3) simulating a practical application using an advanced model to provide valuable insights into the operation of structures subjected to cyclic traffic loads while considering real-world factors from the field.A series of cyclic triaxial tests were conducted to investigate the generation of excess pore water pressure in fine granular materials. Two types of fine granular materials, tailings (a crushed material) and railway sand (a fine granular material) were selected for this investigation. The cyclic characteristics of these materials, including cyclic axial strain and excess pore water pressure, were evaluated in terms of number of cycles and applied cyclics tress ratios (CSR). As a result, the cyclic axial strain and excess pore water pressure were observed to accumulate over time due to cyclic loading. However, the extent of accumulation was found to be significantly dependent on CSR values and material types. In addition, a relationship between excess pore water pressure and cyclic axial strain of the fine granular materials was established and proposed based on the results from the undrained cyclic triaxial tests (including both tailings and railway sand samples).To assess the migration of fine granular materials into overlying layers under cyclic loading, a modified large-scale triaxial system was employed as a physical model test. A quantitative analysis of material migration was based on the mass percentage and grain size of migrated materials collected at the gravel layer. Additionally, cyclic responses (strain and excess pore water pressure) were evaluated. As a result, the total migration rate of the tailing sample was significantly higher than that of the railway sand sample. The migration analysis on tailings also revealed that finer tailings particles exhibited a greater tendency to migrate into the upper gravel layer compared to coarser tailings particles under cyclic loading. This migration could be attributed to significant increases in excess pore water pressure during the final cycles of the physical model test. The findings from this research could make a valuable contribution to the existing literature concerning the accumulation of excess pore water pressure and its effects on the migration of fine particles under cyclic loading.A numerical study was conducted to simulate the complex interactions between tailings materials and cyclic traffic loads on the piers of tailings dams. The integration of experimental data and advanced constitutive models enabled a comprehensive understanding of the behavior of tailings under these loading conditions. The findings focused on the build-up of excess pore water pressures in tailings subjected to cyclic traffic loads while taking into account the effects of truck loads, velocities, and truck resting times. As a result, excess pore water pressures in tailings progressively increased with the number of passing trucks, indicating a cumulative effect of loading cycles. In addition, the effect of truck loads and truck velocities on the excess pore pressure build-up was discovered, with higher truck loads and lower truck velocities leading to increased excess pore pressures, posing a greater risk. Furthermore, through anoptimization process involving variations in truck loads, velocities, and resting times, it was revealed that a combination of increased truck velocity, reduced truck load, and extended truck resting time could effectively minimize the build-up of excess pore water pressures in tailings beneath the pier. These findings offer valuable guidance for optimizing transportation operations on tailings dam piers, enhancing efficiency and safety.