| Author: | Yu, Jiaren |
| Title: | Hydro-mechanical behaviour of an oil-contaminated silt |
| Advisors: | Zhou, Chao (CEE) Yin, Jianhua (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Soil pollution Oil pollution of soils Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxii, 215 pages : color illustrations |
| Language: | English |
| Abstract: | Soil contamination by hydrocarbons, caused by various sources such as oil tailings, industrial activities, pipeline breaks, storage tank leakage, and transportation incidents, has become a global environmental issue. Even in Hong Kong, shipping accidents and industrial activities have caused numerous oil spill incidents, posing long-term environmental hazards. The entrapped oil contamination can lead to groundwater contamination and air evaporation and pose long-term hazards to environment, further complicating soil remediation efforts. In managing these contaminated sites, understanding the hydraulic and mechanical behaviour is crucial for estimating the affected zones and contamination distribution patterns and assessing the impact on the environment and nearby geotechnical structures. Previous studies on oil-contaminated soil have separately focused on the infiltration and mitigation of oil and the oil effects on soil mechanics without considering the coupled effects of both. Moreover, the existing theories are mainly based on the data on clean sand. Therefore, some comprehensive and in-depth experimental and theoretical studies are needed. This study aims to investigate the hydro-mechanical behaviour of oil-contaminated soil and the contamination effects on soil and structures by extending the theories for unsaturated soil (three-phase) to four-phase materials (air, water, oil and solid particles). The principal objectives are to (1) develop some multiphase apparatuses with the ability to independently control/measure air, oil and water pressures for testing the soil in a four-phase system (air, water, oil and solid particles); (2) determine the soil-oil retention curve in a four-phase system and improve the understanding of the oil retention behaviour in the soil with a higher proportion of clay than sand; (3) investigate the impact of fluid type on soil mechanical characteristics such as strength and compressibility, aiming to establish the relationship between mechanical properties and volumetric oil saturation; (4) develop a numerical simulation model that considers the hydro-mechanical behaviour of contaminated soil to explore the effect of oil contamination on fluid flow and soil mechanical characteristics concurrently. To achieve these objectives, two testing apparatus were developed: a suction-controlled triaxial apparatus with the ability to independently regulate and measure three different fluid (air, oil, and water) pressures for determining soil-oil retention curves through the axis-translation technique and an oedometer apparatus for assessing soil compressibility using a similar suction control technique. The soil-liquid (water/oil) retention curves in the three-phase and four-phase systems were measured to study the retention behaviour. Direct shear tests were performed to examine the effects of fluid types on soil shear strength. The oedometer tests, including saturated condition (two-phase) and unsaturated condition (three-phase and four-phase), were conducted to investigate the compressibility of oil-contaminated soil and its influence factors (such as fluid type, degree of compaction, suctions, and saturations). Additionally, an investigation of the microstructure of clean soil and oil-contaminated soil was also carried out to help interpret the impact of fluid-clay particle interaction on pore structure and soil’s hydraulic and mechanical behaviour. On the one hand, the microstructure results show that oil infiltration could affect the soil pore structure by increasing both small and large pore formation, leading to a bimodal pore structure in oil-wet specimens and water-oil-wet specimens, which is different from the unimodal pore structure in water-wet specimens. This fundamental change, induced by oil-clay particle interactions, results in the variation of soil-liquid retention behaviour with oil saturation, whereas the existing retention model cannot capture this oil effect. On the other hand, mechanical tests of oil-contaminated soil reveal that oil infiltration can enhance the soil’s compressibility and shear strength. For example, when oil saturation increased from 0% to 18.9%, 21.3%, and 33.8%, the compression index of the oil-contaminated specimen increased from 0.262 to 0.295, 0.336 and 0.343; and the friction angle of the oil-wet specimen is approximately 30% larger than that of the water-wet specimen. These oil effects on soil mechanical behaviour are primarily attributable to the low dielectric constant of oil and the greater net van der Waals attractive force, and hence the more aggregated structure. Based on the experimental results, a new hydro-mechanical model considering porosity, suction and saturation for oil-contaminated soil is proposed by studying the above experimental data. The numerical implementation and model verification are conducted in COMSOL. Further parametric studies reveal that the leak volume and wetting front of oil increase nonlinearly with an increase in porosity at an increasing rate. The observed nonlinearity can be attributed to porosity effects on soil hydraulic properties by affecting oil’s intrinsic permeability and retention capability. Applying the new hydro-mechanical model in pile safety assessment reveals that the safety of piles in oil-contaminated soil is mainly determined by the specific soil and contaminant types. |
| Rights: | All rights reserved |
| Access: | open access |
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