Reduction In Matric Suction Research Articles

Effect of Moisture Content on the Permanent Strain of Yellow River Alluvial Silt under Long-Term Cyclic Loading

The Yellow River alluvial silt has unique engineering properties and is unstable when encountering moisture. The mechanical properties of silt subgrade can be impaired by the increase in moisture content due to rainwater infiltration, which has a negative effect on traffic safety. To further reveal the influence of moisture content on the deformation characteristics of silt, a series of monotonic and cyclic triaxial tests were conducted on the alluvial silt with different moisture contents. The development law of cyclic accumulative permanent strain and the effects of moisture content, cyclic deviator stress and confining pressure on the axial permanent strain of silt were explored. The study shows that the static strength of silt decreases with the increase in moisture content, and the attenuation of static strength is mainly caused by the decrease in cohesion due to the reduction in matric suction. The permanent strain rises linearly with the increase in moisture content and cyclic deviator stress, and decreases with the increase in confining pressure. An empirical model for predicting the permanent strain of silt under long-term cyclic loading considering the effect of moisture content was established. Compared with the test data and other existing models, the established model has easier obtained parameters, higher prediction accuracy and better applicability.

Effects of Heavy Rainfall on Shallow Foundations in Bukit Timah Granite in Singapore

The increase in rainfall intensities due to climate change affect the entire globe. In particular, Singapore suffers from floods and rising of coastlines. Notably, in the Bukit Timah Region in Singapore, floods are getting more intense, and the region houses multitudes of low-rise constructions with shallow foundations. Damages ranging from physical, in terms of motor vehicle and property damages, to intangible losses such as major traffic delays in both private and public transit were caused by the floods. Few studies have been carried out in Singapore in terms of shallow foundations’ response to rainfall events. When rainfall infiltrates into the soil, the bearing capacity and soil stiffness are affected by the change in matric suction. Thus, the impact of heavy rainfall on shallow foundations in Bukit Timah Granite is investigated numerically using SIGMA/W. Fully coupled flow-deformation analysis with unsaturated soil characteristics, e.g., the Soil Water Characteristic Curve (SWCC) and unsaturated permeability functions, were conducted. A range of rainfall intensities, rainfall durations, and applied loadings were investigated to produce a load–settlement curve that was compared against a semi-empirical model to yield reasonable results. The studies showed that the change in matric suction is affected by the rainfall duration, rainfall intensity, initial groundwater conditions, and hydraulic properties of soil, which in turn affects the settlement response heavily. The bearing capacity is evaluated using graphical methods via the load–settlement response curves, and it was found that the reduction in matric suction heavily reduces the bearing capacity of the soil. Combined with the unsaturated residual soils and transient analyses, the discoveries give insight into the assessment of shallow foundations subjected to water infiltration.

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.

Collapsibility of gypseous soil under suction control

Many geotechnical problems and foundation failures in engineering structures were recorded due to the presence of unsaturated gypseous soils. One of the main reasons, which introduced large volume change and collapse behaviour is the variation of suction pressure and water potential within the gypseous soil mass. In this study, Sandy gypseous soil from Al-Ramadi city (west of Iraq) with gypsum content of more than 70 % is used. In order to investigate the collapse deformation of gypseous soil under suction control by multi-steps wetting procedure, also the effect of sensors positions and applied vertical stress, a special soil-model device is designed and manufactured. In this device, high accurate and sensitive Tensiometers and Time Domain Reflectometry (TDR) sensors with an assistance of high quality Data logger are used for measuring and monitoring of matric suction and gravimetric water content of the soil sample during the deformation processes. The test results indicated that a large volume change and collapse deformation occurred upon reduction of matric suction. The void ratio dramatically reduced stepwise with reducing of matric suction by multi-steps wetting irrespective the value of applied vertical stress.

Drag Force Effect of Surface Runoff on Stability of Shallow Soil Slope

The softening and degradation effects of water on soil mass, increase of pore water pressure, and reduction of matric suction are usually considered for analyzing the contributions of rainfall to the failure of shallow soil slope. However, the effect of drag force at the fluid-solid interface due to slope surface runoff also deserves to be taken into account. To quantitatively investigate this effect, a modified approach based on the limit equilibrium method is proposed by establishing a nonlinear coupled mathematical model. The calculation results show that the effect of drag force at the fluid-solid interface should not be neglected especially when the runoff on shallow soil slopes is relatively deep.

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.

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.

The effect of soil type on matric suction and stability of unsaturated slope under uniform rainfall

It is well understood that rainfall infiltration decreases the unsaturated shear strength of soil slope due to reduction in matric suction as the wetting front moves through it. However, how much the matric suction is reduced that may affect slope stability results is still unknown. The paper presents the numerical results of wetting front depth, matric suction and stability results in term of reliability index for a single rainfall intensity. The analyses are for five soils using the modified Mohr-Coulomb failure criterion in conjunction with a finite element saturated-unsaturated seepage model. The numerical results show that the matric suction results are differently affected for soils with rainfall infiltration. The change in reliability index result is insignificant as long the soil remains unsaturated as the wetting front moves. The reliability index results are however become noticeable if the infiltrated rainfall produces saturation and positive pore-water pressure. The difference in results may be attributed to the wetting front depth and its saturation that depends on saturated-unsaturated hydraulic conductivity and matric suction head.

Spatiotemporal regional modeling of rainfall-induced slope failure in Hulu Kelang, Malaysia

This study demonstrated the use of transient rainfall infiltration and grid-based regional slope stability analysis model incorporated with spatial rainfall distribution model for regional mapping of rainfall-induced slope failures in a slope failure-prone area in Malaysia, namely Hulu Kelang. Infinite slope analysis was incorporated into the improved model to compute the pore water pressure and slope stability of the region. Digital terrain maps, spatial rainfall distributions, soil profiles, groundwater table, and soil properties were input into the model. The results showed that the slope failures in the study area can be grouped into shallow failures (<4 m) and sub-shallow failures (4–8 m). Most of the shallow slope failures (<4 m) were triggered by the northeast monsoon while the NE monsoon that characterized by more prolonged and intense rainfalls. Spatial temporal distributions of shallow and sub shallow slope failures predicted using the improved model showed reasonably good comparison with the historical slope failure regions. Monsoon events have affected slope stability by reduction in matric suction, complete loss of matric suction, and development of positive pore water pressure (PWP). Computation for the effects of rainfall infiltration on PWP response and consequent effects on slope stability is enhanced by the spatial rainfall distribution model. The pore water pressure response to monsoon rainfall is a function of rainfall duration, the rate of infiltration, groundwater depth, soil thickness, and slope inclination. The amount of the local daily rainfall is not the only factor affecting slope stability. The prolonged antecedent rainfall could have a role to play in building up the slope failure mechanism.

Laboratory study on soil behavior in loess slope subjected to infiltration

In recent years, Heifangtai Plateau in Gansu Province, China, has received significant attention as a loess region in which landslides frequently occur. This study aims to examine the soil behavior and failure mechanism(s) of loess slopes subjected to water infiltration. High-quality samples were retrieved behind the failure plane of a landslide in Heifangtai. Three sets of stress path tests were conducted to mimic the process of loess slope failures; these tests include undrained compression on anisotropically consolidated saturated samples, drained shear by decreasing mean effective stress at constant axial load on saturated samples, and wetting by decreasing matric suction at constant axial load on unsaturated samples. Loess behavior was evaluated in terms of shear strength, deformation upon shearing, critical state, state parameter, and soil water characteristic curve. Three major observations were drawn from the test results. First, the anisotropically consolidated saturated loess exhibits a noticeable strain-softening behavior upon undrained compression. The peak shear strength is mobilized at less than 1% axial strain. The ultimate shear strength is only 20–30% of the peak value. Second, when subjected to shear by decreasing mean effective stress at constant axial load, the saturated loess exhibits sudden failure when the confining stress is low. At higher stresses, the soil exhibits progressive failure. Third, unsaturated loess deforms progressively when subjected to a reduction in matric suction (i.e., wetting) at constant axial load. The failure mechanisms of the loess slopes are discussed based on the experimental findings.

Field performance of retaining walls reinforced with woven and nonwoven geotextiles

ABSTRACT: This paper presents an evaluation of the performance of two instrumented sections of a geosynthetic-reinforced soil wall, 5.6 m high, constructed using a lateritic fine-grained soil. Two sections with identical layout of a nonwoven and a woven geotextile were monitored for comparison purposes. The unconfined tensile stiffness of the nonwoven geotextile was three times smaller than that of the woven geotextile. This allowed direct evaluation of the effect of soil confinement on geotextile stiffness. Instrumentation was used to measure face displacements, reinforcement displacement and strains, and soil matric suction. Rainfall occurred both during and after construction, which facilitated evaluation of the effect of soil wetting on the walls performance. Ultimate and serviceability limit state analyses were conducted to gain further insight into the performance of the two walls. The results show that the performance of the section reinforced with nonwoven geotextile was equivalent to the one reinforced with woven geotextile, even after the observed reduction in matric suction after rainfall. For both sections, the overall deformations occurred during construction. Negligible deformations were observed during service. Maximum face displacements were measured in the lowest instrumented layer for the nonwoven geotextile section whereas it was in the highest layer for the woven geotextile section. These behaviours of face displacement distributions can be the result of the significant differences in global stiffness of the sections. Design analyses and field performance show that soil confinement has a beneficial effect on the nonwoven geotextile stiffness. The significant contribution of the soil cohesion of the lateritic soil played an important role in the behaviour of the nonwoven geotextile-reinforced wall.

A Rainwater Redistribution Model to Evaluate Two-Layered Slope Stability after a Rainfall Event

Rainfall infiltration usually leads to shallow landslides due to the reduction of matric suction within soil. However, experience has shown that not all of those landslides occur in the period of precipitation: Some landslides occur hours or even days after rainfall events. This phenomenon has not been widely investigated and addressed in the literature. This paper establishes a general rainwater redistribution model for fine-over-coarse and coarse-over-fine soil stratification profiles based on the one-dimensional Moore's infiltration model and the continuity of soil-water potential at the interface of the two-layered soil. A comparison of the results of the redistribution model with those of the numerical analyses shows that with the use of properly estimated input parameters, the proposed model is quite reasonable. Along with the validated rainwater redistribution model and the infinite slope model, the factors of safety are calculated for two hypothetical landslide cases. The results reveal that both factors of safety decrease slightly in the redistribution process and eventually reach stable values when the accumulative infiltration increases to a certain value. This finding may explain why shallow landslides occur after the rainfall event. Although simple, the analytical model can serve as a useful tool to assess the stability of shallow landslides that occur after a rainfall event.

Shear Strength of Compacted Soil under Infiltration Condition

Landslides in residual soil slopes are commonly induced by rainfall infiltration. These residual soils are typically in an unsaturated state with negative pore-water pressures or matric suctions since the groundwater tables in steep slopes are often deep. The net normal and shear stresses of the soil remain essentially constant during rainwater infiltration into the slope. Failure of the slope during rainfall can be primarily associated with the decrease in the matric suction of the soil. The objective of the study was to investigate the strength and deformation characteristics of a residual soil of the Bukit Timah Granitic Formation during infiltration that leads to slope failure. There were two modified direct shear apparatuses used. One apparatus was used for the determination of shear strength under controlled suction conditions while the other apparatus was used for shearing-infiltration tests. The shearing-infiltration test results were compared with the shear strength values obtained from the shearing tests under constant suction. The shearing-infiltration test results indicate a close relationship between the decreasing matric suction and the increasing displacement rate of the soil specimen. At the initial part of the infiltration process, there is a rapid reduction in matric suction that is accompanied by little movement in the soil. When failure of the soil is imminent, the soil movement will accelerate.

Pore water pressure and streambank stability: results from a monitoring site on the Sieve River, Italy

To investigate the role of pore water pressures in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was monitored by repeated cross-profiling, erosion pins and marked pebbles. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expression was derived for the safety factor with respect to mass movement of the upper bank, incorporating the failure criterion for unsaturated soils and the normal Mohr–Coulomb criterion for saturated conditions. Variations in matric suction have important effects on the stability of the streambank. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle. However, during rainfall and flow events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures. During the rising limb of high-flow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequency of retreat of basal and upper bank zones differ significantly, the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure. Copyright © 1999 John Wiley & Sons, Ltd.