Determination of multimodal soil-water characteristic curve and permeability function

• Main Author: Zhao, Yan
• Other Authors: Harianto Rahardjo
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Unsaturated soil mechanics; Multimodal SWCC; Permeability function
• Online Access: https://hdl.handle.net/10356/177209
• Institution: Nanyang Technological University

The escalating impact of global warming is leading to a surge in abnormal weather events, notably intense drying. These extreme weather patterns heighten the vulnerability to slope failures caused by tension cracks and pose a threat to plant health due to reduced soil moisture levels. Key soil properties, such as the soil-water characteristic curve (SWCC) and the coefficient of permeability, play a vital role in understanding a soil's water retention capacity under varying suctions. Given the anticipated prolonged and intensified dry seasons forecasted by meteorological centers, conserving soil moisture becomes increasingly imperative. Extensive research has focused on soils exhibiting a unimodal soil-water characteristic curve (SWCC), typically associated with a well-graded grain-size distribution (GSD) that leads to a single pore series. Recent studies, however, have unveiled a range of both natural and engineered soils characterized by a gap-graded GSD, resulting in two or more distinct pore series. These soils exhibit bimodal SWCC. Research indicates that bimodal soils possess better water retention capability compared to unimodal soils, attributed to the presence of varied micropore and macropore sizes. Consequently, soils with multiple pore size series are believed to yield a higher SWCC modality, which is expected to enhance water retention capacity. Despite this, the study of soils with multimodal SWCC (i.e., SWCC modality is more than bimodal) remains significantly limited. Hence, there is a critical need to investigate soil mixtures that demonstrate multimodal hydraulic properties and effectively explore these properties. In this study, diverse soil mixtures were meticulously prepared and tested to develop soil with a multimodal soil-water characteristic curve (SWCC). The inclusion of activated carbon, a porous material, led to the successful creation of a soil mixture displaying a trimodal SWCC. However, the use of the conventional experimental setups (e.g., Tempe cell, pressure plate, and triaxial permeameter) for soils with multimodal SWCC become less efficient due to the increased number of air-entry values (AEVs) associated with higher SWCC modality. These setups necessitated extensive testing time (around 1 to 2 months) and repetitive measurement procedures when dealing with soils with multimodal SWCCs. To address this issue, a novel experimental setup employing a continuous evaporation method coupled with high-performance osmotic tensiometers (OTs) was devised and tested in this study. This setup enabled the direct and simultaneous measurement of SWCC and coefficient of permeability up to a suction of 1000 kPa. Furthermore, the newly developed setup was fully automated during the experiments, resulting in the average testing time to range from 1 to 2 weeks. By adopting the continuous evaporation method, the soil suction was transient, closely simulating soil behavior under field conditions. The obtained measurement results from this setup were nearly continuous and ready for analysis with minimal processing and without a repetitive procedure. In situations requiring continuous best-fitting equations for SWCC and coefficient of permeability measurement, two highly accurate best-fitting equations were developed. These equations effectively modeled the multimodal SWCC or the coefficient of permeability with minimal parameters. Moreover, the parameters in both equations have physical meaning where they are directly related to key hydraulic properties of the soil, such as AEVs. These equations were intuitive, easy to use, and successfully validated using both published data and experimental data from this study. Importantly, these equations were deemed general equations applicable to soils with SWCC and unsaturated permeability functions of any modality. Lastly, the performance of the proposed experimental setup and the two general best-fitting equations underwent thorough evaluation, confirming their accuracy, effectiveness, and efficiency.