Douglas G Carson

A METHODOLOGICAL APPROACH TO THE SPONTANEOUS COMBÜSTION OF AGRICULTURAL DUSTS

INTRODUCTION

The most common operations in industries where combustible dusts are produced or handled are grinding, drying, pneumatic transport, filtering, and storage in silos. Unfortunately, these operations all too often give rise to fires or explosions with catastrophic consequences. Spontaneous combustion must be considered to be a potential ignition source for a dust explosion every time that a large mass of combustible dust comes in contact with air.

In this paper we present a methodological approach to the characterization of combustible dusts with respect tp self-heating and give results for certain agricultural dusts.

Our method includes the use of modeling and various laboratory tests such äs Thermal Differential Analysis coupled with Thermo-Gravimetric Analysis (DTA-TGA), Isothermal Oven Tests, and Adiabatic Calorimetry. The objective of these tests is not only to rank the various dusts by order of reactivity but to determine under what conditions a given dust will pose a threat of Spontaneous combustion.

HAZARDS OF COMBUSTIBLE DUST STORAGE OR DEPOSITS

Combustible dusts are found in industry äs finished products or process intermediates created voluntarily by milling or involuntarily by abraison of larger sized solids, such äs feed pellets or äs a by-product of the raw materials (wheat dust). Fire in a storage unit or in a dust layer can be started by an extemal ignition source or by self-heating. Combustion of dust can occur with or without flames depending on the local Ventilation conditions and diffüsion and may create hot oxygen-poor zones where flammable gases can be produced by pyrolysis or partial oxidation. When dust layers are blown into Suspension, a dust explosion can occur if there is a sufficient ignition source present. An extremely dangerous Situation is present if the dust is put into Suspension after having undergone Spontaneous combustion because the ignition source is within. Figure l summarizes some of the hazards associated with agricultural dusts.

SELF-HEATING OF AGRICULTURAL DUSTS

The physical mechanism of self-ignition can be understood by a simple energy balance. Combustible (and fennentable) materials can undergo reactions even at room temperature.

Though the amount of heat produced is not very great, if it is not dissipated to the surroundings, the temperature will rise. As the temperature rises, the oxidation accelerates. If a steady state is not achieved between the heat produced and the heat dissipated to the surroundings, the temperature will continue to climb until ignition occurs.

As for the process of heat dissipation, many agro products and virtually all dusts are excellent thermal insulators. As the size of the dust deposit or storage volume increases, the thermal insulation becomes more efficient. It thus becomes apparent that self-ignition in large volumes is more likely than in small ones.

Ignition Source (open flame, sparks, hot surface...)

Fire in Dust Layer or Storage

Ignition Source (open flame, sparks, hot surface...)

Gas EXPLOSION

Dust EXPLOSION

Figure l.

Hazards of Combustible Agricultural Dusts Self-ignition can develop at any starting temperature. At low starting temperatures, the heat production rate is low and self-ignition occurs only after a long induction time, from several days to many months, in large storage volumes. At high temperatures (such äs can be found in dryers), the reaction rate is much faster; spontaneous combustion can take place in stagnating dust deposits and the induction time is much shorter, from minutes to hours.

Between ambient temperature and 80 °C, reactions other than chemical oxidation , such äs physical or biological processes, can be responsible for rapidly heating the material up to 80 °C.

• The biological processes that lead to a rapid heating of agricultural products are essentially the respiration of living micro-organisms and biological oxidation under aerobic conditions. The latter mechanism is strongly related to moisture content ; below a certain level, biological activity does not take place. Normal storage conditions however are such that the moisture content is below this level for product quality.

• As for all dusts, an equilibrium exists between the moisture content of the dust and the surrounding atmosphere depending on the temperature and the relative hunüdity of the air. When a dust is too dry (overdrying for example) with respect to its equilibrium point, it will have a tendency to pick up moisture from the surrounding air liberating the heat of condenstion of the water. Typically, a 1% gain in moisture content leads to a 20°C adiabatic rise in temperature of the Substrate.

Above 80°C it is chemical oxidation that takes over and leads to self-ignition. However, since chemical oxidation follows the Arrhenius equation (an exponential law), the rate at which heat is produced is much greater at 80 °C than at room temperature.

To have spontaneaous combustion, several conditions must be present at the same time:

1. The substance must be combustible.

2. The combustible substance must have a sufficiently high surface area on which oxygen can react. In many industrial applications, the nature of the combustible dust and its particie size are dictated by the process. But for some agro products, highiy reactive oil from the plant can be dispersed on a porous support. 3. Oxygen must be present. The rate of heat production is a function of the partial pressure of oxygen in the atmosphere. Below a certain limit, the heat production is not sufficient to produce spontaneous combustion. 4. The temperature of the substance or the surroundings must be high enough so that the rate of oxidation is sufficient (heat production). 5. The storage or deposit configuration must be such that the heat is retained. In practical situations, dusts are very good heat insulators and heat transfer is govemed by the storage or deposit size. 6. The residence time of the dust in a given storage or deposit configuration must be long enough to allow spontaneous combustion to take place.

In order to avoid spontaneous combustion, one of these six conditions must be eliminated.

As a conceptual tool, we can construct a "self-ignition hexagon" analogous to the fire triangle (fuel, oxygen, ignition), representing the six main conditions that are needed to obtain selfignition:

COMBUSTIBLE MATERIAL STORAGE OR DEPosrrsizE HIGH SPECIFIC SURFACE

SELF-IGNITION

METHODICAL APPROACH TO THE DANGERS OF SELF-HEATING OF COMBUSTIBLE DUSTS IN DEPOSITS AND IN STRORAGE ÜNITS

The procedure to identify, characterize, and extrapolate the risks related to deposits and storage of combustible dusts is summarized in figure 3 b) Differential Thennal Analysis (DTA) coupled with a Thermo-Gravimetric Analysis (TGA) makes it then possible to rank the combustible dusts by order of risk based on their "self-heating temperature". Normally, three categories of dusts can be distinguished:

1. The least reactive dusts give a "self-heating temperature" greater than 400 °C and need no fürther testing, 2. The moderately reactive dusts give a "self-heating temperature" between 250 °C and 400 °C and undergo complementary testing if warranted by the actual industrial conditions (drying temperature, silo dimensions, storage temperature etc.) 3. Reactive dusts give a "self-heating temperature" less than 250 °C and undergo rurther testing.

Very reactive dusts give a "self-ignition temperature" less than 200 °C.

This ranking scheine has been developed at INERIS based on over a thousand tests on all kinds of combusible dusts. The basic test used at INERIS to eharacterize the oxy-reactivity of dusts is Differential Thermal Analysis (DTA) coupled with a Thenno-Gravimetric Analysis (TGA) (Figure 4). The sample size is from l to 6 grams depending on its density.

The weight of the sample placed in an oven heated at a fixed rate (5°C/min) is measured by an electronic balance äs a fünction of time and recorded by a personal Computer. The temperature of the sample and that of an inert reference substance are recorded äs well. Air pre-heated to oven temperature flows through the oven to maintain an oxidizing atmosphere. In these conditions, by comparing the temperatures of the sample and the inert reference substance (A© temperature difference), it is possible to identify endothermic or exothermic phenomena that may take place äs a fünction of temperature.

Figure 5 shows the results of a rape dust. An endothermic peak is observed at 100°C due to the evaporation of water accompanied by a 8% weight loss. A measurable exothermic oxidation is detected at about 180 °C and accelerates slowly up to the runaway point at 216°C where the sample bums freely. At the end of the experiment, there is an unbumt residue of about 12%. To facilitate the analysis of the results, we can tabulate several characteristic temperatures taken from the figures:

-the temperature at the beginning of the rapid weight loss -the temperature corresponding to the start of the exothermic reaction (usually A© = 0 ° C)

-the temperature at thermal runaway (A© = 50 ° C) taken to be the "self-heating temperature" for this particular test.

These parameters are tabulated in

°C

The self-heating temperature of a given combustible dust is not a unique temperature but depends on many factors such äs the form and dimension of the storage or deposit, the nature of the heat source, the heating rate, etc. This method does however make it possible to make a preliminary ranking of dusts by order of risk to self-heating. Further, it is sufficiently reproducible and reliable to differentiate substances that pose a real danger from those that don't.

One can see that the sunflower dust n°2 is the most reactive dust and that there is a significant difference in reactivity with the other sunflower dust taken from another installation. We would classifiy all of these dusts äs very to fairly oxy-reactive and fürther testing should be carried out based on the environment in which they are found in the industrial facility.

Determination of the Critical Self-Heating Temperature by Isothermal Storage Tests

This method consists of the determination of the critical self-heating temperature of a combustible dust äs a function of sample volume (for füll discussion of experiment, theory, and restrictions see 3.17

Bowes). The dust is placed in cubical cages whose sides are made out of a sieve material sufficiently fine to contain the sample but large enough not to interfere with the diffüsion of oxygen. The cage is placed in an air circulating oven pre-set to the desired temperature. A thermocouple placed at the center of the sample in the cage allows one to determine whether the sample comes to thermal equilibrium with the oven or if there is spontaneous combustion. The test is repeated at an oven temperature higher or lower, depending on the previous results in Order to determine the minimum temperature for spontaneous combustion for that particular volume. This temperature is called the critical temperature.

The critical temperatures for different storage volumes can be fit to a lumped parameter equation based on Prank-Kamenetskii's thermal explosion theory.

In (8c T^r 2 ) = M -N/T (l)

• 8c is a form factor (=2.6 for cubes)

• T is the ambient temperature [K]

• r is the characteristic dimension [m] of the storage volume ; it is the half-edge length of a cube.

(basically this is the distance between the most thermally insulated part of the storage volume and the extemal surface of the volume where heat is carried away from the System). • M and N are constants (independant of temperature and storage size) for a given dust that depend on its oxy-reactivity, heat capacity, particie size, themal conductivity, density, activation energy, heat ofreaction ...

The values ofM and N are found by a least squares fit ofthe experimental results to equation l.

After determining the values ofM and N for a particular dust, we can rearrange equation l to extrapolate the critical characteristic dimension äs a function of temperature:

r =Vo^T exp (M/2 + N/2/T) (2)
Other form factors can be used to fit the real industrial configuration (cy linders, layers ...). The curves that we obtain from the experimental data given in

Temperature (°C) 230 -o-ComDust -»-Wheat Dust -»-Sunflow.Dustn 0 ! -' -•-AlfalfaDust A-Sunflow.Dustn°2
It can be seen from these curves that there is little risk of spontaneous combustion for the wheat or com dusts tested at low or relatively high temperatures. The alfaifa and sunflower N° 2 dusts are more reactive and may pose a threat of self-ignition in large volumes at low temperatures or in dryers if large deposits are allowed to form. However, if mechanisms other than chemical oxidation heat the product up to 65 °C, such äs biological activity or condensation we can have self-ignition problems in moderate sized storage units. Sunflower dust ?2 is very reactive in all cases.

A more detailed analysis than given here is needed to treat borderiine cases or fix safe temperatures for dlyers.

Adiabatic Calorimeter Tests

The adiabatic calorimeter used at INERIS is shown schematically in figure 7. The principal of the method consists of maintaining the dust sample under adiabatic conditions while being traversed by a small flow of air preheated to the same temperature äs the sample. The sample holder is submerged in an oil bath. When there is a temperature rise in the sample the oil bath is heated immediately and automatically to maintain the oil bath at the same temperature äs the sample. In this way, all of the heat generated by the oxidation goes to heat up the sample. The air flow rate must be large enough to provide oxygen for the reaction but also must be small enough not to take any significant quantities ofheat away from the System.

The volume of the glass sample holder is about 20 cm3. The temperature of the sample is recorded äs a fünction of time. The rate of self-heating, which is directiy proportional to the reaction rate, is simply the slope of the temperature-time curve. We can measure self-heating rates from 0.03 to 3 K/min.

The two types of active microorganisms are the mesophiles which develop optimally up to 40 -50°C and the thermophiles which are active up to 70°C. Above 70-80°C, the population drops rapidly and the exothennic effects stop. In a confined environment, äs soon äs the level of CO-t reaches 10 % the respiratory phenonenon is blocked and the heat production falls.

An experimental procedure has been developed to study this type of biological activity.

Substances that can fennent are placed in cubic cages whose sides are made out of a metallic screen that is fine enough to hold the sample but allows the passage of air. The cage is then placed in an oven preset to the desired temperature, 35 °C. The temperature at the center of the sample is recorded äs a fünction of time äs well äs the composition of the gas phase (CO, COz, O; ) at the center of the storage. This is done by aspiration of the gas through tubes at a rate of 10 1/hour using a gas Chromatograph for analysis.

An example of the behavior of sunflower seeds is given below (Burghart and Lodel).

In the first series of experiments, sunflower seeds with a 27 % moisture level were tested in volumes of46, 150 and 300 liters to determine whether the storage volume had an influence on the heating process. The results shown in figure 9 show that the maximum temperature reached in all three cases was about 63 °C, attained after 24 to 48 hours. However it can be seen that the smaller volume heats up more quickly than the larger volumes, perhaps due to oxygen diffüsion. These results show that in a storage unit, the maximum temperature reached is independent of volume.

We can suppose that a storage unit undergoing biological activity will have an effective storage temperature equal to that which is given by the laboratory test even if the surrounding temperature and the initial storage temperature are 20 °C. Storage Volume:

•46 liters -»" 150 liters -*-300 liters

3.22

In a second series of experiments, tests were conducted in the 46 liter volume on samples with varying moisture content (obtained by adding water). The results (Table 3) show that the heating effect if a function of the moisture content, and that above 25%, the maximum temperature reached did not vary. In cases where it may be possible for the agricultual product to pick up moisutre, this test should be conducted to determine the maximum level of moisture that does not lead to seif-heating and the maximum temperature reached for use in detennining the probable storage temperature.

INTERPRETATION OF RESULTS AND CONCLUSION

The interpretation of the results is made based on the seif ignition hexagon.

For a given combustible dust or agricultural product (thus given specific surface area), we can estimate:

• the reactivity of the dust to determine whether further testing is necessary (DTA-TGA)

• the temperature rise due to biological activity (aerobic storage test)

• the temperature rise due to water vapor condensation (adiabatic calorimeter test with saturated air) These two temperature rises must be taken into account along with the process temperatures (storage, drying, ambient,... temperatures) to determine which temperature should be taken äs the probable storage temperature to be used in the following two relationships. • the relationship between storage or deposit size and the critical temperature (isothermal oven tests) • the relationship between induction time and temperature (adiabatic calorimeter test with dry air) (These two relationships are relevant to chemical oxidation and can be established äs a function of oxygen Concentration in the atmosphere if needed)

There is a high probability of spontaneous combustion if the last two relationships show that the probable relevant temperature is greater than the critical temperatures.

This method of characterization has been successfülly used at INERIS in numerous investigations and safety audits to explain and prevent fires by spontaneous combustion in agro-industries and to help in planning early detection Systems.

The results presented here also show that there are large differences in reactivity of the "same" agricultural dust, depending on its origine.

  Figure 2. Self-ignition Hexagon

Figure 3 .

 3 Figure 3. METHODOLOGY FOR THE EVALUATION OF SELF-IGNTTION IN AGRICULTURAL DUSTS

  c) Dusts able»to ferment are subjected to aerobic storage testing in order to determine the temperature rise to expect from biological reactions. d) An adiabatic calorimeter is used to determine : • the rate of öxidation äs a function of temperature which enables us to estimate the induction time before ignition äs a function of initial storage temperature • the temperature rise due to the capacity of the sample to absorb moisture form the air, e) Isothermal oven tests are made to estimate the critical temperature äs a function of storage or deposit volume. f) Tests are carried out under oxygen-poor atmospheres in order to help set the maximum pemüssible oxygen concentration during inerting operations.

Figure 4 .

 4 Figure 4. Coupled DTA -TGA Apparatus

  figure 5a

Figure 6 .

 6 Figure 6. Extrapolation of the Critical Dimension äs a Function of Temperature for Cubic Volumes of Selected Agricultural Dusts. The critical dimension is the half-edge length.

Figure

  Figure 9. Evolution of the Temperature äs a Fünction of Time for an Aerobic Storage Test on Sunflower Seeds at 27 % Moisture.

  Table l for several agricultural dusts :

	TABLE l Characteristic Temperatures from DTA -TGA Experiments on Agricultural Dusts
		Temperature at	Temperature at	Thermal runaway
	Samples	the beginning	the beginning of	temperature
		of rapid weight	exothermic reaction
		loss	AO = 0	°C	A© = 50	°C
	Rape dust	216	°C	180	°C	216	°C
	Sunflower dust n°l	200	°C	157	°C	212	°C
	Sunflower dust n°2	190	°C	110	°C	180	°C
	Wheat dust	239	°C	187	°C	240	°C
	Alfaifa dust	200	°C	150	°C	215	°C
	Com dust	230	°C	200	°C

Table 2

 2

	are presented in figure 6 for

TABLE 2 .

 2 Critical temperatures äs a Function of Volume for Selected Agricultural Dusts.

	Dust\ Volume	8cm 3	120cm 3	340cm 3	1000cm 3
	Alfaifa dust	200	°C	156	°C	148	°C
	Wheat Dust	204	°C	176	°C	169	°C	156	°C
	Rape Dust	193	°C	152	°C	142	°C	128	°C
	Sunflower Dust n° l	236	°C	177	°C	163	°C	154	°C
	Sunflower Dust n° 2	178	°C	120	°C	105 "C	81	°C
	Corn Dust	256	°C	222	°C	215	°C	195	°C

Table 3 .

 3 Aerobic Storage Tests on Sunflower Seeds äs a Function of Moisture

	Moisture in	Oven	Maximum	Maximum C02	Minimum 02
	Sunflower Seeds	Temperature	Temperature	Concentration	Concentration
				Reached
	8%	35	°C	32	°C	0.5%	17.0 %
	14.6 %	35	°C	49	°C	1.3%	16.7 %
	24.8 %	35	°C	63	°C	5.4%	15.0 %
	33.0 %	35	°C	63	°C	17.3 %	7.4%

ADIABATIC CALORIMETER fSAMPLE ^TEMPERATURE

^DRY OR HUMBMFIED AIRT

HE OIL BATH B HEATED SO THAT

Tbath s Isamplc The results can be put on an Arrhenius diagram where we plot the logarithm of the self-heating rate äs a function of inverse absolute temperature. From the slope and the intercept, we can caiculate the activation energy and the preexponential factor given in the Arrhenius equation.

ühder adiabatic conditions, the self-heating rate is at its maximum because by definition, there are no heat losses to the surroundings. We can then use the Arrhenius equation to estimate the minimum time necessary for spontaneous combustion to take place (induction time) :

Knowing A and E, we can extrapolate the calorimeter results to caiculate the induction time äs a function of the initial storage temperature at temperatures lower than those studied experimentally.

For example, when we störe a combustible dust at a given temperature, there is no danger of selfignition if the storage time is not longer than the induction time corresponding to that temperature.

In figure 8 estimates are presented for the induction time for two samples of alfaifa coming from two different industrial facilities. At 20°C, the induction times are estimated to be three weeks for one product and seven weeks for the other. They are significantly shortened however if the storage temperature is above 50°C. The adiabatic calorimeter is also used to provide data on the kinetics of vapor condensation from atmospheric humidity. In this case, the predned sample is traversed by air saturated with humidity at 40°C. The temperature rise and the rate of temperature rise are detennined starting at 40°C.

Generally, the test shows that for a l % gain in moisture content of the dust sample by condensation, the temperature rises 20 ° C (release of the heat of condensation). Depending on the sample and its level of drying, temperature rises of 20 to 30 °C have been observed within 30 to 90 minutes.

It is apparent that this heating mechanism is very fast compared to oxidation at these temperatures.

Biological Oxidation

The respiratory activity of microorganisrns is an exothermic phenomenon that produces heat and COz which develops in an oxidizing atmosphere(aerobic). We can see that schematically from the oxidation of hexose :

C(iHi206+ 60z^6C02+6H20+674 kcal


E Ruffin

Y J Mouilleau

Chaineaux

Large scale characterisation of the concentAtion field of supercritical jets of hydrogen and methane

INTRODUCTION

When an orifice or breach appears in the wall of a tank containing flammable gas under pressure. ajet is created which develops into an explosive cloud. Research has shown that the intensity of the explosion likely to take place in this cloud is then hiehly variable and depends on the cloud characteristics, such äs the concentration of combustible material. the velocity field and turbulence. The experimental study developed at INERIS' sought to characterise the clouds formed by supercritical jets of methane and hydrogen, and the overpressures resulting from ignition of the jets at different points. Only the work on the concentration fields will be reported here. The Parameters determining the composition ofthe explosive cloud in the experiments carried out were: the gas used (methane or hydrogen), the vent orifice diameter (25. 50, 75. 100 or 150 mm) and the time t after the commencement of veniing since the tank is of finite size and does not produce

•>
steady flow conditions. The volume ofthe tank (5 m-3 ) and the pressure and temperature conditions -40 bar and 288 K -inside it prior to the onset ofventing were kept constant.

THEORY

In the case of variable density subsonic jets. the concentration field can be described äs a zone of pseudo-self-preservation. It is now clearly established that the decrease along the axis is a hyperbolic rünction of the distance from the orifice and that it depends strongly on the ratio (Rp) ofthe densities ofthe discharged gas (p,) and the surrounding gas (p^). and the discharge diameter Dj. The equivalent discharge diameter Dgq allows a unique expression ofthe axial concentration profiles in thesejets äs follows:

C l Di r-> l Den MI -^=-L--L-Ri'-^l--Ei-
with D^DiR 17 -and C, -l (la.b)

Cj BX-Xc p BX-Xc eq J p J
where XQ is the virtual abscissa of the hyperbolic decrease and X is the distance from the orifice (discharge section). This behavior was first noticed by [START_REF] Thring | 4th Int. Symp. Comb[END_REF] and later confirmed by [START_REF] Chen | The science & appl. of heat and mass transter[END_REF], [START_REF] Pitts | [END_REF] and [START_REF] Djeridane | Colloque d'Aerodynamique Appliquee[END_REF]. In fact for a given discharge velocity, the equivalent diameter Dgq can be interpreted äs the nozzle diameter ofajet of density pg should have such that its scalar flux N, = p,U,C,D,2 is the same äs that ofajet of density p, discharged through an orifice of diameter D,. In the case of supercritical jets, the discharge section (nozzle) is a sonic throat. At this point, the static pressure Pnoz ls ff^^ tnan °r eq\ial to the critical pressure P^ (Pcrit = 2 Patm approximately). Since the static pressure in the discharge section is thus greater than atmospheric pressure, there is a sudden expansion of the jet (expansion zone) along which the pressure in the jet retums to ambient pressure. Instead ofconsidering the supercritical jet äs frorn its actual discharge section, it is then possible to define fictional jet emission conditions just after the expansion zone, where the pressure in the jet is once again equal to the ambient pressure. Accordingly, [START_REF] Birch | [END_REF] define a fictional jet, starting after the expansion zone and having new characteristics of diameter Dfic, velocity üfic and density pf^. [START_REF] Birch | [END_REF] suggested the p ( -> ^^r^ f-r y' 4 following fictional diameter2:

D" = D"",C, ---- -L (2) V ^mÄY 4 -^ . ^.0ŵ
hich is obtained from considerations of mass flow conservation berween the real discharge section and the fictional discharge section, putting the fictional velocity equal to the velocity ofsound in the gas making up the jet at ambient conditions of pressure and temperature. and by puttina the fictional density equal to the density of the gas under the same conditions. Notice that in the experiments carried out at INERIS, the control parameters P^,, T^o and also Dfic were variable during venting of the tank due to the finite dimension of it. Since the total pressure in the tank during venting was measured in every case. we caiculated the discharge coefficient defined äs follows:

m f v dP,^ /f --- Di"fY-n^ /- C,=-=_---/ y7P,oP,o^--l--i | ^) m,3 W,o dt ;/ ^ 4 v 2 / J
where m,^ is the isentropic mass flow and m the real mass flow which is related to the pressure gradient measured in the tank.

EXPERIMENTAL RIG AND OPERATING CONDITIONS

The experimental rig (figure l) consists ofa 5 m-' lest tank and a horizontal discharge pipe fitted with an orifice at its outer end the experimental nozzle diameter of which could be varied. Since the jets formed by the venting ofthis tank can extend axially for about 100 metres, the rig was placed on the edge ofa low cliffso that the axis ofthe horizontal jets is 5 metres above the ground. With this arrangement it can be assumed that the ground has no effect on the development ofthe jets, at least for the first 50 metres. The concentration Sensors are placed in the subsonic pari of the jet. mounted on thin cables positioned perpendicularly to the flow. Each sensor (figure 2) consists of a pellistor, a metal filament heated by an electric current and coated with a catalytic material. When a mixture ofair and flammable gas comes into contact with the material. oxidation takes place and the temperature of the filament rises. The voltage gradient needed to keep the filament at a constant temperature (about 873 K.) can be easily related to the gas concentration in the mixture. In the measurement sensors developed and used at INERIS. the mixture passes through a sonic throat which prevents any flame from propagating to the outside of the sensor, which can then • '• P. is the total pressure in the tank, P"-. is aunosphcric pressure and C^ ihc discharge coeffieient.

-2- be used in highiy flanunable environments without risk. Twelve of these sensors were built and calibrated for CH -air and H^-air mixtures. Validation tests on the measurement system (monitoring concentration changes) showed that these sensors were capable of following large changes in the concentration of these gases in air, without being affected by changes in the pressure ofthe mixture analysed, with a short response time and with no risk of igniting the mixture.

RESULTS

In each test3 , which involved characterising the concentration field of the explosive cloud obtained by venting the pressurised tank, the gas concentration was measured at different points in the subsonic portion of the jet (M < 0.3), along the axis and also transversely, while venting contihued. The results of the measurements shows the change in the concentration of gas (methane in this case) äs a function of time äs the tank empties. The measurements show that the axial attenuation of the mean mass concentration is a hyperbolic function of X (its reciprocal is linear) and that it can therefore satisfy the relation given at (l). subsonic jets are entirely comparable. In fact. one can show that the mass flow conservation hypothesis used by Birch et al. to obtain the fictional diameter verifies also the hypothesis which leads to the law of pseudo-self similarity in variable density subsonic jeis. i.e. the conservalion of the scalar flux at the exit of the jet. Consequently the fact that a common decay rate is found for the decrease ofthe scalar in supercritical jets and in variable density subsonic jets merely expresses the validitv ofone and the same conservation law which is that ofthe flux ofthe scalar quantity.

' **A**CH^--5-» » • --««rH.-100-2 , TTT H'.-y-'z • SÜCC O Ht-lUO-2 • » « • . * * « * • « <•• .1 *• :)0 4SO HIX:H,-äO-2 ----^H,-7S-2 • -• SSttf r Hi-100-2 II • • '«

CONCLUSION

First of all, the large scale experimental tests carried out at 1NERIS confirm the results obtained by Birch et al in a laboratory. In this way it was possible to validate, in an industrial-tvpe Situation and using highiy reactive mixtures. an original system with short response time for measuring concentrations of flammable gases.

Secondly. it is by no means certain that the fictional diameter proposed by Birch et al is representative ofchanges in quantities other than the mass concentration.

  Figure l: Experimental hg Figure 2: Diagram of a concentrarion sensor

Figure 3 :Figure 4 :

 34 Figure 3: Axial decrease of mass concentration äs a function of distance from the discharge section normalised by the relevant diainctcr (D ) in supcrcritic»! jets of metbane and hydrogen

tesis were made with D^,, equal to 25. 50, 75, 100 and 150 mm for CH«jets and 25, 50, 75 and 100 mm for Hijcis


Bogdan Piwakowski

Jean-Marc Watelet2

David Moreaux

Khalid Sbai

HIGH RESOLUTION SEISMIC PROSPECTION OF OLD GYPSÜM MINES -EVALUATION OF DETECTION POSSIBILITIES

INTRODUCTION

The aim of this work is to evalute the location and detection possibilities of the old Underground gypsum mines, appearing in the region of Paris, and being now unexploited, which represents a real danger both for population and building activities. The experiments* were conducted in the site affected recently by serious accidents, caused by ground collapses. They included the application of the high resolution seismic reflection, nücrogravimetry** and resistivity** methods. The results obtained were tested by the drilling. The positions of the researched galleries were generally unknown and their occurence was only suspected due to the presence ground-collapse (GC) areas caused by the natural collapse of a cavity. Some of the GCs were observed directiy at the surface, the others were detected by means of the comparison of the old and recent aerial photographs. Figure l.a. shows the general geological data ofthis site and the locations ofthe seisnüc profiles S2,S3,S4 considered in this report .

The seismic data was recorded using the 100 Hz geophones and the silenced "betsy gun" äs a source (designed in laboratory). The dosest offset had been individually adapted, based on the approach of the Optimum offset window.

RESULTS

The seismic section ofthe profile S3 (Fig. The several smaller anomalies occuring within the interval CDP( 100-200) are detected by the seismic, which, in fact, seems to be more precise and more sensible. Two ofthem (CDP 135 and CDP 195) The seismic profile S4 ( Fig. 2 a.) shows the clearly identified strong discontinuity between CDP 295 and CDP 330. In fact, this anomaly respresents probably an old gallery, partially collapsed (GC4,Figl.b) and characterised by a "lifting void" located by drilling at the depth of 11 m. A similar image of the cavity, located beyond the first geological marker, can also be also found in [l]. This detection is very well correlated both with gravimetric and resistivity results ( Conceming the detection of tunnel, we point out the basic principle of the imaging which states that the detection of an object of dimension D depends on the D/^ ratio. For the seismic imaging the possibility of detection may be expressed äs a function of the the Sc/Sf ratio, where Sf is the surface of the Fresnel zone [START_REF] Piwakowski | Underground tunnels detection and location by high resolution seismic reflection[END_REF] and Sc is the object surface included in Sf . The comparison of sections (b) and (c) of Figure 3 shows that the tunnel is detected (precisely between the CSPs 258 and 266) only at the section (c) being processed at sufficiently high frequency (Sc/Sf ratio closer to l). The gravimetric method did not give any consistent results in this case.

CONCLUSIONS

-1.The high resolution seismic is quite efficient for the location of the collapsed zones and at the same time it seems to be the most precise and sensible among the compared methods. 2.The seismic detection ofthe single and isolated tunnel of dimensions smaller and compared to the Fresnel zone, seems to be quite impossible in the real field conditions. 3.The gravimetry seems to be less sensible than seismics which detects even karstic anomalies.

However, for the greater anomalies, its results correlate wery well with seismic. 4.The resistivity is more sensible for the diffrent perturbations and remains therefore less efficient. Under good conditions, it give results well correlated with other methods.

  l.c) reveals the expected layers AG, G, M (and S in some locations). Two zones of the strong discontinuities of this strücture, observed in the intervals CDP (30-50) and CDP (200-320), well conrelate with the positions of the ground collapses GC1, GC2 and GC3 (Fig.l.b) and with the gravimetric anomalies (Fig.l.d). The resistivity measurements, having given no consistent results for this profile, are not presented.

  tested by the drilling (SD3 and SD6, Fig.l.b), reveal no artifical void apart from the relatively important natural karstic type anomalies in the MG layer.

Fig. 2 :

 2 Fig.2: (a) Seismic section of the profile S4; (b) Anomalies gravirnetriques for the area along the profile S4 (c) Anomalies of the resistivity for the profile S4

Fig. 3 :

 3 Fig.3: (a) Position of the profile S2 in regard with the location of the tunnel and of the collapsed zone; (b) obtained seismic section for D/^=0.65 and Sc/Sf=0.3 ; (c) Same äs (b) except for D/A=0.9 and Sc/Sf=0.43

  Fig.2.b,c). The seismic section S2 was conducted along the profile (Fig.S.a) including one single tunnel of width D=4 m, the collapsed zon&and un unloaded zone. The seismic image (Fig.S.b) reveals the "normal" geological structure within the zone CDP (250-273). The luck of signal respectively in the areas CDP<250 and CDP>274, is interpreted äs the occurence of the expected collapsed and filled-in ground


F Homand-Etienne

I Mamane

M Souley

P Gaviglio

M Ai Heib

Distinct element modelling and mining induced subsidence: Influence of the major faults Modelisation des affaissements miniers : Influence des faules Die verschiedenen Elemente Methode und Bergbaubodensenkung : Einfluss der Verwerfungen

This study examines the subsidence due to Underground mining works in a coal mine in France.

Coal is mined at a depth reaching 1000 m, according to longwall face method with caving. The seam ranges between 2 and 3.4 m in thickness. The dip strata does not exceed 10°. As predicted by the empirical curves, the numerical and measured maximum subsidence does not exceed 70% of seam thickness. Numerical modelling and empirical methods overestimate the subsidence for extracted longwall width greater than 700 m. RESUME : Cette etude des affaissements induits par l'exploitation de charbon a ete faite pour une mine situee ä une profondeur de 1000 m en France. La puissance de la couche exploitee par longue taille varie de 2 ä 3,4 m et le pendage des strates ne depasse pas 10°. Comme le predisent les courbes empiriques, l'affaissement maximal mesure ou caicule numeriquement n'excede pas les 70% de l'ouverture. Pour des largeurs exploitees superieures ä 700 m, les valeurs mesurees sont nettement inferieures ä celles caiculees. ZUSAMMENFASSUNG : Diese Untersuchung der durch Kohlegewinnung induzierten Bodensenkung ist in dem Rahm eine 1000 m tief französische Zeche durchgeführt. Die Mächtigkeit des durch Strebenmethod abgebauten Flöz variiert zwischen 2 und 3,4 m. Die Längsneigung der Schichten ist nicht grösser als 10°. Wie von empirischen Kurven vorausgesehen überschreitet die abgemessene oder berechnete Höchstsenkung 70% der Mächtigkeit nicht. Für über 700 m abgebauten Breiten sind die abgemessenen Werte entschiedend geringer als die berechneten Werte.

l INTRODUCTION

Knowledge of subsidence engineering is of considerable importance to the planning and development of surface. Subsidence is generally associated with the extraction of minerals and natural resources such äs oil, gas and water. It is vital for population to be aware of the risk of possible surface collapses above the Underground openings. In such situations, knowledge should be established. Prediction methods exist for determining the likely magnitude and extent of subsidence development at the surface. These methods include empirical, numerical and analytical approaehes. Empirical methods were essentially based on experience and field measurements. There are intended to similar other sites. Also, surface subsidence has been predicted using a number of numerical modeis based predominately on simplified elastic and Isotropie assumptions. Many of these methods appear to make assumptions that are invalid and, consequently, give results which oversimplify subsidence prediction.

The main objective of this study is to compare analytical, empirical and numerical methods for a specific site (Provence Colliery) in which the subsidence data were available, with intent to valid the methods of modelling and to evaluate their estimated character in relation to new sites. Empirical or analytical methods can not take into account some local phenomena well known asßight ofstairs near faults. The numerical modelling, especially the one based on the Distinct Element concepts, allows for the presence of discontinuities, therefore permits to display prominently the relative movement of discontinuities and to analyse more accurately this movement. The special feature of the numerical modelling presented here is that we have taken into account the tectonic history affecting the site during the loading sequences. Numerical analyses described in this paper were conducted using the twodimensional UDEC code.

GEOLOGICAL CHARACTERISTICS OP BASIN

The coalfield under investigation belongs to the Gardanne basin, which forms the eastem part of the Are basin, and is an E-W oriented geological unit located 10 km norm of Marseiile.

EUROCK'96, Turin, 2-5 septembre 1996, p. 1305-1312

NE

The geological structure is simple : the asymmetric syncline is strongly affected by a dome-shaped anticiine (Fig. l). In the presently mined area, the structure is overridden by a major thrust sheet overthrusting Northward (Etoile unit), due to the North-South shortening and the major tectonic event of the geological history of the region. The same episode is responsible for strike-slip faulting all over the area [START_REF] Gaviglio | Tectonic structures, strata properties and rockburst occurrence in French coal mine[END_REF].

The lignite seams belong to the Fuvelian (upper part of the Campanian), sequence which is made up of limestone. This occurrence of coal amid calcareous beds is major geological particularity of the coalfield, and is responsible for the great difference in mechanical stiffness between the coal seam and the calcareous beds forming the floor and roof. The Fuvelian limestones are 250 m thick in the mining zone. (Gaviglio etal. 1990).

The investigated zone lies in the autochthonous part of basin, just in front of the tectonic thrust wedge and bounded by the Diote fault and the Safre fault. Numerous detailed geological studies have been conducted in this region, but little attention has been paid to the fracturing pattern. However, due to Underground workings in the lignite field, detailed structural data are now available. To illustrate this, a highest density of faulting lies southward in the vicinity of the Safre fault : this zone has directiy undergone the compressive load exerted by the thrust sheet. Also, the geological history is responsible for the present state of stress, the Variation of the physical and mechanical properties of the calcareous bed, and the geometrical arrangement of the major faults planes. The principal fault planes may have an influence on the behaviour of the strata, through their geometrical characteristics, especially if their effects combine [START_REF] Gaviglio | Tectonic structures, strata properties and rockburst occurrence in French coal mine[END_REF]. This is also, confirmed in a recent study reported by [START_REF] Mamane | Modelisation ä grande echelle de l'influence de la fracturation sur les contraintes in situ et les affaissements miniers[END_REF].

Above the Grande Mine, the overburden is made up of Fuvelian, Begudian, Rognacian and Eocene, below the Fuvelian lie Cretaceous and Jurassic. The coal seam is 1000 m deep and dips East-West at an angle of 8°. The thickness of worked seam varies between 2 and 3 m. Above and below the coal seam, the rock is stratified and consists of very tough beds of limestone.

EMPIRICAL PREDICTIONMETHODS

In these methods, the maximum subsidence (Wm) is caiculated based on the extracted seam height (0), the goaf treatment coefficient (q), the nature of cover rocks and depth (H).

Conceming the influence of longwall width on the maximum subsidence, it seems that the model which is successfully utilised is based on the use of an empirical curve relating the depth (H) to the longwall width (L), äs follows :

W. L_ H m (3.1) 0
Fig. 2 shows the maximum subsidence expressed in terms of extracted seam height and results have been plotted against the raüos of the extraction width over depth below surface (L/H). Fig. 2 represents also the Provence Colliery empirical curves. Note that these curves are qualitatively similar to those obtained in other countries äs reported by [START_REF] Whittaker | Subsidence behavior of rocks structures[END_REF]. However, observations of mining subsidence reported in several countries have shown that the maximum subsidence above the longwall extraction can be appreciably influenced by the nature of the cover rocks. Another feature illustrated in this figure is : the form "S" of the empirical curve which corresponds to three distinct parts [START_REF] Arcamone | Methodologie d'etude des affaissements miniers en exploitation totale et partielle[END_REF]):

• The starting of "S" corresponds to the first effects of subsidence. The maximum subsidence Wm does not exceed 10% of the extracted seam height (0), for widths extraction involving L/H ratios which lie within of 0-0.4. Subsidence is essentially related to the settling of cover rocks. This part of curve represents a highiy effective control parameter of the magnitude of surface subsidence.

• The linear part of curve corresponds to the actual setting of subsidence due to the collapses of cover rocks. In Provence Colliery, Wm varies between 10 and 60% of extracted seam height H, for L/H within 0.4-1.2.

• The third part of "S" corresponds to the final equilibrium. An important feature of this part (in Provence case) is that, for L/H greater man 1.2 ; the value of Wm/0 tends towards a constant of 0.6-0.7. In others words, the maximum subsidence does not exceed 70% of extracted seam height in Provence Colliery case.

In order to predict the shape of subsidence profiles trough for mining dimensions, many authors have proposed some form of equation or graph. Equations may be simple or complex, but generally based on results obtained from case studies. These profiles are all of the form:

W(x) = Wmßr. x. H, C) (3.2)
where Wm is the subsidence at the centre of trough, y is the critical angle, H is the mining depth, x is the horizontal distance from the centre of extracted longwall and C is the profile fünction constant which is adjusted until a best fit Image with the measured profile is obtained.

It is clearly established that geological and hydrogeological factors play a major role on the nature and magnitude of subsidence induced by mining. Also, the empirical methods used for subsidence prediction are unable to take into account the effect of faults on the ground surface movements.

.5 l 1.5 2 2.5 Width/depth ratio (L/H) Fig. 2 -Empirical relationship between the magnitude of subsidence expressed in terms of extracted seam height and longwall width/depth ratio for coal mining operations at the Provence Co//;ery [START_REF] Arcamone | Methodologie d'etude des affaissements miniers en exploitation totale et partielle[END_REF].

NUMERICALMODELLING

Models and loading sequence

As reported in § 2, the area is constituted of Jurassic, Fuvelian, Begudian and Rognacian. In the mined area, the major tectonic events are at the root of variety of fractures, and an accurate study performed by [START_REF] Gaviglio | A fault and stress field analysis in a coal mine (Gardanne, Bouches du Rhone[END_REF] was permitted to define the main fracture trends. In particular, three major reverse faults : Diote, Etoile and Safre form the structure at the southem part.

The two-dimensional geometrical model used to simulate a cross section that strikes North-South, is based on the geological cross section ofthe basin and illustrated in Fig. 3, with respect to the average coal seam dip. The area of interest for the modelling of the "Grande Mine" site is about 4 km long and the model depth chosen is 1.2 km. The deposit is 950 m deep, the height of coal seam is 3 m and the dip strata is 8°. Mining is simulated by instantaneous excavation of 100 m of coal at every step and the excavation of the entire longwall cross section corresponds to 1000 m.

In situ stress measurements were carried out in the mine workings. These investigations have deriioftsträted thäfthe major principal stress is always horizontal, and is much higher (2 to 3 times) than the minor principal stress (03), whereas the intermediate stress is almost vertical fo'2=0v}. Also, the vertical stress is, in any case, smaller than expected from the weight of overburden (0.53 to 0.87 times). In other respects, a recent study reported by [START_REF] Mamane | Modelisation ä grande echelle de l'influence de la fracturation sur les contraintes in situ et les affaissements miniers[END_REF] showed that the in situ stress state results from the combination of the gravity and the participation of tectonic stresses. For these reasons, we performed two modeis : Model A without tectonic stress consideration and which represents the present-day conditions, and Model B involving both gravity loading and a North-South tectonic compression to the value of 5 MPa (Cs=5MPa).

The modelling sequences are performed äs follows : ßrstly, both modeis (A and B) without excavation were consolidated under in situ stresses (Fig. 4b), secondly, for model B a compressive tectonic stress is applied äs illustrated in Fig. 4b, and thirdly, the excavation was carried out (roller boundaries were applied to the sides and the base of the both modeis) and modeis were cycled to an equilibrium state for stress redistribution. Notice that the boundary conditions used during tectonic loading relate the geological history of the region äs reported by [START_REF] Gaviglio | A fault and stress field analysis in a coal mine (Gardanne, Bouches du Rhone[END_REF].

The rock mass is modelled using the fülly deformable blocks assumption. Rock blocks are assumed to be Isotropie and linearly elastic materials. In fact, different materials in Provence Colliery behave elastically [START_REF] Gaviglio | Consequences de l'evolution structurale sur les caracteristiques physiques des calcaires füveliens du bassin de l'Arc (Bouches du Rhone, France): fracturation, proprietes mecaniques, etats de contrainte[END_REF], even if materials overburden the Begudian-Rognacian at the surface are constituted to clay materials which can behave in an elastic-plastic manner.

Choice of materials andjoints characteristics

Concerning the mechanical properties of rock materials, Provence site is constituted of three major sets of materials, though in the lithologic point of view, the basin is formed of five groups (Jurassic, Fuvelian, Begudian, Rognacian and Eocene). Material number of these sets are illustrated in Table l. Mechanical characteristics of these materials also reported in Table l, are derived from laboratory tests. Unlike the rock mass properties, it was not possible to deduct in advance the mechanical properties of joints (faults and joints between materials). Therefore, the choice of these Parameters presented here is based on some approaches encountered in the literature. The normal and shear stiffness kn and ks during the consolidation were caiculated based on a recent study reported by [START_REF] Kulatilake | Use of the distinct element method to perform stress analysis in rock with non-persistentjoints and to study the effect of joint geometry parameters on strength and deformability of rock masses[END_REF]

Results

We then analysed the mining induced movements of ground surface with respect to extracted longwall width according to model A which represents the present-day conditions. A qualitative and quantitative comparison between modeis A and B was also performed. Fig. 5 shows the magnitude of subsidence expressed in terms of height (Wm/O) and the longwall width/depth ratio (L/H), compared to pre-established empirical curves. This indicates that values of Wm/0 ratio have to fall between the lower and upper empirical curves, except for large values of extracted longwall width (L>800 m). In spite of the fact that the model overestimates (much less than 7% of extracted seam height: 0) values of Wm for L>SOO, it is interesting to note that the maximum magnitude of subsidence does not exceed 70% of extracted seam height; this is agreed with field observations. Also, Fig. 5 illustrates that the caiculated values of Wm are virtually identical with those obtairied accoi-dmg to probable curve for L^400, whereas difference between simulated and empirical curves does not exceed 8% of coal seam height, for extracted width ranged between 500-700 m. Consequently, based on these observations, we note a good agreement between simulated and empirical curves. Fig. 6 shows curves of ground surface subsidence W(x), with respect to L. These subsidence profiles lead to the following comments : (i) -W(x) increase with the extracted width and the point of maximuni subsidence (i.e. M in Fig. 6) moves from north to south when the face advances. However, äs illustrated in Fig. 6, this point is not located in the middle of extracted zone (which represents the theoretical point of maximum subsidence) due to the face advance, behaviour of discontinuities and inclination of coal seam. (u) -No significant effect of Diote fault is observed at the surface when L<600 compared to Etoile and Safre faults influence clearly marked in Fig. 6.

.5 l 1.5 2 2.5 Width/depth ratio (L/H) Also, interpretation of results when L>700 shows that while Diote fault increases the magnitudes of subsidence, Safre and Etoile faults tend on the contrary, to reduce the subsidence at the northem side of Safre fault. In other terms, Diote fault effects take place after 700 m of extracted longwall width and tend to reduce the Etoile and Safre faults effects. This is clearly marked for L=1000 in Fig. 6. In order to explain this faults inter-influence with respect to the advancing face, we analysed the behaviour of faults according to extracted width. Fig. 7 shows the direction of block movements and shear displacement through disconünuities at the roof corresponding to L =2 00. Examination of these displacements with respect to L has demonstrated when the extracted width increases, points A, B, C and D reported in Fig. 7 tend to move from north to south, whereas the point E located along Diote fault moves towards the excavation. In particular, when L>700, all joints located at the roof and dipped about 8° have a dextral movement. Consequently, dextral movement occurs along Fuvelian-Begudian interface (2/1), whereas Fuvelian-Fuveliqn iomt (2/2)tends to open due to an increase öFinfIexion of Fuvelian bed at the immediate roof with increase of extracted width. Also, the part of Diote fault overburden the immediate roof Fuvelian has a dextral movement, while Etoile and Safre faults have a normal displacement, but these both faults are reverse in depth. Effectively, behaviour of Safre and Etoile faults has conditioned the evolution (virtually linear) of subsidence in the Begudian block delimited by these two faults äs illustrated in Fig. 6. Note that L=700 represents the minimum extracted longwall width provoking an entire collapse of roof, whereas extraction of 600 m of coal seam induces a maximum roof-floor convergence about 1.9 m.

Evolution of surface horizontal displacement U(x) curves with respect to L is illustrated in Fig. 8. As observed earlier in subsidence analysis, magnitudes of horizontal displacements increase with extracted width and the point of maximum subsidence (intersection of displacement curve and horizontal axis) move from north to south. Also, Fig. 8 illustrates the great part of faults in ground movements. Heaves of Diote, Safre and Etoile faults express fault opening at the surface. It is interesting to note that contrary to Safre and Etoile effects on the subsidence profiles (reducing) observed in Fig. 6 for L^700, Fig. 8 indicates an accumulation of Safre and Etoile effects on the horizontal displacements. It should be noted that constant horizontal displacement ofJurassic block delimited by Safre and Etoile faults clearly indicates that this block behave äs a rigid block, therefore practically govemed by the faults deformabüity äs explained earlier.

Differential movements of the ground surface produces ground strains, and damage of surface structures is generally correlated to the magnitude of these strains. Examination of the horizontal and vertical strain curves (i.e. dü/dx and dW/dy) not presented here, showed that simulated curve fbrms are similar to those encountered in the literature. In particular, horizontal strain curves show two distinct parts : (i) compressive strain part, around the point of maximum subsidence, (U) tensile ground strains on both sides of the previous compressive strain region. Maximum compressive strain (Emax) obtained at the excavation of the entire longwall (L=:1000) is about 1.69 mm/m which approaches the theoretical value given by the empirical relation : k Wm/H (where k is constant ranged between 0.8 and 1.5). About maximum slope of ground surface (vertical strain) and curvature obtained by differentiating the subsidence profile, we got respectively mm/m and Kma^=2.5 10~6 m' 1 . l max~• Then according to the classification of [START_REF] Baochen | Ground surface movements due to Underground excavation in the People's Republic of China[END_REF] applied to surface structures specifically to determine the extent of damage caused to the surface structures äs a result of ground deförmation, previous values of Emax, Tmax äüd Kmax lead to the following Interpretation : structures at the surface are not very sensitive to surface movements and deförmation in the case o! Provence Colliery. Table 3 summarises the values of Wm according to model with respect to extracted width, and indicates that N-S compression reduces the magnitudes of subsidence. Value of this reduction is about 60% for L<200m and tends towards a constant of 5-10% when L>600. In the later cases, shear displacements along discontinuities are qualitatively similar to those observed according to model A (i.e. Fig. 7). However for extracted width L^200, the behaviour of model B is entirely govemed by the applied o,. This has been again confirmed by the examination of displacement and subsidence profiles corresponding to L=100-200 m.

Comparison with in situ measurements

The main comparisons between empirical and numerical predicted and measured maximum subsidence with respect to extracted width are illustrated in Fig. 9. Direct comparison between empirical curves and measured Wm leads to the following observations : five of eight measurements were located between the lower and upper empirical curves, whereas three measures have been overestimated by both predictions (empirical and numeric).

It may be seen that numerical maximum subsidence predicted and the measured data are in fülly dose agreement for three particular extracted longwall widths. The corresponding longwall width/depth ratios are 0.41, 0.43 and 0.65. Also, the measured maximum subsidence corresponding to extracted longwall width/depth ratio higher than 0.71 remains lower than one which was caiculated with modeis A and B. However, recall that the previous ratio (0.7) corresponds to the minimum extracted width occurring an entire collapse of longwall roof, therefore inducing an increase of Diote fault effects äs eariier observed in Pigs 6 and 8.

Note that the induced subsidence and horizontal displacement at the location of Diote fault (i.e. relative subsidence and horizontal displacement between the left and right sides of fault) for L =1000 are respectively about 0.06 and 0.13 m, and increase at the point of maximum subsidence location (combination of Diote fault and excavation effects).

Reasons for these discrepancies between modeis and measurements for extracted width higher than 700 m are not clear, but possibly their are related to either variations of faults behaviour due to immediate roof collapse, and continuum movement of materials towards the longwall betöre the roof collapse process which depends on the extracted width. To illustrate this, an elasto-plastic analysis performed by [START_REF] Mamane | Modelisation ä grande echelle de l'influence de la fracturation sur les contraintes in situ et les affaissements miniers[END_REF] showed that maximum subsidence is reduced due to the roof collapse which occurs before 700 m of extracted longwall, therefore continuum movement of roof in present cases will be reduced due to the actual "artißcial" pillar.

Also, making Diote and Safre faults stiffer reduces the maximum subsidence for L<800 and increases the maximum subsidence for L>900, because in this case, roof collapse occurs with respect to 900 m of extracted width. In other words, maximum subsidence will be correlated to the critical extracted width which provokes the roof collapse.

Fig. 3 -Fig. 4 -

 34 Fig. 3 -Geometrical model based on the geological cross section of the basin.

Fig. 5 -Fig. 6 -

 56 Fig. 5 -Caiculated curve for subsidence with respect to the advancing face compared to empirical curves. Distance x (m) 500 1000 1500 2000 2500 3000 3500 4000

SFig. 7 -

 7 Fig. 7 -Mechanism of slip through discontinuities overhanging the Grande Mine coal seam (L=200 m).

Fig. 8 -

 8 Fig. 8 -Horizontal displacement results; illustrating the influence ofthe extracted longwall width.

Finally

  the existing N-S tectonic compression reduces the magnitudes of W m and W(x), and increases significantly the horizontal displacements U(x) with respect to extracted longwall width.5 DISCUSSION AND VALIDATION

Table l -

 l Elastic properties ofrock materials.

	250m
	ROCK MATERIALS	v	E (MPa)
	BEGUDIAN	.25	3000
	FUVELIAN	.25	24000
	JURASSIC	.25	37000
	COAL	.30	2000

  . In this approach, kn and ks depend essentially on the elastic properties of rock materials. Computed values are reported in Table2. Conceming kn and ks needed for joints during tectonic compression and excavation, it is clearly established that the mechanical behaviour ofjoint is generally non-linear and depend on large parameters such äs : stress level, dilatancy, weathering state of joints, joint wall geometry, strength and deformability of asperities. Based on the relationships suggested byBandis et al.

	(1983) and assuming that all joints have no dilatancy
	and their compressive strength (JCS) is equal to the
	uniaxial compressive strength of rock materials, the
	initial normal stiffness, the maximum normal closure
	and the shear stiffness for each discontinuity were
	caiculated. Therefore it is possible to derive the
	normal stiffness (kn) according to the normal stress

level and the hyperbolic model of

Bandis et al.

Finally, joints parameters used during tectonic loading and excavation are also, summarised in Table

2

. We assume that normal and shear behaviour of joints are linear and the shear stress is limited by Mohr-Coulomb's criterion.

Table 2 -

 2 Mechanical properties of joints.

		DU ^RING
	*	Joint materials l 2 3 ** GPa/m; MPa/m; <(> is joint friction angle. Consc V 188 2625 200 • lidatio n V Ex< ", Kn c: avation . ** ^s 75 1050 3000 7.5 25000 100 80 2500 8	^(°) 20 22 20
	1-Begudian/Fuvelian, Begudicm/Begudian, Begudian/Jurassic
	2-Fwelian/Fuvelian, Fuvelian/Jurassic
	3-Coal/Fuvelian, CoaVCoal, CoaVJurassic.

Table 3 -

 3 Comparison ofmaximum subsidence

	Wm	(cm)	Extracted	longwall	width L (m)
	Model A	L=100 1.88	L=200 8.7	L=300 21.2	L=400 42	L=500 67.6
	Model B	0.7	3.3	16	36.8	62
			L=600	L=700	L=800	L=900	L=1000
	Model A	100	131.5	154.5	171	188.3
	Model B	93.2	123.6	145.5	161.5	179
	Similarly to W m evolution according to model A, o,
	tends to reduce the subsidence magnitudes W(x) for a
	given extracted width. Recall that Cs induces dextral
	movement through faults, whereas mining workings
	generate a sinistral movement of faults. It is
	interesting to note that a combination of the two
	previous phenomena can generate another block
	cinematic at the surface in certain cases.
	In other respects, the existing of <7s develops a large
	horizontal displacement magnitudes. Contrary to
	model A, these horizontal displacements can
	approach the values of Wm-To illustrate this, value
	of horizontal displacement of the right side of Safre
	fault according to modeis B and A are respectively,
	l .24 and 0.76 m. However, horizontal displacement
	curves are qualitatively similar to those observed in
	Fig. 8 particularly for L>500 m except in the region

involving Fuvelian at the left hand side of model (i.e. where <7$ ls applied).

ACKNOWLEDGEMENTS -The authors wish to acknowledge the HBCM Unite d'Exploitation de Provence for the permission to use the subsidence data.

5.2

Comparison with the model ofBaochen (1993) Analytical model of Baochen for subsidence prediction is used to valid the numerical modelling. Validation was made for 1000 m of extracted longwall width and we present the essential results. Parameters of subsidence profiles (maximum horizontal displacement, maximum subsidence and critical angle) needed for the theoretical model, have been derived from the empirical relationships suggested by Baochen (1995). Evolution of subsidence profile according to Baochen's theory has globally the same trend than that observed using modeis A or B (i.e. Fig. 6). Also, the part of analytical and numerical profiles corresponding to the right side of the longwall match qualitatively and quantitatively. Note that empirical relationship of W m given by Baochen underestimates the maximum subsidence compared to simulations. Consequently, Baochen's model tends to underestimate the magnitudes of subsidence, particularly over the longwall. However, both points (analytical and numerical) of maximum subsidence fall in the same ränge.

If we had used the value of Wm obtained from our simulations in Baochen's model, results of this validation will be certainly ameliorated.

CONCLUSION

Ground surface movements due to mining workings were examined for a specific French 'Coal Mine. Results involve numerical, empirical and analytical approaches, and in situ measurements. Based on the analysis reported here, the main conclusions can be summarised äs follows :

• We note a good ägreement between simulated and empirical curves of maximum subsidence with respect to extracted width. As predicted by the empirical curves, the numerical and measured maximum subsidence does not reach 70% of seam height.

• Comparison with the in situ measurements has shown that five ofeight measures are located between 2 L/H the lower and upper curves and consequently agree with the values predicted by caiculations. However, significant discrepancies between numerical and empirical predictions on the one hand, and observed measurements on the other hand were noticed for extracted width greater than 700 m. Attempt of an explanation is made.

• The existing N-S tectonic compression reduces the magnitudes of subsidence and increases the horizontal displacements. • Comparison between the numerical modelling and the analytical model of Baochen for subsidence prediction indicates that the curve forms match qualitatively.


J P Pineau

J F Lechaudel

TOWARDS SHARING OF DATA FROM ACCIDENTS WITH CHEMICALS

For long with the objective of improving the protection of environment and the avoidance of accidents, plant managers, insurers, authorities, consultants, research organisations are dealing 'with learningfrom investigations of accidents with chemicals.

Many databases on chemical accidents are in existence and sharing data is initiated by database developers and end users.

Since 1993, a working group ofEuropean Safety, Reliability and Data Association (ESReDA) is dealing • with data collection, quality and dependability ofdata and networking of databases.. This work will be described, pointing out the possible use ofdata, the requirements äs seen by the end users and the issue ofa directory on existing databases. The emphasis will also be given on the minimal requirements for data collecting including sonne evaluation ofdata and the need of investigations of accidents by multidiscipliniary teams. On this last aspect, lessons drawn from French experience on accident investigations withßammable and toxic gas and liquids in plants, storages and transport will be explained.

INTRODUCTION

For long, very large accidents occurred in plants where chemicals were processed and stored. Further, transportation and use of chemicals were at the origin of very severe accidents. The manufacture of explosives was certainly the first industry manufacturing chemicals in which very strict safety precautions were taken. More recently in Europe, everybody kept in mind the large variety of possible accidents such äs those in Flixborough (explosion with 29 fatalities) in 1974, Sandoz Bäle (fire with release of chemicals in water without fatality) in 1986, La Mede 2nd European Meeting on Chemical Industry and Environment, ^ ".. ^ ^. Alghero, 11-13 septembre 1996, \/v€ 3 ^s»-c ^ .3" a»'' refmery (unconfined explosion with six fatalities) in 1992, Los Alfaques (explosion from a road tank ofpropylene with 216 fatalities) in 1976.

Accidents considered are defined äs any occurrence involving a hazardous substance such äs a major emission, fire or explosion leading to serious damage to human health or the environment including property.

The above mentioned accidents involved very severe damage and are fbrtunately rather rare. In France, every year, chemical accidents statistics are established by Bureau d'Analyses des Risques et des Pollutions Industrielles (BARPI).

For example, from July 1993 to June 1994, 683 accidents occurred in plants or during transportation : 49.20 % dangerous releases, 45.10 % fires, 6 % near misses and 5.70 % explosions. Chemical industry is implied in 6.40 % of all accidents and transport in 21.80 %. As regards the human consequences, water and air pollution, one fourth arose in transportation.

All these accidents involved detailed investigation of their root causes and the development of the sequence of events in order to reassess safety measures for the prevention of the occurrence and the protection against the effects.

In this paper, äs regards chemicals, an overview of the different means of collecting data and analysis on accidents, the improvement of data reliability and the attempts for sharing data and networking ofdatabases will be given *.

COLLECTION AND ANALYSIS OF DATA ON ACCIDENTS

Press dispatches and general newspapers give immediately Information but without detailed analysis and with large uncertainties.

On the contrary, administrative inquiries emphasize detailed information but, in many countries such äs France, are not publically available.

Open databases could be considered äs an interesting mean to have detailed information when reliable data are included. On the basis of an inquiry performed at European Safety, Reliability and Data Association, it was proved that three databases are largely used in Europe :

. ARIA, developed by Bureau d'Analyse des Risques et Pollutions Industrielles including French and abroad accidents . FACTS, which was developed and is still managed by TNO, Department of Industrial Safety, Apeldoorn, NL. * The ideas and opinions expressed in this document are these of the authors at the time of the meeting and do neither commit the French authorities concemed, nor the ESReDA "Accident Analysis" working group.

. MHIDAS, which was originally developed for Health and Safety Executive purposes in UK.

Open litterature information is included in FACTS and MHIDAS.

For ARIA, äs regards the French accidents, only disciosable informations from administrative reports are publically available.

Note : MARS database developed by Joint Research Center, Ispra, for DG XII of the Commission ofthe European Communities, is a confidential database.

All this information allows the preliminary analysis of risks and the getting of statistical data. But, for a detailed analysis of the sequence of events and the development of phenomena, more information is needed .

DIRECT INVESTIGATION OF ACCIDENTS

It is the reason why direct detailed investigation of accidents are always necessary. At many occasions, we were involved in such field investigations and the summary of our findings will be explained.

Recent field investigations dealt with :

. an unconfined vapour cloud explosion (UVCE) in a fire depot at St Herblain, October 7, 1991 (J.F. Lechaudel et al., 1995) with one fatality and extensive damages outside the depot premises . another major UVCE in the TOTAL refinery's fluid catalyting Cracking unit at La Mede (November 9, 1992) (P. Michaelis et al., 1995) with 6 fatalities and 2 billion francs damage . a fire in a fuel depot at St Ouen, June 1991 with 15 fire fighters injured and the evacuation of l.OOOpersons . ammoniac accident release in a chemical plant at Mazingarbe when unloading a railtank (December 16, 1994). About 30 tons were released . fire and explosion following derailment of unleaded füel railtanks on August 13, 1993 in La Voulte. l .000 persons evacuated . fire in aNPK temary fertilizer storage, Nantes in 1987, 30.000 persons evacuated.

Most of these investigations were carried out at the request of French competent authorities, the rest directiy for industrial firms.

Further details on other accidents investigated, either in chemical industries or in other industries are given elsewhere (J.P. Pineau et al., 1993).

Methodological approach to be used

It must be emphasized that the methodological approach is to be carefülly defined especially when the investigation team is a multidisciplinary one. Such an approach is described for La Mede's accident (P. Michaelis et al., 1993) and implies four main steps, with explanation related to this particular case :

. gathering evidence with a search for information (records, chronology, videotapes, snapshots, witnesses interviews, missiles mapping, seismic waves records ...)

. defining potential scenarios (flammable gas mass involved and explosion epicentre, identification of ignition sources, numbering of ruptured equipment, mainty the one frorn which the primary release and the explosion occurred)

. investigating the possible scenarios (four Systems remained selected)

. performing the validation of the assumption (2D modelling of dispersion Simulation, wind tunnel Simulation, dynamic process modelling, use ofmetallurgical and mechanical data ...).

Lessons leamt frorn these various accidents

From the cases in which all the above mentioned steps were used, it was possible to draw some general assumptions.

The accidents at St Herblain and La Mede pointed out the influence of the location of ignition sources when flammable gases can be released and the absolute requisite of the definition of dangerous areas for choosing the adapted equipment.

The accident in the St Ouen füel depot emphasized the role ofrepair and maintenance works and some difficulties for closing valves during fire fighting, mainly in relationship with the design.

The emission of toxic gases äs in Mazingarbe could be considered äs an illustration of a post Seveso accident. A safety System to avoid releases was installed but with a poor reliability implying failures to closing Systems.

The case of Nantes accident is an illustration of a dangerous release of toxic fumes frorn a fire and the necessary emphasis to be paid to proper distribution of goods in storages and use of adapted fire fighting methods.

The railtank accident in La Voulte is dealing with a transportation accident for which the geographical location ofthe occurrence can play a major role : explosion in a sewage System and pollution ofthe drinking water system.

The previous comments are only given äs evidence and examples of the variety of root causes and sequences ofevents to be considered.

A number of other aspects drawn from our experience may be pointed out like enough knowledge of the phenomena (by modelling or experimentation) and possible effects of explosion and fire fümes, the reliability of safety related Systems, the safe operating envelope of a given process, enough consideration of the distribution of adapted equipment in endangered areas and ofthe topography ofthe site.

A case study is always necessary and implies multi-disciplinary experts (fluids mechanics, detonics, mechanics, electricity, metallurgy, industrial risk analysis and assessment, process control).

All these remarks must be kept m mind when considering the use of databases on accidents and the collection ofdata.

EXISTING DATABASES ON ACCIDENTS

A database on accidents could be defined äs any collection of accident data independent from the storage media (paper, CD-ROM ...).

It is a common experience at a Company level that files are existing on past accidents and incidents for the purpose of leaming from the occurrences to avoid their coming back and to mitigate the consequences. The sharing of data in a large Company or between companies could be difficult by reference to different safety cultures or to differences in chemicals processed.

End-users of data could also be other organizations such äs engineering companies, competent authorities, consultants, insurers, research organisations and universities.

Objectives

Objectives are various (J.P. PINEAU et al., 1995):

. to identify accident scenarios . to identify deficiencies ofpotentially hazardous plants, buildings and transportation Systems (in design and Operation)

. to help to identify whether current emergency procedures are appropriate . to assist national and international competent authorities, financial and insurance companies to fonnulate proactive policies . to assist consultants in their tasks dealing with safety cases and experts in accident investigations . to develop quality aspects for data and Software . to improve total quality management ofsafety and training ofoperators and managers . to collect reliability data and failure rates . to look at compliance with regulations, codes ofpractice and Standards . to develop research project for understanding involved hazardous phenomena, hazardous situations and initiating events . to encourage the incorporation and use of databases in the curricula of universities and other academic courses . to improve the right to know oflarge public.

Inquiry on existing databases

A working group "Accident Analysis" ofEuropean Safety, Reliability and Data Association was initiated in January 1993 to deal with all these aspects. It decided first to prepare a questionnaire for assessing strengths and weaknesses of existing databases. This questionnaire was distributed by AA group members in 13 different countries. 93 questionnaires, valid for analysis, were returned and analysed (A.Z. KELLER, 1994).

From this investigation, about 85 databases, either dedicated in house or generic, were identified.

As a conclusion, it was emphasized in a recommendation : "Consideration be given to the future convergence of current accident databases and the development of a future European or international network accident database. Immediate consideration be given to producing an ESReDA Directory of Accident Databases".

The ESReDA Executive Committee accepted and supported by partial funding the preparation ofthe above mentioned Directory.

Directory of existing databases

The basis ofthe Directory (to be issued end of 1996) will be a description in four sections : identification, technical aspects, access conditions and use, database details.

The inquiry phase was launched in 1995 and the form addressed to the previously known database operators. An extension ofcontacts with other database managers was carried out.

Answers were received for 47 database operators. Very surprisingly, only 13 previously identified databases operators sent an answer on the basis ofthe description form.

A first conclusion is to be drawn, the interest of sharing experience is rather limited mainly of the basis ofconfidentiality ofprocesses.

As regards especially chemicals (table l), about one third of existing databases is strictiy devoted to chemicals, but really the vast majority are dealing with chemicals (refinery, offshore, pesticides, water pollution). We are also aware of the work performed at European Process Safety Center -EPSC -level on the sharing of experience from accident between chemical firms. A report was published recently under the title "Use of data on past incidents including, in particular, use to aid hazard identification and control".

Industrial areas and activities

WHICH TYPES OF DATABASES ARE NEEDED ?

When analyzing the findings of detailed field investigations and the data from existing databases, it should be fair to say that the objectives are so various that it is rather impossible that even a very extended database can cover all the fields mentioned in table l.

It implies that specialization with experts in the field to be covered is the only mean to improve the confidence on stored data.

One of the most interesting conclusion drawn from the investigation of existing databases is the very large number of accident reports mentioned in each database (about 12.000). In order to avoid multiple analysis which is time consuming, an important task should be for example the avoidance ofentry ofthe same incident or accident in various databases.

Another requirement is related to the reliability and dependability of data. Questions arose on this aspect at 1994 Autumn ESReDA seminar.

It was proved that the input of the same accident in various databases leads to uncertainties in the data included and induces loss of confidence for the users. Such a drawback should only be avoided by networking of databases dealing with the same field.

Generally speaking, the dependability of data must be improved by using a method in which both the chemicals and process or Systems involved should be checked. Efforts should be devoted to develop a well accepted accident report form.

As a common collection form, the basis could be the one used for the MARS database.

The minimum requirements for inclusion are under consideration at ESReDA "AA" working group and could äs a minimum deal with facts and analysis.

Facts

. date, time of occurrence, location and establishment (or part of it involved, with field of activity)

. immediate effects (consequences) : deaths, injuries, evacuations, material damage . ecological components : Inland, freshwater, shore, offshore, other

. mid term and long term effects . emergency measures : intemal, extemal, sheltering, evacuation, decontamination, restoration, other . specific process, equipment and substances (and their amounts)

. accident type : release, fire, explosion, other

CONCLUSION

In front of the permanent difficulty in Rock Mechanics, to collecttheeompletedatanecessarytosolveaproblem,andparticulady to know the actual geometry of ajointed rock mass, we can now be helped by spedalised Software and by statistical methods.

Then the same difficulty appears when we try to understand and to predict the behaviourofthisjointed rock-mass after reinforeement. SimulationsoftAemechanical behaviour can behelpfüllyundertaken using a numerical model.

Different methodology have been elaborated and tested in two mines (Sotiel and Brusada mines) correspönding to two different geomechanical conditions (hard rock and relaüvely soft-rock) and mining methods.

In the first Situation (Sotiel mine), the geomodel is based upon a statistical teconstitution of the block distribution, coming from some direct structural Statements. The coherence ofthis model has been established by comparing a volume ofblocks fallen in the site. The mechanical behaviour is studied by the way of a staue analysis ofstability for single rigid blocks, identified by the geomodel in the vicinity ofthe excavation. This method proved that it is possible to optimise a roof bolting pattem by testing the influence of several essenüal parameters.

In the second Situation (Brusada mine), the geomodel is reconstituted by grouping together the points of the cables which seems to behave statisdcally in a similar way, and by verifying the coherence of this division in blocks with the structural Statements. The mechanical behaviour of cables is approximated by the use the Distinct Element Method (code UDEC).

The quantitave adjustement between caiculated and measured variables representative ofthe behaviour ofthe reinforced rock mass is not excellent. The inaccuracy or the uncertainty ofsome essential data, äs well äs theassumpdons usedin the modeis have been pointed out.

In the other hand, a qualitative comparison between the theoredcal and actual behaviour is not so bad, so that we can be encouraged to improve and develop this approach.

Table l :

 l Information in the füture ESReDA Directory on accident databases (multiple answers possible)

		Number ofquotations
	offshore	12
	nuclear	4
	refining	16
	chemical	18
	transport	31
	pesticides in agriculture	1
	water pollution	5
	explosives	2
	fires	3
	mines	1

ACKNOWLEDGEMENT

The authors are gratenil to the Sodel and Brusada mines for their technical support and to the Commission of the European Community (DGXII) forits financial support.