Ph.D François Forney

Director, Jean Pierre Amartin

DEVELOPMENT OF A SEISMIC SYSTEM FOR THE DETECTION OF TRAPPED MINERS

Keywords:

When a rockburst occurs, consequences can be of great importance both for the safety of the mineworkers and the extraction in progress. After the recent dramatic accident that occurred in Lassing, Austria (1998), where miners were deadly injured, the French national coal industry has initiated a project for the development of a system for the detection of trapped miners.

We have been asked to develop a system based on the location of microseismic events generated by miners on the sidewalls of a gallery. A preliminary estimate of the quarters and galleries where the miners are expected to be trapped allows the installation of seismic sensors in a sensible place. The system is designed for a real time acquisition and processing of any microseismic event recorded in the rock mass. This paper presents the description of the experimental equipment and results of the preliminary test conducted in the French national coal mining company (Charbonnages de France) test site.

INTRODUCTION

World history of post disaster survival and rescue operations shows that, after a rock fall or rockburst, early knowledge of the number of trapped miners and their location dramatically increases their chances of survival l . Such information could be obtained if there were some kind of communication between the trapped miner and the rescue team, or if location equipment were available. A fast location of the trapped miners, with a reasonable accuracy, increases the efficiency of the mechanised operations for the retrieval of the miners and reduces the time rescue personnel are exposed to a potentially hazardous environment. In many instances, rescue teams have risked their lives in areas where there were no trapped miners only to learn later that if the rescue effort had been directed into other areas of the mine, lives could have been saved [START_REF]Post disaster survival and rescue research[END_REF] . This problem was emphasized for example in the 1968 explosion at the Consol Number 9 Mine in Farmington (US) [START_REF] Einicke | Location and monitoring for personnal safety: LAMPS[END_REF] when the accurate location, the number and the condition of the trapped miners remained totally unknown until their rescue.

More recently (1998), the worst Austrian mining accident since the 1940's happened in Lassing where ten miners were deadly injured when the talc mine caved in. A lack of information provided to the emergency services on the accurate locations of the buried miners contributed to delay the rescue operations.

Following this accident, the French colliery company decided to renew its equipment for the detection of trapped miners developed in the 1970's. The system was designed to work from a set of geophones deployed from the underground and provided a location from a graphical interpretation of seismic signals. Due to the development of new technologies and its expertise in the treatment and seismic data, we developed a system based on the same concept, using seismic sensors, with the objective to enhance the location accuracy whatever the gallery geometry. This paper describes the existing systems for the detection of trapped miners, the main characteristics of the new developed system and its first experimental results.

STATE OF THE ART

Numerous trapped miners detection systems have been developed by the mining countries to match one or all the following objectives:

1. provide information so that rescue efforts could be initiated and directed toward the proper area in the mine; 2. guide the rescue team to trapped or injured miners who might be within a few metres but buried under rocks or obscured by smoke and debris; 3. keep the communication between the members of the rescue team when progress is rendered difficult by the smoke and debris in hazardous environment;

The purpose of this paper is to concentrate on point 1.

The principal requirement for the location of trapped miners is the set up of a communication between the buried and/or injured miners and rescue teams. This establishment of information through the rock mass can either be done by generating a noise source by striking the mine gallery or by carrying some kind of transponder or tag in the miners cap lamp battery. In the first case, a seismic based technique has to be deployed whereas a communication using electro magnetic waves is required in the second case. We recall in the following section the main principles of both techniques [START_REF] Kravitz | Mine rescue and recovery: what works and what doesn't work ?[END_REF] .

Electromagnetic methods

The potential to communicate with underground trapped miners was realised in the 1920's, when simple investigation of underground reception of Long Wave commercial stations where made [START_REF] Durkin | Performance evaluation of electromagnetic technique for the detection of trapped miners[END_REF] . Nowadays, the most commonly developed materials are based on the use of electromagnetic waves between a source carried in the miner's belt and an array deployed by the rescue team 6> 7 . The Integrated Miner Safety Device developed by CSIR 1 uses this technology, which appear to be very suitable for deep conditions in local hard rock mines.

Using a tag transmitter with a 40-60 mW output and a 2.9Mhz operational frequency for the antenna, could provide at least 30 metres location range though the rock. The Bureau's Pittsburgh Research centre developed the Electromagnetic method considering the advantage of this technique for the voice communication [START_REF] Dobroski | Medium radio communication system for mine rescue[END_REF] . Results from this technique showed that the probability of detecting a signal from an underground transmitter is 45 per cent at a depth of 3000 metres and 90 per cent when the depth is reduced to 1500 metres. Further developments by SELECTRONIC (called Sirius) in South Africa (1995) for through-rock communications were carried out using modulation of electromagnetic waves by the heart beat and/or the movement of human chest while breathing.

Seismic Methods

A limited number of systems working with the seismic method have been listed in the literature. The US Bureau of Mines in 1970, following the suggestions of the National Academy of Engineering 9 , undertook the use of the seismic technique for the detection of trapped miners. The principles of this technique consist in the use of a network of geophones placed on the surface above the area where the miners are suspected to be trapped. As displayed on Figure 1, the miners are supposed to hit a part of the mine with any heavy object available in the surrounding. The generated seismic waves are then recorded on the sensors at the surface and the analysis of the travel time differences between the different sensors allows a location of the seismic noise. The resolution of an inverse problem trying to determine the best location with the only knowledge of the travel time of the seismic wave between a source and the receivers at the surface is highly dependent on two main parameters: the sensor distribution relative to the location of the seismic source and the choice of the velocity model. This system, which is still maintained operational and deployed as one element of the Mine Safety and Health Administration, consists in the use of seven subarrays of sensors; each subarray is composed of either 7 or 24 geophones. Tests carried out at various sites have shown that the detection with a single subarray was sufficient to identify the signal as coming from an underground miner 1 0 . The identification can be more precise if several subarrays can detect the signal; a location becomes possible with at least three subarrays but a minimum of five is required for a reasonable accuracy (less than 300 metres). It appeared that the expected accuracy of a few metres for the positioning of the rescue borehole was not realistic and the main source of error was attributed to the topographic relief and geologic conditions that expectedly varied with position.

The equipment developed by "Charbonnages de France" followed the demands from France and Germany to the European Community in 1968 and 1973. One of the objectives was to reduce the duration of the rescue the trapped miners by operating directly from the underground galleries rather than operating from the surface u . The main advantage of this technique is the reduced scale of the trapped miners search, limited to the vicinity of the rescue team gallery (less than a few hundred metres). Sensors were deployed along a line in the closest gallery to where the miners were trapped and the location procedure was based on a graphical interpretation of the seismograms (Figure 2). Depending upon the position of the source relative to the geometry of the sensor array, the accuracy of the location could vary from two metres (when the source was located in a gallery parallel to the geophones and at the position described on Figure 2), up to several tens of metres for perpendicular galleries. The paper graphical display of the seismograms was not constantly working and made the system not optimal for the purpose of the trapped miners rescue.

The new system, SYDEM (SYstème de Détection des EMmurés) [START_REF] Forney | SYDEM: Système de Détection des Emmurés, User's manual[END_REF] developed for "Charbonnages de France" is based on the same hypothesis but uses state-of-the-art seismic acquisition and processing techniques.

SYDEM : SYSTEM INSTRUMENTATION

Equipment description

The operation of the system is displayed in . The signals generated by the miners are recorded on the seismic probes. The geophone used, the Geospace GS-20-DH with a at 28Hz -1kHz frequency range, is housed in the seismic probe as well as the pre-amplifier. Signal is amplified with a selectable gain from 10 to 60 dB depending upon the in situ conditions and signal to noise ratio. The probes are inserted in 50 mm diameter boreholes and are coupled to the rock mass using a plastic wedge. The installation of the probes can be done for a borehole up to a depth of 5 to 10 metres. Metal cables fixed to the probes and wedges allow a fast unclamping of the probes in case they have to be quickly removed due to a damaged borehole or a low signal to noise ratio. The analog signal is sent to the acquisition unit via a cable.

The acquisition unit is composed of a filtering unit that is switch-able by the operator in case of the presence of electrical noise in the gallery (50 Hz characteristic frequency). This initial processing step eliminates interference that would, in some instance, limit the system performance. Waveforms are then stored with a length sufficient for a full recording of the signal for P and S wave arrivals, at 16-bit, 15 kHz sampling. Once the installation of the seven probes has been completed, the data acquisition can be tested using an AUTOtest function.

The response of the system to a ± 2.5 V spike is an essential control of the connection quality and satisfactory operation of each element for the acquisition after the installation.

The number of sensors for the triggering can be set depending upon environment interference (electrical or man-made) during acquisition. The acquisition is set by default to run continuously with an automatic display of the last recorded event. The automatic display option has to be unselected to perform interactive signal processing on the data. Waveform processing was manually done with the picking of the P wave on each channel of the event and the location algorithm is directly run from the acquisition software. The result of the three-dimensional location is automatically displayed on a graphical interface superposed to a drawing of the area of investigation.

All the system settings and the processing are conducted through a lap top personal computer connected to the main acquisition box. The acquisition unit is equipped with two 12 Volt batteries for an operation estimated at a maximum of 12 hours. The PC is limited to the of the battery life of 3 hours.

Location procedure

The location of a seismic source is often a complex issue due to the number of parameters affecting the solution, coming up from a non-linear problem, such as the number and geometry of the sensors, accuracy of the velocity model and the in situ conditions. In the case of a location with an acquisition unit working from the underground excavation, the geometry of the sensor array is forced by the geometry of the gallery i.e. without any boreholes, geophones are necessarily deployed along a Une.

It was possible to overcome the poor sensor coverage by using the hypothesis provided by Charbonnages de France that the system would be used in a case where the location of the trapped miners is unknown but in the limits of a given gallery. This assumption has allowed the development of a "grid search" procedure based on a comparison of calculated travel times with observed travel times (Figure 4). The gallery where the miners are suspected to be trapped is discretised with elementary volumes and travel times are calculated with straight ray paths for each gravity centre of the volume and compared to the observed travel times.

The residual of the two travel times is defined as : R.M.S. -(£ (T calculated propagation"" T observed propagation "*" A Origin)) and it is chosen as the quality criterion for the location. A high value of the R.M.S., in comparison with the rest of data set, is an indication of a large discrepancy between the observed and calculated travel times. The principal origin of these errors is usually attributed to a wrong picking of the P arrival on one or several channels, and/or a significant difference between the effective velocity field and the propagation model. On the contrary, for a given velocity model and with the experimental uncertainties (wrong picking, errors in the sensor locations), the minimum of the R.M.S. criterion is chosen as the location of the trapped miners.

Seismic noise

Seismic noise can at times be a major problem with detecting small amplitude seismic signals.

Since the signal from a trapped miner can be on the order of a few mV, a normal background noise can perturb the signal. There are three common sources of noise typically encountered in the field: (a) natural seismic background noise, (b) manmade seismic noise, and (c) electric interferences. The latter may be readily eliminated by use of the 50 Hz filter previously discussed. Since natural and man-made seismic noises tend to vary widely as a function of the geographic location and time, it is not possible to make any predictions. The solutions adopted to overcome this problem are two fold: (1) use a stacking routine during acquisition in order to enhance the signal to noise ratio, and (2) apply a numerical filter adapted to the characteristic frequency of the noise during the processing.

The data analysis has shown the miners were not likely to strike identical shots at one point of the sidewall. This implies that stacked data should be used very cautiously as the addition of two different signals on the same channel may give a wrong waveform with strong uncontrolled artefacts, perturbing the true location of the trapped miners.

EXPERIMENTAL PROCEDURE

The SYDEM was tested in different geometries with several objectives: to evaluate the efficiency of the system to locate trapped miners, estimate the influence of local parameters such as noise sources and the velocity model, and make a first estimate of the necessary time for a complete installation of the system. Since the situation the miner will face in the case of a rock burst is very unpredictable, we have conducted location tests using various types of sources that a miner may typically have around him, such as metal bars, wood pieces and a plastic helmet.

Experimental site

The system was tested in two gallery configurations : (a) parallel galleries 60 metres long and 20 metres distant, and (b) two perpendicular galleries (Figure 5). The seismic probes were installed on the sidewall facing the trapped miner gallery. The spacing between the probes was chosen to ensure the maximum coverage (10 metres average). The geology of the two test pillars is sandstone. Different sections were selected in the trapped miner gallery in order to measure the location error. Four points were defined for each section, on the sidewalls (facing and opposite to the trapped miners gallery), the roof and the floor (see Figure 5c The required time for all these operations was measured to 4 hours before the first signals were recorded. This time, however, does not take into account a realistic delay for the transport of the equipment to an underground gallery with all the constraints this may suppose. A minimum additional time of 3 to 4 hours would have to be added.

If the detection had to be carried in a mine area with the standard 5000 V in the mine, an adapted 230 V power transformer would have to be installed in the mean time.

Results

Parallel galleries

In our tests the signals were recorded with a maximum amplification of 60 dB and 15 kHz sampling. A typical event recorded on a minimum of 5 channels is presented at figure 6 5a, was placed in order to measure a travel time difference between sensor S8 and S2. For a ray propagating directly across the pillar facing the two sensors, a velocity equal to 1800 m/s was measured. In the case of parallel galleries, the 11 per cent velocity variation does not affect the location result by more than 10 per cent and can, so, be considered as a good approximation for the wave propagation model.

At each test points, miners were asked to hit a minimum of ten times in order to use the information provided by the stacked signals. Locations of some points in the different sections are presented in Table 1. The minimum location error is found when the miners are striking directly on the sidewall facing the sensors (point B2, C2 and E2). The difference for section D shots is not significant as the sidewall (shot point D2) is cemented with a poor coupling with the rock mass, causing a strong perturbation of the waveforms.

Errors on the other points in the sections (points 1, 3 and 4) are due to the variation in the velocity field. The basement of the trapped miner gallery is grouted and a higher velocity model estimated at 3000 m/s improves the location of the point B3 to less than 1.5 metres. In the case of the roof and the sidewall opposite to the seismic base, the distortion of the waveforms generated and travelling around the gallery up to the sensors cannot be taken into account in the location calculations and, as such, explains the discrepancy with the observed travel times. Table I: Locations for parallel galleries at different points in the sections (see Figure 5a and Figure 5c for notations).

The different material tested showed a significant decrease of the signal to noise ratio (40per cent) and, thus, altered the quality of the picking. The soft materials used as sources like a miner helmet caused a decrease of the signal frequency. This induced an increase of the error in the location greater than 2 metres, and up to 7.9 metres when striking the sidewall with a miner safety shoe (Table 2). Table II: Location results with different materials for the source.

Metal support

Perpendicular galleries

Three sections noted from G to I (Figure5b) have been defined in the trapped miner gallery and sources were limited to points 1 and 2. The average spacing between the probes is eight metres. The locations calculated for the six points are presented in Table III. Errors are larger than in the case of parallel galleries and this is attributed to a combination of factors.

Firstly, the seismic array is perpendicular to the direction of the sources and the solution is highly unconstrained in the direction of the trapped miner gallery. Locations found in Table III were determined for a minimum in the RMS for various velocity models. The best location was found for a velocity model of 1450 m/s. We calculated that a ± 3 per cent variation caused a 30 per cent increase of the location error and a 10 per cent velocity variation an error in location of 300 per cent, less than 15 metres in this particular case.

Secondly, the signal to noise ratio in this part of the mine was significantly lower than for the parallel galleries due to a strong attenuation of the seismic energy (Figure 8). This caused an increase in the uncertainty in picking the P arrival. This can be overcome by choosing to pick the first peak, and good quality signals to be distinctly recorded on a smaller number of channels. Table III: Locations for perpendicular galleries at different points of the sections (see Figure5b for notations).

Point

DISCUSSION

In a configuration where miners are confined to a 60 metres long gaËery parallel to the seismic base, the S YDEM is able to provide a location of the miners with an accuracy better than 2 m in 70 per cent of the case and less than 5 m in 90 per cent of the cases. The maximum accuracy (less than a meter) is obtained when the miners are striking the sidewall facing the sensors. If the miners were hitting some parts of the gallery with non-rigid objects, we have always been able to detect and locate them within a 10 metres maximum error around the true location. All these locations, however, are subjected to the choice of a velocity model as close as possible to the true P wave velocity in the rock mass. The addition of a shifted sensor from the seismic base Une allows a good estimate of the velocity when we are considering a ray path travelling directly across the pillar.

The configuration of perpendicular galleries has shown a significant sensitivity of the location algorithm to the input velocity parameter. Further developments are currently in progress for the implementation of additional parameters to help the operator in the choice of the most probable location. These complementary calculations will be validated in a set of tests planned with more complex gallery geometries and in a coal environment.

CONCLUSION

The choice of a detection technique for trapped miners is motivated with the twofold objective of efficiency and accuracy. The other techniques reviewed in the literature were not sufficiently adapted to the requirement of "Charbonnages de France" and, thus, the adopted option was to improve the existing seismic based technique. Compared to the graphical system previously used in the French colliery, the SYDEM is easy to set up with a delay no longer than eight hours between the alert and the recording of the first signals. Its ability to conduct real time and continuous acquisition make this system a more adapted equipment for rescue operations. The accuracy of the system, shown on the tests described, is sufficient to make a decision on the drilling of a rescue borehole: close enough to the trapped miners but at a reasonable distance in order to not injure the miners with the drilling.

AKNOWLEDGEMENTS

The author would like to thank the personnel of the Mine Image at Cuvelette (HBL) for their valuable help and their repeated trapped miner simulations during the tests. Thanks to Sophie

  for notations). Installation When an alert occurs where trapped miners are involved, the following operations have to be undertaken prior to any actual seismic investigations : o meeting of the rescue operating people; o transport of the SYDEM to the gallery identified has being the closest to the gallery where miners are trapped; o drilling of seven boreholes in the direction of the gallery of investigation; o accurate survey of the borehole coordinates. The surveyors also have to provide the three-dimensional orientation of the trapped miner gallery in order to set up the discretisation grid; o installation of the seismic probes and connection to the acquisition system; o testing for successful installation (AUTOtest) and set up of the acquisition parameters (noise level, amplification of the signal, coordinates of the sensors, geometry of the experiment, acquisition data);

  . The signal frequency of the signals ranged between 150 and 270 Hz using a heavy metal piece as a source and 70 to 180 Hz with a plastic helmet. The trapped miner gallery was discretised with 0.5 m elementary cubes. Due to the limited distances between sources and receivers in both experiments, errors in the picking of the arrival and in the velocity model are prone to significantly affect the accuracy of the location. The velocity model is usually unknown when arriving at a site. In this case we have compared the propagation model obtained from direct measurements (2000 m/s) with an indirect measure. A seismic probe located 5 metres ahead of the rest of the seismic line, noted S9 on Figure

FIGURESFigure 1 :Figure 2 :

 12 FIGURES

Figure 3 :CriterionFigure 4 :

 34 Figure3: SYDEM operational description, (a) A seismic event from a trapped miner is recorded on the seismic probes, (b) Probes are clamped to the rock mass through a plastic wedge system, (c) The analogue seismic event is sent to the acquisition system for filtering and A/D conversion, (d) The analysis of the P wave arrivals on the PC provides locations of the trapped miner, (e) Automatic two or three-dimensional display of the trapped miner location.

Figure 6 :Figure 7 :

 67 Figure6: Example of seismic event recorded on four channels at shot point 2 from section E (sidewall facing the rescue team gallery for parallel galleries). The waveforms are 0.6 seconds long with a 15 kHz sampling frequency. The P-wave arrivals have been picked for a grid search location processing.

Figure 8 :

 8 Figure8: Energy attenuation of a seismic shot as a function of distance to the sensors for a shot recorded in the perpendicular and parallel gallery tests. The large decrease of the energy between 10 and 20 metres in the perpendicular galleries, compared to the parallel galleries, is attributed to a significantly damaged rock mass.

Couffin, Eric Guinard and Armand Lizeur for their respective and valuable contributions to the developments and tests of S YDEM.


Keywords: microseismic network, numerical modelling, high stress concentration, rockbursts

Zones of high stress concentration induced by coal mining at a depth of 1250 meters in the Lorraine Collieries are detected ancf monitored using a combination of numerical and microseismic methods. Changes in the stress state induced by coal mining are estimated by means of numerical simulations. The areas of high stress concentration are located and monitored by local microseismic network. The study of microseismic activity recorded during mining made it possible to localise the zones of high stress concentration and validate the calculations done by numerical modelling. The results of this study hold out interesting prospects for using a combination of numerical modelling and microseismic monitoring for detecting, locating and monitoring of the zones prone to rockbursts.

Introduction

Since the middle of the 19 th century the Houillères du Bassin de Lorraine collieries have been extracting the biggest coal deposits in eastern France near the Franco-German border. The geology of the coalfield is extremely complex, in that it is affected by two faulted anticlines (Merleback and Simon) separated by a syncline with highly asymmetric edges (Marienau). This series is overlaid unevenly by the Vosge sandstone formation of a thickness varying between 200 and 300 meters according to location \ 2 (Figure 1). In the zone investigated, the seams worked lie in two clusters: lower and upper of soft coal.

Mining operations modify the equilibrium state of the rock mass. In particular, it leads to a new distribution of stresses, according to the rheological and mechanical behaviour of the structure, and this process is the cause of substantial microseismic activity ' . Some of this microseismic activity is associated with dynamic phenomena 3 ' 4 ' [START_REF] Senfaute | Microseismic Monitoring of Underground Coal Mining at the Provence Coal Mines[END_REF] . The effects of these phenomena are felt mainly in the vicinity of the coalface and can cause significant damage in the workings, such as the collapse of roadways, violent failure of pillars, or coalbursts at the seam. Research has been done to define a methodology for predicting high stress concentration zones based upon large scale numerical modelling [START_REF] Al-Heib | La planification des travaux miniers assistée par des modèles prévisionnels des états de contrainte dans les massifs rocheux[END_REF] ' [START_REF] Josien | The prevision of rockbursts in Gardanne Colliery[END_REF] In the present study, numerical modelling was combined with microseismic monitoring for detecting and locating the high stress concentration zones induced by the coal mining. It was carried out during the mining of the Irma seam at a depth of 1250 meters in the collieries of the Lorraine basin (France).

Description of the site

The Irma coal seam is worked using the longwall caving method. The width of the working face is 290 meters and the total length 1700 meters. The thickness of the seam is from 3 to 4 meters and it slopes at 20°. The working of this face involves 4 different configurations (Figure 2):

• configuration 1: the working face is 535 meters long and is located 180 meters below the old workings of the Louise seam;

• configuration 2: the working face passes through a barrier pillar 195 meters wide, known as the Louise barrier pillar;

• configuration 3: after passing through the Louise barrier pillar, for 390 meters the working face is 180 meters below old workings at the Louise panel and alongside old workings at another adjacent face (the Irma 1140 panel);

• configuration 4: the working face is alongside the old workings at an adjacent face (the Irma 1140 panel) for 580 meters.

Numerical modelling

The zones of high stress concentration are estimated by numerical modelling using the Suit3D computer code8 based on the boundary element method9 . Suit3D is a numerical code developed to calculate induced stresses and displacements dues to mining operations for tabular mines (coal mines, gold mines, etc.). The code uses the displacement discontinuity method. The advantage of Suit3D is its capacity to model group of seams close to each other; to generate mesh for any form of exploitation area and to calculate different energy parameters. The Suit3D code is able to incorporate in the calculations all the panels or seams that influence a particular sector, without limitation. The numerical modelling process at the scale of the Irma panel embraces all the panels worked in the sector (4 seams). Modelling was done in a number of phases corresponding to the different configurations in which the panels were worked (e.g.

Figure 2). The first series of numerical modelling operations sought to identify the high stress concentration zones existing before the Irma panel was worked. It revealed two such zones: one corresponds to the area of the Louise barrier pillar (zonel) and the other to an area of older adjacent worked panels: zone 2 (Figure 3a). In these areas, the normal stress concentration was about 1.2 times the initial stress. The second series of numerical modelling operations was intended to determine the changes in the zones of high stress concentration once work on the Irma panel had begun. It showed that the arrival of the working face in these areas that were already overstressed caused an extension in the overstressed zone and a substantial increase in the value of the stress. This was of the order of 1.7 and 1.9 times the initial stress (Figure 3b).

Local microseismic monitoring network

In order to locate and monitor the high stress concentration zones identified by the numerical modelling process, a local microseismic monitoring network was installed in the vicinity of the workings at the Irma panel. The network consisted of 16 sensors of the vertical geophone type installed in the roofs of the roadways (Figure 4). The geophones have a specific frequency of 14 Hz. The signals are transmitted via telephone pairs to a central data input and processing unit at the surface. Data acquisition employs a computer with a sampling frequency of 3 kHz. As soon as an event is recorded, automatic processing takes place. In this way the location, energy and magnitude of the event are automatically calculated.

Analysis of microseismic activity

Location of events

Six blast were fired in order to determine the propagation velocity of the seismic waves appropriate to precise location of the seismic epicentres in the zone of study.

Interpretation of this series of blast gave a seismic wave propagation velocity of the order of 4700 m/s. The accuracy of the location of the events using this velocity model is of the order of 20 meters. During working of panel, a total of 60,375 microseismic events were recorded, with a maximum magnitude of the order of 3. The events were located close to the working face, both in front and behind. The small seismic events, of magnitude below 1.5 (98 percent of the population) were evident throughout the workings. However the strongest events, of magnitude 1.5 or above (2 per cent of the population) appeared and were located in two zones: the zone of high stress concentration in the barrier pillar (zone 1) and the zone affected by stresses from the old adjacent workings (zone 2).

Seismic energy

The seismic energy of each event is determined using the following equation:

C r 2 V 2 (t)dt in
Where p is the density of the medium, R is the hypocentral distance; V(t) is the ground velocity and c is the group velocity of the P and S phases. This method is valid when the seismic wave is radiated spherically and symmetrically. The quality factor (Q) is not taken into account. The attenuation was estimated as not significant considering the short distances source-geophones and the recorded wavelengths. However, for the energy computation we consider the geometry effect. The calculation is done for each geophone recording a valid signal and then averaged aver that number of geophones.

Accordingly the seismic energy is estimated at the source of the event.

The correlation between the seismic energy and the cumulative advance of the workings demonstrates the different seismic patterns associated with the extraction of the panel.

Four seismic patterns were observed. Figures 5 and6 show the different seismic patterns observed throughout the working of the face:

Pattern 1: This seismic pattern appears after the face has advanced through 50 meters and is associated with the caving of the roof. This pattern shows that the seismic energy released increased as a linear function of the advance of the face for the first 500 meters of working, and is associated with small events of magnitude below 1.5.

Pattern 2: A change in the seismic pattern takes place once the working face arrives beneath the Louise barrier pillar. This new pattern is characterised by a considerable increase in the seismic energy released and is associated with the appearance of strong seismic events, of magnitude 1.5 or more. These events add to the small seismic events that are present during the first seismic pattern.

Pattern 3: A third seismic pattern appears about 25 meters after the working face has moved on from the barrier pillar. It is characterised by a substantial fall in the seismic energy released and is associated only with seismic events of low magnitude. This pattern is perturbed by the appearance of a few strong events. However the strong events are located behind the face and beneath the barrier pillar.

Pattern 4: A change in pattern appears when the working face moves on from the zone located 180 meters underneath old workings. This seismic pattern is characterised by a substantial increase in the seismic energy released and is associated with the appearance of strong seismic events.

Correlation between microseismic activity and high stress concentration zones

The numerical simulations showed two high stress concentration zones before working of the Irma panel, and an increase in stress concentration when the working face arrived in these areas. The location of high stress concentration zones, as estimated by numerical modelling, is confirmed by the microseismic activity recorded during working of the face. Analysis of the seismic energy released as a function of the advance of the face shows a strong change in the seismic pattern in the zone of the barrier pillar (zone 1) and the zone affected by stresses from the old adjacent workings (zone 2); Figure 5. These changes are associated with the appearance of strong seismic events. All the strong seismic events are located in the zones of high stress concentration identified by numerical modelling (Figure 7). The researches [START_REF] Mcgarr | Relationship of mine tremors to induced stresses and to rock properties in the focal region[END_REF] showed that in the Witwatersrand gold mines the position of seismic epicentres coincided with the zones where the changes in stress induced by mining were maxima.

The small seismic events of magnitude below 1.5 that appeared after the commencement of caving are present throughout the working and are grouped in a cluster. This cluster of events is localised in the vicinity of the face, both in front and behind, and advances with the working face. This population of low magnitude events is associated with the ruptures created during caving of the roof and the microcracking of the coal in front of the face 9 ' 10 . The strong seismic events, localised solely in the zones of high stress concentration identified by numerical modelling are associated with the opening or slipping of pre-existing cracks in this zone and/or with the creation of new failures.

Conclusions

The control of the behaviour of the rockmass under difficult conditions of mining necessitated the introduction of a methodology based upon large scale numerical modelling and microseismic monitoring.

The numerical simulations were able to identify the high stress concentration zones induced by the working faces at a depth of 1250 meters. The location of this high stress zones was confirmed by the location of the microseismic epicentres and analysis of the seismic energy released. The changes in seismic pattern and the location of the events of high magnitude were correlated with the zones where the changes in stress estimated by numerical modelling were highest. The results of this study open interesting prospects for using a combination of numerical modelling and microseismic monitoring as a means of detecting, locating and monitoring of the zones prone to rockbursts. 34 375 seismic events (0 < ML < 3) 210 seismic events (ML > 1,5)

Bibliography

Figure 7: Location of microseismic events of magnitude higher to 1.5 recorded during the first 600 meters of working of the Irma panel (the working face was located in the barrier pillar zone) and location of high stress concentration zones identified by numerical modelling.

Figure 1 :Figure 2 :Figure 3 :Figure 4 :

 1234 Figure 1 : Geological section of the Lorraine Collieries

  Irma panel (meters)

Figure 5 :Figure 6 :o

 56 Figure 5: Correlation between seismic energy and the cumulative advance of the Irma panel


A Czyz

J L Zimmer

In order to maintain the safety level of electrical equipment and diesel engines used in French coal mines susceptible to firedamp, INERIS, and Charbonnages de France (CdF) have developed with the contribution of Houillères du Bassin de Lorraine a certification procedure for the workshops which repair such equipment. Due to the decreasing plan of CdF workers, the inadequacy of some manufacturers and the missing of some others, the use of external workshops for the repairing is more and more frequent. So, it is necessary for safety reasons to ensure that such repairs are conducted in optimal conditions. The aim of Saqr-ATEX certification is to ensure that the quality and the traceability of repaired equipment used in gassy coa! mines give an equivalent safety level to the new equipment one. In order to be attractive for workshops, Saqr-ATEX is not only usable for ATEX coal mine equipment but also for ATEX equipment installed in every location where hazardous atmosphere could occur. The setting up of this certification requires:

• the development of a technical referential based upon the IEC standard 60079-19, " the writing of specific procedures for some types of equipment, • the definition of an organisation scheme in order to ensure quality and traceability, • the development of a procedure for the assessment of the workshop and concerned persons, » the development of certification procedures, • the installation of committees with users, repairers and INERIS to ensure the certification transparency. The whole is contained in the certification document Saqr-ATEX which has been registered with the French Ministry of Industry in accordance with the law related to the voluntary certification of products and services. For the time being, about 15 repairer workshops are interested by this quality certification and 4 of them will be certified in October 2001.

INTRODUCTION

The need of a certification for the safety equipment against firedamp was expressed in 1999 by the Lorraine coal mines. This need is motivated by the preservation of the safety of the underground mine exploitations even in conditions of recession of the coal mines and mining industry in France. Indeed, this recession leads to a decreasing of the investments and reduces the interest of the manufacturers of equipment. In the same way, the working units of French coal mines have no more the need to buy new equipment and are obliged to maintain existing equipment. Furthermore, because of the decrease of their manpower, the technical services or the maintenance units of coal mines can not ensure any more all the repairs. In a lot of cases the manufacturers of mining equipment have either disappeared, or abandoned the manufacturing of these equipment, or stopped supplying spare parts. It makes so difficult the repair of the apparatus by the manufacturer or by his subcontractor. The repairs should be made more and more by outside workshops not always informed about the requirements and the compulsory constraints for the electric equipment intended for mines susceptible to firedamp or for explosive atmospheres. The purpose of the certification of the repairers is so to guarantee that the repairs are made according to the state of the art and that the level of safety of the repaired equipment stays the same that of the original equipment.

EVALUATION

While the international standards and the rule relative to the electric equipment and to the diesel engines for gassy mines are very strong and binding for the construction and the installation of these appliances, there is little rules and international standards for the repair. There is an international standard relative to the repair of the electric apparatus for explosive atmospheres, the IEC 60079-19 but this does not apply to the equipment of the group I. However, the use of a single standard is not always sufficient to establish a reliable climate among the repairer and his customer and it is necessary to set up strict working procedures to establish this confidence. It is one of the objectives of the Saqr-ATEX certification. This implementation required:

• the creation of a technical guidance document for the repair based on the IEC 600079-19, • the establishment of specific procedures for various equipment, • the definition of an organisation as a quality assurance system to ensure thetraceability and the quality of the repair works, • the definition of the modes of evaluation of the workshop and the persons, • the implementation of the procedures of certification, 4 the creation of a structure of participation which includes together the certifying body, the users and the repairer to insure the transparency of the certification.

ANALYSIS Creation of a technical guidance document

We adapted the IEC standard the 60079-19 to take into account and to add the requirements relative to the group I as well as the requirements relative to the French mine regulations. However we have excluded the modification of equipment. This technical guidance document will be completed by the establishment of specific procedures for various apparatus and various types of protection. These procedures, more detailed than the technical guidance document, resume the working procedures of the various repairers. They will be validated by INERIS and the technical Unit of "Charbonnages de France". We have also used some foreign Standards on the repairing of such equipment from Australia and Great Britain.

Definition of a quality assurance system

An organisation as a quality assurance system leads to a specific plan of organisation. This plan is inspired mostly on the arrangements taken within the framework of the European directive 94/9/EC relative to equipment for potentially explosive atmospheres as regards to product quality assurance and the survey of the manufacturer. The implementation of Saqr-ATEX guarantees:

• the means and the professional skill necessary for the repairs to be realised,

• the traceability of the repairs, • the means required to realize the necessary tests and measurements, • the marking of the repaired equipment. The organisation scheme of the repairer is based on the following actors: • "the authorised person " who is the final responsible for the repair • "the person responsible for final tests", • "the person responsible for the marking", • "the person responsible for the storage and handling".

Definition of the assessment of persons and of the workshop,

The responsible persons for the repair have to, besides the knowledge in their repairing domain, know the regulations, the standardisation and the concepts of certification for the apparatus for explosive atmospheres. We foresaw so a program of training which could comprise: » awareness on the risks due to the explosive atmosphere, general points on the explosions, regulations in force, the standards guidance documents considered, • information about the protection by enclosure (" d ", " e ", " p " " m ", IP.) • information about the protection of the electronic equipment (" i ", SYST, etc.) • information on," Saqr-ATEX " technical procedures At the conclusion of the training and as soon as the Workshop is ready, the " authorised person" and " the responsible persons ", are evaluated during an audit on the following basis: » individual answers to two questionnaires, one, specific in the types of protection of the repaired equipment, and, another one, specific on the " Saqr-ATEX " procedures. The documents delivered during the training could be used, • checking of a repaired equipment. The equipment shall have an official document issued by an Official testing house, allowing its use in explosive atmosphere. At the issue of the evaluation, every person having acquired the level defined in the Saqr-ATEX procedures receives an individual "CERTIFICATE OF COMPETENCE" established for a duration of 3 years.

As regards the workshop, it is audited by INERIS and it aims at verifying the capacities of the reparing services to make the repairs according to the procedures.

It focuses on the organisation of the company in particular on: • the system of assurance ofthe quality, * the means and the professional skill necessary for the repairs to be realized, * thetraceability ofthe made repairs, • the means required to realise the necessary tests, the marking ofthe repaired materials.

Implementation of certification procedures

These procedures fulfil the French related to the certification of services and industrial products. They describe • the certification process, from the initial request ofthe repairer to the issuing of certificate, • the appeal procedures for the complains and disputes • the mark "Saqr-ATEX" All these procedures and the technical specifications are joined together in one document called "Référentiel". This document and the trademark Saqr-ATEX are registered with the Department of Industry.

The "Référentiel" is given to all the repairers before the certification.

Participation committees

In order to insure the certification transparency, they are two committees, the steering committee and the ratifying committee.

The steering committee is a thinking group. It comprises representatives of INERIS, French coal mines, users and repairers.

The ratifying committee, formed with representatives of INERIS, users, repairers and Administration has the duty to ratify the "Référentiel".

CONCLUSION

The aim of Saqr-ATEX is to become an official national certification. It ensures to demanding users, the competency and the commitment of specialized repairers and a guaranty of safety, quality and traceability. This process is developed on the request of a coal mines user, the French national coal board "Charbonnages de France". It is studied by INERIS to be generalised to all the users of equipment for explosive atmospheres (oil, chemical, food industries...) beyond the survival ofthe French National coal board.

It is hoped, to improve the safety, that a such approach should be used wherever there is a risk of an explosive atmosphere.


B Giovannini

Safety study concerning a phenolic resin injection procedure for ground reinforcement

Bruno

INTRODUCTION

The phenolic resins used in the ground consolidation process are produced in situ by mixing a preparation ofreso! and an aqueous solution of strong acid into an aqueous solution.

Resols are reactive phenolic resins involving methyl alcohol groups (-CH 2 OH) or etheroxy links (-CH2OCH2-). These are produced by fixing formaldehyde on the phenol in an alkaline medium, producing a mixture of alcoholic phenols which react together as shown in figure 1 The polycondensation occuring mechanism shown in figure 1 produces a three-dimensional lattice of the type shown in figure 2 : This lattice structure can be held at a given degree of polymerisation by lowering the temperature.

Figure2.Three-dimensionallatticeproducedbyphénol-formolpolycondensation

The resols used for ground consolidation are reactive liquid resins, of low molecular weight, the polymer lattice of which is of the type shown in figure 2. Adding an acid to a resol at room temperature causes the latter to harden by a curing mechanism of the type shown in figure 1 above. This structural process, which is accompanied by the release of heat, produces a mass of resin of high molecular weight.

It should be noted at once that the kinetics of the reaction between an acid and a resol and the degree of heat release depend on a number of parameters, including :

the acid used and its concentration, the extent of polymerisation of the resols, the number and position of the reactive groups on the resols (-CH i OH), the respective concentrations of formaldehyde and free phenol in the resols used. The phenolic resins used for ground reinforcement are usually obtained by mixing two components -resin (based on resols in an aqueous solution) and catalyst (acid solution) in a volume ratio of 1 to 1.

The reagents (resin and catalyst) arrive on site and are stored at the pithead; they are then taken into the mines and stored near to the point of use, and finally injected into the ground requiring reinforcement.

The two components used for producing the consolidation resin (resin and catalyst) are pumped separately to the injection site where they are mixed and injected into cracks in the ground requiring consolidation.

This system is designed so that the two reagents (resin and catalyst) can be pumped simultaneously in a volume ratio of 1 to 1, the principle being shown in figure 3 hereafter.

• iXl '

Compressed air supply

Catalyst transfer pipe to injection Resin transfer pipe to injection

Catalyst Resin

Figure 3. Operating principle of an injection pump

The two reagents (resin and catalyst) are pumped to the injection site through hoses inserted directly into their respective original containers. The two reagents pumped each through its own line as indicated above are then mixed actually at the injection point by means of a mixing gun in a threebranch Y configuration. Resin and catalyst are fed through the two upper branches of the Y into a static mixer.

EVALUATION Methodology for hazard analysis

The aim of this hazard analysis is to identify any dangerous situations involved in using the equipment and products in the phenolic resin injection system.

The hazard analysis is concerned first with the hazards arising inside the injection process, related to the substances used and the equipment and, secondly, to those of external origin related to activities outside the process.

From a list of the different substances and systems present in the installations concerned by the study the relevant potential hazards are identified.

The hazard analysis is an analytical tool the aim of which is systematically to review facilities within which the process under investigation is applied. This method is applied by a working group in the framework of technical meetings.

The outcome of the group's considerations are set out in summary tables of the type shown in figure 4 The products and systems identified in the phase of describing the installations were reviewed, and the potentially hazardous situations (fire, explosion, leakage, corrosion, contamination, ...) examined.

For each combination of product/system and hazardous situation, the causes leading to the hazardous situation were sought and the impact on the installation and its environment determined.

The review of existing safety devices serves as a check on whether the hazardous situations revealed by the hazard analysis have been foreseen and are detected and, if appropriate, whether protective measures are planned to limit the consequences of potential accidents.

The operating status of each system is also taken into account: this may be normal operation, commissioning, shutdown, infrequent operating conditions (for example, system flushing) or transient. The search for the consequences of the situation under investigation can make use of the knowledge of incidents already noted.

On the basis of the hazardous situations revealed and having regard to the existing safety systems, INER1S is required to give an opinion on the preventive and protective measures liable to improve the safety of the phenolic resin injection process.

Application of the hazard analysis method to the process of phenolic resin injection

In view of the above description, the products liable to be used in the installations covered by the present safety study are:

the component based on resol (resin), the acid catalyst (catalyst), the compressed air supply to the injection pump, the products used for flushing the pumping and injection lines.

The main areas and systems in which the phenolic resin injection process is utilised are: the reagent (resin and catalyst) reception area, the area at the pithead where the above reagents are stored, the route these reagents follow to reach the mine, the zone in the mine where the reagents are stored, the pumping system, the pipes carrying products to the injection site, the system for injecting resin into the rock, the rock into which the resin is injected.

The method already described was applied to each of the areas and installations listed above.

ANALYSIS

The hazard analysis carried out led to the identification of tests for characterising the inherent hazards in using the procedure for phenolic resin injection.

Toxicity

The liquid phenolic resins on the market are usually classified as non-toxic, so long as their free phenol content is limited [1]. Because the process of injecting phenolic resins necessitates a number of handling operations that could lead to one of the reagents used being released, the operator made sure, by means of tests carried out before application, that the chronic exposure limits were observed at all the workstations, notably below ventilation systems where personnel were likely to be present. Checks of this kind must be carried out at intervals, their frequency being determined by the operator's medical services.

As an illustration, the limiting exposure value for formaldehyde is 1 ppm, while the mean exposure value of this product is 0.5 ppm [START_REF]INKS -Note documentaire 1945-153-93 Valeurs limites d'exposition professionnelle aux agents chimiques en France-Décembre[END_REF]. As regards phenol vapours, the mean exposure value is set at 5 ppm [START_REF]INKS -Note documentaire 1945-153-93 Valeurs limites d'exposition professionnelle aux agents chimiques en France-Décembre[END_REF],

Thermal stability

As regards the resins used, Differential Thermal Analysis (DTA) tests were carried out by 1NERIS. These tests showed that the decomposition reaction of the resols becomes significant at about 250°C.

Similar tests revealed that the decomposition reaction of the catalyst becomes significant at about350°C.

Finally, tests indicated that the thermal decomposition reaction of polymerised phenolic resins becomes significant at a temperature of about 270°C.

Reactivity

Reactivity tests were carried out on a number of mixtures containing, first, the basic components of the phenolic resin and, secondly, components of other resins on the market likely to be used in mines. In most of the cases examined, the maximum polymerisation temperatures obtained were lower than those measured for the resin/catalyst mixture, in the same operational conditions.

Changes in the resin-catalyst proportions

Laboratory tests were carried out to examine how a change in the volumetric resin/catalyst ratio affected the maximum polymerisation temperature. The tests showed that for a certain type of phenolic resin the maximum polymerisation temperature, measured on a volume of 800 ml of resin, may be increased when the resin/catalyst mixture involves volumetric proportions of 2/1 of 4/1 rather than 1/1.

Influence of water

According to the laboratory tests, the presence of water tends to reduce the maximum polymerisation temperature of the phenolic resins used by a few degrees.

Influence of the initial reagent temperature Laboratory tests were carried out on 200 ml of mixture to demonstrate that the maximum polymerisation temperature of the phenolic resins tends to increase as the initial temperature of the reagents increases.

Influence of the volume injected

Tests were conducted to investigate how the volume of resin injected affected the maximum polymerisation temperature. For a given type of phenolic resin, the maximum polymerisation temperature measured for a mixture with a volumetric ratio of 1 to 1 can increase by about 10 degrees if the volume changes from 200 ml to 10 litres, under the same operational conditions.

In laboratory tests carried out on volumes between 200 ml and 10 litres of a resin/catalyst mixture the initial temperature of which was 30°C, the measured maximum polymerisation temperature was of the order of 90-98°C. By comparison, the polymerisation reaction of the polyurethane resins involved in incidents occurring in mines led, in laboratory tests on small quantities (a few decilitres) and for an initial product temperature of the order of 25°C, to measure maximum temperatures above 150°C.

On the basis of the foregoing, the risk of coal overheating as a result in a rise in temperature following the introduction of the phenolic resins tested does appear to be less.

Risk of self-heating of coal

Tests were carried out by INERIS to evaluate the capability of phenolic resins to cause selfheating of coal as they polymerised. These tests were conducted on coal encapsulated in a large volume of phenolic resin (about 80 litres) using a procedure already applied for other consolidation resins [START_REF]Essais d'autoéchauffementdu charbon de la Veine ALBERT au contact d'une résine de consolidation de type phénolique -Rapport INERJS -EXI[END_REF]. The test was done on one of the most reactive coals from Lorraine, crushed to a particle size of 200 J i m, the initial temperature of which was previously raised to 40°C. The test was done on a mixture of the two resin and catalyst components in the volumetric proportions that give the highest polymerisation temperature according to the results of laboratory tests. The principle of the test is shown in figure 5 A test similar to that described above was carried out by INERIS on a Lorraine coal preheated to an appreciably higher temperature than that ofvirgin ground [START_REF]Emploi des résines à base de MDI pour la consolidation des terrains -Approche préliminaire des risques -Rapport INERIS[END_REF], to simulate circumstances in which the resin, while polymerising, is in contact with coal in which the self-heating process has already begun.

Having regard to the tests conducted by INERIS on Lorraine coals an initial temperature of the coal of the order of 70°C was adopted. This latter test was carried out on mixtures of the two resin and catalyst components in the most penalising volumetric proportions (ratio corresponding to a degraded situation) and on a one-toone ratio (as recommended by the supplier).

INERJS observed no ignition of the coal under test under the conditions of this test [START_REF]Emploi des résines à base de MDI pour la consolidation des terrains -Approche préliminaire des risques -Rapport INERIS[END_REF] or for either of the two volumetric ratios of mixture tested.

In any event, these considerations are obviously meaningful only insofar as the conditions comply with the supplier's recommendations (1/1 mixture, use exclusively by injection, in limited volumes, ...).

Risk of gas ignition

The issue here is the risk of ignition of any gas present inside the cracks into which the resin is injected while polymerisation is going on. The main flammable ingredient of firedamp -methanehas a self-ignition temperature of the order of 600°C [START_REF] Medard | Les explosifs occasionnels[END_REF].

Other flammable gases such as ethane or propane have minimum self-ignition temperatures of around 500°C [START_REF] Medard | Les explosifs occasionnels[END_REF]. These values are considerably higher than the maximum polymerisation temperatures of the resins investigated during the laboratory tests done. Thus in the light of the tests carried out so far, direct ignition of gas has a result solely of the polymerisation of phenolic resins can be disregarded.

Risk of reaction with gases present in the rock

In view of the fact that formaldehyde -a highly reactive gas -is liable to be released during the polymerisation of phenolic resins, the question here is to consider the risk of reaction with gases that may be present within the rock. According to the literature [START_REF] Walker | Formaldehyde Monograph Series -Third Edition[END_REF], reactions between formaldehyde and alkanes have been observed only with hydrocarbon molecules of molecular weight equal to or higher than that of propane. It may be noted that these reactions, which can lead to the formation of molecules containing alcohol groups, are characterised by relatively slow kinetics and have been observed only in the presence of catalysts based upon organic peroxides. Also, in view of the fact that the quantity of alkanes with a molecular weight equal to or greater than that of propane in fire damp is relatively low, it is unlikely that the exothermic nature of the polymerisation reaction of phenolic resins will be modified significantly as a result of this type of secondary reaction. In the light of the above, it seems likely that the gases present in the rock will not have any significant influence on the polymerisation reaction of phenolic resins.

Moreover phenolic resins, once cured, are relatively inert chemically [7], and are unlikely to react with the principal gases present in firedamp.

Finally, the operator plans to carry out laboratory tests to investigate the interaction between methane and a phenolic resin during polymerisation.

Risk of reaction with local minerals

The question here is to consider the risk of an exothennic reaction between the products used and minerals likely to be present in the rock. This possibility cannot be set aside since the strong acids used as hardeners for resins are likely to react with limestone rocks giving rise to a release of heat and emission of carbon dioxide.

Also minerals such as pyrite are known to react with water in an exothennic manner, notably when they are in a powdery state. Since the reagents used for curing phenolic resin are in an aqueous solution, the operator has had a laboratory test carried out in order to characterise the reactivity of the phenolic resins used and of its basic ingredients with finely divided pyrite.

CONCLUSION

From the hazard analysis carried out on the entire phenolic resin injection process investigated (from arrival of the reagents at the pithead until they are used for ground consolidation), it seems that any hazard arising from the use of phenolic resins will occur essentially during the phase of polymerisation of the resin.

On the basis of laboratory tests, it appears that the rise in temperature resulting from the polymerisation of phenolic resins is appreciably less than that observed during the use of polyurethane resins under comparable conditions. This increase in temperature does not seem such as to give rise to self-sustaining heating of the coal, as part of an operating procedure that complies notably with the supplier's recommendations (l/l mixture, use exclusively by injection, ...) and so long ai> the preventive and protective measures proposed are applied.

These measures involve: Setting out the responsibilities of the different people involved in the process of using phenolic resins, starting from receipt of the reagents through to the injection of the resins into the rock. Indeed it is essential that there should be no "fuzzy areas" between any two successive operations of the above process for which nobody feels directly responsible. Preparing suitable written procedures, setting out in detail the routines to be followed for the application of phenolic resins, from the time they arrive at pithead until they are injected into the ground. The ways and means (both stationary and mobile) of fighting fire at the surface and in the mine.

The collection facilities available on the site for containing any uncontrolled release of products.

Checking that the reagent containers are generally in good condition, as well as their labelling and sealing.

Taking samples systematically from each batch of injection products supplied, with a view to carrying out tests on the physical and chemical properties, the composition and reactivity of the products (measuring the viscosity of each reagent used, measuring the maximum polymerisation temperature of the resin in the laboratory, measuring the degree of expansion during polymerisation of the resin, and the acidity of catalysts). Proper management of empty containers at worksites. Management of storage areas in the mine. The stability of the workstations. Utilising a single type of phenolic resin at any one time in the mine.

Training operators and making them aware of the hazards related to the products used.

Keeping resin ingredients at a temperature of less than 30°C. Checking that safety devices operate correctly (non-return valves, relief valves, hoses, unions, control valves, etc.). Identifying and inspecting reagent transfer pipes.

Communications between the operators responsible respectively for pumping and injection into the rock.

Ensuring that the injection of phenolic resins is strictly limited to normally fissured rock (excluding collapsed or shattered rock). Following the recommendations set out by the resin suppliers.

Quantifying the liability of coal to self-heating for all the panels where the phenolic resins will be used, to verify that it is not significantly higher than that of the specimens on which the tests were carried out.

Figure 1 .

 1 Figure 1. Reaction mechanism of phenol-formaldehyde polycondensation

Figure 5 .

 5 Figure 5. Coal Self-heating test

Figure 4. Typical hazard analysis table used for safety studies relating to the process of injecting phenolic resins

  hereunder.

	No Product or	Hazardous	Causes Consequences Existing safety	Remarks	Proposals for
	system	situation	devices		improvements


Isabelle Zdanevitch

Norbert Gonzalez-Flesca

Emmanuelle Bastin 1 '

Sandrine Bourdet

BTX diffusive sampling to characterise trafBc restriction effects

On September 22 of each year a large number of French and European towns reduce the motor vehicie traffic in their city centre. Diffusive sampling of benzene, toluene and xylene during periods of 10 hours was used to characterise the improvement of the air quality due to these traffic restrictions. Despite the short sampling duration the use of diffusive sampling led to results consistent with the reduction of the number of motor vehicies and the measured concentrations of carbon monoxide. While under normal traffic conditions most benzene roadside concentrations measured during the day under low wind speed conditions exceeded the European limit value of 5 ug/m3, these concentrations decreased to an average of 1.5 u.g/m 3 under restricted traffic conditions with only 2 out of 10 measurement sites showing concentrations exceeding the French air quality target value of 2 ug/m 3 .

l Introduction

Following the example of La Rochelle in 1997 a large number of French towns decided in 1998 to drastically reduce the motor vehicie traffic in their city centres on a demonstration day. This Operation is repeated every year on the 22" d of September, and is now implemented in a large number of European countries. In 1998, for the first national occurrence of 'En ville sans ma voiture' (traffic-free day), the French Ministry of Environment asked INERIS to characterise the improvement in air quality by measuring the decrease in roadside benzene, toluene and xylene (BTX) concentrations due to these traffic restrictions. In association with the local Air Quality Monitoring Network AIRLOR, with whom a previous study had been conducted [l], we carried out a 3 day campaign in the centre of Nancy, one of the largest cities m north-east of France. The city of Nancy was chosen because the traffic restrictions were imposed over a large area. Measurements were taken using sampling on diffüsion tubes [START_REF] Brown | Diffüsive sampling using tube-type samplers[END_REF], followed by GC-FID analysis.

Diffusive sampling appeared to be the best method to fülfil the aims of the Operation because :

• diffusive samplers allow a quasi-simultaneous sampling at different sites • the method is lightweight and therefore easy to use in multi-site applications • the integration time covers well the period of interest.

Experimental

In order to compare the pollutants' concentrations with and without traffic under similar meteorological conditions and to compare them with other available data (traffic counts, CO monitoring), BTX were sampled at 10 different sites over periods of 10 hours on each of the 3 days including the 'traffic-free day'. The sampling points chosen were typical roadside sites dose to the kerbside. Aluminium shelters were placed at sampling heights of 1.7 to 2.6 m, depending on the available support.

Passive samplers were Perkin Eimer tubes loaded with 130 mg of Carbotrap B [START_REF] Bates | Ambient Volatile Organic Compound Monitoring by Diffüsive Sampling. Compatibility of High Uptake Rate Samplers with Thermal Desorption[END_REF]. The difiüsion path length with the Perkin Eimer diffüsion head is typically 21 mm. Each day of the eampaign the tubes were opened early in the moming, placed in the shelters, and closed at the end of the day. On September 22 it was ensured that all the tubes were closed before the traffic started again at 7 p.m. It can be assumed that the amount of compounds trapped during a period of typically 10 hours, especially during the traffic-free day, would be small. Therefore the tube blanks had all been carefülly checked before going to the site, and the analyses were done shortly after retum of the samples to the laboratory. Tubes were thermally desorbed in a Perkin Ebner ATD400 and analysed in a Chrompack CP 9002 gas Chromatograph with an FED and a CP-SDL 5 CB column.

Results and discussion

Measurement results

Concentrations for benzene, toluene and xylenes are given in Table l.

Correlation with other data

We have caiculated the decrease in BTX concentrations for different sites äs where C g is the concentration in ug/m 3 of a compound on September 22 and Q is the concentration on September 21 or 23 . Extreme and mean values for each compound are given in Table 2. We also report variations observed in traffic : number of vehicies and continuously monitored carbon monoxide in the same area. These data were supplied by AIRLOR. Table 2 shows that traffic restrictions reduce both the BTX and CO concentrations in a same order of magnitude äs the number of vehicies. This correlation clearly indicates that traffic is the major source of BTX [START_REF] Gonzalez-Flesca | Pollution urbaine de proximite sur im site parisien. Mesures et modelisation[END_REF]. The larger difference between September 22 and 23 comes from a smaller dispersion on September 23 due to the lower wind speed.

Table 2: decreases in concentrations and traffic on September 22 relatively to Sepftember 21 and 23

Compound or traffic Benzene No data available on 23rd

3.3

Influence of wind direction 6 represents benzene concentrations measured during the 3 days of the campaign at 2 different sites located on the diagonal axis of a crossroad. With the rather high wind speed on September 21 and 22, the concentrations are 2 to 3 times higher downwind than upwind due to the accumulation effects and the influence of the crossroad source. The effect of traffic reduction is dramatic at these sites which were situated dose to the kerbside. On September 23, due to the low wind, the benzene concentrations are comparable on the two sides of the crossroad, indicating no dispersion or accumulation effects.

W in d

Benzene concentrations, Hg/m

Influence of sampling height

Measured BTX concentrations were plotted the measured BTX concentrations against the sampling heights for each day of the campaign. The correlation coefficient of the linear regression is poor because the sampling height is not the only parameter influencing the concentrations at roadside locations : topography, type of traffic and local turbulence vary frorn one site to another. Nevertheless there is a strong tendency : slopes are all negative for the normal traffic days, indicating that concentrations are higher nearer the ground and that the major source of BTX is the vehide traffic. On September 22 concentrations are higher at elevated sampling points indicating that the measured BTX concentrations come from another source than the local traffic.

Conclusions

Results show that, despite a short sampling duration, the use of difiüsive samplers was appropriate to this study. Decreases m BTX concentrations on the traffic-free day were consistent with the reduction in the number of vehicies and the CO concentrations monitored at a roadside location. Finally, under normal traffic conditions, most benzene roadside concentrations measured over daytime periods in the centre of Nancy, under low wind speed conditions, slightly exceed the European Limit value of 5 ug/m 3 . With traffic restrictions, however, these concentrations decrease to an average of 1.5 ug/m 3 , and only 2 sites out of 10 show values which exceed the French Air Quality Target of 2 (ig/m 3 .

We would like to thank A. Frezier, P. Lavrilloux andJ. C. Pinard for technical assistance.

  AIRLOR supplied me meteorological data. Typical wind speeds and directions for the three days of the eampaign are represented in Figurel. The weather was sunny and warm, with a light N-E wind of 4 m/s on September 21 and 22, decreasing to less than 2 m/s on the 23" 1 . The data were recorded in the suburb of Nancy on an open terrain : wind speeds were lower in the streets of the centre, though these are not typical canyon streets because buildings are not high. Influence of wind speed and direction is therefore not predominant. Nevertheless, higher BTX concentrations on September 23 man on September 21 are due to a smaller dispersion. Figures 2 to 5 represent the BTX concentrations for each day of the eampaign. It is evident that these concentrations are much lower on September 22 than on the other two days due to the traffic restriction.

Figure

  Figure l : wind rose for September 22

Table l :

 l BTX concentrations, ffg/m3

						Samplinig	site n°6
	09/21/98	1	2	3	4	5		7	8	9	10
	Benzene	7.43	3.40	3.06	4.23	3.57	2.39	4.74	2.46	2.38	2.31
	Toluene	15.85	4.74	6.18	8.68	6.90	6.01	6.19	4.35	4.34	2.60
	(m+p)-Xylene	11.67	3.70	4.61	6.50	5.00	3.61	4.47	3.17	4.58	1.52
	o-Xylene	3.99	1.38	1.60	2.35	1.66	1.19	1.78	1.49	1.07	0.59
	09/22/98
	Benzene	2.70	0.79	2.09	1.00	2.29	0.67	1.97	1.11	1.29	1.36
	Toluene	2.85	1.26	2.73	1.65	1.69	1.64	3.19	1.13	1.19	1.68
	(m+p)-Xylene	1.97	1.07	1.41	0.53	1.35	1.01	1.08	0.83	0.85	1.08
	o-Xylene	0.75	0.00*	0.61	0.00*	0.60	0.00*	0.70	0.56	0.50	0.32
	09/23/98
	Benzene	7.99	3.52	6.04	6.72	6.12	5.32	6.92	3.68	5.58	3.01
	Toluene	20.23	7.29	8.57	15.44	14.08	12.20	16.91	8.97	13.07	5.47
	(m+p)-Xylene	16.22	5.07	5.86	11.73	9.88	8.59	11.57	6.46	9.63	4.02
	o-Xylene	5.58	1.73	2.49	4.14	3.62	3.21	4.07	2.47	3.60	1.58
	* concentrations were belo~w detection limits

Table 3 /

 3 Figure

  3

	Day	Site n°6	Site n° 7
		(upwind)	(downwind)
	September 21	2.39	4.74	Rue G ustave Sim 01
	September 22 September 23	0.67 5.32	1.97 6.92	©
				tue
				l'A m erval

Table 3 /

 3 Figure 6: Effects o/ wind direction

Table 4 :

 4 influence of sampling height on BTX concentrations

		Slopes : concentration vs. sampling height
		September 21	September 22	September 23
	Benzene	-0.168	0.374	-1.33
	Toluene	-1.18	0.149	-5.17
	m+p-Xylene	-1.27	0.552
	o-Xylene	-0.311	0.139


L Malherbe
email: laure.malherbe@ineris.fr

DESIGNING A CONTAMINATED SOIL SAMPLING STRATEGY FOR HUMAN HEALTH RISK ASSESSMENT

1 Introduction -Human health risk assessment : stakes of a relevant soil sampling strategy ?

Human health risk assessment is a site-based approach which provides a decision support in the framework of brownfield sites management. It is aimed at determining if an old site contamination may have harmful effects on human health in view of the planned or current use of the site. If need be it can help to formulate a remediation strategy compatible with that use.

In practice human health risk assessment commonly involves four steps [START_REF] Hortensius | Entwicklung standardisierter Probenahmestrategien fur Bodenuntersuchungen in den Niederlanden[END_REF] :

1. Site characterization : data providing appropriate information for exposure assessment are collected (nature and extent of contamination, historical review of the site and its users, knowledge of hydrogeology...). Soil sampling comes at that stage. 2. Exposure assessment : the type and magnitude of exposure to chemicals of potential concern present at or migrating from the site are estimated according to (future) land occupation. This stage should include : characterization of exposure setting, identification of exposure pathways, quantification of exposure. 3. Toxicity assessment : qualitative and quantitative toxicity information about substances being evaluated is gathered. This is accomplished in two steps : hazard identification and dose-response assessment. [START_REF] Mate | Guide Gestion des Sites Pollués : Diagnostic approfondi -Evaluations détaillées des Risques[END_REF]. Risk characterization : it brings the toxicity/potency and the exposure data together into an expression of quantitative risk estimation for all receptors.

Obviously the results of risk assessment and the decisions that may ensue are closely dependent on site characterization. This is why a proper soil sampling is of major importance as it can have consequences in terms of public safety and of costs (remediation expenses). Therefore soil sampling needs to fit the objectives of the study and to follow a well-defined strategy. This includes the number of sampling points, their location on site, the depth and the type of samples to be taken. A sampling strategy for health risk assessment is not necessarily intended to locate hot spots as a preliminary site investigation usually is nor to allow the calculation of contaminated soil volumes as required by a soil cleaning project. It should help to evaluate soil concentrations of toxic compounds in the soil compartments involved in people exposure. Besides it must reconcile scientific requirements with resource constraints.

No sampling strategy has been entirely formalized for human health risk assessment yet. National or international guidelines or standards, such as ISO 10381 provide guidance which is often dictated by the suspected level of contamination rather than by the aim of the study. Thus the site assessor has to adapt those guidelines to the specific problem he is in charge of.

The following study is focused on the designing of a soil sampling strategy for human health risk assessment. It does not handle the specific matter of sampling techniques, sample preparation and chemical analysis. We propose to review the components of a soil sampling strategy and to comment them from the angle of human health risk assessment. For each component we will compare several alternatives outlining their advantages and drawbacks. A few examples derived from current practice will be given as an illustration and some recommendations be voiced. The question of a multistage sampling strategy and the problem of representativeness will be briefly discussed.

2

Which soil sampling strategy for human health risk assessment ?

2.1 Sampling pattern : statistical (probabilistic) vs non statistical approaches Among the wide range of possible schemes the well-known systematic grid sampling is commonly used in North-America (USA, Canada). A 25 m grid as recommended by CCME 1 [START_REF] Ccme | Guide pour l'échantillonnage, l'analyse des échantillons et la gestion des données des lieux contaminés[END_REF] ensures a complete site coverage and a homogeneous distribution of samples over the site, hence minimizing bias in the estimation of mean concentrations. In a systematic sampling plan every part of the site is equally viewed as a potential exposure area. The likelihood of missing the presence of contamination is thus decreased.

Besides collected data can be treated with statistical and geostatistical tools, the latter leading to a better understanding of contamination spatial variability and providing 2 or 3D-maps of estimated concentrations. The production of such maps rather applies to the devising of soilcleaning projects. However it can bring out possible high-risk areas or unsufficiently sampled areas where the uncertainty on the estimated concentrations might be critical as regards risk evaluation. If the probability of exceeding threshold values is desired then geostatistical techniques such as conditional simulation can be used [START_REF] Nathanail | A geostatistical approach to spatial risk assessment of lead in urban soils[END_REF].

Though easy to implement in the field systematic sampling can require a large number of samples and be relatively costly unless it is reduced to a loose mesh grid. If so highly contaminated areas may be poorly characterized and the advantages of systematization invalidated.

This is why non probabilistic strategies directly derived from the suspected pollution may prove relevant too. Judgment sampling in areas assumed representative of the whole site should be banned however because of bias imposed by the investigator's subjectivity. Purposive sampling (fig.la) consists in selecting sample locations in regions known to be most contaminated in view of the site history. A few samples may be taken in the remaining part of the site for control only.

Before opting for such a strategy the assessor should wonder whether average concentrations, percentiles or extreme values are of concern. The answer depends upon the approach he decides to favour : a reasonably conservative or a maximising one. In the first case a targeted sampling is not sufficient to calculate mean concentrations or percentiles. In the second case it could be appropriate providing that hotspots have already been properly located through a thourough exploratory phase. If not it may provide misleading information about site contamination entailing a wrong or incomplete risk assessment.

Another approach designated as stratified random sampling is sometimes advocated (e.g.. in Germany or the Netherlands [1] [START_REF] Hortensius | Entwicklung standardisierter Probenahmestrategien fur Bodenuntersuchungen in den Niederlanden[END_REF]). It combines a pollution-oriented and a statistical random sampling (fig. lb). The site is first divided into several areas and/or layers (horizontal and vertical strata respectively) in which pollution is supposed to be relatively homogeneous. Each stratum is then randomly sampled. This approach needs a preliminary detailed study but it helps to evaluate contamination spatial variability within and between strata and to improve the precision of the estimated mean concentrations.

Canadian Council of the Ministries of the Environment Some advice can be drawn from this discussion as regards the choice of a sampling pattern. In addition to a purposive sampling scheme a systematic grid sampling (e.g. a square grid) may give a large amount of information about pollution and be rather easily implemented (fig. lc).

To limit costs sampling density (the mesh size) can be adapted to the suspected level of contamination and exposure being increased in heavily contaminated or heterogeneous areas or even in cleaner areas likely to be highly frequented. A preliminary stratification of the site seems particularly adequate for large and complex sites where a uniform sampling strategy could either be costly (too many samples) or mask some features of contamination (too loose a grid) and make the conclusions of risk assessment unreliable. A sampling strategy as described above might then be designed for each stratum separately. The choice between grab or composite samples can be dictated by soil and contaminants properties and by the analysis costs as well.

A grab sample is an individual sample collected from a single location at a specific time [11]. The collection of grab samples is the best way to get information about the spatial (both lateral and vertical) variability of contamination.

A composite sample is made of a series of discrete equal specimens (aliquots) taken at one location and several depths (vertical composite) or several locations and one depth (spatial composite). It is to be representative of the mean composition of either a vertical profile (vertical composite) or an area (spatial composite). Compositing aliquots reduces the intrinsec variability of the final sample and allows the mean concentration of a profile or an area to be estimated with a better precision. Besides analysis costs are diminished. A composite sample should not however represent an area larger than 1 ha (Jones in [START_REF] Tan | Soil Sampling, Preparation and Analysis[END_REF]) and should be made of more than 4 and less than 9 specimens. With more aliquots concentrations might be diluted [11]. Other technical difficulties are likely to arise when compositing samples (volatilization of some organic compounds, bad homogeneization of humid clay soils) and generate large uncertainties on the analytical results. So that risk is not underestimated grab samples should be preferred in any circumstances to spatial composites unless the absence of volatile compound and the homogeneity of contamination are really well known. The collection of vertical composites should be avoided too (cf. §2.3.).

It is also necessary to take background samples in areas thought to be free of contaminants. They can tell if site concentrations are higher than those observed in natural background conditions and help the assessor to propose realistic clean-up levels if he is asked to.

Sampling depth

The depths at which samples should be taken are linked to the vertical variability of concentrations. They depend on : the soil stratigraphy ; the soil nature and properties which may have an effect on the chemical form and the mobility of contaminants ; the soil nature and properties which may have an effect on the chemical form and the mobility of contaminants ; the physical and chemical properties of contaminants likely to influence their mobility ; the site use and the potential exposure ways.

Thus samples can be taken in the upper layer (over the first centimeters) and/or in subsurface (up to one or several meters depth). Since a contamination profile can be quite uneven it is often recommended to take samples at regular intervals (e.g. every 50 cm or 1 m) and in each stratigraphy unit. Only a few specimens may be then selected for laboratory analysis. The most relevant sampling depths are a function of exposure routes as indicated in table 1. Depths to be sampled 0-5 cm 5-30 cm 30-200 cm between 200 and 400 cm > 400 cm

Exposure routes

Soil ingestion, dermal contact with soil and dust, inhalation of contaminated dust, if soil is not to be paved, and inhalation of volatile compounds.

Soil ingestion, dermal contact and inhalation of contaminated dust (mostly during gardening activities and children games), ingestion of grown vegetables, if soil is not to be paved, and inhalation of volatile contaminants.

Inhalation of volatile compounds Inhalation of volatile compounds

Limited risk, clay does not favour the migration of contaminants towards ground water.

Sampling size

Actually the assessor has to define : the number of sampling points to be distributed over the site; the number of samples to be taken at each of those points ; According to the on-site observations made during the investigations he might then reconsider the total number of samples to be analyzed. The number of samples at each point is linked to the number of selected depths for sampling. As for the number of sampling points littérature shows many recommendations which will not be detailed here. They are based either on experience or on statistics. In the first case they strongly depend on the type of contamination : non suspected, heterogeneous or diffuse contamination. The use of statistics is more of an Anglo-Saxon approach. Determinant factors are then : the probability of success in hitting a hot-spot [START_REF] Tucker | A decision support system for designing sampling strategies for potentially contaminated sites[END_REF].; the precision on the mean that the assessor intends to reach [3][12]; the cost not to be exceeded [START_REF] Gilbert | Statistical Methods for Environmental Pollution Monitoring[END_REF]. The first critérium can be relevant when no prior information about the location of past industrial activities and the soil contamination is available. Otherwise the last two criteria should be preferred: the one allows the site assessor to have a better control over the uncertainties on the calculated risk, the other allows him to integrate cost constraints into the sampling strategy. However the mathematical equations used for determining sampling size assume that concentration variability (expressed by the variation coefficient) is known, which is scarcely the case unless a first sampling stage has been carried out. This is why we suggest that the assessor use all or part of those recommendations and formulas as an indicative basis, making assumptions about contamination variability if need be, and then adapt the number of samples to the site-specific constraints. After samples have been taken and analyzed, and variability has been estimated he may appreciate the relevancy a posteriori of his strategy.

Single or multi-stage sampling ? Complementary methods of investigation

The formulation of a suitable sampling strategy is made easier by a prior knowledge of site characteristics and soil contamination. A two-or multi-stage sampling is often recommended so that sampling strategy can be refined and improved at each step. This idea shows through the assessment processes developped in Europe and North-America. In France a simplified risk assessment based on a first site inspection (initial diagnostic) normally precedes a detailed risk assessment which requires a detailed diagnostic. In Germany and in the Netherlands a distinction is made between an oriented investigation which provides a general characterization of contamination and a further investigation which should give all necessary information for environment and human health risk assessment [1] [START_REF] Mate | Guide Gestion des Sites Pollués : Diagnostic approfondi -Evaluations détaillées des Risques[END_REF]. A third stage {remediation investigation) is devoted to site redevelopment. In the same way risk assessment in Québec is the result of three steps : the review of all available information (stage I), a preliminary characterization (stage II) and an exhaustive characterization (stage III).

As a counterpart a multi-stage sampling strategy requires time and money. To limit sampling to what is strictly needed the assessor should first look into any available information he can have at his disposal : indications about the past activities of the site and possible soil reworking, visual signs of contamination, results of less expensive field screening techniques. All those indicators ought to be integrated into a conceptual model of contamination. It must be kept in mind that screening methods may be used as a support for sampling but on no account as a substitute because they are only of a qualitative nature.

4

Sampling error, the problem of representativeness A sample should be representative i.e. reflect the properties of the soil volume from which it has been extracted. However representativeness is spoilt by sampling and preparation errors. The results of a « comparative evaluation of European methods for sampling and sample preparation of soils for inorganic analysis » [START_REF] Wagner | Objectives, concept and design of the CEEM soil project[END_REF] demonstrate that sampling and sample preparation errors can reach about the same order of magnitude as errors caused in the chemical analysis. Their relative contribution to the total uncertainty in soil contamination studies (and consequently in the assessed risk) is more a subject of speculation than of knowledge even if assurance and control quality (ACQ) sampling aid in quantifying them. However the assessor can take some precautions to reduce such errors, for example by increasing the sampled volume when soil granulornetry is heterogeneous.

Conclusion

Soil sampling of a contaminated site is an essential means of collecting qualitative and quantitative data for estimating people exposure. A cost-efficient sampling plan should be developed with respect to the historical, geological and geochemical conditions on the one hand and the current or planned land utilization on the other hand. Guidance exists on that subject but no sampling strategy has been entirely formalized for human health risk assessment. Given all the constraints the assessor has to cope with, a few recommendations seem more appropriate than the enforcement of a rigid strategy.

A non probabilistic sampling dictated by an initial knowledge of contamination may fit the actual situation of burden at the site but lead to an under or overestimation of concentrations. A statistical approach such as grid sampling ensures a complete coverage of the site but it could induce useless sampling. In fact both methods can be combined. The assessor should thouroughly study all available information (site and soil properties, physical characteristics of contaminants, results of previous investigations...) so as to delimit homogeneous contaminated areas and distribute the sampling points at best in each of them (e.g. using a sampling grid and adapting sampling density to the suspected level and the heterogeneity of contamination and to the precision he intends to reach). He should also take the (future) use of the site into account in order to define areas and layers to be sampled as a priority. Statistical and geostatistical tools can be helpful for constructing of a sampling strategy as well as for the analysis of the collected data.

  Fig.la, lb, lc : targeted, random stratified, and combination of targeted and systematic sampling (after Pellet & al. [8])

Table 1 .

 1 Example of an old industrial site which is to be redeveloped into a residential area

	Soil composition
	coarse-medium
	sand with sandy
	loam soil
	silt
	clay