Moving to the peoplemetered audience. A sociotechnical approach

Keywords: Audience measurement, TV ratings, Actor network theory, Quantification

Using an actor network theory approach, this article argues that the audience exists only from the moment when it is circumstantiated and instantiated. To observe the audience, sociology thus needs to identify its practices, discourses and manifestations. This hypothesis is illustrated though an historical case study: the adoption of a new television audience measurement, the peoplemeter (a new audiometer machine nowadays widely used by professionals) in France in the late 1980s. It opens the technology's black box at a time when changes forced actors to reconsider their collective and individual audience definitions. Through analysing the implementation of the peoplemeter, the article shows how a triple process of routine, consensus and trust led the actors to accept this new system. This process, it is argued, explains how actors translated a state of their own construction of the audience into a technological system. KW :

numerous traces that it leaves behind, making meanings which are never definitive. Some (like [START_REF] Williams | Culture and Society[END_REF] or in a post-modern approach cited in this volume by Jérôme Bourdon) argue that no audience exists outside of its representations, which amount to the texts constituting them. From this perspective, there is therefore a "phenomenon" (watching television, constituting an audience, gathering a public) that materialises only in the discourses, which seek to make it exist. Hence, for the past, it would have to be found in ancient traces. I support a different position here, that there is no audience outside of the agency (see Callon, forthcoming) constituting it. This agency assembles, in a complex way, various elements (such as firms, TV professionals, consumers organization, public authorities, etc.), in a more or less complex composition, connecting heterogeneous elements, from human beings to technical artefacts, calculating tools, rules, machines, programmes, procedures… In TV measurement, this agency proliferates: moments of sharing around the television, comments on a programme, remote participation in programmes, individual choices, measurement of preferences or behaviours, fan organisations, solitary viewing, program cost, etc. There is a host of practices and discourses made possible by television, but which only become an audience if they become human and nonhuman actors, identifiable and localised.

The idea that I defend, inspired by the sociology of science and Actor Network Theory (ANT) [START_REF] Akrich | Sociologie de la traduction. Textes fondateurs[END_REF], is that this opposition between the "real audience" and its representations is inappropriately posited, as is the broader debate between realism and constructivism. ANT has shown that it is futile to oppose these two approaches, with the first claiming that objects (or phenomena) exist since we can use them to act on the world, while the second insists on the fact that they are constructed since work had to go into making them exist. In my type of sociology, science cannot teach us anything about nature when it neither defines its objects nor acquires instruments, rhetoric, laboratories, trades, etc. Likewise, I argue that to observe the audience, sociology needs to delimit its practices, discourses and manifestations. 1 In other words, the audience cannot be considered outside of the empirical descriptions that are made of it. The audience or rather audiences exist from the moment that they are circumstantiated and instantiated. My concern is not so much with their status as an idea (explored in this volume by Sabina Mihelj) as with their actualisation as a agency, that is, as an entity that composes practices, mixes actors -or rather human and non-human agents [START_REF] Akrich | The De-scription of Technical Objects[END_REF] -and redistributes roles and competences. Based on a particular moment in their history, I therefore examine here the systems or arrangements that locally allow for an audience to be constructed, and that empirically and never definitively afford actors the possibility of agreeing on the common definition of a televiewer, so as to further our understanding of what makes "telesviewing".

Note, however, that unlike nature 2 , the object I am concerned with here already carries out part of the work: television and more generally the media cause the systems organising the mediation of audiences to proliferate at all stages of their production [START_REF] Hennion | Programming Music: Radio as a Mediator[END_REF]. To cite but a few: measurements of televiewers' musical preferences through voting systems; pre-tests on new programmes; competitions comparing professionals artists' popularity; etc. Through all these systems, the medium makes an audience exist and makes it present. This audience in turn will then be able to act on the definition it gives of itself, on its productions and on its organisation. This path seems fruitful for a historical approach to audiences, and from a methodological point of view researchers can try to constitute their own mediations. However, I argue that the media offer them a host of mediations on which they can comfortably rely. This type of approach has been neglected, despite offering valuable material for historians -material that facilitates a focus on particular moments, without the risk of anachronism that is always difficult to avoid when it comes to studying ordinary practices. Historians, particularly historians of the media, have shown 1 See as examples of this approach: Méadel, (2010); Wieser, (2013). 2 It is worth qualifying this statement: academic objects, like the ethologists animals, are sometimes more actively involved in their own scientific analysis than is generally thought to be the case (Desprets, 2012).

little interest in the contributions of ANT approaches. Exceptions are work on the history of science and, of particular interest to us here, research on the history of statistics. As [START_REF] Desrosières | The Politics of Large Numbers: A History of Statistical Reasoning[END_REF] has shown, such approaches make it possible to identify the relationships between the epistemology adopted by actors and the network of practices in which they are engaged, and thus to bring out the contingency of notions and their stabilisation (however temporary). This is also how I approach the audience: through the history of the statistical tools that gradually made it emerge (and as I have said, as one of many other means), we can uncover the genesis of its production. For this we need to explore the traces left by the discussions held when the notion of audience was being debated, when it was not yet seen as covered by the seal of obscure evidence or of an equally blind critique of the quantified totalisation of "televiewership". Let me specify however that this is not a unique path, and that other approaches to the audience, including ones involving systems specifically elaborated for a research project, may be equally valid. It is simply that I have chosen to focus on the history of audience ratings and their technologies. My aim is to understand how a "quality" mouthpiece is constructed and functions, that is, outside of any axiological approach; a mouthpiece which provides representations of the audience that may have as much meaning in a magazine as it does is the office of a programme director or in a communication journal. Through what processes do these representations, summarised in a few figures and put into black boxes, operate such a reduction of behaviours so efficiently?

The audimeter, an interested actor

In this chapter I have chosen to study French television actors' adoption of a particular technology, an advanced version of the audimeter: the peoplemeter.

The audimeter is the audience measurement technique most used by professionals; it is omnipresent in the public sphere, has given rise to a large number of controversies, and mobilises considerable resources. More than a technology, we should talk of a system, or better yet, in keeping with the ANT approach explored, of an actor; for the peoplemeter is not only a technological object, but also a stakeholder actively involved in the constitution of diverse networks associating heterogeneous elements: agents (actants) of all kinds and sizes [START_REF] Akrich | The De-scription of Technical Objects[END_REF].

In France, the first audimetric tools were set up in the early 1980s, when television was still exclusively public and measuring its audiences was the responsibility of public authorities (the service in charge of audiences, the CEO, came directly under the remit of the Prime Minister's services). Competition between channels was therefore purely internal. Yet very quickly and very much universally (Bourdon & Méadel, forthcoming), irrespective of the television channels' economic and political organisation, the results supplied by these audimetric machines provided professionals with the main profile of the audience, the one they feverishly checked every morning. This was the contested but recognised authority that was to justify the choice of one professional or talkshow over another within channels. It was the standard accepted by the different actors, which was to determine channels' budgets (both private and public, directly or indirectly). The importance of their role across all television sectors was unquestionable.

From there on, a case was made that such a system was too complicated to be fair, that it would not teach researchers anything they did not already know, and that television was a two-sided market [START_REF] Rochet | Two-sided markets: a progress report[END_REF] selling both its airtime and its televiewers (Smythe, 1077). What did it offer researchers concerned with the constitution of audiences that was not pre-empted by the interest of the parties?

We should first note that from a realist perspective, certain researchers [START_REF] Buzzard | Tracking the Audience: The Ratings Industry From Analog to Digital[END_REF] and professionals alike consider that these figures reflect the truth about the practice (or at least a few of its characteristics). In fact, I should rather say that they consider them from a reflexive perspective: television (as a comprehensive entity encompassing all of its participants) splits into two to better look at itself through a system-mirror, in this case audience ratings. This reflexive point of view is admittedly de facto effective: in a certain type of configuration, the figures do talk for the audience while simultaneously talking about it. Take for example the production of a receivable and commonly shared diagnosis of the success of a politician's televised appearance: the rating, which allows for comparisons both in time and with other performances of the same type, diagnoses a state of citizens' interest through its direct analogy with voting. Televiewer-citizens, in turn (and according to configurations with specificities which historians must study), will potentially appropriate this State; make it an element of their judgement, which may then impact on their judgements. This effectiveness is actually so great that it by far exceeds the scope of those wishing to mirror themselves in it, and ultimately interests far more actors, including in the political sphere [START_REF] Napoli | Audience Measurement and Media Policy: Audience Economics, the Diversity Principle, and the Local People Meter[END_REF]. For the actors themselves, these measurements therefore do talk about audiences, or at least about what they call audiences and construct as such, in each of their programmes, each of their transactions, and each hire or sale of advertising space.

Excluding audience ratings (like any other such internal figures) on the grounds that they speak only for actors who are overly involved and interested, would amount to falling for the illusion of a divide between the audience and its representation. From my perspective, the audience is always the more or less solidified and stabilised outcome of a host of operations instrumented to make it exist. The active interest of the actors who construct audience ratings to build their market and manage their business is part of the definition of this audience; it comes into play in the way this audience is conceived of, and it influences the audience as much as the audience in turn influences it.

Individualising listening or the arrival of the peoplemeter

Let us therefore take a closer look at an episode that may shed light on this way of rereading the history of audiences through ratings. This episode consisted of a change of technology, from the audiometer to the peoplemeter, in France at the end of the 1980s. As a brief reminder, the audimeter is a machine plugged into the TV set of households that are part of a panel, and which records two types of information, generally every second: whether the TV set is on and, if so, on what channel. The data stored are sent to the server by phone. Once the information has been collected from all panel members and the exact sequence of programmes has been analysed to match times and programmes exactly, these data are processed, adjusted and formatted. The main results of the measurements are then delivered to broadcasters every morning for the previous day.

Presented in this way, these operations seem simple. Yet like any technical system designed to produce knowledge, the audimeter required a number of conventions to be put in place for the tool to function [START_REF] Bourdon | Inside television audience measurement: deconstructing the ratings machine[END_REF].

The conventions had to be compatible with professional uses, the distribution of roles, the organisation of the companies in charge of the audio-visual industry, the role attributed to the public media by the authorities, and television practices, at least the most common ones.

This type of survey relies on the techno-scientific equipment of statistics, and on its adaptations by the survey industry, with the same sampling, adjustment and panel management practices, etc. However, unlike surveys, audimeter measurements do not anticipate behaviours (for example electoral behaviours) or opinions, they record an immediate behaviour.

With the first version of the audimeter, in service in France from the early 1980s, it was in a sense the TV set that was being measured rather than the televiewer, since there was no individual differentiation. Then in 1987 the newly established French company Médiamétrie (the outcome of the privatisation of a government service), wished to adopt the state-of-the-art technology: the peoplemeter. With this device, data are individualised as each machine is equipped with a series of buttons, assigned to each member of the household and even to guests. So what does this peoplemeter measure? The answer seems obvious at first: televiewers.

To be more precise: its first version, the household-audimeter, counted the number of TV sets turned on. The second version, the peoplemeter, adds up the seconds of televiewers reporting their presence, whose socio-demographic profile is identified.

The new technology raised many questions that were debated within the framework of a Ratings Committee (Comité Audimat), the archives of which we were able to access 3 . This Committee had decisional power for all questions pertaining to audience ratings. It brought the rating actors together: advertisers, TV channels through their production departments, advertising agencies, and rating professionals employed by Médiamétrie. One of the core questions echoed the above-mentioned problem regarding researchers: that of the machine's "neutrality", since the televiewer's individual intervention was required. Like the Nielsen Institute 4 , at the time everyone thought that only a device that did not require users' cooperation could be considered reliable. The televiewers were presented as the weak link of ratings. Even though the results of studies showing that they pressed the button pretty much correctly 5 were recognised, and there was no reason to refute them, directly involving televiewers in the measurement raised questions: what impact did this have on their behaviour? What did this shift to a measurement seen as non-neutral imply, and what type of audience did it produce?

Why focus only on this particular episode? I could of course highlight the surely interesting concomitance between this change of technology and the economic and cultural transformations the television sector underwent at the time, with its privatisations and industrialisation [START_REF] Bourdon | Du service public à la télé-réalité : Une histoire culturelle des télévisions européennes 1950-2010[END_REF]. I could also focus on this episode to argue that it "reveals" the "reality" of the things hidden by technology, by explaining for example the conventional choices affected by the balance of powers within the company Médiamétrie, with the public service television 3 This chapter therefore draws on the archives of the Comité Audimétrie (hereafter referred to as the Committee). It more specifically reports on dense debates, particularly in the period of interest to us here, the minutes of which are held in the Médiamétrie archives. It also draws on a series of interviews with ratings managers. 4 Broadcasting, 4 May 1987 5 As the Committee's scientific manager explained: "Declarations of presence with the peoplemeter are tallied up with the reality of presence through telephone surveys. The results vary between 94% and 95% coincidence".

company still wielding considerable influence, and Médiamétrie's wariness of anything that might justify accusations of manipulation. No doubt… but a contextual explanation of the kind raises as many questions as it resolves, and would hardly be coherent with the approach chosen since it would suggest a distinction between the measurement instrument and its object, between the audience and its representation, and between the context and the technology. Let us therefore try to open the technology's black box at a time when changes forced actors to reconsider objects' definitions and how they translated into options of the machine, and to potentially understand how these elements, traditionally seen to pertain to the context, were reflected in the technology itself. From this perspective, we identify three modalities in the process of implementation of the peoplemeter: consensus, routine and trust. Consensus emphasises the justifications used by actors to produce collective decisions. Routine stresses the importance of continuity and stability in the audience's definition and construction. Trust clarifies the risk that actors are willing to take in order to carry out their common task: rating audiences.

Consensus or the audience as a collective agreement

In almost all countries, television audience ratings are produced by a specialised agency, which has the particularity of depending on the actors of the very market that it is in charge of evaluating. This agency, organised in the form of a Joint Industry Committee (JIC) [START_REF] Syfret | Television Peoplemeters in Europe[END_REF], sets the rules of the game and the conventions of the measurements, and then subcontracts to a research company, except in France where Médiamétrie is responsible for all operations. In most countries, this has led to a monopoly situation: advertising actors (in the broad sense of the term, including advertising agencies, media production departments, advertisers, and all the intermediaries that test and calculate the results of advertising messages) are challenged as soon as different ratings compete. In early 1988, as the switch towards new technology to individualise results was being discussed, Médiamétrie was just coming out of an acute crisis that could have jeopardised its very existence [START_REF] Méadel | Quantifier le public. Histoire des mesures d'audience de la radio et de la télévision[END_REF]. The members of the Committee were fiercely attached to this co-management of measurement methods, as explained by an advertiser: "there is one principle outlined here which we will not go back on, and that is the principle of audience ratings comanaged by the channels and users. This is a crucial point. This instrument is too strategic, both for you and for us, for us not to have a right of scrutiny. I don't think we can revert to an earlier situation whereby we would turn to an outside company, whatever it may be". Note, incidentally, that the existence of a dual system to rate audiences was not a problem with regard to the legitimacy of calculations (broadly speaking, the actors did not consider that the divergences between the two series were important enough to call the methods into question).

Rather, it was problematic because it created a situation of instability for the actors, as each one chose the figures most convenient to them, depending on the question being asked. In early 1988, the actors finally gathered and represented on the Ratings Committee explicitly set themselves the objective of escaping the past situation of conflict, to avoid reverting to a dual counting system, which they deemed highly unfavourable.

One of the first debates, in March 1988, concerned the second operator to be in charge of managing the panel. The actors considered that this operation could not be entrusted to only one operator and that competition would stimulate the operators. They also explained that a monopoly by one actor before the introduction of the peoplemeter had created a situation of tension. One of the three candidates was in charge of the competing measurement, supported by advertising agencies but disapproved of by advertisers. The issue was then how to escape conflict and thus to reach a collective decision that involved not only a choice of operator but, more broadly and as explained by one of the members, the very definition of the measurement, since the aim was to understand whose object it was, in which network it was embedded, and to what world it was linked. While all the actors' primary goal was to reach agreement between the parties, how could this be achieved? How could a common decision be reached by actors who were competing or had contradictory business relations? While the different partners of the television market were represented on Médiamétrie's Executive Committees, which are the classical structure of organisations in charge of audiences, there was no constraint of representativeness. Or rather, there was no continuous constraint: the first members to take part in the creation of the organisation in 1984 were the main actors of the time, and for some time (until an inevitable and recurrent crisis) remained the only members of the Board of Directors and of the Executive Committees, and hence of the Ratings Committee.

The newcomers had to find their place through an often-critical process, after which their weight in the television marketplace allowed them to make a number of allies.

How did the actors justify their position? Boltanski and Thevenot's (2006) analytical framework equips our analysis by emphasising the plurality of systems of justification articulated by the actors, not according to broad concepts external to them, but through a pragmatic approach to their positions, split into six orders of worth (market, inspired, domestic, reputation, civic, industrial). Since the order of the justifications articulated by Médiamétrie actors matched what Boltanski and Thevenot call the market polity, as they granted importance to competition between the parties, we could expect decisions to be made according to the financial weight of each actor, or their number of shares in the company.

In fact, what we observe in Committee decisions is that their financial "weight" did not come into play, any more than did their market share in the calculation of the prices of studies: these were set for each category of actor. The channels, for example, all paid the same amount, irrespective of their market share and even their coverage rate (which newcomers challenged shortly afterwards).

While the emphasis was more on the part played by Médiamétrie as the Chamber of Commerce of television, with a representative role -in short, while justifications were shifting towards the civic polity with values closer to democracy and the collective will -, the political decision-making model was no longer prevalent, internally, in the Committee's debates. The Committee even explicitly challenged the classical democratic model by explaining its members' hostility towards majority-based democratic management which, as an advertiser explained, "leads to non-transitive choices". This also led the members of the Commission to exclude secret ballot voting which, by not forcing each actor to clarify their choices, made consensus more difficult to reach.

Their objective was not to produce decisions at all cost, but to reach agreement on core issues, with all possible and realistic knowledge of each and every one's positions (both contradictory and competitive). To achieve this, the Commission's organisational mode provided for the exclusion of any voting procedure for as long as possible, and for seeking consensus by all means. When agreement could not be reached, the decision was postponed to the following meeting, and most of the time the extensive negotiation work carried out in the interim by the Médiamétrie executives led to consensual agreement, without the need for a vote. This is what happened for the choice of the second operator. Several unsuccessful meetings and probably numerous interim discussions were needed to reconcile the different points of view. One of the winning arguments in favour of the (temporarily) chosen operator, Telecontrol, was its independence from the market research companies. From an outside perspective, but also probably in the presentation of results and the constitution of the panel, any ties could pollute the results obtained by the peoplemeter, by creating continuity between the television consumer and the consumer of advertised goods. While advertisers would have seen such ties in a highly favourable light, they accepted the idea that they would be so advantageous to them as to destroy the balance between the audience rating partners. This choice therefore reveals audience ratings as a large whole encompassing a multiplicity of actors who convey particular conceptions and constructions of the audience. Agreement on methods, such as that gradually reached on the presentation of results, panel quality control, the minimum participation age, etc., was as important in managing to put the new technology in place as the object measured: the audience. We also see that the audience could function only as a collective agreement that "digested" each opposition and conflict, so that it authorised only a single interpretation of the figures provided. This is further evidenced by the importance of communication tools for the press, or by the role of discussions on the press release published after certain committee meetings.

Moreover, these are all elements which the Commission would seek to stabilise, to make coherent over time.

Routine or the path of normality

As with any statistical series, continuity is crucial here, both to establish norms and to adjust them over time. With the peoplemeter the actors faced not only the conjectural difficulty of rendering the data from the old system (household audimeter) compatible with those of the new peoplemeter, but more significantly and lastingly the formidable problem of continuously measuring behaviour that is in no way continuous. As explained by one of the members of the Commission: "statements on televisual behaviour are some of the hardest to use because when you ask a question regarding a televisual behaviour, you say 'generally, when you watch TV, do you do this or that'. There is never generality6 . Television is a multi-market with multi-moments and multi-programmes". Talking of an audience therefore amounts to isolating a particular way of "watching television" by aligning behaviours to make them measurable over time. This is not so much a case of continuity of series, but of routine. The continuity of statistical series is perfectly relative, even in those that seem the most unchanging: "contrary to what we tend to think, an INSEE7 statistical series is not a paragon of immutability but is subject to incessant changes regarding the conditions in which the survey is conducted as much as the statistical techniques themselves or the survey's political uses" [START_REF] Didier | La solidité des institutions. Les statistiques de « victimation » de l'Insee[END_REF]. These series are consolidated by the constant interventions surrounding the different aspects of the measurements. The Ratings Committee's work thus sought to create routines while also allowing for a number of adaptations, primarily and crucially the individualisation of results.

The Committee endeavoured to establish the continuity of each operation, to get rid of any differences or changes. Take the "return path", for example. The machine actually allowed for two-way communication: the audience data automatically produced were transmitted to the operator, but it was also possible to share information with the panellist. The operator, for example, could send the panellist a message asking them not to forget their duty as a panellist.

However the Committee, which nevertheless gave importance to this characteristic in the call for tenders addressed to the operators, obstinately refused to use it, arguing that it could disrupt panellists' routine and upset their behaviour.

More generally, when processing basic data and turning them into daily bulletins for each client, the Committee decided to exclude anything that could be seen as an atypical case, any behaviour which, in statistical terms, escaped "normality".

Hence, unlike what was happening in other countries, new panellists' results were taken into account only three weeks after the machine was launched (without the panellists actually being informed of this). The idea was for the guinea pigs to have time to get used to the machine, forget about it and therefore no longer produce "atypical" results.

These atypical behaviours can also be linked to new configurations. For example with the spread of new technology like the VCR, very popular at the time, the Committee decided (on 18 May 1988) not to include the data even though they had been well identified and collected, and provided quality information. It chose to reconsider the question only when the Commission deemed that "the phenomenon has become interesting", i.e. that it would become significant within the data (which did not happen).

Refusing the abnormal thus led to the audience being described in as simple and univocal terms as possible, even when the measurement tool's individualisation was opening the possibility of collecting an extensive wealth of personal data. We thus see that audience ratings are not market knowledge tools like any other. In mass markets, many studies have shown the extensive work carried out to qualify and distinguish both the demand and the consumer [START_REF] Callon | The economy of qualities[END_REF]. Here, faced with a televiewer deemed unpredictable, elusive and evasive, the Committee tried to routinize not behaviours (which could hardly be achieved), but the results of the measurement operation, even when this meant losing televiewers, which led it to accept quantitatively inferior results. This routine won the trust of the parties involved by limiting and confining any challenges to the system.

Trust without intervention

The need to work in an atmosphere of trust and to earn all of the actors' trust was constantly stressed by the members of the Commission. It was really a matter not of confidence but of trust, as differentiated by Niklas Luhman. Trust implies a situation in which the actors, faced with a situation in which they cannot control all of the information, make explicit and rationalised choices to obtain satisfaction: "trust is a solution for specific problems of risk" (Luhman,p. 95).

Such confidence presupposes that, as in most ordinary situations, both in public and in private life, actors agree to suspend their expectations and to neglect uncertainty, at the risk of attributing their disappointment or failure to an external factor. For the adoption of the peoplemeter, the members of the Committee did define the options of the new system by taking ownership of their watching television means being in the same room, irrespective of what one is doing. When this definition was being discussed, an advertiser member asked for the notion of attention to be introduced: "would it really be insulting to our fellow citizens to consider that they are not able to understand another instruction that would consist in saying: 'you press on the button to signal your presence when you are in a position to watch television'?". The Médiamétrie CEO answered: "we do not control the attentive audience. (…) We need the most neutral and automatic measurement possible" (17 May 1988). Unlike in Germany, for example, the definition of the televiewer was therefore not left to the panellists, for fear that they would get caught up -as could also be the case of the Commission -in the eternal (French or at least European) debate on the relationships between culture and television. At that point in history, the criterion defining the audience was therefore no longer "watching television", but "living with it". Now is the time to specify that the definition of the audience which developed was local and circumstantial, and provided historians of audiences with a field of investigation that here again meets that of the actors: how can we talk about audiences in general terms when the notion evolves, changes shape, and with it transforms its data, tools and effects? How are we to establish continuity between formats that are not constant? Symmetrically, in our example, measurement actors are required to do bricolage to organise continuity between the figures of household audiences extrapolated to individuals, prime-time programmes seen through the audiometer, and individualised audience figures supplied by the peoplemeter.

It is worth noting that in the late 1980s, against the backdrop of the multiplication of the television offer and of the broadcasting media, the operational definition used by the actors departed from researchers' traditional debate surrounding the audience's passivity/activity (in line with [START_REF] Butsch | The Making of American Audiences[END_REF].

By adopting this very minimal definition of what it means to watch television, the actors signalled that the televiewer's degree of participation, and their capacity to invest themselves in content and produce interpretation or debate, was no longer a dominant criterion in considering the notion of audience.

The Commission was once again faced with a question of trust during several discussions on the machine's "social acceptability". The intensity of the debate was exacerbated by the fact that, at the time, the organisation had invested in work on passive audience measurement (an automatic system to detect the people present in front of the screen, which was a failure), a technology which raised concerns around the theme of "Big Brother is watching you". By involving the panellists in its functioning, the system once again raised the question of surveillance: panellists were now active in their self-surveillance and supplied the data for individual monitoring. This in turn raised the question of how far the survey should go in the degree of knowledge on the people involved and on their habits and consumption patterns. The Committee chose to exclude any complementary survey that would provide better knowledge of the members of the panel but would involve greater intrusion into their intimacy. As an advertiser explained: "anything that might influence the audience should be fiercely proscribed". To challenge these surveys, the members of the Committee referred to the technology's "social acceptability". But this was also a way of confining the pact made with the panellist: their role was limited to pressing on the button (or not): any other form of communication would blur the message and would be seen as an intrusion that would risk disrupting the relationship. This does not detract from the fact that the ratings are also instruments of power, used in the media to justify choices, to "sell eyes and ears" and to put pressure on the authorities. No representative is meant to be neutral; like any mediator, audience ratings must be understood as situated, linked to other forms of representation. Trust is elsewhere: the system will be all the more reliable if risks of intervention and interpretation have been limited. In short, seen through measurements, the audience is all that remains when everything (or almost everything) has been removed. and their formalisations, as well as to each of the parties' role, the quality of the panel (which household stratification should be selected?), the degree of delegation to the operator, the cost of the service, etc. The list of the many questions addressed by the Committee could be extended even further.

Conclusion

Analysing this grammar shows how this triple process of routine, consensus and trust led the actors to accept a new system, which they were reluctant to use to measure audiences. I have used the term consensus process to describe this common construction of a shared definition, both of audiences and of their measurement -a definition that reconciles and aggregates their differences.

Routine refers to the processes through which a phenomenon deemed significant was defined, and shows that this significance could only be built over time.

Finally, the trust process led them to debate the points of agreement and risks borne collectively. During this threefold process, the Committee agreed to abandon the previous, automated audimeter system and to switch to a less neutral system that required the panellist's intervention. The expected benefit was linked to the personalisation of results, no longer extrapolated from household data but activated by the users themselves, with all the questions this raises about their participation, motivations, limits, etc. While the risk was shifted (from one extrapolation to another), the measurement had to continue to be the only collective audience quantification recognised by the members.

Working on such archives sheds light on the functioning of the television market.

For example, using these documents we could show how the basic conditions of the neoclassical market, with pure and perfect competition, undergo significant distortions when it comes to quantifying data on a two-facetted market. We could also show that the search for optimisation, considered as a basic objective of the markets (optimisation of investments, of the number of televiewers, of the number of slots sold, etc.), is not a starting point for discussions, but a result of the construction work carried out by the actors. But this is not the crux of the analysis. My approach has primarily sought to explore how actors translate a state of their own construction of the audience, a state that is collective by definition, as is the audience itself, into a technological system. The threefold process of setting up the peoplemeter echoed the professionals' construction of the audience. Not in its individual dimension, for nothing in the archives I analysed could allow me to make routine, trusting, consensual, etc. beings out of the televiewers… But we can say that, in the collective definition produced both by televiewers and by professionals, this is an active process: what the individual cannot do alone (revealing a practice that is comparable and measurable over time) can be produced through the aggregation of individualities, at the cost of a drastic rarefication of anything that could pollute the "televiewership" practice. These data will not provide the "whole" of viewership, but what defines it in many circumstances and for many actors as a collective practice.

Such a process has a cost: reluctance to embrace innovation. And of course, it proves to be particularly problematic in times of rapid change, as in the world of television since that period. At the end of the 1980s the main changes were

  choice and the risks it presented. Let us consider two of the issues they had to face: the definition of the televiewer, and the machine's intrusive role. How does one define what it means to "watch television"? The peoplemeter opened up uncertainty which the previous machine had embedded: as I have said, with first generation audiometers, television was said to be watched for the exact seconds during which the set was on. The individualisation of measurement required each person to report herself as a televiewer. But when is a person actually a televiewer? What about when the phone rings? If the person goes into the next room but continues to listen, does he or she remain a televiewer? And what about when they are lost in thought and no longer listening? The Committee chose the most univocal definition (or at least the least equivocal):

  Analysing the archives of the Committee for this episode of the introduction of the peoplemeter in 1988 allowed me to identify what could be called (still in keeping with Boltanski and Thevenot's terminology (2006)) regimes of justification, or the grammar of the actors involved in a technological change. This was a major technological change as it entailed modifications to the measuring machine, the task of the panellist operating the machine, the measurement unit (seconds or minutes), the collection of results (what audience presentation?), their processing (what constitutes a valid significance threshold?)

My emphasis.

French agency in charge of public statistics.


Daniel Dias
email: daniel.dias@insa-lyon.fr

M Asce

Jean-Pierre Janin

Abdul-Hamid Soubra
email: abed.soubra@univ-nantes.fr

Richard Kastner
email: richard.kastner@insa-lyon.fr

Three-dimensional face stability analysis of circular tunnels by numerical simulations

The face stability analysis of shallow circular tunnels driven by the pressurized shield is investigated by three-dimensional numerical simulations. Both the active and passive soil failures of the tunnel face are considered in the analysis. A comparison is performed between (i) the present numerical solutions of the ultimate pressure of the tunnel face, (ii) the results of the multiblock mechanism of the kinematical approach in limit analysis and (iii) the values of the ultimate tunnel pressure obtained from centrifuge model tests. The shortcomings of the multiblock failure mechanism are presented and discussed.

INTRODUCTION

The face stability analysis of shallow circular tunnels driven by the pressurized shield has been investigated in literature by several authors. Most approaches are based on a two-dimensional analysis [START_REF] Dias | Slurry shield tunneling : simulation of the effect of slurry infiltration on the face stability[END_REF], DeBorst et al. 1996) and are based on either the limit equilibrium [START_REF] Anagnostou | Face stability in slurry and EPB shield tunnelling[END_REF] or the limit analysis methods [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF][START_REF] Soubra | Three-dimensional face stability analysis of shallow circular tunnels[END_REF], 2002) which require a priori assumptions concerning the form of the failure mechanism. In the three-dimensional two-block failure mechanism investigated by [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF], the assumed failure mechanism considers that only a part of the tunnel circular face (an ellipse inscribed in that circular area) is concerned by failure; the remaining part of the tunnel face surface being at rest. The aim of the present work is to determine the ultimate tunnel pressure and the corresponding soil mass at failure without any a priori hypotheses concerning the shape of the failure mechanism. The approach used in the present paper for the determination of the ultimate pressure of the tunnel face is based on numerical simulations using the Lagrangian explicit finite difference code FLAC 3D . Both the active and passive soil failures in front of the tunnel face are considered in the analysis. The ultimate tunnel pressures obtained in both the active and passive cases are presented and compared (i) to those given by the kinematical approach of limit analysis by [START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF] using a multiblock failure mechanism and (ii) to the results of the centrifuge model tests by [START_REF] Hallak | Etude expérimentale et numérique du renforcement du front de taille par boulonnage dans les tunnels en terrains meubles[END_REF]. In the following, one presents an overview of the recent literature. Then, the numerical FLAC 3D simulations and the comparison between the present solutions and the existing ones follow.

OVERVIEW OF RECENT LITERATURE ON THE ULTIMATE PRESSURE OF THE TUNNEL FACE

Several theoretical models have been presented in literature for the computation of the ultimate tunnel pressure. The most recent approach is the one presented by [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF] and improved later by [START_REF] Soubra | Three-dimensional face stability analysis of shallow circular tunnels[END_REF][START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF]. The model by [START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF] is based on the kinematical approach of the limit analysis theory and used a multiblock failure mechanism. The results given by this mechanism are the best upperbound solutions for both the active and passive cases. On the other hand, centrifuge model tests have been performed by [START_REF] Hallak | Etude expérimentale et numérique du renforcement du front de taille par boulonnage dans les tunnels en terrains meubles[END_REF]. The centrifuge model is composed of a steel container of dimensions 1200mm x 80mm x 720mm filled with Fontainebleau's sand and a steel tube of 200mm diameter to represent the tunnel. The tunnel axis is located at 500mm below the surface of the soil mass. In these tests, the small-scale model is submitted to an acceleration of 50g where g is the gravitational acceleration. This model simulates a tunnel of 10m diameter.

Three phases are performed during the test for the determination of the ultimate active pressure: First, an air pressure equals to that corresponding to the earth pressure at rest at the tunnel center is applied to the tunnel face. Then, the acceleration is increased from 1g to 50g. Finally, the ultimate active tunnel pressure is obtained by reducing the supporting pressure until failure occurs. The experimental results have shown that in the active case, the soil mass at failure does not reach the ground surface and the failure mechanism is confined nearby the tunnel face. However, the soil mass in failure reaches the ground surface in the passive case.

THE NUMERICAL APPROACH

The centrifuge model tests performed by Al-Hallak on a small-scale model of tunnel were simulated using the explicit finite difference code FLAC 3D . For symmetry reasons, only half of the geometry is modeled (figure 1). The size of the numerical model is 0.4 m in the X direction, 0.72 m in the Z direction and 0.90 m in the Y direction. These dimensions are chosen so as not to affect the value of the ultimate tunnel pressure. A three-dimensional non uniform mesh is used. The present model is composed of approximately 23 000 zones. A conventional elastic-perfectly plastic model based on the Mohr-Coulomb failure criterion is adopted to represent the soil. The parameters of the soil mass were deduced from laboratory tests [START_REF] Gay | Modélisation physique et numérique de l'action d'un glissement lent sur des fondations d'ouvrages d'art[END_REF][START_REF] Gaudin | Modélisation physique et numérique d'un écran de soutènement autostable, application a l'étude de l'interaction écran-fondation[END_REF]). The metallic tube simulating the tunnel is simulated by a "liner" structural element. The tunnel is assumed to be rigid and an interface is placed between the soil and the outer surface of the tunnel to allow the slip that may occur on this contact area.

FIG. 1. Numerical model

The characteristics adopted in the numerical simulations for the soil and the interface are given in Table 1. As in the experimental centrifuge tests, a pressure equal to the soil pressure at rest is applied to maintain the stability of the tunnel face. This pressure is taken equals to the experimental value (i.e. 200 kPa). Then the model is subjected to an acceleration of 50g. Finally the ultimate active tunnel pressure is found by gradually decreasing the internal pressure until the failure of the soil mass nearby the tunnel face occurs. This means that a stress-controlled approach is used for the computation of the tunnel ultimate pressure.

THE NUMERICAL RESULTS

As mentioned in the previous section, a stress-controlled approach is used to compute the active tunnel pressure. The computation is performed for the case C/D=0.5, where C and D are respectively the tunnel cover and the tunnel diameter. Several gradually decreasing pressures are successively applied to the tunnel face until failure or plastic flow occurs in the soil. At each pressure, several cycles are performed until a steady state of static equilibrium or plastic flow is developed in the soil. The plastic flow is achieved when both conditions are satisfied as the number of cycles increases: (i) small values of the unbalanced forces for all nodes of the mesh; (ii) a non-constant continuously increasing displacement of a specific point in the soil mass. In this paper, the horizontal displacement of the tunnel center is considered for this analysis. The highest pressure corresponding to this state of plastic flow is called the active tunnel pressure (failure pressure). Notice that for smaller values of the tunnel pressure, a constant displacement of the tunnel center is obtained as the number of cycles increases (figure 2). This indicates that a steady state of static equilibrium (i.e. no failure or plastic flow) is obtained in those cases. From this figure, one can see that the active tunnel pressure is obtained for

kPa t 15 =
for which a non-constant continuously increasing displacement is obtained as the number of cycles increases.

C ycl es H ori zontaldi spl acem ent(m )

16 kPa 15 kPa 14,9 kPa Pressure at the face

FIG. 2 Horizontal displacement at the center of the tunnel face versus the number of cycles in FLAC 3D

An alternative and equivalent approach to identify the state of failure or plastic flow considers (i) the velocity of the tunnel center (horizontal velocity in the present paper) as shown in figure 3 and (ii) the condition on the unbalanced forces. For the active tunnel pressure, the velocity of the tunnel center does not decrease to zero with the number of cycles indicating a non-constant displacement of the tunnel center with the number of cycles (i.e. there is no steady state in this case). However, this velocity decreases to zero (which corresponds to a constant displacement) when the steady state of static equilibrium is obtained indicating that the failure does not occur in those cases. Notice however that the first approach is preferable since it gives information about the value of the tunnel center displacement for the different tunnel pressures.

-

FIG. 3. Horizontal velocity at the center of the tunnel face versus the number of cycles in FLAC 3D

COMPARISON WITH OTHER RESULTS

The results of the active and passive pressures obtained from the numerical simulations using FLAC 3D are compared in figures 4 and 6 with those given by [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF] and [START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF] using the multiblock mechanism in limit analysis. Concerning the collapse case, the pressures calculated with FLAC 3D are smaller than those given by the kinematical approach. A maximum difference of about 20% for C/D=0.5 is obtained. This difference could be related to the a priori assumption concerning the form of the failure mechanism chosen by [START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF]. The intersection between the cone adjacent to the tunnel face in the multiblock mechanism and the tunnel face is an ellipse. This surface represents the zone of the face concerned by failure. In the numerical simulations, the entire tunnel face is concerned by failure (figure 5). This may explain the difference between the ultimate pressures computed by the two approaches in the active case. For the passive case, the ultimate passive pressures calculated by FLAC 3D are smaller (maximum difference of 40%) than those provided by the kinematical approach. This may be explained by the shape of the failure considered by the multiblock mechanism at the tunnel face. Figure 7 shows that in the blow-out case, only the upper-half of the tunnel face is concerned by failure. However, the multiblock mechanism in limit analysis considers an ellipse whose great axis is vertical and equal to the tunnel diameter. Table 2 shows the comparison between the results of (i) the present numerical simulations, (ii) the kinematical approach and (iii) the centrifuge results for the following configuration: C/D = 2, = 42°, = 15.3°, = 15.7kN/m 3 where is the soil unit weight. The solutions by [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF] are not presented since the present upper-bound solutions by [START_REF] Soubra | Kinematical approach to the face stability analysis of shallow circular tunnels[END_REF] are better (i.e. greater) than the ones of [START_REF] Leca | Upper and lower bound solutions for the face stability of shallow circular tunnels in frictional material[END_REF].

The results given by the kinematical approach are not in good agreement with those obtained by the centrifuge tests (difference of about 30 %). This confirms that the failure mechanism used in this approach is not adequate to calculate the limit pressure of the tunnel.

CONCLUSIONS

It was shown in this paper that the kinematical approach does not take into account with accuracy the failure shape at the tunnel face. In the collapse case, the entire surface of the face is at failure. However, the kinematical approach considers only an elliptical surface. In the blow out, the numerical simulations have shown that only the upper-half of the tunnel face is concerned by failure. However, an ellipse with a great vertical axis equal to the tunnel diameter is taken into account in the kinematical approach. Thus, it seems necessary to develop new failure mechanisms in limit analysis in accordance with the experimental and numerical findings.

  FIG. 4. Failure pressure versus C/D (collapse case) -comparison between the kinematical approach (Leca and Dormieux 1990 and Soubra 2002) and FLAC 3D

FIG

  FIG. 5. Velocity field, collapse case, C/D = 0,5

FIG. 7 .

 7 FIG. 7. Displacement velocity field, blow out case, C/D = 0,5

Table 1 . Characteristics of the soil and the interface Soil characteristics Value Interface characteristics Value

 1

	Young Modulus	50 MPa	Normal stiffness	3* 10 11 kN/m
	Poisson's ratio	0.33	Shear stiffness	3* 10 11 kN/m
	Cohesion	0 kPa	Cohesion	0 kPa
	Friction angle	42°Friction angle	28°D
	ilatancy angle	15.3°U
	nit weigth	15.7kN/m 3

Table 2 : Comparison between the present results and those of the centrifuge tests and the upper-bound solutions by Soubra (2002) Authors Centrifuge tests -Al Hallak (1999) Kinematical approach (Soubra 2002) Numerical simulations

 2

	t [kPa]	8	5.8	6.8


Anthony Nouy

Pierre Ladevèze

RADIAL-TYPE APPROXIMATION TECHNIQUE FOR A SPACE-TIME MULTISCALE COMPUTATIONAL STRATEGY

A new multiscale computational strategy was introduced recently for the analysis of structures. It involves an automatic homogenization procedure in space and in time. This procedure requires the resolution of a large number of linear evolution equations. Here, we present a robust approximate resolution technique for these equations, based on the concept of radial approximation. It leads to the construction of a relevant reduced basis of space functions and it is particularly suitable for the analysis of composite structures.

INTRODUCTION

The behavior of heterogeneous structures, such as composite structures, often results from complex phenomena taking place on the microscale. Attempts to predict the behavior of such structures have led to the development of increasingly sophisticated models describing the material on a very fine scale compared to the scale of the structure. Therefore, the numerical simulation of these problems requires the development of efficient computational strategies taking into account these multiscale aspects. For the description of local phenomena with short lengths of variation in space as well as in time to be accurate, the calculations must be performed on a fine discretization of the structure and of the time interval being considered. This condition leads to problems with very large numbers of degrees of freedom, whose computation costs with standard calculation codes are generally prohibitive. One of the main objectives in the last few decades has consisted in developing efficient and robust computational strategies suitable for this type of problem. A new multiscale computational strategy for nonlinear evolution problems was introduced recently. This strategy involves an automatic homogenization technique in space as well as in time [START_REF] Ladevèze | A multiscale computational method with time and space homogenization[END_REF][START_REF] Ladevèze | On a Multiscale Computational Strategy with Time and Space Homogenization for Structural Mechanics[END_REF] which is an extension of previous works limited to space [START_REF] Ladevèze | A micro-macro and parallel computational strategy for highly heterogeneous structures[END_REF]. This strategy, developed in a general framework, makes no a priori assumption on the form of the solution and, therefore, does not have the limitations of standard homogenization techniques. Until now, it has been developed in the framework of small disturbances of (visco)plastic structures under possible contact conditions with or without friction.

THE MULTISCALE COMPUTATIONAL STRATEGY

The first characteristic of this strategy consists in partitioning the structure into substructures and interfaces, each of these entities having its own variables and equations. The change of scale takes place only at the interfaces, where forces and displacements are split into "macro" and "micro" contributions. The macro quantities have a real physical meaning: they are some mean values of the forces and displacements over space and time. The second characteristic of this strategy is the use of the LATIN method [START_REF] Ladevèze | Nonlinear Computational Structural Mechanics -New Approaches and Non-Incremental Methods of Calculation[END_REF], an iterative resolution technique taking into consideration the whole time interval being studied. At each iteration, one must solve a homogenized macro problem defined over the whole space-time domain, and a set of independent micro problems, which are linear evolution problems defined within each substructure, at their boundaries, and over the time interval being studied.

RADIAL APPROXIMATION TECHNIQUE

The resolution of a large number of micro problems at each iteration can lead to prohibitive calculation costs if classical incremental techniques are used. Moreover, when dealing with composite structures containing a large number of similar substructures, the operators and right-hand sides of the micro problems may vary only slightly from one resolution to the next, but classical techniques are unable to take advantage of this characteristic. A possible answer to this problem is an alternative resolution technique based on the concept of generalized radial approximation [START_REF] Ladevèze | Nonlinear Computational Structural Mechanics -New Approaches and Non-Incremental Methods of Calculation[END_REF]. This technique, which relies on strong mechanical bases and is often used in the context of the LATIN method, consists in approximating a solution u(x, t) defined over the space-time domain by a sum of radial functions, each of which is the product of a scalar function of the time variable by a function of the space variable, i.e.

u(x, t) = i λ i (t)U i (x)
Both space and time functions are a priori unknown. First of all, this approach reduces the computation cost drastically by transforming an evolution problem defined over the space-time domain into a series of uncoupled problems defined either over the space domain or over the time domain. It also reduces the storage cost for the variables defined over the space-time domain. Last but not least, it enables the construction of a relevant reduced basis of space functions which turns out to be particularly suitable for composite structures containing families of many identical substructures. Of course, the radial approximation is not defined uniquely. The classical approach is to define this approximation using an orthogonality criterion associated with a primal or dual formulation of the evolution equation [START_REF] Ladevèze | Nonlinear Computational Structural Mechanics -New Approaches and Non-Incremental Methods of Calculation[END_REF]. This definition leads to the "best" approximation only for particular cases, and it provides no simple and systematic way to evaluate the quality of this approximation. Still, this classical approach is efficient in the single-scale framework. However, in some special situations, it may lead to non-convergence of the LATIN method. This lack of robustness is particularly notable for the multiscale computational strategy. Here, we propose a new and extremely robust approach which maintains a completely mixed vision of the evolution problem. In fact, the search directions of the proposed iterative strategy can be interpreted as linearized constitutive relations. A natural and mechanics-based approach then consists in formulating the micro problem as the minimization of the constitutive relation error associated with the search directions. A radial approximation is searched for all the variables on a substructure, static and kinematic. The definition of the best radial functions is simply that they minimize the constitutive relation error. With this new approach, the approximation relates only to the search directions, which are the parameters of the strategy. Compared to the classical method, this approach certainly increases the computational cost, but it also improves the effectiveness and the robustness of the strategy. Moreover, it now provides a simple and relevant error criterion to evaluate the quality of the radial approximation. The construction of the space functions is by far the most expensive step. Therefore, it can be interesting to store and reuse these functions. The space functions constructed at each iteration can be reused systematically during the subsequent iterations. Moreover, in the case of composite structures, the basis of the space functions is shared by a large number of identical substructures. Therefore, the initialization step of a micro problem on a particular substructure can reuse the space functions defined in other substructures during the previous iterations (or even the current iteration). Then, the resolution of a micro problem begins with an initialization step which consists in seeking a set of time functions associated with an already existing basis of space functions. The operators and the right-hand sides of the micro problems vary little from one iteration to the next, and even from one substructure to another. Therefore, this initialization step is often sufficient to obtain a good approximation of the solution of the micro problem. Figure 1 shows a 2D problem of a composite structure containing cracks (unilateral contact with friction). The materials are viscoelastic. Figure 2 shows that very few new sets of radial functions were built at each iteration. Moreover, this number decreases as the iterations progress.

¢¡ ¤£ ¦¥ ¤ § ©¨ ¥ ¢¡ ¢£ ¥ ¤ § ©¦ ¥ ¦ ! ©" $# % & ' ©( $) 0 ! $1 2 3 ! 0 " 54 ( 1 ! ©" 1 6 ¢) 1 7 8 ) " 1 ) 4 ( 9 @ A B C D 9 9 D @ E A F B G C H I P Q $R ©S T U V W X Y Y `W X a b V c d e f 9 @ A B C D 9 9 D @ E A F B G C H I P Q $R gS T U V W X Y Y `W X a b V c d e h

Figure 1. Description of the problem

i p q r s tp u v ¢w yx x ¦ q r ¦v w tq q tp v ¤r s ¤u s g ¦w t p u v ¤w t y tv p s ¢

x d e i p q r s p u v ¢w e i p q r s p u v ¤w gf i p q r s p u v ¢w gh x i p q ¢r s tp u v ¤w yx i p q r s p u v ¤w gi e

CONCLUSION

The presented approximation technique leads to significant savings in terms of computational and storage costs without affecting the efficiency and the robustness of the multiscale computational strategy. It could also lead to the construction of pertinent reduced models for heterogeneous structures and allow analysis such as parametric analysis, optimizations, which nowadays can not be carried out with experiments or computational techniques.

Figure 2 .

 2 Figure 2. Number of new sets of radial functions per iteration and per substructure