| Author: | Liu, Kai |
| Title: | Experimental study and constitutive modelling of unsaturated granular materials |
| Advisors: | Yin, Jian-hua (CEE) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Soil mechanics Granular material Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxxiv, 351 pages : color illustrations |
| Language: | English |
| Abstract: | Granular materials are widely distributed worldwide. In the mountainous western region in China, granular materials are usually used as fill materials in the constructions of high-fill embankments, and these materials are typically unsaturated owing to the arid and semi-arid climates in this area. Moreover, the behaviour of the compacted and intact granular materials is mostly anisotropic. However, previous studies on unsaturated soils primarily focused on fine-grained soils. The experimental study of unsaturated granular materials is challenging owing to complex behaviours, time-consuming tests, and expensive apparatuses. In addition, it is still necessary to develop suitable models to capture the stress-strain, volumetric change, and anisotropic behaviours of unsaturated granular materials. In view of these limitations, this study has been conducted and summarized as follows. Firstly, a novel modular-design-based multifunctional apparatus is designed and manufactured for performing a variety of tests such as pressure plate test, oedometer test, direct shear test, and triaxial creep test under suction-controlled condition. Typical calibration results, test procedures, and experimental results obtained from this apparatus are presented and discussed in detail. It is found that this apparatus is capable of performing any of the tests listed above and measuring accurately the behaviour of both saturated and unsaturated geomaterials. Secondly, a series of triaxial tests were performed on a granular material sampled from a construction site of an airport. Experimental results show that the critical state lines under unsaturated conditions are almost parallel to those under saturated conditions in the q-p̄ plane, where q and p̄ are the deviator stress and net mean stress respectively, suggesting that the friction angle is independent of suction. The total cohesion and hence the shear strength increase with suction. In the v-ln p̄ plane, where v is the specific volume, both the intercept and slope of the critical state line increase with suction. The intercept and slope decrease with increasing suction in the vw-ln p̄ plane, where vw is the specific water volume. Thirdly, a new incrementally nonlinear model is proposed for describing behaviours of granular materials under saturated and unsaturated conditions. An exponential function is used to reproduce the asymptotic relationship between stress ratio and deviatoric strain, and a quantitative relationship considering variable void ratios is adopted to describe changes of degree of saturation during shearing. The proposed model includes nonlinear stiffness, nonlinear stress dilatancy, and critical state. Finally, the proposed model is used to simulate behaviours of Chongqing granular fill and Beijing sand under triaxial stress conditions, showing that this model can reproduce the stress-strain and volumetric behaviour of unsaturated granular materials. The aforementioned incrementally nonlinear model can be calibrated easily but it might have difficulties in modelling anisotropic behaviour of unsaturated granular materials. Therefore, an enhanced anisotropic Barcelona basic model for unsaturated granular materials (ABBM-G) is proposed. An inclined flexible yield surface is proposed to capture the anisotropic behaviour of unsaturated granular materials, in which a parameter is introduced to change the shape of the yield surface between ellipse and drop-shape. A non-associated flow rule is then proposed to govern the direction of plastic flow. In the end, the performance of the proposed ABBM-G is evaluated by modelling the behaviour of Chongqing granular fill and Barcelona silt. |
| Rights: | All rights reserved |
| Access: | open access |
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