Advances on the knowledge of the thermo-hydro-mechanical behaviour of heavily compacted “FEBEX” bentonite

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Abstract

A review of the more relevant laboratory results concerning the thermo-hydro-mechanical (THM) behaviour of the FEBEX bentonite is presented. Most of the THM features of compacted bentonite have been experimentally studied during the FEBEX project. The results about fabric characterisation, water retention, permeability, swelling capacity and mechanical behaviour obtained during the project have been summarised. The influence of temperature and water salinity on the THM behaviour of the bentonite is also considered. In general, the behaviour of the clay may be explained taking into account the changes in its double-level structure, micro and macro. The interactions between these two structural levels are responsible for the main features of the THM aspects of this behaviour.

Introduction

The work presented here has been performed in the framework of the two phases of the FEBEX project (full-scale engineered barriers experiment), whose aim is to study the behaviour of the components of the near-field for a high-level radioactive waste (HLW) repository in crystalline rock (ENRESA, 2000). The experimental work consists of three main parts: an in situ test, under natural conditions and at full scale, performed at the Grimsel Test Site (GTS, Switzerland); a “mock-up” test, at almost full scale, performed at CIEMAT facilities (Madrid); and a series of laboratory tests to complement the information from the two large-scale tests. In the two large-scale tests, the thermal effect of the wastes is simulated by means of heaters, whereas hydration is natural in the in situ test and controlled in the mock-up test.

The main objectives of the laboratory tests were divided in three areas: direct determination of basic parameters; model calibration and determination of parameters by back-analysis and improvement of constitutive laws. This paper is focussed on the advances achieved in the last topic, and specifically, on the influence of porosity, suction, stress paths and temperature on the water retention, permeability and mechanical behaviour of the bentonite.

The material tested is the FEBEX bentonite, which mainly consists of montmorillonite and is being used in the form of compacted blocks in the large-scale experiments of the FEBEX project. In both large-scale tests, the bentonite blocks have been manufactured by uniaxial compaction of the granulated clay with its hygroscopic water content at dry densities close to 1.7 g/cm3. Experimental results obtained in the laboratory on compacted specimens subjected to temperature, suction and stress changes are presented in this paper.

Section snippets

General properties

The FEBEX bentonite has been extracted from the Cortijo de Archidona deposit, in the zone of Serrata de Níjar (Almería, Spain). This deposit was selected in the ENRESA R&D plans previous to the FEBEX project as the most suitable material for the backfilling and sealing of a HLW repository. The criteria taken into account in making the selection were: purity of mineralogical composition, specific surface, plasticity, proportion of fraction of less than 2 μm, thermal conductivity and behaviour

Retention curves

The suction/water content relationship was determined both in confined and unconfined samples under different temperatures. As discussed by several authors, the retention capacity of swelling materials is highly affected by the strain state, and it is higher for samples that can freely swell (unconfined samples) than for those kept at constant volume (Yahia-Aissa, 1999, Villar, 2000, Villar, 2002, Romero and Vaunat, 2000). Although the sealing material will be unconfined only in the outer part

Mechanical behaviour

The mechanical response of compacted bentonite may be analysed following the conceptual model developed by Gens and Alonso (1992). This model considers two structural levels: a microstructural level that corresponds to the clay minerals and aggregates where physicochemical phenomena predominate, and a macrostructural level that involves the ensemble of aggregates. It can be considered that the microstructure remains always water-saturated and that the volumetric deformations at microstructural

Conclusions

The two phases of the FEBEX project included a wide laboratory tests programme that focused on the calibration of the models, the acquisition of parameters by back-analysis and the improvement of the knowledge on the behaviour of expansive clays. But the programme also included tests on new areas. Specifically, the new tests centred on investigations on the influence of the microstructure changes, temperature and solute concentration on the behaviour of the clay.

Mercury intrusion porosimetry

Acknowledgements

Work co-funded by ENRESA and the European Commission, and performed as part of the Fourth and Fifth EURATOM Framework Programmes, key action Nuclear Fission, Projects FEBEX (EC Contract FI4W-CT95-006) and FEBEX II (EC Contract FIKW-CT-2000-00016). The laboratory work was performed by R. Campos and J. Aroz at CIEMAT (Madrid) and by X. Pintado, J.E. Castellanos and J. Suriol at UPC (Barcelona). The helpful discussions with P. Rivas and P.L. Martín (CIEMAT) and E. Romero (UPC) are acknowledged.

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