| Author: | Lou, Kai |
| Title: | Experimental study of the soil improvement on Hong Kong marine deposits using vacuum preloading, prefabricated drains and recycled fibres |
| Advisors: | Yin, Zhen-yu (CEE) Yin, Jian-hua (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Reclamation of land Soil mechanics Soil stabilization -- Materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxvi, 212 pages : color illustrations |
| Language: | English |
| Abstract: | With the rapid development, coastal cities face severe scarcity of land for construction use. Reclamation is a common and effective way to solve the problem. Considering the economic efficiency and environmental impact, dredged sediment from the neighbouring seabed is preferred as filling materials instead of replacing them with expensive granular materials such as sand and gravel nowadays. However, another problem occurs. The soil on the seabed is usually soft soil with high water content, high compressibility, low permeability, and low bearing capacity. These poor engineering properties would induce excessive settlement over a long period. Soil improvement becomes necessary as a result. Vacuum preloading incorporated with prefabricated vertical drains (PVD) and prefabricated horizontal drains (PHD) have been used to improve soft soil ground. However, the effect of the layout of prefabricated drains on the characteristics of vacuum consolidation and treatment efficiency remains unclear. A small-scale model testing apparatus was then developed, with which a series of physical model tests were conducted to investigate the characteristics and the performance of vacuum preloading assisted by prefabricated drains with different arrangements for the treatment of clayey slurry. The settlement, porewater pressure, and water content were monitored during the vacuum consolidation process. In addition, after the treatment, compressibility, permeability and undrained shear strength of treated soils were determined. Results indicated that the PVD accelerates consolidation at the early stage, but the efficiency significantly drops afterwards. On the other hand, the PHD only improves the nearby soil. Combining the advantages of PVD and PHD, the proposed novel hybrid PHD-PVD arrangement was proven to achieve the best performance on soil improvement. Surcharge preloading is another one of the most popular soil improvement methods. However, the treatment efficiency is limited, and it cannot be quickly applied to ultra-soft soils and requires a long time to reach the improvement. Based on the above research, this thesis further proposes the fast soil improvement method combining the advantages of surcharge preloading, vacuum preloading, PHD, and PVD.A new comprehensive testing apparatus was developed, with which the soil behaviour from slurry sedimentation to consolidation can be consecutively studied. The apparatus was used to carry out two tests to investigate the performance of the proposed method for the treatment of soft soil slurry. Test results show that this method significantly shortens the required time and improves the strength. Compared to the conventional expectation, a different pattern of soil particle movement is also observed. In general, the proposed method significantly accelerates the consolidation and reaches better performance than the conventional method. On the other hand, the world was profoundly affected by the global spread of the novel coronavirus COVID-19. It has had a profound impact on the daily lives of people worldwide. One of the most significant changes brought about by the pandemic is the massive use and production of face masks. The disposal of billions of waste face masks has emerged as a severe environmental concern. In addition to the proposed fast soil improvement method above, the fibre-treated soil technique is a way to enhance the engineering properties of the soil itself. Therefore, recycling facemasks for soft soil improvement has been proposed. This is a strategy to kill two birds with one stone by effectively reusing the waste face masks and improving the soil properties. However, the influence of recycled face-mask fibres on the consolidation and mechanical performance of the face-mask fibre-treated soils, such as permeability, compressibility, and stress-strain behaviour, remains unclear. Therefore, this study evaluates the influence of face-mask fibres on the consolidation and mechanical behaviours of soft soils with various fibre contents, through permeability tests, oedometer tests and quasi-Ko consolidated undrained compression triaxial tests with step-changed strain rates. The results show that face-mask fibres improve soil permeability and thus accelerate consolidation. The compressibility and shear strength show nonlinear relationships with face-mask fibre content. With a moderate amount of face-mask fibre (around 0.5%, the mass ratio of fibre to dry soil), the reinforced soil has the highest permeability and undrained shear strength. The strain-rate effect is monotonically influenced by the fibre content, with a higher fibre content corresponding to a higher strain-rate sensitivity. Finally, to validate the feasibility of the face-mask-fibre treatment for soft soils with high initial water content, a series of small-scale model tests have been conducted. Results reveal that the face-mask fibres significantly improved the treatment efficiency of vacuum preloading, with less time, larger settlement, and higher shear strength. The mechanism of how the face-mask fibres work was also discussed by the microscopic analysis. |
| Rights: | All rights reserved |
| Access: | open access |
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