Poulard Frédéric
email: frederic.poulard@ineris.fr

Franck Christian
email: christian.franck@ineris.fr

SCREENING OF FRENCH MINING EXPLOITATIONS: A METHODOLOGY AND A NATIONAL HIERARCHISATION TO EVALUATE THE GEOTECHNICAL RISK

Keywords: Multi-criteria analysis, mining risk, hazard, hierarchisation, ELECTRE's methods, Screening, ground instabilities Hiérarchisation multicritères, risques miniers, aléas miniers, méthodes ELECTRE, Scanning, mouvements de terrains

The French Ministry, in charge of mining and post-mining issues, plans to perform as effectively as possible its future investigations (Mining Risk Prevention Plans, MRPP, in particular). In this way, an ambitious program, called Screening, has been engaged in order to classify and to evaluate all French mining sites according to their mining risk level and then, to perform a quick risk evaluation for the most critical sites. This process included an important stage of hierarchisation. After many months of works, a multi-criteria analysis (based on ELECTRE's method) was finally adopted. It required an appropriated selection of experts, an effective criteria definition, use of adapted tools and a precise validation before application to given mining sites. The different steps of this work are presented in this paper.

Contexts and goals

Because of its rich industrial past and its long mining tradition, more than 4000 mining titles (concessions, exploration agreements) have been granted throughout the whole metropolitan French territory in very different geological configurations. Nowadays, under the effect of diminishing resources and the international competition, most French mining operations are gradually forced to close and only few mines (salt mines) are always exploited. The main issue is to manage these old mining workings, often identified as a source of nuisance that can affect lands, during the postmining period. In France, it is the State responsibility to evaluate and to report the residual mining risks (surface instability, flooding, dangerous gas emission, dangerous ground or water pollution, ionizing radiation).

To draw up rules for managing land use according to the various constraints linked to these mining risks and nuisances, the French State has acquired an operational regulatory tool : Plans de Prévention des Risques Miniers (PPRM) (Mining Risk Prevention Plans -MRPP). Between 1999 and 2007, MRPP have been engaged in the main mining ore basins exploited in France : certain metal mines (copper in Alps, uranium in Centre-West, etc), major iron ore fields (Lorraine, Centre-West), many coal basins (Provence, Centre-South, etc). The study of the major salt basins (Lorraine, Jura, South-West) is programmed for 2008. Taking into account this trend of MRPP implementation, one can easily estimate that MRPP are very long studies. It was essential to define, in a short period of time, the sites that present the most significant post-mining risks in order to assist the State in his land management. Another goal is to establish priorities in the implementation of the future MRPPs (the most sensitive sites could be treated prior to the others). It was also important to identify quickly the critical contexts requiring urgent safety measures.

Hence, the French Ministry, in charge of mining management, asked to carry out simple risk qualification by focusing on surface instabilities. This ambitious program, called "Mining Sites Screening Process", consists in :

• reviewing all French mining sites in order to classify them according to their risk level (considering surface instabilities) ; • selecting the most sensitive mining exploitations and to subject them to quick studies in order to evaluate and to map risk and hazard levels linked with land movement (gradual subsidence, local cave-in, etc). The other phenomena (gas emission, dangerous ground or water pollution, etc) are not considered in detail in these studies. They will be only mentioned in case they are observed.

This paper details the whole steps that allowed to classify mining sites : determination of a classification methodology, construction of hierarchisation tool, validation, analysis of different issues.

The paper is devoted to the first point of the Ministry demand. The second aspect (risk analysis in the most sensitive mining sites) will not be presented here because many other papers have investigated to this topic [START_REF] Didier | The MRPP: a powerful operational regulatory tool to prevent and manage post-mining risks[END_REF] and [START_REF] Didier | Importance of failure mechanisms for management of surface instability risk above abandoned mines. 10th Congress of the ISRM[END_REF]].

Development of a selection methodology and construction of a hierarchisation tool

Hierarchisation methodology

Mining operations consist in extracting large quantities of ore, only a valuable part of the great amount of excavated rocks. These excavations (underground or open-cast) have irreversibly altered the rock formations in which the ore is found and their stability could be threatened at long term. These instabilities could generate disorders at surface (different kind of subsidence events) resulting in damages to structures and potentially dangers for people. The Screening ought to help their evaluation.

The post-mining risk management is based on various knowledge. Many actors and experts have been called to contribute to this risk selection. Like the elaboration of methodological guideline for the elaboration of MRPPs, the methodological works of the Screening were established with the help of an expert college functioning as a steering committee [START_REF] Didier | L'élaboration des Plans de Prévention des Risques Miniers -Guide Méthodologique -Volet technique relatif à l'évaluation de l'aléa -Les risques de mouvements de terrain, d'inondations et d'émissions de gaz de mine[END_REF]. It was composed of mining experts and professionals of geotechnical analysis : BRGM, INERIS, Mining School of Paris and GEODERIS (with JP. JOSIEN who was the organizer of the national experts committee studying land movements for the MRPP methodological guideline elaboration).

In the first time, when the experts have been questioned, more than 10 criteria have been listed in order to evaluate post-mining risks, they were linked with factors of surface vulnerability, significance of effects and susceptibility of underground mining works to collapse. With the experience of the experts committee and taking into account the complex evaluation of mining risks and hazards, it was agreed to use methods and tools based on multi-criteria analysis.

The Screening was expected to provide with a global classification of all the known mining sites.

For this, a multi-criteria Expert-Aid method has been proposed, it was based on ELECTRE III method [START_REF] Merad | Apport des méthodes d'aide multicritère à la décision pour l'analyse et la gestion des risques liés aux mouvements de terrains induits par les ouvrages souterrains[END_REF], [START_REF] Figueira | ELECTRE methods[END_REF]. Indeed, these methods have to generate a classification in a determined sample of mining sites described according to a revelant criteria family (quantitative or qualitative). ELECTRE III allows to classify mining sites from the best to the worst (from the more risky to the less risky). Moreover, these methods have been successfully applied to the mining risk management in Lorraine region (East of France) since 1999 [START_REF] Poulard | Procédure pour la recherche, la hiérarchisation des zones à risque d'instabilité et la prévision des conséquences en surface dans le bassin ferrifère lorrain[END_REF]. Taking into account all these elements ELECTRE III was finally adopted.

Criteria definition and evaluation

ELECTRE III method requires to define a list of criteria, constituting a basis for the classification.

In the Screening, the goal is to select mining sites according to their potential risk level linked with ground instabilities. Therefore, this risk results from the crossing of :

• the existence of a land movement hazard. This idea is based on the evaluation of site's predisposition to be affected by any kind of instabilities (combination of different factors that are favourable or unfavourable to the initiation and to the development of an underground collapse). Moreover, in the Screening, mining hazards involved the quantification of the feared events consequences (size and numbers of disasters…) ; • the existence of human land occupation (constructions, equipment and infrastructures). It aims to characterise the existing vulnerability and to identify the potential future projects which could develop within these areas (main construction projects : allotments projects, etc).

In this regard, 2 criteria groups have been defined. The first tries to give information on mining hazards and the second is linked with the land occupation.

The experts have defined qualitative criteria. They inform on the criteria after an experimental evaluation or an extrapolation, using similarities between mining sites. These qualitative definitions are based on the identification of homogeneous risk levels. For instance, to describe the "geological overburden nature", mining sites were distributed in three levels : weak, fairly strong and strong. That is to say the mining sites could present three similar probability levels of underground workings instabilities (Table 1).

The nature and the quality of mining information available have oriented this criteria definition. Indeed, the information was collected in GEODERIS mining site GIS-assisted database or in Departmental Archive database ; thus, it was useless to establish criteria that could not be informed because missed in databases.

Mining hazard criteria definition

In the Screening hierarchisation, the hazard linked with land movements potentially generated by underground mining works, is described through three phenomena (the most sensitive disorders that may persist at long term in the surroundings of former mining French works) [START_REF] Didier | L'élaboration des Plans de Prévention des Risques Miniers -Guide Méthodologique -Volet technique relatif à l'évaluation de l'aléa -Les risques de mouvements de terrain, d'inondations et d'émissions de gaz de mine[END_REF].

These three phenomena are :

• gradual subsidence (gradual, flexible readjustment of surface layers) ; quantitative criteria taking values based on growing risk levels (Table 2) 1, 2, 4, 8, 12, 16, 24, 36, 48, 72, 144, 432. H • local cave-in (sudden appearance on the surface of a sink-hole). The sudden feature of the instability makes these potentially dangerous events ; • general cave-in (collapse of all or part of surface, often dynamic). These phenomena are potentially very dangerous. General cave-in is a very specific phenomenon that cannot be integrated in a multi-criteria hierarchisation. This rare kind of ground instability is treated and analysed individually when the hierarchisation, linked with a gradual subsidence, leads a "suspect" mining site.

The experts have defined thirteen criteria in order to determine land movement hazards generated by gradual subsidence and local cave-in (three tables above). They are based on :

• geological factors, linked with failure mechanisms : geological contexts, ore and overburden natures, stress states, etc ; • mining exploitation factors : mining methods (methods of treatment of the voids after extraction or those allowing the persistence of important significant voids after closure), depth of workings, mining productions, etc.

Definition of land occupation criteria

This group of criteria is essential but difficult to be informed. It includes the localisation of old mining workings (not always precisely known) and the definition of the land occupation. Screening having to be carried out at a national level and every mining sites in France having to be classed homogeneously, so it was agreed to work with recent topographic maps, called Scan25, from NGI (National Geographical Institute). Indeed, this data has a good quality/price ratio with an appropriate scale (1/25000). Other known maps (aerial photography or digitalised data) have not be selected, because they are not available in all regions of France, they are are more expensive and they give a precision of scale, of 1/10000 or 1/2000, useless for the Screening taking into account the uncertain mining sites localisation.

Thus, five criteria were finally defined (Table 4). Note that the "Nature of the land occupation" was defined using data read on the Scan25. The values of this criteria are objective, except for the areas with potential constructions. It contains a certain subjectivity based on the operator evaluation using Scan25 (tourist sectors, towns proximity, weather characteristics, etc). The Screening implementation, between 2005 and the end of 2007, has shown that, in spite of its initial definition, this criterion has been informed homogeneously ; the only little differences have been erased when the information on the mining sites has been checked by an expert with a global view. The majority of the Screening criteria is defined qualitatively. ELECTRE's method requires converting these modalities into values or codes in order to operate mathematical calculations. For the Screening, it was agreed to give weights to these codes.

Moreover, the different criteria have not the same weights in the final risk evaluation. Thus, criteria must be balanced ones compared to the others by a weight definition.

These two issues are lead to the same question : in which way to associate values (here integer numbers) with individuals in a finite sample (here modalities or criteria). Traditionally, SIMOS's methods are used in these scientific issues [START_REF] Roy | Détermination des poids des critères dans les méthodes de type ELECTRE avec la technique de Simos révisée[END_REF]. These methods, also called "Card's methods", encourage the communication between the experts by : 1. classifying modalities (or criteria) from the more risky to the less risky with the possibility to define ex-aequo ;

2. inserting white cards in order to indicate the levels between modalities (or criteria) classed before ;

3. defining the ratio between the more risky modality and the less risky modality (or criteria).

Simple proportional calculations allow to conclude with the final values (modalities and criteria). In the following, the method applied to the Screening is presented.

Whatever the method used, this stage is commonly organised in two parts : individually (expert) and collectively (experts committee). The first one allows to each expert to give his opinion based on his competence and knowledge. In the second stage, on one hand, the experts share their points of view and, on the other hand, they try to address a single global representation.

Modalities values

In order to calculate these values, SIMOS's method has been applied except for the second point. Instead of classing white cards, the experts have preferred answering to the questions like : "For you, in the evaluation of the mining sites predisposition to a local cave-in, a weak geological overburden is how many times more risky than a strong one ?". The experts have appreciated this enquiries that seem to be more "scientific" and, above all, more efficient than cards. The exploitation and the determination of values are the same.

The final results, established after crossings all expert opinions, are presented in the following table (Table 5). These values have been brought back to a common base of 100 and they are classed with growing risk levels.

These results mean that if a weak geological overburden is ten times more risky than a strong overburden, then a fairly strong overburden is twice risky than a strong one and eight times less risky than a weak one. The same approach, than used for the determination of modalities values, was applied here.

The experts compared and classed criteria into the two groups (mining hazards and land occupations). However, they have distinguished, on one hand, local cave-in, and, on the other hand, gradual subsidence (a same criteria has not got the same weight in the local cave-in evaluation than in the gradual subsidence determination).

The next stage was more complicated, ELECTRE's method requires to compare all criteria between them. This exercise has been very difficult for certain experts and impossible for many others. For instance, no expert has been able to answer to this question : "In the mining risk evaluation, what is the relative weight between the overburden nature and land occupation nature ?".

On the other hand, each expert could determine which in mining hazards or land occupation was the most important for the mining risk evaluation and each expert could give a ratio between them.

Combining all these weights, those between criteria in each group and those between the two groups of criteria, the estimation of the final weights has been carried out.

However, the experts did not reach the single representation as it was expected. They did not agree on the determination of the ratio between mining hazards weight and the land occupation weight. Thus, to tend towards a global classification and to continue the Screening, it was agreed to retain three systems of weights according to the three following scenarii (Table 6) :

• scenario 1 : mining hazard is as important as land occupation ;

• scenario 2 : land occupation is twice more important than mining hazard ;

• scenario 3 : mining hazard is twice more important than land occupation.

Hierarchisation (ELECTRE's theory)

The goal is not to give in detail here the whole ELECTRE's method but only to underline its principal stages that are partly invisible while ELECTRE's software utilisation. The specific choices made for the Screening are explained here.

Using a comparison of mining sites two to two, ELECTRE's method (MERAD, 2004) allows three kinds of relations :

• mining site x a is more risky (or preferred) than x b ;

• mining sites x a and x b are indifferent (meaning that it is not possible to define the most or the less risky mining site between x a and x b ) ; • mining sites x a and x b . are incomparable (meaning that x a and x b, present too many different characteristics to be compared).

Finally, ELECTRE gives a partial final classification where the mining sites are sorted with risk rows (integers) : a n-row mining site will have a risk more higher than a (n+1)-row mining site.

ELECTRE's method leaves open the possibility, even rare, that two mining sites could not be compared. This possibility does not seem appropriate to the Ministry in charge of mines who needs a classification method for whole mining sites.

To limit these kinds of incomparability and to give a global final classification, it is usual to vary the limiting values defined in ELECTRE's method. This, usually studied for each case of incomparability, could not be generalised to the Screening where hierarchisation must be applied homogeneously to thousands of mining sites and repeated in different mining sites samples. With the help of hierarchisation experts, it was decided to define null limiting values. This option does not erase completely the incomparability relation, but in the most cases, it limited them or it transformed the incomparability relation in a relation of preference or indifference.

Thus, between 2005 and the end of 2007, the Screening did not meet any incomparability situation that could have stopped the on-going studies. All these incomparability cases, ELECTRE's method richness, have been detailed by experts in order to finally class these sites. The applied final hierarchisation process is divided in three stages (Figure 1) :

Synthetic representation of the Screening hierarchisation

• first stage where mining sites are characterised in order to describe their mining hazard criteria (local cave-in and gradual subsidence) and their land occupation criteria ; • second stage, during which eight different classifications are carried out :

• for the mining sites exposed to a gradual subsidence : one classification linked with land occupation and three classifications corresponding to the three hazard scenarii defined before ( § 2.2.3.2) ; • four same classifications for mining sites exposed to local cave-in.

• third stage where the hierachisation results are studied by the experts. These results ought to help them to select the most critical sites. For these specific sites, a simplified risk evaluation and a cartography are undertaken systematically ; • last stage, fulfilled only during the validation of methods and tools, where the robustness and the sensitivity of the hierarchisation process are improved.

Validation of the method and the tools of hierarchisation

Application to well-known mining sites

In 2005, Provence Alpes Cote d'Azur region (south of France, PACA) was in advance respect to mining database construction. Moreover, many mining sites have been studied for MRPP. In this region, the Regional Departments of Industry, Research and Environment (DRIRE), managing technical and administrative supervision of mining and post-mining activities, was interested in the Screening results. It was agreed to improve the hierarchisation Screening method in PACA.

This validation followed two goals :

• to check whether the classification based on the hierarchisation tool corresponds to the selection of the most critical sites agreed by the DRIRE and the experts ; • to carry out hierarchisations with MRPP sites and to analyse their risk levels. Note that these sites were not initially included in the Screening (Ministry request).

PACA mining sites

Six MRPP sites have been defined in PACA :

• two sites, called "Gardanne est" et "Gardanne ouest", representing the two main coal exploitations in Provence (Bouches-du-Rhône) ; • sulfur "Camoins" site (Bouches-du-Rhône) ;

• the lead-zinc-fluorite exploitation of "Cogolin" (Var) ;

• lignit site of "Bois d'Asson" (Alpes-de-Haute-Provence) ;

• coal site of "la Tour" (Hautes-Alpes).

There are one hundred and fifty-three mining sites in PACA (without the previous six MRPP sites) :

• eighty-eight mining sites have been "eliminated" (being either far away from any surface structure or without any risk of surface instability) ; • the others sixty-five mining sites :

• are essentially flat exploitations (approximately 60 % of them) ;

• approximately 80 % of mining sites present a minimum depth lower than 50 m ;

• maximal values of deformation criteria and land occupation nature (one of the most important criteria) have never been encountered ; • land occupation nature is essentially insulated constructions (40 %), the sectors with potential constructions (20 %) and suburban sectors (15 %).

3.1.2 Hierarchisation results analysis 3.1.2.1 Land occupation hierarchisation

The mining sites with the most important values on the most significant criteria (land occupation nature and stakes area) are well localised in the most risky rows : row 1 to 13, on twenty-two levels in the whole.

The position of the six last mining sites (the less risky sites) is the consequence either of weak values on land occupation criteria (null value) or of the weak areas of stakes.

To precise this group of criteria, the experts have decided to study the sensibility stakes area criterion because it seems to have a direct effect on the classification ( § 3.2).

Mining risk hierarchisation

Mining sites with the highest values of minimum depth (one of the most efficient criteria for local cave-in) are well localised in the less risky levels : rows 22 to 29, on twenty-nine levels in the whole. This criterion allows to insulate mining exploitations where the workings are not near surface so less critical for local cave-in. On the other hand, there are too many sites with the highest value on these criteria and thus it can not discriminated the most risky sites in the first rows. The criterion of opening seems to be more effective in this regard. Thus, the experts have decided to replace the criteria "opening "and "minimum depth" by a single criterion : H min /opening, more effective in term of classification linked with local cave-in hazard.

Taking into account these elements, mining risk criteria seem to be correctly defined. However, the sensibility analysis of the maximum depth was decided to be performed, this criterion was not really "limpid" ( § 3.2).

Comparison with DRIRE and experts selection

DRIRE has carried out his own selection of the most critical sites in PACA.

The first eleven rows proposed by the ELECTRE's software (on twenty-two rows total for the local cave-in classification), reproduce the whole sites preselected by the DRIRE. These sites are in gray in the following table (Table 7).

In these first eleven rows, the mining sites which are not preselected are :

• the mining sites over-estimated by the most efficient criteria ;

• mining sites not considered by the DRIRE in its preselection (because of the complexity of this work).

The same kind of analysis has been drawn for the gradual subsidence.

These results show that the classification based on the hierarchisation tool, corresponds to the selection of the most critical sites agreed by the DRIRE and the experts

… … … … … … … … … … … …

Sensitivity and robustness analysis

Area of the stakes

This criterion seemed to have a significant effect on the risk classification. To improve this hypothesis, three new hierarchisations have been decided with three new weights for the area of stakes : 13, 17, 21 and 25 (weight defined initially). The results have been presented in a graph where each mining site (X-axis) is correlated with its four rows obtained in four hierarchisations (in Y-axis) :

• the rows of each mining site are not really varying by different weights, the most important variation is approximately two rows in twenty-two rows in total. These variations follow the expected sense : the risk decreases when the criterion "weight" decreases (sites 06_029, 13_106 and 83_054 with the most important areas of stakes). • regarding the whole mining panel, the classifications, from the most risk to the less risky sites, seem to be similar using different weights. Only the site 13_013 goes up significantly in the classification towards the most risky sites when the criterion weight decreases (row 4 for weight 25 and row 2 for weight 13). This situation is natural because this site presents critical values for the significant criteria (maximal value opening of the sample) and an average stakes area.

Maximum depth

Four new hierarchisations have been carried out, with four new weights for maximum depth : 6, 8, 11 (first weight), 13 and 15. The classifications show that these different weights do not modify significantly the results. Moreover, maximum depth seems to distinguish the mining sites having an important workings density at low depth. Indeed, the mining sites presenting the lowest maximum depths (less than 50 m), go up, towards the most risky sites, when the weight of the criterion increases.

Robustness analysis

To improve robustness, it was agreed to integrate MRPP sites in the mining sites of PACA and then analyse their rows. Table 8 presents these results :

• the integration of new mining sites in the sample does not modify the most critical selection sites (lowest rows). Moreover, the precise order is slightly modified but the ten most risky sites before the integration always remains among the most risky after the integration of the MRPP sites ; • MRPP sites are placed in the top of the classification, that is to say among the most risky sites, corroborating the DRIRE choice and opinions (among the 10 first rows for local cave-in and among the eight first rows for gradual subsidence). The only MRPP site appearing further in the classification is "Bois d'Asson". This classification finds its logical explanation because of the low value of land occupation (insulated constructions) which is one of the lowest values observed in PACA. The characteristics of these six sites did classify these sites among the most risky, at least for five of them. Indeed, "Camoins", "Cogolin", "Gardanne Est" et "Gardanne Ouest" and "la Tour", most probably, would have been selected for a quick study in the Screening. "Bois d'Asson", going down in the classification, would not have been selected. The person who has made this MRPP confirms this results, "Bois d'Asson", would not have been studied by MRPP, because of its low risk level, compared to other mining sites in PACA.

Site number

Rows with MRPP sites "13_256" 1 "Gardanne Est" 1 "84_039" 1 "13_256" 2 "13_232" 2 "La Tour" 2 "13_236" 3 "13_232" 3 "13_016" 3 "84_039" 3 "83_065" 3 "13_013" 4 "13_013" 4 "83_065" 4 "06_026" 4 "Camoins" 4 "83_040" 5 "13_016" 5 "83_076" 5 "Cogolin" 5 "06_029" 6 "83_040" 6 "83_041" 6 "13_236" 7 "06_025" 7 "06_026" 8 "83_054" 7 "83_076" 9 "13_024" 8 "06_029" 10 "83_045" 8

Conclusions

In France, mining data are usually centralised at a regional level under the DRIRE responsibility.

To optimise the results reliability and the work effectiveness, the Screening process has been performed, in France, region by region. This choice also allows to give to the Ministry the expected results in due time. When the regional studies will be finished, one final hierarchisation at a national level will be planned.

At the end of 2007, approximately one thousand and hundred mining sites were classified in seventeen different regions. The most important regions were Rhones-Alps with two hundred and one hierarchised sites, Auvergne with one hundred and forty-six hierarchised sites and Languedoc-Roussillon with one hundred and forty-three hierarchised sites.

The hierarchisation method and tools fulfilled the expected objectives. They provided an effective help to the selection of the critical mining sites. The rough results of these hierarchisations have never been used directly ; they required a fine and accurate analysis in order to, in particular, insulate, in the selection suggested by the tool, the sites which had been considered from a safety point of view.

The developed hierarchisation method (and tool) can be described as "adapted" to the issues linked with the Screening. The next stage is to apply this method to the whole national mining sites ; a difficult operation that raises the question of the method robustness regarding the sample extension (more than thousand mining sites).

Figure 1 .

 1 Figure 1. Synthetic representation of the Screening hierarchisation

Figure 2 .

 2 Figure 2. Sensitivity of the stakes area

Table 1 .

 1 Mining hazard criteria (gradual subsidence and local cave-in)

	Criteria C i	Description	Modalities or values
	Ore mechanical	5 MPa (manganese, coal, sulphur…)	TEND
	strength	20 MPa (bauxite, oil shales, fluorite,	MOYR
		anthracite, salt…)
		100 MPa (lead-zinc-silver, copper…)	RESI
	Fields typology	stratiform (deposit layer)	STRAT
		cluster	AMAS
		lenses (maximal extension 50 m)	LENT
		seams	FILO
		disseminated or diffuse	DISS
	Mining method used	rooms and pillars and void rooms	VIDE
		insulated galleries	GISO
		longwall or backfilling	TRF
		open pit works	MCO
	Void percentage	value in en % growing with the risk level	quantitative
	Maximum depth of workings	maximum depth ∈ [0-50 m] maximum depth ∈ ]200-500 m] maximum depth ∈ ]100-200 m] maximum depth ∈ ]50-100 m]	MDa MDc MDd MDb
		maximum depth > 500 m	MDe
	Minimum depth of workings (H min )	minimum depth ∈ [0-50 m] minimum depth ∈ ]100-200 m] minimum depth ∈ ]50-100 m]	mDa mDc mDb
		minimum depth > 200 m	mDd
	Opening of workings	opening < 1 m	Oa
		opening ∈ ]1 à 3 m] opening ∈ ]3 à 10 m]	Ob Oc
		opening > 10 m	Od
	Deformation :
	opening/H min
	(gradual subsidence)

Table 2 .

 2 Deformation criteria values (gradual subsidence)

					Opening
				Oa : < 1 m	Ob : 1 to 3 m Oc : 3 to 10 m Od : > 10 m
	Minimum	depth.	mDa : 0-50 m mDb : 50-100 m mDc : 100-200 m mDd : > 200 m	12 4 2 1	48 16 8 4	144 48 24 12	432 144 72 36

Table 3. Minimum depth (H min )/opening values (local cave-in)

Table 4 .

 4 Land occupation criteria

	Criteria C i	Description	Modalities
			or values
	Nature of	limits drawings based on precise workings plans	PTS
	documents that	limits drawings based on shafts or galleries	OMJ
	helped to draw	limits uncertain	ENVT
	mining sites	only the title limits available (concessions, exploration agreements)	TCU
	limits	no geographical localisation	PTC
	Mining site area	area ∈ ]50 -200 ha] area ∈ ]10 -50 ha] area ∈ [0 -10 ha]	B C A
		area > 200 ha	D
	Mining site	old and local small-scale mining sites, exploration workings,	MINI
	production	small scrapping workings… all mining sites with small
		productions
		all others mining sites	AUTR
	Nature of the	exceptional constructions : schools, hospitals, monuments,	EXCEP
	land occupation	mains plants, etc
		urban centres and villages	HU
		suburban sectors	ZP
		sectors with by potential constructions	ZEU
		insulated constructions	HI
		motorways, national roads et railways	A
		department roads	RD
		no human land occupation	ABS
	Stakes area	quantitative criteria taking values sorted by growing risk levels quantitative
	2.2.3 Definition of modalities and weights values for the criteria

Table 5 .

 5 Modalities values

	Criteria C i	Modality Values
	Ore mechanical	TEND	100	Criteria C i	Modality Values
	strength	MOYR	50	Maximum depth of	MDa	100
		RESI	0	workings	MDb	30
	Fields typology	STRAT	100		MDc	10
		DISS	100		MDd	0
		AMAS	60		MDe	0
		LENT	30	Minimum depth of	mDa	100
		FILO	0	workings (H min )	mDb	30
	Mining method used	VIDE	100		mDc	0
		GISO	30		mDd	Exclusive
		TRF	0

Table 6 .

 6 Different weights of criteria in the different scenarii

					Criteria C i	Weights	Weights	Weights
						scenario 1	scenario 2	scenario 3
					Kind of documents that helped to	4	8	4
	Land	occupation	draw mining sites limits Mining site area Mining site production Nature of the land occupation	13 13 46	25 25 92	13 13 46
					Stakes area	25	50	25
	Mining hazards linked	with local cave-in	Ore mechanical strength Fields typology Mining method used Maximum depth of workings H min /opening Exploitation dip angle Geological overburden nature Presence of shafts or galleries Existence of disorders	2 1 12 12 35 2 15 7 12	2 1 12 12 35 2 15 7 12	5 2 25 25 69 5 30 15 25
	Mining hazards	linked with gradual	subsidence	Ore mechanical strength Fields typology Mining method used Exploitation dip angle Geological overburden nature Presence of shafts or galleries Existence of disorders Deformation : opening/H min	12 3 18 15 9 3 6 35	12 3 18 15 9 3 6 35	24 6 35 29 18 6 12 71

Table 7 .

 7 Extract from the hierarchisation given by ELECTRE software (linked with local cave-in PACA). The mining sites that are in gray are those selected by DRIRE before the implementation of the hierarchisation method

	scenario 1		scenario 2		scenario 3
	Site number Row Site number Row Site number Row
	"13_256"	1	"13_256"	1	"13_232"	1
	"84_039"	1	"84_039"	2	"84_039"	2
	"13_232"	2	"83_054"	2	"13_013"	3
	"13_236"	3	"13_232"	3	"13_236"	3
	"13_016"	3	"13_013"	3	"13_256"	4
	"83_065"	3	"83_065"	3	"06_026"	5
	"13_013"	4	"06_026"	4	"83_040"	5
	"06_026"	4	"83_076"	4	"83_065"	5
	"83_040"	5	"13_236"	5	"13_016"	6
	"83_076"	5	"13_016"	5	"83_076"	7
	"06_029"	6	"83_040"	5	"06_025"	8
	"83_041"	6	"13_024"	6	"13_229"	8
	"06_025"	7	"06_025"	6	"83_041"	8
	"83_054"	7	"06_029"	6	"06_028"	9
	"13_024"	8	"83_041"	6	"13_024"	9
	"83_045"	8	"83_045"	6	"83_012"	9
	"06_028"	9	"13_022"	7	"83_050"	9
	"13_022"	10	"06_028"	7	"06_029"	10
	"13_229"	10	"83_012"	7	"83_045"	10
	"83_012"	10	"83_032"	8	"13_014"	11
	"13_014"	11	"13_229"	9	"13_022"	11
	"83_049"	11	"83_038"	9	"83_003"	11
	"83_050"	11	"13_014"	10	"05_014"	12

Table 8 .

 8 Extract from hierarchisation results (before and after MRPP site integration)

	Hierarchisation scenario 1 (local cave-in)
	Site number	Rows without	MRPP sites

Acknowledgements

The author warmly thanks GEODERIS, the organization in charge of technical expertise in postmining risk assessment, for its contribution and management of the Screening in France. The author also thanks BRGM for its collaboration.


Billaud Pierre

Lansiart Marc
email: m.lansiart@brgm.fr

Artignan Dominique
email: d.artignan@brgm.fr

Baranger Philippe
email: philipe.baranger@geoderis.fr

Mazenc Bernard
email: bernard.mazenc@geoderis.fr

Kimmel Marion
email: marion.kimmel@geoderis.fr

Poirot Nicolas
email: nicolas.poirot@ineris.fr

Quiot Fabrice
email: fabrice.quiot@ineris.fr

Balian Armelle
email: armelle.balian@industrie.gouv.fr

ENVIRONMENTAL HAZARD ASSESSMENT OF FORMER MINES: MAIN METHODOLOGICAL POINTS

Keywords: Post-mining, prevention, hazard, environment, methodology Après-mine, prévention, aléas, environnement, méthodologie

The environmental hazard assessment of former mine sites concerns the surface water, groundwater, soils and sediments and allows the listing and location of the potential areas exposed to one or several contaminations with possible harmful consequences on human and the environment. This assessment needs to evaluate the intensity of the potential pollution and the site predisposition or sensitivity to be affected by this (these) pollution (s). The areas characterized by each environmental hazard classes will be mapped for a better understanding. If necessary the results will be used for the PPRM drawing up.

.

Introduction

The discontinuation of mining activities does not necessarily mean an end to the risks and pollution likely to affect the environment and ecosystems located within the area of influence of the former mines. Many disturbances can arise during the period subsequent to mining, traditionally termed 'post-mining'; sometimes immediately following the end of mining, and also sometimes much later.

These disturbances can take various forms such as ground movement (subsidence, collapse), gas emission (CH4, CO, CO2, H2S, radon), quantitative and qualitative changes in the surface water and groundwater circulation systems (flooding, water pollution), soil and sediment contamination.

For the best management of these risks, the government has available an operational regulatory tool known as the Mining Risk Prevention Plan (MRPP) which enables one to a) identify the areas that are most likely to harbour risks or pollution in the long term, and b) establish regional planning regulations adapted to the various post-mining constraints.

The design and contents of an MRPP in general, which were discussed at the last International Post-Mining Symposium (2005), classically break down into four phases:

the informative phase: collecting all available information, with additional investigations where necessary; the hazard assessment phase: locating and listing areas exposed to potential contamination without taking land use (i.e. type of surface occupation) into account; the factor assessment phase: locating areas subject to one or more risks relative to humans and ecosystems; the regulation zoning phase: defining homogeneous zones in terms of land-use prohibitions, instructions and recommendations concerning both existing and planned projects. To help with the two first MRPP phases, informative and hazard assessment, methodology guides have been prepared concerning the ground-movement, flood and mine-gas emission hazards. They are aimed at all the actors involved in drawing up a MRPP (administrative services, local authorities, consulting firms, etc.).

The methodological guide assessing environmental hazard concerns water, soil and sediment contamination. It is currently being finalized by a working group made up of experts from BRGM, GEODERIS and INERIS assisted by representatives from MEDAD/ DARQSI.

The environmental hazard methodology guide drawn up by the working group provides information and advice on the procedure(s) to follow when carrying out the environmental hazard assessment phase regarding the potential contamination of soils, sediments and surface water and groundwater caused by abandoned mines. It starts by outlining a few major principles, followed by the study of the potential hazards, and their consequences, due to the ore type, the mining and the ore-processing methods. The guide then presents the procedure(s) to follow for qualifying the intensity and predisposition levels when determining and assessing the environmental hazards.

Environmental hazard assessment: definition and some main principles

Definition

Environmental hazard assessment includes phases of identification and qualification followed by the construction of maps localizing and ranking areas potentially exposed to anticipated pollution phenomena.

Hazard assessment requires defining a) the intensity of the phenomenon, i.e. the extent or grade of the pollution likely to result from the anticipated phenomenon, and b) the predisposition that, for its part, reflects the sensitivity of a zone to be affected by one or other of the anticipated phenomena. By crossing intensity and predisposition, the pollution hazard is classified into different levels, generally three (low, medium, high) and plotted for each of the defined phenomena, as illustrated by Table 1 for water, sediments and soil.

Qualitative approach

The principle of hazard assessment is usually based on qualitative studies using available data archived in the offices of central, regional and local authorities, and also from associations, etc. The objective is, as far as possible, to make an inexpensive and rapid assessment in view of the fact that new measurements, surveys, laboratory studies and computer modelling take time and money. The hazard assessment should follow a naturalistic approach.

Nevertheless, depending on the quality and quantity of the available data, some fieldwork with measurements and analyses is generally required in order to reduce the margin of uncertainty. This acquisition of additional data is planned according to the complexity of the studied pollution, the existing or anticipated risks and the available time and funding.

Relationship with stakeholders

The environmental hazard assessment, like all hazard assessments, requires the preparation of simple and operational documents intended to be read and understood by the concerned population and the administrative services in charge of its implementation. Consequently, the work has to be completed in consultation and dialogue with all the stakeholders involved in the management of mining risks.

Potentially dangerous phenomena

Origin

Within the framework of environmental hazard assessments of former mining sites, the phenomena that are potentially dangerous to humans and ecosystems derive from natural factors arising from the geological, gitological, mineralogical, chemical context of the deposit and also from anthropogenic factors related to the mining and processing methods.

Potential pollution sources from mining and processing operations are mine dewatering, mine runoff, new springs containing water from mining aquifer, waste stockpiles, tailings dumps, lowgrade ore stockpiles.

One of the fundamental causes of post-mining pollution and environmental nuisance is the interaction between the mine workings and hydraulic flows leading to soil, surface-water and groundwater contamination.

During the mining phase, dewatering is necessary to stop the mine workings from being drowned.

The impacts here are either hydrodynamic with a lowering of the water table and modification (increase or reduction) of the river flows, or hydrochemical with modification of the water quality although the water is generally (or should be) treated during the exploitation before being discharged into the natural environment. Certain hydrodynamic impacts may remain after mining has ceased, but these are mainly of a hydrochemical nature related to the artificial appearance of surface conditions (air, meteoric water) that develop. Superimposed on this pollution one may find phenomena generated by the high turbidity of the water during mining and likely to result in a high contamination of the sediments once the mine is closed.

When assessing the pollution caused by mining, a major problem is related to the existence of what is called the geochemical background which is generally affected by pollution of anthropic origin.

To assess the slight difference between contamination of natural origin and pollution deriving from the mine workings, it may be possible to have access to various measurement data obtained during the prospecting and exploration stages.

Physical and chemical pollution processes

The main mining-derived pollution processes affecting surface water and groundwater, soils and sediments have been determined the main sources and identified pathways that are:

mine workings (open pits and underground workings;

mine waste storage;

storage of hydrocarbon and chemical products.

For example Table 2 summed up these pollution processes for the mine waste storage. The table indicates the physical and chemical processes (leaching, suspended matter transport, etc.) that are involved between the source (mine workings, waste storage, etc.) and each pathway (surface-or groundwater, air transport, human activity factors, etc.). The overall impacts on the four exposed environmental milieus (surface water, groundwater, sediments and soils) can be deduced by taking into account a primary impact arising from direct interaction between the source and the pathway and a secondary impact following the first one.

The arrows in the table indicate the pollution dynamics.

Ore type and pollution

There is a direct relation between the ore type and the potential pollution. For the French territory a synthesis has been done and gives region by region the nature of the expected pollutions according to the ore type [START_REF] Artignan | Méthodologie pour la réalisation de la phase informative du volet rsique-pollution d'un PPRM[END_REF]). An extract is given in table 3.

Typology of mining-related pollution

Two main types of mining-related pollution can occur during the post-mining period:

diffuse pollution that affects large areas; this is generally due to the spread of liquid or solid materials or due to dust fallout from atmospheric transport; point-source pollution affecting small areas and arising from two main sources: o accidental and point-source pollution associated with the short-term dumping of polluted mining products or materials. It generates severe damage over a limited area and can have a serious, even a tremendous, impact on the different environmental milieus if not treated rapidly and efficiently -an example is the Danube pollution from the Baia Mare gold mine in Romania; o chronic and point-source pollution corresponding to a supply of polluted products or materials over a long period; this is frequently caused by polluted effluents from waste storage, tailings dumps or old processing plants, and also by mine water. The impact is mainly on surface water and groundwater. -Pollution derived from many point sources corresponding to complex phenomena. These notions are important because they show that the contaminated area can extend beyond the mining concession or commune boundary and so raise difficulties in defining the study area (perimeter).

Phenomenon description

To describe the potentially dangerous phenomena it is necessary to take into account the different mining (open pit, underground workings) and ore-processing methods, including the different types of waste storage and tailings dumps with their technical specifications and the potential pollution they can generate in the short, medium and long term. These are summarized in Table 4.

Mine workings and facilities

Groundwater Surface water Sediment Soil Dust Gas

Underground workings

Underground exploitation

Saline exploitation by dissolution cavity

Shaft, Adit

Surface workings

Outcrops with small works

Open pit

Mine facilities

Slag and steril heaps

Tailings dumps

Area of physical processing crushing, screening..)

Are of chimical processing (lixiviation, cyanidation,…)

Storage of chemical products and hydrocarbons

Potential pollution

Impact on Contamination of Generation of

Contamination scenarios

Surface water and groundwater contamination

The main contamination scenarios to be considered where groundwater is concerned are: leaching in underground workings, leaching of waste stockpiles, hydrocarbon and chemical product discharges, percolation of surface water towards the groundwater.

The main contamination scenarios to be considered where surface water is concerned include: mine runoff, hydrocarbon and chemical-product discharges, surface-water runoff on contaminated soils, tailings, waste, etc., effluents from tailings dumps, runoff of a contaminated groundwater.

Soil and sediment contamination

Several contamination scenarios must be considered where soils and sediments are concerned are: contamination from pollutants transported by surface-water and groundwater flows from mining waste and tailing dumps, voluntary (or otherwise) displacement of contaminated mining waste or tailings for use as banking material, deposit of contaminated dust, leakage of chemical products or hydrocarbons, contamination through contact with contaminated surface water.

Hazard assessment

Surface and groundwater pollution hazard

Intensity

Water management on the scale of Europe and France is controlled by a series of directives, laws, acts, decrees, and circulars that have to be taken into account when assessing intensities and hazards. Among other things they define the concept of 'good water status' and provisional Environmental Quality Standards (EQSp), and give the list of substances involved (dangerous, priority, to monitor) in assessing the state of the aquatic environment with values not to be exceeded in order to respect the 'good water status'. Knowledge of the "background level" and the EQSp is essential for determining the maximum permissible concentration for the environmental milieu. The EQSp are given in Circular 2007/23 or can be calculated from the background level. The background level is either measured directly upstream of the concerned site or estimated on the basis of best available information concerning the concentration of the monitored element in the same type of natural environment but with little anthropic pressure. Determining the different levels of concentration can be tied into: -whether or not the water complies with the "good status" as defined in Circular DCE/2007/23 dated 7 may 2007; -whether or not specific requirements are respected; -whether or not there is a deterioration of the reference values characterized by an exceeding of the background levels; -the influence of the measured values on rendering the raw water potable (Decree of 11/01/07). For example, the intensity scale can be constructed on the basis of criteria such as those defined in Table 5. This approach is likely to be adjusted by the person in charge of the study so as to take account of the local situation where certain criteria may not be relevant.

A zero intensity is defined by values equal or lower the background level. The main factors that can be used for the classification of the predisposition are summarized in the following table 6 for the surface water.

According to the pollutant and the previous conditions

According to the flow rate ratio between the source and the receptor

Surface water predisposition

Predisposition factors

Predisposition classes

According to the pollutant

The same kind of table is used for the groundwater

Soil pollution hazard

Intensity

In the case of soils, the concept of intensity is closely related to the concentration of one or more chemical elements (Pb, Cu, As, Hg, Cd, etc.) present on the site and having an impact on the environment and man. The intensity classes can be defined by taking into account, for each chemical element concerned:

-its exceedance of the natural geochemical background;

-certain physico-chemical parameters influencing the mobility of the concerned pollutants, such as reactivity, chemical mobility and physical dispersion;

-ranges of 'ordinary' values and of natural anomalies in the soils provided by INRA;

-the Interpretation of the Environmental Status (IES) developed within the framework of the new contaminated land management policy.

In any event, an expert opinion is necessary at this stage because each selected tool and parameter must be used critically and with care in order not to over-or underestimate the gradation or the intensity level. It is possible to establish intensity classes according to the principle of Table 7.

A zero intensity will be characterized by values that do not exceed the natural geochemical background, excluding the values coming from the source of identified pollutions.

Intensity classes Alteration criteria

Limited

Values exceeding the geochemical background and close to the high natural anomalies but without harmfull effect on human and the environment

Moderate

Values exceeding the high natural anomalies without harmfull effect on the human and the environment

High

Values exceeding the high natural anomalies with potential harmfull effect on the human and the environment

Weighting the intensity

The intensity is related not only to the concentration value of the selected chemical elements, but also to the granulometry and mineralogy of the concerned materials, the chemical soil conditions (pH …) and to the bioavailability/bioaccessibility of the chemical element concerned. As these factors can be regarded as intensity multipliers or dividers, it is proposed to weight the intensity level by applying a purely qualitative step based on expert opinion 4.2.3 Qualification of the predisposition for soil

The principal predisposing factors are:

-the physical accessibility of the materials;

-the permeability;

-the abundance of adsorbent phases;

-the chemical conditions of the soil;

-experience feedback (existence of historical or observed pollution in similar contexts);

-the existence or influence of a spring with mobilizable pollutants

Hazard mapping

One of specificities of the 'water and soil pollution' hazard is that it is able to affect areas that, sometimes widely, extend beyond the area of the mine workings and even of the concession. In fact, the contamination derived from mine workings, insofar as it impacts surface water and groundwater, and also sediments, a priori concerns the entire catchment basin and its structure.

As regards soil pollution, the contamination can also extend beyond the mine site, but over a more limited area, even when one takes into consideration the dispersion of contaminated dust.

Practically, according to the observed and assessed situation, one can have recourse to several types of graphic document. The choice is left to the specialist responsible for drawing up the MRPP documents. He can thus compile a hazard map for each type of phenomenon, which is justified by:

-the great differences between the contaminations in terms of extension, which can justify maps at different scales.

-the level of homogeneity and coherence which needs to be respected in order to provide a comprehensible map.

In many situations the difficulty is in finding a limit to the extension of the pollution phenomena which sometimes extend beyond the boundary of the mine sites, communes or MRPP perimeters. Links have to be established with the "flood" risk for the extension of floods and their relationship with pollutant dispersion. Similarly, in the event of tailings dam rupture, it is necessary to assess the dispersion of the mud.

Table 1 -

 1 Hazard classes for water, sediment and soil pollution

	Intensity	Predisposition	Not vevy sensitive	Sensitive	Very senstive
		Limited
		Moderate
		High

Low hazard High hazard Medium hazard

Table 2 -

 2 Pollution processes and pathways arising from mine waste storage

		Underground flows	Surface flows	Air transport	Anthropogenic activities
		Waste and tailings leaching	Waste and tailings leaching and gullying	Air transport and deposit of dust	Waste and tailings spreading
	Groundwaters	Pollution by contact between waste-tailings and mine runoff	Pollution by surface water percolation	Pollution by surface water percolation	Pollution by waste-tailings unloading and contact soil/water
	Surface waters	Pollution by mine runoff	Pollution by contact waste-tailings and surface water	Pollution by contact soils / waters	Pollution by waste-tailings unloading and contact soil/water
	Sediments	Pollution by contact between waste-tailings and mine runoff	Pollution by contact waste-tailings and water and suspende matter transport	Pollution by dust fall	Pollution by waste-tailings unloading and contact sediment/water
	Soils	Pollution by contact between waste-tailings and mine runoff	Pollution by contact waste-tailings and water and suspende matter transport	Pollution by dust fall	Pollution by waste-tailings unloading as embankment

Primary impact Secondary impact Pollution processes and pathways arising from mine waste storage Pathways Impact on

Table 3 -

 3 Example of relationship between ore type and potential pollution for France's ore deposits(Extract)

	Region	Main extractive activity	Type of the potential pollution
	Alsace		-K, Na, Br n the water,
		Salt, potash, Pb-Ag-Cu sulphides	-pH, metals
			-hydrocarbon in the water and soils
	Auvergne	Au, U, CaF2, BaSo4 sulphides, W-Li-Sn oxides, REE, Coal, As-Sb sulphides, Pb-Zn,Ag	-U in the water and soils -Turbidity, SO4 , As -pH, metals -Fluorine in the water
	Midi-Pyrénées		-pH, metals
		Gas, Coal, U, Au, W oxydes, Zn-	-hydrocarbon in the water and soils
		Pb-Ge-Cd sulphides, CaF2,	-Turbidity, SO4 , As
		BaSO4, FE Mn.	-Fluorine in the water
			-U in the water and soils

Table 4 -

 4 Summary of potentially dangerous phenomena likely to arise in the mining environment.

Table 5 -

 5 Proposed intensity classes for water for one given chemical element

	Raw water	cannot be	rendered	potable			High intensity
								Moderate
	Intensity		Raw water can be	rendered potable if not	Zero intensity	Limited intensity	intensity	Geochemical background Background level EQSp or good status
	EQSp = Provisional environmental quality standard
	4.1.2 Qualification of the predisposition for potential water pollution

Table 6 -

 6 Surface water predisposition factors and classes for surface water

	not very sensitive	sensitive	Very sensitive

Uncertainties on the data, on the phenomenon Phenomenon Generalised X Intermittent X Limited X Hydrology system (Drainage size, flow rate, pluviometry…)

Table 7 -

 7 Soil pollution: intensity classes


Souley Mountaka
email: mountaka.souley@ineris.fr

Mercerat Diego
email: mercerat@ipgp.jussieu.fr

Driad-Lebeau Lynda
email: lynda.driad-lebeau@ineris.fr

Bernard Pascal
email: bernard@ipgp.jussieu.fr

A LARGE SCALE CONTINUUM-DISCRETE NUMERICAL MODELLING: APPLICATION TO OVERBURDEN DAMAGE OF A SALT CAVERN

Keywords: cavern, numerical modelling, continuum-discrete, overburden, damage cavité saline, modélisation numérique, continu-discret, endommagement

With the objective to better understand the evolution of overburden damage on top of an underground solution mining, an in-situ experiment is undertaken above a salt cavity in the Lorraine region (NE of France). The overburden overlying the salt cavity is characterized by a competent layer where most brittle damage, with the associated microseismicity, is expected. This work presents a coupled continuum-discrete modelling approach to simulate the mechanics of fracture initiation and propagation in the rock mass, a continuum approach for the marls and salt layers, and a discrete micromechanical approach for the competent layer. A methodology of a large scale coupled continuum-discrete modelling is proposed and applied to the site of Cerville-Buissoncourt. The numerical verification of the hybrid approach response is achieved by comparing the results with those from the fully continuum model at the site scale when the competent layers behave elastically. The first results of a large scale modelling based on the hybrid approach suggest that the fracture mechanisms in the competent layers are predominantly tensile. Finally, these explicit microcracking simulations open interesting perspectives for comparison with the observed microseismicity of the study area (transient pressure experiment or future final collapse).

Introduction

A large research project has been launched by GISOS1 in order to identify and evaluate the capacity of geophysical (microseismic, hydroacoustic) and geotechnical (extensometer, inclinometer) techniques for monitoring underground growing cavities due to salt dissolution (solution mining). A part of this research program focuses on two complementary axes: (1) the validation of induced seismicity monitoring techniques in salt mine environments, and (2) the numerical modelling of deformation and failure mechanisms with their associated acoustic emissions, as well as the induced microseismicity. Previous studies on induced seismicity of underground growing cavities [START_REF] Johnston | A survey of mining associated seismicity[END_REF]Einstein 1990, Mendecki 1997) confirm the presence of microseismic events associated with the progressive damage of the overburden until its final collapse. Continuous monitoring of such microseismicity may provide crucial information for stability analysis of the area surrounding the cavity. In order to validate such seismic monitoring techniques, a local microseismic network has been installed over a site test at Cerville-Buissoncourt (Lorraine, France), where a stable brine filled cavity is nowadays located [START_REF] Mercerat | Induced seismicity monitoring of an underground salt cavity under a transient pressure experiment[END_REF]. When the salt exploitation restarts, the spatial and temporal evolution of seismic event locations and magnitudes can be analysed and compared with the results of geomechanical modelling. Numerical modelling of such complex process requires a large scale model which takes into account both the cavity evolution within the salt layer and the mechanical behaviour of the overburden layers where high deformation and fracturing are expected. To achieve this scientific challenge (experimental and numerical through geophysics and geomechanics fields), a coupled continuum and discrete modelling strategy is adopted. This paper focuses on the feasibility to achieve the previous scientific objective and presents a methodology of numerical modelling the induced microseismicity location in the competent layers through the microcrack distribution. From numerical point of view and in order to take into account the elasto-brittle behaviour of the competent layers, where most of the seismic damage is expected, we use the code PFC (Particle Flow Code in 2D) based on the discrete element method. To describe the behaviour of the other layers (mainly composed of marls and salt) which present more ductile and/or viscoplastic behaviour, a continuum approach based on the FLAC code (Fast Lagrangian Analysis of Continua) is used to account for different mechanical behaviours. It is interesting to note that recent work published by [START_REF] Cai | FLAC/PFC coupled numerical simulation of AE in large-scale underground excavations[END_REF] followed a similar strategy to take into account explicit acoustic emission simulations near an underground excavation. A large scale continuum model assuming an elastic perfectly plastic behaviour of the competent layers has been firstly performed in order to establish the suitable boundary conditions with regard to the actual cavity size. Secondly, a fully hybrid continuum-discrete model is developed. Comparison between the results of the two mentioned models allows us to validate the developed continuum-discrete model (microproperties used in the PFC inclusion modelling the competent layer, its lateral extension and velocity-force exchanges at the boundaries between continuum and discrete domains).

Site description

Brine cavity, overburden characteristics and microseismic network

The brine cavity of Cerville-Buissoncourt (Lorraine, France) is actually located within a salt layer at about 180 m depth and presents an irregular shape that, in first approximation, can be considered a cylindrical cavity of 180 m of diameter and 50 m high. A scheme of the instrumented site with a schematic geological section of the area is shown in figure 1. Typically, the overburden is characterized by a series of intercalated anhydritic marls lying over the salt layer, and on top of them, the presence of a competent dolomite layer of 8.5 m thickness which presents elasto-brittle mechanical behaviour, and it is located approximately 60 m above the salt layer. This layer corresponds to what is known as the 'Beaumont dolomite', and it represents the level where most of the seismic damage is expected, when the salt exploitation restarts and the cavity will migrate towards the surface.

A microseismic network has been installed over the stable underground brine filled cavity of Cerville-Buissoncourt. The network includes four 3D components and three 1D component seismometers located at different depths (between 35 m and 125 m) in cemented boreholes distributed around the study area (figure 1). The frequency band of the geophones is between 40 Hz and 1 kHz. There is one station per borehole, except for the deepest one at the centre of the array right above the cavity (M6), where one 1D station at the surface and two 3D stations are located at 60 m and 125 m depth (Beaumont dolomite level). The lateral extension of the array is about 400 m by 600 m, and it presents reasonable azimuthal coverage (major gap of ~130° at the centre of the array).

More than 1000 microseismic events were recorded between January 2005 and September 2007. As an event, we denote in this work, a recorded file by the acquisition system used in trigger mode with recording time window of 0.8 seconds. In fact, more than 75 % of the events are grouped in clusters of time duration between 3 to 30 seconds (recorded by almost continuous triggering of the acquisition system). This activity is clearly not related to classical shear slip mechanisms induced by hard rock fracturing.

Transient pressure experiment

Currently, the cavity is full of saturated brine inducing a significant pressure on its walls (~2.5 MPa) to keep the overburden mechanically stable. In October 2005, a transient pressure experiment has been carried out in the cavity, in order to analyse the mechanical response of the overburden and to check the sensitivity of the whole instrumentation. The experiment consisted of a pressure drop of around 0.4 MPa (30 m of brine column height), followed by a stable phase of one week, and a latest phase of pressure raise up to the original level of 2.5 MPa (figure 3). During the pressure raise, two of the clustered groups of events were recorded by all the stations (16 and 18 seconds of duration, respectively). While the pressure drop did not produce any significant seismic activity, except for a small cluster of 3 seconds of duration. This observation suggests that the increase in pressure within some fractured layers on the top of the cavity could produce the observed induced seismicity.

Laboratory characterization

A large number of laboratory tests have been carried out on SOLVAY S.A samples to characterize the geological formations of interest for this study. Some of them are available (by courtesy) for this study. In particular, the Beaumont dolomite and the anhydrite facies present a rather high Young modulus (E≈80 GPa) and uniaxial compressive strength (σ c ≈200 MPa) for typical sedimentary rocks. The tensile strengths of both facies measured by Brazilian tests are also high. On the other hand, the mean values found for samples extracted from the intercalated marls located between the salt layer and the Beaumont dolomite are much lower, with extremely low tensile strengths measured in some samples.

Methodology of large scale numerical modelling of overburden damage

Motivations

The acoustic emissions (AE) and microseismic events are indicators of rock fracturing or damage as the rock is brought to failure under high stress levels. By recording these events, underground excavation induced rock mass degradation or damage can be located and evaluated. A better understanding of the extent and shape of the excavation damaged zone (EDZ) or yield zone around caverns helps to arrive at safe and economic design and construction of the caverns. Consequently, one needs to understand the AE mechanism associated with the excavation process and/or with the microcracks growth and their eventual coalescences.

From the mechanical point of view, the theoretical approach of the physical process of damage can be classified into two main categories : (a) discrete micromechanics approach based on the fracture mechanics (Kemeny and Cook 1987, for example) or the bonded-particle model [START_REF] Potyondy | Simulating stress corrosion with a bonded-particle model for rock[END_REF]); (b) the continuum damage mechanics approach where internal variables (scalar, vector, secondorder tensor and even higher rank tensors) are used to describe the state of damage [START_REF] Zhu | Micromechanical modelling of anisotropic damage in brittle rocks and application[END_REF] provide a synthetic review of this approach). In this case, a damage evolution law is then formulated as a function of stress or strain, in the framework of irreversible thermodynamics and/or some principles of linear fracture mechanics.

Numerical modelling undertaken in this paper is focussed on a fine study of the damage in the overburden of cavity salt works, its spatial and temporal evolution during the exploitation, and the evaluation of the failure mechanisms in the roof of cavern.

It is well-known that numerical models based on the discrete micromechanics or continuum damage approaches are time consumer particularly when performing computation at meso or large scales.

Thus, to reconcile a large scale modelling (with regard to the large dimensions of the cavity) involving numerous different materials properties and mechanical behaviour, and a fine analysis of the phenomena of microcracking being able to locally occur in the overburden, we use a hybrid approach based on a continuum and discrete numerical modelling.

More precisely, two 2-dimensional codes, FLAC a finite difference code and PFC a distinct element code, are coupled. Clearly, the reason to apply the FLAC/PFC coupled approach is to take advantage of each modelling scheme while at the same time minimizing the requirement for computational resources.

Such type of approach starts to develop in the literature. In the recent work of [START_REF] Cai | FLAC/PFC coupled numerical simulation of AE in large-scale underground excavations[END_REF], the rock mass surrounding an AE sensor is modelled using PFC (leading to the authors to study the excavation-induced AE activities) and the remaining rock mass is modelled with FLAC.

Modelling stages and geomechanical model

When the salt exploitation restarts, the cavity will progressively grow leading irreversible damage of the upper layers until its final collapse. As previously mentioned, numerical modelling of such a complex process requires a large scale model which takes into account both the cavity evolution within the salt layer and the more realistic mechanical behaviour (mainly non linear) of the overburden layers where high strains and fracturing are expected.

From the previous mechanical laboratory characterization, the competent layer consisting of dolomite exhibits an elastic behaviour accompanied with a brittle failure. The region of interest in terms of expected damage in the dolomite layer was modelled by an assemblage of PFC particles (figure 3c) in order to investigate the damage initiation and propagation processes. That allows us to represent the damage explicitly by the formation and tracking of microcracks. [START_REF] Hazzard | Micromechanical modelling of cracking and failure in brittle rocks[END_REF][START_REF] Potyondy | A bonded particle model for rock[END_REF]. Compared to work of [START_REF] Cai | FLAC/PFC coupled numerical simulation of AE in large-scale underground excavations[END_REF] where the rock mass surrounding an AE sensor modelled using PFC represents a domain of a few millimetres, herein the PFC inclusion modelling will be 8.5 m height and more than 200 m length.

Compared to the preliminary geomechanical model published in 2005, the PFC inclusion is modelled here with a random packing more than a regular hexagonal packing of particles. This aimed to better represent the microcaraking distribution as discussed in [START_REF] Mercerat | Induced seismicity in a salt mine environment evaluated by a coupled continnum-discrete modelling[END_REF] from the laboratory tests simulations. In fact, we used a random particle packing of particles of decimetric size to create the discrete inclusion of 260 m by 8.5 m. Consequently a new microproperties calibration has been carried out in order to match the mechanical properties (modules, strengths) of the modelled materials at both the laboratory and regional scale.

To describe the behaviour of the other layers (mainly consisted of marls and salt) which present more ductile and/or viscoplastic behaviour, a continuum approach based on the code FLAC, widely used by the scientific and industrial communities, is employed. This leads to approach more complex rheological behaviours based on continuum mechanics (elastoplastic with or without hardening and softening, viscoelastic or viscoplastic). Hence, the brittle layers are modelled in PFC as an inclusion of bonded circular particles embedded in a classical FLAC grid, the applied forces received from PFC act on the grid points along the inclusion boundary. The translational velocities of these PFC boundary particles are received from FLAC (figure 3c). The full coupling between FLAC and PFC boundaries is achieved through a special function written in the internal FISH language.

The geometrical model of the site based on the geological log of the area (figure 3), is essentially composed of :

(a) the salt formation extending from the bottom boundary of the model up to the actual brine cavity roof. The cavity presents a polygonal shape (based on sonar data) that rest fixed throughout the simulations;

(b) on top of the salt layer, a formation of intercalated marls consisted of thin anhydritic to clayley marls with relatively weak mechanical characteristics;

(c) the overburden characterized by the competent layers (Beaumont dolomite and anhydrite level; to simplify, the two competent layers will be assembled into an unique stiff bench), followed by a series of carbonates, sandstones and marls layers up to the surface.

The geomechanical characteristics of the salt, the competent layers and the intercalated marls are known from the laboratory tests, whereas the characteristics of the other materials are chosen according to the literature concerning the regional geological basins [START_REF] Nothnagel | Modélisation des instabilités dans Mécanique des Roches : application à l'exploitation de la concession de Drouville[END_REF].

In a first time and in order to define appropriate model size and boundary conditions with regard to the cavern size, many simulations have been carried out using also a continuum approach for the competent layer. Finally, the selected geometrical model is 4600 m large and 850 m high (figure 3).

Even if there is no symmetry of the cavity, we choose to reduce the 3D problem in 2D for these first calculations. While considering a section in the greatest dimension of the cavity, we place ourselves in conditions of plane strains.

The initial stress state is assumed to be isotropic: this assumption is well justified within the salt layer where null initial deviatoric stress is expected. In absence of data concerning the initial stresses in the other materials, we also admit that they are isotropic. Finally, the vertical stress is equal to the overburden weight. The initial conditions within the PFC inclusion and in the cavity before the exploitation restarts:

(a) within the PFC inclusion, we established a uniform isotropic stress of 3 MPa corresponding to the approximate initial stress found at the inclusion depth (~120 m) due to the overburden weight.

(b) when the cavity is full of brine, a normal stress acting on the cavity walls equal to λ σ 0 is imposed (σ 0 is the overburden weight at the corresponding depth, and λ=0.5). The filling out process is modelled by progressively reducing λ from 1 (no cavity) to 0.5 (brine filled cavity), and later by reducing the wall pressure down to 0 (empty cavity).

PFC microproperties calibration

In PFC, the rock mass is modelled as an assembly of bonded circular particles, that can break apart to simulate explicitly a microcrack generation. The particle contact stiffness, the friction coefficient of the contact and the contact bond model between different particles govern the mechanical behaviour of the material. These are known as the microparameters (or microproperties) of the bonded particle model. They must be chosen in order to reproduce the relevant material properties measured in the laboratory tests of the rock specimen to be modelled. This calibration process is not specified a priori and a trial-and-error procedure is generally used. Nevertheless there is some knowledge and guidance based on previous research on PFC calibration [START_REF] Potyondy | A bonded particle model for rock[END_REF] about the relation between each microparameter and its effect on mechanical properties (see Table 1). Following this guidance and using the previous laboratory test results, we have calibrated our discrete model of the Beaumont dolomite by simulating a Brazilian test and three biaxial compression tests at 0.1, 2 and 5 MPa of confining pressure [START_REF] Mercerat | Induced seismicity in a salt mine environment evaluated by a coupled continnum-discrete modelling[END_REF]. We used a parallel bonded model material because it better approximates the mechanical behaviour of brittle elastic cement joining two bonded particles.

More recently, three-point bending tests performed by [START_REF] Boidin | Interactions roches/saumures en contexte d'abandon d'exploitations souterraines de sel[END_REF] have been simulated in order to better identify the PFC microproperties. In this case, predicted and measured stress intensity factor (K IC ) are compared. Figure 4 shows a simulation of a three-point bending test.

Results of the large scale numerical modelling of overburden damage

Verification of the large scale hybrid FLAC-PFC model response

Before building the large scale model with the hybrid discrete-continuum approach, we firstly carried out a modelling of the draining of the cavity with a completely continuous approach (FLAC).

This preliminary continuum modelling made it possible to optimise the lateral extensions of the geometrical model with regard to the large cavity sizes and also to obtain the first responses of ground movement during the reducing the brine pressure. But also, the confrontation of the results of the continuum model with that based on the hybrid approach makes it possible to validate the discrete-continuum geomechanical model shown in figure 3.

In this section we present a comparison between the continuum and the hybrid models. For that, we compared : (a) the vertical displacement at the surface (subsidence) and the vertical displacements on both sides of the dolomite layer; (b) principal stresses at the base and the top of the dolomite layer. In PFC inclusion, stresses are calculated inside the measurement circles shown in Figure 5.

In figure 5, we compare the vertical displacements and principal stresses at different depths obtained by the fully continuum model (using FLAC code) and the hybrid FLAC-PFC model. We can appreciate the results (at a brine depth of 150 m, just before the plasticity onset in the competent bed) showing an almost perfect match between the two approaches.

Figure 6 represents the distribution of stresses within the overburden of cavity. Profiles of the principal stresses in the model section are qualitatively and quantitatively identical between the two approaches.

This validates the size of the large scale hybrid model, the calibration of the microproperties realized to reproduce the elastic mechanical properties of the assemblage of PFC particles, but also the extension and initial conditions (in terms of stresses) applied in inclusion. As long as the brine level does not exceed the depth of 160 m, there are no developments of microcracks : microcracking remains diffuse in the dolomite layer (Figure 9). The concentration of the forces is not very clear yet, even if we can notice the development of a zone in tension at the central region of inclusion with respect to the abutments (Figure 8).

Starting from 165 m (0.22 MPa of brine pressure at the roof of cavity) we note a coalescence of the microcracks in two zones located at the abutments (top fibre of dolomite bench). When reducing the brine pressure to 0.16 MPa (or 170 m), microcracks coalescence appears in the central part at the base of the competent layer. This is in conformity with the traditional results of the strength of the materials theory which indicate that tensile failure of a fixed beam initially appears at the location of abutments in the top fibre of beam, then to the centre of the beam in bottom fibre; say, primary the behaviour of a fixed beam accompanied to that of a simple beam [START_REF] Diederichs | Damage initiation and propagation in hard rock during tunnelling and the influence of near-face stress rotation[END_REF].

After this stage, for a level of brine of 170 m, microcracks coalescent at the base of the bench and macroscopic fractures start to be propagated upwards. System equilibrium is not reached any more beyond the level of brine of 165 m. The grounds of overburden located above the cavity move to the vacuum, indicating a numerical instability which could correspond to the initiation of a brutal collapse.

Another way of highlighting this instability, for a brine level folded back around 165 m, was the analysis of the numbers of microcracks generated in the dolomite layer during the folding of the brine level. Figure 10 shows a clear increase in the number of the microcracks as soon as the brine pressure at the roof of the cavity is lower than 0.22 MPa (or depth of 165 m). This type of change was observed in simulations with PFC of the laboratory tests, and indicates the creation of one or more macrofractures (inducing significant and abrupt increase in the acoustics emission) which involve the total loss of sample strength [START_REF] Diederichs | Damage initiation and propagation in hard rock during tunnelling and the influence of near-face stress rotation[END_REF][START_REF] Potyondy | A bonded particle model for rock[END_REF]. This abrupt increase of the microcracks number can thus be correlated with an acceleration of the microsismic activity.

In [START_REF] Mercerat | Induced seismicity in a salt mine environment evaluated by a coupled continnum-discrete modelling[END_REF] based on several simulations of the biaxial and Brazilian tests it was pointed out the difficulty to simultaneously calibrate both tensile and compressive macroscopic strengths with a simple PFC models. The main part of the failures awaited in the stiff layer of dolomite being in tension, we privileged the reproduction of the macroscopic tensile strength of dolomite. That led us to the ratio c/ c of 0.6 ( c/ c = 15 MPa/25 MPa, c and c ared defined in Table 1).

However, we wondered about the consequence of such a choice on the generation of the acoustic emissions, since such a ratio value would rather favourite cracks in tensile than in shear (Hazzard and Young, 2002). Thus, a simulation with an increase in the ratio c/ c from 0.6 to 1 was undertaken with the objective to clarify the impact of strengths ratio. This complementary simulation led to a spatial distribution of the microcracks very similar to that presented in figure 9, the only difference being the number of shear microcracks slightly more significant appearing in the bench of dolomite (figure 10). The number of shear microcracks remains marginal compared to that of the microcracks in tension mode. In both cases ( c/ c = 1 or 0.6), the microcracks are however mainly in tension (less than 10 % of microcracks in shear).

Conclusion and perspective

A local seismic network has been installed and further calibrated in the site test of Cerville-Buissoncourt (Lorraine, France). This site has been chosen by the GISOS working group to test and validate some monitoring techniques that could be used in the future in other salt mine environments, and not to pronounce about the consequences, in terms of environmental impact or security, of some specific salt exploitation methodology implemented in the test site.

We have presented a methodology of a hybrid numerical modelling approach to analyse a complex geomechanical and geophysical process in a salt mine-environment. This involves the fracturing and deformation of the overburden due to the brine pumping out and the consequent pressure decrease in an underground cavity.

A hybrid numerical modelling approach FLAC-PFC has been implemented for the specific site test of Cerville-Buissoncourt. The validation/calibration of the hybrid large scale model is achieved by comparing, at the site scale, the elastic response of the discrete inclusion and the response from the fully continuum approach. The results are quite satisfactory. Moreover, the development of microcracking is in accordance with the predicted tensile zones obtained with a fully continuum approach. The sudden rise up of the microcracks cumulative number from a specific brine depth level on, can be interpreted as the fracture development by coalescence of microcracks within the competent bed, which may possible, indicate the beginning of a generalized instability of the overburden. From both continuum and hybrid approaches, the main fractures in the competent layer are tensile. Finally, the coupled continuum-discrete methodology presented herein provides a good opportunity to explicitly simulate the microcraking in the overburden. A conclusive statement about failure mechanisms, with eventually a quantitative comparison, will be achieved when information about microseismic event locations and magnitudes will be available.

Next stage of this work is firstly to back-analyse the transient pressure experiment in order to validate the proposed methodology, and secondly, to compare these numerical modelling results with those of the measurements recorded in situ after restarting of salt exploitation in a few months

Figure 1 .

 1 Figure 1. (Left) View of the instrumented salt cavity. (Right) Scheme of the E-W vertical section with main lithological units and location of microseismic 3D stations (red triangles) and 1D stations (blue triangles)

Figure 2 .

 2 Figure 2. Number of events per day recorded while the transient pressure experiment in the cavity. The activity is concentrated in the pressure raise (two clusters of 16 and 18 seconds of duration)

Figure 3 .

 3 Figure 3. Model of Cerville-Buissoncourt site : (a) geometry and loading, (b) geometry of continuum (FLAC) domain, (c) assembling of PFC particles that models a part of dolomite layers

  n ) and shear (k s ) contact stiffness Young's modulus (E) and Poisson's ratio (ν) Normal (σc) and shear (τc) bond strengths Uniaxial Compression Strength (UCS) Friction coefficient (μ) -Particle clustering Ratio shear/normal bond strength Slope of failure envelope Normal bond strength (σc) -Particle radius Brazilian strength (σ t )

Figure 5 .

 5 Figure 5. Locations of vertical and horizontal profiles used for analysis

Figure 8 .Figure 10 .

 810 Figure 8. Major and minor principal stresses (brine depth of 160 m) indicating tensile stresses

Table 1 .

 1 Relations between microproperties and mechanical properties.

Research Group for the Impact and Safety of Underground Works involving French institutions INERIS, BRGM, INPL and Mining School of Paris.

Acknowledgements

This work is performed within the GISOS project. We acknowledge with gratitude the financial support of the French Ministry in charge of Industry. The authors also thank the company SOLVAY S.A. for the site, the technical support and the data for site characterization.