Behavior Of Compacted Bentonite Research Articles

Oedometric compression and thermal volume behavior of compacted Gaomiaozi bentonite saturated with salt solution

Saline solution has considerable influence on the thermo-hydro-mechanical behavior of compacted bentonite. In this study, the effects of NaCl solution on the swelling deformation, oedometric compression, and thermal volume behavior of compacted Gaomiaozi bentonite were investigated. Results show that compressibility of the specimens decreases with increasing NaCl concentration of the presaturation solution. At an overconsolidation ratio OCR=1 with a vertical pressure σv=0.1 MPa and at OCR=3 with σv=0.1 MPa, the specimens exhibited contraction during heating below the transition temperature, followed by dilation. A larger vertical pressure led to a higher transition temperature (at which the thermal strain turns from contraction to dilation). However, the specimens exhibited only thermal dilation at OCR=10 with σv=0.1 MPa and at OCR=2 with σv=0.5MPa. Additionally, for a given vertical pressure and presaturation solution, the specimens with a higher OCR exhibited a larger irreversible dilation after a thermal cycle. Moreover, the influence of the presaturation solution on the thermal volume change is non-monotonic. A dilute solution (NaCl solution of 0.1 mol/L) and high salinity solution (NaCl solution of 1.0 mol/L) markedly promote the thermal expansion. These experimental trends can be attributed to the fact that attenuation of the diffused double layer repulsion and electrostatic interaction as well as the reduction of the free water at high saline conditions harden the soil.

A numerical study on the coupled hydro-mechanical behaviour of compacted bentonite

Understanding the mechanical behaviour of compacted bentonite upon re-saturation is of outmost importance in most designs of nuclear waste disposal repositories. The behaviour of bentonite is characterized by its stress-path dependency and it is typically interpreted on the basis of its microstructural interactions. Up to now, effective stress-based models have had limited success in reproducing consistently the main responses. Here a recently proposed model for the modelling of volumetric behaviour of compacted bentonites is extended to triaxial stress states. The model is formulated using a conventional effective stress expression and the degree of saturation. Because these two variables are directly related to the water retention, a suitable formulation for bentonites is used. The resulting equations are characterised by a high degree of hydro- mechanical couplings and a low number of material parameters, which can be obtained on the basis of well- established laboratory procedures. In order to demonstrate its capabilities, the model is used to simulate the behaviour of MX-80 bentonite for several stress paths under oedometric conditions. The emphasis is put on the process of parameter determination. The predictive capabilities of the model are also highlighted.

A coupled hydro – mechanical approach for modelling the volume change behaviour of compacted bentonite

The volumetric response of compacted bentonites against environmental actions is a key aspect in most designs of nuclear waste repositories. The safety assessment of such repositories must account for robust and reliable models of stress–strain for bentonites. While many models for unsaturated low activity clays take advantage from the use of a generalized effective stress, its application to expansive soils has not found the same degree of success. One of the possible reasons is the complex water retention behaviour of these materials, which only recently has been successfully reproduced by numerical models. Here, by adopting an appropriate water retention model, a coupled hydro-mechanical approach to simulate the volume change behaviour of compacted bentonites is suggested. An explicit distinction between interlayer adsorbed water and capillary water is used to simulate the water retention behaviour. It is then shown that by using a precise water retention formulation, the volumetric behaviour can be interpreted within an effective stress–degree of saturation based framework. Some interesting results derived from the use of the effective stress include the shrinkage limit, the increase in stiffness of the elastic regime and the use of a single elastic coefficient for both wetting–swelling and reloading stress paths.

A study on the influence of inorganic salts on the behaviour of compacted bentonites

Clay liners are frequently installed at waste disposal sites as a means of preventing pollutant migration and minimizing or eliminating the potential for ground water contamination. Because of their low hydraulic conductivity and high contaminant adsorption capacity, bentonite is used as a liner material to prevent subsurface contamination. However, presence of various chemicals in waste could affect the hydraulic and contamination adsorption capacity of bentonite and in turn reduce its usefulness as barrier material. In addition to the salt solution, a change in the mineralogical composition such as montmorillonite content, cation exchange capacity, specific surface area, exchangeable sodium percentage of the bentonite also significantly influences its swelling and hydraulic behaviour. This study was carried out to study the effect of salt solution and mineralogical composition on the behaviour of compacted bentonite. Two bentonites with varying mineralogical composition, which was reflected in their different liquid limit and free swelling value, were evaluated for their free swelling, Atterberg limits, swelling potential, swelling pressure, and hydraulic conductivity in the presence of various concentrations of NaCl and CaCl2 solution. To study the effect of initial compaction conditions on swelling and hydraulic behaviour in presence of salt solution, studies were also carried out on samples compacted at optimum moisture content (OMC)–maximum dry density (MDD) and 5% dry of OMC–MDD. The result shows that the liquid limit, swelling volume, swelling pressure of the compacted bentonites decreased, whereas, plastic limit and hydraulic conductivity increased with increase in the salt concentration. The results also show that the effect of the salt on the properties of the bentonites depends on the salt type, salt concentration and initial compaction condition of the bentonite. The effects due to salt concentration were found to be more pronounced for the bentonite of higher quality which is marked by a higher swelling capacity, liquid limit, cation exchange capacity, exchangeable sodium percentage and specific surface area.

On the behaviour of compacted bentonite under thermal gradients

This paper presents an experimental and modelling study of bentonite behaviour under thermal gradients. The variables measured have been the temperature during the heating process, the final water content and the change of diameter after heating. The change of diameter and the water content after some months have been measured as well. The test has been simulated with CODE_BRIGHT, a numerical code which has the capability to solve the non isothermal multiphase transport in deformable porous media. The results of the study show that the thermo-hydraulic behaviour after the heating process can be properly simulated. The thermo-hydro-mechanical and the long term behaviour could not, however, be reproduced with the same precision.

Application of Elasto-Plastic Model to Mechanical and Hydraulic Behavior of Buffer Material Under Water Uptake in a Repository

ABSTRACTMechanical and hydraulic behavior of buffer material during water uptake in a repository is a major issue from the viewpoint of mechanical stability of engineered barriers and near-field conditions for performance assessment. This paper presents the results of hydraulic-mechanical modeling of buffer material and the simulations carried out on an engineered barrier system under water uptake.Hydraulic behavior of compacted bentonite of buffer material was modeled as moisture diffusion. An elasto-plastic model was applied to the deformation behavior of compacted bentonite, of which swelling pressure was described by swelling coefficient under restraint condition. The hydraulic diffusivity and swelling coefficient were given based on the result of swelling tests of KUNIGEL-V1 bentonite which contains about 50 % montmorillonite. Being used this model, simulations on re-saturation behavior of an engineered barrier system were carried out for the cases of water uptake from the whole surface of both crystalline and sedimentary rock and from partial surface of opening. The results are : (1) The hydraulic and mechanical behavior of compacted bentonite can be described by a swelling-elasto-plastic model. (2) The distribution of the water content depends on the water uptake conditions. (3) The deformation of compacted bentonite and the displacement of the overpack under water uptake are negligibly small.