Mehrdokht Mohajerani

P Delage

M Monfared

A M Tang

J Sulem

B Gatmiri

Oedometric compression and swelling behaviour of the Callovo-Oxfordian argillite

Keywords: Argillite, saturation, high pressure oedometer compression, swelling behaviour

The Callovo-Oxfordian (COx) argillite is a possible host rock for radioactive waste disposal in which the ANDRA underground laboratory of Bure (East of France) has been excavated. In this paper some aspects of the volume change behaviour of the COx argillite are investigated. To do so, high pressure oedometers with a maximum capacity of 113 MPa have been used. In a first stage, swelling tests were carried out on samples initially compressed at constant initial water content (unsaturated) that were afterwards soaked under vertical loads, respectively, smaller and higher than the in-situ vertical stress. All samples exhibited swelling, even at stress higher than the in-situ one. In a second stage, standard step loading compression tests were carried out on samples previously saturated under the in-situ vertical load, so as to investigate the volume change behaviour under load cycles. The strain-stress curves obtained appear to be different to what is currently observed in overconsolidated or cemented clays with no clear appearance of yield and pre-yield reversible behaviour. The volumetric behaviour during both compaction and swelling is interpreted in terms of damage created by the collapse of pores within a fragile matrix. The amount of swelling observed is related to the extent of damage.

Introduction

The ANDRA underground research laboratory of Meuse-Haute-Marne, located near the village of Bure in the North-east of France [START_REF] Wileveau | Complete in situ stress determination in an. argillite sedimentary formation[END_REF][START_REF] Fouche | Fabric control on strain and rupture of heterogeneous shale samples by using a non-conventional mechanical test[END_REF], is composed of galleries excavated at the depths of 445 m and 490 m in a 200 m thick subhorizontal (1°-1.5° tilting) layer of the Callovo-Oxfordian (COx) argillite, an indurated clay rock dated 155 million years (limit upper-middle Jurassic). This layer is located in between two several hundred meters thick layers of Dogger (bottom) and Oxfordian (top) limestones. The main characteristics of the COx argillite are a very low hydraulic conductivity that restricts water transfer and a low diffusion coefficient that significantly delays solute transport. The COx argillite also has a low compressibility and its high sorption capacity for radionuclide makes it a proper potential site to store high activity radioactive wastes at great depth [START_REF] Escoffier | Caractérisation expérimentale du comportement hydromécanique des argilites de Meuse/Haute-Marne[END_REF][START_REF] Ozanam | Experimental determination of poroelastic parameters of a clayey rock[END_REF][START_REF] Zhang | Experimental study of the hydro-mechanical behaviour of the Callovo-Oxfodian argillite[END_REF]. The in-situ stress state of the COx argillite formation has been investigated in detail in [START_REF] Wileveau | Complete in situ stress determination in an. argillite sedimentary formation[END_REF]. At the depth of 490 m where the cores tested in this paper were taken from, the following total stress values have been obtained: σ v = 12.7MPa, σ h = 12.4MPa and σ H = 12.7-14.8MPa, indicating a nearly isotropic stress state.

Excavation of an underground repository induces shear and tensile cracks and creates an excavation damage zone (EDZ) around the galleries. The hydraulic conductivity of these cracks may be significantly higher than that of the undeformed host rock and this can affect the safety performance of the system. However, a gradual reconsolidation and sealing of the EDZ is expected after backfilling and repository closure, due to the creep and swelling capacity of the COx argillite. Hence, in the long-term view, the self-sealing capacity of clay rocks during the resaturation phase of the EDZ makes them particularly suitable for waste confinement.

Indeed, interesting self sealing behaviour of cracks, apparently due to the expansion of the hydrated clay fraction mobilised in the cracks [START_REF] Skoczylas | Permeability of macro-cracked argillite under confinement: Gas and water testing[END_REF][START_REF] Zhang | Damage and sealing of clay rocks detected by measurements of gas permeability[END_REF] has been experimentally evidenced. As a consequence, it appeared interesting to investigate the effect of damage on the swelling behaviour of argillite. In this context, a recent example of tunnelling in swelling ground has been reported by Carter et al. [START_REF] Carter | Tunnelling Issues with Chilean tertiary volcaniclastic rocks[END_REF]. From field observations and laboratory tests, they [START_REF] Carter | Tunnelling Issues with Chilean tertiary volcaniclastic rocks[END_REF] observed that swelling was highly enhanced in sheared zones and showed that either natural mechanical disturbance (fault, shear band) or man induced disturbance (excavating-trimming) could break down the bonds that in normal rock mass confine the swelling clays from freely expanding, thus promoting swelling. Aversa et al. [START_REF] Aversa | Some aspects of the mechanical behaviour of structured soils and soft rocks[END_REF], in their study on the mechanical behaviour of Italian tectonised shales, performed oedometric tests and observed that samples compressed under higher stresses had a greater swelling index, a feature that they related to the presence of discontinuities.

Oedometers were also used to determine the poroelastic parameters of the COx argillite. Vincké et al. [START_REF] Vincké | Investigation of the poromechanical behavior of shales in elastic domain[END_REF] performed a series of tests including loading-unloading cycles in axial stress (with constant pore pressure) and loading-unloading cycles in pore pressure (under constant axial stress). They obtained values for the Biot coefficient (b) ranging from 0.4 to 0.8 with vertical stress between 9 and 35MPa. Bemer et al. [START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF] found values ranging from 0.45 to 0.65 and an oedometric modulus ranging from 1 to 5 GPa under values of Terzaghi's effective axial stress between 5 and 25MPa. Homand et al. [START_REF] Homand | Pétrofabrique et propriétés mécaniques des argilites[END_REF] obtained values of b between 0.55 and 0.99 with an axial stress between 8 and 24MPa, and a drained oedometric modulus between 1.4 to 6GPa with an axial stress between 2 and 24MPa. All these parameters have been measured under axial stresses below 25MPa, a value lower than the uniaxial compressive strength of the COx argillite [START_REF] Zhang | Experimental study of the hydro-mechanical behaviour of the Callovo-Oxfodian argillite[END_REF][START_REF] Pham | Effects of desaturationresaturation on mudstone[END_REF]. This rather wide range of measured poromechanic parameters is likely to be due to various factors including the natural variability of the deposit (there are some significant changes in clay and carbonate contents with depth), the effects of anisotropy, the different initial state of the tested samples with respect to damage (due to extraction, stress release, drying, storage, transport and specimen trimming), to the initial water content and saturation degrees and to the saturation method applied prior to testing (when performed).

In this paper, the compressive/swelling behaviour of the COx argillite in relation with the effects of damage was investigated by means of oedometric tests. A first series of tests was carried out by soaking specimens that were previously compressed, at their initial water content, under stresses conditions smaller, close or higher than the in-situ one. In well preserved samples, swelling should only appear under a stress lower than the in-situ one and any swelling under higher stress could indicate the existence of pre-induced cracks. A second series of high pressure compression tests was carried out on carefully saturated samples, providing information on the compressive/swelling behaviour along various loadingunloading paths. The 1D configuration of the test also allowed to estimate the sample permeability by using the analytical solution governing the pore fluid dissipation in samples submitted to a constant vertical stress.

Material studied and samples preparation 2.1 Callovo-Oxfordian Argillite

The Callovo-Oxfordian (COx) argillite is an indurated clay of low porosity, low permeability and with a relatively high strength. Its mineralogical composition varies with depth. At a 490 m it contains 45% clay with 30-35% illite, 0-5% kaolinite and chlorite and 60-65% interstratified illite\smectite. COx clay also contains 28% carbonate (calcite), 23% quartz and 5% feldspars, pyrite, iron oxides [START_REF] Guéry | A micromechanical model of elastoplastic and damage behavior of a cohesive geomaterial[END_REF]. The smectite percentage in the clay fraction is responsible for the observed swelling capacity. The total porosity measured by Esteban [START_REF]Anisotropies magnétique et de porosité des argilites du Callovo-Oxfordien du laboratoire souterrain de l'Andra[END_REF] lies between 13% at levels that contain the highest carbonate fraction and 19.5% at levels having the highest clay content. Robinet [START_REF] Robinet | porosité et diffusion des solutes dans l'argilite du Callovo-Oxfordien de Bure (Meuse/Haute-Marne, France) de l'échelle centimétrique à micrométrique[END_REF] mentions that porosities as high as 27.5% can be found in layers containing a significant quantity of pyrite. The porosity of the samples tested here that come from the lower layers of the COx argillite deposit was generally found around 22% (Table 1).

Figure 1: Pore size distribution of the COx argillite (mercury intrusion), after [17].

The clay minerals have a lamellar structure with crystals composed of 10-100 stacked elementary layers of different silicates. Figure 1 presents the pore size distribution curve obtained by using mercury intrusion porosimetry [START_REF] Andra | Dossier Argiles, Référentiel du site Meuse[END_REF] that shows a well represented pore population at 20 nm with a second one at around 3-4 nm. The 20 nm pore population is related to the average entrance porosity within the clay fraction [START_REF] Andra | Dossier Argiles, Référentiel du site Meuse[END_REF]. As quoted by Montes et al. [START_REF] Montes | Structural modifications of Callovo-Oxfordian argillite under hydration/dehydration conditions[END_REF], the hydration and/or dehydration of the swelling clay minerals could generate changes in the clay structure of the rock, with a dependency of the water sensitivity on the pore structure, on the particle size and on the total porosity. By using digital image correlation (DIC), Bornert et al. [START_REF] Bornert | Multiscale full-field strain measurements for micromechanical investigations of the hydromechanical behaviour of clayey rocks[END_REF] recently provided interesting results showing, within the same specimen, significant changes in local stiffness according to the clay percentage of the zone considered.

Sample preparation and setting

The process of saturation of argillite samples prior to testing is of utmost importance given the sensitivity of argillite to changes in water content. Indeed, the results of Pham et al. [START_REF] Pham | Effects of desaturationresaturation on mudstone[END_REF] showed that decrease in water content (obtained by submitting samples to different values of controlled relative humidity RH) leads to significantly increase the unconfined compressive stress of COx samples (from 27 MPa at 98% RH to 57 MPa at 32% RH). The saturation of low permeability argillite samples (with hydraulic conductivity k around 10 -20 -10 -21 m 2 ) that have been desaturated during the processes of in-situ coring, sample storage, transport to the lab and finally sample trimming is difficult and can be significantly long. Also, saturation under zero stress condition can cause hydraulic damage evidenced by the appearance of cracks resulting from hydration and swelling, as shown by Valès [START_REF] Valès | Modes de déformation et d'endommagement de roches argileuses profondes sous sollicitations hydro-mécaniques[END_REF] and Bornert et al. [START_REF] Bornert | Multiscale full-field strain measurements for micromechanical investigations of the hydromechanical behaviour of clayey rocks[END_REF] by using Digital Image Correlation. The quality of the saturation prior to testing is indeed fundamental to investigate the poromechanical behaviour [START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF] with respect to microstructure effects and induced cracks [START_REF] Montes | Structural modifications of Callovo-Oxfordian argillite under hydration/dehydration conditions[END_REF]. The saturation method used by Bemer et al. [START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF] consisted in exposing the samples to an atmosphere close to 100% RH by putting them into a dessiccator containing pore water put at chemical equilibrium with crushed COx argillite from the same initial plug. Resaturation occurred through the vapour phase until the sample weight stabilised, which required 1 or 2 months. A 3% swelling was observed during this phase in which no stress was applied on the sample. In other tests [3?12] resaturation was performed close to in situ stress conditions under a 4MPa confining pressure and a 1MPa back pressure with a chemical composition of the pore water close to the one of the in situ pore water. Since swelling was observed under these conditions, these authors [START_REF] Escoffier | Caractérisation expérimentale du comportement hydromécanique des argilites de Meuse/Haute-Marne[END_REF][START_REF] Homand | Pétrofabrique et propriétés mécaniques des argilites[END_REF] also carried out tests under a confining pressure of 8MPa and a back pressure of 1MPa to prevent swelling.

In this paper, particular care was taken to avoid any swelling prior to testing. To do so, the samples were placed in the oedometer cell with initially dry porous disks that were afterwards water saturated so as to infiltrate the sample while maintaining the vertical strain close to zero by appropriate manually controlled loading. The mineralogical composition of the water injected in clay samples is known to be of utmost importance with respect to the clay-water interactions and to swelling behaviour. Since the specimens to be tested have been somewhat desaturated due to the evaporation of pure water (with salts remaining within the sample), the only way to bring back their pore water to its initial natural chemical composition is to infiltrate pure water at constant volume. This is what has been done here. To conserve afterwards the correct salt concentration, swelling tests should be conducted by infiltrating water with the same salt concentration as that of the pore water. This has not been done here in a purpose of simplicity and pure water was also used in swelling tests (note however that the situation is improved when swelling tests are conducted after compression tests, since some of the natural pore water has been expelled in the porous disk). Compared to natural pore water, the absence of salt in the water used tends to enhance the swelling behaviour.

Two kinds of oedometer cells were used. The three tests EST27396 n°1, EST27396 n°2, EST27396 n°3 were performed in oedometer rings of 50mm in diameter whereas the two tests EST28522 n°1, EST28522 n°2 were carried out in a 38mm diameter rings. Samples were cored to the desired diameter by using a diamond barrel.

The cored specimen was afterwards inserted in the ring and its top and bottom faces were carefully polished by using a sand paper sheet placed on a small plane wooden board. The polishing of the upper and down faces of the sample was completed once the wood piece came in contact with the cylindrical section of the metallic ring, thus achieving exactly the required sample thickness with two flat, regular and parallel faces. The loading direction was taken perpendicular to the stratification of samples.

The initial degree of saturation was determined by carefully measuring the sample volume (by using a precision calliper). The water content was determined by measuring the initial and final weights of a rock piece before and after drying in the oven at 105°C for one day. The initial suction of the samples was determined by using a chilled mirror dew-point hygrometer [START_REF] Leong | Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique[END_REF] fabricated by General Eastern. This device is based on the precise measurement of the relative humidity of the sample. It provides the total suction value within a period of time of about 15 min according to Kelvin's law [START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted unsaturated clay[END_REF][START_REF] Fredlund | Soil mechanics for unsaturated soils[END_REF]:

ln a w RT s u u h gM - = -= (1)
where u a and u w are respectively the air and water pressure, s is the total suction, R is the universal (molar) gas constant (R=8.3143 J.mol -1 .K -1 ), T is the absolute temperature (K), g is the gravity acceleration (g=9.81 m.s -2 ), M is the water molecular mass (M=18.016 g.mol -1 ) and h is the relative humidity. At 20°C, -137.837MPa RT gM =

As any technique based on vapour exchanges, the dew point hygrometer is suitable at high suctions, preferably higher than 3MPa [START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted unsaturated clay[END_REF][START_REF] Romero | Characterization and thermo-hydro-mechanical behaviour of unsaturated Boom clay; an experimental study[END_REF]. In this device, the sample is placed in a sealed chamber containing a mirror that can be chilled with a very precise temperature control. Once the relative humidity of the air in the chamber is equilibrated with that of the sample, the mirror is chilled down to the dew point temperature corresponding to the relative humidity in the chamber (the drier the atmosphere, the lower the dew point temperature). The accurate determination of the dew point is made by detecting the condensation of micro water bubbles on the surface of the mirror by an optical system. The suction is deduced from the relative humidity that corresponds to the dew point temperature.

The parameters characterising the initial states of the samples are presented in Table 1. As expected, with initial suctions as high as 17 to 34MPa, the samples were significantly desaturated just after trimming. According to calculations by using Jurin's law [START_REF] Jurin | An account of some experiments shown before the Royal Society; with an enquiry into the cause of the ascent and suspension of water in capillary tubes[END_REF] based on the pore size distribution curve presented in Figure 1 [START_REF] Andra | Dossier Argiles, Référentiel du site Meuse[END_REF] and adopting the hypothesis of cylindrical pores, one can estimate the air entry value of the COx argillite. Jurin's law [START_REF] Jurin | An account of some experiments shown before the Royal Society; with an enquiry into the cause of the ascent and suspension of water in capillary tubes[END_REF] is written as follows:

4 cos

s a w u u D σ θ -= (2)
in which σ s is the surface tension at the air-water interface and θ is the contact fluid-solid angle (for water, cos θ = 1 and σ s = 72.75×10 -3 N/m at 20°C). D is the diameter of the cylinder. With D = 20nm (see Figure 1), the air-entry value of the COx sample can be estimated around 14MPa, indicating probable air continuity within the argillite at higher suction.

In this study, a high pressure oedometer developed in CERMES was used [START_REF] Marcial | On the high stress compression of bentonites[END_REF]. The apparatus has a double lever arm (with one lever arm multiplying the load by 5 and the second one by 10) able to apply a maximum axial stress of 113MPa on a 38mm diameter specimen. The specimen is placed on a metallic porous disk inserted in the cell base and connected to two lateral valves that allow satisfactory saturation of the porous disk as described later in more details. Another metallic porous disk is placed on top of the sample to allow vertical drainage through the top and bottom faces. Vertical displacements are monitored by using an electronic LVDT transducer connected to a data acquisition system.

An initial calibration of the system was carried out by running a test with no sample in the ring so as to account for the effects of the compression of the top and bottom metallic porous disks, of the cell and of the piston. A reversible response of the system was observed after applying various loading cycles between 0 and 113MPa. During all tests, the compression displacement of the sample was obtained by subtracting the calibrated elastic compression of the system from the total measured compression displacement.

Swelling tests

One of the standard techniques used to estimate the swelling stress of soils consists in running in parallel several oedometric tests in which the samples (at given initial water content) are first loaded under various vertical stresses that are chosen smaller and larger than the estimated swelling stress. The samples are then soaked under these stresses while measuring the volume changes. The swelling stress is interpolated by drawing a curve passing through all the experimental points (either after swelling or compaction) at the intersection of the curve with the no volume change line.

This approach has been applied to the COx argillite. Knowing the in-situ condition (approximated by a total vertical stress σ v = 12MPa and a pore pressure u = 4MPa, [START_REF] Wileveau | Complete in situ stress determination in an. argillite sedimentary formation[END_REF]), three samples were loaded (at given initial water content, see Table 1) to vertical stresses equal to 8, 10 and 12 MPa under which they were subsequently soaked.

Based on the values of the Biot coefficient presented in the Introduction (between 0.4 and 1), the estimated effective stress could be between 8 and 10.4MPa. Hence, the first load (8MPa) corresponds to the lower bound of the in-situ vertical effective stress, the second value (10MPa) to the upper bound whereas the third one (12MPa) is larger. In such conditions, one should theoretically observe some swelling under 8MPa, little volume change under 10MPa and some compression under 12MPa.

The initial characteristics of the three samples are presented in Table 1. As expected, the water saturation is far from being complete in the samples. The initial compression was conducted at constant initial water content around 6% (see exact values in Table 1) by following a standard step loading procedure with the load doubled at each step. Axial stress (MPa)

EST27396 n°1 EST27396 n°2 EST27396 n°3
Loading Soaking Soaking Soaking

Figure 2: Oedometric compression curves and hydration test under various vertical loads (EST27396 n°1, 2 and 3).

Results are presented in Figure 2 in an axial stress versus vertical strain diagram. One can observe that the three compression curves are reasonably similar, probably due to comparable initial unsaturated states (Table 1). Once soaked, swelling is observed under the three stresses applied, including under the higher one (12MPa) that is larger than the in-situ vertical effective stress. The swelling strains are equal to 0.53, 0.26 and 0.35% under 8, 10 and 12MPa respectively. They are not ordered with respect to the stress applied as expected, since a higher strain is observed under 12MPa compared to 10MPa.

It is well known that a sample should not swell under a vertical effective stress higher than the in-situ effective stress in the case of 'perfect sampling' [START_REF] Skempton | The behaviour of saturated clays during sampling and testing[END_REF]. However, the quality of the COx samples has been affected by the excavation, stress release, drying, transport and trimming procedures and all samples hence contain some induced cracks that result in an enhanced swelling behaviour.

During the loading phase, some crack closure can occur under low stress level but at higher stress, new cracks can form and propagate. It is thus expected that after loading the swelling properties during hydration would be enhanced.

High pressure oedometric compression tests

Two oedometric compression tests were carried out on two samples EST28522 n°1 and EST28522 n°2 (see Table 1). The sample was placed into the oedometer ring that was previously coated with grease to reduce friction along the lateral surface of the specimen. The porous disks were kept dry so as to ensure a well controlled saturation procedure under in-situ stress conditions close to the natural ones. To ensure good contact between the sample, the porous disks and the piston, a previous low stress loading-unloading cycle was applied prior to injecting pore water up to 1MPa (a value deemed small enough to preserve the sample from any change). During this cycle, all pore water valves were closed to avoid any drying of sample. For the same reason, this preliminary stress cycle was run rapidly, in less than 5 minutes.

The saturation of the bottom porous disk was carried out by infiltrating water under small pressure into the disk. To do so, one of the drainage valves of the cell base was connected to the pore water reservoir whereas the other one was kept open so as to allow the evacuation of the air from the dry porous disk during water infiltration. This valve was closed once water started to flow out. Two different saturation procedures with minimised swelling were followed prior to carry out the compression test. In test EST28522 n°1, compression was carried out up to 7.09MPa at constant water content. Under this stress, water was infiltrated which resulted in a slight swelling as shown in Figure 3. Once a 2 µm swelling was reached, the load was doubled and compression started with no more swelling. The saturation procedure was improved in test EST28522 n°2 as follows. Water was infiltrated under a lower stress of 3.5MPa (a value lower than the interval proposed for the in situ effective stress between 8 and 10.4). As soon as the monitored vertical displacement reached a maximum value of 2 µm, the applied load was doubled. This procedure was carried out until reaching a load with no further swelling.

As shown in Figure 4 (sample EST28522 n°2), swelling is observed up to 1.75MPa with compression starting at 3.5MPa. Further inspection of Figure 4 shows that the constant volume condition is in fact approximate. Whereas, swelling above the initial sample volume is indeed maintained below 2 µm as planned.

Test 1: Test EST28522 n°1 with one load cycle

A loading-unloading cycle was carried out up to the maximum vertical stress of 113MPa on sample EST28522 n°1. The cycle was conducted by steps with the load doubled at each step. The unloading sequence was performed in the same way. Each loading step lasted up to 2-3 days so as to reach vertical strain stabilization. Strain/time curves under constant load were monitored at each step. The total test duration test was about one month (The saturation procedure adopted is the same as that of Figure 3).The results of test EST28522 n°1 are presented Figure 5 where the changes of the vertical strain and the load are plotted versus time. One can see that a vertical compression strain of 10% is attained under the maximum stress of 113MPa. The behaviour during the unloading phase is also remarkable with a significant swelling observed at each step and a final strain bringing back the sample very close to the initial state. ---------------------------------------------------------------------------------------------------------------------------------------10 This trend is also observed in Figure 6 in which the changes in vertical load versus vertical strain are plotted in linear scale. Under an applied axial stress as high as 113MPa, the sample is very likely beyond the elastic limit. At maximum axial load the compressive strain is 9.6%, a value comparable with that obtained by Heitz and Hicher [START_REF] Heitz | The mechanical behaviour of argillaceous rocks -Some questions from laboratory experiments[END_REF] who obtained an 8% strain under 100MPa in an oedometer test performed on a COx sample.

The unloading curve is strongly non linear and similar to that of swelling plastic soils with a significant swelling mobilised at the end of the unloading phase. Actually, the progressive mobilisation of swelling is observed in the diagram that clearly shows that the swelling brings back the sample volume very close to the initial volume, with more than 60% of the swelling mobilised between 10 and 0MPa.

Test 2: Test EST28522 n°2 with two loading cycles

To confirm the results of the previous compression test and to further investigate the significant swelling observed upon unloading, a second oedometric test was carried out on sample (EST28522 n°2) in which an intermediate loading cycle was performed before reaching the maximum load of 113MPa (the saturation procedure adopted is the same as that of Figure 4). -------------------------------------------------------------------------------------------------------------------------------------- As seen in Figure 7, the sample was first loaded up to 28MPa and subsequently unloaded. It was preferred not to unload the sample below 0.4MPa so as to avoid reaching a void ratio larger than the initial one. The sample was subsequently re-loaded to 113MPa and unloaded down to zero. The test data are presented as a function of time in Figure 7 and the compression curve is presented in a diagram giving the axial stress versus the axial strain in Figure 8. At the end of the first cycle, the second loading curve between 0.4 and 28MPa passes below the first compression curve to join it at the maximum stress applied (28MPa). At higher stresses, the curve is in the prolongation of that of the first loading sequence.

M. Mohajerani et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 606-615 -

The second unloading sequence is carried out from the maximum load of 113MPa. The subsequent swelling under unloading is again significant and comparable to that observed in Figure 6. The shape of the plot is similar to the previous one.

In order to compare the compression curves obtained here with the data already published by Aversa et al. [START_REF] Aversa | Some aspects of the mechanical behaviour of structured soils and soft rocks[END_REF] on Italian tectonised shales and by Heitz and Hicher [START_REF] Heitz | The mechanical behaviour of argillaceous rocks -Some questions from laboratory experiments[END_REF] on various clay shales including the COx argillite, the strain-stress curves of tests 1 and 2 are now presented like these authors by plotting the changes in axial strain with respect to the decimal logarithm of the vertical stress (the standard presentation of oedometric curves in clays) in Figure 9. One can observe that the compression curves are reasonably similar.

Whereas the curved shape of the curve in the semi-log plot would allow the determination of a yield stress, as usually done in Soil mechanics, the linear plots of Figure 6 (test 1) and Figure 8 (test 2) did not show any evidence of yield behaviour. The curves have a slight curvature (much less than what is currently observed in clays) with no significant change of slope that could allow separate an elastic regime from a plastic one. It seems then difficult to define any yield stress for the COx argillite by using the standard interpretation of oedometric tests.

Figure 9: Oedometric strain-stress curves in semi-log plot (EST28522 n°1 and EST28522 n°2).

In Figure 9, one can observe that the average slope of the intermediate unloading curve is less than that of the final one, in accordance with the data obtained by Aversa et al. [START_REF] Aversa | Some aspects of the mechanical behaviour of structured soils and soft rocks[END_REF]. Accordingly, the swelling strain under the same stress decrement along the unloading path (between 28 and 14MPa for instance) is much higher in the sample compressed at 113MPa compared to that compressed at 28MPa (Figure 10). This higher swelling potential at higher density is typical of swelling clays. This trend is illustrated in Figure 10 in which the swelling modulus (the slope of the swelling curve in an axial strain/axial stress diagram) is represented with respect to the axial stress. A clear distinction is observed between the first cycle at intermediate stress and the second one at maximum stress.

Discussion

The main conclusion drawn from the oedometric tests carried out here is that the COx argillite exhibits significant swelling behaviour when hydrated along various hydromechanical paths. Some previous works (e.g. [START_REF] Skoczylas | Permeability of macro-cracked argillite under confinement: Gas and water testing[END_REF]) have linked this swelling capacity to the presence of cracks in the argillite, showing, in a sample with an artificial crack, that the swelling of the hydrated clay fraction (interstratified illite-smectite particles) was mobilised along the faces of the crack. The same phenomenon also explains the good sealing capacity of the argillite, made possible by the filling of the pore volume of the cracks by the swollen hydrated clay particles. In this regard, the swelling capacity can be related to the crack density, i.e. to the damage induced in the argillite. Conversely, significant swelling behaviour reflects significant damage. Argillite is a fragile material and cracks can be created by various phenomena. Besides the cracks induced by shearing that result in a degradation of the elastic modulus as described in standard damage theory, the appearance of cracks has also been visually observed during drying and wetting phases (as an extreme case, a COx specimen placed in pure water progressively disaggregates due to the generation of a crack network along the bedding). Hydration cracks have also been locally observed by Valès [START_REF] Valès | Modes de déformation et d'endommagement de roches argileuses profondes sous sollicitations hydro-mécaniques[END_REF] and Bornert et al. [START_REF] Bornert | Multiscale full-field strain measurements for micromechanical investigations of the hydromechanical behaviour of clayey rocks[END_REF] by using digital image correlation (DIC) techniques. These observations confirmed the necessity of performing preliminary sample saturation under in-situ stresses to minimise any perturbations due to swelling, as observed in stiff swelling clays like Boom clay [START_REF] Delage | Suction effects in deep Boom clay block samples[END_REF].

Swelling was observed in the first series of oedometric compression tests at constant water content (around 6%) and initial degrees of saturation close to 60% when wetting the specimens under a vertical stress higher than the in-situ one. This swelling is related to sample disturbance and drying induced damage during coring, extraction, storage, transport and sample trimming.

A further process of crack generation is suspected to occur during oedometric compression. Based on previous results obtained in sensitive soft clays [START_REF] Delage | A microstructure approach of the sensitivity and compressibility of some Eastern Canada sensitive clays[END_REF][START_REF] Delage | Study of the structure of a sensitive Champlain clay and its evolution during consolidation[END_REF][START_REF] Cerrolaza | Microstructure and volume change behaviour of soft clays: a boundary elements simulation[END_REF] in which it appeared that compression occurred by the ordered collapse of pores, starting from the largest and progressively affecting smaller and smaller pores. This mechanism was confirmed by scanning electron microscope (SEM) observations that also showed that the elastic unloading rebound was due to the local swelling of the collapsed zones.

Although no SEM analysis has been performed in the COx argillite, it is proposed to also consider it as a fragile porous matrix that exhibits pore collapse during compression. Cracks are supposed to be formed due to local pore collapse, resulting in local disturbed zones with enhanced swelling capacity once hydrated. Since compression at higher stress induces the collapse of more pores, the swelling capacity of the compressed argillite is enhanced by the creation of additional cracks. For this reason, a sample compressed under a higher stress exhibits, under the same vertical load, a higher swelling potential (Figures 9 and10).

The two oedometric compression curves obtained here are now compared to those of Bemer et al. [START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF] in Figure 11. The data of test EST28522 n°1 with no loading cycle are quite comparable with one of the curves of Bemer et al. [START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF] whereas those of the test EST28522 n°2 with an intermediate loading cycle exhibit a higher compressibility. Drained elastic oedometric moduli (M oedo ) of the COx argillite are the slopes of stress-strain linear curve calculated from the unloading sequence close to the maximum applied stress, in an area where swelling appears to be impeded by the applied stress, with reasonably low swelling strain. In Figure 12, the calculated drained oedometric moduli M oedo are plotted and compared to that obtained by Bemer. A reasonably good agreement is observed between 28.4 and 14.2MPa axial stress with a value of M oedo = 3.34GPa. This study extends the data with two M oedo values of 8.25 (EST28522 n°2) and 7.64GPa (EST28522 n°1) obtained between 113.4 and 56.7MPa. It also confirms the non linear stress dependency of the oedometric modulus.

Permeability estimation from oedometric tests

Oedometric tests are currently used in Soil mechanics to indirectly determine changes in permeability during compression by back analysing the transient response obtained once the sample has been loaded under a constant total stress. Given the difficulty of measuring permeability in low porosity claystones, this method is now examined based on the data obtained here.

The diffusion boundary value problem corresponding to the changes in excess water pore pressure of saturated geomaterials during transient loading or unloading phases has been considered in detail within the Biot poroelastic theory framework by various authors [33][START_REF] Detournay | Fundamental of poroelasticy[END_REF][START_REF] Hart | A single test method for determination of poroelastic constants and flow parameters in rocks with low hydraulic conductivities[END_REF][START_REF] Hsieh | A Transient Laboratory Method for Determining the Hydraulic Properties of' Tight Rocks[END_REF]. The two transient unloading phases that have been previously used to compute the elastic oedometric moduli are interpreted on the basis of the analytical solution obtained for one dimensional diffusive flow [33]. The first transient phase studied is the unloading phase from 28.35 to 14.17MPa in test 2 and the second one is the unloading phase from 113.4 to 56.7MPa in test 1. The well known general form of the consolidation equation is as follows:

2 2 w w v du u c dt z ∂ = ∂ (3)
The following expression for the coefficient of consolidation (c v ) was proposed by Coussy [33]:

2 1 ( ) 1 v w oedo k c b MC η = + (4)
in which:

1 ( ) s w b C C M φ φ = - + (5) 1 s d C b C = - (6)
C d is the drained isotropic compressibility of the sample, C s is the compressibility of the solid grains (0.02GPa -1 [START_REF] Mctigue | Thermoelastic response of fluid-saturated porous rock[END_REF]), C w is the water compressibility (0.447 GPa -1 ; [START_REF] Spang | Excel Add-In for Properties of Water and Steam in SI-Units[END_REF]), φ is the sample porosity at the beginning of each transient phase (φ = 18.7% in the first unloading phase of test 2 from 28.35 to 14.17MPa and φ = 13.7% in the second unloading phase of test 1 from 113.4 to 56.7MPa). One can observe in Eq.6 that the equivalent isotropic compressibility C d is needed to compute the Biot coefficient. To relate C d to the oedometric compressibility (C oedo oedo ), the stress-strain elastic relationship is written for both isotropic loading and oedometric conditions for a transverse isotropic material [START_REF] Cheng | Material coefficients of anisotropic poroelasticity[END_REF]: [START_REF] Homand | Pétrofabrique et propriétés mécaniques des argilites[END_REF] 12

1 1 1 1 1 23 21 2 2 2 2 2 3 3 23 21 2 2 2 1 1 1 E E E E E E E E E ν ν ε σ ν ν ε σ ε σ ν ν   - -           - -     =                   - -       (7)
where E 2 ,ν 23 are the Young modulus and Poisson ratio in the isotropic plane (x 2 , x 3 ) (the bedding plane) and E 1 ,ν 12 are the Young modulus and Poisson ratio in the (x 1 , x 2 ) plane (perpendicular to bedding plane), respectively. The following relationship holds: [START_REF] Homand | Pétrofabrique et propriétés mécaniques des argilites[END_REF] 21

1 2 E E ν ν = (8)
Under oedometric conditions (ε rad =ε 2 =ε 3 =0, σ rad =σ 2 =σ 3 , ε 1 =ε ax and σ 1 =σ ax ), the strainstress relationship becomes:

12 21 1 1 1 23 1 1 2 1 E ν ν ε σ ν   = -   -   (9)
12 21

1 23 1 1 2 1 oedo C E ν ν ν   = -   -   (10)
Under isotropic loading (σ 1 =σ 2 =σ 3 =σ) ( )

1 2 3 12 23 1 2 1 2 1 4 (1 ) v E E ε ε ε ε ν ν σ   = + + = - + -     (11)
( )

12 12 23 1 21 1 1 4 2 (1 ) d C E ν ν ν ν   = - + -     (12)
Comparing Eq.7 and Eq.9, the relationship between C oedo and C d is written as:

( )

(1 ) 2

d oedo C C ν ν ν ν ν ν ν ν   - + - -     = - - (13)
According to [START_REF] Homand | Pétrofabrique et propriétés mécaniques des argilites[END_REF], the Poisson coefficients of the COx argillite are: Giving, for the COx argillite:

1.5

d oedo C C = (14)
According to these relations, the estimated values of the equivalent drained isotropic compressibility (C d ) and of the Biot coefficient (b) are hence respectively equal to 0.45GPa -1 and 0.96 for the unloading phase from 28.35 to 14.17MPa (test 2) and 0.195GPa -1 and 0.96 for the unloading phase from 113.4 to 56.7MPa (test 1).

To solve the diffusion equation (Eq.3), the boundary and initial conditions must be prescribed. In one dimension compression, the sample is drained from top and bottom with a drainage length (H dr ) equal to half of the sample thickness (5 mm) giving u w = 0 at z = 0 and 2 H dr at any time. At t = 0, the instantaneous response in pore pressure (u w0 ) is undrained and defined by the relation:

0 w u B p = ∆ ( 15
)
where B is the Skempton coefficient [START_REF] Skempton | The Pore Pressure Coefficients A and B[END_REF] and (∆p) is the mean stress increment. In oedometric conditions, this relation becomes:

0 ( 2 ) / 3 w ax rad u B σ σ = ∆ + ∆ (16)
σ rad is calculated from the elastic relationships (Eq.7) assuming ε rad =0;

The Skempton coefficient is defined as:

( ) d s d s w s C C B C C C C φ - = -+ - (17)
The Skempton coefficient B is equal to 0.84 for the unloading phase from 28.35 to 14.17 MPa (test 2) and equal to 0.75 for the unloading phase from 113.4 to 56.7MPa (test 1).

The analytical solution of the consolidation equation that provides the pore pressure as a function of time (t) and altitude (z) is as follows [START_REF] Carslaw | Conduction of Heats in Solids[END_REF]:

2 0 0 2 ( , ) sin v i m T w w i dr u mz u z t e m H =∞ - =     =           ∑ (18)
in which i is an integer and ( Knowing the pore pressure changes, the changes in strain can be computed. To do so the sample is subdivided into different elementary layers with thickness dz at the altitude z. The displacement of this layer at time t (

( ) elem z t dis
) is estimated from the constitutive equation as follows:

( )

( ( , )

).

elem z t oedo w w dis bC u z t u dz = - - (19)
The total displacement of the sample at time t is the integral of

where (2H dr ) is the sample height.

Using the expression of u w (eq. 18), we get:

The observed strain/time data from the unloading sequences have been fitted with the analytical solution by adjusting the permeability value.

Axial strain

Experimental values Calculated values The fitting presented in Figure 13 and Figure 14 have been obtained with permeability values of 2×10 -21 m 2 and 0.6×10 -21 m 2 under loads of 14.17 and 56.7MPa, respectively. These values are reasonable for highly compressed COx argillite [START_REF] Escoffier | Under stress permeability determination of the Meuse/Haute-Marne mudstone[END_REF].

Time (hours)

-0.098 -0.097 -0.096 -0.095 -0.094 In Figure 15 one can observe that the permeability of the sample decreases at higher applied stress. The sample at higher applied stress has also the smaller porosity.

Axial strain

Experimental values Calculated values

Conclusion

Clay shales often exhibit swelling properties, an interesting property regarding the long term safety of radioactive waste disposal. In the literature, swelling behaviour in rocks has often been linked with the development of cracks and damage, either of a natural or of a manmade origin. An experimental investigation of the compression and swelling behaviour of the Callovo-Oxfordian argillite was conducted to further investigate these aspects. To do so, two series of oedometric tests were carried out. In the first series, the argillite appeared to exhibit a swelling behaviour even when soaked under stresses higher than the in-situ stress. In the second series of tests, the swelling capacity appeared to increase with compression. In both cases, swelling was related either to pre-existing cracks due to sample coring, storage, drying, wetting and trimming or to cracks induced during compression. In this regard, compression was suspected to occur by local pore collapse that created micro-cracks that afterwards swelled when hydrated. Indeed, sample compressed at higher stress exhibited higher swelling potential.

Oedometer tests also provided some information on the compression behaviour of the argillite, showing that the determination of a yield stress in a graph giving the volumetric changes with respect to the logarithm of the stress was not as straightforward as in clays. As a consequence, the similarity somewhat considered between argillites and overconsolidated clays is not confirmed. Finally, the transient unloading sequences of the oedometer tests were used to estimate the argillite hydraulic conductivity. This parameter which is essential to assess the long-term safety of radioactive wastes, is difficult to measure and this simple approach provides new useful data.

0 - 0

 0 .005 -0.01 -0.015 -0.02 -0.025 -0.03

Figure 3 :

 3 Figure 3: Saturation phase (EST28522 n°1).

Figure 4 :

 4 Figure 4: Saturation phase (EST28522 n°2).

Figure 5 :

 5 Figure 5: Axial compression and axial stress versus time (EST28522 n°1).

Figure 6 :

 6 Figure 6: Oedometric compression curve (EST28522 n°1).

Figure 7 :

 7 Compression strain and axial stress versus time (EST28522 n°1).

Figure 8 :

 8 Figure 8: Compression curves (EST28522 n°2).

Figure 10 :

 10 Figure 10: Variation of swelling modulus with axial stresses for two consecutive cycles (EST28522 n°2)

Figure 11 :

 11 Figure 11: Comparison of the data obtained here with the oedometric compression tests of Bemer et al. [11].

Figure 12 :

 12 Figure 12: Stress dependency of the oedometric modulus: comparison of the data obtained herewith that of Bemer et al.[START_REF] Bemer | Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states[END_REF]

Figure 13 :

 13 Figure 13: Experimental (EST28522 n°2) and calculated strain changes with time during generated pore pressure diffusion, vertical load equal to 14.17MPa.

Figure 14 :

 14 Figure 14: Experimental (EST28522 n°1) and calculated strain changes with time during generated pore pressure diffusion, vertical load equal to 56.7 MPa.

Figure 15 :

 15 Figure 15: Estimated permeability of COx argillite versus porosity.

Table 1 . Initial characteristics of the tested samples.

 1

	Sample	EST27396 n°1 EST27396 n°2 EST27396 n°3 EST28522 n°1 EST28522 n°2
	Depth (m)		483		481
	Water content (%)	5.9	6.6	6.3	7.1	6.1
	Degree of saturation (%)	54.2	60.6	57.8	68	56
	Porosity(%)	22	22	22	22	22
	Measured relative humidity	78	81	80	87	79
	(%)
	Suction (MPa)	34	28	30	17	32
	Sample diameter (mm)		50		38
	Sample height (mm)		15		10
	Type of test	Compression followed by swelling under various	High pressure compression with
		constant stresses (8, 10 and 12 MPa)	loading cycles

0.5

1.5

2.5

3.5

4.5

5.5

6.5

Acknowledgements

The work presented in this paper has been carried out during the preparation of the PhD thesis of the first author that was partly funded by ANDRA, the French Agency in charge the management of radio-active waste disposal. The PhD thesis was co-funded by ENPC, Paris. The views presented here are those of the authors and do not engage ANDRA in any matter.


Philippe Capet

Thomas Delavallade

Takuya Nakamura

Agnes Sandor

Cedric Tarsitano

Stavroula Voyatzi

In this article we describe the joint effort of experts in linguistics, information extraction and risk assessment to integrate EventSpotter, an automatic event extraction engine, into ADAC, an automated early warning system. By detecting as early as possible weak signals of emerging risks ADAC provides a dynamic synthetic picture of situations involving risk. The ADAC system calculates risk on the basis of fuzzy logic rules operated on a template graph whose leaves are event types. EventSpotter is based on a general purpose natural language dependency parser, XIP, enhanced with domain-specific lexical resources (Lexicon-Grammar). Its role is to automatically feed the leaves with input data.

Introduction

In various fields rational risk analysis is part of the decision making process. It is a fundamental methodological tool which helps economic and political actors to anticipate potential crises. Such an analysis is usually carried out by human experts. The first step in risk analysis is the retrieval of relevant information from available data. The amount of the data may be so large that there is a great need for tools that automate parts of the risk analysis. An early

AUTOMATIC EXTRACTION OF EVENT TYPES A RISK ASSESSMENT SYSTEM WITH

Please use the following format when citing this chapter: Capet, P., Delavallade, T., Nakamura, T., Sandor, A., Tarsitano, C. and Voyatzi, S., 2008, in IFIP International Federation for Information Processing, Volume 288; Intelligent Information Processing IV; Zhongzhi Shi, E. Mercier-Laurent, D. Leake; (Boston: Springer), pp. 220-229.

warning system should help experts to monitor massive flows of events, in the short term, and launch alerts whenever critical event sequences are detected.

For this purpose we are designing ADAC [START_REF] Delavallade | Monitoring Event Flows and Modelling Scenarios for Crisis Prediction: Application to Ethinc Conflict Forecasting[END_REF], an automated early warning system that provides a dynamic synthetic picture of situations involving risk. ADAC is being implemented for detecting weak signals of nuclear proliferation, in order to issue alerts about emerging nuclear risks as early as possible. Risk assessment in this domain has to process large amounts of knowledge -such as educational changes in a particular country, public statements of local leaders, covert information, diplomatic negotiations, satellite observations, etc-that can only be acquired through widely disparate channels of information. A significant amount of this knowledge is directly derivable from the events described in information newswires. At present, data concerning events are introduced into the ADAC system by human analysts.

However, the exponentially growing information flow through the internet no longer allows human analysts to keep abreast of the events referred to in the newswire sources. On the other hand, the more extensively a risk assessment system is populated the more reliable it is. Thus the use of an automatic information extraction (IE) system has become a necessary component of any risk assessment system based on the continuous monitoring of event flows.

In this article we describe the underlying principles of ongoing work: the joint effort of experts in linguistics, IE and risk assessment to integrate Event-Spotter, an automatic event extraction engine, into ADAC. This paper is organized as follows: In section 2 we describe ADAC, the risk assessment component that needs to be fed with automatically extracted events. In section 3 we present EventSpotter pointing out its innovative features compared to other event extraction systems. We argue that these features are necessary in order to meet the requirements of the subsequent risk assessment modules. In this section we underline the importance of the integration of extensive lexical resources into the event extraction system, and briefly describe their form and the principles that lead us to constitute them. We also present an evaluation of the present state of the IE system. In section 4 we present some related work in IE applied to event extraction. Finally, in section 5 we draw some conclusions and show directions for future work.

2.

The Risk Assessment System: ADAC ADAC is a dynamic risk assessment system that monitors the daily evolution of the situation in a particular domain in order to give experts a better understanding of situations involving risk. Based on a library of experts' scenarios describing typical crisis developments and on an ontology representing the domain knowledge, the system monitors a flow of incoming event in order to spot the event sequences that are likely to end up in crisis. This section is dedicated to the presentation of the various components of the ADAC system.

Scenarios

A scenario describes typical developments of a specific type of crisis, i.e. it depicts how a system moves from a normal, sound state to a critical, anarchical state. Scenarios are expressed in the template formalism. The general principle of a template is the description of a complex phenomenon as a combination (conjunction, disjunction, etc.) of less complex phenomena which again are decomposed into a combination of less complex phenomena until elementary phenomena are reached, namely the events directly observed from the input.

Ontology

The scenarios, represented by templates, are part of the knowledge base feeding the system, which is more general than a simple scenario library. It gathers all the information the system has at its disposal to perform crisis detection. It contains all the linguistic terms, organized in a subsumption hierarchy, that are necessary to link the scenarios with the input data. Locations and actors that are under watch are for instance defined in this ontology.

Event Data

The data used to feed the system are structured representations of occurring events, related to the particular type of crisis under study. The structure used to summarize a piece of information is defined by the following fields: the source reporting the information, the date and location of the reported event, the event type, the actors of the event (persons or organizations), the other persons or organizations involved in the event (other participants), the uncertainty of the event from the source's point of view (does the source report the event as a fact, is it an assumption, an opinion... ?). The choice of these fields has been inspired by the work of political scientists concerning the anticipation, monitoring and termination of wars [START_REF] Schrodt | Political Science: KEDS-A Program for the Machine Coding of Event Data[END_REF].

Recognition Engine

The core of our system consists in comparing input event data and known scenarios of crisis developments, through a constrained pattern matching process. This is done by our recognition engine, which assesses the degree of match between event sequences and experts' scenarios, taking into account some, spatial, temporal and operational constraints. It may indeed be important to ensure that two events are considered as parts of the same scenario, only if they occur in a specific area, within a given time frame, while involving sim-ilar actors. The recognition degree is estimated by a similarity degree, which takes its values between 0 (null recognition) and 1 (full recognition). It evolves according to the arrival of new events which confirm or invalidate the hypothesis: each time new data match the template, the recognition degree is updated. Through this information fusion mechanism, guided by experts' knowledge, ADAC enables to detect the early signs of what usually ends up in crisis.

3.

Automatic Extraction of Event Types: EventSpotter

General Properties

As we outlined above, the ADAC system calculates risk on the basis of a template graph whose leaves are event types. The role of the natural language processing (NLP) system is to feed the leaves with input data. We do this by extracting event descriptions from sentences of news articles. These extracted event descriptions are transformed by subsequent operations, which render them suitable for further automatic processing in the template.

The extraction of event descriptions is carried out with syntactic analysis using the Xerox Incremental Parser (XIP) [START_REF] Ait-Mokhtar | Robustness beyond shallowness: incremental dependency parsing[END_REF]. An event is defined in terms of syntactic relationships in the sentences. Out of all the dependency relations produced by the analyzer, we consider an event description to be a predicate (verb, adjective and predicative noun) related to its arguments and modifiers.

The first operation that EventSpotter carries out with respect to the extracted event descriptions is their normalization to a unique representation structure. This operation is domain-independent since invariably every event description is extracted. The unique representation structure consists first in indicating for each event description its information source and factuality as conveyed by the information source. Furthermore we transform each event description into a set of common constituents. The constituents of events are a core, which is the name of the event, and its coordinates, whenever they are present in the sentence. We have defined the coordinates of event cores as agent(s), other participant(s), place(s) and time. The way this normalization is carried out by EventSpotter is described in a previous article [START_REF] Rebotier | Intelligent awareness: event extraction, information evaluation & risk assessment[END_REF].

The second operation that EventSpotter carries out is the association of the extracted event descriptions to the pre-defined relevant event types that constitute the leaves of the template graph in ADAC, whenever it is appropriate. This operation is domain-specific. It is carried out on the event core and its extensions, as we will describe below, and as it is illustrated by the following example taken from the corpus that we have chosen for developing our application. Sentences (2) through (4) indicate extended event cores in bold. These extended event cores are the parts of the sentences associated with (1), one of the relevant event types with respect to our domain defined in ADAC.

(1) to get involved in a cooperation in the nuclear domain.

(2) A delegation from Syria arrives in Iran to begin negotiations on a possible Iranian-Syrian nuclear pact.

(3) The Middle East Newsline reports that Iran is preparing to receive a light water nuclear reactor from Russia.

(4) Former chief nuclear negotiator for Iran Hassan Rowhani says Tehran is ready to negotiate a mutual start for the Natanz nuclear facility.

After the second operation, Table 1 is extracted.

Concept-Matching

In order to match event descriptions in sentences ( 2) through ( 4) with the relevant event type (1) we use the concept-matching framework. Since we have described it previously [START_REF] Sandor | Discourse and Citation Analysis with Concept-Matching[END_REF], here we only recall the basic idea. Concept matching combines the bag-of-words approach with syntactic dependency parsing for extracting complex target concepts. The complex target concepts are coherent and recurrent meaning fragments of sentences, and are expressed in highly diverse ways. Within the concept-matching framework the target concepts (like (1) above) are matched whenever syntactically related chains of expressions conveying -what we call -their constituent concepts (bags of words) occur within the same sentence. We show how the concept-matching framework is applied on (2) through (4) for matching the target concept [START_REF] Ahn | The stages of event extraction[END_REF].

As first step, the target concept is broken down into three constituent concepts: [get involved] in [cooperation] in the [nuclear domain]. We assign general concept labels to these constituent concepts as follows: BEGIN LINK NUCLEAR.

The sentences (2) through (4) all contain words that convey each of these concepts. Moreover, these words form dependency chains in all of the sentences as shown below, thus they do in fact convey the target concept (1):

( We can observe that neither the order nor the type of relationship among the constituent concepts is relevant for the match. The essential constraint of coherence is the existence of a dependency relationship among the words conveying the constituent concepts.

In order to carry out concept-matching automatically we need a general purpose natural language dependency parser as well as domain-specific resources: a set of constituent concepts, lists of words conveying the constituent concepts, a lexicon-grammar to improve the performance of the general purpose parser in the establishment of the argument and modifier dependencies and rules of the co-occurrence of the constituent concepts. The general purpose dependency parser is used for extracting all the possible dependency pairs among the words of the sentences, whereas the domain-specific resources make it possible that the relevant dependency pairs conveying the constituent concepts can be chosen out of all the dependency pairs in the sentences and associated with the target concepts.

In the following sections we will concentrate on the domain-specific resources we have used to extract descriptions of event types that are needed by ADAC to calculate risks of nuclear proliferation.

The Target Concepts and the Constituent Concepts for Nuclear Proliferation. In ADAC 103 event types are listed as relevant for inducing crisis in the nuclear domain. Table 2 is an excerpt of the list. The granularity of the constituent concepts is subject to experimentation. If certain constituent concepts do not assure fine-grained event types, they can be broken down to several types. A word might also be assigned to several constituent concepts. Table 3 shows Table 2 marked up with constituent concepts: Table 3.

to work[PRODUCTION] on secret[SECRET] nuclear[NUCLEAR] programs[PRODUCTION] 2. to sell[MONEY] military[MILITARY] equipment[TOOL] 3. to get involved[BEGIN] in a cooperation[LINK] in the nuclear domain[NUCLEAR]

For the present system we have assigned the list of words to the constituent concepts manually. We have worked on a prototype based on a corpus of news containing 4196 tokens of content words. Lexicon Grammars.

As we pointed out above we have built domainspecific lexical resources in order to ensure the precision of the extraction of the relevant dependencies in the sentences. In the general-purpose XIP parser certain dependencies, especially prepositional phrase attachment and clausal complementation cannot be handled with high precision due to word sense ambiguities on the one hand and the lack of a broad coverage lexical grammar on the other hand. Working on a domain-specific vocabulary allows us to build lexicon-grammars for this vocabulary.

A lexicon-grammar is a dictionary that provides exhaustive and detailed subcategorisation information about the predicates of a natural language such as verbs, predicative nouns and adjectives. Predicates with related syntactic and semantic behaviour are grouped together, for example, in the structure of simple sentences, in the distribution of arguments and in terms of interpretations [START_REF] Leclere | Organization of the Lexicon-Grammar of French Verbs[END_REF]. The lexical, syntactic and semantic features provided by the lexicon-grammars are used for establishing grammatical and dependency rules. Table 5 shows an extract of the lexicon-grammar of English verbs taking a sen-tential complement (e.g. Russia admitted (that Iran's program is of peaceful intent + having discussions with Iran + to providing incorrect information)). Sentence ( 8) is associated to the event-type "nuclear-related agreement" due to specific syntactic features present in the lexicon-grammar, which allow XIP to extract a dependency between "agreement" and "import": (8) An unnamed official of the Russian atomic energy ministry says that Russia has yet to receive Iran's agreement for Moscow to import back radioactive fuel waste from an Iranian nuclear power plant that Russia is building in Bushehr.

dependency chain: ... agreement ... to import back ... radioactive fuel waste

We carried out an evaluation of in what extent the lexicon-grammar resources have influenced the extraction of dependency relationships. We established a gold standard of a 100 sentences, which we compared to the output of XIP with and without the addition of the lexicon-grammar. The improvement of the performance was 36%, which is a significant difference.

Related Work

Event extraction is the subject of an increasing number of information extraction applications. Different systems, however, represent events in different ways. [START_REF] Ahn | The stages of event extraction[END_REF] describes two approaches to represent events: "On the one hand, there is the TimeML model, in which an event is a word that points to a node in a network of temporal relations. On the other hand, there is the ACE model, in which an event is a complex structure, relating arguments that are themselves complex structures, but with only ancillary temporal information.". Our representation is closer to the Automatic Content Extraction (ACE) model, which however, does not describe it entirely. We are not aware of any system that shares our event representation.

Apart from differences of representation, event extraction has been handled with various approaches. It is difficult to make comparisons among differ-ent systems with different purposes in this article. We will give here a brief overview of the latest systems whose main application is event extraction.

Recent applications in the field of event extraction have mainly been carried out with probabilistic or machine learning approaches, which do not need to rely on strict linguistic constraints, but which fail to extract exact semantic relationships based on sentence structure. The following articles describe various event extraction systems, and none of them shares our approach and purposes: [START_REF] Li | A Probabilistic Model for Retrospective News Event Detection[END_REF] built a Retrospective news Event Detection system which merges events with existing similar events with a probabilistic approach based on bag-ofwords and clustering. [START_REF] Xu | Automatic Event and Relation Detection with Seeds of Varying Complexity[END_REF] built a prize-winning event extraction system based on machine learning with limited linguistic constraints. [START_REF] Banko | The Tradeoffs Between Open and Traditional Relation Extraction[END_REF] study the tradeoffs between open and traditional relation extraction. They conclude that it seems more interesting to use traditional IE for a domain specific extraction. Finally, closer to our approach, [START_REF] Aone | REES: A Large-Scale Relation and Event Extraction System[END_REF] built a relation and event extraction system, but only for verb-based events.

Several works in NLP systems argue that acquiring subcategorization information is an important task for the improvement of performance (see [5; 6; 8; 9]). Some of them also put forth that manual acquisition of such resources is time and resource consuming (see [START_REF] Carroll | The Automatic Acquisition of Verb Subcategorisations and their Impact on the Performance of an HPSG Parser[END_REF]). However, manually-developed lexicons (enriched with subcategorization information) prove to be precise [START_REF] Carroll | The Automatic Acquisition of Verb Subcategorisations and their Impact on the Performance of an HPSG Parser[END_REF]. Moreover, [START_REF] Briscoe | Generalised Probabilistic LR Parsing for Unification-Based Grammars[END_REF] estimate that half of the parse failures is caused by inaccurate or incomplete subcategorization information [START_REF] Gardent | Le lexique-grammaire de M. Gross et le traitement automatique des langues[END_REF].

Conclusion

In this article we have described the components of an entirely automatic integrated system of risk assessment concerning nuclear proliferation. It consists basically of two components: linguistic analysis of news articles and computation of nuclear risk. The actual integration has not taken place technically but the evaluation of the output of the linguistic component shows that conceptually it is possible, i.e. the two components are compatible. Once the integration is carried out, ours will be the first system where automatic linguistic analysis of newswire articles is used as input to a risk detection system.

This system is the result of a chain of processes where in each step the enrichment of a lower level analysis makes way to a higher level analysis. Starting from lexical analysis, continuing by syntactic parsing of free text, the output of which is then mapped into semantic role assignment, and coupled with conceptual analysis, the process goes on by carrying out operations on high-level concepts, which yields the final output.

Further work consists in the integration of the two components, which will be followed by an evaluation. We carried out a partial evaluation of the system, which we reported in previous work [START_REF] Rebotier | Intelligent awareness: event extraction, information evaluation & risk assessment[END_REF]. In a longer term perspective, the approach that we propose for nuclear risk assessment can be extended for other kinds of political risk assessment or in general to any kind of risk assessment where the risk is related to event occurrences that are reported in texts.

Table 1 .

 1 Representation of the extracted event[START_REF] Aone | REES: A Large-Scale Relation and Event Extraction System[END_REF]

	Source	Fact. Actor	Core	Oth.pt Place Time	Event type
	Middle East Newsline	F	Iran	receive a light water nuclear reactor	Russia	to get involved in a cooperation in the nuclear domain

  5) A delegation from Syria arrives in Iran to <BEGIN> begin </BEGIN>

	(6) The Middle East Newsline reports that Iran <BEGIN> is preparing
	</BEGIN> <LINK>to receive</LINK> a light water <NUCLEAR>
	nuclear reactor </NUCLEAR> from Russia.
	dependency chain: ... is preparing to receive ... nuclear reactor ...
	(7) Former chief nuclear negotiator for Iran Hassan Rowhani says Tehran
	<BEGIN> is ready </BEGIN> <LINK> to negotiate </LINK> a
	mutual <BEGIN> start </BEGIN> for the Natanz <NUCLEAR> nu-
	clear facility </NUCLEAR>.
	dependency chain: ... is ready to negotiate a ... start for ... nuclear facility.

<LINK> negotiations </LINK> on a possible Iranian-Syrian <NUCLEAR> nuclear pact </NUCLEAR>. dependency chain: ... begin ... negotiations on ... nuclear pact.

Table 2 .

 2 1. to work on secret nuclear programs 2. to sell military equipment 3. to get involved in a cooperation in the nuclear domain In the entire list we propose 18 constituent concepts, whose various cooccurrence combinations cover all the target concepts. They are the following: NEGATIVE, INTENT, BEGIN, CONTINUE, END, POSITIVE, HOSTILITY, LEGAL, SECRET, MILITARY, NUCLEAR, KNOWLEDGE, LINK, MOVE-MENT, PRODUCTION, MONEY, TOOL, STATE

Table 4 .

 4 Some sample constituent concepts and some words associated with them

	Constituent concept	Words
	NEGATIVE	contrary, lie, refute
	INTENT	decide, effort, require
	LINK	ally, connect, negotiate
	CONTINUE	augment, emerge, regular

Table 5 .

 5

Acknowledgments

This research is being funded by the French national project Infom@gic.