Vapour Equilibrium Technique Research Articles

Assessment of the use of the vapour equilibrium technique in controlled-suction tests

This paper presents a study on suction control methodology using the vapour equilibrium technique. To reduce the time needed to reach equilibrium, humid air is forced to flow through the sample or on its boundaries. An analysis of the physical processes that occur in the tested soil samples using a numerical approach is included. The test is numerically simulated and the key parameters of the process calibrated. A sensitivity analysis of the unmeasured parameters and variables is performed. It could be observed that forcing humid air to flow through the sample reduces the equalization time, but the results from the numerical simulations highlight that this flow must be carefully applied to avoid reaching, under steady-state conditions, a different suction than that wished for. When the circulation of air along the boundaries is slow, an increase in air velocity tends to increase the rate of suction changes in the sample. However, if the circulation of air increases over some limit, the flow conditions inside the soil control the development of the process.

Measurements of Unsaturated Hydraulic Conductivity Functions of Two Fine-Grained Materials

Water unsaturated hydraulic conductivity (kw) functions of two remolded fine-grained materials were measured over a wide range of degrees of saturation (Sr) with two methods. The instantaneous profile method (IPM) was used for Sr>50%. An original vapor equilibration method (also known as the vapor equilibrium technique, VET) was used for Sr<50%. Both materials compacted at the standard Proctor optimum water content and maximum density, have saturated hydraulic conductivities (ksat)<10-7 m/s. The VET couples the total soil suction (s) control from desiccators with saturated salt solutions with water mass measurements from a digital laboratory balance. The kw measurements of the two techniques are consistent and complementary. The effect of hysteresis on the kw functions at higher s values was also investigated. The experimental results suggest that the hysteretic effect on the kw-Sr and kw-s relationships cannot be neglected, and that the measured kw are significantly dependent on the initial Sr. The VET tests on the specimens that were initially dried give the lowest values of kw and the tests on the specimens that were initially saturated give the highest values of kw. The relative hydraulic conductivity values are very small (krw<2×10-5) in this saturation range (Sr<50%) for the tested materials.

Vapour Equilibrium and Osmotic Technique for Suction Control

The vapour equilibrium method and osmotic technique have gained widespread acceptance as reliable methods for controlling relative humidity and thereby suction in soil specimens. The ability to impose suction on soil specimens allows for drying and wetting stress paths to be imposed to evaluate resulting changes in strength, deformation and flow characteristics. The two methods presented and discussed in this paper have been adapted for use with a number of traditional laboratory tests including the oedometer, direct shear and triaxial tests. This report provides a summary of some recent developments and knowledge regarding the use of these techniques highlighting the limitations and drawbacks of the methods.

A New Isotropic Cell for Studying the Thermo-Mechanical Behavior of Unsaturated Expansive Clays

Abstract This paper presents a new suction-temperature controlled isotropic cell that can be used to study the thermo-mechanical behavior of unsaturated expansive clays. The vapor equilibrium technique is used to control the soil suction; the temperature of the cell is controlled using a thermostat bath. The isotropic pressure is applied using a volume/pressure controller that is also used to monitor the volume change of soil specimen. Preliminary experimental results showed good performance of the cell.

Evaluation of the transitional inelastic behaviour of unsaturated clay–sand mixtures

This paper examines and compares the mechanical behaviour of two different unsaturated clay mixtures comprised of bentonite clay (Saskatchewan or Wyoming) and quartz sand. The two mixtures have been proposed as compacted barrier materials for reducing groundwater flow in the vicinity of waste disposal repositories. Triaxial specimens were compacted to consistent properties, and then specified suction conditions were applied to the specimens using the vapour equilibrium technique. Following equilibrium at the specified initial suction, specimens were subjected to isotropic and shear loading in a conventional triaxial cell to measure the mechanical response under selected stress paths. The results are interpreted in terms of the yield, strength, and stiffness behaviour at the various suction levels. Results suggest that the clay component of the mixture dominates the behaviour at suctions less than approximately 30 MPa, and the sand component dominates the behaviour above approximately 30 MPa. The transition from clay- to sand-dominated behaviour is attributed to volume strain during application of the initial suction bringing the sand particles into contact. The discussion highlights how the results can be used to modify constitutive models to incorporate the transitional behaviour in numerical modeling.Key words: inelastic, yielding, unsaturated, stress–strain, triaxial testing.

Expansive bentonite–sand mixtures in cyclic controlled-suction drying and wetting

Expansive clay buffers in radioactive waste disposal designs experience cyclic drying and wetting paths during different stages of their design life. Clayey soils subjected to these processes develop swelling and shrinkage deformations, which give rise to the accumulation of compression or expansion strains during suction cycles. Experimental studies were undertaken using oedometer tests on an artificially prepared bentonite–sand mixture (80% bentonite by dry mass). In order to study these processes and to identify the most important features controlling soil behaviour, several wetting–drying cycles with suctions ranging between 130 and 4 MPa were applied using vapour equilibrium technique and covering a wide range of overconsolidation ratios (OCR). The tested samples showed cumulative shrinkage strains along the successive cycles, which became more significant at increasing vertical net stresses (low OCR values). However, no accumulation of expansion strains was detected at elevated OCR values. Test results were interpreted and predicted within the context of an elastoplastic model proposed by Alonso et al., 1999, [Alonso, E.E., Vaunat, J., Gens, A., (1999). Modelling the mechanical behaviour of expansive clays. Engineering Geology, 54, 173–183.] which takes into account the accumulation of strains. A good correspondence between measured soil response and model predictions was observed. The paper also presents the methodology to derive the constitutive parameters.

Controlling suction by the vapour equilibrium technique at different temperatures and its application in determining the water retention properties of MX80 clay

Problems related to unsaturated soils are frequently encountered in geotechnical or environmental engineering works. In most cases, for simplicity, one can study the problems by considering the suction effects on volume change or shear strength under isothermal conditions. Often, under these conditions, a temperature-independent water retention curve is considered in the analysis — obviously a simplification. When the temperature changes are too significant to be ignored, it is necessary to account for the thermal effects. This note presents a method for controlling suction with the vapour equilibrium technique at different temperatures. First, calibration of various saturated saline solutions was carried out at temperatures ranging from 20 °C to 60 °C. A mirror psychrometer was used to measure relative humidity generated by saturated saline solutions at different temperatures. The results obtained were in good agreement with the data from the literature. This information was then used to determine the water retention properties of MX80 clay, which showed that the retention curve shifts downward with increasing temperature.Key words: vapour equilibrium technique, hygrometer, water retention curve, temperature effects, compacted bentonite.