The water-holding and strength characteristics of unsaturated expansive soil and modified soil in a high-fill canal embankment along the central line of the South-to-North Water Diversion Project were investigated using a pressure plate apparatus and a GDS unsaturated triaxial test system. The soil-water characteristic curves (SWCCs) of expansive soil and modified soil were obtained by curve-fitting the results of water-holding characteristic tests, thereby revealing the distinctions in water-holding characteristics of the two soil types. The laws governing the effects of matrix suction on the stress-strain relationships and shear strength of the two soil types were explored through unsaturated triaxial drainage shear tests. According to the test results: (1) The moisture content and void ratio of each soil type decreased gradually with the increase in matrix suction, although the void ratio of modified soil decreased at a slower rate than that of expansive soil. (2) Matrix suction induced a transition from strain hardening to strain softening; (3) The shear strength of both soils increases with the matrix suction and confining pressure, with the increment of expansive soil greater than that of modified soil. Notably, the influence of confining pressure became progressively more significant with increasing matrix suction for both soils; (4) The cohesion and internal friction angle of expansive soil and modified soil increases with the matrix suction, with 200 kPa as the critical point of increasing rate; (5) The expansive soil differs from modified soil in cohesion and internal friction angle under different matrix suctions, with matrix suction of 400 kPa as the critical point. (6) The matrix suction thresholds of 200 kPa and 400 kPa can serve as references for engineering design and construction, as well as seepage prevention and slope reinforcement. This study provides technical parameters and theoretical support for the design, construction, and long-term stability of embankments on the expansive soil in the South-to-North Water Transfer Project site.

An understanding of the mechanical properties and macroscopic behavior of unsaturated soil can be improved through an in-depth microscopic insight of the variables controlling the soil-water characteristic curve (SWCC). In this study, the effects of the initial conditions on the pore structure and SWCC of silty soil was examined. Their relationships to the soil behavior during water loss was addressed from both macroscopic and microscopic perspectives. In this study, patterns different from those of previous studies were revealed; this especially pertained to the effect of the initial water content on the SWCC. The SWCC was obtained using repeated centrifugation and filter paper tests. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were performed to collect the microstructure information. The results showed that soils compacted using the optimal dry side conditions had double S-shaped SWCC for their bimodal pore size distributions, and these pores were classified as intra-aggregate and inter-aggregate pores. Thus, these soils underwent two distinct stages of water loss during drying, and water loss occurred more easily in the first stage because of the presence of many large macropores or inter-aggregate pores. However, soils compacted at the optimum water content produced a single S-shaped SWCC for the multimodal pore structure. Water drained from these soils at a relatively constant rate from a more homogeneous and uniform pore system. This study has provided a comprehensive set of macroscopic and microscopic experimental data and well-established relationships among the PSD, SWCC and initial state of the silty soil.

Rubble deposits with a high concentration of rock debris were created after the powerful earthquakes in Jiuzhaigou. Because of the restricted soil resources, water leaks, and nutrient deficits, these deposits pose serious obstacles for vegetation regeneration. The purpose of this study was to investigate the main mechanisms controlling soil water retention and evaluate the effects of different amendments on the hydraulic characteristics and water-holding capacity of collapsed rubble soils. Fine-grained soil, forest humus, crushed straw, and organic components that retain water were added to the altered soils to study the pore structure images and soil-water characteristic curves. Comparing understory humus to other supplements, the results showed a considerable increase in the soil's saturated and wilting water content. The saturated water content and wilting water content rose by 17.9% and 4.3%, respectively, when the percentage of understory soil reached 30%. Additionally, the enhanced soil's microporosity and total pore volume increased by 45.33% and 11.27%, respectively, according to nuclear magnetic imaging. It was shown that while clay particles and organic matter improved the soil's ability to adsorb water, they also increased the soil's total capacity to store water. Fine particulate matter did this by decreasing macropores and increasing capillary pores. These results offer an essential starting point for creating strategies for soil repair that would encourage the restoration of plants on slopes that have been damaged.

Slope failures caused by changes in soil moisture content have become a growing global concern, resulting in significant loss of life and economic damage. To ensure the stability of slopes, it is necessary to accurately monitor the moisture content and understand the complex interactions between soil, water, and slope behavior. This paper provides a comprehensive overview of advanced soil moisture detection techniques for unsaturated soil slopes, including point-scale measurements and geophysical methods. It first introduces the fundamental concepts of the soil-water characteristic curve (SWCC) and its influence on the shear strength and stability of unsaturated soil slopes. It then delves into the working principles and applications of various point-scale measurement techniques, such as time-domain reflectometry (TDR), frequency-domain reflectometry (FDR), and neutron probe methods. Additionally, this paper explores the use of geophysiDear Editor: The author has checked that the name and affiliation are accuratecal methods, including ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI), for the non-invasive assessment of soil moisture conditions and slope stability monitoring. This review highlights the advantages of integrating multiple geophysical techniques, combined with traditional geotechnical and hydrological measurements, to obtain a more comprehensive understanding of the subsurface conditions and their influence on slope stability. Several case studies are presented to demonstrate the successful application of this integrated approach in various slope monitoring scenarios. The continued advancement in these areas will contribute to the development of more accurate, reliable, and widely adopted solutions for the assessment and management of slope stability risks.

The small-strain shear modulus and shear strength are mechanical parameters crucial in the design of geotechnical structures and in the analyses of soil-structure interactions. This paper proposes a new approach for estimating these mechanical parameters. The proposed approach is predicated on the proportional inverse relationship of mechanical soil properties to the soil-water characteristic curve. The proposed equations supporting the approach incorporate a scaling function, alongside the initial saturated mechanical property. The performance of the proposed equations was demonstrated across a variety of soil textures, utilizing literature soils subjected to varying net normal stresses, and across a wide range of matric suction up to the residual suction zone. It was established that a correlation existed between the scaling function and air-entry value for both small-strain shear modulus and shear strength of unsaturated soils. In addition, the behavior of the scaling function under potential hysteretic effects was demonstrated and recommendations were provided on how to apply the proposed model under such conditions. Finally, the modified equations including the correlation for the scaling function were used to predict additional literature soils.

The use of biochar in earthwork and slope engineering has gained significant interest due to its water and nutrition retention capacity that gives buffer against extreme wetting and drying, as well as helping vegetation growth. Recently, biocharmixed soil has been proposed as an alternative cover material for embankments and cut slopes for roads in tropic regions. Designing the hydraulic barriers with biocharmixed soil in earthwork systems needs clarification of water infiltration behavior into the soil. However, the physical and hydro-mechanical properties needed for application and assessment of hydraulic barrier by biochar-mixed soil are not yet fully understood. In this study, the impacts of biochar amount and types of biochar used on the hydraulic and mechanical characteristics of biochar-mixed compacted clayey sand were investigated by testing two different biochars deriving from rice husk and woodchip. The soil compaction test, permeability test, soil water characteristic tests and numerical study for seepage analysis were conducted for studying the biochar-mixed soil compared with mother soil. The microscopy and mercury intrusion porosimetry provided pore size distribution of biochars to deepen the understanding of mechanisms of water retention in biochar-mixed soils. The results showed that biochar addition increases the micropore which is expected to have higher water capacity, owing to the porous nature of the biochars. It is also shown that the biochar-mixed layer could delay water infiltration when biochar amount addition is large and sufficient. With the same biochar content, rice husk can help to reduce water infiltration more than woodchip.

Soil-water characteristics curves (SWCC) have proved useful in estimating parameters used in modeling unsaturated geotechnical properties of soils including permeability and strength. Either saturation, gravimetric, and instantaneous and initial volumetric water content designation can be used to develop SWCCs. Studies have shown that any of the designations will give good estimates for soils that do not undergo volume change with suction change whereas, for soils that undergo substantial volume change, only saturation and instantaneous volumetric water content designation obtained by incorporating shrinkage curves can give correct estimates. Transition oil sands tailings have fines content that cannot be categorized as sandy or fine materials, and research on volume change with suction change in these materials is limited. In this study, HyProps, Tempe cells, and a chilled-mirror water potential meter were used to measure suction and corresponding water contents for samples that were prepared by mixing coarse sand and Fluid Tailing by ratios that mimic transition zone tailings. Shrinkage tests were also performed to observe the extent of volume change with suction increase. Air Entry Values (AEV) estimated from SWCCs based on gravimetric water content were found to be lower than those estimated from saturation-based SWCCs due to substantial volume changes in these materials with suction increase. The use of saturation water content designation is recommended in estimating AEV for transitional oil sands tailings. This is useful information in predicting the long term unsaturated geotechnical behavior of these materials for environmental management and safety purposes.

Due to climate change, higher rainfall infiltration is expected in the future and it may cause a slope failure. Simultaneously, infrastructure construction and urban redevelopment are rapidly generating large amounts of construction and demolition waste that also contributes to global climate change. To ensure the stability of the slope, it is important to find cost-effective and environmentally sustainable alternatives. Waste material such as recycled concrete aggregate (RCA) can be utilized to protect the slope. The use of RCA for slope protection is that it can be used as a material for the capillary barrier system. The objective of this paper is to investigate the characteristics of pore-water pressure distribution and slope stability with the application of RCA protection during rainfall in comparison with the original slope through numerical modeling. The SWCC for the soil and RCA materials were measured using a high-suction polymer sensor (HSPS) and Tempe cell, respectively. The volume changes of the soil were measured using 3D scanner. SEEP/W was used to conduct the seepage analyses and obtain the change of pore-water pressure distribution due to rainfall infiltration. SLOPE/W was used to evaluate the stability of the slope with different climatic conditions. The use of recycled concrete aggregate (RCA) for slope protection from rainfall infiltration has been investigated in this paper. The results showed that the safety factor of the slope increased with the addition of RCA protection. Rainfall infiltration causes a reduction in soil suction and hence reduces soil shear strength, the safety factor will also decrease since the soil will become weaker.

Rainfall infiltration affects permafrost-related slope stability by changing the pore water pressure in soil. In this study, the infiltration responses under rainfall conditions were elucidated. The instantaneous profile method and filter paper method were used to obtain the soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF). During the rainfall infiltration test, the vertical patters of volumetric moisture contents, total hydraulic head or suction and wetting front were recorded. Advancing displacement and rate of the wetting front, the cumulative infiltration, the instantaneous infiltration rate, and the average infiltration rate were determined to comprehensively assess the rainfall infiltration process, along with SWCC and HCF. Additionally, the effects of dry density and runoff on the one-dimensional vertical infiltration process of soil columns were evaluated. The results showed that the variation curve of wetting front displacement versus time obeys a power function relationship. In addition, the infiltration rate-time relationship curve and the unsaturated permeability curve could be roughly divided into three stages, and the SWCC and HCF calculated by volumetric moisture content are more sensitive to changes in dry density than to changes in runoff or hydraulic head height.

This study compares the performance of various foundation systems in expansive soils, such as mats, granular anchor piles, and concrete piles. Expansive soils experience volumetric changes due to moisture fluctuations, which can lead to structural damage. Abaqus software, in conjunction with the SCV approach, is used to analyze soil-foundation interactions. A custom subroutine enhances simulation accuracy by incorporating empirical data on unsaturated clay behavior, matric suction, and variations in effective stress. The method's accuracy is validated by comparing simulation results to field and laboratory experiments. The findings indicate that increasing the applied load on mats decreases overall heave but increases the differential heave. Additionally, higher soil permeability dereases the differential heave of mats. Granular anchor piles outperform concrete piles by more than 50% in highly expansive soils, suggesting a preference for these foundations. This study provides insights into the behavior of expansive soils, which will assist engineers in designing resilient foundation systems for structures.