Matric Suction Of Unsaturated Soils Research Articles

A framework for estimating the matric suction in unsaturated soils using multiple artificial intelligence techniques

AbstractImplementation of the state‐of‐the‐art understanding of the mechanics of unsaturated soils into geotechnical engineering practice is partly limited due to the lack of quick, reliable, and economical techniques for matric suction measurement. Matric suction is one of the key stress state variables that significantly influences the hydro‐mechanical behavior of unsaturated soils. In this paper, to address this objective, two artificial intelligence (AI) models were developed for estimating matric suction in unsaturated soils based on the particle swarm optimization support vector regression (PSO‐SVR) and multivariate adaptive regression spline (MARS) algorithms. The results suggest that both these models can reasonably estimate matric suction. Compared to the MARS model, the PSO‐SVR model can achieve higher accuracy. Nonetheless, the MARS model facilitates the sensitivity analysis and the selection of essential inputs. A novel integrated framework is proposed and validated, leveraging the strengths, and alleviating the limitations of the PSO‐SVR and MARS algorithms for reliable and rapid estimation of matric suction in the range of 0–1500 kPa for low plastic soils (0 < Ip ≤ 7). Six inputs are required to use this model successfully; some can be measured using conventional laboratory tests, and others can be calculated from mass‐volume relationships.

Numerical Modelling of Matric Suction in Unsaturated Soil under Shallow Foundation Under Varying Soil and Hydrological Conditions

In recent times, extreme hydrological events have disrupted the performance of various structures, particularly the structural foundations responsible for transferring the superstructure's weight to the natural ground. This disruption underscores the significance of matric suction and soil saturation, which are influenced by hydrological conditions like precipitation, soil shear strength, and foundation settlement. These factors are essential when designing structures in specific locations with distinct geotechnical parameters. To address these challenges, our research employs Plaxis 2D numerical modeling to investigate the dynamic changes in matric suction within soil beneath shallow foundations under varying rainfall conditions. Our approach involves a fully-coupled flow method, incorporating the Van Genuchten hydraulic model. In recognition of practical constraints, we utilize fundamental soil classification parameters for model configuration. Our findings reveal the substantial impact of matric suction fluctuations during rainfall on soil deformation, as indicated by displacement patterns. This highlights the critical importance of matric suction in comprehending soil behavior. Furthermore, we observe that higher initial water table levels correlate with reduced variations in matric suction and soil deformation during rainfall, emphasizing the regulatory role of water table depth. In conclusion, this study emphasizes the necessity of considering matric suction and water table depth in structural design and geotechnical analysis, particularly when faced with extreme hydrological events. By comprehending these factors, we can enhance our understanding of soil behavior, improve foundation stability, and develop more effective design strategies for structures in various environmental conditions.

Bearing capacity factor for strip foundations on unsaturated clay

ABSTRACT The matric suction in unsaturated soil depends on the type of soil and various flux conditions i.e., infiltration, evaporation and no flow. The shear strength of unsaturated soil changes with change in matric suction leading to varying resistance of soil under various flow conditions. In this study, the bearing capacity of strip foundations resting over unsaturated clay considering variation of matric suction with depth for different surface flux conditions and position of water table, has been obtained using finite element lower bound limit analysis. Modified Mohr-Coulomb yield criteria based on unified effective stress approach has been employed to incorporate the contribution of matric suction stress to the failure condition. A non-dimensional bearing capacity factor was introduced to estimate the bearing capacity of strip foundations and presented as a function of different influencing parameters such as foundation width, water table depth, soil properties, various flow conditions and their flow rate, and surcharge pressure. The influence of variation of unit weight of soil in the unsaturated zone, air entry and pore size distribution parameters, residual degree of saturation of soil, specific gravity of soil solids, and foundation-soil interface roughness on the bearing capacity has also been examined and found to be insignificant.

Temperature effect on the stability of 3D reinforced slopes under unsaturated flow conditions

Slopes are increasingly exposed to elevated temperatures owing to extreme climates or human activities, such as droughts, heat waves, and shallow geothermal technologies. This phenomenon has worsened with global warming. However, the influence of temperature changes on slope stability has not been fully investigated from the perspective of safety. This study introduces an analytical framework for calculating the safety of three-dimensional reinforced slopes which considers the effects of temperature changes and different unsaturated flow conditions. With temperatures ranging from 10 °C to 40 °C, the matric suction of unsaturated soil is thermally sensitive, which causes change in the shear strength of unsaturated soil. To incorporate the temperature effect into the reinforced slope stability assessment, the temperature dependence of the apparent cohesion of unsaturated soil was quantified using a temperature-related effective stress model and soil–water characteristic curves. Thus, the safety factors and reinforcement strength of the slopes were deduced based on the upper-bound theorem of limit analysis. For validation, the proposed method was compared to two previously reported analytical methods, and its effectiveness was further verified through numerical simulations. Finally, the temperature effect was fully investigated and discussed by performing a series of parametric analyses, and suggestions were provided for practical engineering reference.

Effect of contact angle hysteresis on measuring matric suction in unsaturated sandy soil

The contact angle plays an important role in matric suction of unsaturated soils, but few studies consider the effect of contact angle hysteresis on matric suction. In this paper, taking sandy soil as an example, the influence of contact angle hysteresis on the suction of unsaturated soil matric is studied by soil particle model calculation and experimental method. The results show under the same water content condition, the smaller the contact angle is, the larger the matric suction is and the phenomenon of water repellency appears. Under the low water content of unsaturated soil, when the contact angle is less than 90°, it will also produce water repulsion phenomenon, which is different from the hydrophilic characteristics of the contact angle is less than 90° in the capillary law. When the water content is the same, with the increase of contact angle, matric suction will decrease. With the increase of water content, matric suction is not affected by soil contact angle. In the process of dehumidification, the apparent contact angle of the sandy soil with the same water content less than receding contact Angle; there is a difference in matric suction between continuous measurement method and discontinuous measurement method, and continuous measurement’ matric suction greater than discontinuous measurement’ matric suction. This study provides some theoretical basis and reference for the study of matric suction of unsaturated soil.

Experimental Study on Moisture Migration in Unsaturated Sand Under Cyclic Diurnal Temperature Variation

The phenomenon of mixed migration of liquid water and water vapor appeared in subgrade covered with an impermeable semi-arid rigid base. The soil-water characteristic curves (SWCC) at different temperatures are a key factor in revealing the mechanism of liquid water migration. In particular, the matric suction of unsaturated soil is considerably affected by the change in temperature. A moisture migration laboratory experiment can visually observe the moisture migration process. Therefore, matric suction tests with different temperatures and moisture migration of the one-dimensional soil column were conducted. Then the non-isothermal SWCC model was extended based on the Van Genuchten (VG) model, and the validity was given. Based on the above results, the mechanism of the mixed migration of liquid water and water vapor in moisture migration of unsaturated soil under the effect of the cyclic diurnal temperature variation was revealed. The research demonstrated that the matric suction of unsaturated sand decreases with the increase in temperature. During the daytime, the temperature of the soil column gradually increases. Except for a part of liquid water transformed into water vapor, the liquid water migrated downward under the action of gravity and matric potential. At night, the water vapor accumulated beneath the lid condensed into liquid water under the action of the temperature sharply decreased, which was the main reason for the moisture aggregate beneath the closed layer. Meanwhile, the liquid water and water vapor mutually transformed and dynamically migrated in the soil column under the effect of the cyclic diurnal temperature variation. The moisture in the soil column dynamically migrates between daytime and night, which is named the “moisture circulation of covering effect” in this paper. The “moisture circulation of covering effect” would progressively attain a dynamic equilibrium and dynamic circulatory between the daytime and night with time.

Closed-form modeling of matric suction in unsaturated soils under undrained heating

Matric suction is an important parameter controlling the hydro-mechanical behavior of unsaturated soils. Several emerging geotechnical and geoenvironmental applications involve unsaturated soils subject to undrained heating. This study presents a closed-form model to determine the matric suction in unsaturated soils under undrained heating. The final expression is derived based upon the Young–Laplace capillary principle and only needs the enthalpy of immersion at the reference temperature and as primary input parameters. Under undrained (constant water) conditions, the closed-form model incorporates the impact of temperature on matric suction by considering the temperature dependency of the liquid–gas interfacial tension, the contact angle of the solid–liquid–gas interface, and the enthalpy of immersion. The model is validated against six sets of experimental data for clay, silt, and sand, reported in the literature. The results show a very good agreement between measured and predicted values for all soil types over a wide range of temperatures. The matric suction monotonically decreases with temperature under undrained heating. The proposed closed-form model can readily be employed in numerical and analytical analyses to simulate the undrained behavior of unsaturated soils under elevated temperatures.

Effect of Dolomite Lime on Soil Matric Suction

The most fundamental of unsaturated soil is matric suction parameter. Matric suction in unsaturated soils plays an important role in controlling physical and mechanical properties. This parameter can influence some other important parameters of soil, such as effective stress, shear strength, swelling pressure, etc. Investigation of matric suction was performed on Ayer Hitam soil, at Batu Pahat District, Johor, Malaysia. Field monitoring works were conducted with and without dolomite treatment at the slopes of Ayer Hitam acidic soil. Field testing used some devices to find out the behavior of matric suction during the rainy session. The result shows that the soil matric suction with dolomite treatment is larger than without treatment and matric suction has a significant difference around 20 – 50 kPa. Enhancement of acidity from the dolomite treatment was not only decreasing pores size caused by the alternating of the microstructure of soil but also leading to increasing the matric suction. The treatment of dolomite material is able to control matric suction and increase the stability of slope acidic Ayer Hitam soil.

A bound water model for numerical simulation of SWCC in the wide suction range based on DDA

In this study, Discontinuous Deformation Analysis (DDA) method is extended to simulate Soil-Water Characteristic Curve (SWCC) by considering both capillary water and bound water at the micro-scale. First, a simple and practical bound water model is proposed and incorporated into DDA program to simulate SWCC in the high suction range. Then, to prove the availability of the developed program, the extended DDA is used to simulate the SWCCs of two types of fine sand with kaolin clay in a wide suction range. The simulation results agree with the experimental results, which demonstrate that the extended DDA is an alternative method for simulating and analyzing SWCC of both cohesionless soil and cohesive soil. Finally, one of the influential factors specific surface area is theoretically analyzed. This study provides an approach to simulate the matric suction of unsaturated soil, and the extended DDA could be a potential tool for unsaturated soil research.

A unified approach for analysing surface flow and groundwater induced slope instability

ABSTRACT The combined effects of transient, surface and subsurface flows and infiltration govern the rainfall-induced potential failure of unsaturated soil slopes stabilised by matric suction. In this paper, it is shown how fundamental fluid dynamics can be employed to closely model the above flow effects than the empirical Richard’s approach. The fluid dynamics-based Navier-Stokes formulation also facilitates the integration of the three modes of flow: (1) surface, (2) subsurface and (3) infiltration water flows, into a unified analytical model. Therefore, in contrast to previous studies, this fundamental approach can seamlessly incorporate the effects of continuous interplay between surface and subsurface water flows and the inertial terms of flow on the reduction of matric suction in unsaturated soils. Transient flow effects have been combined with geomechanics of unsaturated soil to demonstrate their impact on the stability of an embankment under a more realistic non-uniform rainfall. The innovative use of the laboratory-evaluated soil properties such as the hydraulic conductivity and porosity to express the drag force in the Navier-Stokes equations facilitates the examination of the sensitivity of slope stability to the relevant soil classification parameters.

Effect of a less permeable stronger soil layer on the stability of non-homogeneous unsaturated slopes

Slope failure occurs due to an increase in the saturation level and a subsequent decrease in matric suction in unsaturated soil. This paper presents the results of a series of centrifuge experiments and numerical analyses on a 55° inclined unsaturated sandy slope with less permeable, stronger silty sand layer inclusion within it. It is observed that a less permeable, stronger silty sand layer in an otherwise homogeneous sandy soil slope hinders the infiltration of water. The water content of the slope just above the stronger layer increases significantly, compared to elsewhere. No shear band is found to initiate in a homogeneous sandy soil slope, whereas for a non-homogeneous slope, they initiate just above the less pervious, stronger layer. A discontinuity of the shear zone is also observed for the case of a non-homogeneous soil slope. The factor of safety of a non-homogeneous, unsaturated soil slope decreases because of the less permeable, stronger layer. It decreases significantly if this less permeable, stronger soil layer is located near the toe of the slope.

Topographic Effects on Three-Dimensional Slope Stability for Fluctuating Water Conditions Using Numerical Analysis

With recent advances in calculation methods, the external factors that affect slope stability, such as water content fluctuations and self-configuration, can be more easily assessed. In this study, a three-dimensional finite element strength reduction method was used to analyze the stability of three-dimensional slopes under fluctuating water conditions. Based on soil parameter variations in engineering practice, the calculation models were established using heterogeneous layers, including a cover layer with inferior properties. An analysis of seepage, deformation and slope stability was carried out with 27 different models, including three different slope gradients and nine different corner angles under five different hydraulic conditions. The failure mechanism has been shown to be closely related to the change in matric suction of unsaturated soils and the geometric slope configuration. Finally, the effect of geometry (surface shape, turning corner and slope gradient) and water (fluctuations) on slope stability are discussed in detail. Emphasis is given to comparing safety factors obtained considering or ignoring matric suction.

Application of improved radial movement optimization for calculating the upper bound of ultimate bearing capacity of shallow foundation on unsaturated soil

Considering engineering practices in terms of decrease in the matric suction of unsaturated soil with increasing embedded depth, this study, employing a new global optimization algorithm on the basis of radial movement optimization (RMO), focused on investigating the critical slip surface and ultimate bearing capacity of a rough strip footing on unsaturated soil subjected to vertical load. Based on upper-bound limit analysis theorem, an equation for calculating the ultimate bearing capacity of unsaturated soil has been established. The data structure of RMO was improved by considering self-feedback of the particles. The results show that the improved radial movement optimization can be successfully applied to accurately and stably search the critical slip surface and subsequently calculate the ultimate bearing capacity on unsaturated soil. It also found that, compared with no matric suction, the critical slip surface determined considering the matric suction of unsaturated soil is larger and deeper, and the corresponding ultimate bearing capacity is also significantly improved.

A Method for Predicting the Matric Suction of Unsaturated Soils with Soil Color Recognition

To overcome shortcomings such as complicated and time consuming existed in methods for measurement of soil suction, a new, simple and quick method for prediction of soil suction is suggested. By measuring soil color and water content simultaneously, a curve for describing the relationship of the variation of soil color with degree of saturation is produced and defined as Soil Color Characteristic Curve (SCCC). By combining the Soil Water Characteristic Curve (SWCC) and SCCC with degree of saturation of soil as an intermediate variable, a new curve connecting the suction and soil color was generated and named as Soil Color-Suction Curve (SCSC). With using SCSC, the suction of an unsaturated soil of definite degree of saturation can be predicted by taking colored image on surface of soil sample and by application of color recognition techniques.

Performance of high-capacity tensiometer in constant water content oedometer test

Matric suction is one of the stress-state variables which governs the behaviour of unsaturated soils. In order to evaluate the effect of matric suction on the behaviour of unsaturated soil under constant water content condition, rapid measurement of change in matric suction during the test is required. High-capacity tensiometer can be used to rapidly measure the change in matric suction of unsaturated soil. In this paper, an oedometer apparatus was modified to incorporate a high-capacity tensiometer to investigate the compression behaviour of unsaturated soil under constant water content condition. The design of the oedometer apparatus was described. Two soils which are kaolin FP grade (commercial kaolin which has a fine grain size) and residual soil from Bukit Timah Granite were used to evaluate the performance of the high-capacity tensiometer. The results showed that kaolin paste is helpful in improving the contact condition between the soil specimen and high-capacity tensiometer. The equilibrium time of the high-capacity tensiometer was about 10 min. Evaporation effect reported by other researchers was not observed with the high-capacity tensiometer. The change in matric suction due to loading and unloading was accurately captured by the high-capacity tensiometer.

Effect of Surface Flux Boundary Conditions on Transient Suction Distribution in Homogeneous Slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope. Keywords: Evaporation, Flux Boundary, Rainfall, Slope Stability, Suction Distribution

Effect of surface flux boundary conditions on transient suction distribution in homogeneous slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope.

In situ monitoring of the groundwater field in an unsaturated pyroclastic slope for slope stability evaluation

In the last 15 years, a series of catastrophic flowslides involving pyroclastic unsaturated soils has caused severe damage and a number of fatalities in the Campania region (southern Italy), where flowslides occur within the pyroclastic cover resting on the limestone massif. Rainwater infiltration is considered to be the triggering mechanism that leads to slope failure by reducing matric suction in unsaturated soils, hence reducing its shear strength. Therefore, knowledge of the groundwater regime is an important factor for slope stability evaluation as well as for predicting slope conditions prone to landslides triggered by water infiltration. The quality of prediction can be greatly improved if observations from in situ monitoring are available. This paper describes the results of an ongoing experimental research project on flowslides based on monitoring a test site in a typical geological environment in western Campania. The site was selected to carry out extensive laboratory and in situ testing. The test programme consisted of field monitoring of climatic conditions, matric suction and volumetric water content. The instrumentation and measuring techniques used in this research are systematically described to provide an example of good practice for use at other sites with similar features. With the use of the collected data, the seasonal fluctuations of the hydraulic regime in the subsoil may be observed and the critical periods for flowslide triggering identified. Measurements of matric suction from the test site are interpreted and directions of groundwater flow vectors in the subsoil are shown. Moreover, once the current state of the subsoil is known, slope stability conditions can be calculated using a simple infinite slope model that would provide continuously updated information on the current slope safety level.

Total and Matric Suction in Unsaturated Soil with the Existence of Different Salts Content

Total and Matric Suction in Unsaturated Soil with the Existence of Different Salts Content

Research on Stability of Slope by Influence of a Rapid Drawdown of Water Table

The importance of analysis of stability of slope during rapid drawdown of water table is firstly introduced in this paper. Through summary of the current theoretical and model test research on stability of slope during rapid drawdown of water table, the important of considering the matric suction in unsaturated soils, the analysis method of transient unsaturated seepage and safety of slope and the changes of slope safety was summarized and introduced in this paper. In addition, the problems existing in the analysis of stability of slope during rapid drawdown of water table are pointed out and some ideas for its further development are suggested.