S Brohez

C Delvosalle

G Marlair

CHEMFIRE: A ZONE MODEL FOR PREDICTING CHEMICAL EFFECTS OF POOL FIRE IN A SINGLE FORCED VENTILATION ENCLOSURE

Zone models have been developed since the early 60s for the prediction of fire parameters such as smoke temperature, smoke filling and movement in multi-compartment buildings. Unfortunately, one major difficulty in current zone models is that heat and chemical species release rates in relation with a given fire source term are usually to be provided as input data. A new zone model for prediction of thermal and chemical effects of pool fire in a forced ventilated enclosure has been developed. The novelty of the approach relies in particular on the provision of three sub-models that are used for reducing the number of input data needed for a given simulation. The burning rate history of liquid pool fire is calculated from a vaporisation sub-model. A solid flame sub-model is used for predicting radiative properties of flame. Yields of chemical species are estimated from a dedicated sub-model of combustion. This zone model has received some validation for use in forced ventilated enclosures only. Promising results have been obtained.

Introduction

A large fire in a compartment is often considered as one of the most hazardous accidental event which may affect safety in industries. The fire damages may be thermal or non thermal. For examples, intense radiation produced by large fires may cause serious burn injuries to the staff of the industrial premises and the fire fighters. Moreover, the fire plume may transport a variety of toxic pollutants which can be very dangerous for people, and polluted extinction waters while unconfined may greatly affect the aquatic environment.

Zone models have been developed since the early 60s for predicting fire parameters such as smoke temperature, smoke filling and movement in multi-compartment buildings [START_REF] Janssens | Room Fires Models. General. In Heat Release in Fires[END_REF]. In this zone modelling approach, the space within each compartment is generally divided into one or two control volumes [START_REF] Cox | Compartment Fire Modelling[END_REF][START_REF] Jones | A Technical Reference for CFAST: An Engineering Tool for Estimating Fire and Smoke Transport[END_REF][START_REF] Quintiere | Fundamentals of Enclosure Fire Zone Models[END_REF]. Following experimental observations of thermal stratification of gases inside the compartment (pre-flashover fires), two-zone models divide the room gas volume into two distinct and uniform layers: an upper layer of hot gases and a lower layer of air. Inside each of those layers, gas-phase physical parameters such as temperature and species concentrations are assumed to be uniform. On the other hand (postflashover fires), one-zone models consider a single layer in the compartment, gases inside the compartment are supposed well-mixed. The physical parameters (temperature and composition of gas) of the layers are predicted by solving mass and energy conservation equations on each layer.

Unfortunately, zone models still have important limitations. One major difficulty in current zone models is that heat and chemical species release rates in relation with a given fire source term have usually to be provided as input data. Consequently, there is no feedback interaction between ventilation conditions and these input parameters, which could be assumed unrealistic in many practical cases since ventilation conditions are generally unknown prior to running the simulation. In order to avoid the mentioned drawbacks, a new zone model has been developed for simulating the behaviour of liquid pool fires in enclosures. The novelty of the approach relies in particular on the provision of three submodels that are used for reducing the number of input data needed for a given simulation. The burning rate history of liquid pool fire is calculated from a vaporisation sub-model. A solid flame sub-model is used for predicting radiative properties of flame. Yields of chemical species are estimated from a dedicated sub-model of combustion. At present time, this zone model has received some validation for use in forced ventilated enclosures only.

Control volumes of the zone model

In the conditions of the experiments conducted for validation purposes of the proposed model (in an enclosure with forced ventilation), fires carried out with low ventilation rates were observed to produce single layer environment in terms of chemical species concentrations [START_REF] Backovsky | Temperature Profiles in Forced-Ventilation Enclosure Fires[END_REF][START_REF] Peatross | Ventilation Effects on Compartment Fire Characterization[END_REF]. Accordingly, in the zone model, the gas phase inside the enclosure is thus supposed well stirred.

Moreover, in classical zone modelling approach, the point source thermal model is considered for the flame, the energy radiated by the flame is specified by the user and is a fraction of the heat release rate (typically 30%). In the development of this zone model, the classical solid flame radiation model initially developed for fire in open space [START_REF] Mudan | Thermal Radiation Hazards from Hydrocarbon Pool Fires[END_REF] is considered for modelling the flame in the compartment. Accordingly, there are two control volumes: one for the flame and one for the fumes (see fig. 1).

Figure 1. Control volumes selected in zone modelling

Vaporisation sub-model

One major difficulty in classical zone models is that mass burning rate (or heat release rate) in relation with a given fire source term is usually to be provided as input data. As this zone fire model is dedicated to simulate liquid pool fires in enclosures, the mass burning rate is predicted from correlation of Zabetakis & Burgess [START_REF] Zabetakis | Research in Hazards Associated with the production and handling of liquid hydrogen[END_REF] valid in open space:

vap comb 3 " f H H . 10 m - = (1)
where " f m = mass burning rate (kg.m -2 .s -1 ); a reduced mass burning rate [START_REF] Peatross | Ventilation Effects on Compartment Fire Characterization[END_REF][START_REF] Nikitin | Variations of Mass Combustion Rate with Oxygen Concentrations and Gas Pressure of a Milieu[END_REF] . The influence of air partly depleted in O 2 on the mass burning rate is introduced additionally by use of the correlation proposed by Peatross & Beyler [START_REF] Peatross | Ventilation Effects on Compartment Fire Characterization[END_REF]:

1 . 1 (%) Y 1 . 0 m m 2 2 O " O % 21 , f " f - = (2)

Combustion (chemistry) sub-model

In classical zone models, the yields of chemical species in relation with a given fire source term are usually to be provided as input data. In this zone model, the combustion sub-model is based on the concept of global equivalence ratio Φ [START_REF] Pitts | The Global Equivalence Ratio Concept and the Formation Mechanisms of Carbon Monoxide in Enclosures Fires[END_REF].

In order to develop the combustion sub-model, experiments were carried out at lab-scale on the Fire Propagation Apparatus operated by INERIS (apparatus in agreement with ASTM E2058) with different mass flow rates of incoming air. For several chemical substances (pyridine, adiponitrile, 1-chlorobutane, thiophène,…), the yields of CO 2 , CO, total unburned hydrocarbons, soot, NO, HCN, SO 2 , and O 2 were measured as function of the global equivalence ratio [START_REF] Brohez | Fire calorimetry relying on the use of the Fire Propagation Apparatus. Part 2: burning characterstics of selected chemical substances under fuel rich conditions, Fire and Materials[END_REF]. As examples, yields of CO 2 and CO are presented in figure 2, as a function of the equivalence ratio for the solvent pyridine. Following the data processing procedure proposed by Tewarson et al [START_REF] Tewarson | Ventilation-Controlled Combustion of Polymers[END_REF], the ventilation controlled fire properties are related to those applying for well-ventilated conditions using the following generalised relationship:

ζ β Φ α - + = exp 1 f f . v . b , i i (3)
where, f i = fire property valid for any ventilation rate determined by the equivalence ratio value; f i,w.v . = the same fire property applying for well ventilated fires; and α, β and ξ = experimental correlation parameters.

The user has to choose a fuel from a chemical substances database file. This file contains empirical correlation parameters (α, β and ξ) of 8 chemicals species (CO 2 , CO, total unburned hydrocarbons, soot, NO, HCN, SO 2 , and O 2 ) as a function of the global equivalence ratio. The yields of N 2 is obtained from the conservation of nitrogen by considering the chemical formulae of the fuel (

f e d c b a S X N O H C
) as well as the NO and the HCN yields: Equivalence ratio

Φ CO2 yield (g/g) Experimental data Maximum CO2 yield Correlation HCN HCN NO NO comb N N M f M f M d M f 2 2 2 - - = (4)
The yields of H 2 O is obtained in a similar way as Beyler [START_REF] Beyler | Major Species Production by Diffusion Flames in a Two-Layer Compartment Fire Environment[END_REF].

It can be pointed out that the database file also contains fuel properties such as heat of vaporisation, specific heat and boiling point. The chemical substances database file today contains fuels such as heptane, adiponitrile, pyridine, TDI, isoproturon, 1-chlorobutane and thiophene.

Solid flame sub-model

In the solid flame sub-model, the flame is modelled as an opaque body. The user only specifies the flame diameter. The flame height is given by the well-known Thomas' correlation [START_REF] Thomas | The Size of Flames from Natural Fires[END_REF]. Air entrainment into the burning region of pool fire is estimated from correlation established in open space [START_REF] Delichatsios | Air Entrainment into Buoyant Jet Flames and Pool Fires[END_REF].

Mass and energy conservation equations

Fumes control volume

Mass flow rate, chemical species concentrations and temperature of smoke are calculated respectively from mass, chemical species and energy balances on a control volume for gas (smoke) inside the enclosure. The conservation of mass in the fumes control volume leads to the following equation: It is convenient to considered all gaseous species as ideal gases. An ideal gas is any gas that obeys the following ideal gas equation:

RT PM T R M P = → = ρ ρ (6)
where P = the pressure; and R = the universal gas constant.

As the pressure inside the compartment can be supposed constant [START_REF] Zukoski | Development of a stratified Ceiling Layer in the Early Stages of a Closed-Room Fires[END_REF], then substituting ρ fum in equation 5 by equation ( 6) leads to the following equation ( 7) :

dt T V d R P m m m m fum fum ent fum pan air = - - + (7)
The conservation of species i in the fumes leads to the following equation:

where k h = molar enthalpy of flux k ; and fum q = net heat transfer to the fumes (W)

Flame control volume

Mass flow rate and chemical species concentrations of plume are calculated respectively from mass and chemical species balances on a control volume for flame. It is assumed that time constant of flame is small in comparison with that one of fumes. Hence, a quasi-steady state is assumed for flame (the volume, composition and temperature of flame are assumed to change slowly over the duration of the time step). The conservation of mass in flame control volume leads to the following equation:

0 m M M f m m pan 1 N 1 i i fuel i fuel ent = - + + = (10)
where N+1 = number of species ; and f i = chemical yields of species i (g of i.g -1 of fuel)

The conservation of species i in the flame leads to the following equation:

0 y m M M f m y m pan , i pan i fuel i fuel fum , i ent = - + (11)
The conservation of energy in the flame leads to the following equation:

0 q h m h m h m flam pan pan fuel fuel fum ent = - - + (12)
where flam q = net heat transfer from the flame (W)

Finite-difference approximation

The differential equations on the fumes control volume are discretised by a classical finite difference scheme. The second-order accurate central difference formula is used to discretise the first derivatives.

Heat transfer sub-model

Radiation heat transfer

Governing equations

The ceiling, the floor and the vertical walls of the compartment are assumed to be made in the same material and to have the same behaviour (same thermophysical properties and same surface temperature). Consequently, the inside surface of the enclosure forms an unique opaque surface. In addition to the wall surface, the flame is modelled as an opaque body (a surface enveloping the flame has to be considered). The wall surface and the flame surface are modelled as diffuse-grey bodies. The layer of fumes filling the enclosure is an absorbingemitting medium. Radiative heat transfer between N surfaces in a participating medium is described by the following equation [START_REF] Siegel | The Engineering Treatment of Gas Radiation in Enclosures[END_REF]:

( ) = - - - - = - - - - = - - N 1 j 4 fum k j j k 4 j k j j k jk N 1 j " j k j j j j k j kj T F T ) F ( q 1 F σ α σ τ δ τ ε ε ε δ ( 13
)
where " j q = net radiative heat flux leaving surface j ; kj δ = the Kronecker symbol ; j ε = emissivity of surface j ; kj F = view factor from surface k to surface j ; k j-τ = fraction of the radiation from surface j to surface k transmitted through the gas ;

The net radiation equation ( 13) applied to the wall surface ( w S ) and the flame surface ( ) leads the following equations:

q 1 F q 1 F 1 ε ε τ ε ε ε - - - - - - - ( ) 4 fum fum 4 flam w flam flam w 4 wall w w w w T T F T F 1 1 1 σ ε σ τ σ τ - - - = - - - - (15) " flam , rad flam " wall , rad flam w w w w flam 1 1 1 q 1 q 1 F ε τ ε ε + - - - -
1 1 1 T T q q fuel flam 4 fuel 4 flam " fuel , rad " flam , rad 2 - + - = - = ε ε σ σ (17)
The net radiative heat flux leaving flame surface flam , rad q is given by the following equation:

" 2 flam , rad flam " 1 flam , rad 1 flam flam , rad q S q S q 2 + = (18)

View factors

The view factor kj F is defined as the fraction of radiant energy leaving surface k and that is incident to surface j. The view factor F -(all the fractions of energy leaving wall surfaces and reaching the surfaces of the enclosure must total to unity):

1 F F w w flam w 1 = + - - (20)

Gas emissivity

Radiation properties depend on several parameters such as gas properties (temperature, pressure, nature of constituents) wavelength and geometry of the gas volume [START_REF] Siegel | The Engineering Treatment of Gas Radiation in Enclosures[END_REF][START_REF] Tien | Radiation Heat Transfer[END_REF]. For engineering applications, the diffuse-grey assumption is used for flame surface and fumes; hence, mean radiation properties (integrated over the whole spectrum of wavelengths) are used. Moreover, the notion of equivalent mean beam length is used for the flame and the fumes in order to simplify the analysis:

Emissivities of smoke and flame are calculated by taking into account main contributing emissivities of CO 2 , H 2 O and soot [START_REF] Tien | [END_REF]:

s g s g ε ε ε ε ε - + = ε ∆ ε ε ε - + = O H CO g 2 2 (22)
The emissivities of CO 2 and H 2 O are calculated using Modak correlations [START_REF] Modak | Radiation from Products of Combustion[END_REF]. The emissivity of soot is calculated according to Yuen & Tien [START_REF] Yuen | A Simple Calculation Scheme for the Luminous-Flame Emissivity[END_REF].

Convection heat transfer

Convection heat transfer is described by the following equation:

) T T ( h q gaz walls " walls , conv - = (23)
where, h = Mean convection coefficient to walls (W.m -2 .K -1 )

The convective heat transfer coefficient can be obtained from empirical correlations for natural convection based on Nusselt, Grashof and Prandtl numbers [START_REF] Atreya | Convection Heat Transfer[END_REF]:

Pr) , Gr ( F Nu = (24)
The convective heat transfer coefficients are approximately the same for vertical and horizontal walls. In order to simplify the procedure, the same weighted average value is taken for the vertical and horizontal walls.

Conduction heat transfer

In radiation and convection heat transfer equations, the wall temperature wall T is still an unknown. This temperature is calculated from the one-dimensional heat conduction model [START_REF] Özisik | One Dimensional Parabolic System[END_REF]:

2 2 p x ) x , t ( T t ) x , t ( T c ∂ ∂ = ∂ ∂ λ ρ ( 26
)
where, ρ , p c and λ = respectively the density, the heat capacity and the conductivity of the wall material ; x = space coordinate ; The user may choose the nature of the compartment surfaces from a thermophysical properties database file (containing values of density, heat capacity, conductivity and emissivity of walls).

In order to be coherent with the discretisation of differential equations obtained on the fumes control volume, the Crank-Nicolson finite difference method is applied for discretising the one-dimensional heat equation. Hence, equation 26 at any of the interior nodes (i=1…M-1) is discretised as follows (the walls are subdivided into M equal parts of mesh size):

n 1 i n i n 1 i 1 n 1 i 1 n i 1 n 1 i rT T ) r 2 2 ( rT rT T )
)) t ( q ) t ( q ( x r 2 rT 2 T ) r 2 2 ( rT 2 T ) r 2 2 ( 1 n " 0 n " 0 n 1 n 0 1 n 1 1 n 0 + + + + + + - = - + λ ∆ (28) x=L, L n M L n 1 M 1 n M L 1 n 1 M r 4 T ) r 2 2 ( rT 2 T ) r 2 2 ( rT 2 γ β β + - + = + + - - + + - (29) with, λ ∆ β M L h x 1 + = and air M L T h x λ ∆ γ =
where,

) t ( q 1 n " 0 +
= net heat flux (W.m -2 ) applied to the boundary surface x=0 at time level t n+1 ; M h = convection heat transfer coefficient at boundary surface x=L.

The net heat flux received by the inside surface is composed of convection and radiation heat transfers to the inside walls:

" walls , conv " walls , ray " 0 q q q - - = (30)
Equations 27 to 29 lead to a tridiagonal system of M+1 algebraic equations with M+1 unknown node temperatures

1 n i T + .
In order to solve this system by the Thomas' algorithm (which is valuable for linear systems), the source term " 0 q in the boundary equation 28 was linearised by the Newton-Raphson iterative method.

As air temperature inside the compartment can be different of ambient air temperature air T , conduction equations must be solved at time level 0 in order to obtain the initial conditions T(x,0). At time level 0, we can point out that we have a convection boundary condition for node x=0.

Validation

Experiments were carried out in a forced ventilation enclosure of 80 m 3 (at INERIS) with pyridine pool fires. The compartment is approximately 4.9 m wide by 4 m deep by 4 m high (figure 3). The walls, floor and ceiling consist of 20 mm thick concrete. The centre of the inlet opening (0.2 m in diameter) is 0.5 m above the floor. The centre of the exit opening (0.5 m x 0.5 m) is 3.25 m above the floor. Pyridine was placed in a steel pan of 0.564 m diameter. This pan was placed on a load cell in the centre of the floor. A total of five tests with different ventilation conditions were performed (ventilation flow rates were decreased stepwise from 1300 to 280 Nm 3 .h -1 ).

The flow rate of air was measured using a Pitot probe and a thermocouple located inside the inlet duct. Humidity of ambient air was also measured. The flow rate of fumes was measured using a bi-directional probe and a thermocouple located in the exhaust duct. Molar fractions of O 2 , CO 2 , CO and NO in the exhaust duct were measured. Molar fraction of THC and generation of HCN in the exhaust duct were measured using respectively a flame ionisation detector and an automatic titrimeter (a heated sampling line was used). Generation of soot was measured using a gravimetric soot measuring device. Molar fraction of O 2 inside the compartment (50 cm from floor -North tree) was also measured.

There were four thermocouple trees to measure gas temperatures inside the compartment. Trees were located in the median plans of the compartment at one meter from the vertical walls. Gas temperatures were measured by Type K thermocouples of 1 mm in diameter [START_REF] Brohez | A two-thermocouples probe for radiation corrections of measured temperatures in compartment fires[END_REF]. East and west trees contain 2 thermocouples located at 1 m et 2.5 m above the floor. South tree contain 15 thermocouples spaced 25 cm apart beginning 25 cm above the floor. North tree contain 8 thermocouples spaced 50 cm apart beginning 25 cm above the floor.

In terms of input data, the model basically requires the knowledge of the mass of fuel, the pool fire (pan) area, the mass flow rate, the temperature and humidity of incoming air and the compartment geometry in relation with a given scenario. In a final step of the initial procedure, the user also chooses the fuel and the nature of compartment surfaces from two database files.

The flame temperature predicted by the model was strongly underestimated. Indeed, the conservation of energy for flame leads to temperature of about 300°C, which is typical of plume temperature. In order to solve this problem, flame temperature must be introduced as an input data. In a forthcoming development, this parameter will be calculated from the conservation of energy on a new control volume, the persistent region of flame.

Figure 4 presents the comparisons for fuel burning rate (for the five tests, in steady state conditions) between the experimental results and the model predictions. The smaller is the ventilation rate, the smaller is the fuel burning rate because of vitiation of air entrained into the base of the flame. The model leads to fairly good predictions of fuel burning rate.

Figure 3. Schematic of the INERIS forced ventilated fire compartment

In order to analyse more finely the combustion sub-model (comparisons of chemical effects between the experiments and the model predictions), the actual fuel burning rate was also fixed as input data (the yields of combustion products depend on the value of the equivalence ratio which depends on the fuel burning rate). The paper now discusses the results obtained for the experiment carried out with an air ventilation rate of 280 Nm 3 .h -1 (the least ventilated test configuration). In this configuration, we can point out that the equivalence ratio was about 0.55. Measured fuel burning rate (g/m2/s) For concentrations of O 2 and CO 2 (see fig. 5 and6), there is a good agreement between the predicted and measured values. Prediction of concentrations of O 2 and CO 2 are respectively overestimated and underestimated by 10%. This difference could be the consequence of an overestimation of the measured ventilation rate of the compartment.

For concentration of CO (see fig. 7), there is a strong difference between calculated and measured values (zone model version 1). The concentration of CO is underestimated by a factor of about 2. We can point out that molar fraction of oxygen at the flame base has an effect on the CO yield [START_REF] Pitts | The Global Equivalence Ratio Concept and the Formation Mechanisms of Carbon Monoxide in Enclosures Fires[END_REF][START_REF] Mulholland | The Effect of Oxygen Concentration on CO and Smoke Produced by Flames[END_REF]. In our case, molar fraction of oxygen at the flame base is equal to about 13%. Hence, in the sub-model of combustion, we have introduced the correlation of Mulholland et al [START_REF] Mulholland | The Effect of Oxygen Concentration on CO and Smoke Produced by Flames[END_REF] in order to take into account the influence of oxygen concentration on the CO yield. As can be seen, there is now a good prediction of CO concentration (zone model version 2).

Conclusion

A new zone model for predicting thermal and chemical effects of pool fire in a forced ventilation enclosure has been developed. The main differences between this model and classical zone models rely on the development of three sub-models that are used for reducing the number of input data needed for a given scenario. The burning rate history of liquid pool fire is calculated from a vaporisation sub-model. A solid flame sub-model has been developed for predicting radiative properties of flame. Yields of chemical species are estimated from a dedicated sub-model of combustion which is based on experimental results obtained at labscale and introduced in a fuel database. This zone model has received some validation for use in forced ventilation enclosures. Promising results have been obtained. For a more general point of view, this is very encouraging as fire safety engineering techniques based on this type of approach are clearly needed for addressing some important safety issues in enclosures such as warehouses.

  the boiling point of the liquid fuel.However, in the configuration of the one-zone model, the air entrained into the fire is partially depleted in oxygen. The reduced oxygen concentrations at the base of flame leads to

Figure 2 .

 2 Figure 2. Yields of CO 2 and CO (g/g) as a function of equivalence ratio for pyridine

  volume, the density an the molecular weight of fumes.

  fraction of species i in flux k.The conservation of energy in the fumes leads to the following equation:

  of the radiation from surface j to surface k absorbed by the fumes ; j T = temperature of surface j ; and fum T = temperature of fumesAs the flame is opaque, the fuel surface only see the surface of the flame base. The flame surface is divided into two surfaces (of same temperature and same emissivity)

  of two equations(15 and 16) with two unknowns ( . The net radiation equation (13) applied to the fuel surface ( fuel S

  the wall surface and the flame surface 1 flam S form an enclosure, we have the following equation for w w

  in the wall at time t.

T

  = temperature of node i at the time level n ; a = material thermal diffusivity Equation 26 provides M-1 algebraic equations but contains M+1 unknown node temperatures. We need also to consider the two boundary conditions (flux boundary condition for node x=0 and convection boundary condition for node x=L): x=0,

Figure 4 .Figure 5 .

 45 Figure 4 . Comparison of measured and predicted fuel burning rate in steady state conditions Figure 5 . Comparison of measured and predicted O 2 concentrations (ventilation rate =280 Nm 3 .h -1 )

Figure 6 .Figure 7 .

 67 Figure 6. Comparison of measured and predicted CO 2 concentrations (ventilation rate =280 Nm 3 .h -1 ) Figure 7. Comparison of measured and predicted CO concentrations (ventilation rate =280 Nm 3 .h -1 )


Christophe Didier
email: christophe.didier@ineris.fr

Leloup Jacques
email: jacques.leloup@industrie.gouv.fr

THE MRPP : A POWERFUL OPERATIONAL REGULATORY TOOL TO PREVENT AND MANAGE POST-MINING RISKS

Keywords: post-mining management, prevention, hazard, regulation, MRPP gestion de l'après-mine, prévention, aléas, principes réglementaires, PPRM

Even if most French mining sites are definitively closed, potential risks remain above the abandoned exploitations. In addition to surface instability, some mining sites may be affected by dangerous gas emissions or by flooding events or environmental degradations. Those kinds of disorders strongly influence the land use management of the concerned areas. In order to post and manage properly those risks, French State has developed a powerful operational regulatory tool : the MRPP (Mining Risk Prevention Plan). MRPP aims to identify the most sensitive areas subject to "post mining hazards" and to define technical and regulation rules able to manage the principles of the future urbanism development on surface. The present paper proposes a methodological description of MRPP in order to explain the nature and objectives of the procedure.

Introduction

For several centuries, the French mining industry, considered to be one of the flagships of national industry, was a source of wealth for the country. In just a few decades, under the combined effect of diminishing resources and international competition, most mining operations were gradually forced to close.

Once a symbol of pride and solidarity, this industry is now often identified with the development of problems or a source of nuisance. As a matter of fact, closing down mining operations has not led to the complete and permanent elimination of risks and harmful effects likely to affect surface land within the geographical limits of old workings. During the period following operations therefore, traditionally known as the "post-mining" period, several kinds of disorders may develop, sometimes as soon as work has ceased but also much later.

Apart from surface instability (subsidence, collapse), some old mining sites may be affected by emissions of gases likely to be dangerous. The irreversible disturbance to underground water circulation caused by mining operations may also be the source of nuisance, both for the water circulation scheme (low point flooding, changes in water course flowrates) and its quality (water or soil pollution). These phenomena may have major consequences for people, activities and property on the surface (support work or development, moving populations). They are also likely to have a major influence on regional development in mining areas. To post and manage these various risks to an optimal degree, the French State has developed a high-performance and operational regulatory tool: Plans de Prévention des Risques Miniers (PPRM) (Mining Risk Prevention Plans -MRPP) are intended to identify the sectors most likely to develop risks or nuisances in the long term and draw up rules for managing land use according to the various constraints linked to the post-mining period.

The present paper gives a brief account of the principles subtending the drafting of MRPPs, with respect to both the administrative procedure and risk evaluation, as well as defining regulatory measures.

The MRPP, a tool for risk prevention management

The MRPP in French risk prevention policy

In France, the State is responsible for posting the risk and including it in regional development management. For this purpose, the State introduced "Plans de Prévention des Risques naturels prévisibles" (PPRN) (Predictable natural risk prevention plans -NRPP) in law n° 95-101 of 2 February 1995.

Based on the successful application of these tools, their field of application was expanded to include mining risks through law n° 99-245 of 30 March 1999. This law introduced the "Plans de Prévention des Risques Miniers" (PPRM) (Mining Risk Prevention Plans -MRPP). The rare differences which justify the introduction of a specific procedure for handling mining risks particularly include the type of risks involved and the funds and procedure for compensating victims in the event of accident.

Administrative procedure for drafting an MRPP

The Prefect, as the regional State representative, initiates the procedure for drawing up an MRPP via a "prescription order". This order defines the limits of the study and the type of risks involved. The prevention procedure is applied, as far as possible, to global physical units, called "risk basins" defined by natural parameters and/or operations and not by administrative limits.

Apart from the administrative departments involved, all town councils in the communities concerned are systematically requested to give their opinion. The Prefect also submits the draft plan for public enquiry among the populations concerned.

At the end of these various consultations, the possibly modified MRPP is approved by Prefectural order. The regulatory constraints it defines take precedence over the Local Town Planning plans drawn up by the individual communities.

The main phases of drawing up an MRPP

The Information phase

The information phase of an MRPP is designed to collect all available information (including further investigations if these are found to be strictly necessary). It requires an on-site investigation and consultation of archives.

This phase gives rise to the production of an informative map which is used as a basis for diffusing essential information to the population, because it helps to justify the prevention procedure undertaken by summarising the disorder and harmful effect which has affected the site in the past.

The Hazard evaluation phase

The hazard evaluation phase is intended to locate and list the zones exposed to potential phenomena, according to the intensity of the predictable events and the zone's predisposition to their development. This evaluation does not include the type of surface occupation.

It ends with the drafting of maps which locate the hazard zones identified by the evaluation procedure.

The Factor assessment phase

This phase includes drawing up an inventory of all existing factors in areas subject to one or more risks and identifying the potential future projects which could develop as a result. It is used to identify populations subjected to a non-zero risk level, particularly listing the most sensitive equipment or public establishments. It results in the production of a map including all the factors concerned.

The Regulatory zoning phase

The regulatory zoning phase defines homogeneous zones in terms of prohibitions, instructions or recommendations concerning land use, for both new and existing projects. The principles of this zoning are particularly based on confrontation between the levels of risks previously identified and the assessment of existing and future factors characterising the surface.

A set of rules is drawn up which is directly linked to this zoning. It is intended to define, clearly and operationally, the regulatory measures which apply to each regulatory zone.

3 Description of the main mining-related risks covered by the MRPP Edict n° 95-101 of 5 October 1995 limits the risks covered by MRPPs to phenomena which are potentially dangerous to people and property. They will be grouped into four main themes detailed in the following paragraphs.

Modification of water flow scheme

Almost all underground mining took place below groundwater level, therefore requiring the use of dewatering pumps to allow ore extraction to take place. At the end of extraction operations, stopping the dewatering pumps allows the groundwater to rise to a level close to its premining level, which leads to the development of an underground reservoir which is discharged at one or more points, into the surface hydrographic network.

During operations, major modifications may have irreversibly altered the initial conditions in the supporting rock formation. Extraction work may also have led to surface subsidence, for instance, or created artificial groundwater vents. In addition to this, benefiting from dewatering pumping and lowering of the groundwater level, some sensitive zones may have taken advantage of this "dry period" to undertake development in terms of surface and/or underground occupation.

It is therefore easy to understand the need to pay particular attention to the possible consequences of rising groundwater levels on surface property and activities once dewatering is no longer taking place.

Several potential risks can be identified. They are described briefly below:

• Modification of the discharge scheme (change in flow, appearance of new discharge points or reactivation of old ones). • Appearance of wet zones or marshes (inadequate drainage capacity for surface land).

• Flooding of subsoil and low points (groundwater level shallow beneath the surface, with possible impact on underground structures such as cellars, etc.).

• Modification of water flow scheme (may increase flood flow in certain water courses or reduce flow in others, particularly at low water). • Sudden flooding (sudden discharge of a very strong water or mudflow, through an exit connected to a mining reservoir).

Surface instability

Mining operations consist of extracting large quantities of material with the intention of marketing part of it in the form of usable ore. These excavations, whether underground or open-cast, have irreversibly altered the rock formations in which the ore is found.

Concerning underground workings, which represent most French mining operations, the method of exploitation used depends mainly on the geological configuration of the seam and the evolution of techniques. From the point of view of residual risks, there are two main methods: operations providing voids backfilling after extraction and those which lead to persistent residual voids once the workings are closed down.

For the first (working, stoping…), surface instabilities are generally limited to low amplitude movements. It is mainly the development of subsidence which is to be feared, as well as damage to buildings or infrastructure. For operations leading to large persistent residual voids (rooms and pillars, dissolution cavities…), the stability of old workings may be compromised by aging material or changes in the environment of underground mining works. Apart from the possibility of subsidence, this type of operation may also be the source of collapses.

In open-cast mining, extraction consisted of digging large pits from which the ore was extracted. The choice of operating method was based on the optimisation of economic profitability (limit the volume of waste to be cleared) and the stability of the mine structure (avoid slopes which are too steep). These rocky fronts often eventually suffer instabilities from simple rock falls to the massive slumping of an entire slope front. The combination of rock fracturing and the morphology of the slopes of the pit generally determine the volume of rock mass which is potentially unstable. The nature of the surrounding rock formation also plays a major part in the development of any instability.

At the same time, mining operations, whether underground or open-cast, have often led to the constitution of bulky mine waste deposits close to the extraction sites. Although waste materials are generally stored in dry conditions and constitute piles of material commonly known as slag heaps or waste dumps, operating residues are mostly discharged into basins confined by retention dykes built round the edge of the storage zone. The rupture of deposit structures of any kind generally leads to the unfavourable evolution of one or more factors governing the mechanical behaviour of excavation or other waste.

The main potential risks characterising the potential for earth movement is described in the following.

• Settling (recompaction of a mound of loose rock affected by underground workings. The induced movement is usually only slight). • progressive subsidence (gradual, flexible re-adjustment of surface layers. Phenomena which often cause damage but are rarely dangerous).

• Sinkholes (sudden appearance on the surface of a sink-hole. The sudden nature of the instability makes these potentially dangerous events). • General collapses (collapse of all or part of an operation, often dynamic. Phenomena which are potentially very dangerous). • Slides or slope movements (characteristics of deposits or slopes of stripping operations in loose rock: slides, gulley erosion).

Mine gas emission on surface

After closing a mine, if not completely flooded, mining voids form an underground reservoir in which mine gasses accumulate. Depending on the ore extracted and the gases present in the deposit, the residual mine atmosphere may differ greatly in composition from one site to another.

Under the effect of various mechanisms, usually resulting from the pressure gradient between the underground workings and the outdoor atmosphere (rising groundwater, atmospheric pressure drop), mine gases may be forced to the surface through natural (fractures, cracks…) or artificial (shafts, drifts…) drains.

Depending on the type and composition of this mine gas, surface gas emissions may constitute several risks or nuisances to people and property. The safety of surface occupants may be affected if the gas is trapped in non-ventilated spaces (cellars, underground networks...). The main dangers for people are: ignition or explosion (methane), intoxication (CO, CO 2 , H 2 S…), asphyxia (oxygen deficit) or irradiation (radon).

• Ignition (CH 4 ): methane, generally trapped in coal, is a non-toxic gas which is physiologically inoffensive. It is essentially its inflammability or explosivity which, when mixed with oxygen in certain proportions, makes methane such a dangerous gas. • Intoxication (CO, CO 2, H 2 S): found in the ore being mined, resulting from the slow combustion or decomposition of organic or sulfuretted material, or resulting from the action of acid water on land, these gases are highly toxic and can be fatal at high concentrations. They attack the nervous system and the body's respiratory system. • Irradiation (radon): Radon is found in old underground uranium mines, but also in other underground spaces if the surrounding land contains even tiny quantities of uranium. Radon is a radioactive element which can be dangerous to humans when inhaled. Its effects increase the risk of lung and bronchial cancer. • Deoxygenated air: Oxygen deficit situations (accumulations of mine gas or oxygen consumption) are likely to be encountered in all kinds of mining operations, particularly coal mines. Mine gas therefore represents a risk of asphyxia for people. An oxygen-poor atmosphere leads to a disturbance of the respiratory and blood systems of anyone exposed to it. A strong deficit can rapidly lead to death.

Water and soil pollution

Environmental modifications and disturbances induced by mining operations are likely to cause more or less important deterioration in the quality of environmental parameters in the area of the mine, reflected in the appearance of abnormal environmental contamination. This deterioration mainly affects underground or surface water and sediments and soils. It may also concern the atmosphere, particularly in the presence of ionizing radiation or toxic particle emissions.

The environmental impact results from complex and varied physico-chemical phenomena, closely linked to the type of substance mined and the mineralogy of the surrounding rock formations as well as the mining method used. To assess the importance of environmental risks and nuisances, it is usual to reason in terms of "sources" (pollutant type and toxicity), "vectors" (water, soil, air) and "targets" in contact with this pollution (human activities, ecosystems…).

The main sources involved result from inorganic chemical contaminants (metals, sulfates…) or organic contaminants (hydrocarbons, phenol compounds...). They may particularly originate in old mine workings (soil leaching as the groundwater level rises), deposits of surface processing residues or structures storing industrial products used during mining operations.

Concerning contamination transfer vectors, underground and surface waters play a major role with air carrying toxic dust or ionizing radiation.

The main targets likely to be affected by these environmental nuisances are humans and, more generally, land and water ecosystems (biotopes and the organisms living in them). These targets may come into contact with various contaminants, either directly or indirectly via the food chain: water, plants, fruit and vegetables, fish, farm animals, etc.

4 Identifying, listing and mapping the hazards and factors involved

Principles for identifying and mapping hazards

Definition

A hazard corresponds to the probability that a given phenomenon will take place on a given site during a reference period, reaching a qualifiable or quantifiable intensity. Characterising a hazard is therefore usually based on the predictable intensity of the phenomenon with its probability of occurrence.

The intensity of the phenomenon corresponds to the amplitude of disorder, sequels or harmful effect likely to result from the feared phenomenon. This notion includes both quantification of the consequences of the feared events (size of a crater, height of water…) and the potential severity of their effect on humans, property and soil use, likely to potentially characterise the site after a certain time (possibility of victims or damage, existence and/or cost of prevention measures...).

The probability of occurrence reflects a site's sensitivity to being affected by any of the phenomena analysed. This is a concept which is particularly difficult to quantify considering the complexity of the mechanisms of evolution, the heterogeneous nature of the environment and the very partial nature of the information available. Therefore, within the context of hazard evaluation linked to the post-mine period, priority is given to the notion of a site's predisposition to suffer disorder or nuisance. Evaluation of this predisposition depends on the combination of different factors which are favourable or unfavourable to the initiation and development of the mechanisms in question.

Qualification of classes of intensity

The intensity of the phenomenon characterises the amplitude of the expected repercussion if the feared event takes place. Each type of phenomenon has one or more corresponding representative physical values which can be used to characterise the consequences of the feared events. It is therefore the size of craters which defines the intensity of sinkholes and the estimated flowrate or depth of water which characterises the impact of flooding.

To evaluate the intensity of the feared phenomena, the expert attempts to evaluate the predictability, even approximate, of the physical values judged to be representative, for each selected phenomenon, using all the information collected. By referring to guide values which define the main classes of intensity, he can then define the class to which the feared phenomenon belongs within the context studied.

These reference values have been defined as part of the production of the methodological guide covering drafting of an MRPP. Some of the main national experts who are well-known in their field met in working groups, each dedicated to a particular theme. The guide values proposed are provided for information and can be adapted to the context by the expert in charge of evaluating risks. They also help to produce a homogeneous approach at national level, by limiting the subjectivity of the analysis inherent in the sensitivity of each expert.

As an example, the following table shows the classes of intensity of various phenomena which are representative of several classes of sequels.

Qualification of classes of predisposition

Determining predisposition is first based on a back analysis of the same type of disorder or nuisance on the site or a similar site. In addition, evaluation is also based on the analysis of scenarios and mechanisms of occurrence of phenomena likely to affect surface land, the combination of parameters promoting this occurrence and helping to raise the class of predisposition. For example, a very thick overburden, limited dimensions of residual voids and the presence of resistant layers in the overburden will make this site less predisposed to develop surface subsidence than a shallow site of operations located under an exclusively marly overburden.

Predisposition to triggering disorder can also be affected by the "incompleteness" of information available, which is often considerable in the case of very old operations. It is regularly necessary to manage the lack of available information by using the idea of "presumption".

While it would not be safe to limit yourself to only proven data, it would also be unreasonable to assign the same level of susceptibility to zones for which you only suspect that the feared phenomenon could develop as with those for which the hazard is confirmed. Therefore, a site's predisposition to the occurrence of disorders can be weighted by this idea of presumption, incorporating doubt resulting from gaps in the information.

Identification of classes of hazards

The principle of hazard qualification consists of combining criteria used to characterise the intensity of a feared phenomenon with criteria used to characterise its class of predisposition. Several principles, explicit or implicit, can be used to combine qualitative values or crossreference qualitative and quantitative criteria. For information, techniques can be noted by rating, upgrading, multi-criteria listing etc.

If the cross-referenced table principle is selected, a synthesis matrix is used, constituted as shown in the following table. To avoid complicating restitution maps, we shall limit ourselves to three classes of hazard level as far as possible: low, moderate and high hazard level.

HIGH HAZARD

LOW HAZARD

Hazard mapping

The hazard has to be mapped for the entire sector concerned by the MRPP so as to reveal which sectors are most likely to develop risks or nuisances. Feedback tends to show that a scale of 1/10 000 is often used to reach a satisfactory compromise between MRPP principles (small scale) and communities' requirements (large scale). 1/5 000 can also be envisaged, mainly in small operations in an urban environment. Depending on the size of the study zone, diffusion could be envisaged in "paper" form but it is increasingly in "computer" form, using the powerful tools of Geographical Information Systems (GIS).

The hazard zoning contours are based on technical parameters (geology, operations…). They therefore have no reason to follow the contours of the plots of land. When, as if often the case, several potential phenomena coexist on the same plot, several risk maps can be drawn up, one per phenomenon, to avoid overloaded graphics and making them illegible.

Hazard mapping includes surface areas concerned by the possible effects of phenomena resulting from mining activities. It therefore takes into account the possible lateral extension of disorders or nuisances initiated in underground spaces and developing as far as the surface.

It also includes the uncertainties inherent in the available information and results of estimates and models required to evaluate the area of influence of the risk.

Principles of assessing and mapping the factors involved

Factor assessment phase

The procedure for assessing factors involved in mining risks consists of identifying the main types of land occupation or activities, both existing or future, which are likely to interfere with the risk prevention procedure. The major factors concerned will naturally include urbanised and particularly urban sectors in which the regulatory measures applied are likely to differ considerably from those defined for open land. The analysis will also identify industrial or urban wasteland which could be strategic for the development of towns with mining risk restrictions on other parts of their territory.

Assessing all the factors concerned also includes evaluating populations in danger and listing the main factors affecting safety, crisis management and vital functions in the area concerned. Therefore, public establishments, particularly the most strategic in terms of emergency aid organisation (schools, hospitals…) are identified, whether they are subject to risk or not, as well as sensitive equipment including networks (roads, electricity, gas, telephone…) so as to identify their sensitivity relative to the expected consequences of the occurrence of disorder.

For example, routes used by traffic circulation which must be secured in priority to facilitate public assistance.

Factor mapping

The map including factors concerned is produced at the scale selected for drawing up the MRPP (often 1/10 000). It locates the main factors identified, giving them references if necessary so that descriptive sheets can be drawn up as required.

Depending on the different objectives to be reached (personal safety, future development of urbanisation…), several maps can be drawn up, giving priority to the factors most concerned by the objective in question.

5 Drawing up the zoning plan and regulations

Principles for regulatory zoning

Posting hazard and drawing up a regulatory zoning plan are two specific procedures with fundamentally different objectives. Whereas the first identifies the different types of disorder likely to develop on the surface and locates them, the second marks out the limits of the zones within which homogeneous instructions can be defined to ensure the safety of existing and future people and property.

The zones are marked out by cross-referencing hazard and factor maps, i.e. combining the type and intensity of predictable disorder with the occupation of surface land. Apart from exceptions, definition of the different zones is based on constructibility criteria (zones which are non-constructible or constructible under certain conditions…). As for hazards, the aim is to avoid multiplying the number of zones, to facilitate the legibility of the maps. For this purpose, the number of zones selected will be limited to three or four (a white zone within which requirements are minimal or non-existent, a red one for sectors considered to be nonconstructible and one or two blue ones in which urbanisation remains possible but with more or less severe constraints and restrictions).

Generally speaking, priority will be given to the development of a new urban area in zones outside areas with mining-related risks. However, although this rule can be applied relatively strictly for areas not yet urbanised, constraints may be relaxed locally in urbanised areas with strong constraints due to mining-related risk over a major part of the area occupied by the communities in question. In this case, building permits may be envisaged in low or moderate risk zones.

The regulatory zoning plan must be drawn up, as far as possible, on a scale which is compatible with studies held during risk evaluation and factor assessment phases.

Drawing up the regulations

At the same time as the zoning regulations are drawn up, the State Department in charge of drafting the MRPP draws up the regulation which defines the regulatory provisions applied to each zone within the regulatory zoning area. These measures are mainly intended to improve the safety of people and property in zones subject to mining-related risks and prevent increasing vulnerability of property and activities in the most exposed zones, reducing it if possible.

The regulations must be as simple and operational as possible to make it easier for the public to understand and assimilate them. It is generally presented by type of regulatory zone (red, blue, white…) after defining the general measures applied to all the plots within the limit of the MRPP.

For each zone, it is usual to organise the measures in two groups: those designed for existing property and those intended for future projects. The different measures may take the form of prohibitions, obligations or simple recommendations.

It will be noted that one of the regulatory specificities of MRPPs, which make them a powerful tool, is their retroactive nature which means that measures can be applied to property existing before it was drawn up. However, if there is no existing restriction to future projects, the work imposed on property built before MRPP approval can only affect limited changes, costing less than 10% of the market value of the property before the risk was posted.

For successful drafting and application of the MRPP, all available expertise (administrative, technical and political) must be federated. A concertation procedure must therefore be systematically used between State Departments and local authorities, to facilitate understanding, appropriation and participation of local authorities in the risk prevention policy.

It is therefore essential to obtain the participation of local representatives when drawing up the regulatory zoning plan and regulations because they are the main authorities who will be in charge of applying it on a daily basis and explaining it to the population.

French law n°95-101 of February 2 nd 1995, relative to environment protection reinforcement (nowadays integrated through articles L.562-1 to L.562-7 to the Code of environment).The elaboration principles are described in the edict n°95-101 of October 5 th 1995.

French law n°99-245 of March 30 th 1999, also called « post-mining law ».The elaboration principles are described in the edict °2000-547 of June 16 th 2000.

Figure 1 .

 1 Figure 1. Major mining fields having been exploited in France.

Figure 2 .

 2 Figure 2. Example of an information map.

Figure 3 .

 3 Figure 3. Discharge through an adit (left) and development of a wet zone close to an other adit (right).

  • Flows: (potentially dangerous disorders likely to affect people and property in the neighbourhood of old deposit structures).• Rockfalls (characteristic of pit slopes in open-cast mines excavated in hard rock formations, with steep slopes).

Figure 4 .

 4 Figure 4. Example of a sinkhole (on the left) and of a general collapse (chalk mine on the right).

Figure 5 .

 5 Figure 5. Effect on surface water of an abandoned coal mine discharge.

Figure 6 .

 6 Figure 6. Example of a hazard map above abandoned iron mines (North-West of France) .

L

  'élaboration des Plans de Prévention des Risques Miniers -Guide méthodologique. Ouvrage collectif sous la direction de l'INERIS. INERIS-DRS-04-51198/R01. 30/06/2004. 128 pages.


D E Gennaro

Claudia Sorgi
email: claudia.sorgi@ineris.fr

Delage Pierre
email: pierre.delage@enpc.fr

AIR-WATER INTERACTION AND TIME DEPENDENT COMPRESSIBILITY OF A SUBTERRANEAN QUARRY CHALK

Keywords: chalk, quarries, suction, viscosity, oedometer craie, carrières, succion, viscosité, oedomètre

In this paper the effect of suction and time on the mechanical response of a quarry chalk during eodometer tests is investigated. The water retention properties of the chalk, giving the changes in fluid content as a function of the suction applied, are studied and related to the changes in relative humidity in the quarry, chalk mineralogical composition and microstructure. Like in partially saturated geomaterials, it is recognised that chalk yielding is suction dependent, in that for decreasing suction (i.e. increasing water content) chalk softens. Preliminary results showing the effect of suction on the viscous behaviour of the chalk are also presented. The importance of the hydromechanical coupling associated to suction cycles and the induced cyclic changes in mechanical strength of chalk is emphasised, with regard to the long term behaviour of chalk pillars.

Introduction

Abandoned subterranean quarries constitute a potential risk in areas where their collapse may affect surface infrastructures and buildings. Quarries in chalk are quite numerous in France, for this reason a specific research is being conducted in the Estreux quarry (North of France) by INERIS, aimed at investigating the possible factors affecting the long term stability of the host chalk formation.

For mineworkings a number of collapse phenomena are documented in literature (among others: [START_REF] Blpc | La craie[END_REF][START_REF] Masson | Pétrophysique de la craie[END_REF][START_REF] Bonvallet | Une classification géotechnique des craies du nord utilisée pour l'étude de stabilité des carrières souterraines[END_REF][START_REF] Bell | A review of selected engineering geological characteristics of English chalk[END_REF][START_REF] Talesnick | The elastic deformability and strength of a high porosity, anisotropic chalk[END_REF][START_REF] Lord | Engineering in chalk[END_REF][START_REF] Grgic | A short-and long-term rheological model to understand the collapses of iron mines in Lorraine, France[END_REF][START_REF] Sorgi | Contribution méthodologique et expériementale à l'étude de la diminution de la résistance des massifs rocheux par veillissement[END_REF]). These collapses are mostly related to the destructuration (weathering) of the rock of the pillars due to external (i.e. physico-chemical processes, environmental conditions) or internal (intrinsic) factors. The timing of these collapses is not easy to predict. They often occur without any manifest precursory symptom or external action, indicating that time effects (weathering, aging) may have serious consequences on the structural stability of the rock.

It seems now well accepted that the collapse of mine pillars is mainly due to the aging and the progressive degradation of the rock material, submitted to a constant state of stress. In the case of Estreux quarry chalk, moreover, the average relative humidity (RH) varies cyclically between 100 and 80%. Due to the well known sensitivity of chalk to water action, it is believed that these changes in RH may affect dramatically chalk mechanical behaviour, increasing degradation phenomena and the risk associated to the occurrence of instabilities. It was then decided to investigate the retention properties of the chalk to check more in detail its state of saturation and its influence on the mechanical behaviour.

In this paper, preliminary results of oedometric tests on samples at various degree of water saturation are presented. Partial water saturation was imposed controlling the suction during the tests. It is shown that increasing water content (i.e. decreasing suction) decreases the yield stress and increases the time dependent sensitivity. Viscous behaviour under constant load (creep) and constant degree of saturation is also analysed. The examination of the experiments is proposed and results are discussed within the general framework adopted to describe the behaviour of unsaturated soils.

General framework

Chalk is considered as a multiphase geomaterial, and it is known from the mechanics of unsaturated soils that multiphase geomaterials behave quite differently as compared to geomaterials that contain only one fluid (saturated soils or porous rocks). In particular, they experience collapse when wetted under a constant load, indicating that an increasing proportion of wetting fluid, mainly located at the inter-granular contacts, soon causes failure.

Because pressure discontinuity through the interface between two immiscible fluids (e.g. air and water) gives rise to capillary pressure, strength loss due to wetting can be viewed as the consequence of the loss of capillary bonding between soil particles. Nonetheless, in clays for example, the chemical activity of the soil particles adds chemical fluid-mineral interactions. Neglecting the gravitational potential, the action of capillary and chemical interactions can be derived from the matric potential and the osmotic potential of the pore water, respectively. Both are part of the chemical potential µ w of the pore fluid, its definition is derived from the Gibbs free energy (e.g. [START_REF] Sparks | Soil physical chemistry[END_REF]:

(1) w o w w a ln RT + μ = μ
where µ w is the chemical potential of the pore water solution, µ o w is the chemical potential of pure water, a w is the activity of the pore water solution (a w = 1 for pure water and a w → 0 increasing solutes concentration), T is the temperature (absolute Kelvin's scale) and R is the constant of the ideal gas (8.3143 Jmol -1 K -1 ). When expressed in terms of pressure, the chemical potential is usually called total suction in unsaturated soil mechanics, and defined as

w w w a ln RT M s ρ - = (2)
where the sign minus is introduced in order to ensure that s is strictly positive, ρ w is the water density and M w is the molecular weight of water (18 gmol -1 ). For partially saturated states, equilibrium of pore water with its vapour phase allows to simplify in eq. ( 2) the expression of the activity coefficient a w of pore water into the ratio between the partial vapour pressure p w of the pore water and the partial vapour pressure p ow of the pure water at a reference state (a w = p w / p ow ), this ratio defines the relative humidity RH (%) = 100a w . If we neglect the effect of dissolved solutes in pore water (accounted in the osmotic suction, s o ), the activity of pore water is only due to its pressure and the geometry of the pore space (i.e. matric suction s m is equivalent to capillary bonding). Thus, total and matric air-water suctions are identical and are computed through

) RH ln( RT M u u s w w w a ρ - = - = (3)
where u a is the air pressure and u w is the water pressure.

To describe the mechanical behaviour of unsaturated soils the usual mono-tensorial approach based on the effective stress concept [START_REF] Terzaghi | The shearing resistance of saturated soils and the angle between the plane of shear[END_REF] is no longer valid (e.g. [START_REF] Wheeler | Constitutive modelling[END_REF][START_REF] Gens | Constitutive modelling : application to compacted soils[END_REF]. Indeed, matric suction is considered as an independent stress variable [START_REF] Coleman | Stress strain relations for partly saturated soils[END_REF]Fredlund & Morgenstern, 1977), and is usually admitted to act as a pure hydrostatic stress tensor. The other possible stress variable is the net stress σ -u a I, being σ the total stress tensor and I the unit tensor (both here are second order tensors). This hypothesis enables to account for hydromechanical coupling in the behavioural features of multiphase geomaterials. Its validity has been verified on multiphase reservoir chalks, where the non-wetting fluid is oil rather than air [START_REF] Delage | Subsidence and capillary effects in chalks[END_REF]Pasachalk1, 2001;[START_REF] Gennaro | Time-dependent behaviour of oil reservoir chalk: a multiphase approach[END_REF][START_REF] Sorgi | Contribution méthodologique et expériementale à l'étude de la diminution de la résistance des massifs rocheux par veillissement[END_REF]Pasachalk2, 2004). Within this theoretical framework, based on equation (3), hydro-mechanical coupling associated to hygrometry changes (i.e. suction changes) has been considered has one of the main factors affecting the stability of the underground quarries in chalk. In this paper we discuss some of the experimental evidences corroborating the proposed general framework for chalks.

Tested chalk: characterisation and sample preparation

The chalk used in this study comes from Estreux quarry (north of France). Blocks of Estreux chalk were retrieved at 25 meters depth. At the temperature and the hygrometry inside the quarry, 11° C and nearly 100% of relative humidity, respectively, the chalk was almost completely water saturated (natural water content w = 20.7%). The SEM image and the MIP curves of Estreux chalk are presented in Fig. 1. SEM picture displays evidence of a clayey matrix (flat platelets) and microfossils (Fig. 1a). The clay fraction was identified as glauconite, which is often present in chalks from the north of France, giving a typical greenish colour [START_REF] Blpc | La craie[END_REF][START_REF] Bonvallet | Une classification géotechnique des craies du nord utilisée pour l'étude de stabilité des carrières souterraines[END_REF].

Probably due to the presence of microfossils, porosity of Estreux chalk is quite high, in the range 35 to 37 % (average void ratio e o = 0.566). MIP curves identify a narrow family of pores, with an average pore entrance radius value in the range 500 to 800 nm (Fig. 1b). We note a certain scatter in the values of the intruded volume at higher pore entrance radii (> 2000 nm) which seems likely to depend on the natural variation of the porosity, and is amplified by the reduced dimension of MIP samples.

Samples used for laboratory tests were cored from a unique block (same orientation) and then resized on a lathe. For water saturated samples this procedure was performed as much quick as possible, in order to avoid evaporation, adding water if necessary. Samples of about 38 mm diameter and 20 mm high were used for the oedometer tests (excepted one test on dry chalk, performed on a sample 50 mm in diameter). For the retention tests, samples had a diameter of about 20 mm and their height ranged between 20 and 25 mm. Dry samples were obtained after a heating period of 24 hours at 105° C.

Experimental techniques and apparatuses

Due to the key role played by suction on the behaviour of chalk, experimentation required a set of testing procedures allowing suction control. The two methods used were: (i) the osmotic technique, (ii) the vapour phase control.

The osmotic technique

The osmotic technique [START_REF] Williams | An evaluation of polyethylene glycol PEG 6000 and PEG 20000 in the osmotic control of soil water matric potential[END_REF]Kassiff & Benshalom, 1971;[START_REF] Delage | Suction controlled testing of non saturated soils with an osmotic consolidometer[END_REF][START_REF] Dineen | A new approach to osmotically controlled oedometer testing[END_REF] allows to control suction by osmosis through a regenerated cellulose semi-permeable membrane behind which a solution at a given concentration of large molecules of polyethylene glycol (PEG) is circulated. The concentration of PEG solution define the corresponding value of suction applied, obtained by previous calibration. To apply the desired suction during experiments, apparatuses should allow the contact of the chalk sample with the semipermeable membrane and the PEG solution. The layout of the experimental setup used to apply the desired suction level to chalk samples during water retention tests is shown in Fig. 2. The method (Cui & Delage 1995[START_REF] Gennaro | Time-dependent behaviour of oil reservoir chalk: a multiphase approach[END_REF] consists in inserting the chalk sample in tube-shaped semi-permeable membrane and in plunging it into a container full of PEG solution placed on a magnetic stirrer. For the determination of the imbibition curve, dry samples were inserted. Due to its wetting properties, water could infiltrate chalk under a controlled suction and expels a given volume of air. In the case of the drainage curve, the samples were saturated with water before being inserted. Due to the higher suction imposed by the concentred PEG solution, the pore water is expelled from the porous network of the chalk, crosses the membrane and is released into the PEG solution. In both cases the fluid transfer come to an end when the osmotic suction imposed by the solution and the matric suction of the sample are at equilibrium.

The vapour phase control

The vapour phase control allows total suction control by means of PEG solutions (Fig. 3a) or saturated salt solutions (Fig. 3b). Suction this case is controlled by the ambience relative humidity imposed by the solution, taking care that isothermal condition is fulfilled. During all the retention tests presented in this paper a temperature of 20° C ± 0.5° C was ensured. The use of PEG or saline solutions depends on the level of desired suction. In this study PEG solutions were used for total suction levels lower than 2.5 MPa (RH = 98.2%), the minimum value of total suction attained by saturated saline solutions being 4.2 MPa (RH = 97% with K 2 SO 4 salt).

The suction controlled oedometer

One sample in this study was subjected to a mechanical test under suction controlled condition. This type of test was performed in the osmotic oedometer (Fig. 4). During oedometric tests chalk samples were subjected to incremental loading (applied instantaneously) and the evolution of the vertical strain during each loading stage was measured. This procedure is well established in soil mechanics. The value of the vertical strain is defined as the strain corresponding to the effective stress state (in saturated soil), i.e. the state of stress for which the excess pore fluid pressure is zero (end of primary consolidation). In a multiphase chalk, excess pore pressures of water during consolidation could modify the suction. Then, the actual vertical strain value should correspond to the condition of equilibrium of suction. In this device suction is controlled via the osmotic technique. The bottom of the sample is in contact with a semi-permeable membrane below which a solution of PEG is circulated (Kassiff & Benshalom, 1971;[START_REF] Delage | Suction controlled testing of non saturated soils with an osmotic consolidometer[END_REF]. In order to control the suction level, water exchanges through the membrane were monitored through visual observation of the water level in a graduated tube placed in the bottle that contains the PEG solution. In order to reach the desired net vertical stress, the sample was loaded using a high pressure oedometer frame with a double lever arm that allowed a maximum vertical stress of approximately 60 MPa on a sample 50 mm in diameter. At each loading stage, the stabilisation of the PEG level in the tube was checked.

Retention properties

The retention properties of Estreux chalk define the relationship between the degree of saturation S rw and the suction s (the retention curve) in an air-water saturated chalk sample. Depending on the driving fluid used to modify the relative saturation of the sample, two curves can be established, following an imbibition path (water driving) or a drainage path (air driving). Retention curves have been obtained using one of the two techniques of suction control described previously.

MIP data were also used to obtain the retention curve following a drainage path. In fact, during mercury penetration the imposed pressure p c is equal to the capillary pressure given by Laplace's equation : p c = (2σcosθ)/r, where σ is the surface tension, θ the contact angle between mercury and chalk (σ = 0.482 N/m and θ = 147°, respectively) and r is the pore entrance radius. The values of surface tension and contact angle in Laplace's equation for chalk in contact with water and air are σ aw = 0.072 N/m and θ aw = 0°. Using Laplace's equation, and imposing the equality of the pore entrance radius, air-water suction s can be derived knowing p c values from MIP as follows

θ σ θ σ = - = cos cos p u u s aw aw c w a (4)
Obviously this relation is valid admitting only capillary interaction between chalk, air and water. Retention curves of Estreux chalk are presented in Fig. 5. It can be seen that a suction level of 1.5 MPa is necessary to expel water and reduce the chalk water saturation degree following a drainage path. This value of suction is also called "air entry value". It is straightforward to observe that retention curve following a drainage path obtained using osmotic or vapour phase technique, give by far higher values of suction than the same curve derived from MIP data using Laplace's equation (i.e. admitting only capillary effects). It follows that air-water interactions of different nature probably exist in this chalk. These interactions are likely to be the consequence of the activity of the clayey matrix of Estreux chalk and may involve other physico-chemical processes (i.e. adsorption, dissolution, etc.). It is believed that the knowledge of the extent of these interactions is of great importance in defining the macroscopic mechanical response of this rock. Another indicator supporting the hypothesis of the existence of possible physico-chemical interactions is the small hysteresis observed in Fig. 5 between the imbibition path and the drainage path. In fact, significant hysteretic behaviour is often associated to capillary interaction and inkbottle effect, leading to the entrapment of a given amount of water in the porous network, whereas physico-chemical interactions often produce reduced hysteresis. Note that, based on eq. ( 3), 1.5 MPa suction corresponds to a relative humidity of 99.8%.

Available measurements of the initial suction of Estreux chalk (at natural state) by filter paper (Fig. 5), gave values of about 40 kPa, in good agreement with the almost fully saturated state of the material. This corroborates also the in situ measurements of RH, indicating almost complete water vapour saturation at the time of the sampling. It is also interesting to observe that the residual water saturation is attained for suction values between 4.2 and 24.9 MPa. The former corresponds to RH = 97% (with K 2 SO 4 salt), the latter to RH = 83,5% (with (NH 4 ) 2 SO 4 salt). Thus, changes of few percentage points in RH may drastically reduce the relative saturation of the rock. Also, available measurements in the quarry gave minimum RH value of about 80%, which is sufficient, based on the retention curves plotted in Fig. 5, to attain almost complete de-saturation. Thus, the natural evolution of the relative humidity inside the quarry can lead to the two extreme saturation states (S rw = 100% and S rw = 5%), with changes in suction values between 0 and 24.9 MPa (i.e. the full range of attainable suction values). Consequently, due to the hydromechanical coupling, suction changes due to relative humidity could act as an additional load on the rock pillars and may affect, at long term, the stability of the underground quarry. The effects of these changes on material strength are quantified in the next section by means of suction controlled eodometric tests.

Results of oedometric tests

Compressibility curves

Figures 6 shows the results of three compression tests performed on chalk samples having various amounts of water: water saturated, dry and partially saturated under 1.5 MPa of controlled suction. The water saturation degrees (S rw ) of the samples and the corresponding level of suction can be estimated from the retention curve of Fig. 5. Thus, the water saturated sample has s = 0 MPa, the dry sample has s > 24.9 MPa and at 1.5 MPa of suction the chalk has S rw ≅ 90% (i.e. is almost water saturated).

The analysis of the consolidation curves (Fig. 6a) for the water saturated sample, using the usual graphical methods employed in soil mechanics, did not permit to identify any significant point where changes in excess pore fluid pressure were apparent. Indeed, these methods seem not reliable for chalk, which is characterised by consolidation curves showing continuous vertical strain of the sample as soon as the loading is applied. Because of the order of magnitude of the permeability in this soft rock is quite high (about 1x10 -8 m/s) and the compressibility of the soil skeleton quite low (due to inter-granular bonding), it is believed that after 24 hours of consolidation any excess pore fluid pressure is reasonably dissipated. This observation is in agreement with the analysis proposed by Lade and de Boer (1997), about the concept of effective stress in soil and rocks, and the general results of poroelasticity [START_REF] Biot | General theory of three-dimensional consolidation[END_REF][START_REF] Rice | Some basic stress-diffusion solutions for fluid-saturated elastic porous media with compressible constituents[END_REF][START_REF] Coussy | Mechanics of Porous Continua[END_REF], showing that in a porous medium, the amount of the applied total load transferred instantaneously to pore water pressure increases when the drained bulk modulus decreases. Thus, it seems likely that the reduced compressibility and the rather high permeability are sufficient in chalk to prevent excess pore fluid pressure generation during instantaneous loading (i.e. stepwise oedometric loading). If this holds true, the delayed volumetric strain of chalk, as observed during oedometric tests (Fig. 6a), should be essentially due to creep and the solid skeleton settles continuously, although the excess pore pressure is zero. This result should be corroborated by pore pressure measurements during applied loading.

In order to reproduce the compressibility curves for Estreux chalk, the time of consolidation was 48 hours for stress levels lower than σ o (conventional yield stress, as defined in Fig. 6b), and it was extended to seven days for loading steps in the plastic regime. The rational behind this choice was to have an insight into the creep behaviour of the rock. The compressibility curves of the chalk show clearly strength dependency on suction. It is interesting to observe that since the imposed suction level in Estreux chalk gives a water saturation degree that is close to the fully water saturated state (Fig. 5), compressibility curve under suction control at 1.5 MPa is also close to that obtained on a fully water saturated sample (Fig. 6b). Nonetheless, the effect of suction is present, and it can be observed that the yield stress σ o (the stress at which rock compressibility notably raises) increases with increasing suction, i.e. with lower degrees of water saturation.

Figure 6b also shows the collapse phenomenon of a dry chalk due to water imbibition. This was obtained by water injection under a vertical load of 29.3 MPa. In this test, the initial air-water suction (certainly higher than 24.9 MPa) was reduced to zero. Thus, based on the results shown in Fig. 5, the initial degree of water saturation of the dry sample, certainly less than 5%, was increased to the final value of 100% under zero suction. Water was infiltrated from the bottom to the top of the chalk sample, imposing few tens of centimetres of pressure head. The final point in Fig. 6b has been obtained after twelve days of continuous infiltration, and defines a total decrease of 0.183 in e/e o ratio (vertical path). This variation of the void ratio corresponds to a compressive volumetric strain of about 6.6%. This collapse is significant, at the end of the process the final point is on the compression curve of the saturated chalk (Fig. 6b), in agreement with the double oedometer approach proposed by [START_REF] Jennings | The additional settlement of foundations due to collapse of sandy soils on wetting[END_REF] to assess the collapse susceptibility in unsaturated soils. The reduced amount of water menisci at the dry state doesn't seem to justify the magnitude of this breakdown as the consequence of the capillary loss solely. Consequently, it seems likely that the inter-granular links have a significant physico-chemical component, and are progressively weakened by the infiltration of water at the level of the inter-grain contacts. Pressure solution creep [START_REF] Hellmann | Experimental pressure solution compaction of chalk in aqueous solutions. Part1. Deformation behaviour and chemistry. In: Water-Rock Interactions, Ore Deposits, and Environmental Geochemistry: A Tribute[END_REF]) is often mentioned to account for physico-chemical processes associated to rock weathering. This type of water-rock interaction bears a number of common features with the phenomena associated to suction. This point should certainly be the object of further researches, in order to better quantify the basic mechanisms at the origin of the chalk deformation.

The results presented so far are compatible with the water weakening effect in chalks, extending this effect to the partial saturation in both fluids under a controlled suction.

Viscous behaviour

Moving from the results presented in Fig. 6a, the analysis of the creep behaviour of Estreux chalk has been conducted quantifying the creep rate developing during each of the loading steps under oedometric conditions. The examination of the curves presented in Fig. 6a for the water saturated sample, plotting the evolution of the ratio e/e o as a function of the logarithm of time, permits to define the classical creep coefficient C α given by

) t (log d C e de i oi α = (5)
where e oi is the void ratio at the beginning of loading stage i, C αi is the slope of the consolidation curve vs time at long-term (Fig. 6a). Since equation ( 5) is valid for each of the hydraulic configuration, C αi coefficients were assessed for all the loading stages of the three tests in Fig. 6b. Results are summarised in Fig. 7, plotting the evolution of C α as a function of the applied vertical stress for all the tests. It is straightforward to note that both suction and vertical stress influence the creep rate. The creep rate varies linearly with the applied vertical stress, while it decreases when the level of suction increases, moving from a fully saturated to a completely dry environment. Similar results are in agreement with findings by [START_REF] Alonso | A review of Beliche dam[END_REF] on the creep behaviour of partially saturated rockfill.

Note that, since suction influences the yield stress of the material, it is very likely that the influence of suction on creep could be reconducted to the similar effect on yield stress, assuming a normalizing factor for the effect of suction. This is the object of ongoing studies.

Parameter C α describes adequately the viscous behaviour of Estreux chalk. Indeed, changes in creep rate due to suction (i.e. relative humidity) and load changes are easily accounted for when considering the results shown in Fig. 7. It is believed that these preliminary results can be useful for both practical applications, such as the assessment of the long-term stability of chalk pillars under various mechanical and environmental conditions, and theoretical developments (modelling approaches).

Conclusions

The experimental data presented in this paper are aimed at showing some effect of suction on the compressibility and the long term behaviour of a quarry chalk.

The role and the importance of the retention properties and their knowledge are emphasised, in order to better understand the coupled hydromechanical behaviour of partially saturated chalks. Compressibility and time-dependent behaviour have been studied in close relation with the influence of air-water suction. It has been shown that a number of common behavioural features exist between unsaturated soils and partially saturated chalks. Chalk is made stronger by increasing suction levels, which in turns means decreasing water contents. Its long-term deformation has consistently been related to the hygrometry changes by means of suction. The water weakening effect in chalk is considered here in a more comprehensive framework, introducing the air-water suction as an independent stress variable capable to account for the water-rock interactions.

Acknowledgements

The results on Estreux chalk have been obtained during the French National Project BCRD coordinated by INERIS. The collaboration of Mrs. M. Audiguier (Ecole des Mines, Paris -SEM images of Estreux chalk), Mr. J.V. Candel Hernandis and Mr. G. Priol is also acknowledged.

Figure 1

 1 Figure 1. (a) SEM picture of Estreux chalk, (b) Mercury Intrusion Porosimetry curves.

Figure 2 .

 2 Figure 2. Experimental setup used to control suction by osmotic technique (Cui & Delage, 1995).

Figure 3 .

 3 Figure 3. Vapour phase control: (a) using PEG solution, (b) using saturated salt solutions.

Figure 4 .

 4 Figure 4. The osmotic oedometer.

Figure 5 .

 5 Figure 5. Retention curves of Estreux chalk.

Figure 6 .

 6 Figure 6. Results of oedometer tests : (a) Consolidation curves, (b) compressibility curves.

Figure 7 .

 7 Figure 7. Evolution of creep parameter Cα vs vertical stress for various hydraulic configurations.