Yu-Jun Cui

Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant-volume and free-swell conditions

Abstract Highly compacted sand–bentonite mixtures are often considered as possible engineered barriers in deep high-level radioactive waste disposals. In situ, the saturation of these barriers from their initially unsaturated state is a complex hydro-mechanical coupled process in which temperature effects also play a role. The key parameter of this process is the unsaturated hydraulic conductivity of the barrier. In this paper, isothermal infiltration experiments were conducted to determine the unsaturated hydraulic conductivity according to the instantaneous profile method. To do so, total suction changes were monitored at different locations along the soil specimen by using resistive relative humidity probes. Three constant-volume infiltration tests were conducted showing, unexpectedly, a decrease of the hydraulic conductivity during infiltration. One test performed under free-swell conditions showed the opposite and standard trend. These observations were interpreted in terms of microstructure changes during wetting, both under constant-volume and free-swell conditions.

Thermo-Hydro-Mechanical Behaviour of Compacted Bentonite

Like every human activity, the nuclear activities generate waste. Because of the particular characters of the nuclear waste, a special geological formation and also plug material for storage of the high level nuclear waste is necessary.

Application of Vertical Strain Control to Measure Swelling Pressure of Clayey Soils

Since the use of bentonite based engineered barriers has been considered for nuclear waste disposal facilities, swelling pressure measurement techniques have been improved. However, some experimental evidence shows that vertical strain control plays an important role on such measurements. In this paper, some results are presented to show that strain control is a key aspect to be considered when measuring swelling pressure.

A New Apparatus for the Measurement of Swelling Pressure Under Constant Volume Condition

A new constant volume cell was developed, allowing the measurement of swelling pressure without any strain adjustment and any effect of the stiffness of the testing device. By employing this new cell, the swelling behavior of compacted soil specimen under different suctions (57, 38, 9 and 0 MPa) was investigated. The results show good repeatability, indicating the reliability of the new cell and validating the test procedures adopted. Moreover, the developed cell is quite convenient to study the long-term swelling behavior of soil since no load adjustment is necessary. The obtained results show that, during the progressive wetting by applying successively the suctions of 57, 39, 9 and 0 MPa, the swelling pressure increases to 0.17, 0.31, 0.46 and 0.89 MPa, respectively. The swelling pressure and the time required to reach equilibrium are function of suction. Vapour-wetting and water-wetting show different hydration mechanisms and result in different swelling behavior: the swelling pressure develops slowly and gradually to finally reach stabilization upon vapour-wetting, while it increases quickly to a peak value and followed by a small decrease upon water-wetting.

Experimental evidences on thermo-hydro-mechanical coupling in engineered clay barrier for deep nuclear waste disposal

In the context of using engineered clay barrier in deep nuclear waste disposal, an experimental work was carried out to observe the behavior of compacted swelling clay under thermo-hydro-mechanical (THM) loading. Firstly, the measurement of thermal conductivity has allowed the elaboration of a theoretical model to estimate the thermal conductivity of unsaturated soils. After an evaluation study on the proposed model and three existing models found in the literature, it was concluded that the thermal conductivity of unsaturated soil could be correlated with only one parameter : the air volume fraction. Secondly, the water retention curve, that influences the water transfer in unsaturated soil, was determined at different temperatures. The results showed a decrease of water retention capacity due to heating. A simple model based on the interfacial tension air-water was formulated to simulate this effect. Finally, the soil volume change under complex THM loading was studied. Thermo-mechanical phenomena, which are often observed on saturated clay, were equally observed on the compacted unsaturated clay.