Investigating structural stability using the soil water characteristic curve

Individual contributions of capillarity and adsorptive surface forces to the matric potential are seldom differentiated in determination of soil water characteristic (SWC) curves. Typically, capillary forces dominate at the wet end, whereas adsorptive surface forces dominate at the dry end of a SWC where water is held as thin liquid films. The amount of adsorbed soil water is intimately linked to soil specific surface area (SA) and plays an important role in various biological and transport processes in arid environments. Dominated by van der Waals adsorptive forces, surface‐water interactions give rise to a nearly universal scaling relationship for SWC curves at low water contents. We demonstrate that scaling measured water content at the dry end by soil specific surface area yields remarkable similarity across a range of soil textures and is in good agreement with theoretical predictions based on van der Waals interactions. These scaling relationships are important for accurate description of SWC curves in dry soils and may provide rapid and reliable estimates of soil specific surface area from SWC measurements for matric potentials below ‒10 MPa conveniently measured with the chilled‐mirror dew point technique. Surface area estimates acquired by fitting the scaling relationship to measured SWC data were in good agreement with SA data measured by standard methods. Preliminary results suggest that the proposed method could provide reliable SA estimates for natural soils with hydratable surface areas smaller than 200 m2/g.

The investigation of unsaturated soil through numerical modeling requires the prior information on soil properties including Soil Water Characteristic (SWC) equation. Acquiring these data for different soil texture classes is laborious and time-consuming. Consequently, there is a need for predefined dataset numerical modeling of unsaturated zone in different soil texture classes. In this study, 32 soil samples were collected from different locations, and soil physical properties like particle size distribution, particle density, bulk density, porosity, and saturated hydraulic conductivity were determined. Six different soil texture classes have been found in this study and average properties were computed for each soil texture class. SWC curves were developed for all soil samples using pressure plate apparatus. Based on average SWC curves, optimal parameters of four different SWC equations (Brooks and Corey, Campbell, Van Genuchten, and Fredlund and Xing models) were determined using non-linear regression with the Trust Region Optimization algorithm with R 2 and RMSE values ranging from 0.9650 to 0.9983 and from 0.0018 to 0.0219 cm3/cm3, respectively. All the four SWC models were compared using Taylor diagrams, which show that Fredlund and Xing models are the best models among the four. Further, the performance of developed SWC equations was compared with the moisture content of UNSODA datasets and results were satisfactory as RMSE ranges from 0.0381 to 0.1265 cm3/cm3. Thus, this study provides a reliable SWC equation for different soils in Bihar.

The engineering behavior of natural untreated and compacted lime-treated expansive soils, which typically exist in a state of unsaturated condition, can be better-explained using concepts from unsaturated soil mechanics. The soil water characteristics curve (SWCC) is the key unsaturated soil property for obtaining unsaturated soil property functions (USPFs). However, there are limited studies on the effect of lime on the SWCC of lime-treated expansive soils. This study investigated the effect of lime on the SWCC of lime-treated expansive soils. The drying portion of SWCCs for untreated natural soil and lime - treated soil samples with three different lime contents (3 %, 6 % and 9 %) with 7 days of curing were studied. The SWCCs were determined by using pressure plate apparatus in the suction range of 0 – 1400 kPa. The shrinkage curve (SC) was also determined to evaluate the change in volume of the different soil samples. The experimental results indicate that SWCC is affected by lime treatment and there is a change in the SWCC parameters and in the shape and position of SWCC as the percentage of lime is changed. The SWCC of the lime-treated soil samples show a higher rate of desaturation as the lime content increases. The Air Entry Value (AEV) and residual water content of lime-treated soil decreases with increase in percentage of lime and the SWCC shifts towards the left side as the AEV decreases. The differences in AEV obtained from gravimetric- water-content-based SWCC (w-SWCC) and degree-of-saturation-based SWCC (S-SWCC) for the lime-treated soil samples were small when compared to the untreated natural soil sample.Keywords: Air-entry value, Expansive soil, Lime treatment, Shrinkage curve, Soil–water characteristic curve, Unsaturated soil.

Different approaches were investigated for estimating the parameters in the Campbell soil water characteristic (SWC) equation from soil attributes such as particle size distribution (PSD), bulk density, and organic matter content. Predicted soil water characteristics were compared with measured values for soils of the wheatbelt of south-eastern Australia. A method of prediction is proposed incorporating an empirical relationship for estimating the slope of the SWC from the slope of the cumulative PSD. A power-law form is assumed for both the SWC and PSD functions. One measured SWC point is then used to locate and thus define the SWC curve. When SWC points predicted with this 'one-point' method were compared with measured values, the mean absolute value of the difference between each measured and predicted SWC point was 0.016 m3/m3 for the Geeves data and 0.027 m3/m3 for the Forrest data. Eight sets of predictive equations, previously developed using multiple regression analysis, were also evaluated. Whilst the equations predicted the slope of the SWC curves reasonably well, predictions of the air entry potential were poor. Although less accurate, the equations developed by multiple regression are less demanding in data requirement compared with alternative SWC prediction methods. The one-point method gave better predictions than the multiple regression approach but was less accurate than the 'two-point' method proposed in the first paper in this series. The one-point method should be considered where PSD data and 1 measured SWC point are available. In most other circumstances it will be more accurate and cost-effective to measure 2 SWC points to define the soil water characteristic function (the two-point method). * Part I, Aust. J. Soil Res., 1996, 34, 195–212.

The effect of soil density on the soil–water characteristic curve (SWCC) is becoming a topic of universal interest due to the heterogeneity of soils and environmental variables. In this study, a simple and effective model based on the idea of translating the particle-size distribution curve to the SWCC is proposed for predicting SWCC change with initial density. There is only one new parameter introduced, and it is easily calibrated using two SWCCs obtained from test data. The SWCCs for the same soil at different initial void ratios can be estimated using the developed model. Several existing models are also thoroughly examined, with an emphasis on the advantages and disadvantages of each model. The validity of the proposed model was then verified by comparing it to three other models and experimental data for eight different types of soils. The proposed model also outperforms other existing models in this extensive study, providing good and consistent prediction performance across various soils. The proposed model is then applied to different engineering challenges involving the estimation of bearing resistance of unsaturated soils. Three typical examples chosen for illustration in this paper are the effect of soil density variation on unsaturated shear strength, bearing capacity of a shallow foundation, and ultimate bearing resistance of an energy pile. The findings of the investigation reveal that the proposed model can be utilized to solve a variety of problems involving soil–structure interaction in unsaturated soils.

An advanced pore-scale model for simulating water retention characteristics in granular soils

Volume change behaviour of expansive clays has been one of the leading causes of damage to civil infrastructures worldwide. Contributing factors that lead to failures relate to changes in water content within the soil. Variations of water content can vary significantly based on an area’s climate regime. Trinidad has two seasons, the dry season (January to June) and the wet season (July to December). This variation leads to volume changes of expansive clay, where they exist mainly within the central and south regions of Trinidad. These areas are densely populated by residential and commercial buildings, which can be susceptible to damages from unsaturated expansive clays. The Soil Water Characteristic Curve (SWCC) for expansive clays is critical to estimate their unsaturated properties for the analysis of water flow movement. This study investigates the SWCCs for two expansive clay soil types in Trinidad. A WP4-T (Water Potential Measurement) is used to measure soil suction. The shrinkage curve (SC) test is conducted to consider the volume change of soil. The Fredlund and Xing (1994) SWCC equation and Fredlund and Zhang (2013) SC equation are used to fit the measured data. The SWCCs in terms of gravimetric and volumetric water contents and degree of saturation are compared. It is found that the normalised degree of saturation SWCC can provide a better display of the SWCC and estimation of the air-entry value.

Core Ideas Replicate‐specimen measurement of SWCCs was performed with an automated technique. Inter‐specimen uncertainty for SWCC parameters was quantified. A comparison of automated evaporation technique vs the standard hanging column is presented. Soil water characteristic curve (SWCC) measurements are used to estimate hydrologic parameters for a variety of engineering and agricultural applications. The SWCC parameterization is often based on a single measurement or with little or no consideration and/or quantification of the uncertainty in collection of SWCC data. Uncertainty in the SWCC data stems from random variability in pore geometry associated with irregular granular media. Additionally, a substantial opportunity exists in SWCC measurements for systematic or experimental errors that can increase uncertainty. This study utilized an evaporation method for measuring 10 SWCCs for each of four different test soils. One low‐plasticity and three high‐plasticity silty sand soils were sampled from the Sagehen Creek Experimental Watershed in Sierra County, California. The uncertainty in the SWCC measurements was quantified by the distribution and variability of van Genuchten parameters evaluated via nonlinear regressions on each set of SWCC data. Despite a careful, replicated specimen preparation, variability in the measured SWCC data was indicated by variability of estimates of van Genuchten's fitting parameters α and n and residual volumetric water content θ r , suggesting that packing specimens to identical dry densities does not produce identical SWCC measurements for a single soil type. For the soils tested in this study, replicate specimen testing resulted in uncertainty in α between ×/1.2 and ×/1.5 the geometric mean estimate for α, between ±2 and ±6% of the mean estimate for n , and between ±2 and ±6% for θ r . Additional testing is required before applying these uncertainties generally to soils of different textural classifications.

Red soils with white net-like veins occur widely along the south of middle to lower reaches of the Yangtze River in China. Up to the present few studies paid attention to hydraulic and unsaturated mechanical aspect in spite of many investigations focusing on age, distribution, and climate significance. However, the soil-water characteristics are critical for understanding the engineering features. Herein, microstructure features of undisturbed red soils with white net-like veins taken from Chenshan laterite section in Xuancheng, China were analyzed by scanning electron microscope (SEM). Also, the soil-water characteristics curves (SWCCs) were discussed by means of unsaturated GDS triaxial stress path system and numerical simulation. The SWCC test of axial pressure 510kPa and radial pressure 505kPa was conducted. The Gardner model, Van Genuchten model, and Fredlund model were used to simulate the measured drying cycle of SWCC. The results of SEM tests showed that microstructures and micropores are of obvious anisotropy, and the measured SWCC of the net-like red soil during wetting process behaves hysteretic characteristics compared to that during drying process. The computed results indicated that the simulation value of Fredlund model has good agreement with measured value.

The soil water characteristics curve (SWCC) represents the relation between soil water potential and soil water content. The shape and range of SWCC according to the relation could vary depending on soil characteristics. The objective of the study was to estimate SWCC depending on soil types and layers and to analyze the trend among them. To accomplish this goal, the unsaturated three soils were considered: silty clay loam, loam, and sandy loam soils. Weighable lysimeters were used for exactly measuring soil water content and soil water potential. Two fitting models, van Genuchten and Campbell, were applied. Two models entirely fitted well the measured SWCC, indicating low RMSE and high R2 values. However, the large difference between the measured and the estimated was found at the 30 cm layer of the silty clay loam soil, and the gap was wider as soil water potential increased. In addition, the non-linear decrease of soil water content according to the increase of soil water potential tended to be more distinct in the sandy loam soil and at the 10 cm layer than in the silty clay loam soil and at the lower layers. These might be seen due to the various factors such as not only pore size distribution, but also cracks by high clay content and plow pan layers by compaction. This study clearly showed difficulty in the estimation of SWCC by such kind of factors. Parameters obtained from the Campbell and van Genuchten models and goodness-of-fit results.

Effects of biochar and straw application on the soil structure and water-holding and gas transport capacities in seasonally frozen soil areas

The development of unsaturated soil mechanics stemmed from the inadequacies of classical soil mechanics in addressing the partial pore spaces found in real-world soil systems. However, assessing soil parameters under partial saturation is resource-intensive. To streamline this, the Soil-Water Characteristic Curve (SWCC) theory was introduced. Widely applied across geotechnical engineering, water resources, and agriculture, SWCC predicts parameters of unsaturated soil systems. In geotechnical engineering, SWCC is vital for designing systems near the Earth's surface, like slopes, clay liners and foundations. Furthermore, most of conventional soil improvement techniques pose a lot of environmental concerns. thus, emergence of green construction materials widely known as bio mediated and bioinspired soil improvement techniques like Enzymatic induced calcite precipitation (EICP). EICP, an emerging eco-friendly as it employs use of simply chemical compound that reduces carbon footprints, bioinspired technique as it mimics natural forming process, was applied to stabilize tropical soil for SWCC determination. SWCC parameters were derived by subjecting EICP-treated soils at varying concentration of cementation solutions (0 – 1.00 M) using pressure plate method. Results revealed that saturated water content θs, retention water content θr, and air entry value Ψa increased with higher cementation solution concentration. The values of θs, θr, and Ψa improved from of 0.663, 0.23405, and 3.4832 at 0 concentration of cementation solution to 0.701, 0.22864, and 7.7086, respectively upon treatment with 1.00 M cementation solution. Finally, it has been demonstrated EICP is capable of improving the SWCC parameters of residual soil, thus the EICP technique can be exploited in stabilization of residual for construction of compacted clay liner.

Temporal dynamics of paddy soil structure as affected by different fertilization strategies investigated with soil shrinkage curve

Considerably long periods of time are required to measure soil-water characteristic curves using conventional equipment such as pressure plate apparatus or a Tempe cell. A commercially available, small-scale medical centrifuge with a swinging type rotor assembly was used to measure the soil-water characteristic curves on statically compacted, fine-grained soil specimens. A specimen holder was specially designed to obtain multiple sets of water content versus suction data for measuring the soil-water characteristic curve at a single speed of rotation of the centrifuge. The soil-water characteristic curves were measured for three different types of fine-grained soils. The three soils used in the study were processed silt (liquid limit, wL = 24%; plasticity index, Ip = 0; and clay = 7%), Indian Head till (wL = 35.5%, Ip = 17%, and clay = 30%), and Regina clay (wL = 75.5%, Ip = 21%, and clay = 70%). The soil-water characteristic curves for the above soils were measured in 0.5, 1, and 2 days, respectively, using the centrifuge technique for suction ranges from 0 to 600 kPa. Time periods of 2, 4–6, and 16 weeks were required for measuring the soil-water characteristic curves for the same soils using a conventional pressure plate apparatus. There is reasonably good agreement between the experimental results obtained by the centrifuge and the pressure plate methods. The results of this study are encouraging as soil-water characteristic curves can be measured in a reduced time period when using a small-scale centrifuge.Key words: unsaturated soils, soil-water characteristic curve, centrifuge technique, soil suction, matric suction, water content.

The unsaturated behaviour of residual soil in Malaysia has been studied, yet the impact of particle size distribution (PSD) on soil-water characteristics still needs to be more adequately explored. Previous research highlights the significance of fine particles in controlling geotechnical properties. This study investigates how the PSD of five soil samples influences the Soil-Water Characteristic Curve (SWCC) parameters, water retention, storage properties and permeability. The study involved five samples, four samples from Ibrahim et al., [1] research, such as reserved forest, gravelly sand, silty sand, kaolin and silty clay soil from the Garinono Formation soil. A pressure plate extractor determined the samples' Soil-Water Characteristic Curve (SWCC) and bulk density. The van Genutchen (1980) model fitted experimental data and the hydraulic response was studied on the five soil samples. The results show that Kaolin and silty clay have higher volumetric water content than gravelly and silty sand due to a higher percentage of silty and clay particles. However, the reserve forest sample has volumetric water content like kaolin and silty clay due to its natural location. Reserved forest, kaolin and silty clay samples have higher water storage, i.e., effective soil porosity (ESP) values than gravelly sand due to higher clay and silty contents. Silty sand, reserve forest and gravelly sand have higher shape parameter (n) values due to their higher sand and gravel content. Clay-rich samples had smaller (n) values, indicating a more uniform distribution. Hydraulic conductivity function (K(θ)) is influenced by scale parameter (α), Air-Entry Value (AEV) and Saturated Hydraulic Conductivity K_s. Kaolin and silty clay samples exhibit lower α values, resulting in higher suction at AEV than other soils as they contain high silty and clay contents. Saturated conductivity values directly affect hydraulic conductivity function, while AEV inversely affects the unsaturated flow. Understanding unsaturated hydraulic responses and SWCC characteristics could assist in anticipating slope instability and ground stabilisation in residual soil and soil behaviour during and after rainfall.