Candice Lagny
email: candice.lagny@ineris.fr

Stéphane Lafortune

Arnaud Charmoille

Zbigniew Pokryszka

François Degrelle

Marion Kimmel

CO 2 gas production understanding above a partly flooded coal post-mining area

Keywords: Noxious gas production, Water, Pyrite, Marcasite, Oxygen, Carbon dioxide

INERIS worked for several years on the characterization of O 2 -defficient and CO 2 -enriched gas production and migration processes above a partly flooded former coal mining area. To understand the origin of this production, a borehole of 90 meters deep was drilled next to the phenomena. Several investigations were conducted to improve the understanding of mechanisms of gas production in relation to the presence of water. This paper deals with the results of experiments conducted in situ and tests done in the laboratory. One hypothesis is proposed due to the results obtained: marcasite, which has been observed on rock samples, is oxidized by contact with the air that migrates through mining fractures in the rock. CO 2 is produced through carbonate dissolution because of the production of acidic fluids in the water. Noxious gas emissions can occur during atmospheric pressure drops and can enter houses, flowing along cracks.

Introduction

Variations of atmospheric composition are observed in some houses located above a former coal mine associated with a rising water table. These variations are characterized by a high content of CO 2 gas (up to 8%vol.) and low content of O 2 (down to 7%vol.). In France, legal exposition limits values are respectively of 1%vol. and 19%vol. These noxious gas emissions occur generally during atmospheric pressure drops. Various hypotheses were first proposed due to the gas emissions location and the hydrogeological context.

Lafortune [START_REF] Lafortune | Emissions gazeuses dans le quartier XXXX[END_REF] carried out isotopic investigations on carbon isotopes, in order to identify the carbon source. A nearby bog was rejected as the source because the carbon isotopic signatures do not match.

Deep water (the groundwater table is about 90 m deep) analysis permits us to exclude a dissolved gas origin, as well. In order to improve the understanding of the mechanisms involved, a borehole was drilled in March 2012. The borehole location was near the houses impacted by noxious gas emissions and in the same geological and hydrogeological context if possible. Hence, different experiments were conducted: during and after drilling (in situ and at the laboratory). This paper deals with these different experiments, the results obtained, and the hypothesis which could be emitted and verified.

Geological and hydrogeological contexts

The noxious gas emission phenomenon occurs in a town in France located in the Lorraine coal basin.

Expected geochemical reaction in link with gas noxious emission

The chemical reaction proposed by INERIS induces several successive geochemical reactions, depending on the following chronology which was confirmed by Agrinier [START_REF] Agrinier | Rapport d'expertise sur l'origine des émanations de CO2 à XXXX[END_REF]. In a first step, ferrous sulphur (pyrite or marcasite) is oxidised, according to:

(1) Then, iron is oxidised, according to:

(2) And then, iron hydroxides precipitates:

(3)

The water is acidified by these reactions by the cumulated production of 16 H + . If carbonates are present, this can interact and generate O 2 consumption and CO 2 production, according to:

(4)

The drilling

The chemical reactions previously described need the concomitant presence of water and atmospheric oxygen which can be observed in the capillary fringe. In order to confirm this, a borehole was drilled during March 2012 down to this zone. The method of continuous core drilling was employed and the core equipment was outfitted with a tube that had fringes to permit the flowing of gas from the strata encountered.

Experimentations after the drilling

In situ experiments

Gas analyses were conducted from the top to the bottom of borehole to obtain gas concentration logs. The gas analyser permitted us to measure simultaneously CO 2 , O 2 CH 4 and CO concentrations. These tests were done during a barometric pressure drop, which is a good condition for gas emission to the surface. Water samples were taken and unstable parameters such as temperature and pH (which was around 7.8) were measured with a multi-parameter probe.

Laboratory experiments

Several analyses were conducted. First, rock samples were chosen to be analysed with a SEM (scanning electron microscope) in order to find sulphur crystals, thin sections were cut to search carbonates. Secondly, rock samples were put in batches with demineralised water to observe if possible the in situ supposed reactions. The gases variations were observed with the same analyser used in situ.

Results and interpretation

In situ experiments

Figure 1 shows the results of one of the concentration gas logs carried out in the drilling. CO 2 gas content varies from 5.6% at the top of the borehole to 11.8% at 50 m depth whereas O 2 varies from 12.8% to 3.4%. Below 50 m, these concentrations stay mostly constant down to the bottom of the borehole.

Laboratory experiments

Marcasite (or white iron pyrite) was found on several samples, in an oxidized state or not. It's a ferrous sulphur which is more oxidizable than pyrite. Carbonates were found on rock samples, after thin sections analysis. Laboratory tests were conducted in batches. After having sterilized the samples in order to eliminate all bacteria, it was observed that O 2 was consumed and CO 2 was produced in the proportion of the theoretic line plots on Figure 2. This has been interpreted as the result of marcasite oxidation followed

Conclusion and outlook

The presence of high content of CO 2 and the lack of O 2 were confirmed by measurements in the borehole. With all the experiments conducted, it was confirmed that marcasite, water, and carbonates are present on the site in the rocks at different levels. Other rock samples still have to be analysed. The hypothesis of marcasite or pyrite oxidation is proposed to explain noxious gas production. Another gas concentration log along the borehole and after an air lift (with nitrogen) will be made to try to locate the gas production zone. Research will be continued on pyrite oxidation reaction because some other assumptions could be made as Schieber [START_REF] Schieber | Oxidation of detrital pyrite as a cause for Marcasite Formation in marine lag deposits from the Devonian of the eastern US[END_REF] has done in a different context.

  The geological and hydrogeological context can be summarized by (down to up): Carboniferous -The Westphalian deposit is constituted by numerous exploited coal seams of different thicknesses. These seams are located 600 m depth below the surface. Dewatering was stopped in 2006. -Stephanian stratigraphic units: they are constituted by conglomerates, shales and sandstones whose thickness reaches 200 m. Permian is constituted by dolomitic conglomerates and sandstones that are not very permeable and represent a stratigraphic level of 25 to 50 m thick. Trias: sandstones from Lower Trias are red or yellow, very porous, around 200 meters thick and they contain an important aquifer used for human activities and as drinking water. In July 2012, groundwater table was around +135 m NGF (i.e. 90 m deep). Obviously, coal mining has induced discontinuous deformations between Carboniferous and Trias stratigraphic levels in several areas, with a maximum collapse of 15 m (see DREAL Lorraine, [2]).

Figure 1 :

 1 Figure 1: O2 and CO2 concentration variations with depth

Figure 2 :

 2 Figure 2: CO2 versus O2 content and main theoretical lines (molecular balance line of: pyrite oxidation and carbonate dissolution in black, CO2 replacement by aerobic respiration in dark grey and CO2 concentration increase by dilution in pale grey)

Acknowledgements

This work is a part of a research program carried out by INERIS for the Ministry in charge of mines and for GEODERIS.


Huguier Pierre
email: pierre.huguier@ineris.fr

Manier Nicolas

Chabot Laure

Bauda Pascale

Pandard Pascal

Caenorhabditis elegans (Nematoda) and Hypoaspis aculeifer (Acari) as new ecotoxicological tools for the assessment of organic materials used in agriculture

The modern society is always producing more and more wastes. Some of them, mainly organic ones, can be valued to be in accordance with sustainable practices. Among organic wastes, agricultural, municipal and industrial organic materials (AMIOM) are intended to be used in agriculture. According to the numerous origins of these materials, the input of undesirable pollutants can't be avoided. Therefore it has been found necessary to assess their potential deleterious effects on the environment prior to their use. For that purpose, ecotoxicological characterization of different AMIOM was set up using routine terrestrial and aquatic bio-assays. In addition, two new model organisms (the nematode Caenorhabditis elegans and the predaceous mite Hypoaspis aculeifer) were included in the test batteries in order to conclude on their relevance in comparison with routine tests. Further work should allow defining a battery of bio-assays for the a priori hazard assessment of AMIOM.

Introduction

In the global context of environmental sustainability, there is an increase of the use of agricultural, municipal and industrial organic materials (AMIOM) in agriculture. These materials are useful to supply nutrients for crops and/or to improve soil physical characteristics. However, AMIOM can be a significant source of pollutants [START_REF] Eriksson | Potential priority pollutants in sewage sludge[END_REF] depending on their origin or production process. Soil quality is of importance, as soil provides key functions in terrestrial ecosystems like decomposition processes, nutrients and energy flows, which are performed by micro-organisms and invertebrates. These crucial functions explain the interest of protecting the quality of soils, which is included in European environmental policy [START_REF]European Parliament resolution on the Commission communication "Towards a thematic strategy for soil protection[END_REF]. That's why, in addition to physico-chemical characterization, there is a need to assess the potential deleterious effects of AMIOM on the environment prior to their use.

Currently, ecotoxicological bio-assays are used to characterize hazards of chemicals and complex matrices on the environment. In this work, the possibility of including two new model organisms (the nematode Caenorhabditis elegans and the mite Hypoaspis aculeifer) in bio-assays test batteries for AMIOM hazard assessment was studied. C. elegans, which is a well-known organism and easy to culture, was selected due to the possibility of testing aqueous as well as solid samples. For the soil mite H. aculeifer, its predaceous function was considered as complementary in term of trophic chain compared to the organisms used in terrestrial tests.

Materials and method AMIOM and test strategy

The AMIOM studied were: limed sewage sludge, ashes (from wastewater sludge, treated wood, paper industry waste and plant residues), deinking sludge, composted household wastes and cow manure (characteristics in Table 1). The test strategy, including terrestrial and aquatic bio-assays, is summarized in Figure 1. Leachates of the mixtures were realized on each dose of each AMIOM. The doses tested were ranging from one to one-hundred times their respective application rates.

Protocol for mixtures preparation, leaching and bio-assays

For soil bio-assays, solid mixtures were prepared with artificial soil, excepted for C. elegans for which the soil used for mixtures was an agricultural soil (LUFA 2.2). Five days prior to leaching, mixtures were moistened up to 20% (d.w.) to reach equilibrium. Eluates were obtained after addition of deionised water to the mixtures (ratio 1:10 w:v), which were put under rotating agitation during 24h. Liquid and solid phases were then separated by centrifugation (30min, 2500g) and the supernatants were tested for their ecotoxicity toward aquatic organisms. For aquatic tests, samples were filtered at 0.45µm excepted for nematode and daphnid tests, for which the supernatants were directly used after centrifugation. These bio-assays were performed following their respective current normative protocol [START_REF]Soil Quality -Determination of the effects of pollutants on soil flora -Part 1: Method for the measurement of inhibition of root growth[END_REF][START_REF]Soil Quality -Determination of the effects of pollutants on soil flora -Part 2: Effects of contaminated soil on the emergence and early growth of higher plants[END_REF][START_REF]Soil Quality -Avoidance test for determining the quality of soils and effects of chemicals on behaviour -Part 1: Test with earthworms (Eisenia fetida and Eisenia andrei)[END_REF][START_REF]Soil quality -Effects of pollutants on earthworms -Part2: Determination of effects on reproduction of Eisenia fetida/Eisenia andrei[END_REF][START_REF]Water Quality -Determination of the inhibitory effect of water samples on the light emission of Vibrio fischeri (Luminescent bacteria test). Part 1: Method using freshly prepared bacteria[END_REF][START_REF]Water Quality -Fresh water algal growth inhibition test with unicellular green algae[END_REF][START_REF]Water Quality -Determination of the inhibition of the mobility of Daphnia magna Straus (Cladocera, Crustacea) -Acute toxicity test[END_REF][START_REF]Water Quality -Determination of the chronic toxicity to Brachionus calyciflorus in 48 h[END_REF][START_REF]Water Quality -Determination of the toxic effect of sediment and soil samples on growth, fertility and reproduction of Caenorhabditis elegans (Nematoda)[END_REF][START_REF]Predatory mites (Hypoaspis (Geogaleops) aculeifer) reproduction test in soil[END_REF], excepted for C. elegans terrestrial tests for which the standard protocol was modified for complex matrices testing [START_REF] Huguier | Improvement of the Caenorhabditis elegans growth and reproduction test to assess the ecotoxicity of soils and complex matrices[END_REF].

Statistical analyses

Mean percentage of effect of each test was compared to their respective significant biological percentage of effect, which was based on the ISO 17616 [START_REF]Soil quality -Guidance on the choice and evaluation of bioassays for ecotoxicological characterization of soils and soil materials[END_REF] and estimated for C. elegans. Then, they were classified as "hormesis effect", "inhibitory effect" and "no effect", depending on the responses observed.

In order to determine the response similarity between bio-assays, hierarchical cluster analyses (HCA) were set up using R software [START_REF]R : A language and environment for statistical computing[END_REF]. For that purpose, the data used were transformed in mean percentage of effect for each test and AMIOM dose. These data were then changed into a dissimilarity matrix using Euclidian distances, which were agglomerated and ranked with Ward method.

Results

The overview of the data collected to date showed that, for both aquatic and terrestrial bioassays, the tested AMIOM were not ecotoxic at their respective application rates. Moreover, terrestrial bio-assays showed a higher sensitivity than aquatic ones.

Among higher plants endpoints, the root elongation test was the most sensitive endpoint. Significant effects were recorded for five times the application rate for limed sewage sludge, ten times the application rate for ashes and composted wastes, and fifty times the application rate of cow manure. Significant effects on biomass were recorded generally at 50 times the respective application rate of the materials (deinking sludge had significant effect from five times its application rate). Concerning earthworms, avoidance behaviour was the most sensitive endpoint (significant effects recorded from fifty times the application rate for limed sewage sludge, composted wastes and ashes). However, both avoidance and reproduction endpoints were correlated (Figure 2). Moreover, they were grouped with the plants ones (S_AveRoo, S_AveBioM and S_BrasBioM), which separate terrestrial organism responses from aquatic organisms. For nematode growth and reproduction in terrestrial tests, significant effects were found from fifty times the application rate of ashes and for one-hundred times the application rate of limed sewage sludge and deinking sludge. However, the data of C. elegans in soil (S_CaeRep and S_CaeGro) were ranked with aquatic tests. It should also be mentioned that some AMIOM had hormesis effects at over-application rates. This unusual ecotoxic response could be ascribed to intrinsic confounding factors, such as the presence of additional food resources (i.e. nutrients, bacteria, growth factors...). Data acquisition concerning H. aculeifer reproduction endpoint is still in progress. Among aquatic bio-assays, the algae P. subcapitata endpoint was found to be the most sensitive. The HCA (Figure 2) demonstrated that this bio-assay response was similar to those of the bacteria V. fischeri (L_VibLum and L_PseGro). Focusing on C. elegans data for both terrestrial and aquatic tests, the reproduction endpoint was found to be more sensitive than the growth. The cluster analysis showed that the nematode responses in aqueous phase (L_CaeGroISO, L_CaeRepISO, L_CaeGroLUF and L_CaeRepLUF) were different depending on the soil used for the mixtures (i.e. ISO and LUFA soil).

Conclusion and perspectives

These preliminary results showed that there was a limited redundancy between routine terrestrial and aquatic tests. Moreover, nematode responses for both environmental compartment were ranked nearly altogether and apart from the other tests. This indicates that this species could be an interesting tool to complement the battery of bio-assays.

Further work should allow defining terrestrial and aquatic batteries of bio-assays for the a priori hazard assessment of AMIOM, as well as concluding on the interest of the two new model organisms for the AMIOM ecotoxicity study.

Figure 1 :

 1 Figure 1: Test strategy for AMIOM testing

Figure 2 :

 2 Figure 2: Hierarchical cluster analysis of the bio-assays responses for the AMIOM assessment (Method: Ward, Distance: Euclidian)

Table 1 : Physico-chemical properties of AMIOM.

 1 Concentrations are expressed on a dry weight basis.

		Limed sewage sludge	Ashes	Deinking sludge	Composted wastes	Cow manure
	Application rate (T.ha -1 )	3	3	5	3	10
	pH	9.3	12.6	8.3	7.3	8.6
	WHC (%)	300	50	100	175	290
	N total (g.kg -1 )	9.4	0.8	3.4	14.6	18.5
	Organic carbon (g.kg -1 )	277	< 1	163	319	433
	P 2 O 5 total (g.kg -1 )	36.6	8.3	1.9	7.6	8.5
	K 2 O total (g.kg -1 )	2.6	8.1	0.7	12.2	33.3
	CaO (g.kg -1 )	148	507	403	60	18.7
	MgO (g.kg -1 )	7	24.1	4.2	5.9	6.9
	As (mg.kg -1 )	4.6	10.1	0.4	3.9	0.4
	Cd (mg.kg -1 )	1.7	3.3	0.1	0.7	0.1
	Cu (mg.kg -1 )	433	383	79.7	96.8	17
	Hg (mg.kg -1 )	0.5	0.6	< 0.01	0.2	0.03
	Ni (mg.kg -1 )	17.8	42.3	1.9	26.1	2.7
	Pb (mg.kg -1 )	39.3	492	3.3	76.8	< 2
	Zn (mg.kg -1 )	458	1549	25.1	395	65.7
	∑ 7 PCB (mg.kg -1 )	< 0.07	< 0.07	< 0.07	< 0.14	< 0.07

Acknowledgements:

The financial support from the French Ministry of Ecology, Sustainable Development and Energy (French Environment and Energy Management Agency, contract 0975C0061) is gratefully acknowledged. The Caenorhabditis elegans strain was provided by the CGC, which is funded by NIH Office Reasearch Infrastructure Programs (P40 OD010440).


Yann Foucault
email: foucault@stc-metaux.com

Marie-José Durand
email: marie-jose.durand@univ-nantes.fr

Karine Tack
email: karine.tack@ineris.fr

Eva Schreck
email: eva.schreck@get.obs-mip.fr

Thibaut Leveque
email: thibaut.leveque@inp-toulouse.fr

Philippe Pradere
email: p.pradere@stc-metaux.com

Sylvaine Goix
email: sylvaine.goix@gmail.com

Camille Dumat
email: camille.dumat@ensat.fr

ECOTOXICITY TEST AND ECOSCORES TO IMPROVE POLLUTED SOILS MANAGEMENT: CASE OF A SECONDARY LEAD SMELTER PLANT

Keywords: sustainable management of polluted soils, metal trace elements, ecotoxicity, landfilling, leaching, ecoscores

Introduction

Originally located on the outskirts of cities, numerous industrial sites, sometimes abandoned, are now in urban areas and are therefore likely to have environmental and health risks to surrounding populations [START_REF] Foucault | Toward rational management of excavated soil contaminated by metal trace elements[END_REF][START_REF] Van Hees | Re-cycling of remediated soil in Sweden: An environmental advantage?[END_REF]. Currently, rehabilitation of the sites frequently entails excavation of polluted soils [START_REF] Colombano | Quelles techniques pour quels traitements -Analyse coûts-bénéfices[END_REF]. Excavated soils can thus follow two different ways: landfilling, expensive and energy intensive, or reuse/recycling, integrated to sustainable development. The choice of a specific track mainly depends on total and leachable concentrations of the pollutant in the soil [START_REF] Van Hees | Re-cycling of remediated soil in Sweden: An environmental advantage?[END_REF]. Among the numerous pollutants observed in urban and peri-urban areas, trace metals are often present in soils [START_REF] Schreck | Influence of soil ageing on bioavailability and ecotoxicity of lead carried by process waste metallic ultrafine particles[END_REF]; atmosphere emissions by smelters being one of the main anthropogenic source [START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF][START_REF] Luo | Distribution, availability, and sources of trace metals in different particle size fractions of urban soils in Hong Kong: Implications for assessing the risk to human health[END_REF]. MTE speciation and compartmentalization in soils can modify their impact on living organisms [START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF]. Now, numerous publications concluded that these two parameters are strongly influenced by soil organic matter (OM) content, pH and texture [START_REF] Fritsch | Spatial distribution of metals in smelter-impacted soils of woody habitats: Influence of landscape and soil properties, and risk for wildlife[END_REF][START_REF] Vega | Influence of soil properties on the sorption and retention of cadmium, copper and lead, separately and together, by 20 soil horizons: Comparison of linear regression and tree regression analyses[END_REF][START_REF] Sammut | Speciation of Cd and Pb in dust emitted from sinter plant[END_REF]. According to Matejczyk et al. [START_REF] Matejczyk | Estimation of the environmental risk posed by landfills using chemical, microbiological and ecotoxicological testing of leachates[END_REF], chemical weathering of soil minerals favours MTE solubilisation and leachates production. Then, these leachates can pollute surrounding soils and waters. According to the council directive n°1999/31/CE, leaching tests with chemical analysis are therefore currently used for the assessment of environmental hazards of polluted soils. But, landfilling is often inevitable for strongly polluted soils, with high "hazard level" (assessed by leached and total MTE concentrations). Moreover, according to Foucault et al. [START_REF] Foucault | Toward rational management of excavated soil contaminated by metal trace elements[END_REF], professionals consider the threshold set as too restrictive and they regret that excavated soils are almost always managed as waste.

In addition to the measure of total and leached MTE concentrations, it appears therefore that knowledge of MTE availability [START_REF] Kashem | Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils[END_REF] and ecotoxicity may carry useful information [START_REF]CEN 14735, Characterization of Waste. Preparation of Waste Samples for Ecotoxicity Tests, European Committee for Standardization[END_REF][START_REF] Buj | Leaching behaviour of magnesium phosphate cements containing high quantities of heavy metals[END_REF][START_REF] Plaza | Ecotoxicological and microbiological characterization of soils from heavy-metal and hydrocarbon-contaminated sites[END_REF] to improve environmental risk assessment [START_REF] Matejczyk | Estimation of the environmental risk posed by landfills using chemical, microbiological and ecotoxicological testing of leachates[END_REF]. Actually, the accurate estimation of metal phytoavailability in polluted soils and solid wastes, using single chemical extraction [START_REF] Meers | Phytoavailability assessment of heavy metals in soils by single extractions and accumulation by Phaseolus vulgaris[END_REF] carry interesting data to perform pertinent risk assessment and remediation efforts [START_REF] Menzies | Evaluation of extractants for estimation of the phytoavailable trace metals in soils[END_REF][START_REF] Arshad | A field study of lead phytoextraction by various scented Pelargonium cultivars[END_REF]. Soil quality integrates both physicochemical and biological characteristics [START_REF] Römbke | Legislation and ecological quality assessment of soil: implementation of ecological indication systems in Europe, Ecotox[END_REF]. Moreover, according to Plaza et al. [START_REF] Plaza | Ecotoxicological and microbiological characterization of soils from heavy-metal and hydrocarbon-contaminated sites[END_REF] microorganisms play important roles in numerous soil functions. Soils are often polluted with a large variety of compounds leading to possible interactions [START_REF] Mansour | Risk assessment of pesticides and heavy metals contaminants in vegetables: A novel bioassay method using Daphnia magna Straus[END_REF], thus as reviewed by Kim and Owens [START_REF] Kim | Potential for enhanced phytoremediation of landfills using biosolids -a review[END_REF] study of leachates ecotoxicity provides a direct functional characterization of various pollutant mixtures. But, only few studies concern the use of ecotoxicological tests to monitor contamination and bioremediation efficiency of polluted soils [START_REF] Leitgib | Comparison of bioassays by testing whole soil and their water extract from contaminated sites[END_REF] and new tests are required by industrial sites managers to assess environmental risks. Among them, microbial bioassays offer quick, cheap and easy ecotoxicity (toxicity and mutagenicity) and bioavailability measurements on bacteria [START_REF] Durand | Specific detection of organotin compounds with a recombinant luminescent bacteria[END_REF][START_REF] Elad | Toxicant identification by a luminescent bacterial bioreporter panel: application of pattern classification algorithms[END_REF]. However, in many cases, microbial bioassays cannot be directly used for the identification and quantification of compounds due to the lack of specificity of the engineered microorganisms [START_REF] Jouanneau | Improvement of the identification of four heavy metals in environmental samples by using predictive decision tree models coupled with a set of five bioluminescent Bacteria[END_REF] and further studies are needed to improve these biotests.

The aim of this study was therefore to assess the ecotoxicity of leachates for landfilling of MTE contaminated soils by various complementary biotests, in addition to usual physicochemical measures. More precisely, the following two scientific objectives were aimed: [START_REF] Foucault | Toward rational management of excavated soil contaminated by metal trace elements[END_REF] what is the pertinence of ecotoxicity tests to assess a more realistic human exposition to contaminated soil leachates? (2) What is the influence of soil physicochemical parameters on MTE mobility and leachates ecotoxicity? The originality of this study was to combine the use of new bacterial strains never tested in a context of the remediation of an industrial polluted site and calculation of eco-scores which facilitates the comparisons between different soils.

Materials and methods

Ten top soil samples were collected in the courtyard of the Chemical Metal Treatments Society (STCM), a secondary lead smelter which currently recycles batteries located in the urban area of Toulouse. This plant was chosen because of its activity and urban location, and many data are already available [START_REF] Foucault | Toward rational management of excavated soil contaminated by metal trace elements[END_REF][START_REF] Schreck | Influence of soil ageing on bioavailability and ecotoxicity of lead carried by process waste metallic ultrafine particles[END_REF][START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF]. These data allowed defining different areas in terms of environmental and sanitary risks that can vary according to past and present activities. Samples were dried, sieved under 2 mm and treated in triplicate. pH, organic matter and limestone contents, cation exchange capacity and texture, were determined for all soil samples. Pb, As, Cu, Cd, Zn and Sb total concentrations were measured by ICP-OES after mineralization in aqua regia.

Normalized leaching test was applied to all soil samples. This procedure consisted of a single extraction with deionised water, using a solid-to-liquid ratio of 1/10. 10 g of soil (granulometry < 4 mm according to the norm) was mixed with 100 mL deionised water during 24h with end-over-end agitation at 5 rpm. After centrifugation at 3,000 g during 15 min, the leachates were filtered with cellulose 0.45 m (Millipore®) filters. 10 mL of each leachates were then acidified with HNO 3 65% prior to analysis by ICP-OES. The other part of leachates was not acidified so as not to disturb microorganisms used for further ecotoxicological tests. Potentially phytoavailable MTE concentrations were estimated by CaCl 2 extractions according to Uzu et al. [START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF]. In 25 mL polypropylene centrifugation tubes, 10 mL of 10 -2 M CaCl 2 were added to 1.0 g of soil. The liquid to solid ratio of 10 is high enough to avoid samples heterogeneities [START_REF] Houba | State of the art and future developments in soil analysis for bioavailability assessment[END_REF]. After agitation end-overend during two hours at 5 rpm at 20 °C, samples wer e then centrifuged during 30 min at 10,000 g. Supernatant was sieved through a 0.22 µm mesh and acidified at 2% with HNO 3 (15 N, suprapur 99.9%). MTE concentrations were finally measured by ICP-OES (IRIS Intrepid II XXDL, analytical errors < 5%).

Acute toxicity tests of leachates were performed on the water flea Daphnia magna according to ISO 6341 and with the Microtox® according to the ISO 11348 regulation specifications. Moreover, new bacterial strains which are dedicated to the specific detection of pollutants or pollutant family were used. Thus, a set of five bioluminescent bacteria namely E. coli Taclux, E. coli Zntlux, E. coli Arslux, E. coli Coplux and E. coli Merlux was used. Monitoring of bioluminescence was recorded and the results were expressed by the logarithm of the induction ratio or the inhibition rate for the inducible strains and the constitutive strain respectively. Decision trees were designed from the learning set of bacterial bioluminescence data using the software "Metalsoft".

Results

Physico-chemical characteristics significantly differ in function of sample origin, i.e. their localization on the industrial site in relation with process. pH value varied between 6.9 to 9.2, CEC value varies between 2.6 to 10.5 cmol(+) kg -1 and amounts of soil organic matter and carbonates (CaCO 3 ) were highly variable: respectively from 0.9 to 46.7 g kg -1 and from 0 to 15.0 g kg -1

. MTE concentrations in polluted soil samples were also very heterogeneous: maximum lead concentration is 42,400 mgPb.kg -1 and other elements are also present at high levels (up to 2095 mgSb kg -1

, 288 mgAs kg -1

, 286 mgCu kg -1

, 294 mgZn kg -1 and 80.9 mgCd kg -1

). Leached MTE amounts in water and corresponding ratios (in comparison with aqua regia extraction, considered as "total") were significantly depending on element nature (Table 2a). The highest extracted concentrations were recorded for lead, antimony and zinc (MTE with high total concentrations): respectively 152.5, 158 and 9 mg kg -1 , i.e. 8.7 %, 7.3 % and 7.9 %. In comparison, copper (at equivalent content) was significantly less extracted than zinc. Although quantitatively low extracted (≤ 5.4 mgCd kg -1 ), Cd was proportionally one of the most water-soluble element (up to 15.9 % for S 6 ). Arsenic was the less extracted MTE with a maximum concentration reached of 2.2 mgAs kg -1 (7.5 % of the total for S 4 ). CaCl 2 extractions results (Table 2b) showed several contrasted behaviours depending both on chemical element and soil properties. The highest lead quantities extracted by CaCl 2 were observed for S 1 , S 3 and S 7 (up to 178.5 mg kg -1

MTE (mg

). However, for S 4 sample with high total lead concentration, the extracted fraction is low (1.3 mg kg -1

). Conversely, antimony extracted concentration reached 306.6 mgSb kg -1 . Other MTE showed a low extractability (in terms of quantity and ratio) whatever the sample, except for S 7 which registered pronounced pools of Cd and Zn associated with high total concentrations. Figures 3-a and3-b show the fraction of the extracted element in relation to total concentration. Cadmium appeared as the most potentially phytoavailable element. Sb and Zn also represented high extracted fractions respectively for S 4 and S 7 . Moreover, compared to the aqua regia fraction, the CaCl 2 fraction remained lower, except for the most potentially phytoavailable Cd element (2-32 %). 178,5 0,71 162,9 1,30 0,52 0,60 89,38 14,69 0,31 3,64 Pb % of the total 0,45 0,05 0,38 0,00 0,05 0,20 0,25 1,02 0,02 0,34 1,54 0,478 1,56 306,6 0,16 0,28 0,77 0,08 0,35 0,11 Sb % of the total 0,07 0,894 0,10 14,10 0,66 2,09 0,04 0,50 0,78 1,20 Ecotoxicity of leachates measured by the inhibition of Daphnia magna mobility was highly variable (Table 3). Whatever the sample tested, the inhibition of daphnia mobility increased between 24 h and 48 h, except for S 3 whose inhibition was near 100% after only 24 h. Ecotoxicity was also maximal (i.e. 100%) for S 1 , S 2 , S 7 and S 8 after 48 h; while the lower inhibition was observed for S 5 (15 %). Ecotoxicity was not simply dependant of MTE concentration: (i) the most MTE-enriched leachates were not always the more toxicant; (ii) leachate of S 8 had low MTE concentrations while the inhibition of daphnia mobility was 100 %.

Leaching

The mean EC 50 -30 min value obtained for zinc sulphate heptahydrate (expressed as Zn 2+ ) was 2.38 mg L -1

, allowing concluding that the invertebrates lot fulfilled the validation specifications. Microtox® test results (Table 3) showed an increase in the number of toxic samples with the contact time: inhibition of bioluminescence was detected in two samples at the beginning of the experiment and for four of them at the end (S 1 and S 7 to 5 min; S 1 , S 4 and S 7 to 15 min; S 1 , S 7 , S 4 and S 3 at 30 min). The measured ecotoxicity also increased over time and was above 90 % for S 1 and S 7 after 30 min of contact. The bacteria were most affected by S 7 with an inhibition of the luminescence of 63% (5 min). Unlike the test on Daphnia, S 2 , S 6 , S 8 , S 9 and S 10 showed no toxicity, as S 5 in both bioassays. As described above, S 1 , S 3 , S 4 and S 7 are among the most contaminated leachates ([Pb] > 80 mg kg -1

; [Sb] > 10 mg kg -1 (except S 7 )) (Table 2a).

The sensitivity and specificity of inducible bacteria were measured after 60 min in contact with leachates. None of the sample induced the luminescence of Coplux strain. Only two samples, S 1 and S 7, showed a slight toxicity as demonstrated by the inhibition of luminescence of the constitutive strain pBtaclux (Table 4). These samples also induced the luminescence of Zntlux, Arslux and Merlux strains. For S 1 and S 7 the maximum IR was recorded for Arslux (IR = 99.6) and Merlux (IR = 138.1) respectively. S 3 increased moderately the luminescence of Merlux (IR = 12.9) and Zntlux (IR = 4.0) while S 2 induced only Arslux (IR = 324.9). The analysis with decision trees was then used to determine the elements potentially responsible for ecotoxicity of S 1 , S 2 , S 3 and S 7 . Crosses between results suggested the presence of arsenic in these four leachates (up to 10 -5 M, i.e. more than by chemical analysis), cadmium for S 1 , S 3 and S 7 , biologically at levels lower than those measured chemically (Table 6-b). Analysis of S 7 also showed the presence of copper and mercury (from 10 -4 to 10 -5 M and 10 -9 to 10 -5 M). According to the previous tests, these results also concluded to the ecotoxicity of S 1 and S 7 , and, to a minor extent, the ecotoxicity of S 3 and S 4 .

Sample

Bacterial strain

Discussion

In this study, soil pH were basic or close to neutral conditions and leaching procedure slightly reduced the pH by water addition. Conversely, CaCl 2 is already known to not modify soil pH and give results closer from field reality [START_REF] Meers | Phytoavailability assessment of heavy metals in soils by single extractions and accumulation by Phaseolus vulgaris[END_REF]. Thus, hazard proposed classification of polluted soils differs between water leaching and CaCl 2 procedures. Several studies already showed that MTE extractability is strongly influenced by the nature of the extracting agent which can control element mobility [START_REF] Meers | Phytoavailability assessment of heavy metals in soils by single extractions and accumulation by Phaseolus vulgaris[END_REF][START_REF] Menzies | Evaluation of extractants for estimation of the phytoavailable trace metals in soils[END_REF]. Moreover, according to Dumat et al. [START_REF] Dumat | Study of the trace metal ion influence on the turnover of soil organic matter in cultivated contaminated soils[END_REF] or Ferrari et al. [START_REF] Ferrari | Predicting uncertainty in the ecotoxicological assessment of solid waste leachates[END_REF], solid-liquid MTE transfers during chemical extractions are complex reactions involving numerous factors that can influence MTE speciation and release. Contact times chosen for chemical extractions were 24h for water and only 2h for CaCl 2 in accordance with the commonly used protocols: these two procedures carry complementary information but the results are not directly comparable.

All MTE were extracted in substantially equal proportions with water (from 0 to 18 %); MTE concentrations in CaCl 2 extracts and corresponding ratios, varied in the range of those reported in the literature [START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF][START_REF] Rodrigues | Evaluation of an approach for the characterization of reactive and available pools of 20 potentially toxic elements in soils: Part II -Solid-solution partition relationships and ion activity in soil solutions[END_REF]37], and Cd was the most available element (up to 32 %). Extracted concentrations were strongly correlated to total concentrations. At the reverse side, for all the other elements no relevant correlation was found between total and extracted fractions. In agreement with previous publications [START_REF] Uzu | Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation[END_REF][START_REF] Meers | Phytoavailability assessment of heavy metals in soils by single extractions and accumulation by Phaseolus vulgaris[END_REF], these results highlighted the influence of soil properties and MTE nature on its behaviour. Differences observed in function of MTE nature can be explained by different OM or CaCO 3 soil contents, CEC or soil pH. In soils, cadmium is generally easy to dissolve which explain its relatively high extractability [START_REF] Beesley | Mobility of arsenic, cadmium and zinc in a multi-element contaminated soil profile assessed by in-situ soil pore water sampling, column leaching and sequential extraction[END_REF]. High correlation factors were observed between exchangeable Cu and Zn fractions and soil organic matter amount. These elements were thus less mobile because of their affinity for this soil fraction [START_REF] Kashem | Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils[END_REF][START_REF] Romkens | Prediction of cadmium uptake by brown rice and derivation of soil-plant transfer models to improve soil protection guidelines[END_REF]. Concerning lead behaviour, no relationship was found between extracted and total concentrations, and the influence of even one soil parameter was difficult to highlight. Nevertheless, low extraction ratios compared to the most concentrated samples (S 1 , S 3 , S 4 and S 7 ) can be explained by stronger bounds on soil phases as mineral fraction [START_REF] Beesley | Mobility of arsenic, cadmium and zinc in a multi-element contaminated soil profile assessed by in-situ soil pore water sampling, column leaching and sequential extraction[END_REF]. Finally, sorption of metalloids as As and Sb, is mainly controlled by mineral phases [START_REF] Wilson | The chemistry and behavior of antimony in the soil environment with comparison to arsenic: a critical review[END_REF]. The high Sb amount extracted from S4 could be explained not only by a higher total concentration but also by the highest CaCO 3 content [START_REF] Denys | Bioaccessibility, solid phase distribution, and speciation of Sb in soils and digestive fluids[END_REF]. Sb could be solubilised under the influence of soil bio-physico-chemical parameters controlling its sorption [START_REF] Collin | Impact of high natural soilborne heavy metal concentration on the mobility and phytoavailability of these elements for sugarcane[END_REF][START_REF] Rodrigues | Evaluation of an approach for the characterization of reactive and available pools of 20 potentially toxic elements in soils: Part II -Solid-solution partition relationships and ion activity in soil solutions[END_REF][START_REF] Luo | The mobility, bioavailability, and human bioaccessibility of trace metals in urban soils of Hong Kong[END_REF]. pH and CEC were already described as influent parameters of element extractability [START_REF] Rieuwerts | The influence of soil characteristics on the extractability of Cd, Pb and Zn in upland and moorland soils[END_REF], retention and mobility in soils [START_REF] Kashem | Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils[END_REF]. Thus, according to the origin of soil sampling, differences in soil parameters were observed (Figure 1): in the industrial site, areas not covered or infiltration zones were the most impacted by MTE. Their organic matter content and CEC were also the higher, thus confirming their role in sorption / desorption mechanisms. The choice of the extractant is thereby an important step to be relevant in risk assessment and to avoid an under-or over-estimation of phytotoxicity. Finally, the data obtained by chemical tests are difficult to interpret because of the many parameters interact. The realization of ecotoxicity tests to measure the impact of pollution on ecosystems seems therefore particularly appropriate in this type of study.

The D. magna ecotoxicity test was more sensitive to MTE impact than the Microtox® test. But, unlike tests on the different bacterial strains, they do not both provide information on MTE quantification. Ecotoxicity differences were measured for some samples, especially S 2 and S 8 . These differences can be firstly explained by water flea sensitivity. Detection capabilities of the ecotoxicity of the leachate are actually dependent on the test used [START_REF] Isidori | Toxicity identification evaluation of leachates from municipal waste landfills: a multispecies approach[END_REF][START_REF] Manusadzianas | Comparative study of indices used in toxicity evaluation of effluents[END_REF] and it has been already shown that V. fischeri was generally less sensitive than D. magna [START_REF] Matejczyk | Estimation of the environmental risk posed by landfills using chemical, microbiological and ecotoxicological testing of leachates[END_REF][START_REF] Boluda | Soil plate bioassay: An effective method to determine ecotoxicological risks[END_REF]. Instead of these tests, experiments by using bacterial strains allowed to determine and quantify the element which was potentially bioavailable and / or toxic for bacteria [START_REF] Charrier | Bacterial Bioluminescent Biosensor Characterisation for On-line Monitoring of Heavy Metals Pollutions in Waste Water Treatment Plant Effluents[END_REF]. Then, response in ecotoxicity tests was not always directly correlated with total or water-soluble concentrations [START_REF] Chapman | Ecotoxicological risk assessment of undisturbed metal contaminated soil at two remote lighthouse sites[END_REF]. These results are in agreement with data previously obtained by Plaza et al. [START_REF] Plaza | Ecotoxicological and microbiological characterization of soils from heavy-metal and hydrocarbon-contaminated sites[END_REF] concerning the influence of pH and CEC on MTE behaviour in soils.

Results of this study have shown that this new bioassay enables the screening of samples in terms of environmental risk during remediation process [START_REF] Jouanneau | Improvement of the identification of four heavy metals in environmental samples by using predictive decision tree models coupled with a set of five bioluminescent Bacteria[END_REF]. However, the drawback of the lack of specificity of one strain and the effect of a mixture of MTE (synergistic or antagonistic effects) could be overcome by using a panel of bacterial strains coupled with a predictive model [START_REF] Jouanneau | Improvement of the identification of four heavy metals in environmental samples by using predictive decision tree models coupled with a set of five bioluminescent Bacteria[END_REF]. Due to the lack of specific bacteria for lead, the introduction of other strains induced by lead like Rastonia Metallidurans AE 1433 [START_REF] Corbisier | Bacterial biosensors for toxicity assessment of solid waste[END_REF] could improve the interpretation of the data.

According to Persoone et al. [START_REF] Persoone | A practical and user-friendly toxicity classification system with microbiotests for natural waters and wastewaters[END_REF] and Matejczyk et al. [START_REF] Matejczyk | Estimation of the environmental risk posed by landfills using chemical, microbiological and ecotoxicological testing of leachates[END_REF], the samples were ranked into one of five classes on the basis of the percentage effect (PE) found in Daphnia and Microtox® tests. Ranking was based on induction/inhibition rates for bacterial strains. A weight score was calculated for each hazard class to indicate the quantitative importance (weight) of the ecotoxicity in that class. The weight score was expressed as percentage.

( 5. Samples were classified as slightly and highly toxic in 10 %, toxic in 30 %, and very highly toxic in 50 %. The percentage of class weight class was above 5 % for only S 7 and S 1 (75 % and 62.5 % respectively). These samples were definitely considered as the most hazardous and acutely toxic to the microfauna. The final classification of ecotoxicity risks was S 7 > S 1 > S 3 > S 2 = S 4 = S 9 = S 10 > S 8 > S 5 = S 6 . Although the toxicity of some samples (S 2 and S 8 , for instance) could be different depending on the test used, the ranking based on total concentrations and leachable contents of MTE was almost the same; samples S 1 , S 3 , S 4 and S 7 presenting the greatest risks while the less contaminated areas were generally the less hazardous. However, as our results demonstrated that only a small fraction of total MTE soil concentrations can be solubilised and phytoavailable; ecotoxicity measures complete therefore efficiently standard performed tests for a realistic risks assessment of MTE-contaminated soils accordingly to [START_REF] Matejczyk | Estimation of the environmental risk posed by landfills using chemical, microbiological and ecotoxicological testing of leachates[END_REF][START_REF] Plaza | Ecotoxicological and microbiological characterization of soils from heavy-metal and hydrocarbon-contaminated sites[END_REF]. Moreover, the use of eco-scores improves the comparisons between bioassays and suggests the use of a restricted battery of tests to perform a cost-effective risk assessment of MTEcontaminated soils.

Score

Conclusion

Biotests and eco-scores improve standard tests performed to assess risk on ecosystems induced by polluted soils. In particular, modified bacteria strains sensitive to metals are useful tools highlighting the presence of different MTE and the influence of soil parameters that lead to synergistic or antagonistic effects. Moreover, eco-scores calculation allows an easy and cheap screening of a large number of polluted soil samples and suggests a restricted battery of bioassays to perform a cost-effective risk assessment. Nevertheless, further researches are required to develop a panel of bacterial strains specific of each MTE. Moreover, the chemical characterisation of leachates could provide additional relevant insight with the use of modelling software for determination of MTE speciation and thus to evaluate the predominant parameter influencing soil availability and ecotoxicity. Finally, the procedure developed in the present work could be used in the context of bioremediation techniques like phytoremediation performed on polluted soils, to assessment their efficiency and favour their use in a context of sustainable development.

Figure 1 :

 1 Figure 1 : Situation of the industrial study site, location and characteristics of the 10 sampling points.

)

  Class weight score = (∑ all test scores) / number of tests performed (= 6) (2) Class weight score (%) = (class weight score) / maximum class weight score × 100 That classification system aimed at the integration of ecotoxicity data obtained in a battery of bioassays as describe by Lors et al. [44]. The classification system is based on two values: a ranking in five acute toxicity classes and a weight score for each toxicity class. The classification of the samples tested in the investigation is reported in Table

Table 1 :

 1 Aqua regia MTE concentrations for the 10 soil samples

	kg -1 )	S 1	S 2	S 3	S 4	S 5	S 6	S 7	S 8	S 9	S 10
	Pb	39800 1425 42400 37250 1020 297 35700 1445 1750 1065
	As	288 28.7 51.5	52.5	5.8 9.3 34.3 8.65 13.3 10.2
	Cu	286 14.7 143.5 116 13.6 16.1 249 19.4 59.5 60.5
	Cd	18.4 2.24 34.3	4.15 3.39 0.69 80.9 3.39 4.7 11.3
	Zn	294 37.1 216	218 42.8 41.9 545	55	116	94
	Sb	2095 53.5 1555 2175 23.5 13.1 1955 15.9 44.5 9.15

Table 2a :

 2a Leached MTE in mg kg -1 and % of the total concentration

	Phytoavailability assessment	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10
	mg kg -1

Table 2b :

 2b Extracted MTE with CaCl2 in mg kg -1 and % of the total concentration

Table 3 :

 3 Results of the Daphnia magna and Microtox® tests for the 10 leachates

Table 4 :

 4 Ecotoxicity results of bioluminescence emitted by the bacterial strains

Table 5 :

 5 Ecoscores calculation and hazard classification


Sandra Dochez
email: sandra.dochez@ineris.fr

Farid Laouafa
email: farid.laouafa@ineris.fr

Christian Franck
email: christian.franck@geoderis.fr

Sylvine Guedon
email: jeanne-sylvine.guedon@ifsttar.fr

François Martineau
email: françois.martineau@ifsttar.fr

Julie D'amato
email: julie.damato@gmail.com

Albane Saintenoy
email: albane.saintenoy@u-psud.fr

MULTI-SCALE ANALYSIS OF WATER ALTERATION ON THE ROCKSLOPE STABILITY FRAMEWORK

Keywords: Water, chemical degradation, rock discontinuity, stability. -

Water is an important weathering factor on rock discontinuities and in rock mass mechanical behavior. The increase of rainfall in frequency or in intensity highlights some problems on the rock slope stability analysis. The aim of this paper is the multi scale analysis of the chemical impact of water on rock (surface roughness and matrix). In this study we show how water induces degradation and thus may decreases the stability of the discontinuous rock mass. Water is known to have significant erosion and dissolution effects on rock surface or rock matrix. The chemical features of water such as temperature, pH or salinity make it a "good" candidate to rock degradation. This study has two main components. The first one is the study of water-solid chemical mechanisms and the other is the analysis of the mechanical response of the discontinuity modified by the water alteration. The stability of the rock mass is naturally a function of the type and the space distribution of discontinuities. The study aims also to evaluate the effect of water flow on the rock slope stability and it is performed at two space scales: laboratory (micro scale and macro scale) and in situ scales. The last one is still under investigation and will be presented.

INTRODUCTION

Roads are frequently closed because of collapsing boulders or more generally by rock fall phenomena. These events are mainly induced by the presence and the behavior of discontinuities in mountains rock mass. As a matter of fact, theses discontinuities play a major role on the stability of the rock slopes [START_REF] Zare | The effect of moisture on the stability of rock slopes[END_REF]. An event as heavy rains is an important factor of rock stability. The Intergovernmental Panel on Climate Change (IPCC) [START_REF] Pachauri | Bilan 2007 des changements climatiques. Contribution des Groupes de travail I[END_REF] related the phenomena of increasing precipitations and rise in altitude permafrost due to climate change will impact the occurrence of the rock fall phenomenon. A number of large-scale slope movements is also linked to precipitation (rain and snow melting [START_REF] Durville | Mécanismes et modèles de comportement des grands mouvements de versants[END_REF]). In this framework water is considered as an important weathering factor because of the specificity of its molecule which confers to it a strong erosive power [START_REF] Javey | L'altération des roches et des monuments. Etude documentaire[END_REF]. Water effects in fractured media are significant by the interactions between water and rock. Many reactions either mechanical pressure or chemical dissolution are involved in rock weathering. Dissolution is certainly the main cause of limestone weathering due to its mineralogy [START_REF] Sjöberg | Calcite dissolution kinetics: Surface speciation and the origin of the variable pH dependence[END_REF][6] [START_REF] Dreybrodt | The kinetics of the reaction CO2 + H20 + H + + HCO3-, as one of the rate limiting steps for the dissolution of calcite in the system H2O-CO2-CaC03[END_REF]. Indeed one of the main mineral of limestone is calcite (CaCO 3 ). This crystal is very sensitive to dissolution, particularly with water acidified. It shows an effervescence due to CO 2 formation thanks to the reaction with acid [START_REF] Javey | L'altération des roches et des monuments. Etude documentaire[END_REF] CaCO 3 +2H 3 O + -> Ca 2+ + CO 2 + 3H 2 O It is known that water pH of natural waters varies between 4 and 9 [START_REF] Javey | L'altération des roches et des monuments. Etude documentaire[END_REF]. The reaction between water and calcite shows dissociation with product of reaction calcium ion and carbonate ion CaCO 3 + CO 2 + H 2 O -> Ca 2+ + 2HCO 3

In addition of chemical reactions, there are also physical mechanisms whose modify the rock cohesion in order to open the way of chemical degradation [START_REF] Vásárhelyi | Influence of water content on the strength of rock[END_REF]. Some features of the rock will be modified by the influence of water [8][9]. This paper will focus on the behavior of discontinuities subjected to chemical action of water. Two experiments reproduced in laboratory showed the influence of water degradation on rock discontinuities. That degradation will be accelerated by a decrease of pH water. The first one considers the alteration of a rock bridge, the rock shape and the shear strength evolution. The second analysis presents the dissolution rate and the roughness evolution. Finally we present the in situ block test under investigation.

ALTERATION OF LIMESTONES IN LABORATORY

In order to examine the behavior of discontinuities subject to water alteration several series of experiments are performed. The alteration is performed in laboratory according to type of stone and water flow. This section shows how water may cause discontinuity degradation. The alteration is performed on six different limestone's sample (Table 1) from French quarries. Four of them are oolitic limestones as Pierre de Lens (Lens Stone) from south of France and Pierre de Caen (Caen Stone) from Normandy, Chamesson and Larrys are from Burgundy. The others as the Comblanchien is a micrite with some stylolitic joints and tension cracks and the sixth named Montagnole is a micrite and locally detritic with tension cracks and stylolitic joints. Some of their properties are given in (Table 1).

UCS * (MPa) 26 50

Total porosity (%) 24 14

(

Porosity Hg (%) 17

(5)

15

(5)

3,5

ND 16.1

ND

Mean radius pore (nm) 1900

(5) 1600

(5)

52

(5) 30 [START_REF] Dreybrodt | The kinetics of the reaction CO2 + H20 + H + + HCO3-, as one of the rate limiting steps for the dissolution of calcite in the system H2O-CO2-CaC03[END_REF] 200

ND Capillarity (g/m 2 /s 0,5 ) 143

(2)

60

(2)

10

(2) 0,4 Two experiments are performed. The first experiment is based on quasi-static water, which means that there is "no" flow of the solution on the sample. In that case it is simulated a water which comes into the discontinuity and remains in it at rest. In the second experiment, the water is flowing in order to model a runoff inside the discontinuity during rains fall. In this experiment the flux is one liter of fluid per day. The different types of limestone are submitted to acidic water (water with 0.035µl sulfuric acid (35%)). Thus acidic water is chosen to accelerate the phenomenon of dissolution. This acid is used in order to increase the rate of chemical rock degradation, it is selected to enhance the degradation kinetics of the rock, and also to reproduce the percolation of rainwater in soil and that loads in humic acid to becomes more acidic than rainwater. This kind of water causes damages to the rocks through discontinuities. Water used for the two experiments has the same pH.

Alteration in quasi-static regime

Experimental procedure

This first experiment consists in leaving limestone's samples at rest in a fixed volume of acidic water (Figure 1). The evolution of weight of each samples during cycles of alteration are measured at different times. A cycle is composed of two steps. During the first step all samples are soaked 2 days in a sulfuric acid solution at pH 2 then during the second step samples are dried at 60°C for 2 days (Figure 1). The acidic solution is renewed. Thanks to that change, the alteration is more effective by repetitions of these cycles, in fine, the samples degradation is increased.

The rock samples are cylindrical (diameter: 12 mm, high 100 mm) (Figure 1). A number of 24 sticks samples are used for each type of limestone. We used four types of limestone: Pierre de Caen (Caen Stone), Pierre de Lens (Lens Stone), Larrys and Comblanchien whose differ by their porosity. Limestones' porosity extend to a range from one to several tens percent. Caen Stone and Lens Stone have a high porosity respectively 20% and 17%, whereas the two others have a low one respectively 7% and 1%. After each cycle of degradation the weight sample is measured in order to follow the evolution during the experiment. A picture of each sample is also taken in order to analyze qualitatively the limestone's aspect between each cycle. This first step of analysis is extended to a smaller scale. During a second step (more quantitative), we performed measures by mercury porosimetry intrusion (MIP) in order to follow the evolution of the porous matrix. The analysis is performed at micro-scale within an environmental scanning electron microscope (ESEM).

Analysis

The evolution of each sample weight is measured during the dissolution process. A decrease of weight is measured for each type of limestone. Figure 2 represents normalized weight (weight (t)/weight (0)) versus time of degradation. We noticed linearity with time of the loss of weight for all the type of limestone (Figure 2). This indicates there is a constant rate of dissolution for limestone. In this figure below, we can observe that whatever the giving porosity the rate of decreasing weight is quite constant. However we can observe the higher the porosity, the higher the rate. This difference of rate means that the dissolution process also affects the pores of the matrix although the pore pressure is very small. Moreover the dissolution process has a higher influence on the more porous sample (Caen stone). We observed on the sample with high porosity (Lens stone and Caen Stone) the fluid at a very small pressure rise into them whereas the fluid does not for the low porosity sample. The visual trailing shows a brownish aspect of the sample mainly for all samples but with an exception for Lens stone which conserve its whiteness. The change of color is due to the presence of impurities specially remobilization of iron often turns the sulfuric acid a yellowish brown color.

At the fluid-air interface recognized by the different of coloring, a recession of the surface is observed for each sample of each limestone (Figure 3, Figure 4). This recession suggests a loss of material. The high degradation in this part of the sample is also attributed to the influence of the CO 2 in ambient air. Actually, CO 2 is considered as an important acidifying component of water. The interface is the place where the exchange between ambient air and the water near the surface is the most important, makes it a zone where degradation is improved.

(Zoom) We observe the same high degradation near the interface, for all types of limestone. This information suggests an influence of CO 2 whatever the porosity. In order to evaluate the different states of alteration of the porous matrix evolution inside the sample, mercury intrusion porosimetry (MIP) measures have been done. This method is commonly used to evaluate the total porosity and the pore size distribution of different materials. MIP results are expressed as the total mercury volume intruded (log diffential intrusion) in a sample versus pore size (mean diameter pore).

Measures were carried out after several cycles of degradation. The Figure 5 shows the volume of mercury intrusion versus the pore diameter. We represent on the graph three of the four limestone used for the study (Larrys (7%) Lens Stone (17%) and Caen Stone (20%)). First of all we can observe the mean pore diameter increases with the limestone's porosity. Then for each type of limestone there is an increase of the mean pore diameter between the initial state and 3 days of alteration. The alteration increase pore volume. Low porosity sample have little mean pore diameter whereas high porosity have their mean diameter higher. Matrix of Lens Stone and Caen Stone samples are modified after the first cycle. Indeed an aperture of the mean diameter pore is measured after the first cycle but the mean diameter doesn't increase for the other cycles.

Nevertheless on Lens Stone, the volume of mercury increases between 30 days and 60 days of alteration. This reveals an increase of pores with same size. Larrys' samples show an increase of the mean diameter pore all along the state of degradation (Figure 5). We observe sample of limestone named Lens Stone at micro-scale with an environmental scanning electron microscope (ESEM) (Figure 6). These three pictures show three states of limestone degradation's at different time, the first represents non-altered sample, the second at 10 days of alteration and the last one after 60 days of alteration. The limestone observed is Lens Stone, it is composed of ooids which are spherical grains composed of concentric layers. We observe along the period of alteration a digging around the ooids, which indicates dissolution of calcite. This kind of feature shows the non-homogeneous fact of dissolution either at space scale.

Shear strength analysis

In order to the observation and measurement of chemical degradation an evaluation of the mechanical response of the sample submitted to a shear loading is performed. The measurements are made with a uniaxial cell in order to evaluate the influence of alteration on the mechanical properties of limestone samples. We used cylindrical samples (Figure 7) as samples in the alteration in quasi-static experiment. Each sample is positioned in a shaped granite base (Figure 7) and put in the uniaxial cell (Figure 8). The loading (kinematically controlled) applied for the shear on the weathered sample is measured and it is called: mechanical strength. Each sample is sheared in order to compare them in function of their porosity in one case and their weathering degree in another case. Shear strength measurements are performed on different non-altered limestone samples on a range of porosity 2% from 35 % (Figure 9). The different curves show an increase in the shear stress versus tangential displacement until a peak is reach and then a decrease followed by an increase to a maximum and then failed. The first stress drop is associated to micro-cracks of the sample and the final stress drop is related to the failure of the sample. All limestone samples seem to have the same behavior.

We noticed the smaller the porosity the stronger slope, this implies that the slope of curves is hardening with the decreasing porosity. It suggests porosity affects the mechanical response of samples. The following experiment focused on non-altered Larrys (7%) sample. The Figure 10 shows the evolution of shear stress versus tangential displacement. The curve shows an increase to a peak at 16 MPa and a decrease to 12 MPa. This first part of the curve with the little peak, indicate a first micro-crack in the limestone sample. After that the shear stress rises to a maximum at 21 MPa and after the shear stress collapse. This part of the curve represents the rupture of the sample. In order to show the alteration influence on limestone mechanical behavior, we performed a test also on weathered limestone. First limestone samples are undergoing a flow of acidic water (pH 2) and others with non aggressive water (pH 7) during several weeks. Two samples of weathered limestone are used for mechanical test.

The Figure 11 shows the evolution of Larrys' samples mean failure peak during alteration period for two different pH degree of water (pH 2 and neutral water pH ( 7)). Which is surprising in that two curves show an increase of mean peak rupture to a maximum and a decrease. They have the same shape except the maximum for the acid water (pH 2) appears before that for rain water (pH 7). The difference between the curves points out the acceleration process of dissolution. The comparison between two different pH degrees of water shows how much the influence of pH water is relevant during the alteration process. Furthermore the slope before the peak for the pH 2 curve is stronger than the slope for pH 7.

Rock bridge alteration in the case of flowing water

Experimental procedure

It exists [11] [12], part in the rock mass which is connected to the block ready to fall. These parts are called rock-bridge and they constitute the block stability. In this experiment, the shape of the sample and the water flow are different from the first one. Limestone's blocks are sawn so as to leave a continuous part, called rock bridge. The rectangular geometry of the bridge is connected to the surface of the block. The above part is taken off. The experiment is realized in open system where CO 2 of the atmosphere can influence the interaction between the acidic solution and the surface of the block because CO 2 is known to acidify water. The acidic solution is stored in tins and the solution flows via a drip system. The water flow is about one liter per day (Figure 12). The experiment is performed in an open system, which means the sample in contact with ambient air. The interaction with the CO 2 of the ambient air increases the phenomenon. The flow of water causes a material dissolution and destructuration (Figure 14). Fine particles are observed in the water of the recovery basin as observed by Kupper [START_REF] Kupper | Les vitesses d'érosion du calcaire dans l'eau: études des phénomènes actuels[END_REF]. The picture shows also an asymmetric flow of the solution. Indeed we observed during the period of runoff that water followed different paths. It is obvious that water prefers the easier path to dissolve crystals less competent that the rest of the rock sample.

Analysis

Each type of limestone received 12 liters of acid solution flowed for a period of 17 days. After 8, 11 and 17 days, a geometric analysis by laser is performed. Many profiles done in order to show the influence of the alteration surface and to establish the decreasing of the rock bridge perimeter. This decreasing of the perimeter is observed during the water flowing for all samples (Figure 15). In addition we noticed an intact part of the rock bridge below. The recession of the rock-bridge perimeter may imply a decrease of the mechanical resistance of the rock bridge. We performed other laser profiles (Figure 16). Profile P1 was taken above the bridge then profile P2 just under the bridge, and the third P3 was measured on the below far from the bridge (Figure 16). These profiles were made at initial state, after 8 days of runoff, 11 days and at final state (17 days). Profile P1 is made near the place where the drop fell. It shows a digging which increases significantly over time. Profiles under the bridge show two diggings (P2) then three (P3). These paths followed small fractures. These ones indicates preferential path of water circulation (P2 and P3).

Localization and geometry of the site

The site is located in Montagnole near Chambery (73) in France. The block is a triangle shaped dihedral with fractures and it is apparently ready to fall (Figure 17). This work aims to evaluate the potential of Ground Penetrating Radar (GPR) measurements to find and estimate the quantity of rock bridges into a block with a discontinuity.

Methods

GPR data were acquired using high frequency (1600 MHz) to estimate the presence of rock bridges (continuity in the rock mass). This high frequency is adapted to detect interfaces such as rock-air-rock in two meters thick. The air tends to disperse the signal, so we can deduce the rock bridge's presence or not. In this paper, GPR measurements consisted in profiles acquired from the block (Figure 18), in order to make a grid of the block and identify the presence of rock bridges.

Results and discussion

The first observation in situ reveals many discontinuities; the major ones represented in black on Figure 17 are those where we suppose there are rock bridges to maintain the stability of the rock mass. The white ones appear to be just on the surface but they are important for the surface degradation. The GPR reveals the possibility to identify the surface discontinuity (Figure 19). As a matter of fact radar-gram present some weirdness, these ones are the proofs of different environment as air or rock mass. These are analyzed and after a treatment, a 3D view of the discontinuity surface could be made as Figure 20 shows.

CONCLUSION

These results allow us to highlight some points on the rock bridges or more generally rock behavior under dissolution caused by water flow.

The experiments in laboratory give us interesting information on the surface, as we showed for the experiment in static regime and also on the behavior of the porous matrix submitted to dissolution. Concerning the loss of weight, we noted that it is related to the available amount of oxonium ion in the solution to dissolve calcite. In this experiment the solution is saturated in 2 days whereas in the experiment in flowing regime the solution is never saturated due the dynamic flow of water.

The shear strength of different rock bridges unaltered limestone indicates that it is a link between physical characteristics of rocks. This is the value of Young's modulus, the average compressive strength and the average force measured at the rupture. These two experiments confirmed this relevance of water in alteration process. The experiment in flowing regime is more efficient than static one. The 60 days of alteration for the first experiment in static regime showed less important damages whereas the 17 days of alteration for the flow regime, the damages are impressive.

We show alteration on the surface, but also inside the sample, which indicate relation between them. During the flowing regime experiment we observed that the reactive water use preferential path no matter what limestone used.

For instant the first results with the GPR method reveal it to be an important tool in characterization of rock bridges and also the visualization of surface discontinuity. Our results need to be correlated with other geophysics methods. The laboratory analysis will be extended to the in situ scale.

Figure 1 :

 1 Figure 1: Schematic representation of the two steps of the experiment "alteration in quasi-static regime" which is composed of one cycle of weathering (left). Picture of the first phase of the experiment quasi-static regime limestone samples sticks are soaking in the acidic water (right).

  1cm

Figure 2 :

 2 Figure 2: Evolution of sample weight during the experiment Alteration in static regime.

Figure 3 :

 3 Figure 3: Pictures of Caen Stone samples: Initial state (E0), after 10 days (E3), 20 days (E6), 30 days (E9), 40 days (E12), 50 days (E15) and 60 days (E17).

Figure 4 :

 4 Figure 4: Pictures of Larrys samples during alteration: Initial state (E0), after 10 days (E3), 20 days (E6), 30 days (E9), 40 days (E12), 50 days (E15) and 60 days (E17).

Figure 5 :

 5 Figure 5: Volume of mercury intrusion versus pore size diameter for samples of Lens Stone, Caen Stone and Larrys and for several state of weathering

Figure 6 :

 6 Figure 6: ESEM pictures from sample of Lens Stone at initial state (left) after 10 days (middle) and after 60 days (right). The dash line circles emphasize the gap between oolites; this gap was created during the alteration.

Figure 7 :

 7 Figure 7: Shape of the granite base with dimension (left) the sample of limestone (middle) are inserted into the granite for the test (right) (after Druon)

Figure 8 :

 8 Figure 8: Picture of the uniaxial cell used for the experiment "alteration quasi-static" (left). Sample in the granite base (middle) and the broken sample after the test (right) (after Druon).

Figure 9 :

 9 Figure 9: Shear stress versus tangential displacement for different limestone's porosity (modified from Druon, 2011 [10] ).

Figure 10 :

 10 Figure 10: Evolution of shear stress versus tangential displacement for the limestone Larrys (modified from Druon, 2011).

Figure 11 :

 11 Figure 11: Evolution of the mean failure peak after period of alteration for Larrys limestones. A comparison between different water, acid (pH 2) and neutral (pH7). Dashes (above and below) represent respectively the standard deviations max and min. The small dashes for pH2, and the large dashes for pH 7(modified from Druon, 2011).

Figure 12 :

 12 Figure 12: Schemes of the shape limestone samples for the experiment Alteration in the case of "flowing water", the acid solution is in drip system, the flow is one liter of solution per day, and run along the discontinuity and around the rock bridge (left). Picture of the runoff along the discontinuity and around the rock-bridge (right).

Figure 13

 13 Figure 13 depicts the time evolution of water pH during two different experiments. A continuous flow on the limestone surface; maintains pH stability around 2 whereas limestone's sample immersion in an acidic solution buffers it at pH 7. This graph shows how time-contact between rock and solution is relevant.

Figure 13 :

 13 Figure 13: Representation of the evolution of pH as a function of time, the dotted curve with open symbols shows the evolution in the case of an alteration in static water regime: limestones (CaCO3) are soaking in an acidic solution (H 2 O; H 2 SO 4 ). The solid curve and filled symbols shows the evolution in the case of an acidic solution flows along the limestone.

Figure 14 :

 14 Figure 14: Picture of Montagnole limestone (CaCO 3 ) sample surface degradation by acidic solution (H 2 O; H2SO4)

Figure 15 :

 15 Figure 15: Comparison between the different states of alteration for each limestone of their rock bridges' perimeter measured with laser.

Figure 16 :

 16 Figure 16: Montagnole sample after acidic water runoff, line P1, P2, P3 represent the place we take the profile by laser (left). These graphs represent the 3 profiles done by laser, the different curves show the evolution of the "topography" of the sample surface during alteration period (right)

Figure 17 :

 17 Figure 17: View of the rock mass, it presents a dislocated area on the left, and major discontinuities are in black and superficial are in white.

Figure 18 :

 18 Figure 18: Depict of the North Face of the block in Chambery (73). The black lines reveal the different discontinuities observed. The black horizontal lines identified each radar profile, profile 3 to 9 from the bottom to the top.

Figure 19 :

 19 Figure 19: Radar gram acquired after profiles from the block, profile 3 to 6, the black lines represented the weirdness observed and used for the 3D representation.

Figure 20

 20 Figure20show horizontal dots, which represented the separation between rocks and void i.e. the discontinuity inside the block.

Figure 20 :

 20 Figure 20: 3D view of the surface discontinuity after analyze and treatment of the data.

Table 1 : Some properties and features of the different limestones used in the experiments.

 1

	Name		Caen	Lens Stone Larrys Comblanchien Chamesson Montagnole
			Stone
	Density (kg/m	3 )	2050


Cyrille Balland
email: cyrille.balland@ineris.fr

Jacques Morel
email: jacques.morel@andra.frmountaka.souley@ineris.fr

Mountaka Souley

Ultrasonic sounding and monitoring of the excavation damaged zone in relation with drift support

Keywords: damaged zone, ultrasonic, tomography, velocity survey

INTRODUCTION

Tunnel excavation in Callovo-Oxfordian argillites produces a damaged area called EDZ (Excavation Damage Zone) which depends primarily on the physical properties and the mechanical behaviour of the host rock and the initial stress field. The nature of this area has been observed many times in underground laboratories [START_REF] Maxwell | Seismic imaging of rock mass responses to excavation[END_REF]Young 1996, Balland et al. 2009). The goal is now to study its evolution over the intermediate term and more specifically about the ability of the rock to return or not towards its initial state depending on the selected support. In this context, Andra carried out the OHZ experiment (Observation of the hydromechanical behavior and EDZ), which aims in particular to monitor the evolution of two galleries oriented in the direction of the major horizontal stress, one equipped with a soft design support (bolts, shotcrete and yieldable wedges) in the GCS drift and the other with a support of rigid design for GCR drift, same as the GCS support but with a concrete ring (Figure 1). The task of INERIS in this experiment was to characterize the damage around these two galleries by ultrasonic tomography sounding and then to monitor its evolution by analyzing the propagation of ultrasonic waves in the rock. Measurements were performed in an identical configuration for both drifts. The ultimate goal was to compare the evolution of the EDZ in these two cases. Sensors chosen are best suited to clays encountered. P-wave sensors of R6 type (Mistras) have a 60 kHz bandwidth centred

Rigid Designed Gallery

Soft Designed Gallery

σ h =12.4MPa σ H =16MPa σ v =12.7MPa

SUMMARY

Under high in situ stresses, the excavation of underground openings generally causes the creation of a disturbed (EdZ: Excavation disturbed Zone) and/or damaged (EDZ: Excavation Damaged Zone) zone, resulting from the initiation and growth of cracks and fractures and from the pre-existing stress redistribution.

The EdZ or EDZ changes the mechanical and hydromechanical properties which in return, constitute a potential risk for the performance of the geologic and/or engineered structures in the context of waste underground storage. Ultrasonic experiments have been implemented to characterize the EDZ extension around drifts and its evolution in time according to the structural support type (soft or rigid) and the environmental conditions. Those studies consist of two experimental components: (1) the prior auscultation of the floor and sidewalls of the gallery by ultrasonic transmission tomography, (2) the monitoring of the time-dependant evolution of EDZ and the analysis of measurable changes in the propagation of ultrasonic waves in the medium term. A code for computing these continuous in situ velocity measurements into the elastic has been developed. Then, the five dynamic elastic constants for the assumed transverse isotropic character of the rock are derived as a function of time and the distance from the drift wall. Performed a few months after the excavation of the galleries, the tomography shows that ultrasonic velocities are higher in the orthoradial direction (both in the concrete support and rock). This velocity field highlights clearly the damaged zone and induced stress shift.

response chosen as compromise between the Fresnel zone (corresponding to the influence zone of the wave train) and attenuation of high frequencies in argillites. For S-waves, the sensor of SH6 type has been specifically developed to produce a pulse in the same P-wave frequency scale. The acquisition system [START_REF] Pettitt | Development of the tools and interpretation techniques for ultrasonic surveys to monitor the rock barrier around radioactive waste packages in geological repositories: Omnibus Final[END_REF]) consists of three main components:

• the data acquisition unit trigering and recording on ;

• the sensor interface unit, which manages the multiplexing and sensors alternatively used as emitters or receivers;

The acquisition system enables particularly the transmission and recording of the waves in the active mode.

RESULTS

Velocity field anisotropy

Figure 3 and Figure 4 present four stereonets computed from all velocity measurements for the three tomography planes in the GCR drift sidewall. Each stereonet corresponds to areas of interest selected the tomographic images: the ring of cast concrete, shotcrete, damaged rock (EDZ) and finally the undisturbed rock.

In the undisturbed rock (Figure 4), the velocity field shows mainly transverse isotropy as expected for the argillites [START_REF] Ghorbani | Effects of desiccation on the elastic wave velocities of clay-rocks[END_REF]), the axis of isotropy (or main direction of the lowest P-wave velocity) does not seem vertical. It rather dips northeast-east in the direction of the minor horizontal stress (N65°). Stereonet also shows anisotropy in the horizontal plane with the main directions of velocity close to those of the stress field, what might emphasize the impact of the stress field. The same anisotropy scheme, but amplified, is observed in the EDZ. In that case, the velocity field is mainly disturbed by preferential fracturing at gallery side along the axis of the major horizontal stress, as was observed on the boreholes cores. It might also be influenced by stress redistribution around the drift. The velocity stereogram in shotcrete (Figure 3) is rather heterogeneous and therefore does not show well defined anisotropy. On the contrary, the concrete ring shows a marked anisotropy in the direction of the gallery itself i.e. in the direction of the major horizontal stress. The highest velocities are measured on the tangential raypaths to the gallery. The question is therefore whether this velocity anisotropy of the concrete ring is intrinsic (structure, installation, sealing ...) or due to mechanical loading.

Tomography

The acquisition of the tomographic data (Figure 5) has been carried out both through the concrete (65 cm thick) and the rock. P-wave velocities, calculated from the arrival time, vary from 2000 to 5000 m/s. However, these velocities are well differentiated between concrete and rock. In argillites, they vary in a range comparable to previous experiments (2000 to 3500 m/s). The concrete itself is differentiated by the ultrasonic velocities ranging from 3500 m/s and 4500 m/s in the shotcrete and around 5000 m/s in the concrete ring. These first images of the velocity field clearly show the contrast between the rock and the two types of concrete. The tomographic images were then corrected for anisotropy calculated in each plan except for the horizontal plane where no natural anisotropy is expected in the argillites. Anisotropy correction takes into account only the anisotropy found in the rock. This correction was also applied to the concrete (the device does not allow to discriminate the anisotropy in different areas). The velocity scale was adjusted to reflect only the velocity range observed in the rocks (1600-3500 m/s). The image is saturated in concrete areas because the velocity is over this scale. The purpose of these images is mainly to identify the velocity variations in the rock. The 3 tomographic panels show lower velocities in the rock near the shotcrete. In the first 30 cm of rock (0.6 m to 0.9 m depth from the face of the gallery), the velocities are very low (less than 2000 m/s) and then gradually increase up to 1.2 m deep (0,6 m in the rock). This area may be associated with the EDZ generated during excavation of the gallery. Another area between 1.6 and 1.9 m (1 to 1,3 m in the rock) deep with low velocities can also be identified. This anomaly is especially well marked in the horizontal plane between 1.7 and 1.8 m depth. This anomaly can be associated with an isolated crack observed before the experiment in three horizontal boreholes.

Inversion of wave velocities data in GCR drift

A code for the inversion of these continuous in situ measurements of five elastic wave velocities (i.e. V P (0°), V Sh (0°), V P (90°), V Sv () and V P (), where  is off-axis) has been developed with the help of Mathematica software. Then, the five dynamic elastic constants for the transverse isotropic character of the argillites (assumed in this paper) are derived as a function of time and the distance from the gallery wall. From these elastic constants, some indicators of damage rate were proposed with respect to the space and the time. Finally, in addition to the experimental data analyses, synthetic data (generated from the given elastic constants and velocities derived from the theoretical formulation) as well as data from literature (measurements of ultrasonic velocities in the laboratory on samples) allowed us to successfully evaluate the consistency of the results obtained with this program. Indeed, the measures of wave velocities performed on samples (resulting from cores drilled in the vicinity of the experiment area) in laboratory showed that the average values of modules perpendicular (E1) and parallel (E2) to the stratification and the shear modulus in the anisotropic plane (G12) are respectively 7, 10 and 4.5 GPa. The inversion of the first measures of velocities recorded in situ give the values of 4, 8 and 3.8 GPa for E1, E2 and G12 near the wall (at the distance of 0,5 m from the gallery wall). They are 7, 12 and 4.4 GPa at 3 m of the wall (in the undamaged domain). The next stage will consist in analyzing all the in situ measures with respect to the space around the gallery and the time.

CONCLUSIONS

Ultrasonic tomography measurements were performed successfully in argillites around a rigid support drift. Tomographic measurements highlight the damaged zone of the rock on gallery side. The lower velocities may be due to fracturing or preferential opening of fractures parallel to the gallery, in agreement with visual observations on drilling core. Finally P and S wave velocities was traced back to the stiffness matrix versus depths, in the rock. However impact of the anisotropy in stiffness computation must be verified because the method is based on the hypothesis that the rock presents a purely transverse anisotropy and we highlight that is not entirely true.

More generally, ultrasonic tools and methods associated with the inversion of the velocity field is an interesting approach to investigate the performance of the rock face and the support of the deep mining structures (drift or shaft). It can provide both diagnosis and monitoring of the EDZ. It can then help to detect unexpected breakage or damage of the structure support and then prevent associated risks

Figure 1 :

 1 Figure 1: Plan of the underground laboratory of Meuse / Haute-Marne and experimental galleries DEVICE AND METHOD Two devices were selected (Figure 2): • a triplet of facing horizontal drilling in the gallery side, extending to 3 m depth, with two holes 30 cm apart (about 25 cm between sensors) located in the same facies (OHZ 1841, 1842 and 1843); • a pair of vertical boreholes in the middle of the floor of the gallery (OHZ 1844 and 1845).

Figure 2 :

 2 Figure 2: Ultrasonic device installed around the galleries.

  Figure 3: Stereonet in support.

Figure 4 :

 4 Figure 4: Stereonet in rock.

Figure 5 :

 5 Figure 5: Tomographic panels around the rigid supported gallery with two different Vp scales.