Excess pore water pressure generation in fine granular materials under cyclic loading -A laboratory study

• Main Author: Do, Tan Manh
• Format: Others
• Language: English
• Published: Luleå tekniska universitet, Geoteknologi 2021
• Subjects: Excess pore water pressure; migration of particles; fine granular materials; cyclic loading; Geotechnical Engineering; Geoteknik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-83517; http://nbn-resolving.de/urn:isbn:978-91-7790-803-6; http://nbn-resolving.de/urn:isbn:978-91-7790-804-3

Abstract Excess pore water pressure can be generated in subgrades of both railway and pavement sub-structures under cyclic loading caused by heavy traffic. When saturated subgrades are subjected to cyclic loading, excess pore water pressures accumulate over time which then could lead to migration of particles into overlying layers. The migration of subgrade soil particles to the upper layers would lead to clogging of pores and reducing the upper layers' drainage capacity. Both excess pore water pressure accumulation and migration of fine particles could negatively affect the long-term performance and service life of the sub-structures and eventually may lead to failure. Understanding the mechanism of both excess pore water pressure and migration of fine particles under cyclic loading is, therefore, essential for not only designing but also further proposing efficient and economical maintenance methods. The main objectives of this research are to (1) investigate excess pore water pressure generation in fine granular materials under cyclic loading and (2) evaluate migration of fine granular materials into overlying layers under cyclic loading.  A series of undrained cyclic triaxial tests were performed to study the excess pore water pressure generation in fine granular materials. Two types of fine granular materials, i.e., railway sand (natural granular material) and tailings (artificial granular material), were selected for this investigation. The cyclic characteristics of these materials, e.g., accumulated strain and excess pore water pressure, were evaluated in terms of number of cycles and applied cyclic stress ratios (CSR). As a result, axial strain and excess pore water pressure accumulated over time due to cyclic loading. However, its accumulations were significantly dependent on CSR values and material types. Finally, a relationship between excess pore water pressure and accumulated strain of the fine granular materials was discovered based on all outputs from the undrained cyclic triaxial tests (both tailings and railway sand samples).  In order to evaluate the migration of fine granular materials into overlying layers under cyclic loading, a modified large-scale triaxial system was used as a physical model test. Samples prepared for the modified large-scale triaxial system composed of a 60 mm thick gravel layer overlying a 120 mm thick subgrade layer (tailings and railway sand). The quantitative analysis on migration of the fine granular materials was based on the mass percentage and grain size of migrated materials collected at the gravel layer. In addition, the cyclic responses (strain and pore water pressure) were evaluated. As a result, the total migration rate of the railway sand sample was found to be small. There were no migrated sand particles pumped up to the gravel surface, i.e., no mud pumping, after the test terminated. The migrated sand particles were observed and collected at the bottom half of the gravel layer. The total migration rate of the tailing sample was much higher than that of the railway sand sample. In addition, the migration analysis revealed that finer tailings particles tended to be migrated into the upper gravel layer easier than coarser ones under cyclic loading. The migrated tailings particles were observed at the surface of the gravel layer after the test ended. It could be involved in significant increases in excess pore water pressure at the last cycles of the physical model test. The findings obtained in this research may provide an additional contribution to the literature dealing with the excess pore water pressure accumulation and its effects on the migration of fine particles under cyclic loading.