Vapour Equilibrium Technique Research Articles

Study on the effect of pore structure changes induced by freeze-thaw-wetting-drying cycles on water retention characteristics of compacted loess

Deterioration of earthworks (including compacted soil collapse and loss of suction) as a result of climate change has had a significant negative impact on engineering. To explore the water retention characteristics and microstructural evolution of loess after freeze-thaw-wetting-drying (FTWD) cycles. In this paper, the soil water characteristic curve (SWCC) of unsaturated loess in the full suction range is obtained by using the filter paper method (FPM) and vapor equilibrium technique (VET). Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were used to test the two-dimensional images and pore size distribution (PSD) curves of the specimens. The results of the study showed that the SWCC of compacted loess before and after FTWD cycles showed an unimodal pattern. Compacted loess has inter-aggregate pores and intra-aggregate pores, and the PSD curves show a bimodal pattern. The FTWD cyclic action reduces the water retention capacity of loess in the low to medium suction range (0kPa-288kPa). Inter-aggregate pore porosity decreases with an increasing number of FTWD cycles. Finally, to easily predict the SWCC of loess under the condition of pore variation, a prediction model was established in the double-porosity media framework, considering the changes of inter-aggregate pores and intra-aggregate pores. The rationality and applicability of the present model were verified by experimental data. The present model can reasonably predict the water retention properties of compacted clays with unimodal or bimodal type SWCC under microscopic pore changes, which is suitable for the engineering application of compacted clays.

Strength and dilatancy of an unsaturated expansive soil at high suction levels

A series of suction-controlled triaxial tests was conducted on Nanyang expansive clay to investigate the effects of dry density and suction on dilatancy and strength. The suction of the soil samples was controlled using a vapour equilibrium technique, with four suction levels ranging from 3.29 MPa to 198.14 MPa, where water retention is dominated by adsorption. The experimental results show that the tested soil exhibits a brittle failure mode under high suction, significantly distinguishing the hydro-mechanical behaviour of the soil at high suction from that observed at low suction. This brittle failure mode significantly increases the contribution of suction to peak strength compared to residual strength, causes the soil to fail before reaching the critical state, a phenomenon not observed in soils under high suction, and results in dilatancy caused by damage to the soil particle aggregates rather than particle rearrangement. The dilatancy data obtained from the triaxial tests reveal that significant soil dilatancy occurs during shear after reaching peak strength, with the maximum dilatancy angle increasing with suction and decreasing with confining pressure. However, the initial dry density has a negligible impact on the soil's dilatancy under high suction levels. This observation further supports that, for unsaturated soils under high suction levels, dilatancy is attributed to damage to soil particle aggregates rather than the rearrangement of soil particles.

Thermal effects on the strain rate-dependent behavior of highly compacted GMZ01 bentonite

Investigation of thermal effects on the strain rate-dependent properties of compacted bentonite is crucial for the long-term safety assessment of deep geological repository for disposal of high-level radioactive waste. In the present work, cylindrical GMZ01 bentonite specimens were compacted with suction-controlled by the vapor equilibrium technique. Then, a series of temperature- and suction-controlled stepwise constant rate strain (CRS) tests was performed and the rate-dependent compressibility behavior of the highly compacted GMZ01 bentonite was investigated. The plastic compressibility parameter λ, the elastic compressibility parameter κ, the yield stress p0, as well as the viscous parameter α were determined. Results indicate that λ, κ and α decrease and p0 increases as suction increases. Upon heating, parameters λ, α and p0 decrease. It is also found that p0 increases linearly with increasing CRS in a double-logarithm coordinate. Based on the experimental results, a viscosity parameter α(s, T) was fitted to capture the effects of suction s and temperature T on the relationship between yield stress and strain rate. Then, an elastic-thermo-viscoplastic model for unsaturated soils was developed to describe the thermal effects on the rate-dependent behavior of highly compacted GMZ01 bentonite. Validation showed that the calculated results agreed well to the measured ones.

Critical state behaviour of an unsaturated kaolin mixture

In this paper, a series of suction-controlled triaxial tests were conducted to investigate the hydromechanical behaviour of an unsaturated kaolin mixture. The studied variables included the mean net stress, suction, and initial dry density. The suction was controlled over a wide range – in the span of 0–390 MPa using the axis translation method and the vapour equilibrium technique, aiming to holistically study the mechanical response of unsaturated soils. The discussion focused on the critical state behaviour and dilatancy characteristics of the tested soil. The experimental results indicate that the ductility of the soil decreases and shifts towards brittleness with increasing suctions. The critical state stress ratio (Ma) of the soil exhibits a significant increase with suction, and this increase becomes more pronounced with higher initial dry density. Conversely, the impact of suction on the critical state line in the specific volume-mean net stress plane (v-lnpnet plane) diminishes with higher initial dry density. Furthermore, suction notably influences the stress-dilatancy relationship of kaolin mixtures, while the effect of confining pressure appears to be marginal. The influence of suction on the stress-dilatancy relationship primarily stems from its impact on the critical stress ratio (Ma), which can be normalized by dividing the stress ratio by the critical state stress ratio. Lastly, the stress-dilatancy equation of the modified Cam-clay model is extended to capture the effect of suction on the dilatancy characteristics of unsaturated soils.

Structural evolutions of MX80 bentonite pellet/powder mixtures under wetting and suction-controlled oedometer loading

The compression behaviour of MX80 bentonite pellet/powder mixture was investigated by performing suction-controlled oedometer tests. The suction from 113 to 4.2 MPa was imposed by vapour equilibrium technique (VET), and zero suction by water circulation (WC). After instantaneously unloaded from various target vertical stresses, the structural observation was conducted by mercury intrusion porosimetry and micro-computed tomography. Results showed that during wetting, the intra-grain micro- and macro-pores increased due to the swelling of pellets and powder grains. During loading, the variation of intra-grain micro-pores was characterised by the loading effect, while that of intra-grain micro-pores was controlled by the suction and loading effects. The inter-grain pores, for the VET samples, were reduced by the swelling of pellets and powder grains during wetting on one hand, and by the movement of pellets and powder grains during loading on the other hand. For the WC samples, most inter-grain pores were closed during wetting, with a few pores in the top closed by further loading. The global compression of bentonite mixture samples during loading was mainly governed by the filling of inter-grain pores for the VET samples, but controlled by the compression of intra-grain pores for the WC samples.

Study on the Effect of Soil Type and Salinity on Water–Soil Characteristic Curves of Saline Soils Based on the Vapor Balance Technique

The soil–water characteristic curve (SWCC) describes the relationship between water content and soil suction, which is an important tool for determining the engineering properties of unsaturated soil. By measuring SWCC through vapor equilibrium technique, which could obtain soil suction up to 309 MPa, this paper explored the influences of the soil type and salt content on SWCC of saline soil. The experimental results illustrate that fine‐grained saline soil had a good water‐holding capacity, which had a higher air entry value and slower dehydrate rate. The study also brings out the observation that salt had negligible influence on soil suction and parameters effecting SWCC in low suction range; however, this effects was slight until it disappeared with the suction increasing.

An experimental study on the measurement of gas diffusivities in un-saturated clay based materials

Gas migration in porous media in the context of nuclear waste disposal is dominated by diffusion. The availability of abundant data on diffusion coefficients of different gases in many clay based materials such as Boom Clay, Opalinus Clay and Callovo-Oxfordian Clays provides a good overview of the rates of gas transport through these different clay rocks [1]. Such formations are being considered as potential host rocks for the disposal of high and intermediate level radioactive wastes across Europe. Diffusion coefficients have been studied with particular interest due to the unavoidable gas generation phase in the nuclear waste repository [2]. However, all the diffusion measurements performed so far have been under fully saturated conditions, with little experimental data on gas diffusivities in partially saturated conditions, which have been hypothesized to exist during the early part of the life cycle of the repository [3]. The objective of the current study is to provide an overview of the effect of desaturation on the rate of diffusive transport of gases in partially saturated clay based materials.
 The experimental methodology is based on the double through diffusion setup used in previous studies to measure gas diffusivities in saturated clays [1]. However, the setup is modified so as to keep a fixed level of unsaturation in the clay sample. This is done by employing the vapour equilibrium technique, using oversaturated salt solutions in both upstream and downstream reservoirs to maintain a constant relative humidity in the entire setup [4].
 Preliminary pore network modelling has already shown that the gas diffusivity increases with a decrease in water saturation of the clay material. However, the predicted trend also suggests that this increase is only marginal and should not normally exceed an order of magnitude. This is shown in the figure below.
 All the diffusion measurements are carried out on synthetic clay samples of dimensions 3.8 cm (diameter) and 2 cm (height). In Table 1, the Sw stands for water saturation. The compositions of the synthetic clay samples (60% clay, 20% sand, 20% silt) have been adjusted so as to most closely mimic the mineralogical compostion of Boom Clay. Synthetic samples are used instead of natural samples like Boom Clay because of their better physical response to drying than Boom Clay.
 The selected control variable to lower the saturation of the material is suction, which is imposed by changing the relative humidity of the environment using oversaturated salt solutions [4]. Suction can be kept constant given constant ambient temperature and pressure. Thus, the clay sample is conditioned at a certain suction, which in turn leads to desaturation in the range of 70-100% of total saturation depending on the level of suction. For this study, the saturation range has been fixed in between 70-100% of total saturation. This range has been chosen because of what is expected in a real life nuclear waste repository. The first measured diffusion coefficients in two partially saturated samples are shown in Table 1. The initial measurements of gas diffusion coefficients in unsaturated synthetic clays show that the increase in diffusivity of helium is higher than that of argon. Helium is the second lightest known element and has an extremely small molecular size and hence the impact of desaturation on helium diffusivity could be more pronounced than the same for heavier gases.

Soil–water characteristic curve of unsaturated collapsible soils

AbstractCollapsible soils are almost found in unsaturated states and involved significant engineering problems. Geotechnical challenges of such soils are represented by the hydro-mechanical behaviour during wetting–drying cycles due to the humidity and climate conditions. The main objective of this paper is to investigate the soil–water characteristic curve (SWCC) of unsaturated collapsible soils. In this study, three types of collapsible soils were investigated such as natural soils of sandy gypseous, silty loess, and artificial soil of gypsum–sand mixture. Determination of soil–water characteristic curve represented by wetting and drying paths has been done using a combination of the axis-translation technique (i.e. pressure plate device) and vapour equilibrium technique (i.e. salts solution desiccators) to cover a wide range of applied suction. The test results show that the air-entry value for all soils occurs at a very low suction range. At the boundary effect zone, the coarse grain size of the soil mass cannot hold the water molecules in the pore space, even with a low value of imposed suction. Moreover, the amount of hysteresis varied based on the geological formation and homogeneity of the soil fabric. Furthermore, SWCC has been interpreted by insignificant volume change and a slight reduction in void ratio, especially at high applied suction.

Impact of sand on the water retention properties of a bentonite/sand mixture by NMR characterization

Blocks of compacted bentonite/sand mixture, with a 40/60 proportion in dry mass, are being studied as an elementary component in the construction of seals in the French concept for high-levelradioactive waste disposal. In the disposal, the seal will be subjected to water infiltration from the host rock. Water retention behaviour in the mixture is governed by bentonite. Nevertheless, the impact of the sand fraction, especially at high concentrations, is still unclear. The water retention curve is found experimentally for the wetting path by using vapour equilibrium technique. Moreover, NMR relaxometry tests executed atfull saturation state are presented. As a result of the comparison with experimental data reported in the past, for different mixtures -ranging from pure bentonite to 20/80 bentonite/sand mixtures-, it is concluded that all porosity in the mixture can be correctly attributed to bentonite. This conclusion simplifies mixtures to equivalent pure bentonite specimens with a dry density equal to the bentonite dry density (𝜌𝑑,𝑏) of the mixture. Moreover, an original method to compare NMR relaxation times for different bentonite/sand -or pure bentonite- mixtures is proposed. It provides the longitudinal relaxation time (T1) in saturated mixtures as a function of 𝜌𝑑,𝑏 and the montmorillonite concentration in bentonite.

Investigation of water-retention and uniaxial compression behaviour of MX80 bentonite pellet at various suctions

MX80 bentonite pellet/powder mixture has been regarded as a candidate sealing material in deep geological high-level radioactive waste disposal. As single pellet plays an important role in the global hydromechanical behaviour of the mixture, a series of single pellet compression tests were conducted on cylindershape MX80 bentonite 32-mm pellet after reaching suction equilibrium. The changes in volume and water content during the suction equilibration with the vapour equilibrium technique allowed the water-retention properties to be investigated. X-ray μCT observations were also made on pellets after reaching the target suctions. Results showed that upon drying the water content, void ratio and volumetric strain decreased slightly, but increased significantly upon wetting. Moreover, upon drying the network of cracks presented insignificant change and the cracks were mainly located in the marginal areas of pellet, while on wetting, cracks substantially propagated from the marginal areas to the core of pellet. The uniaxial compression tests on single pellets showed that the axial strain εaxial contributed to the contractancy, while the lateral strain εlateral to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young’s modulus E were found to decrease with suction decreasing, evidencing the suction effect through softening and cracking. Because of the combined effect of suction and wetting-induced cracks, the Poisson’s ratios v increased until the suction decreased to 38-25 MPa, and then decreased with further suction decrease. Uniaxial loading closed the horizontal wetting-induced cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

Elastic and cracking behaviour of MX80 bentonite pellet at various suctions in uniaxial compression

MX80 bentonite pellet/powder mixture has been considered as a candidate sealing/filling material in geological high-level radioactive waste disposal. As a single pellet can play an important role in the global hydro-mechanical behaviour of the mixture, the compression behaviour of a single MX80 pellet was investigated by performing uniaxial compression tests on cylinder-shaped pellets at different initial suctions, imposed by the vapour equilibrium technique. Large cracks were observed at suctions ≤ 9 MPa. Results from the compression tests showed that the axial strain εaxial contributed to the contractancy, whereas the lateral strain εlateral contributed to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young's modulus E were found to decrease with decreasing suction, providing evidence of the suction effect through softening and cracking. CI was considered as the inflection point of Δεlateral and the onset of dilatancy. Because of the combined effects of suction and wetting-induced cracks, the Poisson's ratios v increased until the suction decreased to 38–25 MPa, and then decreased with further suction decrease. Loading closed the wetting-induced horizontal cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

Soil–water characteristic curves and hydraulic conductivity of Gaomiaozi bentonite pellet-contained materials

The bentonite pellet-contained material (PCM) is a desired joint sealer for high-level radioactive waste repositories. During the operation of the repository, however, the PCM is unsaturated for most of the period. The water retention capacity and permeability of PCM can thus directly affect the bentonite buffer barrier seepage, nuclide migration, and joint healing. Moreover, the performance of PCM can be strongly influenced by the particle size of bentonite pellets and dry density. In this work, the pressure plate method and vapor equilibrium technique were both utilized to test the soil–water characteristic curves (SWCCs) of the PCMs with varying particle sizes and dry densities. The unsaturated hydraulic conductivity of the PCMs was then predicted by combining the SWCC model and saturated hydraulic conductivity. The results showed that in the low suction range (20–1150 kPa), the dry density and particle size are anti-correlated with the water content. In the high suction range (4200–309,000 kPa), the SWCC is almost insensitive to the variation of dry density and particle size. The Gardner model is suitable for describing the SWCC of PCM. In addition, the hydraulic conductivity of the PCM increases with the rise in particle size, while it decreases as the dry density grows. Finally, the influence mechanism of the SWCC and hydraulic conductivity was further discussed based on the scanning electron microscopy images and pore size distribution curves.

The application of vapour equilibrium technique to control suction to study the shrinkage and water retention of compacted Claystone-Bentonite mixtures

Understanding soil shrinkage and retaining water is essential for learning more about the possibility of cracking of liner. Moreover, the factors that influence it are also important to know to improve the material tested as hazardous waste liners. The vapour equilibrium technique is widely used to control suction of compacted soils experiencing drying-wetting phenomena. It is considered to be inexpensive, simple, and has the ability to adequately control the suction applied to soil samples. This paper, therefore, describes its application in studying the shrinkage and water retention in compacted claystone-bentonite mixtures. This involved using five saturated salt solutions including potassium sulphate (K2SO4), potassium chloride (KCl), sodium chloride (NaCl), potassium carbonate (K2CO3), and magnesium chloride (MgCl2.6H2O). The sample was allowed to be in equilibrium with the relative humidity salt solution and a calliper was used to measure the dimensions every day up to when this was achieved. The results showed the bentonite in the mixture affects the amount of shrinkage and water retention while the sample’s initial moisture content was also found to be very influential on the magnitude of the primary and residual shrinkage. Moreover, the sample’s ability to hold water was almost the same without differentiating the initial water content at a total suction of more than 41084.91 kPa.

Water retention characteristics of unsaturated bentonite–sand mixtures under controlled-temperature

Unsaturated bentonite–sand mixtures (BSMs) are accepted generally as ideal buffer/backfill materials in geologic repositories because of its excellent performance. Water retention characteristics are the important factor that affect the engineering properties of BSMs, but little attention has been devoted to that of Gaomiaozi (GMZ) BSMs under controlled-temperature in high suction range. This study evaluated the soil–water characteristic curves (SWCCs) of compacted GMZ BSMs using the vapour equilibrium technique with different sand content and temperature under unconfined conditions. It was found that the sand grains size had a negligible effect on the SWCCs in the high suction range. As the temperature increased, the decreasing adsorption and the increasing evaporation capacity promoted the decrease of water content. The BSMs changed from the compact structure to a loose structure with increasing sand content, which strengthened the flow of vapour and evaporation of water from bentonite and leaded to an observable reduction of water retention characteristics of GMZ BSMs. We presented a new empirical equation for unsaturated BSMs in the high suction rang considering effects of temperature and sand content, and that processed the data more efficiently and described satisfactorily the effects of temperature and sand content on the SWCCs of GMZ BSMs.

Hysteresis of the water retention curves of geosynthetic clay liners in the high suction range

This paper examines two needle-punched geosynthetic clay liners' water retention behaviour at high suction ranges using the vapour equilibrium technique where super-saturated salt solutions controlled the relative humidity. This study shows that the bentonite form and its mineralogy affect the absorption/desorption of GCLs and their corresponding water retention curves. In particular, a granular bentonite-based GCL was found to absorb more and release less water than a powdered bentonite-based GCL due to its higher montmorillonite content and larger pores. The water retention curves of both GCLs exhibited very little hysteretic behaviour at high suction. Repeated wetting-drying cycles shifted the WRCs of both GCLs slightly downward with minimal impact on their degree of hysteresis.

Effects of various relative humidity conditions on copper corrosion behavior in bentonite

Bentonite is low resistance water retaining conductive clayey material often used as grounding enhancement material, particularly in a troubled environment. Due to its unique property, bentonite retains water and dissipate unwanted electrical current to ground. However, bentonite's water retention capacity would inevitably induce corrosion to the grounding electrode and may jeopardize the grounding system's performance in the long run. This study presents bentonite properties on copper electrodes' corrosion behavior. In this study, copper corrosion behavior buried in Andrassy bentonite specimens was investigated under varying relative humidity conditions using the vapor equilibrium technique. The corrosion rate was calculated at equilibrium, and FESEM later characterized the corroded surfaces. The copper corrosion rate in Andrassy bentonite was found to be between 2.2 and 2.8 × 10-3 mm/year. Test results showed that more water might not necessarily induce and expedite copper corrosion. However, it was noted that copper corrosion was somewhat limited under extremely dry conditions and very wet conditions. On the other hand, RH = 75% was the optimum copper corrosion condition to occur. It is believed that the corrosion of copper specimen may be influenced by other complex mechanism presence of such as chemicals species and microbes contained within the bentonite and surrounding environment.

Osmotic Oedometer Using Sodium Chloride Solution and Reverse Osmosis Membrane

Abstract There are two suction application techniques in laboratory testing of unsaturated soils: (1) the axis-translation technique in which pore-air pressure is elevated above atmospheric pressure and pore-water pressure is at an atmospheric or a small positive value to give a matric suction and (2) using salt solutions. Salt solutions can be used to control the relative humidity of the air surrounding the soil specimen in the vapor equilibrium technique or as an osmotic suction applied through a semipermeable membrane in the osmotic technique. The salt solution in the vapor equilibrium technique applies a total suction, whereas the salt solution in the osmotic technique induces a matric suction on the soil specimen. Unlike the axis-translation technique and vapor equilibrium technique, the osmotic technique does not require a sealed chamber, but it is not as popular as the axis-translation technique and vapor equilibrium technique because the commonly used poly(ethylene glycol) solution and cellulosic membrane are susceptible to degradation during long-duration tests. This paper investigates the osmotic technique using sodium chloride solutions and a reverse osmosis membrane in a conventional oedometer. The performance of the updated osmotic oedometer was evaluated using kaolin–bentonite specimens. A kaolin–bentonite specimen was first mechanically consolidated, and thereafter the water within the base of the oedometer was replaced with sodium chloride solution. The results show that osmotic technique causes the soil specimen to be compressed further. However, the measured matric suction of the soil specimen at equilibrium is less than the applied suction. The difference is attributed to the membrane resistance effect. The updated osmotic oedometer is attractive, as it eliminates the drawbacks of the previous osmotic oedometer, and it can theoretically apply matric suction as high as 5,000 kPa.

Simple Testing Method for Measuring the Triaxial Stress-Strain Relations of Unsaturated Soils at High Suctions

Abstract Unsaturated soils at high suctions are widespread in arid and semiarid areas. Test data on the mechanical behavior of unsaturated soils are scarce at high suctions. In this article, the effect of void ratio or density on the soil-water retention behavior was discussed on the basis of measured data of various compacted soils at high suctions. According to the test data, the soil-water retention behavior is independent of the dry density or void ratio over a high suction range, namely, if the water content of a soil specimen is kept constant, the suction will be constant and independent of the void ratio (deformation) at high suctions. Therefore, a simple testing method for measuring the stress-strain relations of unsaturated triaxial specimens at high suctions is proposed by combining the vapor equilibrium technique (VET) with saturated salt solutions and the triaxial shear tests at constant water contents. First, a known initial high suction is imposed on the specimen by using the VET with a saturated salt solution. Then, the specimen is triaxially sheared under a constant water content, thereby resulting in a constant high suction during triaxial shearing. The proposed testing method can substantially shorten the testing period for one triaxial test. Finally, the results of triaxial shear tests on silt, silty clay, and expansive soil at high suctions obtained by the proposed testing method are presented.

Effect of Temperature on Hysteretic Behavior of Water Retention Capacity for Clay–Sand Liners

The effect of temperature on the hysteretic behavior of water retention is of significant importance for unsaturated clays used as barriers for high-level radioactive waste disposal or as cover layers in waste management facilities. This study aims at investigating the effect of ambient temperature on the hysteretic behavior of water retention curves of sand-expansive clay mixtures that can be used as barriers. Two specimens consisting of mixtures of sand and expansive clay with clay contents of 15% and 30% and a specimen of pure expansive clay are considered in the study. To identify the possible thermal effects, drying–wetting soil–water retention curves for these soils are determined at selected ambient temperatures of 7, 24, and 47 °C. The vapor equilibrium technique is used to cover a wide range of suction. Different saturated salt solutions and concentrations are prepared and calibrated under nominated temperatures. The area of the hysteresis enclosed between the drying and wetting curves is calculated and used as a measure of the degree of hysteresis induced under different temperatures. The study indicates that ambient temperature has a significant impact on the hysteretic behavior of soil–water retention curves. The degree of hysteresis between drying and wetting curves tends to decrease linearly with an increase in the temperature for the investigated range of 7–47 °C.

Water retention behaviour of tailings in unsaturated conditions

Tailing dams are complex geotechnical systems comprising of an embankment and a basin containing the waste products from the mining processes. These structures are characterized by a wide surface exposed to the atmosphere whose interaction governs the position of the phreatic surface within the basin. A detailed knowledge of the hydro-mechanical properties of the tailings is fundamental to reliably assess the stability of the tailing dams. While most of the previous studies have dealt with the response of tailings in saturated conditions, this research provides an extension of the hydraulic behaviour in unsaturated and nearly saturated state of tailings collected after the failure of the Stava basins. The hydraulic behaviour in unsaturated conditions was investigated by means of tests where the suction was imposed and the water content was monitored (axis translation technique and vapour equilibrium technique), and tests where the water content was imposed and the suction was measured with psychrometer (dew point method). To account for the in-situ heterogeneity of tailings, the dependency of the water retention relationship on the grain size distribution, the preparation technique and on the initial density / void ratio was studied. Denser tailings showed a higher water retention behaviour than that given in looser specimens. Similarly, the increase of the fine content was demonstrated to improve the water retention capability. As for standard soils, also statically compacted Stava tailings reveal lower retention capability than the slurry samples, thus confirming the importance of the preparation method in determining the hydro-mechanical response of such soils.