BCV bentonite hydromechanical behaviour and modelling

Abstract

It is assumed that clay materials such as bentonites will be used as the buffer and backfill of high-level radioactive waste disposal. Due to the very long timescale, the design can be based only on mathematical models that have been thoroughly validated via the use of high-quality laboratory data. The presentation of an extensive laboratory dataset, accompanied by the validation of a bentonite constitutive model, comprises the main topic of this paper. The objectives of the laboratory research were to determine the basic hydromechanical properties of BCV bentonite and to study the stress path behaviour of this bentonite. The laboratory results revealed that the hydromechanical behaviour is highly dependent on the stress path assumed, as well as the method of water introduction into the sample. The results further indicate that the micro- and macrostructures are constantly evolving, which exerts a significant impact on the determination of the material properties and the development of mathematical models. Moreover, it was demonstrated that swelling pressure tests, although they are typically considered to be element tests (with constant fields of state variables), when adopted by researchers for the calibration of constitutive models, do not in fact constitute element tests. A hypoplastic model of the behaviour of bentonite was implemented and validated using the experimental data obtained. It was demonstrated that the hypoplastic model accurately predicted the main behavioural features of bentonite that are important in terms of the prediction of the behaviour of the buffer in nuclear waste repositories.

Enhancement of a hydro-mechanical hypoplastic model for unsaturated fine-grained soils accounting for small strain stiffness

“…In the present work, the coupled hypoplastic model for unsaturated fine-grained soils developed by Wong and Mašín [7] was further extended to include a smoothed hysteretic water retention curve formulation proposed by Svoboda et al [9] The model shows better predictions of the hydraulic soil behaviour, because it is able to capture the non-linear dependency of suction on the degree of saturation.…”

“…The reference model adopted a hysteretic bi-linear water retention curve following the formulation by Brooks and Corey [8], see Fig 1 . However, experimental evidence has indicated that the dependency of suction on the degree of saturation is non-linear [10]. For this reason, a smoothed WRC proposed by Svoboda et al [9] was incorporated into the original model. The smoothed formulation also improves the numerical performance of the model, avoiding abruptly changes at the intersection of the main wetting/drying curves with the scanning curve and at the air entry/expulsion suction value due to the derivates of .…”

“…In this work, a hysteretic water retention curve was incorporated to the constitutive model proposed by Wong and Mašín [7] using a non-linear formulation proposed by Svoboda et al [9] to predict the non-linear dependency of the degree of saturation and suction observed in experimental evidence [10]. Additionally, the capabilities of the extended model to predict the cyclic behaviour of unsaturated soils were evaluated using element tests simulations to predict the experimental results of constant water cyclic triaxial test from Zhou [11] at three different suctions on a completely decomposed tuff.…”

Experimental and numerical study on homogenisation of BCV bentonite using dual density samples

“…The cation exchange capacity reaches 60.9 meq/100 g [11], the liquid limit is 135 ± 5% and the plastic limit 48 ± 5% [11,12]. Further details on the geochemical and geotechnical parameters can be found in [11] and [13].…”

“…The laboratory experiments were simulated by means of a thermo-hydro-mechanical hypoplastic model for bentonite [14,15] implemented in the SIFEL coupled finite element code [16]. The calibration and validation of the model using data obtained from BCV bentonite laboratory (single-density) samples, details of which are available elsewhere [13], proved its high level of performance with respect to experiments of various types when modelled using a single parameter set. The experiments used for calibration purposes included the testing of swelling pressure and oedometric swelling under constant loading at differing initial dry densities, the oedometric loading and unloading testing of saturated samples, water retention tests under confined and free swelling conditions at various initial dry densities and the uniaxial compression testing of unsaturated samples.…”