Reduction In Suction Research Articles

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.

Analysis of excess water impact on the structural performance of flexible pavements

Excess water in pavement foundations is one of the major factors contributing to pavements deterioration. In recent years, a number of research studies have been carried out to understand the water movement into pavements and to assess its detrimental effects on the mechanical response of the foundations. However, these studies do not quantify the impact of suction reduction or pore pressure buildup in the foundations on the pavement response. In this paper, coupled finite element analyses are carried out to investigate the effect of the excess water in the granular foundations on the structural performance of flexible pavements. The coupled analyses simulate critical features governing the foundations' hydromechanical response including the transient unsaturated flow and porous nonlinear behaviour of the foundations under moving wheel loads. Furthermore, a parametric study that examines the influences of various loading and foundation parameters on the performance of pavements subjected to excessive moisture scenarios is carried out. The numerical analysis results obtained in this paper are qualitatively in line with the empirical observations reported in the literature.

Swelling pressure–suction relationship of heavily compacted bentonite–sand mixtures

This paper presents results of an experimental work to determine a relationship between swelling pressure and suction of heavily compacted bentonite–sand mixtures. For comparison, tests were also carried out on heavily compacted bentonite specimens. A series of swelling pressure tests were performed using multi-step constant-volume method where suction of the specimens tested was reduced in a stepwise manner toward a zero value. The suction reduction was induced using vapor equilibrium and axis-translation techniques. It is shown that compacted specimens did not exhibit any collapse upon suction decrease and exhibited maximum swelling pressures at zero-equilibrium suction. The development of swelling pressure with decreasing suction of the specimens showed threshold suctions below which a further reduction in suction yields an increase in the swelling pressure of the same magnitude. The magnitude of threshold suction was found to be a function of bentonite content in compacted specimens.

The Stability Studies of Soil Slope under the Action of Short-Time Heavy Rainfall

Based on the saturated-unsaturated seepage theory and the unsaturated shear strength, the numerical simulations of soil slope seepage field in short-time heavy rainfall was carried out, and on this basis calculated the slope safety coefficient at different rainfall time. The research shows that: the rainfall infiltration will cause the slope soil matrix suction decreased or even disappeared which leads to the slope shear strength reduce; Rainwater infiltration increases the transient water load of infiltration area; and under the dual role of matrix suction reduction and transient water load increasing, the slope safety coefficient has the downward trend obviously.

Small-strain stiffness of Zenoz kaolin in unsaturated conditions

An experimental study has been carried out to investigate the effects of isotropic compression, wetting, and drying on the initial shear stiffness of Zenoz kaolin, an unsaturated lean clay, both in normally consolidated and overconsolidated conditions. The proposed study was conducted using fixed–free resonant column – torsional shear (RCTS). Specimens were compacted using the undercompaction technique. Initial shear stiffness was measured almost continuously along complex stress paths including (i) an initial equalization stage to a suction value of 0, 50, 150, and 300 kPa; (ii) an isotropic compression stage at constant suction, up to a net stress high enough to move the loading collapse line; (iii) an isotropic unloading stage at constant suction; (iv) a wetting and (or) drying path. The mentioned stress path allowed elimination or determination of the overconsolidation effect on the initial shear stiffness measured. The behavior observed is qualitatively similar to that of saturated soil, while wetting data clearly indicate that G0 depends significantly on volumetric behavior. In normally consolidated samples where wetting is accompanied by collapse, reduction in suction has no remarkable effect on G0. Conversely, in overconsolidated samples G0 reduces significantly as suction decreases.

Modelling the initial shear stiffness of unsaturated soils as a function of the coupled effects of the void ratio and the degree of saturation

Existing models for predicting the small strain behaviour of unsaturated soil are not capable of predicting the initial shear stiffness during suction reduction under normally consolidated conditions. This problem has been addressed in the present study by combining an existing elastoplastic model and recent experimental data to provide a new model for the initial shear stiffness. The model, which is similar to that typically adopted for saturated soils, uses the average skeleton stress and an additional function of the degree of saturation. This new model not only captures the behaviour of the new experimental results, but it also describes a unique relationship between saturated and unsaturated soils.

Collapse behavior of soil in a Brazilian region affected by a rising water table

Collapsible soils are usually nonsaturated, low density, and metastable-structured soils that are known to exhibit a volume reduction following an episode of moisture increase or suction reduction. This paper describes the collapsible behavior of clayey sand based on controlled soil suction tests carried out on undisturbed samples from the city of Pereira Barreto, in the State of São Paulo, Brazil. Foundation settlements due to soil collapse are common in this region and occurred during the filling of the reservoir of the Três Irmãos Dam, which induced the elevation of the groundwater table in different parts of Pereira Barreto. This paper shows that collapse strains depend on the stress and soil suction acting in the sample and that saturation is not necessary for a collapse to occur. The influence of soil suction, gradual wetting, and the wetting and drying cycle on the collapsible behavior of the soil is also shown and discussed.

Response parameters for characterization of infiltration

Rainwater infiltration is of particular interest with respect to slope stability hazard management. The process of rainfall infiltration into unsaturated soil is complex due to the number of soil parameters involved and the random nature of moisture flux boundary conditions. In this paper, the effect of rainfall intensity on infiltration is investigated through the use of finite element seepage modeling for a 5 m high soil column subjected to various rainfall intensities. The numerical modeling study identified three response parameters that can be used to describe one-dimensional infiltration into an unsaturated soil. The response parameters are the depth of wetting front, the matric suction reduction depth, and the sectional infiltration rate. The practical application of the rainfall response parameters in slope stability analysis is illustrated.

Predicting the performance of gas-sparged and non-gas-sparged ultrafiltration

Abstract Tubular and hollow fibre membrane ultrafiltration is commonly used for waste water treatment and potable water production often with the introduction of gas bubbles into the filtration feed to improve flux. For design purposes, accurate models of the process are required. The one dimensional (1D) flat plate boundary layer analysis remains a simple tool for mass transfer analysis of these processes. However, this analysis neglects effects of wall permeation or curvature of circular closed-conduits e.g. hollow fibre and tubular membranes. Another pitfall is use of bulk fluid properties for flux prediction. Consequently, flat plate analyses severely under-predict ultrafiltration flux in such membranes. In this work we analytically include curvature (edge) effects of circular conduits into flat plate analyses. Flux estimates incorporating both edge and suction effects were reliable and predicted flux increases from 47.2% to 131.2% over the classical analysis. It was also shown that use of bulk diffusion coefficients in flux estimates led to reduction of suction- and edge-inclusive flux by up to 61%. The improved 1D model derived here enabled more accurate, yet simple flux prediction. With regard to gas-sparged systems a mechanism of flux enhancement is proposed for hollow fibre membrane ultrafiltration. This mechanism is determined from previously published experimental and computational fluid dynamics studies of the capillary slug flow process, as well as from dimensional analysis of the process. A physicochemical model for flux prediction is designed around the postulated enhancement mechanism. Again the boundary layer analysis is adapted to include the effect of wall suction. The flux-prediction model enables estimation of upper and lower bounds which were correctly found to encapsulate experimental values.

Hydro-mechanical response of a bentonite pellets/powder mixture upon infiltration

A series of infiltration tests on a 50/50% bentonite pellets/powder mixture compacted at different dry densities are reported and analysed. This mixture was proposed as sealing material in the Belgian concept for radioactive waste disposal. The tests have been performed with the French FoCa bentonite at two laboratories following analogous methodologies. The water intake, swelling pressure and axial strain have been measured during the tests. A common pattern of swelling pressure development has been found in all the tests, regardless the density of the mixture, the dimensions of the specimen, or the initial fabric, since a similar behaviour has also been found for specimens of compacted powder. At the beginning of saturation the swelling pressure increases sharply and then decreases, what gives place to an initial peak pressure. This decrease is interpreted as a collapse of the macrostructure on suction reduction. As saturation proceeds the swelling pressure increases again and eventually reaches a steady final value, what is a result of the redistribution of water towards the microstructure. The dry density and the initial water content of the mixture are two key factors affecting the particulars of this behaviour. After full saturation, the mixture becomes homogeneous with respect to water content and dry density, and the swelling pressure developed is analogous to that expected for a specimen of powder compacted at the same dry density.

The more closed the bypass system the better: a pilot study on the effects of reduction of cardiotomy suction and passive venting on hemostatic activation during on-pump coronary artery bypass grafting

Cardiac surgery with cardiopulmonary bypass (CPB) leads to a powerful activation of the hemostatic system. We assessed to what extent this activation can be attenuated by comparing three different perfusion regimens for on-pump coronary artery bypass grafting (CABG): 1) use of a closed CPB system with aspiration of blood from the operation field via the cardiotomy suction line and active venting of the heart via a roller pump; 2) use of a closed CPB system avoiding aspiration of blood from the operation field via the cardiotomy suction line, but with active venting of the heart; and 3) use of a closed system, avoidance of aspiration of blood from the operation field via the cardiotomy suction line and with passive venting of the heart into the collapsible venous reservoir. Our data show that avoidance of aspiration of blood via the cardiotomy suction line significantly reduces hemostatic activation during on-pump CABG. However, further attenuation of hemostatic activation can be achieved by further closing the system and minimizing the blood/air interface by passive venting of the heart.

Thermo-hydro-mechanical behaviour of two heavily overconsolidated clays

An experimental study on the thermo-hydro-mechanical behaviour of two heavily overconsolidated clays is presented. Laboratory tests have been conducted on a Spanish bentonite (FEBEX bentonite) and a Belgian kaolinitic/illitic clay (Boom clay), statically compacted at different initial dry densities and water contents. Volume change behaviour of the soils during suction reduction paths at different temperatures and during heating–cooling cycles at constant water content or suction have been investigated through the use of suction and temperature controlled oedometer cells. In addition, the volume change response under unconfined conditions and constant water content has been measured to determine thermal expansion coefficients. The results show similarities and differences between the observed behaviour of the two types of clays that have been interpreted on the basis of their different structures and regarding their proportion of intra-aggregate water.

A Study on Suction-rainfall Response of a Cut Slope in Unsaturated Residual Soil Using a Field Rain Simulator

This research describes the design and construction of an artificial rain simulator system and the results obtained with regards to suction-rainfall response of a cut slope in unsaturated residual soil of weathered sandstone of various weathering grades. From the field measurements, the shallow tensiometers are found to record an immediate initial increase in matric suction within the first 20 min of the simulated rain before a gradual drop in suction takes place. However no such initial spikes in suction readings were recorded by the deeper tensiometers. With constant rainfall for the next 130 min, a clear drop in soil suction was observed in all cases illustrating the effect of net infiltration process. Positive pore water pressure was even recorded by some of the deeper tensiometers. The suction recorded by the shallow tensiometers was always higher than those of the deeper tensiometers, indicating a reduction in suction with depth below the ground surface. In term of weathering grades, residual soil of weathered sandstone of weathering grade III has the highest infiltration rate indicating the more porous nature of this soil material. Like wise there is a steeper drop in suction during the rain at the said location.

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.

Laboratory Study of Loose Saturated and Unsaturated Decomposed Granitic Soil

Weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as Hong Kong. The mechanical behavior of loose decomposed fill materials, particularly in the unsaturated state, has not often been investigated and is not yet fully understood. The objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state, the stress path and the soil suction on the stress-strain relationship, shear strength, volume change, and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens. It was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing. Strain-softening behavior is observed in loose saturated specimens. In unsaturated specimens sheared at a constant water content, a hardening stress-strain relationship and volumetric contractions are observed in the considered range of net mean stresses. The suction of the soil contributed little to the apparent cohesion. The angle of friction appeared to be independent of the suction. In unsaturated specimens subjected to continuous wetting (suction reduction) at a constant deviator stress, the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation. It was revealed that the dilatancy of the unsaturated soil depends on the suction, the state, and the stress path.

Bidimensional Swell Effect on Accuracy of Footing Heave Prediction

Abstract The direct method (the Texas Highway Department Method TEX-124-E) and the Jennings and Knight “Double Oedometer Method” are widely used by practicing engineers for heave prediction of footings located on expansive soils. These methods are simple to use and yet simulate field conditions too. These two methods, however, are characterized by overpredicting the value of footing heave. The degree to which these methods overpredict heave is dependent on the soil conditions, as well as the applied footing pressure. The present work studied the influence of footing pressure on the precision of the two mentioned methods. It was found that the factor needed to adjust the predicted footing heave by these methods (“Heave Reduction Factor Rf”) decreases markedly with increasing footing pressure. The heave reduction factor Rf decreased from 0.92 and 0.88 to 0.62 and 0.53 for the direct method and the double oedometer method, respectively, as the footing pressure increased from 25 to 50 kPa. Soil strength depends upon the level of soil suction. Soil wetting contributes to reduction in soil suction, and consequently, to a loss in bearing capacity. Therefore, a test experiment with footing pressure of 100 kPa suffered bearing capacity failure after about one day since the beginning of the test experiment.

An Elasto-Plastic Model for Unsaturated Soil in Three-Dimensional Stresses

A three-dimensional elasto-plastic model for unsaturated soil is presented, using a transformed stress tensor based on the Extended SMP criterion. The influences of suction on the mechanical behavior of unsaturated soil are taken into account in the model by using a parameter named suction stress σ0(s), which is added to net stress to form an effective stress, and associating a yield stress and model parameters with it. The proposed model needs few material parameters, which can be easily determined from the results of ordinary tests, for example, oedometer and direct shear tests or triaxial test. It can predict the stress-strain-strength behavior of unsaturated soil under the conditions of three-dimensional stresses and constant or variable suction. A considerable number of triaxial compression and extension tests on unsaturated compacted kaolin were performed by using a suction-controlled triaxial apparatus, to measure the stress-strain behavior under different stress paths with constant and decreasing suction. The prediction of the proposed model shows good agreement with the measured behavior of unsaturated soil not only in triaxial compression but also in triaxial extension under the conditions of constant suction and reduction in suction, which may induce an irrecoverable volumetric compression (collapse).

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.

A numerical investigation of the stability of unsaturated soil slopes subjected to transient seepage

Abstract Slope instability in unsaturated residual soils and loose fills has attracted increasing attention in recent years around the world such as Brazil, South Africa, Japan and in the Far East. Rain-induced failures are the most common ones. Rainfall leads to the development of perched water table, rising the main groundwater level and soil erosion (due to concentrated water flow), resulting in an increase in pore water pressure or a reduction in soil matric suction. This, in turn, results in a decrease in shear strength on the potential failure surface to a point where equilibrium can no longer be sustained in the slope and then failures occur. However, the present understanding of the influence of transient seepage in unsaturated soils, due to water infiltration under various boundary and ground conditions, and hydrogeological regimes on slope stability is still relatively poor compared with other elements of geomechanics. To investigate the influence of various rainfall events and initial ground conditions on transient seepage and hence slope stability, a parametric study has been conducted using the finite element method. A typical steep unsaturated cut slope in Hong Kong has been adopted for the parametric study. Variables considered in the parametric study include isotropic and anisotropic soil permeability, initial water table at upslope boundary, rainfall intensity and duration. Pore water pressures or suctions predicted during the transient seepage analyses are then used as input ground water conditions for subsequent limit equilibrium analyses of the stability of the slope. Factor of safety is calculated using Bishop’s simplified method, with modified Mohr–Coulomb failure criterion to allow for shear strength variation due to the presence of matric suction. Infiltration due to rain water causes a reduction of matrix suction, but an increase in moisture content and soil permeability in unsaturated soils. Perched water table is developed above the main water table. The factor of safety is not only governed by the intensity of rainfall, initial ground water table and the anisotropic permeability ratio, but it also depends on antecedent rainfall duration. A critical rainfall duration can be identified, at which the factor of safety is the lowest.