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The soil-water retention curve (SWRC) is fundamental in presenting the hydromechanical characteristics of soils, which are closely connected with soil deformation, permeability, and shear strength. The Fredlund and Xing (FX) model accurately fits the SWRCs of different types of soils over a wide suction range. However, experimental comparisons of the fitting showed that the obtained parameters differ from the physical meanings assigned by Fredlund and Xing. To address this issue, the traditional FX model has been improved, resulting in the proposal of a two-step FX model. Firstly, the FX model is applied without taking the correction coefficient c(ψ) into account to fit the measured SWRC. The values for α, n, and m are then determined and substituted into the FX model to refit the experimental data. Finally, the last parameter Cr can be obtained. The curves resulting from these two steps have a good agreement with the experimental results, and the obtained parameters align better with their physical meanings.

Wildfires can significantly affect mountain hillslopes through the combustion of trees and shrubs and changes in soil properties. The type and magnitude of the associated post-fire effects depend on several factors, including fire severity and soil physical–mechanical-hydraulic features that, coupled with climate and topographic conditions, may cause increased runoff, erosion, and slope instability as consequence of intense rainfall. The post-fire response of slopes is highly site-specific. Therefore, in situ surveys and laboratory tests are needed to quantify changes in key soil parameters. The present study documents the post-fire physical and hydromechanical properties of pyroclastic topsoil collected from three test sites that suffered wildfires and rainfall-induced post-fire events in 2019 and 2020 in the Sarno Mountains (Campania Region, southern Italy). The tested pyroclastic soils in burned conditions show (i) no significant changes in grain size distribution, soil organic matter, and specific gravity; (ii) a deterioration in shear strength in terms of decreased soil cohesion caused by the fire-induced weakening of root systems; and (iii) a decrease in hydraulic conductivity. Accordingly, it can be argued that the documented post-fire erosion responses were mainly caused by the reduced cohesion and hydraulic conductivity of the burned topsoil layer, as well as by the loss of vegetation cover and the deposition of fire residues. Although deserving further deepening, this study can represent the necessary background for understanding the initiation mechanism of post-fire erosion processes in the analyzed area and on several natural slopes under similar conditions.

Many civil engineering projects are related to hydro-mechanical behavior of unsaturated expansive soils over a wide suction range, which was investigated by imposing suctions on an expansive soil using the axis-translation technique and the vapor equilibrium technique with saturated salt solution in this paper. Water retention test results on compacted expansive soil show that void ratio keeps decreasing along with increasing the suction in an entire suction range (from 0 to about 1000 MPa), and the soil–water retention curves in terms of gravimetric water content versus suction relation are independent of the dry density or void ratio when the suction is higher than 250 kPa. Therefore, the mechanical tests on an unsaturated expansive soil with constant water content can be considered as that at constant suction when suction is higher than 250 kPa. The stress–strain behavior at different constant suctions in the entire suction range measured from triaxial shear tests under a constant net confining stress shows that the strength and stress–strain curve of specimens with higher suction are higher than those with lower suction and the higher the suction, the more dilative the specimens. The strain softening appears when the suction is higher than a specific value and the appearance of strain softening is related to the sliding surface. The tested compacted expansive soil with extremely high suctions (i.e., 38.0 and 368 MPa) shows distinct peak strength, strain-softening and dilative behavior.

Nature-based solutions are popular techniques for managing stormwater. Most of them allow porous media as their main layer. The description of the Soil Water Retention Curve (SWRC) as the Unsaturated Hydraulic Conductivity Curve (UHCC) is often required to run the hydrological simulations with the physically based models. Using the unimodal and bimodal models to assess the SWRC and UHCC of soils is a widespread technique but their evaluation is often present in literature only in terms of curve fitting. Based on these assumptions, this work presents the performance assessment of the van Genuchten unimodal and bimodal models by functional evaluation of them based on the runoff from several substrates. Four substrates were investigated to define the structure, the SWRC, and the UHCC. Results showed that all substrates had a bimodal behaviour with lowest values of RMSE (RMSE_Θ = 0.0023 to 0.0037, RMSE_K = 0.0636 to 0.1284). Finally, a numerical simulation using the HYDRUS-1D model was performed for a three-month data set to check the effectiveness of the unimodal model instead of the bimodal one. The findings have shown that the unimodal model must be preferred instead of the bimodal because it has fewer parameters and assured low discrepancies in runoff volume (ε=0.00% to 6.25%).

Many civil engineering projects are related to hydromechanical behavior of unsaturated soils over a wide suction range, which was investigated by imposing suctions on clayey silt specimens using the axis translation technique and the vapor equilibrium technique with saturated salt solution in this paper. Firstly, the effect of void ratio or density on the water retention behavior was discussed based on the measured data. The test data indicate that shear tests under constant water content can be considered as those under constant suction in high suction range. Secondly, the mercury intrusion porosimetry tests were also performed to observe the pore-size distribution of compacted specimens with different void ratios. The experimental results show that the inter-aggregate pores of compacted specimens decrease with increasing the initial void ratio, but the intra-aggregate pores remain unaffected, which can explain the water retention behavior of compacted specimens with different initial void ratios over a wide suction range. Finally, for triaxial shear tests, the shear strength increases with increasing the suction until a maximum strength is reached at a specific suction value. When the suction is higher than the specific suction value, the tendency of strength change depends on the density in high suction range.

The dynamic effects in soil water retention curves (SWRCs) have been the focus of much research. However, most studies implemented short column tests in a few centimeters, under which a semi-permeable porous media inevitably minimizes or magnifies the dynamic effects. In this study, full-scale sand column tests were conducted to eliminate this flaw by preparing a saturated zone under the unsaturated one. The soil suction and moisture profiles were monitored using high-precision tensiometers and spatial time-domain reflectometry, thereby providing a rational overshooting range of the dynamic SWRC. The results confirm that the dynamic primary drainage curve overshoots the static one. The dynamic effects were estimated quantitatively from the soil moisture re-equilibrium time ( τ S ) and dynamic coefficient ( τ p ), falling within reasonable ranges from previous studies. The τ p increases log-linearly with decreasing moisture content and can be estimated well from the corresponding τ S and the first derivative of SWRC. Also, the τ p increases as the soil becomes finer and better graded, which agrees with more-prominent dynamic effects for lower-permeability reservoirs from petroleum studies but disagrees with more-significant dynamic effects for higher-permeability sand from soil-hydrology studies. The analysis shows that the dynamic effects are not dominated solely by the τ p or permeability but also by the groundwater dynamics, which can be seen as a pressure boundary from the saturated zone. This finding explains the significant dynamic effects for both high- and ultra-low-permeability geomaterial. Therefore, the present full-scale soil column setup with a prepared saturated zone is recommended for academic investigations of dynamic SWRCs.

Biochar-amended soil (BAS) is increasingly considered an effective cover material for various geotechnical and geo-environmental applications, owing to its potential to minimize infiltration, support vegetation and sequester carbon dioxide. However, the detailed hydraulic characterization of unsaturated tropical soil amended with biochar remains largely underexplored. This study investigated the hydraulic properties of residual silty sand (SM) amended with varying biochar contents (3.5, 5, 10, and 20% by weight) derived from corncob feedstock. This research focuses on crucial unsaturated soil parameters, including soil-water retention curves (SWRCs), saturated permeability (k sat ) and permeability (k) functions. The addition of biochar increased volumetric water content by 8–10% for suction ranges between 0 and 100 kPa compared to bare soil, due to the intraparticle voids and porosity of biochar. However, this effect was less pronounced with biochar percentages below 3.5%, especially for suctions below 5 kPa. The SWRCs of the BAS had bimodal shape with their fitting parameters nonlinearly increasing with biochar percentage. Biochar addition lead to notable reductions (about 20 times) in both saturated and unsaturated permeabilities, enhancement in suction stress and reduction in volume change upon drying. The implications of these observed behaviours were also discussed in terms of water holding capacity of biochar cover and environmental benefit in terms of CO 2 sequestration.

The paper investigates the changes in the physical, mechanical, and hydraulic properties of coarse-grained pyroclastic soils, considered under both wildfire-burned and laboratory heating conditions. The soil samples were collected on Mount “Le Porche” in the municipality of Siano (Campania Region, Southern Italy), hit by wildfires on 20 September 2019. The area is prone to fast-moving landslides, as testified by the disastrous events of 5–6 May 1998. The experimental results show that the analyzed surficial samples exhibited (i) grain size distribution variations due to the disaggregation of gravelly and sandy particles (mostly of pumice nature), (ii) chromatic changes ranging from black to reddish, (iii) changes in specific gravity in low-severity fire-burned soil samples different from those exposed to laboratory heating treatments; (iv) progressive reductions of shear strength, associated with a decrease in the cohesive contribution offered by the soil-root systems and, for more severe burns, even in the soil friction angle, and (v) changes in soil-water retention capacity. Although the analyses deserve further deepening, the appropriate knowledge on these issues could provide key inputs for geotechnical analyses dealing with landslide susceptibility on fire-affected slopes in unsaturated conditions.

The soil water retention curve (SWRC) or soil–water characteristic curve (SWCC) is a fundamental feature of unsaturated soil that simply shows the relationship between soil suction and water content (in terms of the degree of saturation and volumetric or gravimetric water content). In this study, the applications of the SWRC or SWCC have been extensively reviewed, taking about 403 previously published research studies into consideration. This was achieved on the basis of classification-based problems and application-based problems, which solve the widest array of geotechnical engineering problems relevant to and correlating with SWRC geo-structural behavior. At the end of the exercises, the SWRC geo-structural problem-solving scope, as covered in the theoretical framework, showed that soil type, soil parameter, measuring test, predictive technique, slope stability, bearing capacity, settlement, and seepage-based problems have been efficiently solved by proffering constitutive and artificial intelligence solutions to earthwork infrastructure; and identified matric suction as the most influential parameter. Finally, a summary of these research findings and key challenges and opportunities for future tentative research topics is proposed.

Water resources are increasingly limited and less available, so improving water use efficiency is particularly pressing in the agricultural sector, where inefficient irrigation systems are often in operation. Drip irrigation is one of the most water-saving systems that can optimize distribution according to the actual needs of the crop. In the present study, an engineered drip irrigation system was applied to a poplar crop dedicated to the production of biomass for energy use. The aim was to evaluate the influence of the supply of different volumes of irrigation water on crop growth. Four levels of water supply were tested (theses T1, T2, T3, and T4), each determined by different intervention thresholds established by using four soil moisture sensors, one for each thesis (PLOT) (20, 30, 40, and 50 percent, respectively). In the third growing season, the effect of the different amounts of irrigation water supplied on plant growth was particularly evident between control T1 (104 m3 ha−1) compared with T2 (540 m3 ha−1), T3 (1924 m3 ha−1) and T4 (4549 m3 ha−1). Regarding biomass production, no statistical differences were found between T2 (8.97 Mg DM ha−1 year−1), T3 (9.99 Mg DM ha−1 year−1) and T4 (10.02 Mg DM ha−1 year−1), but they were all different from T1 (7.09 Mg DM ha−1 year−1. This leads to the conclusion, at least up to the third growing season, that with equal satisfactory levels of biomass production, water and energy savings can be achieved by choosing intermediate levels of water supply (e.g., with the sensor at 30%) as compared to the maximum value tested.