Soil Matric Suction Research Articles

Mechanics of Rainfall-Induced Landslides after a Prolonged Dry Period Based on Laboratory Tests and Numerical Models Incorporating Soil-Water Characteristic Curves

In India, particularly within its Northeastern territories, landslides triggered by rainfall following dry periods are a major concern, consistently causing extensive damage to both life and infrastructure. This study focuses on mitigating their impact through preemptive measures, with an emphasis on analyzing slope stability to determine critical intervention points. The investigation includes experimental tests on soil samples to assess key parameters, such as soil matric suction and unconfined compressive strength, alongside an analysis of slope failures during the 2017 monsoon in Mizoram’s Lunglei district. Employing Soil-Water Characteristic Curves (SWCC) derived from ASTM D5298-10 standards and a microwave drying technique for preparing soil samples, the research evaluates the condition of the slopes before and after monsoonal rains. This study utilizes a blend of numerical modeling and empirical laboratory investigations to explore the factors contributing to slope instability. The findings underscore the necessity of advanced landslide warning systems, suggesting that a deeper understanding of rainfall-induced slope failures could significantly enhance disaster preparedness and reduce potential damages.

Investigation on parameters affecting reduction coefficient of groundwater buoyancy in clay layers

The buoyancy of groundwater in clay layers is a critical factor that influences the behavior of geotechnical engineering structures. However, there is still a lack of understanding regarding the factors that contribute to the reduction of groundwater buoyancy. This study aims to elucidate the influence of soil parameters and matric suction on groundwater buoyancy, employing laboratory model tests and innovative experimental methods. The research findings uncover noteworthy variations in the reduction coefficient of groundwater buoyancy contingent upon the clay type, ranging from 0.45 to 0.85. Furthermore, experimental results indicate that matrix suction effectively diminishes pore water pressure but exerts minimal influence on groundwater buoyancy. It is elucidated that pore ratio, permeability coefficient, and water content evince a robust positive correlation with the buoyancy reduction coefficient, whereas dry density and wet density exhibit an inverse trend. Conversely, parameters such as cohesion, saturation, internal friction angle, specific gravity, liquid limit, and plastic limit manifest minimal correlation with the buoyancy reduction coefficient. These findings have practical implications for anti-floating design in geotechnical engineering.

Prediction of hysteretic soil and water characteristic curve of loess based on multifractal theory and improved physical statistics

Soil and water characteristic curve (SWCC) is an important soil parameter which is used to estimate various parameters to describe unsaturated soil hydrodynamic properties. Due to the large span of soil matric suction, this procedure is time-consuming and difficult to measure it in the laboratory. However it is of great significance to find a scientifically efficient and economically viable method to predict SWCC of loess soil. Based on the measurement of soil–water characteristic curves in the laboratory, this study predicted the drying curve (DC) based on the soil particle size distribution (PSD) fractal theory. It also used the domain model to predict the wetting curve (WC) based on the pore distribution, and finally obtained the complete soil–water characteristic curve of loess. These procedures improved the research status which using the drying curve to instead of the wetting curve, and this replace usually leads to a large calculation error and underestimates the safety of the slope. The method proposed in this study can quickly obtain the hysteretic soil–water characteristic curve of loess based on only the soil particle size distribution, improve the correlative calculation accuracy of unsaturated loess, and play an important role in predicting the hydraulic parameters of loess soil.

Tree morphology dependent transpiration reduction function of Schefflera arboricola for landfill cover restoration

Abstract Changes in hydrological processes and water resources required to sustain vegetation for ecological restoration of landfill covers and post mining sites in arid environments pose challenges in the context of extended droughts. Knowledge of actual threshold and wilting suction values based on tree morphological feature or plant age is essential for understanding the variation of root water uptake with drought stress and numerically predict the pore water pressure profile in root zone. The objective of this study is to quantify the transpiration reduction function (TRF; in terms of stomatal conductance (SC) and xylem sap flow (SF)) of Schefflera arboricola, considering the effects of tree morphology. Continuous drought condition was applied on the plant quantified with leaf area index (LAI) values at 0.5, 2 and 3.5, wherein each LAI represent tree age. The soil matric suction (ψ) and volumetric water content were measured by embedded sensors in the root zone. Based on the TRF obtained from SC values, a unique threshold suction (ψ NTR t) ranging from 30 to 50 kPa was identified. Beyond this ψ NTR t, measured leaf abscisic acid concentration increased up to 35 ng/mL, indicating the start of water stress avoidance mechanism. It is evident that ψ NTR t is independent of tree morphological parameter- leaf area to root length ratio (LA/RL). On the contrary, a threshold suction (ψ SAP t), depending on LA/RL ratio, can be determined, indicating the start of xylem cavitation. This ψ SAP t values ranging from 80 to 500 kPa depending on the LA/RL value, imply that the plant could significantly resist xylem embolization at higher LA/RL. In contrast, the plant with low LA/RL values have less tolerance of drought stress and hence low survivability. The results from this research study can be vital for devising and predicting plant available water in water scarce arid environments by a flux-based approach which is dependent on the tree age.

Experimental and Numerical Simulation Study of Water Infiltration Impact on Soil-Pile Interaction in Expansive Soil

A laboratory model of a single pile embedded in Nanyang expansive soil and subjected to water infiltration is applied in this study to examine the interaction between the expansive soil and pile foundation upon water infiltration. The soil matric suction decreases as a result of the rising soil-water content. The amount of soil ground heave reaches its peak of 10.7 mm after 200 hours of water infiltration. As matric suction decreases, pile shaft friction also declines, which causes more of the load at the pile head to be carried by the pile base resulting in more pile settlements. A new numerical simulation method is provided to simulate this issue by coupling the subsurface flow, soil deformation, and hygroscopic swelling to investigate the expansive soil-pile response upon water infiltration. From the numerical simulation model, hygroscopic strain arises as a result of elevated moisture levels resulting from the entry of water, and due to ground heave and the mobilization of lateral soil swelling, the shear stress at the interface between the soil and the pile gradually increases over time. It reaches its maximum value of 4420 Pa at upper depths around 200 hours after the infiltration. The comparison between the lab model testing data and the numerical model results demonstrates a good level of concurrence.

Contribution of soil matric suction on slope stability under different vegetation types

Contribution of soil matric suction on slope stability under different vegetation types

Analysis of Slope Stability with Different Vegetation Types under the Influence of Rainfall

Rainfall-prone shallow landslides account for one-fifth of the global land area, and rainfall is critical to the mechanics and hydrology of shallow slopes. In typical geological disaster-prone areas, the hydrodynamic responses of slopes with different vegetation types under rainfall conditions require further study. The purpose of this study was to analyze the hydraulic stability of soils with different vegetation types under rainfall conditions and their effects on slope stability. Thus, the soil–water characteristic curves and water-stable aggregate characteristics of soils with three vegetation types were analyzed. A two-dimensional finite element model was used to simulate the slope stability of extreme rainfall environments with different rainfall durations. The results showed that the matric suction of soil with trees was less affected by rainfall with a better stability of water-stable aggregates than that of soil with shrubs and grass. The plastic strain cloud map showed that the maximum plastic strain occurred at the toe of the slope. In addition, the potential slip depth of slopes with trees was smaller than that of slopes with shrubs and grass. Under the two rainfall durations, the factor of safety (FoS) of slopes with trees changed by 0.06, whereas that of slopes with shrubs and grass changed by 0.1. The findings of this study provide valuable insights into changes in the stability of slopes with different vegetation types under varying rainfall conditions. It is of great significance to provide a scientific basis for the application of ecological measures in the prevention and control of mountain disasters and guide the implementation of appropriate land management measures.

Prediction of hydraulic and petrophysical parameters from indirect measurements of electrical resistivity to determine soil-water retention curve – studies in granular soils

The characterization of unsaturated soils using hydromechanical methods is an essential requirement in soil science. However, current laboratory techniques used to obtain soil water retention and unsaturated hydraulic conductivity curves are time-consuming. To address this issue, a method based on indirect measures (electrical resistivity/electrical conductivity) was developed to quantitatively characterize soils. A novel unsaturated semi-empirical hydrogeophysical model of soils was developed by incorporating the hydrodynamic, geophysical, and petrophysical characteristics of soils. The model assumes that the parameters influencing the variation in the volumetric water content with matric suction and electrical resistivity are the same. The electrical resistivity characteristic curve (ERCC) defines a function that correlates environmental variables, electrical resistivity, soil water status, matric suction, hydraulic and petrophysical parameters, and fluid electrical resistivity. Model validation confirmed that the proposed approach can estimate the soil water retention curve (SWRC) via the indirect measures, and the results agreed with the experimental data. This indicates that it is possible to determine the SWRC and unsaturated hydraulic conductivity function of soil using the described approach.

Development of a Multiphase Numerical Modeling Approach for Hydromechanical Behavior of Clay Embankments Subject to Weather-Driven Deterioration

Clay embankments used for road, rail, and flood defense infrastructure experience a suite of weather-driven deterioration processes that lead to a progressive loss of hydromechanical performance: micro-scale deformation (e.g., aggregation and desiccation), changes in soil-water retention, loss of strength, and macro-scale deformation. The objective of this study was to develop a numerical modeling approach to simulate the construction and long-term, weather-driven hydromechanical behavior of clay embankments. Subroutines within a numerical modeling package were developed to capture deterioration processes: (1) strength reduction due to wet-dry cycles; (2) bimodality of the near-surface hydraulic behavior; (3) soil-water and soil-gas retentivity functions considering void ratio dependency; and (4) hydraulic and gas conductivity functions considering void ratio dependency. Uniquely, the modeling approach was comprehensively validated using laboratory tests and nine years of field measurements from a full-scale embankment. The modeling approach captured the variation of near-surface soil moisture and matric suction over the monitored period in response to weather cycles. Further, the developed model approach could successfully simulate weather-driven deterioration processes in clay embankments. The model predictions manifested the ability of the modeling approach in capturing deterioration features such as irrecoverable increases in void ratio and hydraulic permeability near surface. The developed and validated numerical modeling approach enables forecasting the long-term performance of clay embankments under a range of projected climate conditions.

Environmental Temperature Effect on Hydraulic Behavior and Stability of Shallow Slopes

This study established a study framework to quantify the safety factors of unsaturated shallow slopes at different temperatures. This study is based on a non-isothermal soil water characteristic curve model quantifying the temperature-dependent hydraulic properties of soils. The hydraulic coupling analysis models HYDRUS 2D and The Slope Cube Module were used for finite element modeling. A slope stability analysis was performed based on the local factor of safety (LFS) theory. An increased temperature decreased the soil matric suction, suction stress, effective stress, and LFS, weakening the soil strength. Slope modeling analysis showed that soils were dominated by different water retention mechanisms before and after rainfall infiltration, and the trends caused by temperature changes also changed accordingly. This study provides insights into the relationship between soil mechanical properties and temperature, which is valuable for maintaining soil stability and preventing geological hazards.

Evaluating innovative direct and indirect soil suction and volumetric measurement techniques for the determination of soil water retention curves following drying and wetting paths

Soil Water Retention Curves (SWRC) are a main characterising component in constitutive models developed for describing unsaturated soils behaviour. Their determination requires establishing a combination of accurate soil matric suction measurements and volumetric measurements. While direct suction methods are always preferred, current methods, such as high capacity tensiometers, are limited to a measuring range of only 1.5 MPa. In this work, the novel Northumbria High Capacity Tensiometer, with an extended measuring range of up to 3.5 MPa, is used in a simplified continuous-drying setup, with proposed modifications to enhance the equalisation times and volumetric measurements. The results are then compared and validated with direct and indirect methods in both drying and wetting paths. At the same time, different volumetric measurement techniques were assessed in order to establish an understanding of Soil Shrinkage Curves (SSCs) that describe the moisture-voids relationship during drying and wetting and has the potential to be used for indirect determination of volumetric dimensions of soil specimens. Hysteresis was observed in the water retention behaviour but not in the volumetric behaviour. An established SSC model is proposed in this work and is fitted to experimental data in order to indirectly estimate volumetric measurements from the water content of small-scale specimens. The proposed novel approach standardises volumetric measurements determination, leading to uniform and complete-range SWRCs.

Characterizing thermo-hydraulic behaviors of seasonally frozen loess via a combined opto-electronic sensing system: Field monitoring and assessment

Frost damage in mid-latitude loess regions is mainly caused by the coupled soil water and heat transfer during freezing-thawing (FT) processes. In this paper, to better understand the thermo-hydraulic (TH) behaviors of seasonally frozen loess, field monitoring and assessment were performed on the Loess Plateau, China with a quasi-distributed opto-electronic sensing system. This system successfully captured the spatiotemporal variations in multivariables of seasonally frozen soil in near real-time, including in-situ soil temperature, unfrozen water content, and ice content. The monitoring datasets collected during the 2020/2021 winter offer unique insight into the TH behaviors of freezing point (FP) depression, in-situ soil-freezing characteristic curve (SFCC), and cryosuction. The findings highlight the importance of acquiring SFCCs under natural boundary conditions and confirm the applicability of laboratory-obtained FP in determining in-situ soil freezing depth. Furthermore, the study reveals that soil matric suction, considering the presence of ice, is the primary driving force of water migration during FT. Finally, errors in field monitoring are discussed, which arise from two aspects: the special features of loess and imperfections in monitoring schemes. This assessment of the coupled TH responses to climate changes in loess offers valuable theoretical guidance for optimizing engineering construction schemes and mitigating geohazards in mid-latitude loess regions.

Estimating Unsaturated Hydraulic Conductivity in Unsaturated Soil Using Soil Moisture Sensors

Unsaturated hydraulic conductivity (K) is a measurement of soil water-retaining ability in unsaturated soil. The unsaturated hydraulic properties are necessary to predict the movement of water in unsaturated soil. The current study aims to create an alternative experimental device to estimate theKusing soil moisturesensors. The experimental set up was designed and fabricated in order to measure the volumetric water content using GS3 (Campbell) sensors at different heights. The data obtained from the sensors was validated using mass of moist sand column measured from electronic balance.TheKwas estimated with the help of retention and conductivity functions. The primary materials used in the study were solute-free water and sand. It was found that the reading from the sensors was in good match with the reading of the volumetric water content calculated from the mass of water in the sand column with a root mean square error of 0.0091. Soil water retention curve and hydraulic conductivity were plotted using Peters-Durner-Iden (PDI) model.The results were compared to the van Genuchten function. TheKprediction wasinitially equivalentfor both models, but the lateroverpredicts the falling rate of hydraulic conductivity with the increasing soil matric suction before underpredict after 200 cm matric suction on a sandy soil.PDI function was able to extrapolate the hydraulic conductivity film flow region of sandy soil.

Feature and mechanism analysis of dispersive soil disintegration impacted by soil water content, density, and salinity

AbstractDispersive soil has caused innumerable water‐induced erosion failures of earthen structures in arid regions up to now, among which disintegration, as the first response process, is commonly deemed as the primary inducement but with inadequate understanding. In this study, the disintegration characteristics of a compacted dispersive lean clay with sand were investigated by carrying out laboratory disintegration tests under a water immersion regime. Soil mass water content (ω), dry density (ρd) and total soluble salt content (η) were considered. Results suggest that for dispersive soil, (1) the disintegration process exhibited two distinct modes, which was divided by the ω near the optimum water content; (2) higher ρd delayed disintegration monotonously; (3) increasing η first weakened and then enhanced the disintegration, in which pattern the η at the inflection point increased with increasing ρd, showing a density dependence. Evolutions of representative soil morphology were illustrated. More importantly, the underlying influential mechanism of ω and η to disintegration were elaborated emphatically: (1) the initial clay dispersion level, soil consistency change, and conceptual “constraint force” exerted by soil matric suction jointly determine the disintegration modes; (2) increasing η makes the soil pore solution concentration higher and microstructure simpler, competition between the two leads to nonmonotonic change patterns of disintegration completion time (Td) and defined velocity (Vd); the interactive effect of ρd and η manifests that η produces an additional influence on the base of microstructure created by ρd. Finally, the grey relation entropy analysis was introduced to evaluate the contributions of the three test variables to Td and Vd. The obtained results and proposed perspectives are expected to be conducive to a deeper comprehension of the same or relevant behaviours of dispersive soils in the presence of water, so as to provide new insights for the targeted prevention and control of dispersive soil catastrophes.Highlights Disintegration of dispersive soil shows two distinct water content‐dependent modes. Soil salinity has a two phase impact on disintegration of dispersive soil. Dry density and soil salinity interact affecting disintegration. Relationship between soil dispersion and disintegration is elaborated.

Physical Modelling of Rainfall-Induced Sandy and Clay-Like Slope Failures

Small-scale slope modelling was performed to evaluate the failure process of a landslide triggered by artificial rainfall. The model platform 2.3 m long, 1.0 m wide, and 0.5 m deep was used to build small-scale slope models with the same geometric conditions but different soil types/materials, including sand and two sand-kaolin mixtures with the same slope angle. The hydraulic response of the slope models under simulated rainfall conditions was monitored using volumetric water content, pore water pressure, and matric suction sensors installed at different depths and along different profiles. Slope surface deformation and failure development was also monitored. This paper discusses the factors affecting landslide initiation and propagation, and their relationship to the slope material, infiltration process, and overall soil resistance in a slope related to soil strength, effective stress, and matric suction contribution in the unsaturated part of the slope. Rainfall infiltration caused increase of volumetric water content, dissipation of suction in initially partially saturated materials of the small-scale slope models, resulting in a decrease in effective stresses and shear strength, which in turn led to the occurrence of movements and initiation of slope failures. The main observations arising from the results of the conducted tests relate to initiation and development of the observed instabilities of sandy and clay-like slopes. The test results have shown that within the slopes built from clean sand failure occurs due to groundwater level rising at the slope foot and further retrogressive failure towards the top of the slope, while in the slopes built from sand-kaolin mixtures, instabilities occur in the form of cracks in unsaturated conditions and are the result of matric suction dissipation due to rainfall infiltration.

Study on erosion and stability of the ecological slope

Rainfall is the main influencing factor causing slope erosion, landslide, and instability in loess; thus, it is vital to comprehend the process of rainfall erosion on various slope surfaces and water penetration inside the slope. In this paper, the loess sample is from Heifangtai in Gansu Province, and triaxial shear tests were conducted on loess with roots under varying water contents to evaluate the slope-reinforcing impact of roots. The slope surface erosion process was analyzed using a soil moisture sensor and matric suction meter to monitor the variation of matric suction in the middle slope and slope foot in response to varying precipitation levels. The numerical simulation approach is utilized to analyze the fluctuation of slope stability under the effect of varying rainfall intensities and humid heat, and the analytical solution of the safety factor is compared to the model solution. The results indicate that the shortest generation time for bare slope runoff is 6 min, whereas the greatest generation time for the Bermuda grass slope is 12 min; the shorter the period, the less water penetration and the simpler it is to reach the slope erosion stage. The slope’s rise increases runoff velocity, strengthening water resistance on the slope surface. When the test slope is 30°, the maximum mass of scouring sediment on the bare slope is 15.2 g from 24 to 36 min, compared to 14.7 g from 24 to 36 min when the test slope is 60°. The amount of scouring reduces as the slope increases. The slope safety factor declined from 3.51 to 2.84 after 24 h of heavy rain, and the loss rate accelerated as the rainfall intensity increased.

Developing a novel geophysical tool to investigate the influence of vegetation on slope stability

Vegetation is important for managing shallow geotechnical assets. However, root water uptake-driven changes in slope hydrology and the near-surface (soil water content, matric suction, and hydraulic conductivity) are highly complex. Improved knowledge of these processes is increasingly important as society faces the threat of a greater prevalence of climate-driven extreme rainfall and drought events. Intrinsic factors affect slope stability, including geometry, soil properties, groundwater, and vegetation-driven matric suction. Field evidence shows that engineered slopes are susceptible to hydrometeorological instability mechanisms and pose a potential failure hazard to asset operation and public safety. This study considers the combination of a novel geophysical monitoring system and geotechnical point sensors for use in controlled laboratory conditions to assess the influence of vegetation on soil-water dynamics in the context of geotechnical infrastructure. The geophysical monitoring system, referred to here as PRIME (Proactive Infrastructure Monitoring and Evaluation system), uses electrical resistivity tomography (ERT) technology to non-invasively image changing subsurface moisture-driven processes. The PRIME system and point sensor arrays are being developed for near real-time data acquisition of transient soil moisture conditions in a suite of soil column experiments. Through addressing the challenges associated with designing integrated geophysical-geotechnical laboratory-scale monitoring experiments, this research aims to provide new tools and approaches to further our understanding of vegetation-driven soil moisture movement to better assess slope instability risk.

When nature meets technology: AI-informed discovery of soil-water-root physical interaction

Nature-based solution using vegetation has been considered as a sustainable and environmentally friendly approach to improve slope performance through root reinforcement and variations of soil matric suction upon transpiration. During plant growth, roots explore soil pore space. How fundamentally the pore structure might evolve with time following root growth dynamics and how this dynamic soil-root interaction may modify the hydraulic properties of unsaturated soils remain unclear. This paper reports the use of advanced technologies including artificial intelligence (AI) to aid the discovery of soil-root-water physical interaction and the characterisation of the hydraulic properties of rooted soils. A newly developed miniature unsaturated triaxial apparatus that enables rooted soil samples to subject to simultaneous in-situ loading and X-ray imaging is introduced. An AI-informed image processing technique is illustrated, aiming to enhance the reliability of phase segmentation of X-ray computer tomography (CT) images of four-phase unsaturated rooted soils for quantifying 3-D pore structure and root phenotype. New discoveries of how roots interact with the pore space, including the dynamic changes in the distribution, orientation and connectivity of soil pore sizes, and how this pore-level information can be used to explain the hydraulic properties are discussed.

High Capacity Tensiometers: performance and behaviours

High capacity tensiometers (HCT) allow the direct measurement of soil matric suction, and their major limitation is the occurrence of cavitation. In this paper HCT designs using different ceramic filters with varied air entry values (AEV), varied reservoir sizes, and different pressure transducers are assessed as to determine the impact that each component may have in the HCT performance. Moreover, the effectiveness of first saturation and resaturation processes is discussed in relation to the time required to prepare/recover HCTs. The results obtained with the different designs show that the measuring range is directly linked to the AEV of the ceramic filter and that the choice of materials used for the various components may affect the reliability of measurements in field installations if the thermal performance is not accounted for in the calibration procedure. The use of a 1hr high vacuum pre-stage followed by overnight water pressurisation at pressures equal or above the AEV of the ceramic filter was found to be the quickest process to fully saturate an HCT for the first time. While resaturation time for an HCT can be reduced to as little as a few minutes if the HCT is resaturated immediately after cavitation has occurred.

A three-dimensional experimental study on bank retreat: The coupled role of seepage and surface flow

The coupled role of seepage and surface flow on bank retreat has long been neglected, partly due to the concealment and complexity of seepage erosion. To fill this gap, we set up a three-dimensional laboratory experiment to explore bank retreat process in response to seepage and surface flow. During each experiment, we measured the changes of total soil stress, matric suction, and water content within the bank, as well as flow velocity and suspended sediment concentration near the bank. Results show that a rapid decrease in matric suction, the bank toe undercutting consequent to seepage erosion, the formation of tension crack on the bank top, and the occurrence of toppling or shear failure is the typical sequence of the observed bank retreat process under seepage flow. The inclusion of surface flow erodes slump blocks and so promotes cantilever formation, leading to additional bank failure. Compared with the case where only seepage is considered, the frequency of toppling failure under the coupled effect of seepage and surface flow decreases, but the contribution to the bank retreat increases by 37 %. The time taken to collapse in three-dimensional experiments is at least 1.5 h earlier than that of two-dimensional experiments, indicating the importance of preferential flow pathways of seepage. Overall, this research illustrates how surface flow interacts with seepage flow to control bank retreat process and is indeed a first step toward a fully understanding of multifactor-driven bank retreat.