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Title: Experimental study on compression behaviors of saturated and unsaturated granular fill materials under static and cyclic loadings
Authors: Chen, Wenbo
Degree: Ph.D.
Issue Date: 2019
Abstract: Granular fill materials are widely used in the subgrades of transport infrastructures, including flexible, semi-flexible and concrete pavements, ballasted and ballastless railways. The coexistence of a large portion of coarse-grained particles and a high fraction of fines for the locally-procured fill materials is common in many practical cases. Besides, the fill materials, on one hand, are normally subjected to seasonal and large variations in water content, which render the materials to be characterized as saturated or unsaturated. On the other hand, these materials will experience a large number of cyclic traffic loads in the service life, thereby possibly inducing excessive or differential deformation in the subgrades. However, most existing studies only focus on the unsaturated compression behaviors of the materials with fine-grained particles. The cyclic compression responses of granular fill materials with high fines content are also less investigated. In this study, two granular fill materials from two construction sites were obtained as test materials. One, termed as Fill 1, is used in the high embankment of an airport, and another one, termed as Fill 2, is adopted in the bottom layer of subgrade of High-speed Railway, The first principal objective of this study is to investigate and model the compression behaviors of unsaturated granular fill materials subjected to static loading. Fill 1 was used in this part of the study. A newly-designed triaxial double-cell system for measuring the volume change of a saturated or unsaturated soil specimen was developed. Also, an existing unsaturated direct shear apparatus was modified for unsaturated oedometer tests. One series of suction-controlled drained triaxial tests and several suction-controlled multi-stage oedometer tests were performed on Fill 1. Furthermore, one series of constant-rate of strain oedometer tests and one series of multi-stage oedometer compression tests without suction control were conducted on a portion of Fill 1 to study the particle crushing and flooding effects on the time-dependencies of the granular fill material.
The results from unsaturated triaxial tests correlated the effective stress parameter and matric suction. The suction-controlled multi-stage oedometer tests illustrated that the compression index and effective preconsolidation stress increased significantly when the specimens were changed from saturated to unsaturated. Besides, the multi-stage oedometer tests without suction control proved that the particle crushing and flooding effect contribute significantly to the deformation of the material. Furthermore, the mechanism of particle crushing was found to be the disaggregation of clay minerals, which bond small quartz particles together to form a single grain of particle. A linear relationship between the time-dependent compression coefficient and normalized effective vertical stress was established. Also, a nonlinear function was utilized and proved to be capable of predicting well the development of time-dependent compression of unsaturated granular fill material. The second principal objective is to investigate and model the compression behaviors of saturated and unsaturated granular fill materials subjected to cyclic loading. Fill 2 is adopted in this part of the study. One series of static triaxial compression tests was conducted to understand the basic static mechanical properties of this material. Followingly, one series of cyclic triaxial tests with stepwise increasing cyclic deviator stresses was carried out to study the permanent axial strain and resilient modulus behaviors of Fill 2. Another series of cyclic triaxial compression tests with large cycle number was conducted. Lastly, a series of cyclic triaxial tests was supplemented to investigate the influence of water content on the resilient modulus. New criteria to characterize the compression behaviors of granular materials under cyclic loading based on shakedown theory were proposed and validated by the tests data of Fill 2 and the existing literature. These new criteria produced more accurate characterization results than that by the existing criteria. A new empirical model was proposed to correlate the stress states and the accumulated permanent strain. This new model was verified by single stage and multi-stage cyclic triaxial tests and a full-scale physical model test. Lastly, a modified prediction model is proposed for describing resilient modulus, considering the effects of water content and stress paths, to capture the non-monotonic relationship between resilient modulus and stress states. In the last chapter, the main findings and conclusions of the study have been summarized and listed. Recommendations for further study are also presented.
Subjects: Hong Kong Polytechnic University -- Dissertations
Granular materials -- Testing
Road materials
Embankments -- Materials
Pages: xxx, 240 pages : color illustrations
Appears in Collections:Thesis

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