Osmotic Suction Research Articles

The Water Retention Characteristics of an Unsaturated Compacted Loess Material

The water retention curve of an unsaturated compacted loess material has been obtained by combining the filter paper method, the vapour equilibrium method and the axis translation method. It was indicated that the combined method is efficient to investigate the water retention characteristics of compacted loess materials. The results showed that the osmotic suction in the compacted loess varies slightly with the increasing water content. Higher matric suction occurs in the compacted loess compared with the natural loess at the same moisture.

Physico-Chemical Effects on Shrinkage Behavior of Compacted Expansive Clay

AbstractPhysico-chemical effects have a significant impact on the behavior of clay barriers due to the interactions between the pore fluid and clay particles, and they pose a great challenge because the efficiency of clay barriers and cover systems may be altered. Therefore, this paper highlights the effect of physico-chemical factors on shrinkage behavior of compacted expansive clay. To achieve this, the compacted clay specimens were inundated with distilled water, NaCl, and CaCl2 salt solutions in separate oedometer cells at a vertical pressure of 6.25 kPa and allowed to swell. The swollen specimens were gradually shrunk until the specimens attained constant mass, and the changes in void ratio and water content were monitored during drying. The experimental results showed that magnitude of induced osmotic suction and type of pore fluid had a significant impact on shrinkage behavior of compacted clay specimens due to the changes in soil structure. The average pore size of compacted specimens significantl...

Hydrochemical Effect of Different Quality of Water on the Behaviour of an Expansive Soil During Wetting and Drying Cycles

AbstractThe effect of quality of water on deformation, pH, EC (electrical conductivity) and osmotic suction was studied for an expansive soil during wetting and drying tests. The cyclic wetting and drying tests were conducted on samples of an expansive soil in a modified oedometer flooded with distilled, acidic and saline water. During the tests axial deformation of the samples was recorded continuously. pH and EC of pore water and reservoir water were measured through duplicated samples in a conventional oedometer. Osmotic suction was calculated based on the values of EC. The results show that the magnitude of deformation depends on the quality of the water and the deformation attained an equilibrium condition after almost four cycles. pH, EC and osmotic suction decreased with increasing suction. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of pore fluid osmotic suction on matric and total suctions of compacted clay

The experimental results of the present study on compacted expansive clay illustrate that matric suction increases with the increase in pore fluid osmotic suction due to changes in soil structure. Scanning electron micrographs show that particle stacking (aggregations) increases with increase in pore fluid osmotic suction and cation valence due to reduction in diffuse double layer thickness, and thus results in a reduction in micro pore size with a corresponding increase in macro pore size. At a given water content, the macro pore degree of saturation reduces with the increase in osmotic suction owing to the increase in macro pore size and leads to an increase in matric suction, as the matric suction is characteristic of the macro pore and the degree of saturation of the macro pore. The matric, osmotic and total suction measurements using the filter paper method also confirm that the matric and osmotic suction components are additive. This demonstrates that the method of obtaining matric suction from the difference between total suction and pore fluid osmotic suction determined using non-contact filter papers is reliable.

Effect of pore fluid osmotic suction on matric and total suctions of compacted clay

Effect of pore fluid osmotic suction on matric and total suctions of compacted clay

Assessment of Molding Moisture and Suction on Resilient Modulus of Lime Stabilized Clayey Subgrade Soils

A comprehensive laboratory study was undertaken to evaluate the effects of molding moisture and suction on the resilient modulus (MR) and unconfined compressive strength (UCS) of lime stabilized clayey subgrade soils. Two subgrade soils, AASHTO class A-6 and A-7-6, are stabilized with three percentages (0, 5, and 7 %) of lime on the basis of pH test and used to prepare cylindrical MR samples at three moisture contents: optimum moisture content (OMC), dry state (OMC-2 %), and wet state (OMC +2 %). MR tests on lime stabilized soils are conducted with a modified stress sequence incorporated in the AASHTO T307 procedure based on past literature and laboratory experience gained in this study. Test results revealed that the effects of moisture on MR and UCS values of lime stabilized soils were less than those on untreated soils. MR and UCS values increased due to lime treatment, but improvement varied with soil type and lime dose. A filter paper method was used to determine the total and matric suctions at different moisture states in this study. It was observed that osmotic suction increased to 15 % of the total suction due to lime treatment. Finally, an existing MR constitutive model was revised by incorporating total suction. The revised model was shown to have better predictive capability over existing MR models.

Calibrations for volume change measurements using osmotic suction control technique

Evaluation of the volume change behavior of expansive soils under controlled suction and boundary conditions is important for the proper design of different geotechnical systems. Osmotic technique was used by several researchers for suction controlled testing of expansive soil specimens. This technique involves circulating polyethylene glycol solution (PEG) over a semi-permeable membrane underlying a test specimen. Different suction controlled testing apparatuses incorporating osmotic techniques were developed included triaxial and oedometric testing conditions to simulate different boundary conditions. This paper describes a series of calibration performed on suction controlled modified oedometer and triaxial equipment used for testing expansive soil. The modified oedometer, developed by the authors, comprises of thin-wall oedometer ring instrumented with strain gauges to measure the lateral stresses evolving during the test. The first part of this paper introduces the errors that arise during osmotic suction testing, concerning the device flexibility and PEG solution losses. The second part of the paper highlights the calibration procedure for estimating lateral stresses in modified oedometer from instrumented strain gauges as well as introduces a correction for the temperature fluctuation. Finally, the effect of triaxial cell flexibility on the volume change measured during osmotic suction controlled testing as well as temperature effect on volume change measurements were evaluated and correction procedures were introduced.

Osmotic flow in compacted expansive clay

Depending on the relative chemical concentrations of clay pore water and landfill leachate, three osmotic flow scenarios may be anticipated to dissipate the osmotic suction gradient: outward osmotic flow, inward osmotic flow and no (osmotic) flow. This study examines the volume change behaviour of clay subjected to all osmotic flow conditions. Results show that osmotic suction gradient decreases swelling strains when the osmotic flow is outward, whereas swelling strains increase in case of inward osmotic flow. The increase or decrease in effective stress from osmotic suction gradient is transient and reduces to zero with dissipation of matric suction and osmotic suction gradient. In the no (osmotic) flow condition, the swelling strains are governed by matric suction and pore fluid osmotic suction. The experimental results bring out the dependency of swelling strains on applied stress, stress paths and osmotic suction gradient.

Role of osmotic suction in volume change of clays in salt solution

The volume change behaviour is usually expressed by different curves of e–p when concentrated pore fluids are introduced to clays. Increase in effective stress from osmotic suction in salt solution and increase in inter-particle attraction from the pore-water salt concentration in salt-clay system. The void ratio is smaller in the salt solution with higher concentration under the same vertical overburden pressure, and it results the different curves of e–p in salt solution with different concentration. This paper aims to express the e–p relationship using a unique curve for clays in salt solution with different concentration. A conceptual model is proposed to explain the influence of osmotic suction on the volume change of clays in a quantitative way. A new formula for effective stress is proposed as pe=p+pπ incorporating with osmotic suction (π), and pπ=(p/π)D‐2π. Experimental data for volume change of clays in NaCl solution with different concentration can be expressed by a unique curve using the proposed effective stress.

Soil-water characteristic curves of clays

The drying suction – water content soil-water characteristic curves (SWCCs) of three clays (MX80 bentonite, yellow bentonite, and Speswhite kaolin) were experimentally determined using axis-translation, vapour equilibrium, and osmotic techniques. The shrinkage paths of the clays were established from Clod tests. The suction – water content SWCCs in conjunction with the Clod test results enabled establishing the suction – degree of saturation SWCCs and further determination of the air-entry values (AEVs) of the clays. Chemical analyses of the polyethylene glycol (PEG) solutions in the osmotic tests revealed an imbalance of the osmotic suctions between the expelled and the retained salts on either side of semi-permeable membranes. A decrease in the water content due to an applied suction for clays with significant osmotic efficiencies was explained by two mechanisms. In mechanism 1, the water content decrease prior to the air entry is controlled by the interparticle repulsive pressure, and in mechanism 2, a decrease in the degree of saturation following the air entry is primarily due to the matric suction. The agreements between the AEVs of the clays determined based on the osmotic suctions corresponding to various applied suctions and that determined from the suction – degree of saturation SWCCs were found to be very good.

Chemo-hydro-mechanical behaviour of compacted Boom Clay: joint effects of osmotic and matric suctions

An experimental programme involving osmotic and matric suction controlled tests is carried out to investigate the effects of increasing pore fluid concentration, chemical cycling (salinisation/dilution) and matric suction on the liquid retention properties, shear strength and volume change behaviour of Boom Clay. This Tertiary Clay formation has been thoroughly investigated as a possible host material for the disposal of Eurobitum bituminised radioactive waste (BW) in Belgium. Within this context chemical impact and partially saturated conditions are studied in a combined way to cover a wide range of in-situ states induced by underground construction, ventilation and diffusion of a large amount of sodium nitrate (NaNO3) (contained in the bituminised waste). Statically compacted specimens are prepared at different sodium nitrate concentrations or are exposed, at constant vertical stress, to sodium nitrate salinisation or pore water dilution. An aggregated structure with a double porosity network is induced on salinisation and subsequent compaction even at high water contents. An increase in pore fluid concentration enhances the aggregation of the microstructure and reduces the liquid retention properties of the material. Results of oedometer tests performed under constant matric suction and increasing osmotic suction show shrinkage of the sample, a decrease of the compressibility and an increase in the preconsolidation stress. Shear strength results have been interpreted using a single constitutive stress (average skeleton stress) to take into account matric suction and chemical effects on degree of saturation and on microstructural changes. A unique linear shear strength failure envelope has been obtained for the tests performed at different osmotic and matric suctions. The modification of the liquid retention behaviour of the material induced by salinisation has proven to be a crucial aspect for the interpretation of the obtained results.

A Study on Tensiometer Measurements in Salt Laden Soil Used for Irrigation Scheduling

Tensiometer is one of the most popular and simple instrument for in situ and laboratory measurement of negative pressure or suction in soils. Such measurements in the top soil or root zone are useful for automated optimized irrigation scheduling for different vegetation. The top soil accommodates various salts/contaminants whose main source is storm water runoff, irrigation water and application of crop fertilizers. These salts accumulate in the soil due to evaporation. It is an established fact that tensiometer measures only soil matric suction with the help of a ceramic interface. It is believed that osmotic effect of salts present in the soil does not influence tensiometer measurements. However, there is no systematic experimental verification in the literature to understand the influence of salt laden soil on tensiometer measurements. It needs to be ascertained that the pore size of the tensiometer ceramic will not exhibit semi-permeable membrane characteristics leading to osmotic effect. Such verification is important as it would influence the irrigation scheduling of crops. It is also of interest to know whether the osmotic suction (due to the salts) would increase the water retention characteristics and hence the irrigation requirement. If so, then the tensiometer based irrigation scheduling would be an underestimate of the actual irrigation requirement of crops. To investigate these issues, the present study has performed controlled laboratory suction measurements using tensiometer in a locally available soil, contaminated with known concentration of inorganic salt solution. The total suction of the same salt laden soil has been measured using the WP4 dew point potentiameter technique. The results indicate negligible influence of salts present in the soil on tensiometer measurements. The study demonstrates the incapability of tensiometer to assess the osmotic component of salt present in the soil. This would result in the under prediction of water retention and hence irrigation requirement of the soil.

An experimental and constitutive investigation on the chemo-mechanical behaviour of a clay

Engineering issues for which the understanding of the chemo-mechanical behaviour of soils is relevant include wellbore stability problems, the salinification of groundwater, and nuclear waste storage. However, despite the vast number of situations in which couplings between chemistry and mechanics occur, the available constitutive models rely on limited experimental evidence. This paper presents the results of an experimental programme on the chemo-mechanical behaviour of a non-swelling illite. The osmotic suction is controlled through the ion concentration of sodium chloride in the pore water. Stress paths include mechanical loading at a constant osmotic suction, and an increasing osmotic suction at a constant mechanical stress. The experimental results point out a correlation between the osmotic suction and initial oedometric modulus, as well as between the osmotic suction and yield stress. A constitutive framework for soils is extended to take the observed chemo-mechanical couplings into account. The numerical model has been calibrated for the illite using the parameters obtained through the tests under mechanical loading at a constant concentration, and validated using more elaborate stress paths. The presented experimental and constitutive investigation builds a basis for the assessment of engineering issues in which pore liquid chemistry plays a major role.

A chemo-mechanical constitutive model accounting for cation exchange in expansive clays

The paper presents a chemo-mechanical model for expansive clays that takes into account the effects of cation content and cation exchange. These factors play a key role in the mechanical behaviour of very active clays, particularly with regard to volumetric behaviour. The model is based on an existing double-structure formulation that distinguishes specifically between microstructure and macrostructure. Chemical effects are defined at the microstructural level, the seat of the basic physico-chemical phenomena affecting highly swelling clays. The microstructural model accounts for changes in both osmotic suction and in cation content. Microstructural strains are considered to be reversible; material irreversibility arises from the interaction between the two structural levels. The formulation is developed for general unsaturated conditions; saturation is considered as a limiting case. The model is successfully applied to the reproduction of experimental behaviour observed in oedometer tests on saturated bentonite subjected to chemo-mechanical loadings, and in hydration tests of unsaturated bentonite performed using different solute concentrations.

Double-structure effects on the chemo-hydro-mechanical behaviour of a compacted active clay

This work presents an insight into double-structure effects on the coupled chemo-hydro-mechanical behaviour of a compacted active clay. In the first part, selected pore size distribution curves are introduced, to highlight the influence of solute concentration on the evolution of the microstructure of compacted samples. An aggregated structure with dual-pore network is induced by compaction even at relatively high water contents. This structural arrangement is enhanced by salinisation, and has a notable influence on transient volume change behaviour – that is, the occurrence of different stages of swelling upon pore water dilution and higher volume change rates upon salinisation. A coupled chemo-hydro-mechanical model, taking into consideration double-structural features from a chemo-mechanical viewpoint, is described and then used to interpret these behavioural responses and present complementary information on local transient processes. The model is designed to identify an intra-aggregate and an inter-aggregate domain, and assigns different values of hydraulic pressure and osmotic suction to each domain. Distinct constitutive laws for both domains are formulated, and the flow of salt and water between the two domains is accounted for by a physically based mass exchange term. The model is used to simulate salt diffusion tests run in an oedometer at constant vertical stress. Parameters used in the formulation are calibrated based on separate experimental evidence, both through direct test results and through back-analyses of laboratory experiments.

Osmotic Swelling and Osmotic Consolidation Behaviour of Compacted Expansive Clay

Compacted clay soils are used as barriers in geoenvironmental engineering applications and are likely to be exposed to salinization and desalinization cycles during life of the facility. Changes in pore fluid composition from salinization and desalinization cycles induce osmotic suction gradients between soil–water and reservoir (example, landfill/brine pond) solution. Dissipation of osmotic suction gradients may induce osmotic swelling and consolidation strains. This paper examines the osmotic swelling and consolidation behaviour of compacted clays exposed to salinization and desalinization cycles at consolidation pressure of 200 kPa in oedometer assemblies. During salinization cycle, sodium ions of reservoir fluid replaced the divalent exchangeable cations. The osmotic swelling strain developed during first desalinization cycle was 29-fold higher than matric suction induced swelling strain of the compacted specimen. Further, the diffusion controlled osmotic swelling strain was 100-fold slower than matric suction-driven swelling process. After establishment of ion-exchange equilibrium, saturated saline specimens develop reversible osmotic swelling strains on exposure to desalinization cycles. Likewise the saturated desalinated specimen develops reversible osmotic consolidation strains on exposure to cycles of salinization. Variations in compaction dry density has a bearing on the osmotic swelling and consolidation strains, while, compaction water content had no bearing on the osmotic volumetric strains.

The Effect of Contact on the Filter Paper Method for Measuring Soil Suction

Abstract The filter paper method is one of the most commonly used and critiqued techniques for measuring soil suction. However, many aspects related to its use still require some clarification. The results of a comprehensive study on the effect of the contact between the soil grains and soil water and the filter paper are presented herein. We investigated the influence of the equilibration time, the texture of the porous material and the degree of contact, or lack thereof, between the soil grains and the filter paper using Whatman #42 and three different types of porous material. To enhance the difference between the total suction and the matrix suction, osmotic suction was induced by saturating the specimens with a sodium chloride solution.

Revisiting total, matric, and osmotic suction in partially saturated geomaterials

An accurate quantification of negative pore pressure (commonly referred to as ‘suction’) in the pore network is necessary for modeling the mechanical response of unsaturated geomaterials. Traditional definitions and formulations of total, matric, and osmotic suction suggest incorrect pore fluid pressures under certain conditions. In this paper, the notion of suction is revisited by deriving an expression for pore fluid pressure in a simple osmotic, capillary tube using the framework of mixture theory in conjunction with the fundamental laws of thermodynamics. Based on the derived expression for the tube, expressions are derived for total, matric, and osmotic suction for partially saturated geomaterials. Particular attention is given to osmotic suction since confusion regarding its mechanisms has apparently contributed to its misapplication in geomechanics. The new expressions derived herein adequately explain behavior that is incorrectly explained by the traditional formulations and unifies two approaches to modeling osmotic suction previously considered to be in contradiction.

Plastic shrinkage cracking of concrete − roles of osmotic suction

Plastic shrinkage cracking of concrete occurs when the stresses arising in the concrete, due to a combination of suction and restraints of deformation such as reinforcement or formwork, equal its strength. However, three different types of suctions should be distinguished, namely total, matric and osmotic suctions. Although the total suction comprises matric and osmotic suctions, it is often used interchangeably with matric suction, with the underlying unconfirmed assumption that either the osmotic suction or its effect is negligible. In this paper, after a discussion of the pore moisture suctions and strength of unsaturated early-age concrete, experimental investigations of the suctions arising in, and the tensile strength and shear strength of, fly ash mixed with solutions of different osmotic suctions are described. It was found that osmotic suction has negligible effect on the shear and tensile strength, and hence, by inference, the inter-particle stresses in the fly ash mixture and early-age concrete. This strongly suggests that the role played by osmotic suction in the plastic shrinkage cracking of concrete is minimal and, accordingly, justifies the focus of earlier researchers on matric suction only.

Critical Review of the Methodologies Employed for Soil Suction Measurement

Modeling the behavior of unsaturated soils necessitates the measurement of soil suction and the establishment of its variation with the water content, which is commonly known as the soil-water characteristic curve (SWCC). Several methodologies have been developed for measuring either total suction ψ (sum of matric suction ψm and osmotic suction ψo) or ψm. While employing different methodologies for suction measurement, there is a possibility that various factors (viz., type of the soil, measurement methodology, range of the suction measurement, equilibration time, and presence of salts or contaminants in the soil) may influence the results and hence the SWCC. Therefore, it is essential to investigate the uniqueness of SWCC, determined by using some commonly adopted suction measurement methodologies. This study indicates that the SWCC established by adopting different methodologies may not be unique and is primarily influenced by the range of suction measurement. As such, it is essential to highlight the range of suction values involved for establishing the SWCC, to facilitate unambiguous modeling and to precisely understand the behavior of unsaturated soil.