| Author: | Jiao, Kangfu |
| Title: | Theoretical and experimental investigations on cemented soil at small and large strains |
| Advisors: | Zhou, Chao (CEE) Yin, Jianhua (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xvii, 174 pages : color illustrations |
| Language: | English |
| Abstract: | The deep cement mixing (DCM) method has garnered increasing attention in recent years due to its environmental benefits and high efficiency. The strain of DCM columns under working conditions at the serviceability limit state is typically within the small strain range. Therefore, understanding the behaviour of cemented soil at small strains is crucial for predicting foundation displacement and evaluating performance under working conditions. Many experimental and theoretical studies of cemented soil have been carried out, but they mainly focused on large-strain behaviour, such as strength and post-peak strain softening. These studies also have some major limitations. First of all, some constitutive models for cemented soil exist, mostly within the classical elastoplastic framework, focusing on strain-softening. However, these models fail to capture the nonlinearity of cemented soil behaviour at small strains, such as the non-linear stiffness. Secondly, the impacts of curing temperature (Tc) and stress (Sc) on the behaviour of cemented soil over a wide strain range remain unclear. According to the field data, the temperature inside DCM columns can reach 40°C, and the effective confining pressure ranges from 0 to approximately 200-300 kPa. Ignoring the distinctions between laboratory and field curing conditions may result in inaccurate parameters and improper designs. The principal objective of this study is to investigate the impact of curing conditions on the properties of cemented marine clay and improve the theoretical modelling of cemented soils from small to large strains. More specifically, it aims to (i) investigate the strength and small strain stiffness of cemented soil cured under various temperatures and stresses, reflecting field conditions; (ii) develop a constitutive model incorporating the small strain behaviour and strain-softening of cemented soil; (iii) develop a numerical code based on the proposed theoretical model; and (iv) conduct a parametric study of DCM-stabilised foundation using the proposed advanced constitutive model to improve design methods in current engineering practice. To achieve these objectives, a new temperature and stress-controlled curing apparatus was developed to prepare cemented marine clay at three temperature levels (20, 30, and 40°C) and three stress levels (0, 150, and 300 kPa). The specimens underwent unconfined compression (UC) tests, undrained triaxial tests, Brazilian splitting tests, and direct tension tests after 28 days of curing, with local strain to measure small strain stiffness. Additionally, microstructural analysis was employed to interpret experimental results. Test results reveal that increases in Tc and Sc can enhance strength and stiffness, decrease the elastic threshold strain, and increase the rate of stiffness degradation within the small strain range. These findings are attributed to the fact that higher Tc can accelerate the cement hydration reaction, enhance the pozzolanic reaction, which generates stronger cementation products between soil particles, and refine the pore structure. While Sc has minimal impact on cement hydration, it enhances density and contact area between soil particles, leading to more effective cementitious bonding. The findings suggest that considering the effects of Tc and Sc can lead to more cost-effective designs. A new constitutive model (denoted as the full version model in this thesis) for cemented soils suitable for both small and large strains was developed. To enhance the modelling of nonlinear stiffness at small strains and strain-softening at large strains, some new formulations were proposed, including (i) the elastic shear modulus over a wide range of stress conditions and (ii) the non-linear degradation of bonding strength (pb) with damage strain (εd) in the pb-εd plane. Furthermore, the new constitutive models were implemented in finite element software (FEM) in a modular approach. The full version model consists of a simplified version (denoted as the basic version model in this thesis) with two additional modules. The full version and basic version models allow users to select the appropriate model based on test data and engineering requirements. The performance and advantages of the proposed model are highlighted by simulating different boundary value problems using the full version model, the basic version model, and the Mohr-Coulomb (MC) model. Both the full version and basic version models can simulate small strain modulus degradation and strain-softening behaviour well. The full version model simulates the dilatancy behaviour well, while the basic version model shows some limitations in simulating the dilation behaviour. However, the MC model is inadequate in simulating the strain-softening and the degradation of the small strain modulus of cemented soils. Through parameter analysis, the study compares the bearing capacity and settlement calculations of DCM-stabilised foundations using the advanced model with those based on current design guidelines. The results show that the linear calculation methods commonly used in current standards result in considerable inaccuracies in settlement predictions as the safety factor varies. Specifically, overestimates settlement by approximately 50% when the safety factor is 2.0 and progressively underestimates settlement by 30% when the safety factor is 1.0. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/13924