Decrease In Matric Suction Research Articles

Mechanical behaviour of soil under drying–wetting cycles and vertical confining pressures

A system for preparing soil specimens subjected to drying–wetting cycles while under vertical confining pressures was introduced with clayey soil specimens subjected to different drying–wetting histories, and then the relative performance was tested using consolidated undrained triaxial shear tests. Meanwhile, their soil water retention properties were also measured. The experimental results indicated that drying–wetting cycles lead to a rise in the matric suction for soil with a high moisture content and a decrease in matric suction for soil with a low moisture content. Partly owing to the higher pore water pressure, peak shear strength reduces gradually during drying–wetting cycles. The impacts of drying–wetting cycles on the hydromechanical properties of soil specimens during the first cycle are greater than those during the second and third cycles, as the highest matric suction of soil occurs during the first cycle. The vertical confining pressure is shown to limit the impact of drying–wetting cycles on the hydromechanical properties of soil effectively because of its restricting effects on the volumetric deformation of soil during the cycles.

Soil Water Characteristic Curve Analysis of Weathered Granite Soils with Various Fine Content

Determining the parameters related to unsaturated soil properties considering capillary and adsorption phenomena is challenging. In addition, the matric suction generated in unsaturated soil increases the stability of the slope; however, the decrease in matric suction owing to heavy rain causes a decrease in slope stability. Therefore, it is crucial to determine the parameters of unsaturated soil accurately. In this study, the engineering characteristics in the unsaturated state were analyzed using weathered granite soil, which is typically distributed in mountainous areas in Korea. Four samples were prepared in which the compositions of the fine contents of weathered granite soil were 0%, 10%, 20%, and 30%, and a soil characteristic curve test was performed on the samples. In the coefficient prediction method, the widely applied van Genuchten model was used to analyze the air entry value and parameter values according to the fine content of weathered granite soil in the range of the drying process. As a result, a correlation between the fine content in the weathered granite soil and the air entrainment value and parameter values was identified.

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.

Estimation of Active Earth Pressure for Narrow Unsaturated Backfills Considering Soil Arching Effect and Interlayer Shear Stress

Affected by climatic conditions such as rainfall, evaporation and air temperature, most of the backfill soil is in an unsaturated state, and the decrease in matric suction leads to the failure of the retaining structure. In view of this, this study takes the vertical rigid retaining wall with narrow unsaturated backfill as the research object, assuming that the backfill behind the wall forms a circular soil arch and considering the interlayer shear stress; thus, the analytical solution of the active earth pressure of narrow unsaturated soil is derived based on the thin layer element method. The reliability of this method is verified by comparing with the experimental and existing theoretical results. A parameter analysis demonstrates that with the increase in the interface friction angle of the moving wall–soil, the average shear stress coefficient of zone I and zone II increases gradually, but with the increase in the interface friction angle of the fixed wall–soil, the average shear stress coefficient of zone I decreases; with the increase in effective internal friction angle and effective cohesion, the active earth pressure decreases and the tension crack depth increases; with the increase in the interface friction angle, the active earth pressure in the upper part of the retaining wall increases slightly, while the active earth pressure in the lower part decreases obviously; with the increase in matric suction, the active earth pressure first decreases rapidly and then increases gradually, and the tension crack depth first increases and then decreases, but the distribution pattern of the horizontal active earth pressure remains unchanged; the active earth pressure decreases with the decrease in the aspect ratio, and when the aspect ratio is smaller, the attenuation is more obvious; until the aspect ratio reaches a certain value, the active earth pressure is basically unchanged.

Algorithm Implementation of Equivalent Expansive Force in Strength Reduction FEM and Its Application in the Stability of Expansive Soil Slope

Loss of matric suction during rain has an important effect on the instability of expansive soil slope. A new device was designed for testing the expansive force in order to propose and determine the equivalent expansive force corresponding to the matric suction. First, the internal relation between the matric suction and the corresponding equivalent expansive force of unsaturated expansive soil was analyzed. Then, numerical algorithm implementation of the equivalent expansive force was discussed, and the equivalent expansive force was introduced into the strength reduction finite element method (FEM). Finally, the equivalent effects of the matric suction and the equivalent expansive force were compared and analyzed by evaluating the stability of an unsaturated expansive soil slope. The results show that the new testing device significantly improves the accuracy of the expansive force test and shortens the testing time. The relation between the matric suction and the equivalent expansive force with the change in initial water content is obvious. The equivalent expansive force can reflect the macro contribution of matric suction to unsaturated expansive soil, and the developed strength reduction FEM based on the equivalent expansive force can be used to evaluate the rainfall-induced instability of an expansive soil slope caused by the decrease in matric suction resulting from the rainfall infiltration.

Three-dimensional modeling of the mechanical behavior of a single pile in unsaturated expansive soils during infiltration

In this paper, a novel coupled hydro-mechanical model for unsaturated expansive soils and a constitutive model for unsaturated pile-soil interfaces are implemented into ABAQUS for performing 3-D finite element analysis of single piles in expansive soils during infiltration. Some key features of coupled thermal–mechanical model from ABAQUS are used to formulate coupled hydro-mechanical model developing five independent user-material subroutines. User-defined field subroutine (USDFLD) is used to access the stress and strain and transfer these values to other subroutines. Three subroutines including user-defined material subroutine (UMAT), user-defined thermal material subroutine (UMATHT), and user-defined thermal expansion subroutine (UEXPAN) are developed and used to calculate the stress-deformation, the hydraulic behavior, and the expansion strain, respectively. In addition, an unsaturated soil-structure interface model is implemented into user-defined friction behavior subroutine (FRIC) to calculate the friction stress between soil and pile. The program is successfully validated using experimental results performed on a single model pile in an expansive soil. The modeling results are consistent with the experimental results which suggest that a decrease in matric suction can contribute to the reduction in the shaft friction as more load is transferred to the pile resulting in an increase in the pile head settlement and pile base resistance. The modeling approach presented in this paper is promosing and can be extended by the practitioners in the rational design of pile foundations in expansive soils.

Methodology to Characterize the Seismic Compression of Unsaturated Sands under Different Drainage Conditions

Abstract This study focuses on a new experimental setup and methodology to characterize the volumetric contraction of unsaturated sands during cyclic shearing under different drainage conditions. A new cyclic simple shear device was developed with suction-saturation control using a hanging column suitable for testing unsaturated granular soils with a maximum suction of 11 kPa. The pore water and pore air pressures are monitored during cyclic shearing using an embedded tensiometer and a gauge pressure transducer protected by a hydrophobic filter, respectively. In addition to describing the specimen preparation techniques and testing methodology, typical results for the seismic compression of nearly saturated, dry, and unsaturated sand specimens during cyclic shearing under different drainage conditions are compared. The differences in the response of these sand specimens were interpreted using the changes in mean effective stress and secant shear modulus during cyclic shearing. In drained conditions, the unsaturated sand specimen showed lower seismic compressions than the dry sand specimen and the nearly saturated sand specimen. In undrained conditions, a greater contraction was observed for the unsaturated conditions due to decreases in mean effective stress and secant shear modulus. The decrease in effective stress occurred due to a decrease in matric suction associated with different rates of increase in pore water and air pressure and an increase in degree of saturation with volumetric contraction.

Hydraulic characterization of an unsaturated vegetated soil: The role of plant roots and hydraulic hysteresis

Flowslides and debris flows in granular soils pose a serious threat to human life and man-made structures. Due to rainwater infiltrating into superficial unsaturated soils, rainfall is the most common triggering factor of such landslides, causing a decrease in matric suction and hence in soil shear strength. Early warning systems based on accurate analyses of groundwater response to meteorological factors are widely used to mitigate landslide risk. In such a context, the accuracy of the model adopted to calculate the groundwater field is closely related to the reliability and meaningfulness of hydraulic soil characterization. In this paper, an extensive laboratory investigation regarding the hydraulic behaviour of pyroclastic unsaturated deposits from a vegetated slope monitored on Mount Faito (Campania, Southern Italy) is presented to highlight the importance of hydraulic hysteresis and the presence of roots in shallow soils. Water retention properties and hydraulic conductivity functions were determined, focusing on a drying–wetting cycle. Tests on specimens sampled in the top ten centimetres of the soil profile were also carried out to assess the effects of plant roots on soil hydraulic properties. Inverse analyses were used to estimate the parameters of a hysteretic hydraulic model. Finally, parametric numerical analyses, carried out via a finite element code, were used to highlight the potential effects of the hydraulic characterization on the stability of sloping pyroclastic covers, including all above factors.

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.

Influence of rainfall on transient seepage field of deep landslides: A case study of area II of Jinpingzi landslide

Rainfall is one of the primary factors that induce landslides. Rainfall infiltration forms a transient seepage field in slopes, thereby leading to an increase in water content, a decrease in matric suction, and an increase in the volume weight of slopes. The increase in water content in rock and soil leads to the degradation of the mechanical properties of the slope medium and reduces the shear strength, thus causing slope instability. In this study, a large-scale accumulation landslide is taken as the research object. The front edge of the landslide is high and is not eroded by river water. Hence, rainfall is regarded as the main external factor that changes the groundwater and seepage field of the landslide. According to the landslide’s topography, geological data, local rainfall, and hydrological monitoring results, the saturated–unsaturated seepage theory of rock and soil and the finite difference method are used to analyze the impact of rainfall on the landslide’s seepage field and groundwater. The evolution law of the transient seepage field of the landslide under the condition of rainfall duration is studied. The results provide a basis for the analysis of landslide deformation and stability.

Large-Scale Test Model of the Progressive Deformation and Failure of Cracked Soil Slopes

A large-scale test bed (LWH=6 m×3 m×2.8 m) instrumented with various sensors is used to examine the effects of rainfall infiltration and evaporation on the deformation and failure of cracked soil slopes, taking the Anhui area along the Yangtze River as a field example. The results indicate that (1) during rainfall, the soil around the shallow shrinkage fissures attains transient saturation, and the attendant decrease of matric suction is the primary cause of the shallow slope failure; (2) slope deformation continues during post-rainfall evaporation; (3) if a period of evaporation is followed by heavy rainfall, soil creep is concentrated near the deepest cracks, and two zones of steep gradients in pore pressure form at the crest and toe of the slope. Finally, a saturated zone forms near each crack base and gradually enlarges, eventually forming a continuous saturated layer that induces the slope instability or failure.

Effects evaluation of grass age on hydraulic properties of coarse-grained soil

This study investigates the effects of grass age on hydraulic properties of coarse-grained volcanic soil, namely: Komaoka soil. The soil-water characteristic curve (SWCC), unsaturated coefficient of permeability, outflow, runoff, and the variations in volumetric water content/matric suction under different rainfall intensities varying from 15, 30, 60, to 120 mm/h are measured by carrying out column tests. In addition, permeability tests are performed to determine the saturated coefficients of permeability of bare soil and grassed soils. In these tests, grassed soils with grass ages varying from 1 to 6 months are used to study the influence of root volume ratio on the hydraulic properties. The experimental results reveal that the grass age has an influence on the hydraulic properties of Komaoka soil. At the same volumetric water content, grassed soil with higher grass age has higher matric suction. Both saturated and unsaturated coefficient of permeability are significantly reduced with the increase in the grass age. Moreover, there is a decrease in the outflow and an increase in the runoff for grassed soil with higher grass age. Matric suction decreases later in grassed soil than that of bare soil under rainfall events. The higher the grass age, the longer time is taken to observe the change in matric suction. However, the matric suction of grassed soil is dramatically reduced while there is a gradual decrease in matric suction of bare soil. Furthermore, even though the matric suctions remain constant at somewhere in the vicinity of 2 kPa under long and intense rainfall events, higher volumetric water contents are recorded for grassed soil with higher age. The new model is proposed to estimate the unsaturated coefficient of permeability of coarse-grained soil due to the effects of grass roots. The good agreement between estimations and experimental results shows that the proposed model is useful to capture the decrease in unsaturated coefficient of permeability of grassed soil.

Impact of the wetting process on the hydro-mechanical behavior of unsaturated residual soil from flysch rock mass: preliminary results

Slope instabilities in the form of shallow and deep-seated landslides in flysch formations throughout Europe typically occur after prolonged periods of heavy rainfall. The Rjecina River Valley, Croatia, is characterized by the presence of flysch material in the lower part of the Valley, where numerous historical and recent landslides have occurred. The weathering process and climate conditions result in a complex engineering geological profile of flysch slopes in the Valley, with unsaturated residual soil covering the slopes. To investigate the behavior of residual soil existing on the flysch slope under increasing water content due to the rainfall infiltration process, undisturbed soil samples collected at natural water content were tested in the modified direct shear apparatus. Under imposed stress conditions, samples of low hydraulic conductivity were subjected to a prolonged wetting process simulating the rainfall infiltration process in the field. The obtained results suggest that a gradual decrease of matric suction and an increase of water content resulted in an increase of displacement rates under constant shear stress, which was interpreted as a failure of samples in partially saturated conditions. A unique shear strength envelope expressed in terms of Bishop’s effective stress equation was found to be able to predict stress conditions at the slip surface at the time of failure, while the relationship between measured matric suction and water content closely matched with the main wetting curve. Although the testing results did not point out any special characteristics of residual soil from flysch rock mass behavior, the data about hydro-mechanical behavior of unsaturated residual soil from flysch rock mass, as well as similar fine-grained soils, are very rare and presented results would be valuable for further research. The presented testing procedure and obtained results are useful for studies of rainfall-induced landslides triggered in fine-grained soil materials in zones above the phreatic line, such as shallow landslides occurring in natural flysch slopes or in physical landslide models built in laboratories.

Landslide analysis of unsaturated soil slopes based on rainfall and matric suction data

Shallow slope failures that occur as a result of a decrease in matric suction after water infiltration from intense rains (unsaturated soil conditions) are the main causes of slope instability in tropical and subtropical regions. This study analyzes the occurrence of shallow landslides in a highway cutting of residual soil originating from aeolian sandstone. Characterization of failure mechanisms and stability analyses have been carried out in different scenarios and supported by surface and subsurface investigations, instrumentation and monitoring of rainfall, matric suction, and water level, field tests, and laboratory tests. The results indicate that the reduction in matric suction induced by rainwater infiltration is the triggering mechanism of slope failure. Conventional slope stability analyses and analyses incorporating unsaturated seepage models present results compatible with the hydrological and geotechnical data collected in the study. On the basis of these analyses, and considering the frequent rainfall events in the study area, a critical geometric configuration is proposed for the triggering of landslides in highway cutting slopes of residual soils of aeolian sandstone.

Numerical simulations of triaxial shearing-infiltration tests

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.

Prediction of the wetting-induced collapse behaviour using the soil-water characteristic curve

Collapsible soils go through three distinct phases in response to matric suction decrease during wetting: pre-collapse phase, collapse phase and post-collapse phase. It is reasonable and conservative to consider a strain path that includes a pre-collapse phase in which constant volume is maintained and a collapse phase that extends to the final matric suction to be experienced by collapsible soils during wetting. Upon this assumption, a method is proposed for predicting the collapse behaviour due to wetting. To use the proposed method, two parameters, critical suction and collapse rate, are required. The former is the suction value below which significant collapse deformations take place in response to matric suction decease, and the later is the rate at which void ratio reduces with matric suction in the collapse phase. The value of critical suction can be estimated from the water-entry value taking account of both the microstructure characteristics and collapse mechanism of fine-grained collapsible soils; the wetting soil-water characteristic curve thus can be used as a tool. Five sets of data of wetting tests on both compacted and natural collapsible soils reported in the literature were used to validate the proposed method. The critical suction values were estimated from the water-entry value with parameter a that is suggested to vary between 0.10 and 0.25 for compacted soils and to be lower for natural collapsible soils. The results of a field permeation test in collapsible loess soils were also used to validate the proposed method. The relatively good agreement between the measured and estimated collapse deformations suggests that the proposed method can provide reasonable prediction of the collapse behaviour due to wetting.

Forming condition of transient saturated zone and its distribution in residual slope under rainfall conditions

Rainfall, as one of the most significant factors triggering the residual soil slope failure, leads to not only the reduction of soil shear strength, but also the increase of soil weight and the decrease of matric suction as well. All these modifications in soil properties have important influence on the slope stability. The water infiltration and redistribution inside the slope are the preconditions of the slope stability under rainfall conditions. Based on the numerical simulation via finite element method, the water infiltration process under rainfall conditions was studied in the present work. The emphases are the formation, distribution and dissipation of transient saturated zone. As for the calculation parameters, the SWCC and the saturated permeability have been determined by pressure plate test and variable head test respectively. The entire process (formation, development, dissipation) of the transient saturated zone was studied in detail. The variations of volumetric water content, matric suction and hydraulic gradient inside the slope, and the eventually raise of groundwater table were characterized and discussed, too. The results show that the major cause of the formation of transient saturated zone is ascribed to the fact that the exudation velocity of rainwater on the wetting front is less than the infiltration velocity of rainfall; as a result, the water content of the soil increases. On the other hand, the formation and extension of transient saturated zone have a close relationship with rainfall intensity and duration. The results can help the geotechnical engineers for the deeper understanding of the failure of residual slope under rainfall condition. It is also suggested that the proper drainage system in the slope may be the cost-effective slope failure mitigation method.

Hydraulic-mechanical properties of loess and its behavior when subjected to infiltration-induced wetting

Water retention, unsaturated permeability, and deformation-failure characteristics induced by infiltration were studied by analyzing stress-dependent soil–water characteristics and the wetting stress path of unsaturated intact clayey loess in Baoji, China. Test results in Baoji and Heifangtai were compared. The results show the following: (1) The soil–water characteristic shape is related to stress, and suction decreases with water content; (2) the van Genuchten model is applicable to the clayey loess in Baoji, (3) the unsaturated permeability coefficient is related to stress, which increases with axial net stress; (4) under certain confining stress, hydraulic properties of loess vary considerably by region and, compared with loess in Heifangtai, the water retention capacity of clayey loess in Baoji is greater; (5) brittle deformation is dominant in Baoji but plastic deformation is dominant in Heifangtai; and (6) deformation behavior and the strength of loess is related to the confining stress, and the combined action of that stress and suction cause breakage of loess cementation bonds. The decrease of matric suction is not necessary for loess deformation, but the breakage of cementation bonds is the essence of the decreasing strength and final deformation failure of loess.

RESPONSE OF SUCTION DISTRIBUTION DUE TO VARIATIONS OF PERMEABILITY IN RESIDUAL SOIL SLOPE

A landslide in residual soil normally occurs immediately after heavy rainfall. Previous studies have shown that decrease in matric suction during rainfall decreases the shear strength of soil and results in landslides. One of the factors that contribute to infiltration of water into soil is permeability of the soil which varies with depth. The variations of permeability can either prevent or allow water to infiltrate into deeper soil layer. Therefore, the aim of this study is to determine the suction distribution in a two- layered residual soil system with variable permeability function using laboratory physical slope model. The Ksat for Grade V varies from 5.11 x 10-4 m/s for relict joint of 100 mm spacing to 5.40 x 10-5 m/s for relict joint of 300 mm spacing. Meanwhile the Ksat for Grade VI represent Grade VI without burrow holes, 5.00 x 10-7 m/s and Ksat with burrow holes, 6.98 x 10-4 m/s. The infiltration tests were conducted for 12 series of experimental program. The suction distribution due to variations of permeability and rainfall intensity were determined. The results illustrated that suction distribution responded in various ways depending on permeability of the layered soil and also the rainfall intensity.

Probabilistic back analysis of rainfall induced landslide‐ A case study of Malin landslide, India

On 30th July 2014, a devastating landslide resulted in the burial of a village called Malin in western India and led to about 160 deaths. The catastrophic failure was triggered by heavy rainfall over the last 3days. Site investigation was conducted to obtain representative information about the area. To understand the relevant physics, which initiated landslide, seepage and back analysis of the unsaturated slope was performed. Transient seepage analysis was carried out using finite difference method to simulate the slope behavior during rainfall infiltration. A coupled fluid flow analysis was employed to capture the fluid/mechanical interaction in unsaturated soils accurately. The results show that antecedent rainfall played a role in slope instability; rainfall intensity and duration also have a big impact on slope instability. This paper presents a methodology to back analyse the slope so as to identify the mechanisms responsible for landslide initiation. Back analysis is performed using a probabilistic method based on Bayesian analysis. Probabilistic methods can back-analyse numerous sets of uncertain slope stability parameters. The uncertain parameters are imported as random variables in the analysis and are described by their probability distributions. Bayesian analysis updates these distributions based on the observed slope behavior. The method evaluates the reduction in matric suction which initiated landslide in Malin slope. The results show that the decrease in matric suction of about 100% in addition to the development of positive pore pressures is the main mechanism which has triggered the landslide. This amount of reduction in matric suction and the development of positive pore pressures decreases the mobilized shear strength in soil below the threshold value required to maintain equilibrium in the slope and hence the slope failed.