Author: Pan, Yu
Title: Experimental and numerical study of hybrid prefabricated horizontal drains and prefabricated vertical drains assisted with vacuum preloading method for treatment of clayey slurry
Advisors: Yin, Zhen-yu (CEE)
Yin, Jian-hua (CEE)
Degree: Ph.D.
Year: 2024
Subject: Marine sediments
Clay soils
Vacuum technology
Reclamation of land -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxv, 168 pages : color illustrations
Language: English
Abstract: Reclamation plays a crucial part in the development of Hong Kong. However, the scarcity of high-quality granular filling material for such projects presents a substantial challenge. On the other hand, the dredged Hong Kong marine deposit (HKMD) extracted from the seabed poses potential environmental issues and required reasonable disposal. Beneficial reusing these dredged HKMD as a filling material in reclamation projects emerges as an attractive solution to address both issues. However, HKMD possesses poor engineering attributes, characterized by high initial water content, low permeability and strength, and high compressibility. Regarding the engineering background, this thesis proposed a novel method for treatment of clayey slurry with high water content using hybrid prefabricated horizontal drains (PHDs) and prefabricated vertical drains (PVDs) assisted with vacuum preloading. Physical model tests are conducted to verify the feasibility of the proposed method and to evaluate its performance for improving the clayey slurry with high water content, followed with numerical modelling investigations to optimize key design parameters of the proposed method. Furtherly, the first field trial test applying vacuum preloading assisted by PHDs was conducted in Hong Kong to verify the applicability of vacuum preloading assisted with PHDs in Hong Kong. The thesis is organized as follows:
To examine the feasibility of the proposed method, a large-scale physical model test is designed and conducted, in which the high water content Hong Kong marine deposits (HKMD) are used as clayey slurry. First, three layers of slurry are blown-filled into the model tank with three layers of grid PHDs preinstalled and vacuum preloading followed step-by-step, up to a steady settlement. Then, the PVDs are installed with vacuum preloading applied up to another steady settlement. During the test process, the settlement, water content, undrained shear strength, pore water pressure, pore water pressure, and vacuum pressure of the soil in the tank are measured at different stages. The results find that the undrained shear strength increases from nearly 0 kPa to 22~34 kPa, and the average water content decreases from 200% to 55%, demonstrating that the proposed method can achieve a desirable treatment performance.
The impact of the spacing configuration of the PHD grid on the efficiency of the proposed method is investigated through a series of model tests. A test without the installation of PVD was also set for comparison, and in this case, only two phases of vacuum preloading are applied sequentially through the PHD layer installed in stage. The other three tests involve three phases, with the addition of a vacuum preloading stage through PVD and variations in arrangement pattern of grid PHD layer (vertical and horizontal spacing). Results show that decreasing the vertical spacing of grid PHDs results in larger settlement rates during the early consolidation stage and greater final strength. Reducing the horizontal spacing of grid PHDs leads to increased early-stage settlement rates, while has less impact on the final strength. Furthermore, the model tests using this proposed approach yields a final average undrained shear strength of soil of up to 30 kPa, meanwhile reducing the average water content from 200% to around 50%, evidencing the effectiveness of this novel method again.
To furtherly verify the applicability of PHDs vacuum preloading method in practice in Hong Kong, a field trial test has been conducted in Tung Chung reclamation site. In this field trial test, the HKMD was used as filling material with an initial water content of 150% and four layers of slurry were filled into the field trial step by step, vacuum preloading was applied via the PHD layers after a period of self-weight consolidation. The undrained shear strength and water content were measured regularly to assess the performance of the vacuum consolidation assisted with PHDs for the improved soft soil. Considering the unsatisfactory improvement on the surface soil by the vacuum consolidation assisted with PHDs based on the test results conducted before, the field trial was subjected to the sunlight irradiation to enhance the surface layer of soil after the vacuum consolidation. A crust layer of soil was formed after the sunlight irradiation, obvious improvement can be observed in the surface layer of soil. The test results show that the maximum undrained shear strength reached to 40 kPa, and the average water content decreased from 150% to 60%. It can be proved that the PHDs assisted by vacuum preloading method is applicable to future reclamation projects.
To investigate the optimal configurations of the proposed method, numerical simulations are conducted using the finite element software PLAXIS 3D. Numerical models are built based on the construction stages of model tests, simulated settlement, and water content, are compared with the measurements from the experiments to calibrate the numerical model. Then, the development of excess pore water pressure distribution, suction line are studied to furtherly explore the vacuum pressure distribution of proposed method. Lastly, the parametric studies based on the numerical modelling on the key construction parameters, such as the initial water content, vertical and horizontal spacing of PHDs and PVDs, are conducted to propose the optimal design parameters of this method for the treatment of clayey slurry with high water content. The numerical results agree well with experimental results. Moreover, proper design parameters, e.g. the vertical spacing of PHDs in the scope of 0.5 m to 1 m, and the horizontal spacing of PHDs in the range of 0.5 m to 1 m are suggested based on the balance of cost and treating efficiency.
Rights: All rights reserved
Access: open access

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