Thermo-hydro-mechanical characterisation of a bentonite from Cabo de Gata. A study applied to the use of bentonite as sealing material in high level radioactive waste repositories.

Abstract

The FEBEX Project, initiated by ENRESA in 1995, is\n\t\t\t\t a test designed to demonstrate the technical feasibility\n\t\t\t\t of manufacturing and installing the engineered\n\t\t\t\t barriers for a high level waste disposal facility in\n\t\t\t\t crystalline rock and to study their behaviour. The\n\t\t\t\t Project consists of a full-scale in situ test, performed\n\t\t\t\t at the Grimsel underground facility (Switzerland), of\n\t\t\t\t a large-scale mock-up test carried out under controlled\n\t\t\t\t conditions at the CIEMAT installations (Madrid)\n\t\t\t\t and of a series of laboratory tests designed to\n\t\t\t\t complement the information acquired from the two\n\t\t\t\t large-scale tests on the performance of the sealing\n\t\t\t\t material and provide a basis for the modelling work\n\t\t\t\t carried out simultaneously.\n\t\t\t\t This work is part of the set of laboratory tests performed\n\t\t\t\t within the FEBEX Project, and focuses on the\n\t\t\t\t Thermo-Hydro-Mechanical (THM) characterisation\n\t\t\t\t of the sealing material. The Spanish reference bentonite\n\t\t\t\t has been used, selected by ENRESA from previous\n\t\t\t\t suitability studies and extracted from the\n\t\t\t\t Cortijo de Archidona deposit in the Cabo de Gata\n\t\t\t\t region (Almer??a). Attempts have been made to reproduce\n\t\t\t\t the behaviour of the bentonite under conditions\n\t\t\t\t similar to those that will be found in the disposal\n\t\t\t\t barrier during the saturation transient. For this\n\t\t\t\t purpose, work has been carried out with the bentonite\n\t\t\t\t compacted to densities close to those of the barrier\n\t\t\t\t blocks, the material has been subjected to different\n\t\t\t\t saturation conditions and the temperature\n\t\t\t\t factor has been introduced in certain tests.\n\t\t\t\t The determinations were carried out on the granulated\n\t\t\t\t bentonite compacted, in most of the cases\n\t\t\t\t with its hygroscopic water content, at dry densities of\n\t\t\t\t between 1.30 and 1.85 g/cm3, this covering from\n\t\t\t\t the initial density of the compacted blocks used for\n\t\t\t\t the large scale tests of the FEBEX Project to the density\n\t\t\t\t of the barrier once saturated at the emplacement,\n\t\t\t\t including even the local variations to be expected.\n\t\t\t\t The repercussion of material density, water\n\t\t\t\t type and anisotropy on hydraulic conductivity, on\n\t\t\t\t swelling pressure and on strain upon saturation has\n\t\t\t\t been tested. Permeability to gas and thermal conductivity\n\t\t\t\t have been measured in samples with different\n\t\t\t\t degrees of saturation. The retention curve for\n\t\t\t\t different dry densities has been determined under\n\t\t\t\t free and constant volume conditions, following drying\n\t\t\t\t and wetting paths, at 20 and 40 C. An important\n\t\t\t\t part of the work contains the results of suction\n\t\t\t\t controlled oedometric tests performed at 20 C following\n\t\t\t\t different stress paths (vertical loads from 0.1\n\t\t\t\t to 9 MPa, suctions from 0 to 500 MPa, as a result\n\t\t\t\t of which practically all the hydration states of the\n\t\t\t\t bentonite have been explored). The results of suction controlled oedometric tests performed at 40,\n\t\t\t\t 60 and 80 C are also included.\n\t\t\t\t New experimental techniques have been fine tuned\n\t\t\t\t up ???like those for measurement of the permeability\n\t\t\t\t to gas, determination of the retention curve at constant\n\t\t\t\t volume and performance of suction and temperature\n\t\t\t\t controlled oedometric tests??? as, due to the\n\t\t\t\t special behaviour of this type of materials (high\n\t\t\t\t expansibility, very low permeability, very high suction),\n\t\t\t\t the soil mechanics standard testing techniques\n\t\t\t\t are difficult to apply.\n\t\t\t\t The analysis of the results dealt with in this work underline\n\t\t\t\t the repercussion of the microstructure on the\n\t\t\t\t macrostructural behaviour. The tests results represent\n\t\t\t\t an advance in the knowledge of the properties\n\t\t\t\t of expansive clays and may be useful for those researching\n\t\t\t\t the flow and strain phenomena in the\n\t\t\t\t clayey materials used as barriers in waste repositories.\n\t\t\t\t In particular, with respect to the characterisation\n\t\t\t\t of an expansive material, the following conclusions\n\t\t\t\t can be highlighted:\n\t\t\t\t When the clay saturates at a constant volume,\n\t\t\t\t expansion being prevented, the water entering\n\t\t\t\t acquires a density in excess of 1.00 g/cm3.\n\t\t\t\t The increase of the dry density of the sample\n\t\t\t\t gives rise to an exponential decrease in permeability.\n\t\t\t\t Hydraulic conductivity increases slightly when\n\t\t\t\t saline water is used instead of granitic or distilled\n\t\t\t\t water as the permeating agent.\n\t\t\t\t The intrinsic permeability of samples having the\n\t\t\t\t same porosity varies with the degree of saturation,\n\t\t\t\t with differences of up to eight orders of\n\t\t\t\t magnitude between dry and saturated samples.\n\t\t\t\t The suction of the FEBEX bentonite is very high\n\t\t\t\t and fundamentally of the matric type, with the\n\t\t\t\t contribution made by osmotic suction being inappreciable.\n\t\t\t\t The water retention capacity of the clay is conditioned\n\t\t\t\t to a large extent by the state of confinement.\n\t\t\t\t The difference between the water\n\t\t\t\t contents reached for a given value of suction\n\t\t\t\t under free volume and confined conditions becomes\n\t\t\t\t more appreciable for suctions below a\n\t\t\t\t given value, which depends on dry density and\n\t\t\t\t indicates the limit between intra and inter-aggregate\n\t\t\t\t suction.\n\t\t\t\t The suction/water content relation determined\n\t\t\t\t under free volume conditions may be expressed\n\t\t\t\t by means of a logarithmic law.\n\t\t\t\t 9\n\t\t\t\t Abstract\n\t\t\t\t The retention curves at constant volume show\n\t\t\t\t hysteresis.\n\t\t\t\t The swelling pressure of bentonite relates to\n\t\t\t\t dry density by way of an exponential law.\n\t\t\t\t The compacted blocks do not show anisotropy\n\t\t\t\t with respect to either hydraulic conductivity,\n\t\t\t\t swelling pressure or retention capacity.\n\t\t\t\t The change in volume during hydration ???whose\n\t\t\t\t main component is microstructural??? decreases\n\t\t\t\t in intensity the higher the overload under\n\t\t\t\t which it occurs. The expansion in the microstructure\n\t\t\t\t as a result of hydration under low\n\t\t\t\t load gives rise to a reorganisation of the\n\t\t\t\t macrostructure, which becomes more porous,\n\t\t\t\t an irreversible increase in volume occurring.\n\t\t\t\t During this microstructural expansion there is\n\t\t\t\t also a decrease in preconsolidation pressure.\n\t\t\t\t The swelling pressure of the unsaturated sample\n\t\t\t\t increases logarithmically in the interval of\n\t\t\t\t suctions of between 130 MPa (the suction corresponding\n\t\t\t\t to hygroscopic water content) and\n\t\t\t\t 10 MPa, although below this value the increase\n\t\t\t\t is attenuated. For suctions in excess of\n\t\t\t\t 200 MPa, there is hardly any development of\n\t\t\t\t swelling pressure.\n\t\t\t\t The drying of the sample by decreasing suction\n\t\t\t\t to values of around 500 MPa involves practically\n\t\t\t\t no variation in volume. For suctions\n\t\t\t\t above this value, the external load does not\n\t\t\t\t cause any important consolidation in the sample,\n\t\t\t\t which becomes rigid.\n\t\t\t\t Drying of the sample by increasing suction to\n\t\t\t\t 500 MPa does not modify either its swelling\n\t\t\t\t capacity during subsequent hydration or its\n\t\t\t\t swelling pressure once saturated.\n\t\t\t\t The increase in temperature to 80 C in the\n\t\t\t\t controlled suction oedometric tests performed\n\t\t\t\t would appear to induce a hardening of the\n\t\t\t\t material, increasing its preconsolidation pressure\n\t\t\t\t and limiting its swelling capacity.\n\t\t\t\t The main conclusions concerning the use of bentonite\n\t\t\t\t as sealing material in HLW repositories are summarised\n\t\t\t\t below:\n\t\t\t\t The high value of suction of the bentonite installed\n\t\t\t\t in the disposal facility in the form of\n\t\t\t\t high density blocks manufactured from the clay\n\t\t\t\t granulate with its hygroscopic water content\n\t\t\t\t will be the fundamental force driving the saturation\n\t\t\t\t of the barrier with the water from the\n\t\t\t\t surrounding massif. However, as the clay saturates,\n\t\t\t\t its intrinsic permeability will decrease, as\n\t\t\t\t the size of the flow channels becomes smaller,\n\t\t\t\t this being accompanied by a decrease in the\n\t\t\t\t suction gradient, as a result of which the saturation\n\t\t\t\t process will become steadily slower. This\n\t\t\t\t reduction in flow will be compensated only in\n\t\t\t\t part by the increase in relative permeability occurring\n\t\t\t\t with increasing saturation.\n\t\t\t\t Simultaneously with hydration there will be expansion\n\t\t\t\t of the bentonite in those areas in\n\t\t\t\t which there is space for this to occur. In this\n\t\t\t\t way, the swelling and high level of suction of\n\t\t\t\t the bentonite will also prevent the joints between\n\t\t\t\t blocks becoming preferential channels\n\t\t\t\t for the flow of water, since water coming into\n\t\t\t\t contact with the bentonite will be immediately\n\t\t\t\t absorbed, causing the latter to expand and to\n\t\t\t\t seal any type of joint. Likewise, the gap between\n\t\t\t\t the bentonite and the wall of the drift will\n\t\t\t\t be closed by this same process. In those areas\n\t\t\t\t in which the bentonite is confined, the swelling\n\t\t\t\t pressure exercised by the clay will increase\n\t\t\t\t logarithmically with saturation, causing compression\n\t\t\t\t of the drier areas and an increase in\n\t\t\t\t their density.\n\t\t\t\t The controlled suction oedometric tests h

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