Véronique Merrien-Soukatchoff

Yann Gunzburger

Cécile Clément

Natural thermal strains close to surface of rock slopes Measurement and modelling of the "Rochers de Valabres" case

Keywords: Roche, versant, thermo-élasticité, modélisation, solution analytique, forage, cellule CSIRO Rock, slope, thermo-elasticity, modelling, analytical solution, borehole, CSIRO cell

Les variations naturelles de température sont souvent mentionnées pour leur rôle sur les instabilités de pente. Pendant plusieurs années, des mesures thermomécaniques in situ ont été réalisées sur le site des Rochers de Valabres" (Alpes-Maritimes) afin d'étudier les phénomènes thermomécaniques de surface. Le site expérimental est d'abord présenté. 8 cellules CSIRO regroupées en 3 stations ont été installées à des profondeurs variant de 0 à 50 cm. Pour comprendre les déformations des jauges des cellules CSIRO soumises aux variations de température, une solution analytique et des modélisations du problème d'un forage, dans un milieu élastique semi-infini, soumis à des variations de température sont présentés. Les données in situ sont ensuite analysées au regard de ces calculs. L'ordre de grandeur des déformations mesurées est comparable à celles calculées, confirmant les phénomènes pris en compte dans les calculs. L'analyse précise des données est, par contre, plus problématique en raison de l'influence de la température sur la cellule CSIRO ellemême, de problème de dérive et de variation dans le temps du comportement des jauges de mesures. L'influence de la déformabilité de la cellule CSIRO sur la mesure a été modélisée et confirmée par des tests de laboratoire et des modélisations.

ABSTRACT. Natural temperature variations are often are thought to play a role in slope instabilities. For several years, in situ thermo-mechanical measurements have monitored the "Rochers de Valabres" (Alpes-Maritimes, France) site to examine thermomechanical surface phenomena based on in situ measurements. First, a description of the experimental site is presented in this paper. Eight CSIRO cells grouped in three stations were installed at different depths varying from 0 to 50 cm. To understand CSIRO cell strains under temperature variations, analytical solutions and modelling of the problem of a borehole subjected to temperature variations, in a semi-infinite elastic media, are presented. The in situ data were analysed based on these computations. If the order of magnitude of the strain measurements is quite similar to those computed, confirming the computed phenomena, an accurate analysis of the data is difficult due to several instrumental problems: drift, the influence of the temperature on the CSIRO cell itself, and the variation in the time of the gauge behaviour. The influence of the CSIRO cell deformability on the measurements was modelled and confirmed through laboratory tests and modelling.

Introduction

The expected climate changes in the future raise questions on the impact of climatic factors on the stability of rock slopes, especially those alongside important thoroughfares or threatening nearby populations. The role of climate and paleoclimate changes is presumed to be significant in slope evolution and instability activation (see, for example, [START_REF] Soldati | Landslides and climate change in the Italian Dolomites since the Late glacial[END_REF][START_REF] Borgatti | Landslides as a geomorphological proxy for climate change: A record from the Dolomites (northern Italy)[END_REF]. Thus, the accurate effect of the temperature variation is often invoked in a qualitative way, but its impact is currently difficult to quantify.

The work undertaken in recent years on the "Rochers de Valabres" (French Alps) experimental site have aimed to improve the understanding of thermomechanical phenomena and the surface characterisation of their potential impact as a preparatory factor of rock slope instability.

The thermo-mechanical behaviour of rock masses has mainly been studied at high stress levels with controlled temperature variation. Subsurface rock-masses of rock slopes submitted to natural temperature variation are in a different context:

-rock-masses are at low stress level, and, consequently, stress variations due to temperature fluctuations may be of the same order of magnitude as the in situ stress; -fractures at the surface of rock slopes may be widely open; -the temperature at surface is not controlled; it varies continuously with different periods (day, season, year, and multiyear) and never reaches a steady state; and -mechanical and thermal boundary conditions are linked to a given topography and are thus 3D and complex. Surface temperature oscillations are known to contribute to rock falls through:

-the accumulation of thermally-induced micro-cracks [START_REF] Berest | La thermomécanique des roches[END_REF];

-the non-reversible movements of rock blocks along pre-existing fractures when submitted to thermal loading [START_REF] Matsuoka | Direct observation of frost wedging in Alpine bedrock[END_REF]2008;[START_REF] Gunzburger | Influence of daily surface temperature fluctuations on rock slope stability: case study of the Rochers de Valabres slope (France)[END_REF][START_REF] Watson | Temperature influence on rock slope movements at Checkerboard Creek. In: Landslides: evaluation and stabilization[END_REF]Vlcko et al., 2005); and -freezing and thawing process [START_REF] Coutard | Rock temperature measurements in two alpine environments: implications for frost shattering[END_REF][START_REF] Matsuoka | Direct observation of frost wedging in Alpine bedrock[END_REF][START_REF] Matsuoka | Frost weathering and rockfall erosion in the southeastern Swiss Alps: longterm (1994-2006) observations[END_REF][START_REF] Anderson | Near-surface thermal profiles in alpine bedrock: Implications for the frost weathering of rock[END_REF][START_REF] Bost | Altération par le gel des massifs rocheux : Etude expérimentale et modélisation des mécanismes de génération des contraintes dans les fissures[END_REF][START_REF] Frayssines | Failure mechanisms and triggering factors in calcareous cliffs of the Subalpine Ranges (French Alps)[END_REF].

The objective of this paper is to investigate the mechanical effects of natural variations of the temperature field on a rock slope. Only the effect of positive temperatures was studied, and freezing and thawing processes were excluded.

The analysis is based on analytical solutions and numerical modelling and is compared to thermal and mechanical time series data from the "Rochers de Valabres" field site. The understanding of rock temperature and especially the role of insulation will be presented by Gunzburger et al. (in prep.) based on a the same temperature time series.

Due to the monitoring devices on which the investigations are based, the inquiry is limited to almost continuous rock faces. The first paragraph describes the site and monitoring method. Analytical and modelling computations are presented in the next two paragraphs as a reference for the monitored strains presented subsequently.

The "Rochers de Valabres": a field laboratory for surface thermomechanical phenomena investigations

The "Rochers de Valabres" (Figure 1) is a gneissic rock slope located in the Mercantour Massif (southern French Alps). Since 2002, this site has been monitored by various instrumentations (e.g., geophysical devices, geotechnical apparatus, and a meteorology weather station, see Figure 2) to better understand rock slope behaviour [START_REF] Clement | Monitoring of natural thermal strains using hollow cylinder strain cells: The case of a large rock slope prone to rockfalls[END_REF][START_REF] Clement | Monitoring of natural thermal strains using hollow cylinder strain cells: The case of a large rock slope prone to rockfalls[END_REF]Clement et al. 2006, Dünner et al. 2006;[START_REF] Clément | Stress measurement by overcoring at shallow depths in a rock slope: The scattering of input data and results[END_REF][START_REF] Gunzburger | Rôle de la thermique dans la prédisposition, la préparation et le déclenchement des mouvements de versants complexes. Exemple des Rochers de Valabres (Alpes-Maritimes)[END_REF][START_REF] Merrien-Soukatchoff | Influence of topographic roughness on the stress state in a sloped rock-mass[END_REF], Merrien-Soukatchoff et al. 2007[START_REF] Senfaute | Impact of climate change on rock slope stability: Monitoring and modelling[END_REF], especially the role of climatic factors in this behaviour. A stress measurement campaign had been also undertaken.

The site experienced two rock falls in May 2000 and October 2004 with volumes of 1000 m 3 and 40 m 3 , respectively. Among the potential causes for these failures, temperature variations have been cited as one possible preparatory factor [START_REF] Gunzburger | Field investigations, monitoring and modeling in the identification of rock fall causes, Landslides: Evaluation and stabilization[END_REF][START_REF] Gunzburger | Influence of daily surface temperature fluctuations on rock slope stability: case study of the Rochers de Valabres slope (France)[END_REF]Clement et al. 2009).

Figure 1. Location of the "Rochers de Valabres" experimental site in the Southern

French Alps (From Clement et al., 2009)

Figure 2. Location of the different monitoring devices at the "Rochers de Valabres" experimental site

In 2005, CENARIS (The French National Geohazard Monitoring Centre of INERIS) carried out the installation of thermometric and thermomechanical measurement devices. The installation consisted of both a thermometric multipoint sensor, which enables the measurement of rock temperatures at various depths (up to 0.5 metres), and three groups of CSIRO (Australia's Commonwealth Scientific and Industrial Research Organisation) strain cells installed at depths of 0.2, 0.3 and 0.5 metres in boreholes [START_REF] Dünner | Field natural and thermal stress measurements at « Rochers de Valabres" Pilot Site Laboratory[END_REF]. The measurement devices are organised in 3 stations (Figure 3), with each station in a different topographic configuration and consisting of cells at different depths between 30 and 50 cm.

Figure 3. Location of CSIRO strain stations

Each cell (CSIRO Hi 12, hollow cylinders), comprising 12 strain gauges (Figure 4), was glued with epoxy cement into a 38-mm diameter borehole. Two of the gauges (gauge n°1 and 7) lie parallel to the cylinder axis and measure axial strain ( zz ), five are orthoradial (gauge n°2, 6, 8, 11 and 12) or annular (  ), and the other five are diagonal ( 45 and  135 ). The strain gauge resolution is ±2 10 -6 . Each cell is also equipped with a thermal gauge.

Axial gauge

Diagonal gauge

Orthoradial gauge (  )

Figure 4. Orientation of the strain gauges within a CSIRO cell.

Hourly thermomechanical measurements were recorded between April 2006 and May 2008. Due to acquisition breakdowns, the network was operational for 58% of the total experiment duration, and the longest period of continuous auscultation was 182 days. Some gauges were out of service since the beginning of the acquisition. Over time, the number of gauges in use declined, and the operating rate of each cell varied from 25 and 100%.

Analytical computation of thermal surface strains with and without a borehole

Thermal surface stresses and strains

Let us consider the case of a semi-infinite medium whose straight free surface is submitted to temperature variations (Figure 5a). Temperature variations decrease in amplitude with depth [START_REF] Gunzburger | Influence of daily surface temperature fluctuations on rock slope stability: case study of the Rochers de Valabres slope (France)[END_REF]. Assuming thermoelastic behaviour, when the surface temperature changes, the free surface moves: for a temperature increase, the free surface moves outward and vice-versa. Due to boundary conditions (and not taking into account any weight), the stress in the z direction (perpendicular to the surface) is zero. If x and y are the directions perpendicular to z, then there is no strain in theses directions.

Considering elastic behaviour and applying the thermo-elastic relationships: The stress in x and y directions (or r and , using cylindrical coordinates) are

          E νEE 1 ν 12 ν 1 ν 12 ν tr ij ij ij ij T                [1]   ij ij ij ij ij 1 ν ν .. t r . σ σ EE T       [2]
      νEE 12 ν 1 ν 12 ν xx yy zz rr T                  [3]
The strain in the z direction can be deduced from the previous relationships:

   zz1 1 ν . 1-ν T     [4]
The stresses in the x and y directions (or r and , using cylindrical coordinates, which will be useful below) are

  11 -E 1 ν xx yy rr T               [5]

Stresses and strains along a borehole in a semi-infinite medium

Influence of a borehole

Let us now consider the case of a borehole, drilled in a semi-infinite medium submitted to temperature variations (Figure 5b) with the hypothesis that the borehole has only a mechanical influence and no influence on the temperature distribution in the medium.

In elasticity, due to the superposition principle, drilling a borehole before varying the temperature is equivalent to drilling after temperature variation.

The stress and strain variations caused by temperature variation (stage 1) are the same as those computed in the previous paragraph. The stress and strain variations caused by borehole drilling (Stage 2) after temperature variation can be considered according to the Kirsch solution, assuming a long hole. In cylindrical coordinate, during the drilling stage:

- 2 varies from its initial value to twice its initial value according to the Kirsch solution ( 2 = 2 1 - 1 =  1 ), - rr2 varies from its initial value to zero ( rr2 = 0 - rr1 = - rr1= = - 1 ), - zz2 and  zz2 are zero and thus:

        2 1 22 2 2 1 1 1 ν 1 ν νν .. .. σσ EE EE rr zz                      [6]         2 1 2 222 11 1 ν 1 ν νν .. .. σσ EE EE rr rr rr zz                   [7]
Therefore, the orthoradial and radial strains are

          1 2 1 ν 1 ν 1 ν -E .. . σ EE 1 ν 1 ν TT                 [8]           1 2 1 ν 1 ν 1 ν -E .. . σ EE 1 ν 1 ν rr TT              [9]
The total strain ( zz ,  rr ,   ) due to temperature variation in a borehole is the sum of the strain due to the temperature variation without the borehole ( zz1 ,  rr1 ,  1 ) and the strain due to borehole excavation without the temperature variation but with initial stress due to the temperature field ( zz2 ,  rr2 ,  2 ).

  

12 1 1 ν . 1-ν + zz zz zz zz T         [10]     12 2 1 ν . 1 ν 0 T                 [11]     12 2 1 ν . 1 ν 0 rr rr rr rr T            [12]

Remarks on the analytical solution

Several objections could be raised against the translational invariance hypothesis:

-The borehole is not infinite; and -The temperature varies along the borehole, and thus, a cross section at a depth z will differ when z is varied.

This analytical solution shows the following:

-the axial and orthoradial strains along a borehole, due to temperature variations at the surface, are, at a point, of the same amplitude but of opposite sign (shortened for one, elongated for the other); and -considering a sinusoidal temperature variation [START_REF] Carslaw | Conduction of Heat in Solids[END_REF], Gunzburger et al., 2005) with an average value T0, a period  of 24 hours and a temperature amplitude A (See Table 1):

  0 ,c o s z z Tz t T A e t            [13]
and

   1 ν . 1-ν cos z zz z Ae t                  [14]
Figures 6 and7 show the curve computed for a temperature variation between 0°C and T(z,t) according Equation [13], with T 0 =0°C. ) versus depth for a variation of temperature from O°C to T(z,0) at t= 0, given by ( 9) Computed axial strain (*10 6 ) over 48 hours versus time (in hours) at z=0 and z=0.5 for a variation of temperature from 0°C to T(z,0), at t=0, as given by ( 9)

Numerical modelling

The analytical solution was completed by a thermo-elastic computation performed with CESAR-LCPC-v4 to investigate the influence of the length of the borehole. A 50-cm-long borehole was modelled in an axisymmetric numerical model. The dimensions of the model are given in Figure 15. A variation of the temperature from zero to T(z,0) was applied. Figures 8 and9 show the results, and the comparison between Figure 6 and Figure 8b shows the weak influence of the borehole length. The computed axial and orthoradial strains are equal in magnitude and of opposite sign as in the analytical solution, and their magnitudes are similar to that of the analytical solution. The numerical modelling confirms the analytical solution and shows that the influence of the length of the hole is negligible.

In situ data

Rough data

The analysis of the in situ data showed different types of behaviour. Figure 10 presents the example of Cell 4 between 28/04/2006 and 08/05/2006. Globally, the axial strains increased with temperature, whereas the orthoradial strains decreased; however, the upper value of the axial strain seemed to reach a maximum and to remain at this maximum for several hours, which may be an indication of sensor saturation.

Figure 10. Micro-strain variations of gauges of Cell 4 (30 cm depth) between 28/04/2006 and 08/05/2006

Over a longer period, the gauge behaviour was more inconsistent. Figure 11 presents the case of the axial gauges of Cell 5. If, during a short period, the axial strains increased with temperature, then during a long period, the gauges can be subjected to drift: between 3/7/2006 and 7/12/2006, no measurements were performed, and Gauge 7 varied according to the temperature variation, whereas Gauge 1 increased. Such a drift phenomenon is visible in the behaviour of numerous gauges.

As an example, Figure 12 also shows the variation of Gauge 7 versus Gauge 1 of Cell 5 during two different periods. The gauges varied in the same direction during both periods, but the ratio between the strain of Gauge 1 and the strain of Gauge 7 was globally very different between 28/04/2006 and 3/7/2006 as well as between 7/12/06 and 22/07/2007. Over shorter periods, for example, a period of eight consecutive days (see Figure 13); the ratio became more consistent around 0.65. For the daily strain amplitude versus temperature variations, the axial strains almost behave according to the analytical computation. Despite the large scatter, Figure 14 shows a linear relationship between the axial strains and temperature variations for Cell 6, and according to Equation [10], the ratio between the axial strain and temperature is approximately 10, which corresponds to a Poisson ratio of about 0.18. However, the orthoradial strains (Figure 15) had a significantly higher ratio than the axial strains, and a ratio greater than 21 leads to a computed Poisson ratio, according to Equation [11], of greater than 0.5. This trend was found for almost all cells.

Instrumental artefacts

Laboratory test

To investigate the role of instrumental artefacts, some laboratory tests were performed on hollow cylinders containing a CSIRO cell and equipped with external strain gauges (Figure 16). Internal and external gauges should measure the same deformation.

The cores were those previously drilled during overcoring experiments (Clement et al., 2009). Hollow core cylinders of gneiss and PMMA (polymethylmethacrylate or "plexiglas") were submitted to controlled thermal variations in a climate store. PMMA was chosen as a reference because it is a homogeneous material, and its mechanical and thermal properties are close to that of the epoxy.

Figure 17. Measured micro-strains due to imposed thermal variations

Figure 17 shows the axial and orthoradial strains induced by the temperature variations. If the contrast between the coefficients of thermal expansion of the two materials is important, then their differential expansion can produce the opposite behaviour than expected. The core of gneiss, unlike the PMMA, exhibited elongation during cooling and vice versa when shortening was expected.

The measured CSIRO deformations were the result of the thermal expansion of the rock and epoxy. The axial strains were weakly affected by the epoxy layer, whereas the orthoradial strains were greatly affected.

Numerical tests

The role of the CSIRO cell in a drill-hole was investigated through numerical modelling. The CSIRO cell was represented as a 4-mm-thick cylinder of epoxy (Figure 18). The properties of the rock and the epoxy are shown respectively in Table 1 andTable 2. The modelled thickness is greater than the real one (1 mm epoxy + 1 mm of glue), but, due to mesh problems, it was difficult to reduce the thickness of the cylinder. However, the purpose of the modelling was conceptual: the intention was to investigate the type of perturbation induced in the strains by a CSIRO cell.

Table 2. Thermo-elastic properties of epoxy

The influence of the CSIRO cell is shown by comparing the strain along the drill hole without (Figure 19) and with (Figure 20) a 4-mm-thick cylinder of material with the thermo-elastic properties of epoxy. The comparison was performed for a variation of temperature from zero to T(z,t1) with t1 = 6 hours. The axial strain is influenced by the epoxy cylinder, but, in the middle of the 10-mm cylinder, the value changed only slightly due to the epoxy cylinder. For the orthoradial strain the influence of epoxy cylinder completely modified the strain in the middle of the 10mm cylinder. The disturbance is more significant when the temperature variation is large than when it is small. To take into account the influence of the cylinder thickness, other computations varying the Young modulus and dilatation coefficient of the epoxy were performed. They clearly showed that the CSIRO cells and epoxy glue used to seal the cell have an influence on the measured deformations. This influence is linked to temperature (for weak temperature variation, the influence is low) and cannot be simply corrected.

Thermal strain interpretation

The existence of instrumental artefacts leads to doubt in interpreting the measurements. The axial thermomechanical strains seem to be weakly influenced by the CSIRO cells. They are in quite good agreement with the analytical and modelled solutions. The computed Poisson ratio (0.18) deduced from axial daily strain variation (Figure 12) was consistent with the expected value.

The orthoradial thermomechanical strains were affected by artefacts as a function of temperature.

Moreover, all of the measurements could be affected by drift and were not consistent over time (Figures 11 and12).

The order of magnitude of the strains was, however, in good agreement with the computed solution; the influence of the thermal boundary conditions and the decrease of strains with depth were very clear (Figure 21). The daily strain at the surface amounted to roughly 2 10 -4 , which is an important value that is equivalent to the effect of loading and unloading approximately 150 m of rock, considering the parameters listed in Table 1. Up to now, based on measurements, it was not possible to deduce the influence of the geometry due to the differences between the 3 stations, as initially predicted.

In the absence of a borehole, thermal stresses (Equation [5]) are liable to inducing fatigue rupture as shown by [START_REF] Clément | Auscultation d'un versant rocheux soumis aux sollicitations thermiques naturelles. Cas des Rochers de Valabres[END_REF], which may explain some observed instabilities.

Conclusions

The role of climate variations on slope evolutions and instabilities is difficult to grasp. Investigations undertaken at the "Rochers de Valabres" experimental site were designed to contribute to the understanding of thermo-mechanical phenomena. Yet given problems of measurement our attention focused on the expected strains and consistency of thermal strain measurements through CSIRO cells.

Analytical solutions and modelling developed herein are in good agreement and showed the importance of strain variations at the surface. The order of magnitude of the strains was confirmed by measurements, and they are well correlated with the thermal boundary conditions on the free surface of the slope.

However, the gauges of CSIRO cells, used to investigate the phenomena, were subjected to drift and sudden unexplained changes. The influence of the cell itself on the measurements is important and was determined though laboratory tests and numerical modelling. Possible corrections can be applied; however, the measurements remain too heterogeneous to accurately study deformations due to temperature variations and especially the role of the geometry and fractures on site which was the intention of the monitoring devices. Some alternative devices should be considered to investigate these aspects. At the same site, optical fibber monitoring was also used in other boreholes to investigate the role of forced thermal conditions. The results of this instrumentation have not yet been analysed, and it is not possible to state whether this device will allow more than axial strain to be measured

Figure 5 .

 5 Figure 5. a-Semi-infinite medium submitted to temperature variations. b-Semiinfinite medium with a borehole perpendicular to the surface submitted to temperature variations.

Figure 6 .

 6 Figure6. Computed axial strain (*10 6 ) versus depth for a variation of temperature from O°C to T(z,0) at t= 0, given by(9)

  Figure7. Computed axial strain (*10 6 ) over 48 hours versus time (in hours) at z=0 and z=0.5 for a variation of temperature from 0°C to T(z,0), at t=0, as given by(9)

Figure 8 .Figure 9 .

 89 Figure 8. Computed axial strain in an axisymmetric numerical model. a: Isovalues. b: cross section of the boundary of the borehole down from the top

Figure 11 .

 11 Figure 11. Micro-srain variations of Cell 5 between 28/04/2006 and 22/07/2007

Figure 15 .

 15 Figure 15. Orthoradial daily micro-strains amplitude versus temperature variations of Cell 06 between 28/04/2006 and 14/05/08

Figure 16 .

 16 Figure 16. Photography (a) and diagram (b) of an instrumented hollow cylinder core with a CSIRO cell inside and external strain gauges. R1 is the internal radius (R1 = 17.5 mm), R2 is the drill hole radius (R2 = 19 mm) and R3 is the external radius of the core (R3 = 73 mm)

Figure 18 .Figure 19 .Figure 20 .

 181920 Figure 18. Axisymmetric numerical modelling of a borehole and an epoxy cylinder within a semi-infinite medium. Size of the modelled zone (left) and enhancement of the borehole zone (right) showing the mesh refinement

Figure 21 .

 21 Figure 21. Mean daily strain versus depth for Station n° 3 (Cells 6, 4 and 5 at, respectively, 20, 30 and 50 cm of depth)

Table 1 .

 1 Thermo-elastic parameters: Definitions and values for a gneissic rock, used in the analytical and numerical computations (modified from[START_REF] Gunzburger | Influence of daily surface temperature fluctuations on rock slope stability: case study of the Rochers de Valabres slope (France)[END_REF].

	Parameter	Symbol	Value and Unit or (Unit)
	Temperature	T(x,t)	(K)
	Average temperature	T 0	(K)
	Thermal amplitude at the surface Period of temperature solicitation Pulsation of temperature solicitation Thermal conductivity Penetration depth Specific heat capacity Expansion coefficient Thermal diffusivity Density Young's Modulus Poisson ratio	A   = 2 /  2a   c p  a = /(.C p ) 1.3 10 -6 m 2 .s -1 (K) 24 h (86400 s) (s -1 ) 2.95 W.m -1 .K -1  (m) 836 J.kg -1 .K -1 7.5 10 -6 K -1 2.7 10 3 kg.m -3  E 20.7 GPa  0.24

Acknowledgements

This work program was performed in the framework of a research project (entitled STABROCK) thanks to the financial support provided by the French Ministry of Ecology, Energy, Sustainable Development and Sea (Ministère de l'Écologie, de l'Énergie, du Développement Durable et de la Mer : MEEDDM) through the RGU (Réseau Génie Civil et Urbain). This project seeks to assess the influence of climate change on rock slope stability (Senfaute et al., 2007).

All authorisations and assistance from the Mercantour National Park and the national EDF electric utility are gratefully acknowledged.


F Delvare
email: franck.delvare@ensi-bourges.fr

B Durand

J L Hanus

S Richomme

IMPORTANCE OF MODEL SELECTION FOR PROCESSING DYNAMIC THREE-POINT BENDING TESTS

The Split-Hopkinson Pressure Bar apparatus remains the standard test to investigate high strain rate materials properties. To determine the tensile strength of quasi-brittle materials three-point bending tests are frequently employed. In this paper, we compare different analytical models for processing the recorded experimental wave signals. It is concluded that only the semi-infinite beam model and the modal superposition analysis with several modes are relevant to get precise strain rate and material dynamic characteristics.

INTRODUCTION

Safety analysis of structural systems need to adress a wide range of malevolent attacks or accident scenarios. Due to the extreme nature of blast or impact loadings, the deformation and fracture behaviour of materials differ signif cantly to that observed under quasi-static conditions. Precise rate dependent material characteristics, used as constitutive models parameters, are required to perform accurate numerical simulations of the response of structures to transient loadings. Hopkinson bar experimental techniques have overcomed limitations of conventional servohydraulic testing instrument or drop weight impact test at high strain rates [START_REF] Field | Review of experimental techniques for high rate deformation and shock studies[END_REF]. The classical conf guration, with a short specimen placed between long input and output bars, used to perform compression test, can be modif ed for the study of dynamic mechanical behavior in tension and torsion test [START_REF] Gray | Classic split Hopkinson pressure bar testing[END_REF]. Even with a SHPB facility, several factors inf uence the accuracy of the results, including impedance mismatch, friction, specimen size effects, wave dispersion, stress equilibrium, among others. In this context, data processing is a key feature. This paper aims specif cally at adressing the relevant model issue for processing three-point bend test with SHPB. The paper f rst presents the experimental set-up. The different relevant model for analysing the data are then discussed. Finally, a test with a concrete specimen is fully analysed with the different models and the obtained results are compared.

HOPKINSON THREE-BAR/THREE-POINT BENDING TEST

The modif ed SHPB apparatus for dynamic bending used in this study has been proposed by [START_REF] Yokoyama | A novel impact three-point bend test method for determining dynamic fracture-initiation toughness[END_REF]. A scheme of the set-up used in the present study is shown in Figure 1. The two transmission bars were used to support the specimen, and the load was applied via the incident bar. The strain waves in the bars are measured using resistance gauges. The impact velocity is measured via photoelectric cells. The main advantage of SHPB is that it can be used to apply a dynamic load to a specimen and to measure both the force and the velocity at the impact point between the incident bar and the test specimen. The impact velocity and force are determined using the following classical formulae:

V e (t) = -C B (ε i (t) -ε r (t)) (1)
F e (t) = -Z B C B (ε i (t) + ε r (t)) (2)
where C B = E B /ρ B is the wave speed and Z B = E B A B /C B the characteristic impedance. ε i and ε r are the incident and the ref ected waves at the input bar/specimen interface. However, special precautions are required for processing raw data. The strain gage, glued in the middle of the long incident bar (Figure 1), records successively an incident and a ref ected wave. It is necessary to be able to perform highly precise time shifting procedures to transport the recorded pulses towards the specimen/bar interfaces. Indeed, a synchronization based on a simple alignment of the peaks of the pulses does not yield the correct time delay [START_REF] Zhao | On the use of SHPB techniques to determine the dynamic behavior of materials in the range of small strains[END_REF], [START_REF] Mohr | Evaluation of stress-strain curve estimates in dynamic experiments[END_REF].

MODELS FOR BENDING TEST INTERPRETATION

SHPB dynamic tests differ from quasi-static ones in the sense that neither a controlled force nor a displacement are imposed. The loading results from dynamic interactions between the deformed specimen and the incident bar at contact zone. The relation between the incident and ref ected waves (equations ( 1) and ( 2)) is linked to the relation between V e and F e . In the case of compression tests this relation can be written:

ε r (t) = Z C -Z B Z B + Z C ε i (t) where Z C = S ρ E and Z B = S B ρ B E B (3)
The ref ected wave is concomitant and proportional to the incident wave. The elastic simulation is used to verify the slope of the ref ected wave and determine the start time of the transmitted wave [START_REF] Zhao | On the use of SHPB techniques to determine the dynamic behavior of materials in the range of small strains[END_REF]. In bending tests, the elastic response of the test specimen viewed as a structure, which relates V e to F e , determines the ref ection of the compression wave and hence the loading conditions. The accuracy of calculated intrinsic characteristics such as the tensile strength of the material, depends mainly on the level of realism of the specimen model used. Four models are presented below, along with the corresponding coupling equations.

Simply supported beam in a quasi-static state

Considering a simply supported beam in a quasi-static state, the bending moment in the central cross-section is:

M = F e L 4
where L denotes the distance between the supports (4)

The relation between the force and the displacement at the end of the incident bar is:

U e = F e k e
where k e = 48 E I L 3

The relation between the incident and ref ected waves is given by the convolution integral:

ε r (t) = ε i (t) - t 0 2 ε i (τ ) δ(t -τ ) - 1 τ s e -t-τ τs dτ where τ s = Z B k e (6)

Beam modelised by a single degree of freedom (SDOF) model

The previous model is improved by taking the inertial forces into account. The beam response is approximated via a single degree of freedom (SDOF) system, assuming the shape of the dynamically deformed beam to be similar to that of the quasi-static deformed beam.

The coupling relation, at the center of the beam, between the force and the displacement is:

U e (t) = 1 m e ω e t 0 F e (τ ) sin(ω e (t -τ )) dτ where ω e = k e m e (8)
The relation between the incident and ref ected waves is given by the convolution integral:

ε r (t) = ε i (t)- t 0 2 ε i (τ ) τ e e -t

Modal superposition model

The motion of the beam is described in this model by superposing vibration modes. The coupling relation imposed at the center of the beam between the force and the displacement is the sum of convolution integrals :

U e (t) = ∞ j=1 1 m j ω j t 0 F e (τ ) sin(ω j (t -τ )) dτ (10)
where ω j and m j are respectively a vibration pulsation and the modal mass associated to a simply supported beam. The relation between the incident and ref ected waves can be expressed as:

ε r (t) = ε i (t) -2 t 0 ε i (τ ) L -1 B(s) τ m + B(s) t -τ dτ (11)
where

B = ∞ j=1 s s 2 + ω 2 j and τ m = m j Z B
and L -1 denotes the inverse Laplace transform.

Semi-infinite beam model (SIBM)

During the f rst times of the impact, the stress wave does not reach the supports and the situation is equivalent to a one-point bending test. Therefore, in order to make the best possible use of the experimental data during the beginning of the test, it is proposed to consider a semi-inf nite elastic beam. Using the time Laplace transform, the transient dynamic elastic response of the test specimen can be expressed in terms of the velocity V e and of the force F e :

w(x, t) = t 0 G 1 (t -τ ) Ω 1 (x, τ ) dτ - t 0 G 1 (t -τ ) + G 2 (t -τ ) Ω 2 (x, τ ) dτ (12)
where

G 1 (t) = t 0 V e (τ ) π (t -τ ) dτ , G 2 (t) = t 0 F e (τ ) 4 E I α 3 dτ with α 4 = ρ S 4 E I (13)
and

Ω 1 (x, t) = 1 √ π t cos α 2 x 2 2 t , Ω 2 (x, t) = 1 √ π t sin α 2 x 2 2 t (14)
Since the behavior of specimen is supposed to be elastic, the rotation at the origin remains zero. We obtain the following relation for the coupling imposed at the end of the incident bar between the force and the velocity during the f rst times:

V e (t) = 1 2 η t 0 F e (τ ) π (t -τ ) dτ where η = 4 E I α 3 (15)
With the use of ( 1) and ( 2), we get the relation between the incident and ref ected waves:

ε r (t) = ε i (t) - t 0 2 ε i (τ ) √ τ f 1 π (t -τ ) - 1 √ τ f e t-τ τ f erf c t -τ τ f dτ (16)
Where

τ f = 2 η Z B

PROCESSING THE RAW SIGNALS USING VARIOUS MODELS

Time-shifting procedure

The recorded strain histories must f rst be shifted in time. In a f rst step, from the experimental incident wave, we evaluate elastic ref ected waves from the different models (Figure 3). In a second step, we use these simulated ref ected waves as reference to translate the recorded ref ected wave (Figure 4). The velocity V e and the impact force F e can now be deduced from the fundamental relations ( 1) and (2) (Figure 5 to 8).

Tensile strength

During the dynamic bending process, the stress state is not homogeneous. Consequently, the maximum tensile stress is obtained by performing a structural analysis. With the various models, the maximum stress can be expressed in terms of the displacement, in terms of the force or in terms of the incident and ref ected waves as follows:

-in the case of the quasi-static beam model:

σ(t) = 6 a E L 2 U e (t) = a L 8 I F e (t) = - a L S B E B 8 I (ε i (t) + ε r (t)) (17)
σ(t) = 6 a E L 2 U e (t) == 6 a E C B L 2 t 0 (ǫ r (τ ) -ǫ i (τ ))dτ (18)
-in the case of the modal analysis model:

σ(t) = a Eπ 2 ρSL 3 +∞ n=1 (2n -1) 2 ω n t 0 F e (τ ) sin(ω n (t -τ ))dτ (19)
-in the case of the semi-inf nite beam model:

σ(t) = a α 2 E V e (t) = a α 2 E C B (ε r (t) -ε i (t)) (20)
The time evolution of the maximum stress is reported on Figures 9 and10. Time in s quasistatic model SDOF model modal approach (1 mode) modal approach (3 modes) modal approach (5 modes) modal approach (7 modes) modal approach (9 modes) semi-infinite beam model

Strain rate

The strain rate is evaluated in the most highly strained f ber of the beam: -in the case of the quasi-static beam or that of the SDOF beam, the strain rate is proportional to the velocity:

ε(t) = 6 a L 2 V e (t) (21)
-in the case of the modal superposition model, the strain rate is given by:

ε(t) = aπ 2 ρSL 3 +∞ n=1 (2n -1) 2 t 0 F e (τ ) cos(ω n (t -τ ))dτ (22)
-in the case of the semi-inf nite beam model, it is proportional to the acceleration: Strain rate in /s Time in s quasistatic model SDOF model modal approach (1 mode) modal approach (3 modes) modal approach (5 modes) modal approach (7 modes) modal approach (9 modes) semi-infinite beam model

ε(t) = a α 2 Ve (t) (23)

RESULTS AND DISCUSSION

From the previous evaluations we can determine material dynamic characteristics (table 2). As expected, the dynamic tensile strength obtained is higher than the static one (7 MP a). We observe a wide disparity in the estimated values for both the strain rate and the tensile strength. It seems that the modal analysis converges slowly, with an important number of modes to the same value as the semi-inf nite beam.

The results demonstrate that the choice of an analytical model for processing three-point bending test is not trivial. A key feature is the equivalence with a one-point bending test during the early stage of the impact, which must be taken into account for the time-shifting procedure.

Figure 1 .

 1 Figure 1. SHPB bending test set-up

m

  e Üe + k e U e = F e where m e = 17 35 ρ S L

e -t 2

 2 dt denotes the complementary error function.4 EXPERIMENTAL SET-UPExperimental tests are carried out with bars made of aluminium. The characteristics of the bars are specif ed in table1. The specimen are unnotched micro-concrete beam with nominal length L = 14 cm, a square cross-section S = 16 cm 2 , a density ρ = 2000 kg m -3 and a Young modulus E = 12 GP a. An example of the raw signals recorded is presented in Figure2. Length of the striker L I = 1.25 m Length of the incident bar L B = 3 m Diameter of the bars φ B = 40 × 10 -3 m Bars cross-section S B = 12.57 × 10 -4 m 2 Young's modulus E B = 74 GP a Density ρ B = 2800 kg m -3 Elastic waves celerity C B = 5140 m s -1 Impedance Z B = 18094 kg s -1

Figure 2 .

 2 Figure 2. Raw signals measured

  05 2e-05 3e-05 4e-05 5e-05 6e-05 7e-05 8e-05 9e-05 0.0001 Deformation waves (elastic simulation) Time in s incident wave quasistatic model SDOF model modal approach (1 mode) modal approach (3 modes) modal approach (5 modes) semi-infinite beam model

Figure 3 :Figure 4 :

 34 Figure 3: Experimental incident wave and simulated ref ected waves

Figure 5 :Figure 6 :

 56 Figure 5: Impact speed V e estimated by using the incident experimental wave and the simulated ref ected waves

Figure 7 :Figure 8 :

 78 Figure 7: Impact forces F e estimated by using the incident experimental wave and the simulated ref ected waves

Figure 9 :

 9 Figure 9: σ(t) estimated using the incident experimental wave and the simulated ref ected waves.

Figure 10 :

 10 Figure 10: σ(t) estimated using the incident experimental wave and the time shifted experimental ref ected waves

Figure 11 :

 11 Figure 11: ε(t) estimated using the incident wave measured and ref ected waves obtained by the various elastic models.

Table 1 :

 1 Characteristics of the SHPB system

Table 2 :

 2 Dynamic properties evaluated with various models.

	Elastic model	Maximum impact force Yield stress Strain rate
		kN	MP a	s -1
	Quasi-static beam	12	40	68
	SDOF	40	15	58
	Modal analysis
	1 mode	40	12	46
	3 modes	36	24.5	95
	5 modes	37	27.5	98
	7 modes	37	29	105
	9 modes	37	30	110
	Semi-inf nite beam	37	35	115

ACKNOWLEDGMENTS

The authors would like to thank the French National Research Agency (ANR) for supporting this research (in the framework of the VULCAIN ANR-07-PGCU project).


B Durand

Franck Delvare

Jean-Luc Hanus

Patrice Bailly

Analytical and numerical methods for processing hopkinson bar loaded bending test on concrete : a comparative study

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Analytical and numerical methods for processing Hopkinson Bar

Loaded Bending test on concrete: a comparative study B. Durand 1,2 , F. Delvare 1 , J.L. Hanus 1 , P. Bailly 1 1 Institut PRISME, ENSI de Bourges, France, {bastien.durand,franck.delvare,jean-luc.hanus, patrice.bailly}@ensi-bourges.fr 2 Institut National de l 'Environnement Industriel et du RISque, Verneuil-en-Halatte, France Risk assessment tools for concrete structures subjected to dynamic loading such as explosions or impacts require information on dynamic characteristics of concrete. Various techniques have been used to test concretes at high strain rates. Among these different techniques, various tensile tests based on Split Hopkinson Pressure Bars (SHPB) have been developed for testing the strength of different materials. The modified SHPB apparatus for dynamic bending used in this study has been proposed by [Yokoyama & Kishida (1989)]. The aim of this study was to make a critical review and a comparative study of analytical and numerical methods for processing three-point SHPB bend test with a particular focus on wave shifting. An illustration and application of the method to micro-concrete is given. The input velocity V i and the input force F i at the contact point between the incident bar and the test specimen boundary were determined using the following classical 1D theory formulae:

V i (t) = -C B (ε i (t) -ε r (t))
(1)

F i (t) = -C B Z B (ε i (t) + ε r (t)) (2)
where

C B = E B /ρ B is the wave speed and Z B = E B A B /C B the characteristic impedance.
The ε i and ε r are the incident and the reflected waves at the input bar/specimen interface. For practical reasons and to allow an easy separation of the waves in the input bar, the strain gages are glued in the middle of the incident bar (fig. 1). The first step of analysis is to shift the incident and reflected waves towards the specimen/bar interfaces. In a dynamic bending test, the mechanical transient response of the specimen imposes a coupling relationship between V i and F i :

G τ∈[0,t] (F i (τ),V i (τ)) = 0 (3)
With the use of the equalities (1) to (3), one derives an implicit relation between the incident and reflected waves. During the first instant, the specimen behavior is supposed to remain elastic. Several methods can be used to characterize the elastic coupling relationship (3) :

• simply supported beam in a quasi-static state [Ruiz & Mines (1985)], [Bacon & al. (1994)];

• beam approximated by a single degree of freedom (SDOF) system (Rayleigh's method) [START_REF] Dutton | Analysis of the Hopkinson pressure bar loaded instrumented Charpy test using an inertial modelling technique[END_REF]], [Jiang & al. (2004)]; • modal superposition [Sahraoui & Lataillade (1998)], [Rokach (1998a)];

• long beam model (derived from results established by [START_REF] Ditkine | Calcul opérationnel Editions MIR: Moscou[END_REF]).

The responses of the various models to analytically given loads are compared. For an imposed force ramp, the results are compared to reference solutions obtained using the finite-element method (fig. 2). The different techniques of wave time shifting are applied to process the experimental recorded data obtained with a micro-concrete specimen. It is found that, in this case, the failure occurs during the first few instants, when bending waves have not yet reached the supports (the mechanical state is identical to that resulting from a one point bending load). As a consequence, only the long beam model and the modal analysis with a relatively large number of modes, can be used to accurately time shifting the waves.

Identifying the best model is an important issue because a small time shifting error involves a large error in the estimation of the input force and in the mechanical strength.

Figure 1 :

 1 Figure 1: Schematic of bending test set-up and strain gages location

Figure 2 :

 2 Figure 2: Maximum stress predicted by various models and FEM simulations

Acknowledgments

The authors would like to thank the French National Research Agency (ANR) for supporting this research (in the framework of the VULCAIN ANR-07-PGCU project)


Myriam Merad
email: myriam.merad@ineris.fr

Frédéric Marcel

Contribution of organizational approach and decision aiding theory for assessing risks and benefits of outsourcing: implementing an action plan for contracts optimization in a company submitted to authorization

Keywords: Outsourcing, risks, decision aiding, organizational analysis and comprehensive diagnosis

Outsourcing support activities in companies and industries is a key trend with core business strategy. This trend has increased with the hardening of the current financial crisis.

The issue of outsourcing support is a major theme in the field of risk prevention. Even if, outsourcing is considered by Preventers as a key factor of risk, Managers still consider this solution as cost-effective and in some cases that subcontractors have more expertise. This paper will report on a methodology developed for studying the risks (financial, judicial, availability, safety) induced by outsourcing and the conclusion of a real case study performed in 2009 within a company submitted to authorization in order to optimize the management of outsourcing contracts.

We will first analyze the state of the art about risks and benefits induced by outsourcing. A description of the company and the management's request will be described in a second time.

We will then present our comprehensive methodological approach based on decision aiding framework and on the achievement of an organizational diagnosis to consider impact on working relationships and organizational conditions in managing outsourcing service delivery contracts.

Finally, we will discuss the way our recommendations were taken into account by the management staff more than two years after proposing an action plan aiming at sustaining the quality and the performances of the services delivered to the Companies.

Introduction

Outsourcing became in the industrial sector one of the most valued solution these last decades. More flexibility, less expenses and more concentration on core business are expected and lauded. Another trend is the risk externalization and contractualization of key resources both (human and materials ones).

Looking back to scientific literature (Schema 1), one can notice that the number of publications about how to optimise outsourcing has not stopped growing up from 1995 to 2010. [START_REF] Carta | Socio-technical study about subcontracting impact on railway safety at the RATP[END_REF] has tried to summarize motivations presented by companies when they wanted to appeal to outsourcing. She identified three main reasons that we have summarized:

• Economical constraint: reduce costs and optimize productivity.

• Knowledge and technical constraints: learning by building collaboration with experts. • Legal constraints: export risks.

One of the solutions suggested in the papers for optimising outsourcing is to limit the number of subcontractors and condense both the different activities and subsidies on larger companies. This solution was considered as a mean to improve the quality of outsourcing governance structure: increase the control, reduce costs and export juridical risks. Now looking back to scientific works done on outsourcing and safety, we were surprised to notice that the number of publication has increased at the same time of the publication on outsourcing and optimization. It was like if thinking about cost reduction for example has an immediate corollary that is an impact on health and work conditions and therefore on environment. More precisely, let notice that this number of publication has doubled in 2004 compared to the number of publication about outsourcing and optimisation (Schema 1). Schema 1. Number of scientific publications about "outsourcing and optimization", "outsourcing and safety" and "outsourcing and major accidents"

Based on four case studies, [START_REF] Mayhew | The effects of subcontracting/ outsourcing on occupational health and safety: Survey evidence from four australian industries[END_REF] suggest that factors of hazards and risks can be pre-detected when the companies appeal to subcontracting. The fact is that self-employed workers are prone to an intensification of their work and "survival prerogatives" like economical pressure, disorganization, inadequate regulatory control and the inability of workers to organize themselves.

In 2004, Pasman and Suter have emphasized these observation by noticing how outsourcing has contributed to transfer risk from company staff to contractor staff and have insist on the negative influence of outsourcing on safety. Especially, when the Management has not given enough attention to the inherent difficulties of subcontracting and when the subcontractors and the company staff have not the same safety culture.

The important part of the literature about outsourcing and safety focus on the Occupational Health and Safety (OHS) consequences (Mayhew and al,1997;[START_REF] Carta | Socio-technical study about subcontracting impact on railway safety at the RATP[END_REF]. However, one can notice that the scientific literature trying to make a link between outsourcing and major accidents started to be significant in 2001 (Schema 1). Indeed, major accidents like Bhopal (1984), NASA Challenger (1986) and AZF in Toulouse (2001) seems to have significantly stressed the perception of the negative effects of outsourcing [START_REF] Dechy | First lessons of the Toulouse ammonium nitrate disaster, 21st September 2001, AZF plant, France[END_REF][START_REF] Papadopoulos | Occupational and public health and safety in a changing work environment: An integrated approach for risk assessment and prevention[END_REF]. The Proposal 6 of the French Parliamentary Commission report after AZF accident suggests to look back, as early as possible, to the impact of a reorganization, such as for example the use of outsourcing, on safety (Dechy andal, 2004 and[START_REF] Bourrier | The Contribution of Organizational Design to Safety[END_REF].

Our aim in this paper is to consider if that is possible, before restructuring the appeal to outsourcing due to economical constraints, to reveal what can be the potential impact on safety and working conditions. In fact, many authors suggest different methodologies (Hawkins, 1984;[START_REF] Bourrier | The Contribution of Organizational Design to Safety[END_REF][START_REF] Llory | L'accident et l'organisation[END_REF]. We have decided to choose a comprehensive methodology [START_REF] Merad | Aide à la décision et expertise en gestion des risqué[END_REF] to perform an organizational diagnosis and analysis.

A methodology to investigate a real case study

In 2009, we have been mandated by a Company submitted to Authorization (hereafter denoted "the Organization") to investigate, before reorganizing and optimizing the way outsourcing contracts are managed, the equilibrium between safety, working conditions and the imperative reduction of expenses. This Organization has always try to respect all the normative constraints such as ISO 9001 certification, accreditation such as NF IN 45011, ISO CEI 17025, Good laboratory practice…

The same year, the Organization was engaged in a Corporate Social Responsibility (CSR) approach that aims at finding equilibrium between economic, social and environmental considerations. By "consideration" we mean both the risk and the resilience aspects generated by reaching what can be considered as contradictory objectives.

With all these considerations, the management of the Organization is becoming more and more complex due to the increasing number of:

(i) regulations to fulfill, (ii) other actors that are impacted by the decision of the Organization who must be consulted, (iii) indicators (parameters or criteria) to follow.

When Managers looks to classical economic and management theories, they can notice that the Organization has to make economic profit in order to maintain its activity within the market. According to this point of view, for a long time, management theories have focused on [START_REF] Fayol | Administration industrielle et général[END_REF]:

Forecasting. Implement, following a strategic perspective that consists of predicting and analyzing the future resulting in the implementation of a plan of action.

Organizing. Create a structure for the Organization considering both human and equipment considerations. Commanding. Maintain monitoring and control of employee activities.

Coordinating. Harmonize and create cross links in order to maintain appropriate means and efforts. Maintaining. Ensure that the rules and directives are followed.

Management needs to adopt a viable posture in the face of a variable environment. CSR approach has changed the way profit, costs and risks are considered. Indeed, profit is not only financial quantification (e.g. euro), but also represents social and environmental gains (e.g. more diversity within the organization and less pollution).

The CSR approach needs a strong political engagement (of the Top staff Management) because of the constant need to seek equilibrium between economic, environmental and social concerns (see Schema 2) that cannot be obtained without the implementation of a participative governance model [START_REF] Merad | About the use of participative framework for managing organizational change in response to environmental demands: example of the implementation of a sustainable development plan[END_REF].

Schema 2. Sustainable Development principle

To be able to implement this approach, it is necessary to develop methods and tools. Decision aid methodology [START_REF] Merad | Aide à la décision et expertise en gestion des risqué[END_REF] can help both to build an appropriate diagnosis of the Organization and try to frame proportioned recommendations.

In what follow we will focus on the contribution of decision aid methodology for dealing with outsourcing problems within Organizations.

The Organization constantly seeks equilibrium (to reduce risks and increase benefits) with respect to interactions within its external environment. Different actions can be taken at different levels of decision-making within the Organization. These levels are differentiated according to: (i) the practical objectives that are sought, (ii) the nature and the level of information/knowledge that is available and (iii) the potential impacts:

• The strategical level. At this level, the objective of decision-making is action planning. The decision is a long term decision (more than 8 years) and is dominated by both political and regulatory dimensions. At this level, the information is abundant but imprecise and difficult to sort and select. Looking back to the case of optimizing outsourcing, we can consider that the Top management staff objective is to reduce costs and concentrate on the core activity of the Organization and maintaining a quality of service to the final customer.

• The tactical level. The decisions/actions are less influenced by political and regulatory dimensions than at the strategical level. At this level, the decisions are under the constraints and objectives within the Organization (social, technical and economic). This level is represented by the intermediate staff of management. Going back to the optimization of outsourcing contract, Middle staff management objective is to optimize the use of their allocated budget to increase to do more tasks knowing the limited number of their internal executive staff.

• The operational level. Decision-making pertains to short-term goals achieved in less than 1 year. The decisions made are more concrete involving technical information which is often specialized, precise, and more specific than the higher levels. Looking back to the case of optimizing outsourcing, we can consider that the executive staff objective is to produce task with more added value considering time and organizational constraints.

For each level of decision, different choices can be make seeking out an equilibrium between economic, social and environmental concerns (risks/costs, benefits).

The difficulty here is to both (i) clarify what measures (actions) must be taken at each level of decision-making and (ii) consolidate all the measures at each level in order to choose the right strategy for reorganizing outsourcing with respect to the CSR approach.

In order to go beyond these limits, we have chosen to use a decision aid methodology. The methodology is based on a two-step approach (Schema 3).

The first step consists in "outlining and structuring the problem". To do so, one must identify what is at stake, the constraints, the actors concerned or affected, and to choose the appropriate method according to the level and the nature of information and knowledge.

The second step is the "implementation of a method". This step consists in restructuring the available information according to the method to be used and analyzing the results of the method in order to provide the adequate recommendation to the decision-maker.

The figure given below describes these two steps.

Schema 3. Decision aid process framing a link between the Analyst and the Decision Maker

The problematic of "Description and structuring" (Schema 3, item 2) in decision sciences is a challenging problem. We have chosen to use an Organizational Analysis (OA) technique crossed with a "contextual diagnostic" to describe the SD problem. These two techniques are imported from social sciences and help to make explicit what is at stake and what are the rationale between actors inside and/or or outside of the Organization.

A. About the use of Organizational Analysis on structuring problems

OA (see [START_REF] Llory | L'accident et l'organisation[END_REF] helps in making a diagnosis about system dysfunction/pathology (e.g., production pressures) before prescribing a remedy for an obvious symptom (e.g., excessive reliance on outsourcing). The expert (analyst) thus makes an effort at the same time by contextualizing (to circumscribe) pathology by giving a causal explanation of a reported dysfunction and at the same time by decontextualizing by highlighting the "reasons" of the emergence of the latter [START_REF] Ricoeur | Ecrits et conférences 1. Autour de la psychanalyse[END_REF].

By widening the range of problems identified initially by the Decision Maker (DM), OA re-interrogates methodologies and the conditions of investigation. Thus, the OA reintroduces the creative idea of a "case study" to the observational sciences through the "history of problems". The "history of problems" is the result of an interpretation, a judgment, or "giving sense to" of an investigator, of an entire set of reported experiences, experienced or perceived by operators, management staff, etc. "Giving sense to", suggested by the OA, is made possible by using [START_REF] Ricoeur | Ecrits et conférences 1. Autour de la psychanalyse[END_REF]):

• "Method of investigation" and exploration of the Organization.

• "Network of theoretical concepts" giving intelligibility to "pathological episodes" reported by the actors of the Organization.

• "Method of analysis", interpretation of the facts experienced by the actors of the Organization.

"Giving sense to" is based on both the perceptive aspect of the judgment of the investigator and the "conditions of validity" of the reported facts given by the actors according to their experiences within the Organization.

We can thus argue that the OA helps to go beyond the only individual evaluation to a collective evaluation of one situation (Douglas, 1986;Godard, 1999). This points to the delicate problem of scientific objectivity [START_REF] Merad | Apport des méthodes d'aide multicritère à la décision pour l'analyse et la gestion des risques liés aux mouvements de terrains[END_REF]. With respect to this complexity resulting from incomplete information and even evolutionary partial knowledge (Khun, 1972), the investigator adopts an attitude of choice: "one does not construct what he chooses, but chooses with what he uses to construct"1 (Moles, 1958).

Once the context is described, the Analyst/investigator should move to a prescriptive model (Schema 3, point 3). That means giving or framing an action plan for optimizing outsourcing contract with respect to social, environmental and economical considerations.

B. The investigation: reframing the problem?

The DM initial demand was to elaborate an action plan for optimizing the outsourcing contracts of the Organization (the Organization staff is equal to 600). The first step of our investigation was to reframe the initial question asked by the DM in a way that is more coherent with CSR approach. We have suggested framing an action plan to improve the management of outsourcing and more precisely to look at the impact of outsourcing on the work organization and its effects on business continuity. This consists in considering if the service provided by the Organization, considering the appeal to subcontracting, maintains its quality during the time with respect to the global performance.

To be able to identify proportioned actions aiming at optimizing the management of outsourcing contracts and considering the potentialities and the limits of the Organization_ and then avoid loss of energy and means_ we have used a comprehensive approach that consists in:

• Performing semi-directive or open interviews focusing on the "work" done by the operators/contract managers. • Using approaches that focus on the "work" and "activity" organization.

• Using an organizational diagnosis and analysis.

We have fixed our working plan in January 2009 and our operating very early in order to be able to mobilize the staff concerns and/or impacts directly/indirectly by the outsourcing optimization. Indeed, we have considered that when looking to all outsourcing aspects, it was necessary to interview different Divisions and Units within this Organization:

• General Secretaries division (GSD) that is responsible of the maintenance, the technical assistance (Unit X1), projects owner and engineering consulting (Unit X2) and central services (Unit X3). 12 interviews were performs within this division. The staff interviewed was responsible of following, at that time, more than 190 outsourcing contracts.

• Health and safety department (HSD) (1person was interviewed), Juridical department (JD) that is responsible of designing the administrative side of the "call for contribution" but also the selection of subcontractors (2 person were interviewed), the Financial Division (FD) (1person was interviewed) and the number 2 of the Organization.

We have decided to widen our investigation by consulting document and other sources of information like: outsourcing contracts databases, the history of organization and reorganization of the activities within the Company and incidents and accidents database. Our objective was principally to let emerge based on the perception and the history of the interviewees their point of views and their visions of what are both the positive and the negative sides of having recourse to outsourcing.

The Organization appeal to subcontracting/outsourcing for different objectives: maintenance, work control prerogatives (normative, health and safety), energy distribution …

The 190 outsourcing contracts were followed by 9 employees working on different divisions and units. We have mapped for each employees (e i , i=1 to 9) the number of contracts they have to follow. The conclusion was that: GSD: 51 contracts followed by employee e 1 . X1: 3 contracts followed by employee e 2 . X2: 6 contracts followed by e 3 + 21 contracts followed by e 4 +21 contracts followed by e 5 . X3: 2 contracts followed by e 6 + 80 contracts followed by e 7 +3 contracts followed by e 8 +3 contracts followed by e 9 .

Some observations and findings

We were surprised to notice that some employees have to follow sometimes more than 80 contracts alone.

A. Some answers to key questions

We have then asked both the management staff and the operators to tell us what "managing and following an outsourcing contact means for them". The answers to this question were different according to their Division of affiliation:

• For GSD:

Organize the consultation (choosing the right subcontractor) and develop the specifications, asses a comparison of the costs and choose the right subcontractor. They have precised that their work is dependent on the arbitration of the JD that is in charge of lowering, according to them, the cost of outsourcing. Their role is to precise the technical specification with JD. Once the outsourcing contract is operational, members of the team are responsible for the liaison with the subcontractor.

Some single technical operations that are done with a subcontractor are not identified by a continuous contract.

Outsourcing can be done for curative tasks (e.g. maintain the facility in life) and controls tasks (preventive).

The follow-up depends on the contract: it can be bi-annual, quarterly,... Management Assistants: responsible for the administrative and financial follow-up.

• For HSD: Follow all the tasks dealing with quality and safety.

• For JD:

They consider that their role is to be an "opponent" and to be able to delimit the contractual and the legal risks of subcontracting.

As we can notice, being in charge of an outsourcing contract seems to be really complicated and complex due to the different technical dimensions to follows and the different interaction to respect within the department. We have than asked them "What are the necessary competencies to have when been in charge of an outsourcing contract?", The answers were the following:

• To be able to master both the technical and the regulatory aspects.

• Be able to develop contract specification.

• Be able to check if the response of the subcontractors is realistic knowing that JD will select the subcontractors on basis of "lowest bidder". • Monitor the progress of the contract: check, charge, and know whether to claim. • Have enough clear time to perform the set of actions.

It appears to us that the employees in charge of following the outsourcing contracts seem to do it fulltime. We have then asked them to tell us what they do think about "Interest and the complexity they see in this work?".They have considered that managing an outsourcing contract presents different dimension of richness and diversity:

Master both administrative and technical constraints. Provide de possibility to work in a collaborative framework with different departments GHD, HSD, JD and FD. Work for Internal clients: staff and laboratories.

Elements of complexity come when:

dealing with several subcontractors and different issues (e.g. regulatory); negotiate both technical and administrative aspects with subcontractors; working for difficult internal clients; Monitoring within the time different contracts at the time.

There is a lack of means (staff and time).

We have then try to map the distribution of the 190 contracts on the 89 subcontractors (Table 1). We have noticed that, with each subcontractor, the Organization can have from 2 to 15 contracts meaning that the subcontractors can be in charge of different tasks/activities. Surprisingly, the two subcontractors having more than 11 and 15 tasks (contracts) were managed within the Organization by e 7 that have more than 78 other contracts to follow at the same time. These complex contracts are respectively maintenance contracts and energy distribution contracts. We have then asked the employees "why, according to them, our intervention for optimizing outsourcing contracts, were asked by the General Direction?". They have informed us that it was due to an accident within a subcontractor in 2005, to several problems encountered with a subcontractor and to a problem of reorganizing an activity. More precisely:

Following a problem with company B 1 : the offer was not well detailed and the subcontractor has misused the Organization (technically and financially seeking). A staff member has detected the drift. It was decided with JD to give more precisions in the specifications.

A need therefore to closely examine all the elements of the chain of relationship between the Organization and the set of subcontractors.

The Gas distribution was followed by one employee. This employee has retired and the Organization has no visibility and legibility since his departure.

Accidents of a subcontractor followed by e 7 in 2005: person responsible for the maintenance of the facilities. Drop from less than 3 m height. Contusion to the head, the front, and shoulder.

The employees have also insisted on some other outsourcing contracts that reveal, according to them, in-depth problems. They have indicated that the management staff always takes repair actions where preventives actions are needed (see failures to learn and implement corrective actions in Dechy and al, 2010). Indeed, they precise that there is a significant and chronic lack of staff for example the activities like gas distribution. They have specified that the decision of externalizing the activity of gas distribution was a strategic decision and not an operational one (see chapter Methodology to investigate a real case study). For Health and Safety support, the employees have indicated that they need a strong support from the HSD and they admit their technical and regulatory limits for e.g. asbestos case and also administrative limits during the framing of the technical subcontracting documents.

B. Going back to the history of the Organization

Our investigation makes us go back 7 or 8 years ago, where an audit on the general services Organization was conducted in order to reduce the number of employees in this support division based on the principle of economy. The staff has specified that this audit was not the only one mandated to decrease the number of employees within this department. Indeed, more than 3 audits were conducted and have leaded to what the staff has qualified as a "natural" decrease of the workforce and a progressive externalization and transformation of the activity using outsourcing.

Following historical incidents with subcontractors, Middle Management staff has only taken curative measures contract by contract. The need for "preventive approach" is strongly sought and advocated. By preventive approaches they mean to rethink the way the structure, the organization and the management of contracts are structured/framed. The employees interviewed also proposed to limit, prevent and manage in a proportionate manner the dependence on external service providers in order to be able to reduce the risk of loss of availability of the working tools.

We have asked the employees to think about the dual problems of what can be qualified as a success, in terms of both good quality of work respecting safety, of the present organization and what can be qualified as a failure (giving an example). For GSD staff, the positives aspects of the actual organization are that each person contributes to the functioning of the whole organization. Indeed, the work of their staff is recognized positively by other employees (good score in the satisfaction survey). However, workloads increase shows that they will not be able to work with these constraints for a longtime: there may be a need to reorganize. They have also noticed that there is a problem of data and knowledge sharing between department (JD, FD). For more preventive actions, they have signaled that the outsourcing databases_ that are not shared and are not structured the same way for all the departments_ must be federated. Good example of good collaboration and good management of outsourcing contracts are the contract A 1 (maintenance) and radiation protection and the contract A 2 of gas detection.

However, strong difficulties are encountered. The staff of the different divisions indicated that different typologies of risk remain:

Individual risk (/person): according to the significant number of contract to follow, the risk of forgetting contract and of not being technically competent to follows all the needed dimensions increased significantly; and the risk of being accused of error increased proportionately. They have given the example the regulatory contracts and in particular radiation protection. They have indicated "We are general practitioners, not specialists".

Collective risks (/all the Organization or for the Division). They have identified different categories:

• Financial risk: control invoices (e.g. energy distribution case).

• Contracts "hygiene".

• Risk of gas leak.

• Contracts "pressure machinery" likes autoclave and compressed air. They were frightened by the risk of forgetting a material.

• Risk dealing with handling. The subcontractors' risks and regulatory control. • Risk of forgetting an internal inspection. E.g. case of fire extinguishers.

• Management of drinking water and discharges (risk of legionellosis).

Following theses interviews, two other observations have emerged. First, is that the "contract risk" is not integrated with workplace' safety studies. Second, is that a lot of good practices framed by the National Institute of research on Safety in working conditions (INRS) were neither considered nor taken into account in internal procedures at that time.

According to the interviewed staff, the "energy distribution" contract B 1 (see Table 1) is the most difficult one. Principally because it is a big contract (cost) that fluctuates during the time and impact all the employees. Both the person in charge of this contract and his Management have qualified this contract as "wear contract" where many difficulties remains with "internal customers". They have given the example of electric heating where the employees have difficulties in understanding that noncompliance with the level 21°C have an impact on th e cost of the energy subcontract and on the environment pollution. They indicated that the employees are "not factual on perceptions". At the same time, the subcontractor is minimizing his work compared to what is fixed within the contract and use only one technician to follow the energy distribution for more than 600 people.

The staff of GSD has indicated some curatives measures that can improve the B 1 and other contracts management. More generally, the staff insists on the need of more internal communication on topics such as expenses, safety and contracts risks. Specifically, the staff highlights the need to: Recruit a person for gas contract. Balancing externalization and internalization consideration. Anticipate the retirement of key-employees to prevent the loss of knowledge on outsourcing management.

The JD staff has insisted on the need to develop a common database or to share and update the existing DB on outsourcing; to be consulted on the follow-up of outsourcing contracts to choose the right preventive actions and avoid conflicts; to organize coordination meetings between GSD and JD in order to develop a common language in terms of outsourcings contracts.

The HSD has insisted on the need to recruit a person who will be a gate-keeper between health and safety problematic and general secretaries problematic.

C. Some elements of discussions with the Top Management staff and some recommendations

After more than 3 months of investigations, we have exposed the organizational diagnostic and analysis to the General Secretary, the Financial Director, the Juridical Director and the Health and Safety Delegate. Some observations made by the employees were considered as difficult to hear by the Management staff and were sometimes not considered at its rights positions. The management staff has considered that it was just a problem of perception and overestimation of risks.

After 1 month of discussion, we came to propose an action plan to the top management staff. This action plan was co-framed with the support of the 9 employees that have a considerable role to play within the following up of outsourcing contracts. Four actions were proposed:

1. In terms of organization of work to reduce vulnerability: it was suggested to avoid concentrating contracts on one person. We have also suggested organizing a redundancy in contract management. We have also insisted on the need to develop a stronger collaborative framework between GSD, HSD, JD and FD by using for example:

• collaborative and brainstorming meetings;

• putting in place a person in charge of reinforcing links between HSD and GSD; • federate GSD and JD databases.

2. In terms of structure and organization: we have proposed to recruit more staff in GSD to strengthen the existing and avoid tense flow management. The transition to a preventive mode requires an investment which can be hopefully recovered within the time. We invited the Management staff of GSD to speak frankly about the need of more staff. In case of a failure to recruit, for some reason or other, organize a minina a binomial assignment of work in terms of outsourcing management.

We have also invited the top Management staff to sponsor an external audit on both financial aspects (the subcontracting solutions seems at short terms less expensive but is very expensive with a long-term point of view) and structural aspects (given the Organization evolves, what should be the organization and the need of GSD to be able to support all the expected evolution).

In terms of internal communication:

It was suggested to communicate about the contributions and the internal constraints of GSD activities.

4.

In terms of risk mapping and availability of the system. We have suggested to assess the risks of the different contracts (internal and external, and different typologies) and their impacts on the availability by both identifying short term risks (e.g. link with the business internal continuity plan and be sure that subcontractor have also frame a continuity plan) and medium/long term risks.

Summary & Conclusions

This paper has offered us the opportunity to discuss on a real case of organizational analysis of how a Company can optimize the management of outsourcing contracts.

Based on the perception and the experience of the Organization staff, we were able to identify a set of both organizational deficiencies and local adaptations of the employees to a continuing externalization of their activity. This externalization of the activity, that has end up at appealing more and more within the time to outsourcing, has deeply transformed the nature of the activity of the Organization' employees from "technical" to "contract management" and has strongly contributed to a significant loss of competency and then of self-confidence of the employee on both their limits and their knowledge.

More than two years after this organizational investigation of outsourcing contracts, the Top Management staff has challenged some of these organizational objectives and has taken some actions to improve the situation. A new staff member was recruited at the HSD to support the employees in the Health and Safety (HS) dimensions. The activity of this new member was not only devoted to outsourcing, but it has significantly contributed to reduce the perception of the employees of being alone when facing difficulties with HS within outsourcing activities. Outsourcing database were also revised and improved sharing the information coming from the different departments (HSD, GSD, JD and FD).

In 2011, an accident has occurred with a subcontractor that has made patent the factors of organizational vulnerabilities (like for example managing different outsourcing contracts at the same time). However, the different incidents, accidents and recurrent problems that are in fact "symptoms" of a "deeper organic/structural problems" have not reoriented the tendency to appeal to outsourcing but have just contributed to change the configuration of outsourcing to more concentration on "big subcontractors".

However, the trend is that "big contractors" use "subcontractors" to reduce their costs; and the Organization is facing again a new complex configuration of outsourcing: cascade of subcontracting.

Table 1 .

 1 Distribution of the contracts within the subcontractors

	Number of contracts/	Number of
	subcontractors	subcontractors
	2	12
	3	9
	4	7
	5	1
	6	3
	8	1
	11	1 (contract B1)
	15	1 (contract B2)

« il ne construit pas ce qu'il lui plaît, mais il choisit ce qu'il lui plaît de construire »(Moles, 1958).


Application of an integrated approach to evaluate health risks for toxic chemicals by linking multimedia environmental and PBPK models

Introduction

The paradigm of health risk assessment may consist of two main pillars, i.e., the exposure and doseresponse assessments. Human exposure to chemicals via multiple pathways can be estimated by environmental multimedia models, which calculate the distribution of chemicals in the component media. Combined with the information about human behaviours such as dietary habits, the multimedia models can provide an estimation of the daily chemical intake via inhalation or ingestion by humans. Physiologically based pharmacokinetic (PBPK) models are used to estimate the body burden of toxic chemicals throughout the entire human lifespan, integrating the evolution of the physiology and anatomy from childhood to advanced aged [START_REF] Beaudouin | A stochastic whole-body physiologically based pharmacokinetic model to assess the impact of inter-individual variability on tissue dosimetry over the human lifespan[END_REF].

A main objective of the European project called 2-FUN is to improve the approaches currently used in exposure and dose-response assessments. According to the aim of that project, an environmental multimedia model and a generic PBPK model are coupled as an integrated tool (2-FUN tool) and built up on a platform system, Ecolego®. This study aims at demonstrating the first application of the integrated tool to perform the full-chain risk assessment of a chemical for human health, considering multiple exposure pathways of chemical via inhalation of out-door air, and ingestion of water and foods. For this application of the tool, a case study was designed based on the information obtained from a region situated on the Seine river watershed, downstream of the Paris megacity and Benzo(a)pyrene (B(a)P) was selected as a target chemical substance. This study focuses especially on the propagation of uncertainty and inter-individual variability along the modelling chain. A probabilistic simulation was then performed to identify the input parameters and exposure pathways sensitive to specified model outputs.

Materials and methods

The 2-FUN tool consists of six compartments; air, freshwater, soil/groundwater, plant (root, potato, leaf, grain, and fruit) compartments, which constitute a multimedia environmental model, and human compartment, which is corresponding to a generic PBPK model. The multimedia model calculates the daily B(a)P exposure via inhalation and ingestion to humans. The calculated exposure outputs are automatically used as inputs in a PBPK model to estimate final model outputs.

In the designed case study, there are two model inputs associated with B(a)P levels; one is B(a)P concentration in river water at the upstream zone, and the other is B(a)P concentration in the air flow entering the target region. The long-term time series of B(a)P concentrations in the upstream of zone (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) were obtained from the database made by the Seine-Normandy Water Management Agency (SNWMA). The long-term time series of B(a)P concentrations in the air flow was made up based on the information in [START_REF] Quéguiner | Contribution of atmospheric emissions to the contamination of leaf vegetables by persistent organic pollutants (POPs): Application to Southeastern France[END_REF]. A simulation by the 2-FUN tool was performed over the period corresponding to that for these model inputs (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008).

The model outputs of interest in the case study are the concentration of B(a)P in the liver and lungs, and the total quantity of metabolites formed in liver and lungs. To perform a global sensitivity analysis and an uncertainty analysis for the defined model outputs, only the key parameters potentially influential to the model outputs were in advance selected and assigned their corresponding probability density functions (PDFs).

Results and discussion

The magnitude of sensitivity is shown by relative sensitivity index. Figure 1 presents the average index value over the simulation period for each parameter. The result for each output is as follows:

Concentration of B(a)P in liver: The most influential parameter is maximal velocity for metabolism in liver (Vmax liver ), followed by growth rate of leaf (K g, leaf ), and maximal velocity for metabolism in lungs (Vmax lung ).

Concentration of B(a)P in lungs:

Maximal velocity for metabolism in liver (Vmax liver ), followed by growth rate of leaf (K g, leaf ), and Michaelis constant for metabolism in lungs (Km lung ).

Metabolites in liver: Partition coefficient between air and water in leaf (K AW,leaf ), followed by Octanol-water partition coefficient in grain (K OW,graiin ) and growth rate of leaf (K g, leaf ).

Metabolites in lungs: Maximal velocity for metabolism in lungs (Vmax lung ), followed by Michaelis constant for metabolism in lungs (Km lung ) and liver (Km liver )

Conclusions

The model outputs of 2-FUN tool are not explicitly able to indicate health risks (e.g., risks of cancers in liver and lungs) at this stage of model development. Nevertheless, it can be concluded that 2-FUN tool has the potential applicability for health risk assessment, taking into account multiple exposure pathways via inhalation and ingestion. For the further development of the 2-FUN tool, the following points should be considered in future studies:

To compare simulated results (internal B(a)P concentrations and accumulated quantities of metabolites in the target organs) with the corresponding bio-monitoring data, and to evaluate the model performance To incorporate dose-response functions into the 2-FUN tool to facilitate the evaluation of potential risks of human disease occurrences To add dermal intake as a main exposure pathway

Figure 1 :

 1 Figure 1: A global sensitivity analysis for specified model outputsThese results indicate that the parameters used in human and leaf compartments have relatively high sensitivity to all the specified model outputs.


Julien Michel
email: julien.michel@ineris.fr

Claire Rollin

As transport characterization in the vadose zone of the soil: a combined study between field and laboratory experiments

Introduction

In industrialized countries, a lot of brownfields polluted with trace metals are identified. In France, site management strategies are based on a site specific approach. This corresponds to a combined approach between field investigations and modeling. In these studies the chemistry and hydrology of the vadose zone of the soil are not considered though the pollutants are entering the soil from the unsaturated zone. That is why it seems necessary to have a good knowledge of the mechanisms responsible for heavy metal retention and transfer in soils from the vadose zone to the water table, in order to put forward potential risks for groundwater. Most of the time, pollutant transport is evaluated at the laboratory scale with batch or column experiments. Nevertheless, these systems are of small dimensions, the soil used is generally sieved and these studies are mainly carried out under saturated flow conditions. We therefore may wonder whether these conditions are representative of what occurs in the field. That is why experiments at a pilot scale are necessary to assess heavy metal transport at the field scale. But these kinds of experiments are rather time consuming and quite expensive, and as a consequence cannot be used when doing risk assessment studies. Therefore, the aim of this work was to characterize As transport in soil in the field and to put forward mechanisms responsible for its migration in the vadose zone of the soil by setting up an original laboratory experiment. This study focused on As release as it was the trace element released on site at concentrations above the EU guidelines for drinking water quality.

Materials and methods

Site description

This study was carried out on an in-service wood preserving facility site, located in the north east of France. The wood preserving activity for phone and power line poles started in 1929. The main chemicals used were chromated copper arsenate compounds (CCA) and creosote distillates. On site, soils are sandy clay loam. The mean depth of the water table is 6 m from the ground surface; the water table level variation is about 1 m over a year depending on the season. A regulatory monitoring of the groundwater quality was initiated by a prefectural order in 1998 and 4 water wells are monitored: a well was installed upstream (W83), an other one downstream (W84), and 2 were placed in the centre of the site (W82 and W14). The well W14 is an industrial well: a pump is installed at about 9 m below the ground surface and water is pumped for the need of industrial activities, in order to dilute CCA compounds. Groundwater quality is measured twice a year.

Soil sample for laboratory experiments

Soil samples were taken between 0 and 6 m depth, right upstream the industrial well where the main pollution source was supposed to be localized after the analysis of the As groundwater concentrations over time. A quantity of 21 kg of soil was sampled with a helical auger, placed in plastic bags and stored in a cooler before being analyzed in the laboratory. These samples were homogenized by quartering and separated in 2 sub-samples (S1 and S2). A small quantity of each sub-samples was dried, sieved, grinded and submitted to a sequential and a total extraction. Total concentrations for As were 202 and 253 mg/kg for S1 and S2 respectively. As was found in soluble and exchangeable form (about 8 %) and mainly bound to manganese and iron oxides (more than 50 %). In addition, soil physico-chemical parameters such as grain size distribution, pH, TOC, CEC and total trace element content were determined on each fraction.

Experimental set up

Two laboratory columns were used in this study. They were all the same size and were made of transparent PVC and composed of 3 parts (Figure 1). The middle part was filled with 5.4 kg of As polluted soil. This part was 30 cm high and 12 cm in diameter. The lower part of the set up was composed of a funnel.

Experimental strategy

Two kinds of experiments were conducted with two columns. The first one was an infiltration column (C1) and was filled with the sub-sample S1. Cycles of rain water infiltration and drying periods were simulated: DI water was applied at the top of the column thanks to a peristaltic pump, and samples were collected at the bottom. Between the upper and middle parts, a plate was sealed; it was composed of holes, and pipes were set in these holes, so that the soil in the middle part was homogeneously fed. The water fluxes were controlled and adjusted to the fluxes recorded on site. The influence of the flowrate (between 0.04 and 0.25 mL/min), the feed duration (between 2 and 10 days) and the drying period (between 6 and 90 days) was studied. As the flowrate was very low, the column ran under unsaturated conditions.

The second column (C2) was filled with sub-sample S2 and saturated upward with calcium nitrate (2.5 10 -3 M) thanks to a peristaltic pump. The flowrate varied between 0.02 and 0.1 mL/min and was adjusted to the water table level rise speed measured on site. When the soil column was fully saturated, the flow was stopped, the pump was disconnected and water was collected at the bottom of the column. Several cycles were applied to the column and the flowrate influence on As release was studied.

pH, conductivity, redox potential, TOC and As content were measured at the outlet of both columns.

Results and discussion

Results from field experiments

Arsenic concentrations in the wells upstream and downstream were below the EU guidelines for drinking water quality (10 µg L -1

). Concentrations were a little bit higher in W82, showing only one value above the EU guidelines. Nevertheless, arsenic concentrations in W14 were much higher and above the EU guidelines, varying between 5 and 100 µg L -1

. Given the small radius of influence of the industrial well (below 1 m), the pollution source was supposed to be located right upstream this well. In addition As release was highly correlated to the water table level: the higher the water table level, the higher the As concentration in groundwater. This indicated that when the water table level was high, water was in contact with the pollution source allowing As to migrate in solution.

Results from laboratory experiments

At the outlet of C1, As concentrations were constant over time and were mainly around 3 000 µg/L. No influence of the flowrate, the feed duration and the drying period on As release was noticed. The same trend was observed for C2: As concentrations at the outlet of the column did not show any particular evolution over time and were mainly around 2 000 µg/L, with no influence of the flowrate. These results indicated that As release in this soil was not rate limited. Therefore we could hypothesize that it resulted from a thermodynamic equilibrium and that As could mainly come from the soluble and exchangeable fractions.

Conclusions

In this work, As release was studied at the field scale and at the laboratory scale. Experiments in the field helped us to identify the main pollution source location and showed us that As release in groundwater was highly correlated to the water table level. An original laboratory column allowed us to identify phenomenon responsible for As release in the unsaturated zone of the soil. In a future work As release in these soil columns will be modelled in order to confirm the hypothesis of As being released from soluble and exchangeable fractions. This study paves the way for a better understanding of trace elements behavior in the vadose zone of the soil during risk assessment studies and gives a new approach to study this part of the soil.

Figure

  Figure 1. Laboratory column