G Sbierczik

J P Josien

W Kammer

J P Piguet

OPTIMISED LAYOUT AND ROADWAY SUPPORT PLANNING WITH INTEGRATED INTELLIGENT SOFTWARE S. Kouniali (A) and

Experience with knowledge-based systems for Layout planning and roadway support dimensioning is on hand in European coal mining since 1985. The systems SOUT (support choice and dimensioning, 1989), SOUT 2, PLANANK (planning of bolt-support), Exos (layout planning diagnosis, 1994), Sout 3 (1995) have been developed in close cooperation by CdF 1 , INERIS 2 , EMN 3 (France) and RAG 4 , DMT 5 , TH -Aachen 6 (Germany); ISLSP (Integrated Software for Layout and support planning) development is in progress (completion scheduled for July 1996). This new software technology in combination with conventional programming systems, numerical models and existing databases turned out to be suited for setting-up an intelligent decision aid for layout and roadway support planning. The system enhances reliability of planning and optimises the safety-to-cost ratio for : • deformation forecast for roadways in seam and surrounding rocks, consideration of the general position of the roadway in the rock mass (zones of increased pressure, position of operating and mined panels).

• support dimensioning

• yielding arches, rigid arches, porch sets, rigid rings, yielding rings and bolting / shotcreting for drifts • yielding arches, rigid arches and porch sets for roadways in seam • bolt support for gateroads (assessment of exclusion criteria and calculation of the bolting pattern) • bolting of face-end zones (feasibility and safety assessment; stability guarantee).

INTRODUCTION

With declining ore grades, exhaustion of near surface reserves and rising labour costs the underground coal and metal mining industry in the European Union requires efficient planning tools that will improve its performance since mechanisation has almost approached 100% so that machine development will only give marginal improvements in performance. Major improvements can only be achieved by the re-development of the whole production process through careful and precise mine planning that will utilise simulation and production system's analysis. Mine planning involves a variety of disciplines, phases and operations and a multitude of parameters to be considered (e.g. for designing a support system at least 10 parameters must be evaluated that include rock mass structure, rock strength properties, stress analysis, geometry of excavation, rock-support interaction analysis, mine life etc.). Errors in planning are frequent. Within the German coal mining activities they are estimated to be at least 10% in the decisions taken. As a consequence to that mine workings, as an example, are located in highly stressed zones, which means subsequent expensive repairs and maintenance. Furthermore planning errors cause production losses, affect safety and might have environmental impacts by subsidence. These supplementary costs are difficult to quantify. Accordingly, INERIS and DMT co-operate closely with TU-Aachen and the mining groups CdF and RAG since 1985 on improvements of the planning process in order to avoid the above-mentioned planning errors and resulting costs to a far-going extent. For fielding and supporting the planning process improved in the intended way, software programs are being developed since the sixties. The more and more widespread use of conventional calculation programs, numerical models, data bases, and programs for 3dimensional representation of the deposits and the mining infrastructures allow, since the late seventies, significant steps towards improved planning. The planning process was accelerated and made cheaper. These programs however do not allow consideration being given to the specific know-how and experience of the specialists which is of foremost importance for mine. Therefore, INERIS and DMT work, since 1985, on knowledge based systems for mining. This technique -an artificial intelligence (AI) technique -has been chosen because, in contrast to conventional data-processing algorithms, such a system makes it possible: • to formalise knowledge and experience, • to integrate the know-how of the experts and to guarantee that it is coherently implemented, • to facilitate the updating of this know-how, which is absolutely essential in a context which evolves as the deposit is worked and as technology develops.

Several knowledge-based systems : SOUT (1989), SOUT2, PLANANK, EXOS (1994) and SOUT 3 (1995) have been developed in close cooperation by CdF, INERIS, EMN (France) and RAG, DMT, TH-Aachen (Germany).

Since early 1996, these knowledge based systems are integrated with conventional calculation programs, programs for 3-dimensional visualisation, databases, and numerical models for rock pressure calculation within an intelligent system for layout and roadway support planning.

Industrial use of this integrated software system is supposed to reduce cost substantially, about all in roadway maintenance, support, and by avoiding production standstill.

SOFTWARE SYSTEMS FOR MINE PLANNING 1 Conventional Programs

A variety of programs for general calculation are a standard use in mine planning since a number of years. Based on longyears underground observation, DMT developed e.g. programs for roadway deformation forecasts. The experience gained in this way and the following treatment of the statistical methods of variance and regression analysis led to empirical formulae describing the roadway deformation as a function of geological (e.g. seam thickness, rock strength) and mining technical parameters ( e.g. roadside packing, support backfilling, planning of the roadheading relative to the face advance) and the strata pressure. Furthermore we should mention here those programs which allow support planning and dimensioning: standing support, bolt support in gates, bolting in baseroads (coalface is starting from base roads) and bolting for securing face ends.

2 Databases for the 3-D representation of the deposit and the mine

In the late sixties, RAG started to gather technical data (in particular in the field of mine surveying) for certain evaluation purposes. These approaches are integrated in the DlGMAP system. This three-dimensional interactive graphic system allows managing and processing the data on existing and planned mine workings and on the deposit. The main activities of that system are mining-specific mapping and deposit evaluation for the survey side of layout planning. Since 1993 that system has been introduced by 100% with RAG.

Several other commercially available databases exists (e.g. SuRPAC); CdF is using a company specific 3-D database under AutoCAD.

Numerical models

In the context of this application, numerical models are to be understood as representations of the field under investigation -including the mine working-which are in conformity with the dimension and the limiting conditions. In all points of these models the differential equations (or the partial derivations) which govern the physical phenomena involved are solved. This solution is arrived at numerically, by means of various approximation methods mainly associated with appropriate discretisation on the field, a discretisation of certain calculated functions, and to ponominal interpolations of subfields.

A priori this approach is based on analysis and formalisation of the physical reality of the involved phenomena. This approach allows at the present and to certain point to integrate the most complicated aspects of these phenomena as well as their interdependencies, and is trying to be an exact representation of physical reality. It is possible today, e.g. by finite element methods, to investigate mechanical, elastic, plastic and viscous behaviour, flows, thermal effects, and how to consider anisotropy and heterogeneity.

The capacity of the computer hardware allows, at present, to take on problems applying thousands degrees of liberty either two or three dimensional.

In this field, DMT developed e.g. the software GEDRU, « Gebirgsdruckvorausberechnung : rock pressure forecast » (Figure 1). It is a planning tool which helps to head mineworkings in low-pressure zones of the mine in view of keeping deformation at a minimum. The calculations are based on three-dimensional finite-differences programs which allow only for vertical shifting. The calculations covers a block with surface area of approximately one square kilometre and a vertical range of 500 meters. This zone may contain up to 10 seams each of them mined or earmarked for mining, over large areas. Coal and rock are supposed to be elastic. In the zones of mining, piecewise, linear correlations between pressure and shifting are assumed (variable by the user according to the kind of stowage practised).

Figure 1 : Rock pressure forecast by the software GEDRU

Introduction

Before 1983, the task of producing an expert system generally consisted in asking an expert for his expertise and in coding it immediately; every rule represented the reply of the expert to a problem in a given context. The knowledge and the know-how were closely intermixed. This led to systems which were certainly functional but whose development was more or less anarchic and particularly "fragile" and difficult to maintain. Since 1983, one has talked of second generation knowledge based systems to describe these systems where knowledge is encoded in a declarative form, regardless of the form of use. Models of certain areas are drawn up (classification, design, diagnosis), making it possible to define typical methods of knowledge acquisition for these areas.

The declarations provided by the system so designed are much more than a simple reproduction of the rules. They are also more durable, because they tend essentially to be based on the characteristic principles of an area rather than on point by point heuristics.

It can also be said that, in order to acquire the knowledge of areas of application, one tends to turn to specialists rather than experts (the classic difference between the specialist and the expert being that the former demonstrates his knowledge publicly and through books and publications, whereas the expert's knowledge is essentially of a private nature and is based on personal experience difficult to impart". Since the difficulty in knowledge acquisition is more a problem of interpretation and modelling than the simple transfer to the computer system, collaboration with the specialists in the relevant discipline simplifies this phase. This procedure, in conjunction with systematic use of object-oriented methods enable one today to design systems which use modules developed for other applications, even if they were originally intended for areas bearing little similarity. The intention is even to take over knowledge bases.. With new architecture it is possible to address the problem of heterogeneous specialist knowledge arising from the difference in the theoretical solutions proposed for a given case, and also the problem of the heterogeneous specialist knowledge arising quite simply as the result of the difference between the experts asked. Reference is made then to multi-expert systems.

The graphic instruments normally available for most computer systems have simplified the creation of user-friendly interfaces to a great degree.

In the final analysis the systems do not stand alone: they can communicate with standard applications (calculation and spreadsheet programmes) more suitable for numerical calculations and with the companies databases. This integration in the "classic" computer system of the user simplifies the introduction of systems in practical terms. The system is thus more practice based and easier to handle.

The "art of mine"

The shape and management of a mining concern is more an "art" than a process or configuration of processes. The areas of knowledge which are of relevance have many different layers. The "art of mine" is a promising area of use for artificial intelligence methods, and especially methods which enable one to develop knowledge-based systems.

Where classic optimisation procedures only provide partial solutions, the expert systems help integrate the wide variety of areas of knowledge addressed to solve the problems arising. Designed as intelligent systems to help in decision-making, they perform an auxiliary function for mine operators without "replacing" specialists, as was attempted in the early 80s.

In fact, the development of research into expert systems such as we have referred to above was in fact only belatedly taken into consideration in our SOUT project; the obvious reason is the fact that the development was the work of engineers and researchers involved mainly with the geotechnics of mines and rather than the work of artificial intelligence professionals.

HISTORY OF THE PROJECT « KNOWLEDGE BASED SYSTEMS FOR MINING »

The « knowledge-based systems for mining » project was started in 1985 and it may be said that, in terms of artificial intelligence and its applications, it went through the following three periods :

1 The beginnings: the myth of the artificial expert (from 1985 to 1988) In 1985, at the time of the great epoch of artificial intelligence, and expert systems in particular, Charbonnages de France (CdF, the French coal board), decided to commission CERCHAR 7 to devise and implement an expert system to help with the choice of mining support structures with the idea, quite widespread at the time, of "replacing" the experts on supporting structures. This is how the SOUT project was born [Waterman 86].

2 The disappointment: Analysis of the failure of the first phase (1989 to 1992) At the end of the 'eighties it came to be noticed that the expert systems were having difficulty keeping their main promise ("replacing the experts"), and a general feeling of disappointment set in. The SOUT project did not escape this. Apart from a cruel lack of user-friendliness and a lack of integration (no direct interfacing with existing databases, little direct access to analytical or numerical computation programs) it was quickly observed that there were few chances that SOUT would "replace" the support expert. The SOUT 0.1 system remained in this state until 1992.

3 The reality: An Intelligent Decision-Supporting System (from 1992)

Since the beginning of the 'nineties, we have witnessed a new development of artificial intelligence and a redefinition of the role of expert systems; they are now regarded as help and support systems rather than systems designed to replace the experts. A collaboration has been set up between INERIS, and our colleagues in the German coalmining industry: the DeutscheMontanTechnologie (DMT, German mining technology, DMT). This collaboration, and the research and engineering works carried out since 1992 are described in detail in the section which follows.

EVOLUTION OF THE PROJECT SINCE 1992

Version 0.1 of SOUT, completed in 1988, has unfortunately continued to be little used and little effort has been devoted to maintaining it. The new interest since July 1992, in expert systems in the general mining field, and in SOUT in particular, has enabled us a) to keep an up-to-date eye on the technology in the field of expert systems and b) to improve the dataprocessing implementation of SOUT considerably. This has enabled us to "professionalise" the SOUT 0.1 experimental prototype produced for an university doctoral thesis [Baroudi 88] and come out with version SOUT 2.0, which represents genuine robust, ergonomic and "ready-to-use" software.

At the same time the systems Exos (TU-Aachen and DMT) and PLANANK (INERIS and DMT) were developed for allowing a diagnostic of layout and roadway planning and for the dimensioning of standing and / or bolting support.

1 Computer re-programming of the expertise Version 0.1 of SOUT was intended mainly to demonstrate the value and feasibility of an expert system in the mining industry.

Although this SOUT version 0.1 contained excellent expertise in the fields of geotechnics and mining operations, it did not take full advantage of the new methodologies of expert systems.

Nor did the data-processing implementation meet the current requirements regarding ease of use and robustness. The interfaces were not very user-friendly; incorrect use of the prototype, or the introduction of unplanned data caused the program to "crash".

In SOUT version 2.0 [Thoraval & al 94], [Sbierczik & al 95] we placed the emphasis on programming with NexpertOject® 8 > using the accepted practices in artificial intelligence, while taking into account of recent research results in this field and incorporating the expertise of SOUT 0.1.

Finally, with Toolbook® 9 we have created true user-friendly graphical interfaces, taking maximum advantage of the possibilities offered by the mouse, scrolling menus, graphic screens, push buttons etc. (cf. Figure 2). A particular effort has been made during the development of version 2.0 of SoUT to illustrate the argument interactively and to explain and gives reasons for the results of the system (Figure 3). Users in fact only have confidence in the results if they can obtain proof of them. This is very important in order that the tool becomes accepted and, above all, used. Another asset is the fact that the product can be used for training purposes. Thus by pressing, for example, the "?" button of Figure 2, we can be given reasons for SOUT'S conclusion (cf. Figure 3). In 1994, DMT decided, in close collaboration with INERIS, to create the expert system: PLANANK (Planning of an Anchor Support system, Figure 4). The ultimate purpose of the project is to incorporate, within a single system, all the knowledge available to the German coal industry on bolting of mining galleries with a view to producing a complete software package dealing with all aspects of bolting, and capable of being directly used by engineers.

Acquisition des caracteristiques du terrain

In accordance with the above objectives, this system contains the knowledge of German experts on the subject of the bolting of gate roads in mines. This knowledge proved to be complementary and not contradictory to that contained in SouT. Furthermore, the PLANANK 0.1 system has been especially adapted for German conditions :

• it takes account of the special geological features of German deposits (thin seams, highly stratified formations...), • it directly integrates specific computation programs produced by DMT for the calculation of the length and density of the bolts, • and it meets the statutory German requirements for anchor bolting systems. The main objective of the knowledge based system Exos is to realise a diagnostic of layout planning and dimensioning of roadways.

The diagnostic of gateroads start with the criterion « position within the strata ». It is checked whether the parameters which strongly influence the strata pressure effects are considered (e.g. strength and composition of the associated strata). Subsequently, working boundaries and pillar edges in the roof as well as faults are investigated.

Under the criterion « influence of working faces » the strata mechanical influence of working faces on the roadway is analysed with consideration being given to earlier and synchronous mining in other seams and working of both seams alongside the roadway. Subsequently the controllability of the roadway is investigated (the convergence to be expected is assessed, convergence-reducing measures are proposed).

Eventually, the intended support is examined (e.g. the support techniques: rigid arch, yielding arch, and porch-set support). The diagnosis of base roads is analogous to gate roads with the main difference in the influence of the working faces in the same seam. As a main difference between roadways in seam and drifts there is the criterion position within the strata. For support dimensioning, the support systems yielding ring support and rigid ring support are investigated in addition.

A diagnosis report, including, if necessary, several recommendations to improve the examined layout planning situation, constitutes the overall result.

5 The SoUT 3 Franco-German multi-experts system for roadway support planing Based on the SOUT 2.0 expert system and the Planank 0.1 expert system, an expert system for the planning of bolting supports for a haulage road [Kouniali 95], a Franco-German multiexpert system for the calculation of bolting supports in mining galleries was developed.

As was stated above (cf. § 1.5), the multi-expert approach makes it possible to model the knowledge and know-how of several experts within a knowledge base containing the rules of reasoning used by specialist engineers in the field to define and calculate the bolting scheme in question. Such a system made it possible to integrate the complementary knowledge and experience of several proven specialists and to provide a quite unique tool for the training of engineers of different disciplines (cf. Figure 5).

The objective of this multi-expert system has been to integrate the knowledge and experience of French and German experts in the bolting field in order to be able:

• to store the knowledge and know-how of these specialists for their own benefit and that of others, • to provide continuous training of engineers and foremen within CdF, RAG and DMT, • to have a tool to aid decision-making in the absence or temporary unavailability of human experts.

-Conclusions : Longueurs - 1 Definition ISLSP (Integrated Software for Layout and Support Planning) is an "intelligent" decision support system (IDSS) for layout planning and roadway support dimensioning in underground mining with the principal purpose of optimising the safety-to-costs ratio.

The system comprises five independent software components developed on the basis of various software technologies. These software components co-operate : e) The data processed in this way (layout planning, mine infrastructure, deposit, strata pressure and roadway convergence forecast) are then cycled automatically to knowledgebased system Exos which runs diagnostics of the layout planning with respect to strata mechanics . If the result of this diagnostic work qualifies the layout variant as unfavourable, automatically proposals for improvement are drafted. f) The user may then improve layout planning by means of the calculated recommendations. g) If the result of the diagnosis is satisfactory to the user the latter can demand support planning. The knowledge-based System SOUT 3 then calculates type and dimensions of support.

3 Planned developments

The recent and conclusive results of the ISLSP project as an aid to decision-making has urged us -in collaboration with our German (DMT and RAG), French (CdF), Spanish (AITEMIN) and Greek (NTUA and LARCO) colleagues -to contemplate an intelligent system for the planning and simulation of a mining operation.

The planning of a mining operation involves, firstly, a series of decisions which we will classify as "first-order decisions" and which are very closely determined by considerations relating to the geology and history of the deposit being worked: these concern the geometry of the deposit, its regularity, the position and size of the joint-planes, the existence or location of former workings, the existence, location and state of pre-existing infrastructures... These "first-order" decisions have a bearing, for example, on the cutting of panels, the working sequence, the speed of working... They may be modified at a later date. These first choices will more or less automatically involve a certain number of consequences (for example, with regard to creations of new infrastructures or distribution of "pressures in the solid rock"..). These consequences are subject to change in the sense that they will occur as the "first-order" decisions are implemented. The whole thing represents to some degree an "initialisation" of the system. From that point on, the specific working project will require a certain number of "secondorder" decisions to be made. The purpose of these is to create the conditions under which the "first-order" decisions can be implemented. These choices concern: the methods to drive the infrastructures, the support of the roadways and galleries, the mining methods, the support of the workings, the ventilation, ... A second series of consequences result from these decisions. These have to do with labour, commodities, and the necessary investments, and will determine production levels and costs.

The operators wish to have an intelligent decision-supporting tool, permitting advanced 3D modelling, in order to carry out simulations which provide an insight into the consequences of the planning decisions they have in mind.

It is this kind of an intelligent system for the planning and simulation of a mining operation which we are planning now to create.

CONCLUSION

The design of an knowledge-based system is a complex process which cannot be improvised. The rules of the art involved in the creation of such a system are ultimately the same as those which govern the management of a project whether this be data processing or general engineering. The specific skills relating to the cognitive nature of the project are also difficult to bypass; this is why we have decided to take genuine stock of the fact that the experts and the operators have begun to consider SOUT, Exos and ISLSP as a tool capable of helping their profession and that they have given their approval to the follow-up projects we have mentioned.

The expected benefits through the large utilisation of integrated knowledge-based planning software are aiming to reduce planning errors and increase planning reliability. At the same time planning costs, particularly for adaptation to new and unpredictable earlier situations, will become cheaper. Repair and maintenance costs of roadways are also expected to be reduced in a significant way, while a reduction in the accident rate is predicted to an unspecified at present figure. More specifically the following results are expected : 1. Improved layout and support planning systems, which will provide greater roadway stability. 2. Reduction of the environmental impact of mine activities until mine closure. 3. Improvement of working and safety conditions.

Figure 2 :

 2 Figure 2 : Acquisition of the basic data concerning the roadway and the strata 10

Figure 4 :

 4 Figure 4 : The DSS : PLANANK O.I 12 .

Figure 6 :Figure 7 :

 67 Figure 6 : The conventional program DlFLAN for digitalising mine surfaces.

  Results of SOUT concerning the possible length of the bolts 13 .

	Sout vous propose les longueurs suivants :
	Auteur	min	max	Fact Cert
	Etienne (CETU)	2.09	2.09	1.00
	Barton	2.56	2.56	1.00
	Lang	3.00	non determinee	LOO
	Farmer	3.00	4.00	0.40
	Dejean Raffoux	2.00	3.00	1.00
	DMT/RAG	2.22	3.00	LOO
				111
	Figure 5 :

INTEGRATION INTO A COHERENT SOFTWARE PACKAGE : ISLSP

Centre d'etudes et recherches des Charbonnages de France, now INERIS.


F Marliere

ACUTE TOXICITY OF SMOKE: STUDY OF GASEOUS INTERACTIONS

A previous paper entitled " Toxicity of fire smoke" and presented at the European workshop held in Cadarache (Industrial fires II, 1994) described the first phase of a two phases project supported by the French Ministry of Environment. This first phase was dealing with the contribution of smoke particulates (soots + aerosols) in acute toxicity and, more precisely in the degree of incapacitation, by using an original behavioural study of exposed animals (Wistar rats). The present paper concems the second phase of the project. It is carried out entirely in laboratory and comprises a behavioural post-exposure study considering the degree of incapacitation. It consists in the reconstitution of toxic atmospheres in laboratory from synthetic gases. Selected gases (CO, NO, N02, HCN, S02) are narcotics or irritants, so different types of combinations can be done. Oxygen and carbon dioxide levels are kept constant respectively to 19 % and 1,5 %. An Inhalation installation has been designed so äs to obtain toxic mixtures of known composition held constant for 30 minutes and to permit animals to be exposed. The purpose is to evaluate the different types of interactions (synergy, antagonism, additivity) occuring during Inhalation of non-lethal gas mixtures. These interactions are revealed on diagrams obtained by computation treatment of behavioural data (descriptive method) which gives a general view of the degree of incapacitation for each tested atmosphere, and involved interactions. Revealed interactions are expressed in term of "tendancy" and are quite well correlated with animals clinical follow-up and some biological analysis ( blood gases for example). Main results and conclusions are presented and show that toxic interactions are a complex subject which need to be further investigated (in a biochemical way for example) in order to identify physiological mecanisms involved in interactions.

. INTRODUCTION

In the event of a fire, the formation and the dispersion of a plume containing toxic components is one of the major issues (among other things). The consequences can be serious for the environment and more especially for population living in the surrounding.

Two types of exposures to toxic fire smoke are to be considered in the assessment of the toxic impact of smoke study :

-the first relates to the confined environment. Fire-fighters and trapped people are concerned by this exposure to undiluted effluents, so to high concentrations of toxic components.

-The second type relates to the neighbouring and populations located at a distance of 50 meters to several kilometers from the fire place. These people are exposed to the cloud issuing from the dispersion of the plume of smoke in the atmosphere, so to relatively low concentration levels. Even in that case, inhalation of the polluted air may be an actual hazard.

Acute toxicological consequences of these inhalations are different and ränge from lethality to nausea or headache for example.

Studies dealing with the improvement of our knowledge of the toxic effluents of fires have enabled considerable significant progress to be made in areas regarding the emission of gases and their significance in actual fires, with the surveying of toxic components and their mechanisms of formation, animal experimentation techniques, the respective effects offire gases,... Among these research fields which must be continued, effects of toxic gases remain an important subject of discussion. It is still required to perfect our current knowledge of the interactions beetwen gases ( synergetic, additive or antagonistic effects observed on exposed organism) and consequences ensuing from a short time exposure. The toxic effects of the inhalation of individual component or gas mixtures can be studied from different points of view, either through the intermediary of biological responses obtained from the exposure of in-vivo or in-vitro modeis, or by behavioural responses of in-vivo modeis. Each of these different methods provides precious information. However, the high costs of these studies means that very few research programmes focussing on the measurement of the toxic impact include these different aspects at the same time and it is therefore difficult to correlate and compare these data issuing from different origins. More generally, the limited scope of the data prevents from reflecting a complete picture of involved phenomena.

It was with this feeling in mind that INERIS (Institut National de l'Environnement industriel et des RISques, France) developped a programme financially supported by the French Ministry of Environment. Started in 1991, it studied firstly the toxic impact of smoke of füll scale fire tests involving rats äs animal model. These result were briefly presented at Cadarache (Industrial Fires II, 1994). The second phase, ended in 1995, studied toxic impact of gas mixtures inhalation by using a behavioural method developped and improved during the first phase, and carried out biological investigations in order to identify some early indicators of intoxication. This paper presents the behavioural part of this study.

. OBJECTIVES

In a first time, the objectives of the study were to perform animal sublethal exposures of 30 minutes enable to reveal the toxic effect of single narcotic and irritant gases. Selected gases were carbon monoxyde (CO) and hydrogen cyanide (HCN) äs narcotic gases, sulfür dioxyde (S02) and nitrogen oxydes (NOx) äs irritant gases. NOx effect was considered mainly irritant and they were used äs unique gas. In a second time, more or less complex mixtures were realized by binary, ternary,...combinations ofselected gases in the aim to observe interactive effects. NOx components, nitrogen oxyde (NO) and nitrogen dioxyde (N02), effects were succinctiy studied. Gases concentration were adjusted by reference to füll scale fest results and adequate scaling factor. Toxic impact was assessed through the intermediary of a behavioural test giving results in term of degree of incapacitation. This was completed by a clinical follow-up of exposed ammais, blood gases analysis (CO and CN~) and biochemical investigations (not discussed here).

. EXPERGMENTAL

Animal exposure device

The animal exposure device used all long the inhalation programme and enabling known concentration mixtures to be made is shown in figure n°l. It is entirely set under a large hood. This device comprises a specific Teflon line for each gas (included air of dilution). Each gas flow is controlled by a mass flow-meter and a solenoide valve is inserted in the line for safety reasons.

Concentrated gases are stored in pressurized cylinders :

-CO 4129 ppm in air -C02

19,25 % in l'air -NO 4920 ppm in N2 -N02

533 ppm in air -HCN 879 ppm in air -S02 1800 ppm in air Whatever the mixture is, toxic gases flows are very low compared to the total flow passing through the exposure chamber. The air flow represents more than 90% of the total flow which is about 1500 l/h. Each line is connected to the blender which enables mixtures to be homogenized.

Figure l: animal exposure device

Once homogeneized in the blender, gas mixtures reached the exposure chamber shown in figure n°2. It is designed äs a cylindrical volume of 20 liters, divided in two levels, each level being itself divided in 8 compartments. Physico-chemical analysis are performed inside the exposure chamber in order to characterize the actual exposure conditions. These conditions are characterized in terms of ambiant temperature measurements at both levels, infernal relative pressure, on-line analysis for CO, C02, 02 NO, N02 and HCN. All information are collected and centralized on a PC Computer, and are thus immediatly accessible.

When they leave the exposure chamber, gas mixtures flow through 2 bubbiers filled with soda solution in order to absorb soluble gases, then through a pump to balance relative pressure inside the device, and finally through an active carbon filter set before being discharged to the atmosphere.

Figure 2 : Anhnal exposure chamber

Test mixtures

The composition of test mixtures is presented in table l. Each mixture contains C02 at the concentration level of 1,5 % in order to keep the hyper-ventilation effect influence constant, and 02 level is kept at the concentration of 19 % minimum in order to avoid hypoxia phenomena. Groups n°l and 13 are exceptions to these ruies because they are control animals exposed to ambiant air.

Table l: mixtures composition

Animal model

Exposed animal were WISTAR ICO/WI male rats. Eight animals by group were required for each toxic atmosphere exposure. The same animals were biochemically investigated just after the end of the behavioural test. The sole control animal groups exposed to air required 10 rats.

Exposure conditions

Animals were exposed during 30 minutes to toxic atmospheres at constant concentration and ambiant temperature. During exposure, ammais were followed visually to set up the clinical follow-up. At the end of the exposure, ammais were immediately transfered into their individual behavioural fest device.

. BEHAVIOURAL STUDY

Principle

Toxic effects of smoke and soots produced in a fire can be revealed by the analysis of behavioural variations of exposed animals. The protocol proposed is to reveal and measure behavioural signs which are characteristic of inhaled mixtures while carrying out a previous learnt task.

It looked judicious to reveal these impacts when the animals are placed in an emergency Situation. This Situation is here to get food.

The experimental device used, called Skinner box, is presented on figure 3. It comprises different areas usefüll in data treatment. There is the cup area (C), the lever area (L), the round-trip area and the "elsewhere" area. The animals move freely inside this device. Limits of areas are virtual. Sidewalls are made of plexiglas.

To get food, animals are trained to push on the lever with sufficient vertical force to induce a food pellet delivery in the eup.

A well-conditioning is considered achieved when the animals stay within areas which enable them to get food. The "elsewhere" area has no interest for such an animal.

:-Elsewheres 1 -

Figure 3: Skmner box

Animal conditioning is obtained äs follow :

1) on arrival, two-month old rats are put in Standard individual cages at a temperature of 20 °C and subjected to a 14/10 photoperiodical System.

2) for the first 5 days after their arrival, rats are given food and drink ad libitum. They are weighed at the same time each day.

3) for the following 5 days, they are put on a gradual fast so that they will loose 20 % of their initial weight.

4) for the next 5 days, at the same time each day, the animal is placed in a skinner box for 15 minutes. All behavioural signs are videotape recorded on the last day.

5) on the following day, group of conditioned animals are exposed to the toxic atmosphere for 30 minutes.

6) at the end of the exposure, animals are immediately retumed to the skinner box to be individually filmed for 15 minutes.

Then "performances" of each animal (8 by test) are compared to those got the previous day before exposure with the help of videotape recordings. All the acts of each animal are registered, then acts are sorted and gathered according the specific area where there were done and the type of act in order to leave 9 fundamental descriptive signs in the behavioural grid. These signs are listed here under :

-acts near the cup (Coup) -acts near the lever (Levi) -acts in the round-trip area (Alre) -acts in the elsewhere area (Aill) -average immobilization duration (Dmoy) -total immobilizations duration (Dtot) -immobilizations number (Fimo) -total activity (Acti) -number of food pellets eaten (Boul)

These fundamental signs are also used to compare groups performances to each other.

Data treatment

Because of the low number of animals by group and the relatively large interanimal behavioural variations observed sometimes, statistically significant results were not obtained for all tested groups by direct classical statistical computation. In order to compare animal groups each other, we used a descriptive method of data treatment which is the principal components analysis (PCA). It gives a synthetic view of the information hold in the behavioural grid and the results must be considered äs tendancies.

The PCA results are represented on 2 dimensions graphics because the 2 principal axis are sufficient to explain from 85 to 90 % of behavioural variations in all performed analysis. Two types of graphics are issued from this statistical data treatment.

The first one presented on figure 4 is the result of the PCA of fundamental behavioural signs. Axis l is characterized by the Opposition beetween activity and conditioning signs (Acti, Levi, Coup, Alre, Boul) and the total immobilization duration (Dtot). It can be considered äs the conditioning axis where high performance groups (or less incapacitated groups) are located on the left side of the axis, and the low performance groups (or more incapacitated groups) are located on the opposite side of the axis. Axis 2 is explained by the Opposition beetween the number of immobilizations (Fimo) and average immobilizations duration (Dmoy), or more clearly, high frequent immobilization but brief lasting groups and low frequent immobilisation but long lasting. Through this figure, it seems that fundamental behavioural signs were correctiy selected to reveal the incapacitation state and that they are strongly correlated following axis l. Most ofthem are located on the high performance side ofthe figure and indicate thus that these signs are inter-correlated. Axis l traduces the animals performance versus their initial conditioning and indicate the incapacitation level of exposed groups. The right end side location of groups is associated with animals presenting the highest level of incapacitation and thus exposed to the highest toxic atmosphere.

Axis 2 rather sorts the incapacitation level according to the type of animal response. Two exposed groups may have similar location according to axis l (so are equally incapacitated), but may be distinctiy located according to axis 2. This may mean that their types of toxic impact induce different physiological responses (narcotic or irritant effect for example).

PCA of all exposed groups is presented on figure 5. Test mixtures are identified by their composition. A perfect virtual group (T) is included in the graphic in order to be a reference point. This point represents the location of a non-incapacitated group. The interpretation of this figure, and more precisely the mixtures arrangement according to axis l and 2 should give answers on interactive effects. ------------------y-----------------p--------------,--^------------------y-------------------^--------- The groups arrangement covers all parts ofthe figure 5, and is thus indicating that gaseous toxic effects seem to be of various types and of different incapacitation levels. Before concluding on gaseous interactions, we must recall that PCA results are only tendancies, and that they are linked to tested concentrations and to the rat for animal model.

AXE2(14.4%) 3 -• 2 - l - 0 - -2- .3-(-

mdividual gases

Individual gases effects are presented on figure 6. It can be seen that:

-CO seems to produce very low toxic effect.

-HCN seems to be very incapacitating.

-NO tends to increase animals activity and can be considered äs an exception among other gas toxic effects.

-NO 2 can be considered äs a strong irritant gas. The incapacitating effect observed is in the same order of magnitude äs HCN.

-S02 appears äs a less powerfüll irritant gas than N02, and induces a weak incapacitating effect in the same order of magnitude äs CO. -mixed with an other gas, with the exception ofNO, HCN remains a powerfüll incapacitating gas. The CO concentration level is too low to allow us to give a ruie on the likely additivity of the narcotic effects of CO and HCN .

The same comment can be applied to the HCN/S02 mixture but this time for a supposed antagonistic effect. Moreover, clinical follow-up results seems to indicate that the interaction beetween HCN and S02 may be simple additivity.

-the mixture of NO and N02 to make (the so called) NOx induces a moderate effect, at intermediary level to the respective effects ofNO and N02. This may lead to suppose that an antagonistic effect appears with this mixture.

-the CO/S02 mixture reveals a very low toxicity, dose to the control group (T), according to their respective individual toxic impact previously observed. A simple additivity interaction may be considered. When mixed to more than one other gas, HCN effect seems to be lowered. This is particularly clear in the NO/CO/HCN mixture where an antagonistic interaction beetween NO and HCN decreases the toxic impact of the mixture to the lowest incapacitation level. Such an explanation may also be put forward in mixtures like CO/S02/HCN and NOx/S02/HCN. In both cases the HCN impact is reduced but the overall impact is more important than mixtures without HCN. However, mechanisms and natures of these antagonistic interactions are not easy to identify because of the low number of performed tests in this study.

Most of the tested mixtures including NOx äs a component seem to present a high immobilisation frequency (Fimo). This may be characteristic of the NOx presence in a mixture.

The CO/NOx mixture reveals an important toxic impact which could be the result of a synergetic effect.

The NOx/S02 mixture seems to reveal a simple additivity interaction.

Finally, the most complex mixtures (4 and 5 components), like CO/SOa/NOx/HCN or NOx/SOa/HCN, show the existence of interactions, generally antagonistical if the HCN exposure group is taken for reference, which are impossible to identify. In these cases, though mixtures are complex, revealed toxic impacts are in the same order of magnitude äs an individual moderate toxicant exposure. The results of this study are in relatively good agreement with specialized literature on the subject ofgaseous toxic interactions.

However, examination of tendancies show that there are no general ruies that can be deduced froni them in order to define toxic interactions appearing in the three possible types ofmixtures :

-narcotic + narcotic -irritant + irritant -irritant + narcotic

Global effects observed are issued from numerous and various interactions which seem to depend on the nature of the gases contained in the mixtures. General ruies on interactive phenomena cannot be drawn-out with the low number of performed tests in this study. So this research need to be continued.

This paper shows that the behavioural method developped in the study is adapted to the defined objectives, and that data treatment results are relatively easy to translate into terms oftoxic impact and likely associated interactions.

This study need to be continued with new parameters such äs concentration levels ofCO and S02 increased to reach about 50 % oftheir respective LC50 with the aim to reveal more easily interactive effects, and with variations of concentration of each gas to approach these innuence on interaction levels.

It must also be reminded that clinical follow-up results (not shown here) are quite well correlated with behavioural results, and that CO and CN blood analysis are quite well correlated too. These complementary measurements appear neccessary to confirm the behavioural results.

The biochemical investigations must also be kept in order to identify mechanisms involved in interactive phenomena.

Figure 4 :

 4 Figure 4: PCA of behavioural signs

  Figure 5: PCA of exposed groups

  Figure 6: individual gases effects

  Figure 7: binary mixtures

  Figure 8: complex mixtures effects

Acknowledgements

INERIS wishes to thank the üniversity of Toulouse (Pr. A. GALLO) and the F. WIDAL hospital (Dr F. BAÜD) for their respective help in data collecting, treatment and interpretation.


M Al Hed3

J P Piguet

La Planification des Travaux Miniers Assistée par des Modèles Prévisionnels des Etats de Contraintes dans les Massifs Rocheux

L'évaluation de ce que l'on a coutume d'appeler "pressions de terrains", et de leurs conséquences sont devenues de la plus grande importance, dans les exploitations européennes de charbon, à couches multiples, de plus en plus profondes et affectées par des conditions tectoniques sévères. '•

• Les principes du calcul des contraintes induites par les travaux d'exploitation ont été révisés grâce aux développements de la modélisation, numérique, associés avec les performances permises aujourd'hui par les outils informatiques. Un code de calcul a été développé en collaboration avec l'institut de recherche sur les études géomécaniques et géologiques de Saint-Pétersbourg. D permet de calculer le champ de contraintes autour d'exploitations mono ou multicouches, eh trois dimensions, en utilisant les principes de la méthode des éléments frontières (discontinuités de déplacement). Il permet d'intégrer, sans limitations, l'ensemble des panneaux influençant un secteur donné et se caractérise par l'interactivité, la facilité d'emploi et de lecture des résultats.

Des applications à des exploitations des charbonnages français sont présentées et montrent les avantages de cette méthode pour la planification des "travaux futurs, qui doivent être dimensionnés pour obtenir la meilleure sécurité d'exploitation, malgré la difficulté et la complexité des conditions. '''••

INTRODUCTION

La planification des travaux d'exploitation tient compte de nombreux facteurs liés à l'organisation des chantiers, à la ventilation et aux pressions de terrains.

Avec l'approfondissement des exploitations, les pressions de terrains deviennent l'un des principaux paramètres qu'il convient de maîtriser, particulièrement dans des zones influencées par d'ancienne exploitations (AL Heib, 1993). La réalisation d'un projet d'exploitation nécessite une étude géotechnique des conditions naturelles pour la planification des travaux. Cette étude doit comprendre : une étude géologique détaillant l'environnement tectonique, la présence des structures telles que failles, plis, ...

-une analyse détaillée des carottes de sondage du toit, et du mur et de la couche exploitée.

Des essais mécaniques sont recommandés pour connaître la ou les lois de comportement de la roche sous divers sollicitations.

-la réalisation de mesures des contraintes naturelles. Par ailleurs, la maîtrise des pressions de terrains passe par l'expérience acquise complétée par des études spécifiques pour des nouvelles situations. La modélisation numérique est de plus en plus employée à la fois dans la phase d'élaboration du projet et pendant l'exploitation. Elle aide à la compréhension des mécanismes qui surviennent autour des ouvrages existants (rétro-analyse) et contribue à la planification des nouveaux chantiers. Pour répondre à cet objectif, différents types de modèles sont disponibles, chacun d'eux permettant de simuler un phénomène physique.

La démarche de modélisation consiste donc à choisir le ou les modèles adaptés, à partir de l'analyse des phénomènes les plus importants dans le comportement observé ou attendu de l'ouvrage. Cette démarche qui a été mise au point pour la conduite de l'exploitation montre que ces techniques peuvent être maintenant opérationnelles.

DESCRIPTION DE L'ENVIRONNEMENT

DESCRIPTION DU PHENOMENE

Un coup de charge d'une magnitude de 2, s'est produit dans le chantier de traçage d'Erna 3-1250, Aile Nord, le 23/11/95. Il'se traduit par un soufflage de la sole entre les cotes à 120 m et à 210 m variant de 10 à 50 cm. Un glissement de la couche de charbon du mur vers la sole et le toit et un dégagement important de grisou ont été observés. Trois autres coups de charge ont été enregistrés après ce phénomène avec des magnitudes et des effets moins importants. Des travaux dans le chantier ont été réalisés après l'incident (dégazage, remise en place du soutènement, renforcement, rabassannage de la zone soufflée). Des trous de détente ont été réalisés avant de démarrer le chantier et ensuite avec l'avancement du chantier.

COMPREHENSION DU PHENOMENE

Le caractère dynamique du soufflage du mur dans le traçage d'Erna 3 peut être dû à la présence au mur de bancs de caractéristiques géotechniques contrastées (sandwich charbon-grès). Les bancs les plus résistants servent d'armatures et freinent les déformations ; dans le cas d'un état de contrainte défavorable, la rupture des bancs raides ou le glissement à leur interface entraînent du rattrapage brutal de déformation. L'état de contrainte dans cette zone est lié : -aux contraintes naturelles élevées qui ont été mesurées dans le compartiment central avant le début de l'exploitation dans cette zone (Revalor, 1985, figure 1).

-aux modifications des contraintes par les exploitations aux différents étages et en particulier dans le compartiment central. -à l'interaction possible avec l'exploitation en cours dans la veine Cécile (figure 2). Le traçage d'Erna avait été planifié selon les règles habituelles de façon que l'exploitation de Cécile n'influe pas sur le traçage. Ces règles ont été établies pour des profondeurs moins importantes et en l'absence des contraintes horizontales élevées (Enchayan, 1977) 4 montre la distribution des contraintes normales au plan des veines, dans la zone du traçage Nord et Sud. L'Aile Nord, dans les premiers 100 m, traverse une zone très proche de l'état initial avec des contraintes dans un rapport inférieur à 1.10. Au fur et à mesure que le traçage approche le panneau de 1 NE -1250, cette contrainte augmente dans les derniers 100 m, la contrainte varie entre 1.4 à plus de 1.5 fois la contrainte initiale (38,5 à 52,5 MPa), pratiquement deux fois la contrainte verticale initiale. La contrainte normale s'accroît rapidement lorsque l'on rapproche de la zone exploitée par les trois coupes de charge dans l'Aile Nord. 2). On a considéré le pendage des veines égal à 60° pour les trois veines. D'après le mécanisme supposé du soulèvement brutal de la sole, le critère retenu pour analyser les résultats des calculs est la valeur des contraintes de cisaillement maximal susceptible de rompre l'équilibre entre les bancs du mur du traçage.

Résultats

Situation lors de l'événement * Cas d'un tenseur initial : le calcul a été réalisé avec le tenseur mesuré à l'étage 1250. Les résultats analysés correspondent à la configuration d'une coupe à 227 m de TB où les déformations ont été maximales. La distance verticale entre les deux traçages est égale à 8 m et la distance orthogonale est égale à 24 m (figure 2). A cet endroit la distance orthogonale est la plus faible. Les contraintes de cisaillement se développent au mur de la veine Erna 3 jusqu'à une profondeur égale à 18 m (figure 5). Cette zone, où la contrainte de cisaillement vaut deux fois le cisaillement initial, est beaucoup plus importante comparée à un traçage réalisé tout seul (hors influence de Cécile).

* Cas d'un tenseur induit : d'après le calcul réalisé à grande échelle avec SUIT3D, la zone affectée du traçage est soumise à un régime de contraintes plus important que les contraintes initiales. Les calculs avec un tenseur plus important montrent que la zone de forts cisaillements se développe davantage. Le cas d'une contrainte horizontale de 1,5 fois la contrainte initiale est le plus critique en terme de contrainte de cisaillement. Le mur de la veine Ema 3 est sollicité par de fortes contraintes de Influence de la contrainte horizontale : Deux calculs ont été réalisés pour examiner l'influence de la contrainte horizontale, l'un où elle est égale à la moitié et l'autre où elle est égale à deux fois la contrainte verticale. On constate que :

-la zone du mur du traçage se trouve en surcontrainte horizontale de 20 % dans le cas d'une contrainte horizontale élevée (figure 6). En revanche, pour une contrainte horizontale faible, cette zone est dans un état de contraintes proche de l'état initial (figure 7).

-la contrainte du cisaillement maximal est plus importante dans le cas de fortes contraintes horizontales.

Les calculs confirment que le comportement observé est dû à une contrainte horizontale élevée aggravée par les exploitations antérieures. Les contraintes de cisaillement induites au mur du traçage sont suffisantes pour être à l'origine du phénomène.

PROJET DU TRAÇAGE DE L'AILE SUD D'ERNA 3

Le calcul réalisé à l'aide du code SUIT3D a montré que l'Aile Sud se situe dans des conditions générales de contraintes plus favorables que l'Aile Nord puisque hors influence du panneau adjacent. D'autre part, l'étude menée pour analyser le comportement de l'Aile Nord a montré l'influence de la position de traçage d'Ema 3 par rapport à l'exploitation du panneau de Cécile. Une étude paramétrique a été réalisée pour étudier l'influence de la position du traçage. Quatre configurations ont été étudiées : Les traçages se trouvent au même niveau (0 m), 5 m, 10 m et 15 m. Tous les autres paramètres ont été maintenus constants. Les résultats ont été analysés en terme du cisaillement maximal autour du traçage dans le plan de Mohr (figure 8). Traçage au même niveau : la zone de modification de la contrainte de cisaillement est moins étendue en couronne mais plus étendue au daisne que dans le cas du traçage seul ; cependant à 5 m au mur les modifications restent faibles et le cisaillement maximal est du même ordre que pour le traçage isolé soit 10 MPa. Traçage à 5 m au-dessus du niveau du démarrage de Cécile : la zone de modification de la contrainte de cisaillement s'étend beaucoup plus loin dans le banc du mur, au daisne du traçage. Le cisaillement maximal à 5 m dans le daisne passe à 18 MPa du fait de l'annulation de la contrainte verticale dans cette zone. Traçage à 10 m au-dessus du démarrage de Cécile : on revient à une répartition de contrainte proche du cas du traçage isolé avec une répartition des contraintes de cisaillement à peu près symétrique en couronne et au daisne. On retrouve un cisaillement maximal à 5 m au mur dans le daisne de 10 MPa. Traçage à 15 m au-dessus du démarrage de Cécile : les résultats sont peu différents du cas précédent. L'analyse des quatre configurations montre que la situation où le traçage s'effectue à 5 m est la plus défavorable. Donc il semble préférable d'éviter la position du traçage à 5 m au-dessus du démarrage de Cécile et de choisir une position plus favorable en se plaçant soit au niveau du démarrage de Cécile, soit à 10 m au-dessus de ce démarrage.

a -Aile Nord Cas de l'événement de novembre 1995 • l'importance des contraintes induites à grande échelle,-par l'exploitation sus-jacente, des étages situés au-dessus et d'autres secteurs.

• l'influence de la position du traçage d'Erna 3 par rapport au traçage de Cécile, tant au niveau de la distance verticale qu'orthogonale.

• l'analyse des quatre configurations pour le traçage de l'aile Sud montre que la situation où le traçage s'effectue à 5 m est la plus défavorable. La position du traçage à 10 m audessus de ce démarrage permet de trouver un état des contraintes beaucoup plus proche d'un traçage hors influence de la veine de Cécile. Le traçage de F Aile Sud a été réalisé à 10 m au dessus du TB, position préconisée par l'étude. Il s'est déroulé sans problème.

  DU TRAÇAGEERNA3 L'Unité d'Exploitation Vouters des HBL exploite à l'étage 1250 la veine Cécile 1SE et prépare le 1 lème Colloque Franco-Polonais en Mécanique des Sols et des Roches, Gdansk, 7-10 septembre 1996 panneau en veine Erna 3. Ces chantiers se situent dans le compartiment central limité par les failles Dora et Reumeux (figure 1). Le traçage d'Erna 3 à l'étage 1250 a été démarré après l'exploitation de 20 m du panneau Cécile 1250. La veine Cécile est une veine de faible puissance (2.5 m) dont l'exploitation a pour principal objectif la détente des autres veines exploitées au toit et au mur. La veine Erna 3 a un toit constitué de grès fins. En revanche, son mur est constitué de charbon, de schistes et de grès avec des épaisseurs très variables et des caractéristiques géotechniques contrastées.

4. 2

 2 Modèle local à l'échelle du traçage 4.2.1 Présentation du modèle L'objectif de la modélisation à l'échelle du traçage est d'éclairer le comportement mécanique des terrains, en particulier les observations au mur du traçage d'Ema 3, en fonction de la géométrie et de l'influence des contraintes initiales. Il s'agit d'étudier et préciser la zone d'influence de deux exploitations (le panneau de Cécile et le traçage d'Ema 3) en dressant par une modélisation numérique basée sur la méthode des éléments finis (CESAR). La géométrie du modèle réalisé correspond à un modèle en deux dimensions d'une coupe verticale, parallèle au travers-banc. Les dimensions du modèle sont 150 m de large et 110 m de hauteur (figure

Fig. 4 :Fig

 4 Fig. 3 : Modèle réalisé avec SUTT3D (6000 x 2000m)

Références 1 -

 1 Piguet J.P. 1983. La modélisation en mécanique des terrains et son application à l'exploitation minière . Thèse de docteur es science. INPL. Ecole de Mines de Nancy. 2. Al Heib M.'M. 1993 : Les nouvelles méthodes de modélisations numériques et le volume d'influence des exploitations minières en conditions complexes. . Thèse de docteur INPL. Ecole de Mines de Nancy. 3. Enchayan B. 1977 : Le volume d'influence des exploitations en dressants. Thèse doc. Ingénieur INPL.

  . Pour analyser le comportement de traçage d'Erna 3 Nord, deux types de modèle ont été réalisés. Le premier modèle est à grande échelle tenant compte de l'ensemble des panneaux exploités du compartiment central. Le deuxième modèle est à

	permettant l'utilisation de la géométrie à partir
	d'Autocad.
	• 150m
	• Schistes
	E23	Charbon
			une	échelle	locale	permettant	d'analyser
		s=	l'influence du panneau de Cécile sur le traçage
	.Traçage Ema	o	d'Erna 3.		Puits
						dlle
						Galerie
						d'essai pour
	Fig. 2 : HBL -U.E. Vouters Modèle réalisé avec CESAR-LCPC					mesures de contraintes
	Pour étudier la distribution des contraintes
	induites dans le chantier, nous avons réalisé un
	modèle à grande échelle avec SUTT3D qui prend
	en compte l'exploitation du compartiment central entre les failles Dora et Reumaux (figure 3). Les secteurs modélisés correspondent à : 1 SE, 2 SE, 1 NE, 2 NE, et 3 NE, des étages 686, 826, 1036 et 1250. D'autre part, les veines exploitées ont été	Zone de plis d'entrainem ent	~~~-	al = 36,5 MPa o2 = 27,5 MPa a3 = 17,5 MPa
	regroupées pour simplifier le modèle en quatre	Fig. 1 : Bloc diagramme schématique au siège Vouters
	groupes. Les dimensions du modèle sont 6000 m		Zone de mesures de contraintes
	x 1200 m. Le toit et le mur sont considérés
	comme homogènes, isotropes. Les contraintes
	induites correspondent au tenseur mesuré à l'étage
	1250. La distribution de la contrainte normale, au
	plan des veines, a été examinée dans le plan de la
	veine (Cécile, Erna 3). La figure
						des
			chambres et piliers. Les paramètres nécessaires
			sont réduits-' au minimum et la géométrie de
			l'exploitation peut être modélisée en détail. Ce
			logiciel a été développé afin d'être utilisé par
			l'exploitant. D'autre part, ce code fonctionne sur
			PC, sous environnement Windows 95 et NT.
			Les résultats sont fournis sous forme de
			graphiques et d'iso-valeurs facilitant l'analyse des
			résultats. Un logiciel utilitaire a été conçu

4.1 Modèle à grande échelle

L'INERIS, avec la coopération du VNIMI 1 , a développé un code de calcul (SUIT3D) basé sur la méthode des éléments frontières (Discontinuité des Déplacement). Il s'applique aux problèmes liés aux calculs de pressions de terrains (contraintes-déformations) dans les cas des mines exploitées par la méthode des longues tailles foudroyées ou remblayées, et la méthode

VNIMI : State Research Institute of Mining Geomechanics and Mine Surveying (Russie).


D Oberson-Geneste

C Lambré

Aspects médicaux de l'exposition à l'oxyde d'azote

Les oxydes d'azote sont souvent désignés par la formule générale NOx.

On considère que parmi eux, seuls le monoxyde d'azote (NO) et le dioxyde d'azote (NO 2 , aussi appelé peroxyde d'azote) représentent un risque potentiel pour la santé.

Propriétés chimiques

En général, ces deux polluants coexistent dans l'atmosphère, bien que du fait de sa grande réactivité, le NO s'oxyde spontanément en NO2 avec l'oxygène atmosphérique. Cependant, dans le cas où par exemple les émissions automobiles sont impliquées, on peut trouver dans les voies à grande circulation, des concentrations en NO 5 fois supérieures à celles de NO 2 .

Le dioxyde d'azote est en condition normale assez stable, et il réagit avec l'eau pour donner de l'acide nitrique (HNO 3 ) et nitreux (HNO2). Ces composés vont alors conduire à une acidification du milieu, notamment au niveau des muqueuses, et entraîner des effets irritants au niveau des yeux, du nez et de la gorge.

Enfin, parmi les propriétés chimiques importantes du NO2, il faut citer son fort pouvoir oxydant qui va entre autres conduire à l'altération des membranes cellulaires et à l'oxydation du matériel génétique.

Valeurs d'expositions

Etant donné les effets déjà connus de ces polluants sur la santé, il a été établi aussi bien en milieu professionnel qu'environnemental, des valeurs limites d'exposition.

En milieu professionnel, la valeur moyenne pour une exposition de 6 heures par jour, 5 jours par semaine se situe pour le NO à une concentration de 25 ppm (30 mg/m 3 ), tandis que la valeur limite d'exposition est de 3 ppm (6 mg/m 3 ) pour le NO2 (JO, 1990). La différence importante des valeurs d'exposition entre ces deux polluants est le reflet d'une toxicité reconnue beaucoup plus faible pour le NO que le NO2.

On peut remarquer que les valeurs limites dans l'environnement sont de 10 à 15 fois plus faibles qu'en milieu professionnel, puisque de 0,25 ppm (300 ug/m 3 ) pour le NO et 0,1 ppm (200 ug/m 3 ) pour le NO 2 (Directive CEE, 1985).

Si les teneurs en oxyde d'azote extérieures sont issues des sources de combustion automobiles, industrielles et thermiques, des études épidémiologiques se sont également intéressées aux concentrations "indoor" de NO 2 , qui sont elles essentiellement dues à l'utilisation de chauffage au fuel ou de cuisinière à gaz. On peut, en effet, trouver dans les habitations des concentrations de 0,01 ppm de NO 2 sur plusieurs jours avec des valeurs maximum sur 1 heure de 0,5 à 1 ppm.

En ce qui concerne les mesures de ces polluants, ils ont été pendant de nombreuses années, dosés indistinctement par des méthodes colorimétriques comme celle de Gries-Saltzmann après oxydation de NO en NO2.

Depuis 1975, des appareils automatiques basés sur le principe de la chimiluminescence permettent le dosage simultané de NO et NO 2 . Cependant, le port de badges individuels, permettant d'estimer l'exposition cumulée à NO 2 , reste rare.

Impact des NOx sur l'appareil respiratoire

La pénétration des gaz dans l'appareil respiratoire humain va, entre autre, dépendre de leur solubilité.

Par exemple un gaz comme le dioxyde de souffre (SO 2 ) qui est très hydrosoluble va réagir rapidement avec les sécrétions et rester au niveau des voies aériennes supérieures, représentées par la bouche, les fosses nasales, le rhino-pharynx et la trachée. A l'inverse un gaz beaucoup moins soluble tel que l'ozone, aura une action beaucoup plus distale jusqu'au niveau du poumon profond, qui est constitué par les alvéoles pulmonaires.

Le NO2 plus soluble que O3, pénètre moins profondément dans les voies aériennes, sa toxicité ciblant davantage les bronches et bronchioles que les alvéoles [START_REF] Overton | Physiological processes and the formulation of dosimetry models: fondamental extrapolation modelling of inhaled toxicants, ozone and nitrogen dioxide[END_REF].

Cependant, le mode respiratoire intervient dans la pénétration du NO 2 . Ainsi une respiration nasale entraîne une absorption plus importante dans les voies aériennes supérieures, alors que la respiration buccale permet une pénétration plus distale du gaz.

Dans le cas particulier d'un exercice musculaire tel que la pratique d'une activité sportive, la ventilation forcée qui en résulte, facilitera une plus grande pénétration alvéolaire du NO 2 [START_REF] Sandstrom | Respiratory effects of air pollutants: expérimental studies in human[END_REF].

Le NO, peu soluble, aura pour cible l'alvéole pulmonaire, zone d'échange entre l'oxygène de l'air inhalé avec le CO 2 véhiculé par les globules rouges. C'est dans ces alvéoles que se trouvent les macrophages alvéolaires. Ces cellules sont, entre autre, capables de capter tout élément étranger (comme les particules et les bactéries) de les phagocyter et les détruire.

Les débris sont ensuite remontés grâce à un film tensioactif présent dans les alvéoles et synthétisé par les cellules qui les constituent. Cette sécrétion très riche en phospholipides s'appelle le surfactant. Il permet la remontée des résidus depuis les alvéoles jusqu'au tapis mucco-ciliaire présent dans la trachée et les bronches. Ce tapis est constitué de cellules ciliées qui provoquent par leur battement un mouvement ascensionnel des sécrétions broncho-pulmonaires et permettent aux résidus englués dans un "tapis roulant" de remonter vers la trachée et l'oesophage où elles sont habituellement dégluties. D'autre part, l'acide nitrique, issu de l'hydrolyse du NO 2 , se transforme en ions nitrites dans l'appareil respiratoire avant de pénétrer dans la circulation sanguine. Les nitrites réagissent avec l'hémoglobine pour former la nitrosylhémoglobine. Le NO quant à lui pénètre sous forme non transformé dans la circulation pour également former de la nitrosylhémoglobine.

Dans les deux cas, la majeure partie des nitrates formés à partir de la nitrosylhémoglobine sont excrétés dans l'urine par les reins, tandis qu'une partie peut atteindre l'estomac où ils sont transformés en azote gazeux, puis éliminés.

De plus, par leurs propriétés oxydantes, les oxydes d'azote vont avoir un effet délétère sur l'intégrité des membranes cellulaires et le surfactant pulmonaire.

Effets sur la morphologie du poumon

Chez l'animal, les expositions aiguës (quelques heures), inférieures à 10 ppm de NO2 sont sans effet chez le rat. Par contre, des expositions de 10 à 25 ppm, pendant 30 minutes à 36 heures, se traduisent par une perte ciliaire, une hypertrophie de l'epithélium bronchique, ainsi qu'un épaississement des cellules alvéolaires de type I, qui constituent la paroi des alvéoles, et un oedème pulmonaire qui peut être responsable de mortalité.

Les mêmes symptômes sont observés chez le cobaye à partir de 2 ppm, de tel sorte qu'il constitue un modèle plus sensible que le rat. Chez l'homme, dans le cas d'exposition accidentelle, l'intoxication suraiguë est généralement mortelle en quelques instants par arrêt cardio-respiratoire. L'intoxication aiguë évolue le plus souvent en trois phases :

1. irritation plus ou moins marquée des voies aériennes supérieures avec des bronchospasmes, toux, souvent associée à une irritation oculaire. 2. il s'en suit une phase de récupération qui dure de quelques heures à quelques jours.

Effets sur la fonction respiratoire

La plupart des résultats disponibles sont issus d'études en exposition contrôlée chez des volontaires sains ou asthmatiques ou atteint de bronchite chronique obstructive. a) chez les sujets sains, les résultats diffèrent d'une étude à l'autre [START_REF] Sandstrom | Respiratory effects of air pollutants: expérimental studies in human[END_REF]. Si cette variabilité peut être liée à la susceptibilité individuelle, elle est aussi le reflet d'un manque d'harmonie des études entre elles : concentrations testées, conditions d'analyses et types d'analyses. On retiendra cependant qu'une exposition de 15 minutes à 2 ppm de NO2 est sans effet sur la fonction respiratoire. Pour des expositions plus longues à des concentrations allant de 1,5 à 5 ppm, on enregistre une augmentation de la résistance bronchique qui est réversible après arrêt de l'exposition. Récemment, on a pu mettre en évidence une augmentation de la réactivité de sujets sains à la méthacholine suite à une exposition de 2 ppm de NO2. Chez les enfants, la fonction ventilatoire ne semble pas être altérée par de faibles concentrations de NO 2 , bien que les résultats soient contradictoires chez les très jeunes enfants [START_REF] Bâtes | Adverse health impacts of air pollution -continuing problems[END_REF]. b) chez les sujets volontaires asthmatiques, une hyper-réactivité bronchique est établie à partir de concentration supérieure à 0,2 ppm (380 ug/m 3 ), avec une diminution de la capacité vitale forcée, quel que soit l'agent stimulant (agents pharmacologiques, exercice ou air froid) [START_REF] Strand | Immédiate and delayed effects of nitrogen dioxide exposure at an ambient level on branchial responsiveness to histamine in subjects with asthma[END_REF]. c) les sujets atteints de bronchites chroniques obstructives présentent une diminution des paramètres fonctionnels respiratoires à partir d'une exposition à des concentrations de l'ordre de 0,3 ppm de NO 2 .

Mécanismes biochimiques d'altération du poumon

Certaines études réalisées sur l'homme (par l'intermédiaire notamment de lavages bronchoalvéolaires), sur l'animal, ou in vitro, au niveau cellulaire ou moléculaire, ont permis d'appréhender certains des mécanismes biochimiques conduisant à l'altération du poumon.

On a pu observer chez l'homme un recrutement de polynucléaire neutrophiles dans la fraction bronchiolaire pour des expositions de 1,6 à 3,2 ppm de NO2 [START_REF] Helleday | Differnces in bronchoalveolar cell responses to nitrogen dioxide exposure in non-smokers and smokers[END_REF]. De même une réponse inflammatoire alvéolaire précoce se traduit par le recrutement de mastocytes et lymphocytes, qui sont ces cellules impliquées dans le développement de la réponse allergique [START_REF] Sandstrom | Inflammatory cell responses in bronchoalveolar lavage fluid after nitrogen dioxide exposure of healthy subjects: a dose-response study[END_REF]. D'autre part, des études réalisées aussi bien in vitro (plasma humain) qu'm vivo, montrent des effets protecteurs de la vitamine C et E, qui sont des agents antioxydants, vis-à-vis des altérations provoquées par une exposition à NO 2 [START_REF] Haliwell | Interaction of nitrogen dioxide with human plasma. Antioxydant depletion and oxidative damage[END_REF] [START_REF] Hatch | Nitrogen dioxide exposure and lung antioxidants in ascorbic acid déficient guinea pigs[END_REF].

Enfin, des études in vivo ont permis de montrer des effets directs sur certaines protéines connues :

-une modification du collagène et de l'élastine qui constituent le tissu pulmonaire, et dont l'augmentation peut affecter l'élasticité du poumon, comme dans le cas des fibroses [START_REF] Last | Synergistic interaction between nitrogen dioxide and respirable aérosols of sulfuric acid or sodium chloride on rat lungs[END_REF], -une augmentation des protéines d'adhésion des granulocytes favorisant leur migration via l'endothélium vasculaire et donc le développement d'une réaction inflammatoire locale [START_REF] Strand | Immédiate and delayed effects of nitrogen dioxide exposure at an ambient level on branchial responsiveness to histamine in subjects with asthma[END_REF], -la diminution de l'activité de l'histamine N-méthyltransférase, qui n'est autre que l'enzyme qui dégrade l'histamine [START_REF] Hoshi | relatif à la teneur minimale en oxygène ainsi qu'aux teneurs limites en substances dangereuses admissibles dans l'atmosphère des travaux souterrains (AE-l-A, article 4[END_REF]. L'histamine est une molécule très impliquée dans l'asthme, qui intervient directement dans la broncho-constriction (stimulation des fibres musculaires ). Il est clair qu'une augmentation de la concentration en histamine ne peut que favoriser l'hyper réactivité bronchique et l'asthme.

Effets sur tes mécanismes de défense de l'appareil respiratoire II a été décrit chez l'homme pour des concentrations de 1,5 à 3,5 ppm de NO2 pendant 20 minutes, ou chez l'animal, une baisse ou même une perte des battements ciliaires au niveau de l'arbre trachéo-bronchique [START_REF] Helleday | Nitrogen dioxide exposure impairs the frequency of the mucociliary activity in healthy subjects[END_REF]. Ceci limite les capacités d'épuration décrites ci-dessus. D'autre part, des études in vitro ont montré une action sur l'intégrité de la membrane et la viabilité du macrophage. Cette cellule ne pourra donc plus assurer son rôle d'éboueur, via la phagocytose, de même que son activité bactéricide. Enfin, ces cellules seront moins à même de réguler leur sécrétion de cytokines pro-inflammatoires telles que l'IL-lp, l'IL-6, le TNFoc, TGFp, très impliquées dans la surveillance immunitaire du poumon profond [START_REF] Kienast | Modulation of IL-ip, IL-6, EL-8, TNF-a, and TGF-b sécrétions by alveolar macrophages under NO2 exposure[END_REF].

Les atteintes à différents niveaux de ces mécanismes de défenses naturelles du poumon, pourraient du moins en partie expliquer la prédisposition observée aux infections par des micro-organismes.

En effet, des études réalisées sur la souris ont montré une augmentation de la sensibilité aux infections bactériennes consécutive à une exposition au NO2. In vitro, des macrophages humains exposés au NO2 ont une capacité moindre à inactiver le virus de la grippe. Chez l'homme, on a pu faire le lien entre exposition au NO2 et infections pulmonaires, dans le cas de travailleurs exposés professionnellement.

Pouvoir génotoxique et cancérogène

In vitro, il a été montré expérimentalement que le NO et NO2 sont mutagènes sur des souches bactériennes (Salmonella typhimurium TA 100). In vivo, NO2 induit des mutations et des aberrations chromatidiennes sur les cellules pulmonaires de rats exposés à des concentrations de 8 à 27 ppm (15 à 51 mg/m 3 ).

En ce qui concerne le pouvoir cancérogène des NOx, il n'existe pas de preuve épidémiologique. De plus, expérimentalement la production de tumeurs pulmonaires chez la souris reste critiquable dans la mesure où elle a été observée que dans le cas de l'utilisation de souris A/J présentant spontanément un taux de tumeurs pulmonaires élevé.

Conclusions

Les effets actuellement connus des NOx sur la santé humaine peuvent être synthétisés comme suit :

• une relative résistance de l'appareil respiratoire pour l'exposition à des concentrations faibles de NOx,

• des lésions inflammatoires et prolifératives des voies aériennes qui sont le plus souvent réversibles sauf en cas d'exposition prolongée et importante,

• un pouvoir irritant marqué (bronches et yeux), favorisant l'hyper réactivité bronchique chez les asthmatiques,

• une augmentation de la susceptibilité aux surinfections, particulièrement sensible chez les jeunes enfants où elles peuvent laisser des séquelles respiratoires à distance,

• pas de preuve définitive d'une activité cancérogène, malgré une activité oxydante avérée qui peut altérer le matériel génétique.

  3. enfin une détresse respiratoire avec toux et oedème aiguë du poumon qui peut être ou non fatale. Dans le cas favorable, on observe souvent une guérison totale ou une broncho-pneumonie de pronostic le plus souvent favorable.Dans le cas d'expositions subchroniques et chroniques, allant de quelques semaines à plusieurs mois, des effets sur la morphologie du poumon sont observés chez la souris et le rat pour des concentrations faibles de l'ordre de 0,5 ppm de NO 2 .Les symptômes observés incluent la perte des cils au niveau de l'arbre trachéo-bronchique, une hypertrophie de l'epithélium entraînant une obstruction des bronches et une atteinte alvéolaire (remplacement des pneumocytes I par les pneumocytes II traduisant la sensibilité des pneumocytes I au NO2 ) qui perturbent les échanges gazeux. A plus long terme, entre trois et douze mois, le tissu pulmonaire se détruit progressivement ce qui mène à un emphysème chez la souris. Pour obtenir des modifications pulmonaires identiques, il faut multiplier par 5 les concentrations en NO. Chez l'homme, il semble que l'exposition prolongée à des concentrations insuffisantes pour induire un oedème puisse favoriser un emphysème. Ces résultats sont cependant discutés du fait que les NOx sont rarement émis seuls, mais en général avec d'autres polluants et il est souvent impossible d'évaluer les effets synergiques dus aux associations de polluants : NOx + O3 + particules...


Zbigniew Pokryszka

Christian Tauziède

André Carrau

Rémy Bouet

Eric Saraux

APPLICATION OF NUMERICAL GAS FLOWS MODELLING TO OPTIMISATION OF NITROGEN INJECTIONS IN THE GOAF

In Lorraine Collieries (France), the risk of spontaneous combustion in the goaf of longwall coalfaces is relatively high. Nitrogen injections into the goaf are used to control this risk. However, this practice remains relatively empirical in terms of choice of injection locations and flowrates.

Carrying out experiments in the goaf is both difficult and expensive. Thus, the CFD modelling of gas flows (air, methane and nitrogen) is an investigation method well adapted to the situation.

For 5 years, a research project has been carried out at INERIS with financial funding from Charbonnages de France and European Coal and Steel Community. So a numerical model has been developed and calibrated using a large quantity of field data and then used for various simulations.

The results of these simulations indicate that numerical modelling using CFD codes, such as PHOENICS , is a very efficient tool for improving our knowledge of flows in the goaf and for perfecting the various processes and techniques used to control firedamp and spontaneous combustion. For the particular conditions of the modelled faces, optimal nitrogen injection parameters values can now be determined, allowing increased production, diminution of costs and simultaneously safety improvement. This research is still in progress, in order to make the model more realistic. But computer modelling has already become a powerful and practical tool enabling better understanding of the complex phenomena occurring in goaf of coalfaces , thus being a valuable aid for the mining engineer.

MODELLING OBJECTIVES

The spontaneous combustion of coal and fires in the goaf of coalfaces are very serious hazards in French collieries. Quite apart from the serious implications for workers, they can cause the shutdown -sometimes permanently -of the faces, leading to loss of production or equipment.

One of the most common ways of controlling these phenomena is to inject nitrogen into the goaf.

Although there is extensive experience of this technique, its practical application to a particular face configuration is still fairly empirical, in terms of choosing the most suitable and effective approach, i.e., the number and location of the nitrogen injection points, as well as the flowrates to be used.

To improve knowledge in this field, there is continuing research programme, involving physical and numerical modelling of gas flows in the goaf of Lorraine retreat faces [START_REF] Pokryszka | Modélisation des Circulations de Gaz en Taille Rabattante[END_REF][START_REF] Jones | A Physical Scale Model of Flows in the Waste[END_REF]. One of the ways explored during this research work was the development of a CFD numerical model with the particular objectives of :

finding the optimum values of the parameters governing the injection of nitrogen for preventing and/or controlling spontaneous combustions ; -improving firedamp drainage bearing in mind the risk of spontaneous coal combustion in the goaf.

A number of results have already been obtained and are summarised here, as an aid to decision-making by mining engineers. This picture is not complete and will be added to and/or revised as the project proceeds.

OUTLINE OF THE MODELLING PRINCIPLES

The availability of CFD packages on the market has indeed made their use relatively common. CFD models have been used not only for ventilation simulation in headings [START_REF] Oberholzer | The Evaluation of Heading Ventilation Systems Through the Use of Computer Simulations[END_REF] and around machines [START_REF] Cook | Methane Safety Research for Continuous Miners in South Africa[END_REF] but also for the concerned matter [START_REF] Kershaw | Methane Control for High Production Retreat Faces[END_REF]. Specific models have also been designed for the same purpose [START_REF] Sulkowski | Position of Flammable Zone in the Goaf of a " Z-Ventilated " Face[END_REF][START_REF] Michaylov | On Void Aerodynamics in a Porous Media of a Gob Area[END_REF]. Other ways have been explored such as physical scale models (Jones et al., 1995) and specially adapted ventilation network softwares [START_REF] Banik | Ventilation Control of Self-Heating in Retreating Longwall Coal Mines[END_REF].

The INERIS numerical model is based on the use of a standard CFD package known as PHOENICS , into which the appropriate equations representing the phenomena of interest have been incorporated. The modelling of a particular configuration requires a number of operations to be performed, obviously involving simplifications. Thus it is necessary :

-to describe the geometry and intrinsic characteristics of the working. This is done using a grid technique. An elementary calculation of transfers to and from the neighbouring meshes is performed for each mesh of the grid. These exchanges depend on the incoming and outgoing flows and on the intrinsic characteristics of the zones represented : for example porosity and permeability as concerns the goaf, and resistance for the roadways and the face.

The model is only two-dimensional, first in order to simplify the problem, but also because of the 3D spatial variation of these parameters, which is difficult enough to determine in actual conditions. Accordingly, for the data introduced and the results obtained, it is assumed that the variables at one point in the two-dimensional space are a resultant of the values along the missing direction (perpendicular to the plane of the seam) ;

-to describe the boundary conditions. This part of the task is simpler, covering the incoming flows of air, firedamp and nitrogen as appropriate, the outgoing flows (air return and firedamp drainage) and the atmospheric pressure at the system inlet. However there is still a problem in describing the methane flow distribution ;

-to calibrate and validate the model by adjusting the values of the parameters : porosity, permeability and methane emission, at each point in space. To validate the model in this way, the results of the calculations (concentrations of different gases, flow velocities and directions and hence the transit time between different points) are compared with the actual data available. These data include the values of gases concentrations at a certain number of points (air return, drainage, and so on). They also include information obtained by specific tests using tracer gases : leakage flows in the goaf, transit times, etc. [START_REF] Tauziède | Ecoulement des Gaz dans la Zone de Foudroyage des Tailles Rabattantes[END_REF], 1997, Pokryszka et al., 1995).

The parameters are fitted by successive iterations (based upon intuition, knowledge of the model's sensitivity and mining engineering experience) until the differences found are regarded as acceptable.

The model can then be used for simulating particular cases or simply to investigate the sensitivity of a result to a change in a parameter whose real value is difficult to evaluate.

More details on the principles and methods used in this modelling procedure can be found in the final report of the ECSC Research Project 7220/03/277 entitled "Modelling of gas circulation in the waste of retreat coalfaces" [START_REF] Pokryszka | Modélisation des Circulations de Gaz en Taille Rabattante[END_REF]. The theoretical aspects of the modelling process have also been described in detail by [START_REF] Tauziède | Modelling of Gas Flows in the Goaf of Retreating Faces[END_REF].

DETERMINING THE OPTIMAL NITROGEN INJECTION PARAMETERS

Simulations done

After calibration on a particular face configuration, the numerical model can be used for simulations. The calculations then involve varying the parameters whose influence is to be investigated (in this case, the location and flowrate of nitrogen injection).

We describe here nitrogen injections simulated for two faces : Dora 1 Sud and Frieda 5 Sud. These faces are representative of conditions in the Lorraine Collieries. For Dora 1 Sud the simulations were done for two separate positions of the face corresponding to distances from the start line of 210 m and 340 m respectively. The Frieda 5 Sud face was simulated for the situation corresponding to a distance from the start line of 230 m.

The calculations were done assuming drainage values corresponding to actual conditions at the faces analysed. The principal features of the configurations investigated are shown in the following table: The aim of these simulations was to investigate the influence of the injection flowrate and the injection point position on the effectiveness of goaf inerting. In the presented cases of simulations, the injection point was located at the bottom of the goaf (abandoned intake roadway).

The following values were used in the calculations :

• nitrogen flowrate : 0.3, 0.6, 1.2 and 1.8 m 3 /s ;

• distance from the face to injection point : 15, 25, 50, 75 and 95 m.

From all the results obtained, we give here only those concerning the two problems most interesting from the practical point of view : the effectiveness of goaf inerting and the influence of nitrogen injections on the release of methane from the goaf to the atmosphere of the workings and to the drainage.

Effectiveness of goaf inerting

For characterising the effectiveness of goaf inerting, the parameter selected was the "width" of oxygen penetration into the waste zone. This parameter is (arbitrarily) expressed as the maximum distance between the face and the 1% oxygen contour in the goaf.

The set of results obtained for the Dora 1 Sud for two situations of face advance are given in figures 1 and 2 respectively, where the distance from the face to the 1% oxygen contour is plotted as a function of nitrogen flowrate and injection point location.

These results show that whatever the length of the goaf, the two main parameters -the distance between the face and the injection point, and the nitrogen injection rate -show limiting values. These limiting values turn out to be practically identical for the two positions of the face investigated and are about 25 m for the distance and more than 0.3 m 3 /s for the flowrate. If effective inerting is to be possible, the parameters used must be greater than these limiting values. Otherwise it is impossible to completely inert the goaf, as the maps of oxygen concentration in the goaf in figure 3 show.

The calculations show that an injection flow of 0.3 m 3 /s is insufficient to completely inert the goaf. In this case the 1% oxygen front penetrates the goaf to a minimum of 140 m.

However if the flowrate is 0.6 m 3 /s or higher, the maximum penetration of the 1% oxygen front is limited to 50 to 60 m and the effect of inerting is practically unchanged by further increases in the nitrogen flowrate (in steady conditions).

As regards the influence of the injection position, the simulations show that for nitrogen flowrates above the critical value (i.e., exceeding 0.3 m 3 /s), the inerting effect is virtually the same for injections made at distances in the goaf lying in the range 25 to 95 m.

The results obtained in this way have been combined in the form of a nomograph, showing the approximate position of the oxygen front penetrating the goaf as a function of two variable parameters : the nitrogen flowrate and the distance between the face and the injection point (figure 5a). For the Frieda 5 Sud face, the results of the simulations carried out show similar trends to those observed for Dora 1 Sud (figures 4 and 5b). However there are some significant differences:

-first, it is practically impossible to inert the goaf in such a way as to reduce the width of the zone penetrated by oxygen behind the face to less than 60 m ;

-secondly, the limiting values of nitrogen flowrate and the distance from the injection point to the face are higher than those determined for Dora 1 Sud, even though there is no drainage in the goaf of Frieda 5 Sud.

The explanation of these results stems from the difference in air leakage into the goaf. Since this rate is higher in the case of Frieda 5 Sud, the relatively high air flow circulates further in the immediate goaf area, resulting in a greater optimal distance between the face and the injection point in order to avoid diluting the nitrogen "at source". At the same time, in order to replace a greater quantity of air flowing throughout the goaf, it is necessary to utilise a proportionately higher flowrate for the nitrogen.

Influence of nitrogen injections on methane emission

The effect on methane emissions of nitrogen injection is characterised by the changing methane concentration in the air return and in the drainage system.

As regards dependence of the methane concentration in the return air on nitrogen flow, the calculations show that this falls practically in proportion with the injected nitrogen flow, implies that the total flow of air and nitrogen is higher (dilution effect).

In the case of the coalface with firedamp drainage (Dora 1 Sud), a slight but nevertheless appreciable increase in methane concentrations is calculated when the injection point is displaced towards the goaf (figure 6a). The reason for this tendency is an increasingly large nitrogen circulation in the distant parts of the goaf and a gradual replacement of methane by nitrogen in the gaseous mixture collected by the drainage systems (see This shows that when inerting a goaf which has substantial firedamp drainage, it is preferable to avoid injecting nitrogen too far into the goaf.

SUMMARY

Using a CFD code, it is possible to identify the optimal parameters for inerting before starting a face. The first step -reasoning by analogy with old faces having similar conditions -is to determine the expected leakage rate of air into the goaf. To do this, an attempt will be made to compare the face with others at which this leakage flowrate has already been measured, mainly with regard to the way in which the goaf will be opened up and then recompacted. The estimated leakage rate will make it possible to determine the extent of nitrogen injection, if any needed for the new face.

Secondly, when the face has started, specific tests will be done using a tracer gas to measure the actual leakage rate and thus normalise the face in question to the cases already analysed. Repetition of these measurements at a later date may be useful to confirm the results.

The principles to be applied in order to minimise the extent of the oxygen penetration zone in the waste are as follows:

-nitrogen injection should be done from a point upstream in the ventilation of the zone to be inerted : e.g. for faces with up cast ventilation, the injection point should be located at the bottom of the goaf (in the abandoned intake roadway) ;

-injection should be done with a minimum flowrate and at a minimum distance from the face so that the nitrogen is not diluted "at source" by the air circulating in the nearby goaf. The order of magnitude of nitrogen flows to be used for faces with a total air flow of 30 to 35 m 3 /s is as follows :

for a leakage rate of about 15% (the case of Dora 1 Sud), in order to ensure that the not-inerted or poorly inerted zone does not extend 60 m behind the face, a flowrate of 1.0 m 3 /s must be injected at 30 m from the face or 0.7 m 3 /s at 40 m from the face;

for a leakage rate of 25% (the case of Frieda 5 Sud), to obtain the same result, inject 1.8 m 3 /s at 40 m from the face or 1.3 m 3 /s at 100 m from the face. If a non-inerted zone 80 m wide is sufficient, an injection of 0.5 m 3 /s at 60 m or 1.2 m 3 /s at 40 m will suffice.

-for a given flowrate, injection should be done as near as possible to the optimal distance given above. With injection further back, the inerting would be practically as effective, but it would have the result of replacing the methane by the nitrogen and would therefore reduce the firedamp concentration in the drainage system, consequently increasing the firedamp concentration in the air return from the face.

As a general rule, the outline values given above should provide an initial approximate solution to the problem. For complex cases, the ideal approach would be to model the face numerically taking all relevant conditions into account (geometry, ventilation, effective drainage, and so on).

5.

Figure 1 .

 1 Figure 1. Influence of nitrogen injection parameters on effectiveness of goaf inerting.Simulation 1-coalface Dora 1 Sud ( June 1992 position).

Figure 2 .

 2 Figure 2. Influence of nitrogen injection parameters on effectiveness of goaf inerting.Simulation 2 -coalface Dora 1 Sud (September 1992 position).

Figure 3 .

 3 Figure 3a. Injection at 15 m, flowrate 0.3 m 3 /s

Figure 4 .

 4 Figure 4. Influence of nitrogen injection parameters on effectiveness of goaf inerting. Simulation 3 -coalface Frieda 5 Sud (December 1992 position).

Figure 5 .Figure 6 .

 56 Figure 5aFigure5b0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0Nitrogen flowrate (m3/s)

  Figure 6a

Table 1 :

 1 Characteristics of Studied Faces

	Characteristic	Unit	DORA 1 SUD	FRIEDA 5 SUD
			June 1992	September 1992	December 1992
			situation	situation	situation
	Face length
	Distance from the start line
	Average dip
	Total air flow
	Rate of air leak through the waste
	Total firedamp inflow