Cauvin Maxime
email: maxime.cauvin@mines-nancy.org

Van Der

Merwe J Nielen
email: nielen.vandermerwe@up.ac.za

Didier Christophe
email: christophe.didier@ineris.fr

Mothibi Oupa
email: oupa.mothibi@up.ac.za

LONG TERM STABILITY ASSESSMENT OF A ROOM AND PILLAR COAL MINE -A FRANCO/SOUTH AFRICAN APPROACH

Keywords: Mature mine, Pillar failure, Surface stability, Uncertainty, Probabilistic analysis Mine mature, Rupture de pilier, Stabilité des terrains de surface, Incertitude, Analyse probabiliste

As abandoned mines, "mature" mines are very challenging for geotechnical engineers with respect to long term stability issues that are part of the management of the "post-mining" phase. This paper presents a risk assessment study that has been carried out in an underground coal mine by French and South African experts in the framework of the PROTEA Programme funded by the institutions of these two countries. This study associates classical geotechnical methods of pillar stability analysis with tools allowing an integration of uncertainties in the analysis and a probabilistic expression of hazard issues. Technical regulations, aimed at managing the consequences a pillar collapse may have on public safety and future land use management, are also discussed.

Introduction

"Mature mines" constitute an important challenge today in the Republic of South Africa. The current reserves of those mines have almost been depleted and the challenge for mining engineers is to stretch the exploitation to its limit in order to save time until new coal fields have been prepared to replace the current production. Such an extent in mining should however respect different issues regarding both the current safety of workers and the management of the "post-mining" phase.

The issue of "post-mining" management is today clearly international. All over the world, mining countries face, or start to face, problems related to their old mining activity. However, even if national approaches are progressively developed, there is still a total lack of formalism, at an international scale, on the methods used to assess and manage the "post-mining" hazards.

The paper presented herein illustrates a risk assessment that has been undertaken by both French and South African researchers. France has worked for about ten years on "post-mining" issues and the methodology developed with the aim of predicting, preventing and managing the potential hazards related to the disused mining activity, is today successfully applied. On its side, the Republic of South Africa, where mining is still valuable, is currently facing problems similar to the ones in France and is trying to anticipate them for the near future.

The case of application being chosen concerns the evaluation of the likelihood of a surface collapse above a South African room and pillar coal mine. Technical regulations aimed at managing the consequences it may have on public safety and future land use management will also be discussed.

Presentation of the mine

The mine being studied is located in the Mpumalanga Province. It belongs to the Witbank coal basin. The geological sequence in the area includes five coal seams but only one is currently exploited. The mine can be described as a shallow mine (17 to 88 meters below surface) exploited using the room and pillar mining method.

The study focuses in an area where mining ended in December 2006. The exploitation is presented in Figure 1. The area can be divided into three distinct zones (A, B and C) according to their pillar dimensions. Pillars in Zone B were originally larger but they have been split as part of the sweeping process. The direction of advance was from Zone A to Zone C. Following an underground visit in November 2006, widespread roof falls and substantial pillar scaling were observed in some areas. The scaling rate was investigated and a reasonable agreement was found in comparison to the predicted one (van der Merwe, 2003b). It should be noted that, as a consequence of the fact that the mine had changed ownership twice in the last decade, all the information required for such an investigation was not at hand. In particular, the original mining maps are no longer available. All that could be founded were digitalised versions of the plans that were supplied by the mine during a previous investigation. Such a situation is common in France where most mines are very old and information are usually missing. In our case of study, the mine was then requested to measure the road widths and heights and subsequently supplied that information measured over 80 splits. These were then compared to the original dimensions from digitalised plans.

The concrete objectives of that study are the followings:

Estimate the current stability of the mining area and evaluate the impact of a reduction of the pillars dimensions on the global stability; Estimate the long term stability of the workings; Complete the approach that is used in South Africa by the integration of uncertainties and expression of probabilities in order to tend towards a real assessment of "hazard".

The methodology that has been used to reach those objectives may be detailed in several steps: a "local" study: the stability of a single pillar from Zones A, B and C is assessed and the "most critical" area is then identified; a "regional" study: the impact of the failure of a single pillar on the whole mining area is investigated; a "temporal" study: remedial actions are finally proposed in order to answer the initial objectives of the study.

Pillar stability assessment

The conventional methods used to assess the stability of mine pillars are based on deterministic (empiric or analytic) approaches. They usually adopt the Safety Factor (SF) as an indicator of the stability of the pillar. This Safety Factor is defined as the ratio of the pillar strength over the mean vertical stress acting on pillar. Theoretically, a SF value greater than 1 means the system is stable while a SF value lower than 1 means it is unstable. In practice, a threshold value higher than 1 is generally used at the design level to incorporate uncertainties. The stress acting on the pillar is generally determined using the Tributary Area Theory which considers the total overburden load directly over the pillar and the portion of the galleries at its perimeter. The long term strength of a coal pillar has been investigated in South Africa in several research works. Three different formulae that all integrate the width-to-height ratio of the pillar [START_REF] Salamon | A study of the strength of coal pillars[END_REF][START_REF] Bieniawski | The effect of specimen size on compressive strength of coal[END_REF]van der Merwe, 2003a) will then be used in that study.

While practically using this approach, it appears that input data as well as formulae are uncertain. This is partly due to the nature of the system being studied which is highly concerned by natural variability (spatial and temporal). However, this uncertainty also results from a lack of knowledge about the data and about the mechanisms of failure, usually named 'epistemic uncertainties'. [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF] presents a methodology that can be used in order to integrate different natures of uncertainties. It basically consists in performing Monte Carlo simulations in which data as well as models are considered as random values. Using such a methodology may allow to integrate in this study (1) the existence of several models to estimate the strength of a mining pillar, (2) the fact that each model combines the input data in a certain manner and that it only allows to approach reality in an approximate way, (3) the uncertainty on the parameters that calibrate models, and (4) the uncertainties on input data that are integrated in models. It also allows to estimate p f , the probability of 'unsatisfactory performance', defined as the probability that the computed safety factor is less than the target value of stability, i.e. 1.

Table 1 presents the characterisation of the input parameters as well as the results of the stability assessment of pillars from Zones A, B and C. The mean values and the standard deviation have been determined from the analysis of the measures realized on 80 splits, or from the interpretation of topographic maps. Coefficient of variation of model parameters have been assumed to equal 10%. The scattering of SF values may be explained by a lack of knowledge as well as by the existence of spatial variability in some data (Table 2). Therefore, in order to interpret the value of p f (48.2%) for pillars in Zone C, that is "linked" with the variance of SF, it is important to distinguish between the part of the final variance that is due to natural variability and the part which is due to epistemic uncertainty.

Table 2 allows doing that. The variance explained by a spatial variability only concerns in our study the input data, namely the geometrical and mechanical properties of the mining workings. It varies between 0.017 and 0.028. If we want to give a practical meaning to the p f value, only the "variability part" of the variance has to be taken into account and interpreted [START_REF] Cauvin | Prise en compte des incertitudes et calcul de probabilité dans les études de risques liés au sol et au sous-sol[END_REF]. Therefore, using the hypothesis that the Safety Factor is lognormally distributed, and that the mean value is 1.05, the frequency of pillars that may be expected to be in a unstable state ranges between 43 and 45%, corresponding respectively to a variance of 0.017 and 0.028 (figure 2). The practical implications of that is that almost 4 pillars on 10, anywhere in the area, have a SF value less than 1.

Consequences of a pillar failure in Zone C

In order to determine the consequences of a pillar failure occurring in Zone C for the whole mining environment, the mode of pillar failure must be investigated. This mode is indicated by the post failure modulus, which is in turn linked to the width-to-height ratio of pillars (van der [START_REF] Van Der Merwe | Rock engineering for underground coal mining[END_REF]. The post failure modulus can be estimated at a value of -2.05 GPa in Zone C. The implication of such a negative figure is that it means that once failure has been initiated, there is sufficient energy in the system to drive the continuation of the process without the addition of additional energy. The failure process is thus expected to be continue rapidly, resulting in what is commonly called a pillar run. Using the French terminology, the consequence of such a pillar collapse will be the apparition of a "brutal surface collapse".

The process of failure can spread to others areas if the load is transferred to unfailed pillars. This can only happen if the overburden does not fail, because if it does, there is no continuity in the overburden to transfer load to adjoining areas. During former stooping trials at the mine, it was observed that the overburden had a tendency to hang up periodically. Consequently the potential for load transfer exists. The only way to estimate the magnitude of load transfer is by numerical modelling. The model chosen for this was LAMODEL, a pseudo 3-dimensional boundary element code (Heasley, 1988).

Risk management

From a problem which seemed only to relate to Zone C, it has been shown that the whole mining area could be affected by pillar failures. The problem is all the more problematic as a pillar run could occur and consequences regarding the safety of miners or, at a longer term, for people or infrastructures settled in the vicinity of the mine could be expected. In such a context, it appears particularly important to integrate a temporal dimension to this analysis and to try to forecast when the collapse can occur. This 'time" factor may have direct consequences on decisions that will be taken.

The literature appears relatively poor concerning the integration of time into risk analyses related to the wide topic of geotechnical issues. Two strategies appear however possible. The integration of time in the study may be done either directly, through the introduction of a temporal parameter in the mechanical models being used, or indirectly, through a study of similar events that occurred in the past. The two strategies have been used in the paper.

Van der Merwe (2003b) developed a formula to evaluate the life of coal pillars. This empirical formula is based on the observation that the scaling process highly contributes to the weakening of pillars over time. Scaling is thus integrated in the mechanical modelling of pillar failure through the introduction of a reduction of pillar dimensions at a constant annual rate. Using a database of failed pillar in South Africa, a statistical analysis can be performed and a formula to assess life expectancy of pillar can be determined. Life of pillars in Zone C may be evaluated to be 70 years in this study.

The analysis of collapse events that occurred in the past in similar mining and geological contexts may also be performed to precise this result. Figure 4 presents the repartition (over time) of 31 pillar failures that occurred in the Witbank Coalfield since 1919. It should be noted that some failures appear very early after mining whereas others occurred later. For [START_REF] Carter | Some observations on the time dependency of collapse of surface crown pillars[END_REF], the first failures are due to mining "defects" while the last may be explained by "wearing out" effects. In a context of post-mining management, the last type of failures must therefore be analysed. The small number of events (21) makes it difficult to perform a real statistical analysis of the database and to develop a temporal model to assess the probability that a pillar failure occurs over time after the end of mining. Practical qualitative conclusions may however been drawn from the analysis of Figure 4. Firstly, it appears that pillar failures occurs later when seam n°2 of the Witbank sequence has been mined. Secondly, no "average" life can be identified. That signifies that a collapse can occur at anytime. Thirdly, a pillar failure can still occur 52 years after mining ended. Zone C, in which mining of the seam n°2 started 31 years ago, is thus currently particularly concerned by the phenomenon of pillar failure.

The two methods allowing to estimate the possible date of a pillar failure lay on very empirical methods and must be interpreted with a lot of care. The formula proposed by van der Merwe (2003b) has for example a confidence level that does not exceed 80%. Moreover the communication of its results to mining engineers may appear delicate as those latter may not be very familiar with statistical analyses. The practical meaning and application of results that have been obtained using statistical tools is still difficult for many actors working in the field. Adding another method of analysis into the problem may therefore, if the two methods correlate, give a supplementary weight to the results. This is the case in that study.

Technical recommendations

This study has shown that even if Zones A and B are stable at the time of the analysis, a failure in Zone C may have consequences on the whole area. Due to the presence of important spatial variability in the area, such a failure can occur everywhere in Zone C. Two temporal analyses have even illustrates that a pillar failure is likely to occur in a small amount of time.

The practical implications of those conclusions are that workings must be stopped and abandoned in the studied area. In a short term, the implementation of a monitoring system aimed at protecting the crews working in the other sections is essential. "Reverse extensometers" installed from the surface into a borehole drilled through a pillar and anchored in the rock below the pillar will for example allow to monitor any compression in a pillar and can be used to trigger an alarm.

Actions have also to be taken in order to protect the population and the infrastructures that may settled above the workings at a longer term. In France, in order to post and manage the consequences of the events induced by a past mining activity, the French Ministry in charge of Industry has charged several actors to elaborate Mining Risk Prevention Plans. Those are aimed at identifying hazards and risks that may affect old mining areas and at defining rules for urban development and for land and water use management [START_REF] Didier | The MRPP: a powerful operational regulatory tool to prevent and manage post-mining risks[END_REF]. Such a scheme of methodology may be applied in the South African context in order to define technical regulations concerning the mine being studied.

Conclusions

The joined study between France and the Republic of South Africa presented in this paper constitutes a real opportunity at realising an exchange of knowledge and a strengthening of the current skills of the two countries. The mining induced hazard assessment made at the studied mine has indeed taken advantages from the mining and geomechanical South African knowledge as well as from the French experience in terms of the management of the post-mining issues.

In addition to the practical results that may be obtained concerning the studied mine, more conceptual questions have also been tackled. The use of probabilities and the concrete meaning of the probability of unsatisfactory performance have thus been discussed. An emphasis has been laid on the importance of communication between the geomechanical expert in charge of the hazard analysis and the stakeholders, either the mining company or the Authorities. The evaluation of the place and the time of the possible failure appears therefore essential for a better hazard management and a good planning of the future land use.

Figure 1 .

 1 Figure 1. Area of study.

Figure 2 .

 2 Figure 2. Interpretation of the probability of 'unsatisfactory performance'.

A

  first calibration modelling led to results close to the current situation with Safety Factors of approximately 1.0 in Zone C and 2.3 in Zone B. A second simulation was then done by artificially weakening pillars in Zone C to create a reasonable situation where they had already failed. As a result, the stress on the larger pillars of Zone B increases to 7.5 MPa and their Safety Factors decreased to 1.1. Those pillars would therefore also be subject to failure and would not be able to stop the pillar run. Figure3illustrates the areas the potential directions of a pillar run initiated in Zone C.

Figure 3 .

 3 Figure 3. Potential for a pillar run initiated in Zone C.

Figure 4 .

 4 Figure 4. Number of pillars failure VS time elapsed since the beginning of mining

Table 1 .

 1 Input parameters, Safety Factors and probability of 'unsatisfactory performance' for the 3 Zones of the studied area (mean value and (standard deviation)).From Table1, pillars in Zone C appear to be in an unstable state. Their Safety Factors are indeed close to 1 and their probability of 'unsatisfactory performance' is high. An analysis of variance has been carried out to complete those results. Table2presents the relative influences of each of the different sources of uncertainties on the total variance of the Safety Factor computed for a pillar in Zone C. 45% of the variance is explained by the uncertainties on data whereas 55% are explained by the use of models and the uncertainties existing on the parameters that calibrate those models.

	Parameter	Distribution	Zone A	Zone B	Zone C
	General parameter
	Number of pillars		18	16	46
	Age (years)		25	44	31
	Input data
	Pillar width (m)	Normal	5.4 (0.3)	8.5 (0.5)	6.2 (0.4)
	Pillar centre (road + pillar		12	15	14
	width) (m)
	Pillar height (m)	Normal	3.2 (0.2)	3.7 (0.4)	3.9 (0.3)
	Mining depth (m)	Uniform	29 (0.9)	36 (2.3)	45 (2.9)
	Volumic weight (kN.m -3 )	Normal	25 (2)	25 (2)	25 (2)
	Model parameter
	Salamon & Munro (1967)
	K	Normal	7.2 (0.72)	7.2 (1.4)	7.2 (1.4)
		Normal	0.5 (0.05)	0.5 (0.1)	0.5 (0.1)
		Normal	0.7 (0.07)	0.7 (0.1)	0.7 (0.1)
	Bieniawski (1968)
	a (Mpa)	Normal	2.8 (0.28)	2.8 (0.6)	2.8 (0.6)
	b (Mpa)	Normal	1.5 (0.15)	1.5 (0.3)	1.5 (0.3)
	Van der Merwe (2003a)
	k (Mpa)	Normal	3.5 (0.35)	3.5 (0.7)	3.5 (0.7)
	Result
	Safety Factor		1.72 (0.38)	2.63 (0.61)	1.05 (0.25)
	p f (%)		0.3	0	48.2

Table 2 .

 2 Nature and influence of the various sources of uncertainties on the computed Safety Factor of a pillar in Zone C.

	Parameter	Nature of uncertainty Spatial variability Lack of knowledge	Variance	Variance / Total variance
	Input data
	Pillar dimensions	X		0.017	28%
	Mining depth	X	X	0.005	8%
	Volumic weight	X	X	0.006	9%
	Models
	Model parameters
	Salamon & Munro (1967)		X	0.013	21%
	Bieniawski (1968)		X	0.001	2%
	Van der Merwe (2003a)		X	0.003	6%
	"Reliability" of models		X	0.016	26%

Acknowledgements

The authors want to acknowledge the PROTEA Programme for the funding of that international cooperation and the mining company for the availability of the data.


Ha Nguyen
email: nguyen.ha-dat@cermes.enpc.fr

Dat

D E Gennaro

Sorgi Claudia

Pierre Delage

EXPERIMENTAL AND MODELLING INVESTIGATE ON THE BEHAVIOUR OF A PARTIALLY SATURATED MINE CHALK

Keywords: chalk, quarries, suction, water retention, oedometer craie, carrières, succion, rétention d'eau, oedomètre

 allowing to account the swelling and collapse under wetting at constant vertical load.

Introduction

In the context of research into the stability of abandoned subsurface cavities in chalk by INERIS (Institute National de l'Environnement Industriel et des Risques), the abandoned Estreux mine (Northern France) has been conducted. In relation with the regular environmental changes (i.e. water table, hygrometry) induce a variation of degree of water saturation in the pillars in the mine, the study of the behaviour of the Estreux chalk under unsaturated states was found necessary.

Various investigations have been carried out on the collapse phenomena for mineworking [START_REF] Raffoux | Stabilité générale de la carrière souterraine d'Estreux[END_REF][START_REF] Bonvallet | Une classification géotechnique des craies du nord utilisée pour l'étude de stabilité des carrières souterraines[END_REF][START_REF] Bell | A review of selected engineering geological characteristics of English chalk[END_REF][START_REF] Talesnick | The elastic deformability and strength of a high porosity, anisotropic chalk[END_REF][START_REF] Sorgi | Contribution méthodologique et expérimentale à l'étude de la diminution de la résistance des massifs rocheux par vieillissement[END_REF][START_REF] Priol | Comportement mécanique différé et mouillabilité d'une craie pétrolifère[END_REF]. There are two main factors to create these collapses on the geomaterial, which are external (environmental conditions) and internal (intrinsic). This is certainly related to the water weakening effects, described specially for the oil reservoir chalk in the North Sea [START_REF] Newman | The effect of water chemistry on the laboratory compression and permeability characteristics of some North Sea chalks[END_REF][START_REF] Andersen | Petroleum research in North Sea chalk[END_REF][START_REF] Schroeder | Water/chalk (or collapsible soil) interaction: Part II. Results of tests performed in laboratory on Lixhe chalk to calibrate water/chalk models[END_REF][START_REF] Gutierrez | The effect of fluid content on the mechanical behaviour of the fractures in chalk[END_REF][START_REF] De Gennaro | Time-dependent behaviour of oil reservoir chalk: a multiphase approach[END_REF][START_REF] Sorgi | Contribution méthodologique et expérimentale à l'étude de la diminution de la résistance des massifs rocheux par vieillissement[END_REF]. [START_REF] Delage | Subsidence and capillary effects in chalks[END_REF] showed that the mechanics of unsaturated soils could be used to investigate the behaviour multiphase chalks.

In this page, using the methods and the concepts of the mechanics unsaturated soils investigate the determination of the water retention properties of Estreux Chalk. From these results, the compressibility of a partially saturated mine chalk is investigated through running the controlled suction odometer. These results have been simulated by means of the Barcelona Basic Model -BBM [START_REF] Alonso | A constitutive model for partially saturated soils[END_REF].

Experimental investigation

Material characterisation

The study was carried out on the Estreux chalk specimens extracted from the Estreux abandoned underground mine in Northern France, 10 km East of Valenciennes in the vicinity of the A2 highway relied on Valenciennes and Brussels. Blocks of Estreux chalk were retrieved at 20 meters in depth. The Estreux chalk formation belongs to the late Cretaceous geological period, which dated from 89 to 94 Ma years ago. One square pillar (side 1.4 m and 1.8 m in height) has been continuously monitored in relationship with the research programme conducted by INERIS on "ageing phenomena" in geomaterials since 2003 [START_REF] Sorgi | Contribution méthodologique et expérimentale à l'étude de la diminution de la résistance des massifs rocheux par vieillissement[END_REF], Auvray et al. 2004). These measurements show the relative humidity inside the mine included between 80 and 100% and the temperature is almost constant 11°C.

Table 1 presents the index properties of Estreux chalk. By using a helium picnometer, the specific gravity Gs of the Estreux chalk is obtained 2.74. As compared to the specific gravity of pure calcite (Gs = 2.71), this higher value is related to the presence of the glauconite fraction (with Gs = 2.99 for glauconite), which is often presented in the northern French chalk [START_REF] Masson | Pétrophysique de la craie[END_REF][START_REF] Bonvallet | Une classification géotechnique des craies du nord utilisée pour l'étude de stabilité des carrières souterraines[END_REF][START_REF] Hazebrouck | Particularité du comportement mécanique des craies: rôle de l'eau -rupture sous contrainte hydrostatique[END_REF]. It explains the high value of specific surface measured using methylene blue absorption (Ss = 13 m²/g, as compared to 9 m²/g for a pure chalk as Lixhe chalk). The average porosity n about 37% shown as a good agreement with the values proposed in the literature [START_REF] Masson | Pétrophysique de la craie[END_REF][START_REF] Bonvallet | Une classification géotechnique des craies du nord utilisée pour l'étude de stabilité des carrières souterraines[END_REF]. The high value of degree of saturation S rw = 97% presents probably the chalk completely saturated in the mine, which could indicate a loss of water during testing.

Retention properties of Estreux chalk

The water retention properties of Estreux chalk were determined by cylindrical samples of 20 mm in diameter and from 20 mm to 25 mm in height. Because the relative humidity changes in the mine from 80 to 100%, we carry out for the suction value between 0 and 24.9 MPa by using 3 methods: the osmotic methods applied for the low suction (from 0 to 1.5 MPa) [START_REF] Williams | An evaluation of polyethylene glycol PEG 6000 and PEG 20000 in the osmotic control of soil water matric potential[END_REF][START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted unsaturated clay[END_REF][START_REF] Marcial | Comportement hydromécanique et microstructural des matériaux de barrières ouvragées[END_REF], the vapour equilibrium method with high suction (from 2 to 24.9 MPa) [START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted unsaturated clay[END_REF][START_REF] Cui | L'eau dans les sols non saturés[END_REF][START_REF] Marcial | Comportement hydromécanique et microstructural des matériaux de barrières ouvragées[END_REF] and the filter paper method with contact for intact sample [START_REF] Fawcett | A filter paper method of determining the moisture characteristics of soil[END_REF][START_REF] Chandler | The filter papar method of suction measurement[END_REF][START_REF] Houston | Laboratory filter paper suction measurements[END_REF][START_REF] Bulut | Soil suction measurements by filter paper[END_REF].

The water retention curve of Estreux chalk is described in Figure 1, in terms of changes in degree of saturation (S rw ) as the function of the logarithm of suction (lg(s)). As compared to the wetting path, a small discrepancy of the dry path is related to a rather slight hysteresis. In fact, this is often associated to a variability of the porous network (inkbottle effect) interpreted by the capillary interaction, drop effect (different of contact angle between fluid and solid for wetting path and dry path) [START_REF] Cui | L'eau dans les sols non saturés[END_REF] and the presence of diverse nature in the material (physico-chemical interaction), which regularly produce reduce hyteresis. [START_REF] Priol | Comportement mécanique différé et mouillabilité d'une craie pétrolifère[END_REF] also found the small hysterisis in Lixhe chalk on the system air -water. This is related to the similar porous media of Estreux chalk, suggested by the Environmental Scanning Electron Microscope observation carried out [START_REF] Sorgi | Analyse microstructurale au MEB environnemental d'une craie soumise à chargement hydrique et mécanique[END_REF] and the presence of the clay fraction (illite) in the Estreux chalk.

Measurements of paper weights have made after 15 and 30 days; the average matrix suction of the intact sample equal to 40 kPa with the high value of degree of saturation (S rw = 97%) shown as the intact chalk in the mine remained saturated. The air entry value of Estreux chalk can be estimated at approximate 1.5 MPa. This is a suction level required to throw out water and reduce degree of saturation following a dry path. Desaturation starts above 1 MPa and the degree of saturation at suction of 2.5 MPa is 10%. It means that along the wetting path or the dry path, the starting of saturation or desaturation produces rather brutally which is caused by changes of few percentage points in the relative humidity. At the highest suction (s = 24.9 MPa) as the lowest relative humidity (hr = 83.5%), the degree of saturation equals to 2 -5 %, the chalk is nearly desaturated. Thus, with the changes of the relative humidity in the Estreux mine between 80% and 100% can generate important changes of the state of saturation of the Estreux chalk, at least in the surface of the pillar in contact with the relative humidity of atmosphere. Consequently, it is related to affect the stability of the mine. The effects of these changes are quantified by running the controlled suction oedometer.

Odometer tests

A high stress double lever arm odometer equipped with a suction control system was used (see [START_REF] Marcial | On the high stress compression of bentonites[END_REF] to investigate the compressibility of a partially saturated Estreux chalk samples. The control of the suction was carried out either by using the osmotic method (suctions smaller than 1.5 MPa) (see Kassiff & Benshalom, 1971;[START_REF] Delage | Suction controlled testing of non saturated soils with an osmotic consolidometer[END_REF][START_REF] Dineen | A new approach to osmotically controlled oedometer testing[END_REF][START_REF] De Gennaro | Time-dependent behaviour of oil reservoir chalk: a multiphase approach[END_REF][START_REF] Priol | Comportement mécanique différé et mouillabilité d'une craie pétrolifère[END_REF]. The same cell was also used at higher suctions with the vapour equilibrium method [START_REF] Moratilla | Caracterizacion experimental de la expensividad de una roca evaporitica[END_REF][START_REF] Oteo Mazo | Laboratory tests and equipment with suction control[END_REF][START_REF] Oldecop | A model for rockfill compressibility[END_REF][START_REF] Marcial | Comportement hydromécanique et microstructural des matériaux de barrières ouvragées[END_REF] for suctions higher than 4.2 MPa. In this case, air with controlled RH was circulated in the odometer cell under the bottom of the sample (see Figure 2). Samples of 38 mm in diameter and 19mm ± 2mm in height were reshaped on a lathe. A dry sample was obtained after a period of 48 hours in an oven at 60°C following the recommendations of the International Society of Rock Mechanics. Based on the experimental results from the determination of the water retention properties, four oedometer compression tests were carried out as follows: two tests in dry conditions (T1 & T2), one test in controlled suction (T3: s = 4.2 MPa with vapour equilibrium method, K 2 SO 4 ) and one test in saturated conditions (T4). We considered that the suction of the saturated sample equals to 0 MPa and its dry sample can be estimated at 30 MPa. The compressibility curves of Figure 3 shown some responses are compatible with these of unsaturated soils:

-increase in yield stress with increased suction -increase in compressibility with decreased suction -slight suction dependency of pseudo-elastic compressibility module -slight swelling due to suction release in the elastic zone -significant collapse when soaking under high stress when the sample is located on the LC curve. Interestingly, the position of the collapsed sample is close to the saturated compression sections of tests T2 and T4.

The corresponding numerical values are given in Table 2. These trends illustrate the sensitivity of the mechanical response of the Estreux chalk. They are in good agreement with the water weakening effects described by [START_REF] Matthews | Influence of intact porosity on the engineering properties of a weak rock[END_REF] and with earlier observations on reservoir chalks (with water and oil as pore fluids) by De Gennaro et al. ( 2004) and [START_REF] Priol | Comportement mécanique différé et mouillabilité d'une craie pétrolifère[END_REF]. Water sensitivity is denoted by the swelling observed in test T1 (soaking under 441 kPa) and by the collapse observed in T2 when soaking under 29.28 MPa. The increase in compressibility and decrease in yield stress with increased degree of saturation (decreased suction) are two other manifestations of the water weakening effect.

BBM Modelling

General framework

Alonso et al. (1990) have proposed the Barcelona Basic Model (BBM), an elasto-plastic constitutive model describing the stress-strain behaviour of partially saturated soils. This model is formulated in the framework of hardening elastoplasticity and extends the modified Cam-Clay model for an anisotropic stress state. Two independent sets of stress variables used in the investigation of the mechanical behaviour of partially saturated soils: the net stress (p = p total -u a where p total and u a are the total mean stress and the air pressure respectively) and the suction (s = u a -u w where u w is the water pressure). and s o are the stiffness parameter for changes in net mean stress for saturated state, the stiffness parameter for changes in suction foe virgin states of the soil, the preconsolidation stress for saturated conditions and the hardening parameter of the suction increase yield curve.

• The elliptic yield surface takes into account the effect of suction by introducing an additional net stress (p s = ks) which defined the difference of compression strengths between saturated and unsaturated specimens. It can be formulated as:

where M is a slope of critical state lines.

• The loading collapse (LC) and suction increase (SI) controlling the irreversible deformation are expressed by: LC: • Introducing a parameter α which assume that the flow rule predicts zero lateral strain for odometric path, to avoid the shortcoming, a non-associated flow rule is given by: 2 2 ( 2)

p s p vp s o d q dM p p p ε α ε = +- (7)

Modelling

Using the experimental results, Figure 5 shows the determination in a manner of the parameters ,,() s κλλ . For the Estreux chalk studied the model parameters are given in Table 3. The BBM response for an odometer test with suction constant is depicted in the Figure 6. In the elastic state, the loading path of the odometer tests is considered as linear between p and q by the lateral stress ratio at rest (K o ). From the plastic region, the slope (p,q) of loading path tends to way asymptotic towards the one of virgin consolidation odometer. Thus, this slope can be proposed by:

( )exp( ) oed elas oed qA x p ηη η =+- - ⎡⎤ ⎣⎦ (8) with 3(1 2 ) 1 elas v v η - =
+ where v is a Poisson coefficient, A is a parameter which controls the rate of slope of loading path and x is a loading increment. According to the LC yield curves and the hardening laws, a decrease in suction within the plastic region will result in the volumetric strain to account the swelling and the collapse under wetting at constant vertical loading in the odometer tests:

In the Figure 7, the experimental results are compared to the numerical simulation at three states (saturated -s = 0MPa, suction at 4,2 MPa and dry -s = 30 MPa). At dry states, the water injection is also modelled. A qualitative good agreement between experimental and numerical results is obtained.

Conclusion

In this paper, the water retention properties and compression behaviour of partially saturated chalks from an abandoned underground were investigated in relation with the long term stability of abandoned underground quarries.

The water retention properties of Estreux chalk confirmed the changing values of the ambient relative humidity in the quarry (between 80% and 100%) can definitely lead to significantly unsaturated states, at least the surface of the pillar directly in contact with the ambient relative humidity.

Four tests on odometer tests shown the compressibility of the chalk depends on the suction: chalk strength is increased by increasing suction; the water weakening effects in the chalk is examined in more details by the presence of swelling and collapse in water injection.

The comparison between experimental and numerical results using BBM were confirmed the application of the methods and concepts of mechanics of unsaturated soils for analyse the compressibility in chalks.

  Figure 1: Water retention curve of Estreux chalk

Figure 2 .Figure 3 .

 23 Figure 2. Scheme of the vapour equilibrium oedometer.

Figure 4 :

 4 Figure4: BBM yield locus in p-q-s plane

Figure 5 :

 5 Figure 5: Suction controlled oedometer tests

Figure 6 :

 6 Figure 6: Loading path in p-q-s plane

Figure 7 :

 7 Figure 7 : Modelling compressibility curve odometric tests

Table 1 . Index data of Estreux chalk

 1

	Properties
	Specific gravity, G s	2.74
	Degree of Saturation, S rw (%) Intact dry density, g s (Mg/m 3 )	97 1.73
	Porosity, n (%)	37
	Natural water content, W sat (%)	20.7
	Specific surface, S s (m²/g)	14

Table 2 .

 2 Compressibility data taken from odometer tests.

	State	Stiffness Elastic Plastic	Yield stress (MPa)
	Dry (T1)	0.0022	0.1082	16
	Dry (T2)	0.0055	0.094	13.5
	Suction controlled (T3)	0.0095	0.1137	11.4
	Saturated (T4)	0.0039	0.135	7.5

Table 3 :

 3 Model parameters

	λ(0)	κ	β (1/MPa)	r	p c (MPa)	p o (0) (MPa)	k	ϕ'
	0,120	0,006	0,500	0,944	0,010	10,700	0,185	27
			E (MPa)	ν	κ s	λ s	Μ
			2399,4	0,19	0,01	0,002	1,069

Acknowledgements

The results on Estreux chalk have been obtained during the French National Project BCRD coordinated by INERIS.


Yann Gueguen

Marwan Al Heib
email: marwan.alheib@ineris.fr

Benoît Deffontaines

Bénédicte Fruneau

Marcello De Michele
email: m.demichele@brgm.fr

Daniel Raucoules
email: d.raucoules@brgm.fr

Yves Guise
email: y.guise@brgm.fr

Pas -De Calais

France Guéguen

A L Heib

Michele M De

THE INTERFEROMETRY TECHNICS APPLIED ON RESIDUAL SUBSIDENCE ANALYSIS MEASUREMENT OF CLOSURE COAL MINES, EXAMPLE FROM NORD

Keywords: coal mines, subsidence, residual, interferometry mines de charbon, subsidence, résiduel, interférométrie

This paper describes the residual movements associated with deep coalmines (France).

The Nord-Pas-de Calais basin has been monitored since 10 years by traditional method. The interferometry technics are applied on Nord-Pas-de-Calais coal basin. In this study, both differential SAR Interferometry (DINSAR) and Persistent Scaterrers Interferometry (PSI) are used to estimate the induced deformations during 12 years (1992 to 2004) after the end of exploitation. 88 images of ERS scenes, distributed on two adjacent tracks, are processed, using DIAPASON software for DInSAR and GAMMA-IPTA for PSI. The area undergoes high temporal decorrelation due to the high amount of vegetation. Deformations are well detected; they present low amplitude with a maximum rate of only 1 cm/year during 7 years after the end of the exploitation. They show a good agreement with the traditional methods of levelling. A robust methodology can be developed to use Interferometry for surveying surface above abandoned mines.

Introduction and objective

Subsidence of the ground surface induced by mining is an environmental problem in many countries. To study the magnitude of total and residual subsidence, traditional survey methods are generally used. Recently interferometrics methods have been employed to determine depression due to water pumping. These techniques can be a useful tool to survey the surface of large area with low cost. The objective of this study is to know the precision and the potential of the interferometrics techniques to measure residual subsidence of closer abandoned mines. Charbonnages de France use high precision levelling methods to measure the vertical movements having a magnitude less than centimetres. The Nord and Pas-de-Calais coal basin has been also monitored by differential interferometry SAR (DINSAR) and Persitent Scatterers Interferometry (PSI). Indeed, these methods provided, in accurate area, a precision of displacements less than centimeter. In this study, both (DINSAR and PSI) are used to estimate the induced deformations during 12 years (1992 to 2004) after the end of exploitation. 88 images acquired by ERS satellite are processed, using DIAPASON software (CNES-Altamira Information) for DINSAR and IPTA (GAMMA software) for PSI. The area undergoes high temporal decorrelation due to the high amount of vegetation. The evaluation of atmospheric artifacts is also difficult. Nevertheless, deformations are well detected; they present low amplitude with a maximum rate of only 1 cm/y during 5 to 7 years after the end of the exploitation. The results obtained by the interferometrics methods are then compared to traditional levelling measurements.

Mining subsidence

The underground excavation induces the bending and the deconsolidation of the overlying strata. The induced displacements are transmitted on the surface and generate a subsidence trough (figure 1). This subsidence trough extends on the surface above the exploited area; its volume remains less than the extracted one due to the bulking effect. Figure 1 indicates schematically the geometry of the subsidence profile. The mains characteristics of the subsidence trough are: influence angles (or limit angle, γ) and maximum subsidence amplitude Am. The final subsidence can be divided into three phases as illustrated in figure 2, (Wojtkowiak, 1997).

Phase I known as initial subsidence, corresponds to the period when the face of extraction penetrates the cone of influence of a considered point located on surface. The influence angle and the depth of the considered point define this cone, until passing to the vertical of this one. This phase is generally associated to 10 to 15 % of final subsidence. Phase II known as principal or accelerated subsidence, corresponds to the period when the face of extraction moves away from the vertical of the point, until it leaves the cone of influence of this point. In the United-Kingdom coalmine, 97 % of final subsidence is reached at this stage. From the bibliography, the corresponding value of subsidence (phase I to II) varies between 90 to 95 % of Am [START_REF] Whittaker | Subsidence, Occurrence, Prediction and Control[END_REF]. Phase III, known as residual subsidence (also named differed or delayed subsidence) constitutes the final phase of subsidence. It continues after the extraction phase or when the front of the face is completely out of the area that influences the considered point. Residual subsidence represents only some fraction of the total subsidence. After this last phase, subsidence is supposed finished and surface is stabilized.

Residual subsidence

Amplitude and shape of residual subsidence

The amplitude of residual subsidence is generally about 1 to 10 % of final subsidence (figure 2). This value is largely allowed by the whole French and foreign authors. The amplitude depends on many factors: thickness of the mined seam, depth, backfilling and goaf methods as shown in figure 3. The amplitude of residual subsidence is maximum in the vicinity of the inflection point of the subsidence trough. On both sides of this point, the amplitude of residual subsidence decreases significantly. Generally, residual subsidence decreases with the mine depth. We can highlight, during the last two or three years of residual subsidence phase, that the amplitude of subsidence is negligible. The settling speed is decreasing over this duration. A great part of residual subsidence is given at the end of the first year (40 to 90 %), sometime before, (80 % after one month in a USA mine).

Duration of residual subsidence

Table 1 presents bibliographical data related to the duration and amplitude of the residual subsidence phase [START_REF] Heib | Residual subsidence analysis after the end of coalmine work. Example from Lorraine colliery, France[END_REF]. We specified for each case the treatment method (goaf, backfilling) and the nature of strata. The duration of the residual subsidence phase is variable from months to years, i.e. about 3 to 4 months when the exploitation is carried out in an already disturbed zone (several seams, goaf...). There are some isolated cases resulting from geological contexts and/or particular exploitation, for these cases, the duration of residual subsidence can appreciably be raised and spread out over one period of 4 to 6 years.

Cases

Mining conditions Residual subsidence U.K. (Many basins) [START_REF] Whittaker | Subsidence, Occurrence, Prediction and Control[END_REF] Long-wall method and plastic layers Less than 12 months; 5-6 % of the total subsidence UK (Durham Coalfield) [START_REF] Orchard | Time dependence in mining subsidence[END_REF] Tow mines with high resistance strata 1) 8 % of total subsidence after 4 years, and 9 % after 6 years. 2) 6.8 % of total subsidence after 4 years West Germany Kratzsch, 1983 Many mines using long-wall method 5 years, with 75 % from the first year Australia (Whittaker and Reddish, 1989) Long-wall with caving method 3 to 7 months Long-wall in virgin ground 10 months to 15 month for 10 to 30 % of total subsidence India [START_REF] Singh | Subsidence in Kamptee coalfield[END_REF][START_REF] Saxena | Subsidence management in Jharia Coalfield, India. A concept[END_REF] Long-wall method -in extracted zones. 2 to 4 months; 5 to 10 % of total subsidence USA [START_REF] Hasenfus | A hydrogeomechanical study of overburden aquifer response to longwall mining[END_REF] Long-walls method 10 % of total subsidence 1 month and 12 % after 17 months France: Nord-Pas-de-Calais Plastic strata; in already extracted zones. 99 % of total subsidence after 3 to 4 years France: Provence, Lorraine And Blanzy

Resistant strata; in already extracted zones 2 to 3 years Except some specific context [START_REF] Heitfeld | Ground heave by mine water recovery[END_REF], residual subsidence does not generally affect the surface constructions because the residual horizontal strains, which are more prejudicial, are practically small. We don't have specific study undertaken on the damage that mining works can induce during residual phase. We can however roughly estimate the horizontal strain and the tilt starting from relations established for the active phase. Maximum residual subsidence reaches only a small proportion of maximum subsidence (about 10 %) in active phase [START_REF] Heib | Residual subsidence analysis after the end of coalmine work. Example from Lorraine colliery, France[END_REF]. The residual maximum strain and tilt result from those in active phase in the same proportion. Since the horizontal strains reach in active phase, 0.1 to 3 mm/m (more rarely higher values), the order of magnitude of the additional horizontal strain, will be 10 -2 to 10 -1 mm/m in residual phase.

Case study: Nord and Pas-de-Calais Basin

The basin is located at the centre of the powerful coal belt, which joins together the Ruhr basin (Germany) to the basin Kent (England). It extends from West to East on about 120 km. The exploitation began in 1720 to finish at the end of 1990. The exploited coal seams are numerous; their thickness are low and variable, ranging between 0.5 and 2 meters. The depth of the exploitation increased according to time, of 200 m to 1100 meters. Geology is marked by the presence of several faults of more or less important rejections. The grounds of surface consist of more or less chalky marls (to 20 m of depth), beyond, one notes the presence of a white layer of 50 m thickness chalk. The overburden is characterized by a plastic behaviour (deformation spread out in time). Mining work is in the course of submergence, but the water level, is very low compared to the level of final submergence.

Charbonnages de France has the complete cartography of the exploited veins on the whole basin at 300 meters below the surface. This database also contains the years of exploitation as well as the exploited thickness and thus makes it possible to locate the last exploited zones and to consider the awaited compressing rate. This step was carried out during previous work undertaken by INERIS [START_REF] Degas | Etude des affaissements résiduels -Application au bassin houiller du Nord et du Pas-de-Calais[END_REF] and allowed to draw up cartography of the zones which are most sensitive to residual subsidence.

High precision levelling data

Since 1991, the French National Geographic Institute (IGN) realizes precise measurements each year. These five high precision levelling lines are usually measured in the beginning of October, except in 1999 where two campaigns have been carried out during spring and autumn in order to reveal seasonal effects. Two years (1992 and 1995) are missing. Three zones were then monitored initially (Wallers/Arenberg, Billy-Montigny and Estevelles/Carvin/Courrières), which were later extended to Lens and Bruay-la-Buissière in 1998 (figure 4). Concerning the Estevelles/Carvin/Courrières line, some points with a six centimetres amplitude can be identified between 1992 and 1999, but only in the restricted area of the "Cité Saint Paul". Some points in surrection are also highlighted in particular for the line of Estevelles/Carvin/Courrières where the movements reach two centimetres for a period of seven years (92-99). After 1999, the stabilisation of the majority of the leveling points of the three studied lines is observable. Only a few points of the "Cité Saint Paul" sector (Estevelles/Carvin/Courrières lines) still present subsidence of low amplitude (0.25 centimetres per year). Some points of the Lens and Billy-Montigny levelling lines present new displacement between 2002 and 2004 reaching amplitude of the order of the centimetre over this period.

Differential and persistent scaterrers interferometry

Since the beginning of the 1990's, the utility and the effectiveness of the radar interferometry treatments for the monitoring of the topographic surface movements, in particular in mining areas, were shown with many recoveries [START_REF] Carnec | Three years of mining subsidence monitored by SAR interferometry, near Gardanne, France[END_REF][START_REF] Raucoules | ReSUM -Site OP5 : Nord-Pas de Calais -Interférométrie radar et confrontation avec le nivellement[END_REF][START_REF] Raucoules | Monitoring of slow ground deformation by ERS radar interferometry on the Vauvert salt mine (France) -Comparaison with ground-based measurement[END_REF][START_REF] Colesanti | Detection of mining related ground instabilities using the permanent scatterers technique -a case study in the east of France[END_REF]. Two methods are used here for this study: differential interferometry radar (DInSAR) and Persistent Scatterer Interferometry (PSI), with CNES Diapason and GAMMA Remote Sensing IPTA software respectively. The satellite data used in this study are coming for ERS-1 and ERS-2: 88 scenes distributed on two frames are available with an approximately three months periodicity between 1992 and 2002. Unfortunately, due to the zero-gyro mode of ERS-2, interferometric processing could not be carried out after 1999.

DINSAR and PSI results

According to the last areas exploited, we focused this study on the centre of the coal basin, around the important conurbation of Lens. In addition, this area presents the advantages to be monitored by three levelling lines. The major of studied area presented important temporal decorrelation difficulties. The presence of an important part of fields and forests as well as a low urban density leads to low coherence, which limits the use of DInSAR. Nevertheless, the area of Lens provides a quite good coherence and concentrated our attention. We thus clearly could highlight three principal zones of movements localized on the towns of Courrières, Billy-Montigny and Lens. They show a depression phase between 1992 and 1996 with a deformation of about four centimetres over this period, corresponding to one centimetre per year velocity (figure 6).

A.

B. After 1996, no interferometric fringes can be observed on the Courrières area. But we can't exclude that small movements remain with amplitude under the detection threshold of the method. On the other hand, both zones of Lens and Billy-Montigny still present movements between 1996 and 1999 (end of the exploited data). The subsidence rate of these areas, initially of about 1 centimetre per year, decreases after 1996 and reaches 0.5 centimetres per year. Thus, the total subsidence is then of about 6 centimetres over the whole 1992 to 1999 period. The PSI method allows confirming the results obtained by DInSAR and to identify more precisely the moving areas. Indeed, although the three zones quoted previously are again identified and present the same order of magnitude of deformations, a new area at the North of the Courrières city appears active. Located in the "Cité Saint-Paul", this one shows an average velocity of about 1 centimetre per year all along the considered time period (figure 7).

Discussion and methods comparison

DInSAR measurements are compared to precise levelling data provided by Charbonnages de France in areas where both are available. This could be carried out only for the zones of Courrières and Billy-Montigny, because the levelling of Lens becomes available only since 1999. They are in good agreement, and show same areas of displacements with the same order of amplitude. On Courrières area, DInSAR method provides clear results on the city and highlights a subsidence of about one centimetre per year. But, on the other hand, it does not provide any result for the "Cité Saint-Paul" due to a too low temporal coherence. However, some PS pixels have been found in the "Cité Saint Paul", and can be compared with levelling points in a 150 meters neighbourhood. They present very good agreement and proved the effectiveness of this method in low temporal coherence areas (figure 8). Concerning the results for the Billy-Montigny area, we observed depressions of the same order of magnitude for the two methods allowing the validation of the interferometric results in this quite urbanised region.

Interferograms analysis

After unwrapping, vertical displacements maps are obtained. Spatial profiles, calculated on a 11 pixels large band allow to compare the different subsidence area shapes through time (figure 9). We can then highlight important differences between Lens and Billy-Montigny behaviour. Indeed, the Lens subsidence area shows some quite abrupt slopes at the difference of Billy-Montigny, which looks smoother. These differences may be due to different exploitation types and extent. On the other hand, using numerous profiles allows realizing three-dimensional visualization. The latter is very interesting to clearly visualize the shapes and spatial extent of the subsidence areas and confirm the more smooth slopes of the Billy-Montigny area according to the Lens one.

10 km 5 km (1992-1996 in blue; 1996-1999 in green)

Conclusion

The residual subsidence of Nord and Pas de Calais coal basin is mostly stabilized after less than five years of closure mine. The measured movements using precise levelling method and SAR interfermotry conformed this conclusion. The amplitude of subsidence velocity is less than 1 centimetre per year. The precise levelling method is traditionally used for low topographic surface movements monitoring. It already proved its ability to monitor residual phase deformation and provides very precise order of magnitude. However, this method is limited by its small spatial extent and its expensive cost. DInSAR and PSI offer new possibilities as both gives access to displacements that are of the same orders of magnitude than levelling and allow to investigate areas of larger spatial extension. It is also possible to take benefit from the ERS archive, with images acquired since 1992. In addition, the description of a new moving zone by the PSI shows that this method is particularly effective in areas of low temporal correlation where DInSAR fails and is then complementary. The realisation of profiles, velocity and displacement maps have moreover provided a great help for the analysis of the observed phenomenon.

Figure 1 :

 1 Figure1: Theoretical subsidence trough above a single panel at critical width in horizontal seam (after[START_REF] Proust | Étude sur les affaissements miniers dans le Bassin du Nord et du Pas-de-Calais[END_REF]

  Figure 2: (a) Typical evolution of a subsidence profile with time; (b) cone of influence of a point located on the surface

  Figure 3: Residual subsidence for various coverings according ratio length exploited/depth (after NCB, 1975)

Figure 4 :

 4 Figure 4: Leveling monitoring of the Nord and Pas-de-Calais coal basin.

Figure 5 :

 5 Figure 5: Example of the Billy-Montigny leveling line. The stabilization of the residual subsidence is clearly observed after 1999.

Figure 6 :

 6 Figure 6: Identification of subsiding areas with DINSAR. A.: 6623-25504 interferogram (21/10/1992-31/05/1996) highlighting three principal subsidence areas. B.: 8837-21362 interferogram (28/12/1996-22/05/1999) showing only two subsidence areas still active after 1996.

Figure 7 :

 7 Figure 7: PS average subsidence rate between 1992 and 1999.

Figure 8 :

 8 Figure 8: Comparison between PS displacements and levelling monitoring in the "Cité Saint Paul" area for PS closer than 150m of the levelling point. A.: Localization of the "Cité Saint Paul" area and considered data (red square). B.: Comparison of PS and levelling results where dashed lines represent the levelling points and continued lines, the PS.

Figure 9 :

 9 Figure 9: Localization (A) of the profiles (B) obtained from the unwrapping interferograms for two different time period(1992-1996 in blue; 1996-1999 in green)

Table 1 :

 1

Duration and amplitude of residual subsidence phase (after

[START_REF] Aissaoui | Amélioration de la prévision des affaissements dans les mines à l'aide des approches empiriques[END_REF]


Rabia Badreddine

Célia Brunel

Marguerite Munoz

Christine Destrigneville

Assessment of long term mining waste behaviour by a dynamic leaching test : application to contrasted physico-chemical conditions

Keywords: modified soxhlet, equivalent time, inertisation time, tailing drainage water

Mining activity has generated large amounts of mining wastes abandoned on the site after extraction, concentration or metallurgic processes. In this study assessment of the long term behaviour of mining wastes has been performed by the realisation of a dynamic leaching test performed with a Soxhlet device on mining wastes from two contrasted sites : Bentaillou and Saint-Salvy from south-west France. Sphalerite and galena are the source of heavy metals, (Zn, Cd and Pb) in both sites. Pyrite and pyrrothite are the main potential acid source in Saint-Salvy and Bentaillou respectively. Although both wastes display high contents of carbonate minerals associated to the metal bearing phases, their composition is different, calcite for Bentaillou and siderite for Saint-Salvy allowing different minerals transformations and drainage mining water compositions. Due to different impoundment type, dumps for Bentaillou and a dam for Saint-Salvy, oxidation reactions are favoured in the former compared to the latter. Waste drainage waters generated in situ display lower pH, Eh and higher Fe and heavy metals in drainage waters from Saint-Salvy compared to those from Bentaillou. The leaching experiments lasted 62 days and 84 days for Bentaillou sample and Saint-Salvy sample respectively corresponding to an equivalent time of 288 and 183 years respectively calculated from pluviometry in situ. During the experiment, the amounts of elements released was higher for Saint-Salvy sample ( 0.2 % of Zn, and Pb and 1,4 % of Cd) than for the Bentaillou one ( 9.7 % of Zn, and 0.15 % of Pb and 46 % of Cd). The comparison of experimental leaching results with site data on waste drainage waters demonstrated the validity of laboratory results under oxidic conditions at Bentaillou while the duration experimentally estimated for total sulphide oxidation overestimates the time calculated from the sulfate flow in field drainage waters under reducing conditions at Saint-Salvy.

Introduction

Different processes of mineral industry such as prospecting, extraction, treatment and storage of mineral resources, induce large amounts of mining wastes enriched in heavy metals such Hg, As, Pb, Cd. They constitute potential sources of metal contamination for surrounding soils, groundwater and superficial. Evaluation of the long term behaviour of mining wastes is being a lasting challenge for researchers, indeed besides the analysis of contaminant contents, their mobility has to be addressed to assess the potential impact of contaminants on the environment. In this study, the assessment of the long term behaviour of wastes mining and the kinetic determination of the metallic and not metallic elements has been performed with a dynamic test. This test using a Soxhlet device was used formerly on Solidified/Stabilized wastes [START_REF] Badreddine | Long term retention of trace metals as affected by the weathering of solidified/stabilized wastes[END_REF], De Windt et Bardreddine, 2007).

The results exploitation and the validation of dynamic test were undertaken by calculation of equivalent time and inertisation time and the comparison of data obtained from laboratory and site drainage waters. This approach has been applied to treatment wastes from two distinct Pb-Zn mining sites, both resulting from flotation process of Zn-Pb ores and displaying similar physical parameters (grain size distribution, permeability,…) but differing by the type of impoundment and the mineralogical composition.

Description of study areas

Bentaillou site

The orphan mining Zn-Pb site of Bentaillou is located in the upstream catchment of the Lez stream (French Pyrenees, SW France, Fig. 1). The Bentaillou site has been the second most important Zn-Pb mining site of the Pyrenees mountains. The sulphides are associated to gangue minerals, dominantly calcite and ankerite and secondarily illite and chlorite. The mineralisation is hosted by Palaeozoic and metamorphosed limestone. The carbonated context of the Bentaillou mining site is representative of most of the Pyrenean Zn-Pb mining sites. The ore was treated in a plant downhill by flotation process since 1920. More than 410,000 tonnes of treatment tailings were produced.

They were impounded in four dumps on the Lez stream banks, varying from 10 to 26 m in height. Pluviometry averages 1347 l.m -2 .y -1 .

Saint-Salvy site

The Saint-Salvy mining district is located 70 km ENE of the city of Toulouse, at the southwestern in south-west France (Fig. 2). The mine has been operating between 1975 and 1994. It has produced 450,000 tons of Zn with Pb, Ag and Ge as byproducts. The ore was composed of sphalerite and galena associated with minor pyrite and chalcopyrite. Associated gangue mineral are mainly composed of quartz and siderite. Host rocks are composed of black shales and aplopegmatites sills. Four million tonnes of flotation wastes have been impounded in a storage dam, located in a small catchment 4 km to the south of the mine, covering 2 hectares at altitude around 300 m. Pluviometry averages 1200 l.m -2 .y -1 .

Material and experiments

Solid samples collection and characterisation methods

A representative sample was collected from both sites taking into account the centimetric-scale variations of the laminae composition. The height, width and thickness of sampling were around 70 cm, 40 cm and 10 cm respectively. This representative sample was collected on a profile at the edge of the dump after removal of the surface (15 cm) tailing at Bentaillou, and, for Saint-Salvy, at the northern edge of the storage dam, where it is not covered by the clay land layer. Sub-samples were used for characterisation and experiment after mixing and quarting.

Qualitative and quantitative characterisation of metal-bearing mineral phases was carried out, after gravimetric separation, using optical metallographic microscopy, Xray diffraction, Electron Probe Micro Analysis.

Dynamic leaching test

The samples of mining wastes from Bentaillou and Saint-Salvy mines were leached during 62 days and 84 days respectively in a water extractor, similar to the Soxhlet device. The system is adapted to mimic cyclic wetting and drying in open conditions. A detailed description of the extraction procedure was given by [START_REF] Humez | A new assessment of mobility of elements in sediments and wastes -Chemical Speciation and Bioavailability 9[END_REF], [START_REF] Humez | A new experimental approach to study the long-term behaviour of solidified/stabilized wastes -Chemical Speciation and[END_REF] and [START_REF] Badreddine | Long term retention of trace metals as affected by the weathering of solidified/stabilized wastes[END_REF] who used it to study the leaching of heavy metals from mainly Solidified/Stabilised (S/S) wastes. This extractor allows to keep the leaching water temperature at 18°C. In the apparatus employed, distilled water is introduced into the extractor after evaporation from a flask and condensation in a condenser open to the air. The extraction vessel contains 120g in average of mining wastes sample and the water flow is 250ml/hour. The progressive addition of water dribbling on the sample results in a decrease of the solution concentration in the extraction vessel and as a result, in the diffusion of solutes from the pore solution of the solid to the bulk solution. By analysing the leachate in the evaporation flask at fixed intervals, it is possible to follow the kinetics of elements release from the solid matrix. pH and Eh, fhe concentrations of SO 4 , Ca, Mg, Fe, Zn, Pb and Cd were measured.

Tailing drainage water sampling, analysis and modelling

On Bentaillou site, tailing drainage waters (TDW) were collected nine times, in contrasted moisture conditions, between march 2002 to February 2004 from the permanent source. At Saint-Salvy, TDW sampling was carried at the dam outlet under base and high flow conditions between October 1998 to august 2002. Five samples have been collected. In both sites, redox potentials, pH and alkalinity were measured in situ. Chloride, nitrate and sulphate concentrations were measured using ion chromatography (HPLC, Dionex 2000i), base cations concentrations using atomic absorption Perkin Elmer 5100PC spectrometer, and dissolved Organic Carbon (DOC) using a Carbon Total Analyser (Shimadzu TOC 5000). Trace metal concentrations were determined by inductively coupled plasma mass-spectrometry (ICP-MS, Perkin Elmer, Elan 6000). Geochemical modelling of aqueous data was realised with CHESS geochemical code (Van der Lee et al., 2002) using SUPCRT92 and MINTEQ databases (Johnson et al., 1992), in order to determine theoretical distribution of metal species in solution and the saturation state of TDW relatively to different metal-bearing species.

Results and discussion

Mineralogical characterisation

Mining wastes form Bentaillou are mainly composed of quartz and calcite which amounts estimated by X-ray diffraction are about 50 % and 20 % respectively. The clay minerals are present in small amount in illite and chlorite forms. The mining wastes were composed equally by ankerite, amphiboles, diopside, micas, feldspars and minor dense minerals as magnetite, zircon and ilmenite. Pyrrhotite, sphalerite, galena and scarce willemite, chalcopyrite, arsenopyrite and nickelite were identified as primary minerals of the ore. Sphalerite contains in average 5.4 wt% of Fe and up to 0.2 wt% of Cd. Cerusite, Fe-(oxy)hydroxide and scarce smithsonite were identified as secondary minerals [START_REF] Brunel | Dynamique des éléments traces métalliques (Pb, Zn et Cd) sur un petit bassin versant contaminé par des déchets miniers-cas du bassin versant amont du Lez (Ariège, Pyrénées[END_REF]. Bulk chemical composition was obtained by analysing four samples. Only the analyses concerning the representative samples BEN-R and STS-R are presented (Table 2). These mining wastes are mainly composed of silica (49.3 -52.9 wt%). They are enriched in Ca (13.4 -17.8 wt%) and in inorganic C (14.8 -16.6 CO 2 wt%). Pb, Zn are dominant metals with 4.8 -10.9 g.kg -1 and 13. 3 -29.0 g.kg -1 respectively. Cd concentration is in the range 38 -92 mg.kg -1 . Mining wastes from Saint-Salvy are mainly composed of quartz, siderite, mica and kaolinite associated to sulphide minerals. Siderite (FeCO 3 ), the dominant carbonate, is enriched in Mn with around 29 g.kg -1 . Pyrite (FeS 2 ), sphalerite (ZnS) and galena (PbS) are the sulphide minerals identified in these wastes. Fe and Cd cations are substituted to Zn in sphalerite, hence iron content ranges between 2.2 and 3.3 wt%. and cadmium content averages 2500 mg.kg -1 [START_REF] Munoz | Geochemical and mineralogical evolution of the Zn-Cd-Pb bearing tailings from the Saint-Salvy mine[END_REF].

Only Fe-(oxy)hydroxide compounds have been identified as secondary minerals. Chemical analysis of this compound evidences Zn, Pb, and Cd enrichment (up to 10 g.kg -1 , 0.045 g.kg -1 and 0.015 g.kg -1 respectively) [START_REF] Munoz | Geochemical and mineralogical evolution of the Zn-Cd-Pb bearing tailings from the Saint-Salvy mine[END_REF]. Bulk chemical composition indicates that these wastes are mainly composed of silica (60.0 -67.8 wt%) and are enriched in Fe (10.0 -18.1 wt%) and in total C (5.56 -10. CO 2 wt%). Among heavy metals, Zn is dominant with 13.6 -18.7 g.kg -1 while Pb and Cd concentrations range between 221 -550 mg.kg -1 and, 25 -53 mg.kg -1 respectively. Sample STS-R is the most representative one and used for the comprehensive study. Its carbon organic content amounts was 0.54 wt%.

4.2.Tailing drainage waters (TDW) and geochemical modelling

TDW from Bentaillou are lightly basic (7.2 -7.5). On field, Eh potential (450 mV) reflects oxidising conditions within tailing dump. TDW present a relatively low TDS (112 mg.l -1 as a mean). Calcium and hydrogenocarbonate ions are the main cation and anion, respectively. This bulk composition reflects the carbonated feature of tailings with calcite dissolution. Sulphates are the second main anions reflecting sulphides oxidation. Zn and Pb which are the main heavy metals in the tailings are also dominant in the TDW with concentrations ranging between 34 and 269 μg.l -1 and between 0.1 and 21 μg.l -1 respectively. Cd concentration ranges between 0.2 and 1.8 μg.l -1 . The average discharge is 10.3 m 3 .day -1 . Geochemical modelling indicates that TDW from Bentaillou are under-saturated with respect to sulphate minerals (Saturation Index (SI) lower than -2.O). Carbonate minerals display higher saturation index. Cerusite (PbCO 3 ) is close to equilibrium in agreement to mineralogical observations (SI ranging from 0.0 and -0.6). Although the TDW are undersaturated with respect to smithsonite (SI ranging from -1.5 and -2.0), it has been occasionally found in these mining wastes probably because it was formed localy during drought periods or at the dryer edge of the tailing dump limiting the mobility of Zn. Similarly, ferrihydrite might precipitates in such circumstances as TDW are very slightly under-saturated with respect to this iron phase (SI ranging from -1.0 and -0.2) and ferrihydrite can be considered as a potential scavenger for heavy metals in these TDW.

TDW from Saint-Salvy are slightly acidic (6.3 < pH < 6.5) with a redox potential (200-244 mV) indicating reducing conditions. They are high mineralised Ca-SO 4 dominated waters (TDS = 1013-1264 mg/l -1 ), the lower values corresponding to the rainy period in spring (Table 4b They are enriched in Fe (total Fe = 13.4-23.1 mg.l -1 ), Mn (4.7-10.5 mg.l -1 ) and Zn (1.9-3.3 mg.l -1 ). They presents rather low Cd (2.3-4.9 g.l -1 ) and Pb (0.1-1.4 g.l -1 ) contents. The average discharge is 360 m 3 .day -1 .

Fe(II) and Fe(III) concentrations, determined assuming that the redox potential refers to the Fe(II)/Fe(III) system, were used to calculate the degree of saturation for potential iron secondary minerals. In the measured pH and Eh range, the TDW are close to equilibrium with respect to ferrihydrite, Fe(OH) 3(s) , indicating that Fe(III) concentration in drainage water is controlled within the waste material by this amorphous phase (SI ranging from 0.0 and -0.4), while the high log SI values for goethite are the result of none, or very slow crystallisation, because of kinetic constraints (SI ranging from 4.0 to 4.6). The waters are under-saturated with respect to Fe(II) mineral phases.

In spite of the high sulphate concentrations, drainage waters are slightly under-saturated with respect to gypsum, CaSO 4 , (SI ranging from -0.4 and -0.7) and under-saturated with respect to jarosite, KFe 3 (SO 4 ) 2 (OH) 6 , mineral commonly found in this type of site (SI ranging from -0.8 and -1.8). None of the sulphate minerals present in the data base appears to control the sulphate content of the drainage waters and in this case, this species behaves conservatively. However, as gypsum SI values are slightly negative at the dam outlet, it is possible that locally saturation is reached.

In addition, drainage waters are strongly under-saturated with respect to all oxide, hydroxide, carbonate and sulphate minerals of Zn, Cd and Pb included in the database, indicating there is no secondary phase other than ferrihydrite which precipitates from this solution in the tailings. Hence, the scavenger potential of this mineral phase is to be considered for the control of heavy metals in the drainage waters.

Dynamic leaching test results

For the waste sample from Bentaillou, the dynamic leaching test was performed during 62 days with analysis of the leachate after 1, 3,6,13,20,27,34,41,48,54 and 62 days.

At the end of experiment, the amount of elements released was very low in comparison with the initial content of mining wastes Figure 3. It reached 0.2% for Zn and for Pb and 1.4% for Cd, 3.6 % for SO 4 2-and 2.5 % for Ca. The metals amount released into solution were very small reflecting a retention mechanism of Pb, Zn and Cd in the mining wastes. The pH value of eluates varied from the beginning to end of experiment between 9.3 and 8.4 whereas, during the experiment, the lower value reached is 7.1. Half of sulphates was released in the beginning of the experiment, probably resulting from the dissolution of sulphate mineral such as gypsum rather than from sulphide dissolution.

The dynamic leaching test was performed on the mining wastes of Saint-Salvy during 84 days. Lead was mainly retained in the solid matrix while the release of Zn and Cd during the experiment was more important than for the Bentaillou Site. The release amounts of Pb, Zn and Cd were 0.15%, 9.7% and 46 % respectively. Third of the liberation was operated in the beginning of the experiment (Figure 3). The sulphates leaching was fast in the beginning of the experiment. The total amount liberated is 1500 mg.kg -1 corresponding to 5% of initial amount of sulphur. The high calcium release was operated in the beginning of the experiment whereas 30% was mobilised by the end of experiment. The pH values varied from 7.1 to 5.3 from the beginning to end of the experiment.

Equivalent time and inertisation time estimations

The determination of equivalent time is based on an extrapolation of the experimentation duration to the leaching duration in the field by comparison of the The inertisation time of system is defined as the time necessary for oxidation of the total amount of sulphide minerals which are the metallic contaminant providers. Its estimation is based on exportation of sulphate which must behave conservatively [START_REF] Igarashi | Estimation of pyrite oxidation rate by sulfate ion discharged from a catchment[END_REF][START_REF] Munoz | Geochemical and mineralogical evolution of the Zn-Cd-Pb bearing tailings from the Saint-Salvy mine[END_REF][START_REF] Brunel | Dynamique des éléments traces métalliques (Pb, Zn et Cd) sur un petit bassin versant contaminé par des déchets miniers-cas du bassin versant amont du Lez (Ariège, Pyrénées[END_REF]. Although a few percent of sulphur might occur as sulphate mineral in the waste sample and is released at the beginning of the experiment, the solution/solid ratio does not favour saturation of sulphate mineral during the experiment. If the sulphur release is linear with time, the inertisation time from site data is determined from the following equation :

T inertisation = C S /F SO4 C S : Total molar stock of sulphur in the waste sample F SO4 : calculated mean molar flow of SO 4 2-annually exported.

Hence, the inertisation time determined experimentally for Bentaillou waste dump is of 7730 years approximately.

The inertisation time determined from the experimental data for Saint-Salvy wastes is of 4050 years approximatively.

Validation of the modified Soxhlet experiment

For the validation of modified Soxhlet test, the estimation of inertisation time of mining wastes based on field data was compared to the estimation based on experimental data. The inertisation time of system was defined as the time necessary for oxidation of the total amount of sulphide minerals in the dump and the dam of Bentaillou and Saint-Salvy respectively. Its estimation is based on exportation flow of sulphate by drainage waters in the sites. Assuming the sulphur release is linear with time, the inertisation time from site data is determined from equation :T inertisation = C S /F SO4 C S : Total molar stock sulphur in the mining wastes drained in the site F SO4 : mean molar flow of SO 4 2-annually exported by drainage waters in the site.

The inertisation time estimated on the Bentaillou site is around 9400 years. The inertisation time determined from site data of Saint-Salvy is approximately 1440 years.

Finally, the estimated inertisation times from experimental and site data are 7730 and 9400 years for Bentaillou and 4050 and 1440 for Saint-Salvy respectively. The same order of magnitude from the experimental and site date implies the efficiency of the modified soxhlet test for mining wastes characterisation aiming the prediction of their behaviour. However some uncertainties such as the flow rate, the representativity and not totally conservatives character of sulphates infer that the results are only approximate.

Hence experimental and field results are close for Bentaillou taking into account various assumption and approximations. Validation of the modified Soxhlet method for long term behaviour assessment can be proposed.

On the contrary, for Saint-Salvy wastes the duration experimentally estimated for total sulphide oxidation overestimates the time calculated from the sulphate flow exported by drainage waters in situ..

In the Saint-Salvy impoundment waste system, subsequently to sulphide oxidation and neutralisation reactions, elements are released and transported downwards by infiltrated waters to more reducing conditions. Oxidation reactions are allowed by infiltrated waters transporting oxygen which decreases with depth. The oxidation products of sphalerite, pyrite and galena are acidity and liberation of SO 4 , Zn, ferrous iron, Cd and Pb. The H + ions supply generates siderite dissolution whose products are HCO 3 -, Fe 2+ and Mn 2+ . In the Eh and pH conditions of site, only a part of ferrous iron oxidise favouring the ferrihydrite precipitation which can act as a screen protecting sulphides grains from further oxidation. Such a mechanism may explain the lower rate of oxidation obtained in the experimental leaching procedure performed under oxidising conditions while drainage waters in the field evidence more reducing conditions. Consequently the modified Soxhlet procedure is not validated for dam impounded wastes, this type of impoundment lowering oxygen availability.

Figure 1

 1 Figure 1 Bentaillou site localisation Figure 2. Saint-Salvy site localisation

  passed through the sample during the experiment with the rainfalls flows of the area. The 62 days lasting experiment performed on the Bentaillou mining wastes sample corresponds to an equivalent time of 288 years. The equivalent time determined for the Saint-Salvy site from the experiment of 84 days would correspond to a leaching duration of 183 years in situ.

Table 1 :

 1 Chemical composition of representative samples

  7

	SAMPLE	BEN-R	STS-R
		% wt
	SiO 2	49.43	67.82
	Al 2 O 3	6.57	6.40
	Fe 2 O 3	4.58	12.76
	MnO	0.24	0.60
	MgO	1.63	0.77
	CaO	16.27	0.50
	Na 2 O	0.06	0.09
	K 2 O	1.72	1.79
	TiO 2	0.44	0.32
	P 2 O 5	0.07	0.16
	S tot	0.14	1.04
	CO 2	15.24	7.06
	C.org	0.36	0.54
		mg.kg -1
	Zn	18 290	18 560
	Cd	53	50
	Pb	7 520	550
	As	44


Steel production generate great amounts of by%products as steel slags. The use of Basic Oxygen Furnace slags (BOF slags) has been restrained due to insufficient volume stability, and due to the lack of environmental regulations. The purpose of the PRECODD%ECLAIR research program is to develop a behavior model based on a multi%scale physico%chemical, mechanical, hydrodynamic and ecotoxicological characterizations of a BOF slag used in a public works scenario. This paper aims at presenting the overall ECLAIR research program, the equipped experimental platform constructed using a BOF steel slag, and the first results of the slag characterization.

" #$

Steel slag, civil engineering, chemical analysis, hydrodynamics, environment.

%&

Steel production generates great amounts of by%products as steel slags (2.2 million tons in France in 2004). Conversely to blast furnace slags which have been extensively used in cement industry, building trade and road construction, the use of Basic Oxygen Furnace slags (BOF slags) has been restrained due to insufficient volume stability, and due to the lack of environmental regulations. Ageing processes have been developed in order to control the volume instability and the use of this product is developing in civil engineering to save natural resources.

Mainly composed of tricalcium and dicalcium silicates (Ca 2 SiO4), dicalcium ferrite (Ca 2 Fe2O5) and solid solutions containing iron and manganese oxides ((Fe, Mn,Mg,Ca)O) [START_REF] Goldring | Petrology and stability of steel slags[END_REF], steel slags also contain free lime (CaO) and periclase (MgO) coming respectively from the additions during ore reduction and from the refractory wall of the oven. Leaching tests carried out on steel slags have shown that the pH of leachates could reach high values (10%13) and trace elements could be released from some slags [START_REF] Motz | Products of steel slags an opportunity to save natural resources[END_REF][START_REF] Chaurand | Speciation of Cr and V within BOF Steel slag reused in road constructions[END_REF]. Besides, weathering promotes the hydration and carbonation of BOF%slags inducing a modification of the mineralogical assemblage and of the trace element speciation [START_REF] Chaurand | Apport de la cristallochimie et de la spéciation du chrome et du vanadium à la modélisation de l'altération de granulats artificiels[END_REF].

Few studies deal with the transfer of contaminants from the by%product layer to the underlying soil, or with the evaluation of the potential impact on the ecosystems at the pilot or work scale. They essentially concern the municipal solid waste incineration bottom ashes [START_REF] Ph | Chemical changes and leachate mass balance of municipal solid waste bottom ash submitted to weathering[END_REF] or electric arc furnace slags [START_REF] Lind | Environmental impact of ferrochrome slag in road construction[END_REF]. A study about the human health and ecological risks posed by the uses of steel%industry slags has shown that direct risk for human is low, but on site evaluation is necessary as regards the environment [START_REF] Proctor | Assessment of human health and ecological risks posed by the uses of steel%industry slags in the environment[END_REF].

The aim of the PRECODD%ECLAIR project is to develop a behavior model based on the physico%chemical, mechanical, hydrodynamic and ecotoxicological characterizations of a BOF steel slag used in a public works scenario, within the frame of a research approach integrated in the comprehensive problems of the better knowledge about steel slag behavior.

In 2006, the PRECODD%ECLAIR research program has been launched for three years in order to develop a behavior model of a BOF slag used in a public works scenario. The study deals with the metallic trace elements contained in the slag mineral phases and that might be mobilized during fluid percolation through the structure. A multi% scale characterization (macro, micro and molecular) will be carried out to evaluate the crystallo%chemical evolution of the BOF slag mineral phases and of the associated metals. These results, together with the evaluation of specific parameters (volume stability, hydrodynamic flows), will allow to use relevant data to develop a model characterizing the fluxes and their migration from the structure. Finally, a study of the impact on the earthworm, which is a widely used model for the evaluation of polluted soil ecotoxicological risks, will be performed. The overall model will be validated with the results obtained from the follow up of a full%scale experimental civil engineering structure.

The project comprises seven different steps:

Step 1: Follow up of an experimental civil engineering structure constructed using a BOF steel slag. The quality of water percolating through the structure will be monitored for at least one year in order to evaluate the fluxes of elements originating from the slag.

Step 2: The understanding of geochemical mechanisms of metal release from the slags needs the knowledge of the metal speciation and of the mineral matrix evolution. Initial material and materials resulting from weathering in situ at the experimental site, or coming from a lysimeter and from laboratory leaching tests, will be studied. Multi%scale structural techniques will be used, including X%ray diffraction and electron microscopy. Microscopic synchrotron radiation tools will also be used to characterize metals in slags, especially as regards valence and speciation.

Step 3: A first step of modeling will be performed to simulate the weathering and transport phenomena observed during leaching reactions. It will allow to model the evolution of the mineral matrix for major constituents as well as for contaminants carrying phases. The CHESS/HYTEC model coupling chemistry and transport will be used.

Step 4:The hydraulic characterization of steel slags is necessary for the modeling of pollutant transport. The hydrodynamic characteristic curves, suction versus water content (h(θ)) and hydraulic conductivity versus water content (K(θ)), will be determined.

Step 5: The modeling of pollutant transfer in the soil under a civil engineering structure will be carried out using the PHREEQC model and the data from the previous steps.

Step 6: The toxicity of leachate from the steel slags will be evaluated on the earthworm thanks to a set of not very specific or non%specific biomarkers, including genotoxicity, biochemistry, physiology or behavior of worms.

Step 7: This last step will be dedicated to the proposal of a conceptual model to simulate the long term behavior of pollutants at the work scale, depending on the implemented scenario (work structure and size, mechanical and environmental conditions…).

(& ) (&% *

The studied material is a BOF steel slag screened and treated to remove iron in 2003. The slag was dumped outside in a 3 m heap from 2004 to 2006 for ageing. The used mixture was made with three available granular fractions of 0%2 mm, 2%4 mm and 4%6 mm combined into equal portions in march 2007. Figure 1 shows the result of the grain size analysis of the slag mixture. The optimum moisture content (normal Proctor test) is 10% with a bulk density of dry material of 2.11 t.m %3 .

+ (&' ,!

The experimental site consists of an outside industrial platform, 10.9 x 6.4 m and 0.52 m thick, located in the southern part of France. The bottom of the work has been covered with a geomembrane and equipped with a drainage system in order to collect the stormwater percolating through the BOF slags. Two 0.26 m%thick layers of slags have been dumped in the platform and compacted (total mass about 70t). The bearing capacity of each layer was about 50 Mpa after compaction (plate%bearing test). The platform is unpaved and there is no traffic. The level of the water table is generally below 2 m at the site. The dry bulk density average 1.88 t m %3 , which is not so far from the optimum proctor bulk density.

In addition to the platform, a 2%m 3 lysimeter has been installed outside and filled with the same compacted BOF slag. This approach will allow a multi%scale comparison of the results regarding potential pollutant release from the slags and the fitting and validation of predictive models.

(&( -# # . One of the purposes of this research program is to develop a specific laboratory setup that affords a complete hydrodynamic characterization. In addition, in situ infiltration experiments (infiltration of water in the material through a disc, under control of the water pressure at the surface ) have already been performed through the use of a simple ring and SW 080 infiltrometer. Cumulative infiltrations (total infiltrated water head versus time) were modelled to derive the water retention h(θ) and hydraulic conductivity K(θ) curves.

(&/ # The total trace metal concentration in slags was measured using HNO 3 /HCl (aqua regia) sample digestion. The solutions were analysed, after filtration through a 0.45%Nm membrane, using inductively coupled plasma atomic emission spectroscopy (ICP%AES or ICP%MS) according to NF EN ISO 11885 standard method. The major elemental composition of the slag was determined by ICP%AES, after alkaline fusion as regards silicon. Chloride, fluoride, sulphate and phosphate were determined using ionic chromatography according to NF EN ISO 10304 standard method. Various characteristics such as pH, total Carbon, loss of ignition were also determined. NF EN 12 457%2 leaching test has been carried out in order to characterize the initial pollutant potential of each fraction of BOF slag (single 24h%extraction with deionised water, liquid/solid ratio of 10). Bulk mineralogy of the steel slags was determined by X%ray diffraction. Thin sections were prepared for microscopic analysis. Optical microscopy and scanning electron microscopy (SEM) coupled with X%ray spectroscopy micro% analyses (EDS) was used to study the localization of mineralogical phases in slag grains. More details are given in [START_REF] Deneele | Natural weathering of Basic Oxygen Furnace slags: Characterization and consequences[END_REF].

/& /&%

. Table 1 shows the results of the chemical analysis of the three fractions of steel slags and of the studied mixture. The pH%H 2 O, loss of ignition (LoI) at 550°C, Carbon content, major element and trace element contents have been determined. As compared to other data [START_REF] Chaurand | Apport de la cristallochimie et de la spéciation du chrome et du vanadium à la modélisation de l'altération de granulats artificiels[END_REF], the composition of the studied BOF steel slag was representative of the worldwide production, as regards major element content. Slags were characterized by a high content of calcium, iron and silicon. The composition was quite homogeneous whatever the grain size fraction. The chemical composition of steel slags depends on the production source, ores and nature of added materials during production process. So, trace element contents vary very much [START_REF] Chaurand | Apport de la cristallochimie et de la spéciation du chrome et du vanadium à la modélisation de l'altération de granulats artificiels[END_REF][START_REF] Proctor | Assessment of human health and ecological risks posed by the uses of steel%industry slags in the environment[END_REF]. Nevertheless, results in table 1 confirmed the presence of vanadium, chromium, barium and boron in steel slags. Besides, the content of chromium, vanadium and barium seemed to increase with the grain size. The conductivity of leachate was high and the pH was very alkaline due to the high content of lime and carbonates in slags. In the conditions of the leaching test, the release of trace elements was generally low, concentrations of chromium and vanadium were below the detection limit in leachates. The leaching of elements was lower in the 4%6 mm fraction, probably due to a grain size effect.

/&' -# # . The infiltrations were performed at both initial stage (August 2007) and 6 months later over smooth%like and coarse%like zones. The presence of these different zones may result from the emplacement of the material. During the first campaign (initial stage, figure 3) the infiltration differed according to the kind of zones, the coarser zone being more conductive. Six months later, infiltrations were reduced for both zones. Clearly, hydrodynamic behaviour was inhomogeneous at the initial stage and then became more homogeneous with a reduction of the hydraulic conductivity of both zones. Such a reduction could be attributed to the chemical evolution of the material: clogging of the porosity due to precipitation of certain phases. Due to the rather low precipitation depth at the experimental site during the study period and to the high water retention capacity of the slags, only three samples of seepage water have been collected and analysed. Table 3 presents the results of the analysis of the water samples collected in June (1360 L), October (120 L) and December (650 L) 2007. The pH of seepage waters was very alkaline, more than 12, and the conductivity was rather high. The concentrations of Total Organic Carbon and sulphates were quite low. The calcium content was very high and, except for barium, the concentrations of trace elements were low, below or close to the detection limit.

3&

The ECLAIR%PRECODD project has been launched for 3 years in 2006. The purpose of the project is to develop an integrated approach to assess the environmental behavior of a BOF steel slag used in a civil engineering scenario. An experimental platform has been constructed and equipped to collect seepage water. Hydrodynamic characterization pointed out the heterogeneity of the slag layer and the effect of weathering inducing a decrease of the hydrodynamic conductivity. Results of the chemical characterization revealed the presence of Cr and V in steel slags, but leaching test and first results of seepage water analysis seemed to show a very low release of trace elements in water.

  has been in operation since May 2007.

  The results of leaching test performed on each slag fraction are shown in table 2.

				!
		0	1 '	' /	/ 2	1 2
	!--'		12.7	12.8	12.1	12.7
	3314	%	4	1.0	1.0	3.0
		%	1.78	0.63	0.97	1.46
	' (	%	2.77	1.69	3.54	2.1
	* '	%	14.52	14.86	12.4	15.97
	5 '	%	0.58	0.47	0.39	0.48
		%	42.02	43.85	44.75	40.7
	)	%	7.45	6.52	7.16	6.46
	6 '	%	0.06	0.06	0.05	0.07
	' 3	%	1.62	1.43	1.14	1.4
	" '	%	0.04	0.05	0.03	0.06
	) ( /	%	2.95	2.54	2.15	2.59
	' (	%	28.46	27.84	27.47	29.82
		mg/kg	<2	<2	2.87	<2
		mg/kg	96.7	124	137	128
		mg/kg	113	135	124	118
		mg/kg	<1	<1	<1	<1
		mg/kg	782	877	917	910
		mg/kg	5.59	6.68	7.51	7.86
		mg/kg	4.21	2.49	<2	2.7
	)	mg/kg	4.03	3.45	4.01	4.02
	6	mg/kg	7.44	5.35	4.78	4.04
		mg/kg	6.83	5.75	6.54	5.74
	7	mg/kg	596	740	777	741
	8	mg/kg	9.13	<6	<6	<6
	-	mg/kg	<0.2	<0.2	<0.2	<0.2
			"
			0	1 '	' /	/ 2
	#		mg/kg	18680	18700	13200
		#	NS/cm	8250	8100	7490
	!-		%	12.6	12.5	11.4
			mg/kg	15.32	15.09	31.99
	*		mg/kg	22.4	14.2	17.9
			mg/kg	<0.1	<0.1	<0.1
			mg/kg	5.18	5.52	3.16
			mg/kg	7520	7578	7040
			mg/kg	<0.05	<0.05	<0.05
			mg/kg	<0.1	<0.1	<0.1
			mg/kg	1.16	0.63	0.38
	7		mg/kg	<0.1	<0.1	<0.1
	-		mg/kg	<0.01	<0.01	<0.01

* Author to whom correspondence should be addressed: michel.legret@lcpc.fr

This paper aims at presenting the overall ECLAIR research program, the equipped experimental platform constructed using a BOF steel slag, and the first results of the slag characterization. The geochemical characterization of the slag is presented elsewhere[START_REF] Deneele | Natural weathering of Basic Oxygen Furnace slags: Characterization and consequences[END_REF].