Influence of dry density and water content on the swelling of a compacted bentonite

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1 Available online at Applied Clay Science 39 (2008) Influence of dry density and water content on the swelling of a compacted bentonite M. Victoria Villar a,, Antonio Lloret b a Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Avd. Complutense 22, Madrid, Spain b Universitat Politècnica de Catalunya (UPC), C/ Jordi Girona 1-3, Barcelona, Spain Received 24 November 2006; received in revised form 13 April 2007; accepted 19 April 2007 Available online 27 April 2007 Abstract In the context of a project for the study of the behaviour of the clay barrier in a nuclear waste repository, the swelling properties of compacted bentonite have been investigated. Results on the influence of initial dry density and water content on the swelling pressure and swelling deformation of a compacted bentonite are presented. Swelling pressure is exponentially related to dry density but is rather independent of the initial water content of the clay. The swelling capacity is mainly affected by the vertical load under which saturation takes place. It increases with initial dry density but decreases as the initial water content is higher. The effect of the initial water content on the final swelling strain is less important for the low dry densities and the high vertical loads, becoming negligible for vertical loads close to the swelling pressure. These features of behaviour agree with the predictions of conceptual models that consider the interaction between the responses of the microstructure and the macrostructure of the material to suction and stress changes Elsevier B.V. All rights reserved. Keywords: Compacted bentonite; Swelling pressure; Swelling deformation; Water content; Density 1. Introduction The design of high-level radioactive waste (HLW) repositories in deep geological media includes the construction of a barrier around the waste containers constituted by a sealing material (Fig. 1). Bentonite has been chosen as sealing material in most disposal concepts because of its low permeability, swelling capacity and retention properties, among other features. One of the options is to place the bentonite in the facility Corresponding author. Tel.: ; fax: addresses: mv.villar@ciemat.es (M.V. Villar), antonio.lloret@upc.es (A. Lloret). in the form of high-density compacted blocks made from granulated clay. Since this barrier will be manufactured with the clay at its hygroscopic water content, the bentonite will be initially unsaturated, i.e. air and water will coexist in the pores of the barrier. This confers the bentonite a very high suction that promotes and conditions the saturation of the bentonite, which will take water from the surrounding geological medium. At such densities, the hydraulic conductivity of bentonite is extraordinarily low, while its swelling capacity is very high and will depend on its initial density, on the confining conditions, on temperature and on the quantity and composition of the water supplied by the host rock. During the saturation of the barrier, as the clay becomes hydrated, expansion occurs. The swelling capacity of the /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.clay

2 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 1. Appearance of the bentonite barrier of an in situ test during assemblage: central container (simulated by a heater), bentonite blocks around it, and granite walls of the gallery that will provide water during operation. The diameter of the gallery is 2.4 m. Note the gaps between blocks, in the upper part and around the heater (Fuentes-Cantillana and García-Siñeriz, 1998). bentonite will allow the filling of voids and gaps in the system for example the voids between the bentonite blocks or between the blocks and the host rock (Fig. 1) closing quick preferential pathways for the migration of fluids. As a result, the density of the barrier will decrease in these areas, compared to the manufacturing density of the blocks. However, this reduction in the density of the bentonite will be local, since the overall barrier is confined, this meaning that when hydration occurs swelling pressures will be developed and, therefore, there will be an increase in the radial stresses in the barrier. During this transient state, these stresses cause the internal parts of the barrier to compress, thus increasing their density, what is enhanced by the heating caused by the waste. In this way a density gradient is established in the system, with lower densities in the areas affected by water, where the bentonite will have expanded, and higher in the innermost areas not yet affected by water which are pushed by neighbouring zones. These gradients can be observed in Fig. 2, where the water contents and dry densities of the bentonite measured upon dismantling of the barrier of a 5-year duration in situ test are plotted (Villar et al., 2005). These modifications in void ratio and water content affect increasingly larger areas as hydration progresses and have consequences on the local hydraulic and mechanical properties of the bentonite and consequently on the behaviour of the barrier. Theoretical descriptions of the swelling mechanisms of clay soils that influence their macroscopic mechanical behaviour have been provided since long, and are usually based on physico-chemical considerations (e.g. Bolt, 1956; Aylmore and Quirk, 1962; Barbour and Fredlund, 1989; Pusch et al., 1990). Several authors have dealt with the influence of water content and dry density on the swelling characteristics of clays from a macroscopic point of view and, in the last years particularly of compacted bentonites (Gens and Alonso, 1992; Cui et al., 2002; Sánchez et al., 2005). Back in the seventies the swelling behaviour of plastic clays was investigated as a function of suction (Kassiff and Ben Shalom, 1971; Kassiff et al., 1973) or water content (Brackley, 1973). More recently, several authors (Komine and Ogata, 1994, 1996, 1999; Komine, 2004) have studied the swelling behaviour of compacted Na-bentonite and of bentonite/sand mixtures as a function of smectite content, void ratio, initial water content and solute concentration and have tried to predict it by using equations based on the diffuse double layer theories (Sridharan and Choudhury, 2002; Tripathy et al., 2004). The swelling capacity of these mixtures has also been studied by Agus and Schanz (2005), Graham et al. (1989), Gray et al. (1985), Villar and Rivas (1994), among others. Mollins et al. (1996) and Studds et al. (1998), focussed on sand/ bentonite mixtures compacted to low dry densities, in the first case with different water contents. The work presented in this paper refers to the influence of dry density and initial water content on the swelling capacity of heavily compacted bentonite, for which swelling pressure tests and swelling deformation tests have been carried out. It was performed in the framework of FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock), which was a project for the study of the near field for a HLW repository in crystalline rock according to the Spanish concept: the waste canisters are placed horizontally in Fig. 2. Distribution of water content (w.c.) and dry density (d.d.) along different radii of the bentonite barrier shown in Fig. 1 after 5 years of operation at repository conditions.

3 40 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) drifts and surrounded by a clay barrier constructed from highly-compacted bentonite blocks (ENRESA, 2000). The experimental work of the FEBEX Project consisted of three main parts: an in situ test, under natural conditions and at full scale (at the Grimsel Test Site, Switzerland, Fig. 1); a mock-up test, at almost full scale (at CIEMAT, Madrid); and a series of laboratory tests to complement the information from the two large-scale tests. All these activities served as a support for a farreaching programme of modelling work, based on the study of processes and variables and the development, verification and validation of numerical models. In fact, the results of hydro-mechanical laboratory tests performed in samples compacted with hygroscopic water content to dry densities similar to those of the blocks used in the large-scale tests have been analysed following conceptual models developed during the Project (Lloret et al., 2003). In addition, the dismantling of the large-scale tests will provide the opportunity to calibrate these models and check their predictions, as well as to observe the modifications experienced by the bentonite in a direct and representative way. To determine the possible changes in the hydro-mechanical properties of the bentonite occurred during the experiments, due to the combined effect of temperature, water content, joints and solutes, it is necessary to have a complete database on the behaviour of the bentonite at different initial conditions. Accordingly, the aim of the work presented here is twofold: to quantify the influence of density and water content on the swelling capacity of the FEBEX bentonite in order to be able to calibrate models, and to collect a database useful to check if the swelling properties of the bentonite have changed after the thermohydraulic treatment to which it is being subjected in large-scale and laboratory tests. For that, it is necessary to have simple relationships characterising the dependence between bentonite density and water content and its swelling pressure or swelling capacity. The relationships obtained in this work allow the comparison of the swelling capacity obtained for bentonite previously submitted to thermal, mechanical or geochemical processes with that of the original material, letting thus an evaluation of the possible deterioration of the properties of the bentonite as a consequence of different treatments. Furthermore, the results published up to now on the swelling characteristics of the FEBEX bentonite (Villar, 2002; Lloret et al., 2004; Villar and Lloret, 2004) were obtained in samples with initial hygroscopic water content, whereas in this paper a broad range of initial water contents has been investigated, because the samples subjected to thermo-hydraulic treatments display a variety of final water contents. Besides, the work provides an extensive data set of the swelling behaviour of heavily compacted bentonite which can be useful to check qualitatively and quantitative conceptual and mathematical models on the hydro-mechanical behaviour of expansive materials. 2. Materials The tests have been performed with the FEBEX bentonite, which was selected by ENRESA (the Spanish Agency for Radioactive Waste Management) as suitable material for the backfilling and sealing of HLW repositories. The FEBEX bentonite was extracted from the Cortijo de Archidona deposit (Almería, Spain). The processing of the material at the factory consisted on disaggregation and gently grinding, drying at 60 C and sieving by 5 mm. For the largescale tests of the FEBEX Project, the bentonite blocks were manufactured by uniaxial compaction of the granulated clay with its hygroscopic water content, at dry densities close to 1.7 Mg/m 3 (ENRESA, 2000). The FEBEX bentonite has a content of montmorillonite higher than 90%. Besides, it contains variable quantities of quartz, plagioclase, K-feldspar, calcite and opal-ct (cristobalite-trydimite). The cation exchange capacity (CEC) varies from 96 to 102 meq/100 g, and the major exchangeable cations are: Ca (35 42 meq/100 g), Mg (31 32 meq/100 g), Na (24 27 meq/100 g) and K (2 3 meq/100 g). The liquid limit of the bentonite is 102±4%, the plastic limit is 53±3%, the specific gravity 2.70±0.04, and 67±3% of particles are smaller than 2 μm. The hygroscopic water content in equilibrium with the laboratory atmosphere is 13.7 ± 1.3%. The value obtained for the external specific surface area using the BET technique is 32±3 m 2 /g and the total specific surface area obtained using the hygroscopicity method (Keeling, 1961) is about 725 m 2 /g. The hydraulic conductivity of the saturated FEBEX clay can be related to the dry density through an exponential law. The values of permeability to deionised water for densities around 1.6 Mg/m 3 are in the order of m/s. The most relevant thermo-hydro-mechanical and geochemical characteristics of the FEBEX bentonite are shown in detail in ENRESA (1998) and Villar (2002) and summarised in ENRESA (2000). 3. Experimental procedures Soils containing clay minerals of laminar or expansive structure are characterised by their high deformability on hydration. If the material is saturated at constant volume and its deformation is prevented, the particles will exercise a pressure on the confining structure. This is known as swelling pressure, and reaches its equilibrium value when the sample is completely saturated and is at zero suction. If a constant mechanical pressure lower than the swelling pressure is applied during saturation, the soil will expand, which is the case of the swelling deformation tests. Both kinds of

4 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) laboratory tests have been carried out in this work in oedometers at laboratory temperature following the procedures described below. Deionised water has been used in all the tests in order to establish a clear reference condition, since swelling is also affected by the salinity of the hydrating water. To produce water contents greater than the hygroscopic one, the granulated bentonite with its hygroscopic water content (about 14%) was mixed with deionised water in different proportions so that to obtain different initial water content conditions. The mixtures prepared were allowed to stabilise several days in plastic bags kept in a humid atmosphere, in order to facilitate a uniform distribution of moisture. The initial suction of the specimens decreases as the initial water content increases. Thus, the samples of dry density 1.5 Mg/m 3 have initial suctions ranging from 120 MPa (for the hygroscopic water content) to 18 MPa (for 22% initial water content). For the same water contents, the samples of initial dry density 1.6 Mg/m 3 have initial suctions from 130 to 22 MPa, and the samples of initial dry density 1.7 Mg/m 3 have initial suctions from 140 to 26 MPa. These values have been obtained from the water retention curves determined at constant volume for the FEBEX compacted bentonite (Lloret et al., 2004) Swelling pressure The swelling pressure test makes it possible to determine the equilibrium swelling pressure exercised by a sample on complete saturation at constant volume. For the performance of this test, oedometer frames and conventional oedometric cells were used, in which the surface of the sample was reduced in order to be able to counteract the high pressures expected. The initial height of the specimens was 12.0 mm and their section was 998 or 1140 mm 2. The sample is confined in a ring preventing it from deforming laterally, and between two porous stones at its upper and lower surfaces. The piston of the cell, which is adjoined to the upper porous stone, is in contact with the loading ram, whose displacement may be accurately measured by means of a dial gauge. The sample can be loaded via the ram by means of the system of levers of the oedometric frame on which the cell is located. The maximum applicable pressure in the equipments used is of 16 MPa, in several or in a single step. With this system the swelling pressure exercised by the sample is determined from the load that has to be applied in order for the volume of the sample to be kept constant during saturation. The samples were prepared by means of uniaxial compaction of the clay directly in the oedometer rings. Specimens of different initial dry densities were obtained by varying the compaction pressure. The sample contained in the oedometer ring is placed inside the oedometer cell and the lower porous stone is covered with deionised water, such that the sample begins to saturate from the bottom upwards, allowing the air in the pores to escape from the upper part. The displacement recorded by the dial gauge as the sample saturates is observed, and swelling of the sample is prevented by the application of loads. Ideally the reading of the dial gauge shall not drift excessively from the initial value (within a margin of ±0.005 mm), thus preventing both the swelling and the consolidation of the sample. The test is considered to be completed when, under a constant vertical load, no strain is observed for at least 24 h. The duration of the tests depended on the dry density of the samples, with an average of 4 days. It is a known fact that expansive materials may develop swelling in a non monotonic way. With the equipments used in this testing program it was not possible to follow the detailed evolution of pressure over time. However, tests performed with the FEBEX bentonite in high-pressure oedometers in which the time evolution can be measured during saturation by means of a load cell (Lloret et al., 2004) showed that, for samples of dimensions similar to those tested in this work compacted to dry densities in the same range, the swelling pressure evolution may record intermediate periods in which the value of pressure decreases or in which the rate of pressure increase is lower (Fig. 3). These periods take place usually after h of hydration and well before the clay reaches full saturation. Since full saturation has been confirmed in all the tests reported in this work, the attainment of the equilibrium swelling pressure value at the end of the tests is assured. The dry density of the specimens may vary slightly during the tests, due to failure to keep the volume of the sample perfectly constant, since the application of loads is manual. Also, the deformation experienced by the equipment upon loading is calibrated and taken into account when the final strain of the specimens is computed. The corrections made to the final height of the samples based on the calibration results range from 0.01 mm for vertical load of 0.1 MPa to 0.26 mm for vertical load of 13 MPa, what would correspond in the extreme cases to a maximum change of the dry density of 0.02 Mg/m 3 during the test. Consequently, the swelling pressure of the sample is obtained for the dry density at the end of the tests and the water content at which the specimen was manufactured. Once the sample is dismounted its water content is checked by oven-drying at 110 C during 24 h. The tests were performed at laboratory temperature. Fig. 3. Evolution of swelling pressure during saturation at room temperature and under constant volume of a sample of FEBEX bentonite compacted with its initial water content to dry density 1.50 Mg/m 3 (modified from Lloret et al., 2004).

5 42 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 4. Vertical stresses applied to manufacture specimens of different dry density and water content by uniaxial compaction. The estimated suctions after compaction are indicated in MPa Swelling deformation The influence of density and water content on the swelling capacity of the clay has been checked by swelling deformation tests (also called soaking under vertical load tests). They have been performed in the standard oedometers described in the previous section. The initial water content of the samples was hygroscopic, 17, 20 and 22%. The material was compacted inside the cell ring using static uniaxial compaction until reaching nominal dry densities of 1.50, 1.60 and 1.70 Mg/m 3. The average vertical stresses applied to obtain specimens of mm diameter and 12 mm height and the densities reached are shown in Fig. 4. Once in the oedometer, vertical pressures of 0.1, 0.5, 1.0 and 3.0 MPa have been applied to the samples. Immediately afterwards, the samples are flooded with deionised water at atmospheric pressure from the bottom porous plate. The swelling strain experienced by the specimens upon saturation is recorded as a function of time until stabilisation. The final result is the percentage of strain of a sample of given initial dry density and water content on saturating under a fixed load. On completion of the test, the water content of the specimen was determined. The tests have been performed at laboratory temperature. Swelling pressure was determined using specimens compacted uniaxially inside the ring of the oedometer to different densities and with initial hygroscopic water content (13.2±1.2%). The values obtained are shown in Fig. 5 as a function of the dry densities of the bentonite at the end of the tests, which may differ slightly from the dry density to which the sample was initially compacted, due to the small deformations allowed by the equipments and to inaccuracies in the manual application of loads. With the points determined it is possible to obtain an exponential fit between swelling pressure (P s, in MPa) and final dry density (ρ d, in Mg/m 3 ): lnp s ¼ 6:77q d 9:07 r 2 ¼ 0:88 : ð1þ This equation allows the prediction of the maximum pressure that the barrier will exert depending on the density to which the bentonite is compacted, which is a parameter for the design of the repository. The maximum difference between the experimental values and the fitting of this equation is 25% on average, as is also shown in the figure. This dispersion, which is wider for the higher dry densities (Fig. 6), is due both to the natural variability of bentonite and to the measurement method used, which 4. Results 4.1. Swelling pressure Fig. 5. Values of swelling pressure versus dry density of the bentonite at the end of the test, obtained for saturation with deionised water, and exponential fit. The initial water content of the samples was hygroscopic (13.2±1.2%).

6 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 6. Difference between the swelling pressure measured and calculated with Eq. (1) as a function of the dry density of the bentonite. does not allow for high degrees of accuracy in the determination of the final dry density. Added to this is the fact that the equipment was occasionally used at the limit of its capacity especially for dry densities in excess of 1.6 Mg/ m 3 allowing for a certain deformation of the sample, to which swelling pressure is highly sensitive, as it will be pointed out below. This translates into a wider dispersion of the values obtained for dry densities in excess of 1.6 Mg/m 3. Dispersions of a similar magnitude in the swelling pressure values determined in the laboratory have been observed by other authors (Dixon et al., 2002). At the end of the tests it was checked that the specimens were completely saturated. In fact, the degrees of saturation computed considering the density of water to be 1.00 Mg/m 3 are higher than 100%. This is because in the vicinity of clay laminae, the structure of water molecules is disturbed, their properties differing from those of free water and its density being higher than the value for free water (Martin, 1962; Low, 1976; Sposito and Prost, 1982; Skipper et al., 1991; Hueckel, 1992; Ichikawa et al., 1999; Hueckel et al., 2002; Marcial, 2003; Villar and Lloret, 2004). The average density of the pore water of the FEBEX bentonite has been calculated from the final water content of the samples tested, by adjusting the final degree of saturation to 100% in each of the tests performed (Villar, 2002). The values obtained are represented in Fig. 7, where, despite the dispersion, it is shown that the average water density increases when the volume of pores, and consequently the amount of free water, decreases. To check the influence of initial water content on the swelling pressure value, several tests have been performed. The clay was mixed with different proportions of deionised water and stabilised for several days before manufacturing the specimens by uniaxial compaction at different dry densities. The results obtained are shown in Fig. 8 and are in the order of those for the samples compacted with hygroscopic water content. Despite the dispersion of the data, it seems that there is no trend for the swelling pressure to change as a function of the initial water content Swelling deformation The saturation (or swelling) under load test makes it possible to determine the strain capacity of the soil when it saturates under a pressure previously applied. To check the influence of dry density on the swelling capacity a first set of tests was performed in which specimens obtained from the granulated material with hygroscopic water content compacted at different initial dry density were saturated under different loads with deionised water. Fig. 9 shows the evolution of strain versus time in the tests performed with specimens of nominal initial dry density of 1.60 Mg/m 3. The figure illustrates how Fig. 7. Values of average density of pore water calculated for specimens of bentonite subjected to swelling pressure and swelling deformation tests (modified from Villar, 2002). Fig. 8. Influence of initial water content on the swelling pressure of bentonite (the dry density of the samples is indicated in Mg/m 3 ).

7 44 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 9. Evolution of vertical strain during saturation under different vertical pressures of bentonite compacted with its hygroscopic water content to an initial dry density of 1.60 Mg/m 3. The figure on the right shows the strain normalised with respect to the final value. swelling develops more rapidly in the samples subjected to lower loads; this is due to the fact that these samples are able to expand more, this causing a greater increase in pore size and, therefore, in permeability. The duration of the tests was on average 18 days. At the end of the tests all the specimens were verified to be completely saturated, the degrees of saturation computed considering a value of 1.00 Mg/m 3 for the density of water being between 100 and 115%. They are lower than those measured in the swelling pressure tests (Fig. 7) because the final dry density of the swelling deformation tests is also lower. The final strains reached in the tests performed with bentonite compacted with hygroscopic water content and saturated under different vertical pressures are shown Fig. 10. The data can be fitted to the empirical Eq. (2), which has been used to draw the lines displayed in the figure. The equation relates final swelling strain (ε, %) to vertical pressure (σ, MPa) as a function of the initial dry density (ρ d0, Mg/m 3 ) for samples compacted with hygroscopic water content: e ¼ ð5:40q d0 1:32Þ lnr þð 48:25q d0 þ 63:69Þ: ð2þ To check the influence of initial water content on the swelling capacity, the same kind of tests was performed with bentonite previously mixed with different proportions of deionised water. The final strains upon saturation with deionised water under a vertical pressure of 0.1 MPa reached by specimens compacted at different initial dry densities are plotted as a function of the initial water content in Fig. 11. The results for vertical pressures of 0.5, 1.0 and 3.0 MPa are shown in Figs. 12, 13 and 14, respectively. Fig. 10. Vertical strain after saturation under different vertical pressure of bentonite compacted with its hygroscopic water content at various dry densities (indicated in Mg/m 3 ). Fig. 11. Final strain reached in soaking tests under a vertical pressure of 0.1 MPa for specimens compacted at different initial dry density and water content.

8 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 12. Final strain reached in soaking tests under a vertical pressure of 0.5 MPa for specimens compacted at different initial dry density and water content. An empirical relation has been found between swelling strain (ε, %), initial dry density (ρ d0,mg/m 3 ), initial water content (w 0, %) and vertical pressure (σ, MPa): e ¼½ 12:12 ð ln q d0 þ 1:89Þ lnr þ ð36:81q d0 53:59ÞŠ lnw 0 þ ð38:27 ln q d0 1:25Þ lnr þð 149:05q d0 þ 211:42Þ: ð3þ The lines displayed in Figs have been obtained with this equation. The predicting capability of the equation for the interval of density, water content and vertical pressure tested is very good. 5. Discussion The swelling pressure tests have shown, as expected in an expansive material, that this property is highly dependent on dry density (Pusch, 1979; Komornik et al., Fig. 14. Final strain reached in soaking tests under a vertical pressure of 3 MPa for specimens compacted at different initial dry density and water content. 1980; Graham et al., 1989). On the other hand, from the figures of swelling deformation tests it becomes clear that, indeed, the swelling capacity decreases as the vertical stress increases. Other authors have found relations between swelling deformation and vertical stress similar to those obtained in this work. Mollins et al. (1996) found that bentonite swells to reach a final state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, what has been confirmed in this work, as shown in Fig. 15, in which the results of Fig. 10 are replotted in terms of final void ratio. Moreover, the results presented here show that for a given water content and vertical pressure the higher the dry density, the larger the final strain (Kassiff and Ben Shalom, 1971). Fig. 16 represents schematically the paths followed in swelling pressure and swelling deformation tests in a suction/vertical stress space: the decrease of suction Fig. 13. Final strain reached in soaking tests under a vertical pressure of 1 MPa for specimens compacted at different initial dry density and water content. Fig. 15. Results of the swelling deformation tests performed in samples compacted with hygroscopic water content to different dry densities (indicated in Mg/m 3 ).

9 46 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 16. Generic stress paths followed in the tests for determination of swelling pressure. (hydration) and the increase of vertical stress do not follow the same sequence (Justo et al., 1984; Gens and Alonso, 1992). An alternative value of swelling pressure can be deduced from the swelling deformation tests by calculating through Eq. (3) the vertical pressure for which there is not macroscopic deformation of the bentonite for each dry density. The value calculated in this way is rather independent of initial water content, as it is also the experimental evidence of Fig. 8. For an initial water content equal to the hygroscopic one (Eq. (2)), the vertical pressures (σ, MPa) for which there is no deformation can be related to the initial dry density (ρ d0, Mg/m 3 ) through the following equation: r ¼ 0:09q 8:97 d0 : ð4þ The values obtained with this equation i.e. the swelling pressure values derived from swelling under load tests are plotted in Fig. 17 together with the empirical curve obtained from swelling pressure tests at constant volume (Eq. (1)). The influence of the path followed in the determination of swelling pressure on the final value obtained has been underlined and demonstrated repeatedly (Gens and Alonso, 1992). Justo et al. (1984) and Sridharan et al. (1986) report three methods to determine swelling pressure: the swell-load test, the swell under load test and the constant volume test. Among them, the first one, in which the sample is initially wetted under a small load and subsequently loaded to recover the initial void ratio, is reported to give the highest values of swelling pressure. The difference between the other two methods, which are the ones used in this work, is not so clear, although generally the constant volume test is considered to yield higher values than the swell under load test. In the case of the FEBEX bentonite the influence of the stress path on the swelling pressure value is small, but it seems noticeable for dry densities above 1.65 Mg/m 3. From this value on, for a given dry density, the final vertical pressure in a swelling pressure tests (a constant volume test in which the load is gradually applied) is higher than in a swelling deformation test (in which the sample is initially loaded and subsequently saturated). These small differences in swelling due to the different loading conditions during the hydration of the bentonite can be explained as a consequence of the coupling between the microstructural and macrostructural levels of plastic materials in the conceptual framework proposed by Gens and Alonso (1992) and Sánchez et al. (2005). According to this conceptual model, the swelling strain is caused by both microstructural and macrostructural strains. The former depend on the applied load and the suction change, and the latter on the proximity of the applied load to the apparent preconsolidation pressure (Lloret et al., 2003). In the swelling pressure tests, the load applied at the beginning of saturation is low and the deformation of the microstructure induces macrostructural strains that have to be compensated by applying vertical loads. If the initial suction is high enough, the load applied may match the preconsolidation pressure and the macrostructure collapses, what translates in this kind of tests in a decrease of the measured swelling pressure. Eventually, when suction is already low, the microstructural strains reach a maximum and they cannot be counterbalanced by the collapse of the macrostructure, what causes a further increase of swelling pressure. These three phases become clear in the swelling pressure evolution shown in Fig. 3 and have been illustrated in the generic stress path drawn in Fig. 16. In the swelling pressure tests, the sample can experience collapse or swelling strains during saturation, depending on the proximity of the load applied to the value of the preconsolidation pressure. Fig. 17. Vertical pressure for null vertical strain obtained in swelling pressure tests and swelling deformation tests in samples with initial hygroscopic water content.

10 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) Fig. 18. Final strain reached in the swelling deformation tests performed with samples of initial dry density 1.70 Mg/m 3 : experimental results and predictions with Eq. (3). The results displayed in Figs show that, for a particular vertical pressure, the influence of initial water content on the swelling capacity is more noticeable for the highest initial dry densities and that for a given dry density the swelling capacity decreases with water content, what stands patent in Fig. 18, in which the results for the initial dry density 1.70 Mg/m 3 have been replotted. This figure shows also that the influence of the initial water content is less evident as the vertical pressure is higher. In fact, for each initial dry density, there is a vertical pressure above which there is no influence of the initial water content on the swelling capacity. These values can be obtained from Eq. (3), and are plotted in Fig. 19, in which again the vertical pressure that must be applied to avoid deformation in the swelling under load tests is also represented (Eq. (4)). It can be observed that the vertical pressure above which there is no influence of the initial water content on the final strain is below the vertical pressure that has to be applied to avoid any deformation. This would be in accordance with the fact that the swelling pressure is apparently not affected by the initial water content of the clay. Several authors have observed a decrease of the swelling capacity of the clay as the initial water content increased (Kassiff and Ben Shalom, 1971; Day, 1998). Kassiff et al. (1973) present results of tests in which specimens of a plastic clay (w L =72%) compacted at dry densities of 1.5 Mg/m 3 with different initial water contents are saturated under different vertical loads. The results show that the swelling capacity of the clay saturated under a given vertical load decreases as the initial water content increases, although this difference becomes negligible when the vertical pressure approaches the swelling pressure value, what leads to the deduction that, although not explicitly measured, the swelling pressure value is not affected by the initial water content of the clay. The same idea is supported by Brackley (1973), who finds that the swell pressure of a weathered norite (w L =89%) is solely related to void ratio, while free swell is a function of original moisture content. Furthermore, the dependence of swelling on initial water content observed by Brackley (1973) and Kassiff et al. (1973) is much higher than that detected for the FEBEX bentonite, maybe because the vertical stresses applied by these other authors are lower. However, Komine and Ogata (1994) found that the maximum swelling deformation of a sodium bentonite compacted at dry densities between 1.24 and 1.99 Mg/ m 3 is almost independent of initial water content but that it increases in proportion to the initial dry density at a constant vertical pressure. They also found that the maximum swelling pressure increases exponentially with initial dry density but is independent of the initial water content. Sridharan et al. (1986) also observed an independence of swelling pressure with respect to the initial water content. The dependence of swelling deformation on water content and the lack of correlation between initial water content and swelling pressures that have been found in this work can also be analysed using the double-structure models proposed by Gens and Alonso (1992) and Sánchez et al. (2005). In the swelling pressure tests, when the initial suction is high, the preconsolidation pressure is reached during hydration and the macrostructure collapses. These collapses prevent any further increase of the swelling pressure, thus the swelling pressure reaches values close to the preconsolidation pressure in saturated conditions, which in turn depends on the initial dry density of the sample. As a result, the swelling pressure paths tend to converge towards a reduced range of mean stresses, what explains the lack of Fig. 19. Vertical pressure that must be applied to avoid deformation (Eq. (4)) and vertical pressure above which there is no influence of initial water content on the swelling capacity (both deduced from swelling deformation tests).

11 48 M.V. Villar, A. Lloret / Applied Clay Science 39 (2008) correlation between initial water content and swelling pressures (Gens and Alonso, 1992). On the other hand, in most of the swelling under load tests performed in this work, the vertical load applied during saturation was much lower than the apparent preconsolidation pressure. For this reason, the volume changes of the microstructure which depend on the initial suction (i.e. water content) have been large and have induced important volume changes in the macrostructure. As the vertical loads applied during saturation are higher and closer to the preconsolidation pressures, both the microstructural and the macrostructural strains tend to diminish, with what the effect of initial water content also becomes negligible. 6. Conclusions The influence of initial dry density and water content on the swelling properties of a heavily compacted bentonite has been examined by means of swelling pressure and swelling deformation tests. The swelling deformation of the bentonite is mainly related to the vertical pressure applied during saturation, between both variables there is a logarithmic relationship. The increase of dry density rises exponentially the swelling pressure of the bentonite and also the swelling deformation, although the increase of the latter is not so remarkable. The swelling pressure seems not to be affected by the initial water content of the clay. On the contrary, the swelling deformation for a particular dry density after saturation under a fixed vertical pressure decreases with initial water content. However, the influence of initial water content on the swelling capacity is smaller than the influence of initial dry density. Moreover, the influence of initial water content on the swelling deformation becomes negligible above a given vertical pressure, which is higher the higher the dry density. This value of vertical pressure is very close to the swelling pressure value. These observations suggest that the development of swelling pressure after saturation in the bentonite barrier of a HLW repository, which is a confined system in which swelling will take place always under a certain pressure, and in which the initial suction of the clay is very high, will be mainly related to variations of dry density. The dependence of the swelling pressure value on the suction/stress path followed has been also underlined, although it is significant only for dry densities higher than 1.65 Mg/m 3. These features of behaviour agree with the predictions of conceptual models that consider the interaction between the responses of the microstructure and the macrostructure of the material to suction and stress changes (Gens and Alonso, 1992; Sánchez et al., 2005). In addition, a comprehensive database on the swelling behaviour of the FEBEX bentonite has been collected, what allows the study of the mechanical modifications induced by thermo-hydraulic treatment on bentonite subjected to the conditions of a HLW repository, by comparison of the reference values collected in this work with those obtained in samples treated (Villar and Lloret, 2007). Acknowledgements Work co-funded by ENRESA and the European Commission and performed as part of the Fifth EURATOM Framework Programme, key action Nuclear Fission ( ), Project FEBEX II (EC Contract FIKW- CT ). The laboratory work was performed at Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT, Madrid) by J. Aroz and R. Campos. References Agus, S.S., Schanz, T., Swelling pressure and total suction of compacted bentonite sand mixtures. In: Bilsel, H., Nalbantoglu, Z. (Eds.), Problematic soils. Proc. Int. Conf. on Problematic Soils Geoprob 2005, vol. 1. 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