Diaspora as a multi-tiered resource for migration policy and its confrontation to migrant strategies: the case of Omsk

Anne Le Huérou

Anne Le Huérou, CERCEC, Paris, France Diaspora as a multi-tiered resource for migration policy and its confrontation to migrant strategies: the case of Omsk

As labour migrations became an issue of public policy and a quite disputed matter of public debate in the Russian Federation by the mid 2000s (Vitovskaya & al. 2009 ;Zajonchovskaya & Tjurjukanova, 2010, Le Huérou & Regamey, 2007, Schenk, 2010) the implementation of those policies and their relation to migrants experience is also a key to understand the position of "new" countries of emigration -Central Asia and to a lesser extent the Caucasus in their relation to the Russian State and society.

Once Republics with proclaimed equal rights inside the Soviet Union, they had been nevertheless part of the Soviet nationalities policy and thus submitted to a set of rights and duties, promotion and repression policies, embedded in complex processes of modernization and domination. We can choose to stress the colonial nature of the Soviet statehood with its proactive construction of (Hirsh, 2005) , to point the emancipatory features of the "affirmative action Empire" [START_REF] Terry | The Affirmative Action Empire: Nations and Nationalism in the Soviet Union, 1923-1939 Ithaca[END_REF] or to suggest no more but a "family resemblance" [START_REF] Mark | Soviet empire as 'family resemblance[END_REF] between the Soviet Empire and other imperial experiences, but we cannot anyway escape the fact that today's new independent States of Caucasus and Central Asia have inherited a specific place in their relationship to the new Russian State. Partly due to Tsarist Russia and Soviet legacy, this place has been and is being also renegotiated since 1991 according to multiple factors.

Based on field research in the Omsk oblast (Western Siberia, bordering Kazakhstan) in 2009 and 2010, my communication will focus on the multiple ways the notion of "diasporas" is used and perceived in the migratory policy management and implementation in the region. Having studied various elements of the local authorities' migration policy (UFMS, regional and municipal economic bodies...) and looked into employers and migrants' strategies and attitudes, I happened to be, in the course of my research, more and more puzzled by the fact that diasporas were appearing in many discourses as well as in concrete attempts to manage migrant workforce.

I decided to choose this angle to see how it helps in understanding the conducts of the different actors (authorities, employers, labour migrants, intermediary of different kinds...) in the migration field in the way they are embedded into a scheme that includes a lot of "old style" Soviet constructs and stereotypes, -often similar to the colonial stereotypes we can find in the attitudes towards migrants in European countries (at least France). Meanwhile, they also include new "post-soviet" features, built in the two past decade, that reflect the evolution of the Russian state and society (new xenophobia mixed with a liberal approach to the labour market needs), the type of relations settled between the Russian Federation and the migrants' countries of origins and the more globalized economy and migration field.

I will not discuss here the essence or the validity of the concept of diasporas per se but use the term because and in the way it is used on the ground by the authorities, the employers or by migrants' communities for their ambiguous search of a balance between flexibility, security and control. I-My first point will address the way both authorities and employers use, voluntarily or not this concept of diaspora or so called diaspora.

a) Local authorities : ensure flexibility and control

After 2006, Migration policy in Omsk has followed the line of the general federal trends and has elaborated mechanisms that try to simplify procedures for migrant workers thus allowing a more flexible scheme for the needs of the labour market that can be qualified as more "liberal" for the employers (in terms of extending quota and simplifying procedures), as well as more protective for the migrants (fewer dependence from employers, legalizations of large numbers of migrants in 2007 and 2008 before the crisis...).

Thus, as it resorted from the interviews I conducted as well as from regional media and official documents or reports, it is possible to say that Omsk regional authorities are quite proud of their policy. They describe it as an example of clever and moderate approach that could be followed. Besides the everyday bureaucratic job of issuing residence or work permits by the UFMS 1 , and besides the task for municipal and regional economic authorities to establish lists of firms asking for migrants quota, investigating their needs, according or refusing them etc., local branch of Federal Migrations Service, has established an advisory council (Obshchestvennyj sovet pri FMS), which they like to present as a corner stone of their policy.

-What is interesting to notice is that apart from law enforcement agencies and representatives from the employers' side (mostly people from the local entrepreneurs Union) they have convened representatives of non Russian religious authorities and "diasporas". The creation of advisory bodies under administrations is the result of the general reform of administration implemented in the Russian Federation in the middle of the 2000s, giving the civil society a possibility to have their say and even to exercise a kind of control (often called obshchestvennyj control) over the work of administration. In the case of the Omsk, the council comprises mainly representatives from "nationalities" or "local communities" who are part of the Russian Federation, such as Siberian Tatars, local Kazakhs, or even long settled Armenians who become Russian citizens years ago. Those representatives who are not concerned by migrations issues more than any Russian citizen , are asked to approve the UMFS policy or to formulate recommendations. The construction is done as if their "ethnic" status gives them a special mandate or a special capacity to do so.

-This is particularly salient -from the civil council activity reports -in the way regional FMS legitimate these "diaspora" on the public security agenda. This position given from outside is reinforced when administration or law enforcement agencies expect some disorders linked (or supposedly linked) to labour migrants (massive arrests in a market, street fights, aggression or murder reported by local media...).

-The protective mission is not forgotten, and UFMS also praise the work with "diaspora" as a very useful collective intermediary to warn and denounce abuses against labour migrants, would they come for local policemen, employers or even fake or dishonest intermediary firms/individuals.

-Local administrations try also in some way to be in contact with representatives of the labour migrants communities, mostly through those who have acquired Russian citizenship and often act as middlemen. They ask for their practical assistance in translating instructions posted in UFMS offices or in giving to their fellow nationals some basic information about policy changes. More unofficially, law enforcement agencies do have some informants from the migrants community especially through those who have become intermediaries.

-An other elements that I did not investigate further2 could be mentioned to add as a part of this approach: the amount of expertise, opinion polls and monitoring that are ordered by regional administration to different academic or independent research centres about the "interethnic" climate and/or tensions in the region, and that encompass labour migrants together with Russian citizens considered as "ethnic minorities" (Savin, 2010).

Should we conclude from this that "local minorities" are embedded as symbolic go-between between labour migrants and administration? At least partly yes, and trying to understand the reasons for this, we could suggest that the local administration's efforts for achieving efficiency and pragmatism appear to be constrained by traditional Soviet schemes giving a particular weight to nationalities constructs and to their representatives supposedly organised in diaspora 3 . There is also a probability for a kind of discrepancy between the political and proclaimed policy at the federal level and the way it is implemented on the ground, local authorities resorting to people and modalities they are used to deal with.

In this respect, it could be interesting to go further on the way Soviet nationalities policy and its scientific construction of nationalities [START_REF] Hirsch | Empire of Nations: Ethnographic knowledge and the Making of the Soviet Union Ithaca[END_REF]) could still have an influence in the administrative practices. Some French scholars suggest a strong relation in the building of the "inner enemy" in the French society for the French police as a result of the Algerian war [START_REF] Mathieu | L'ennemi intérieur , Des guerres coloniales au nouvel ordre sécuritaire[END_REF] and in the elaboration of the French migration policies in the 60s and the 70s by high ranking civil servants infused with representations and administrative practices inherited from the colonial times [START_REF] Sylvain | Une politisation feutrée. Les hauts fonctionnaires et l'immigration en France[END_REF].

b) Employers

Employers of labour migrants play a very important role in the dissemination of perceptions at the local level. Those we interviewed during field research were often retired military officers or policemen who have turned into small/middle range business. Interviews allow to draw a contrasted picture: on the one hand, their career background leads them to perform views that are not very far from administration representatives' in terms of acquired stereotypes about minorities and/or migrants. On the other hand, their present professional position get them to adopt a much more pragmatic attitude that can be summarized by the basic idea that for their practical needs, labour migration is not so much a problem but a solution.

Regarding observance of the migration laws, they often try to take advantage of some of them (bargaining for official quotas,...) but also often bypass law, resort to deals with middlemen (brigadiry) from migrants communities who provide them with migrant workforce. Being reliable, hardworkers and cheap, migrants do have a comparative advantage, on the condition that they remain quiet, the less visible as possible, in order to not attract attention of law enforcement agencies and not creating any public trouble.

Though they live quite close to them, -they are often small size firms-, and though migrants and their employers were not long ago part of the same Soviet society, the latter demonstrate very vague knowledge and idea of the migrants life or cultural habits, often confounding their country of origin and their language, speaking of them as a collective using the plural, or put forward quite unexpected comparisons: a former criminal police officer 4 , law abiding and sympathetic to the few Kyrgyzstan workers he employs regularly, complains about employers' abuses toward migrants. He immediately attempts to equalize this situation with the one the Russians who were subjected to "massive labor slavery" by people from the North Caucasus, some case he witnessed when he was on duty as a policeman... Xenophobic attitudes and acts that have become widespread in Russia especially in the last decade, are also becoming more and more marked by confusion between , the more no difference is seemed to be made between labour migrants and Russian citizens of non "ethnic Russian" or "Slavic" appearance, as demonstrated the events of the Manezh Square on Dec. 11 th 2010. Nevertheless, public discourse, media coverage, misused statistics (Malakhov, 2007[START_REF] Vladimir | Lukavye tsifry i obmanchivye teorii : o nekotoryh podhodah k izucheniu migrantov (chiffres biaisés et théories trompeuses : à propos de quelques approches de l'étude dans migrants)[END_REF], show that there is also a specific anti-migrants attitude that has grown further after 2008 crisis. Thus, the problem of xenophobia is both related to old recipes linked to Russian nationalist discourses, but also to contemporary ways to raise in the political arena migrations issues. This can be observed in Western states living in liberal and globalized economies. So is the fact that Russia has "joined" the club making it to be an ordinary Western state conducting post-colonial policy? There is no doubt about a certain rapprochement of Western and Russian policies.

The obvious ethicized and racialized stereotypes that leak from all the absolute majority of the sayings and attitudes from both employers and local authorities is a / shared inheritance from the Colonial component of the Soviet empire construction, with the 2 faces of the mirror -affirmative action and elite promotion as described [START_REF] Terry | The Affirmative Action Empire: Nations and Nationalism in the Soviet Union, 1923-1939 Ithaca[END_REF] and help by scientific knowledge [START_REF] Hirsch | Empire of Nations: Ethnographic knowledge and the Making of the Soviet Union Ithaca[END_REF] to construct a classification and hierarchy of nationalities.

In any case, it shows quite a strong State capacity to construct and implement norms of public action even when it is towards "liberalization". In this latter case, it will be by establishing close ties with emigration neighbouring States, that are to control that law enforcement and security apparatus is still closely tied to their Russian counterpart. In this scheme, a new form of management by diasporas can be seen as a rather clever tool to ensure social control over the migrants, and political security between the States.

That will be the subject of my second part.

II/ The revamping of traditional diasporas into useful intermediaries

We had the opportunity to observe at least two organisations that have seemed to us emblematic of this tension observed in the State and Employers' practices. Representatives of the local migrants communities -or self-proclaimed ones-can act as intermediaries at different levels and with different strategies between migrants and authorities as well as between migrants and employers.

a) Local Kirgyz diaspora :

Run by a resident of Omsk since he completed university in the 60s, an explicit Soviet Union nostalgic, the official Omsk Kyrgyz diaspora appears to be a part of the Soviet scheme of nationalities management but got adapted to the new conditions, by including younger people who work as recruitment intermediaries, interacting with migration service or law enforcement when facts of abuse are found concerning workers from Kyrgyzstan, and hoping to be enlisted by a local politician close to United Russia who promised to invite diasporas to participate in the new municipal legislature as a part of the civil society.

The complete achievement for Kyrgyzstan citizens working as middlemen and at the same time active members of the official Kyrgyz diaspora organisation will be the Russian citizenship they were waiting for, still they were already integrated and self-assured to the point that they became unnoticeable for the police, who "think they are Kazakhs".

b) The Centre for migrants' recruitment (Centr po trudoustrojstvu migrantov) AND society for Uzbekistan friendship (obshestvo sodruzhestva Uzbekistana): matching offer and demand?

The organisation, founded in 2007 and registered in 2009, happened to be at the crossroads of almost all issues and interactions 5 . Created and run by a ex-member of the prokuratura and former Afganets, coming in the late 1980s from Uzbekistan towards Uzbekistan and Uzbekistan citizens -of all "ethnic" origins including Russians. It reveals a new model of hybrid institution, mixing at the same time several activities: doing business controlling, protecting. The organisation seeks to be indispensable with employers and the authorities, in selecting and recruiting migrants directly from Uzbekistan, "cleaning the place" from non verified intermediaries and trying to avoid the arrival by train of "unwaited workers", establishing ties with official Uzbek migration agencies in several regions. The center was also hoping to extend its activities in other oblasti of the Siberian Federal District, using Afgantsy networks in the regional administrations and legislatures.

The "diaspora" part of the job -the two being located in the same house-is more oriented towards cultural activities, generally organised in an official way in official premises and with official blessing from both sides; but Community control is also an important dimension of the activity, since the organisation is regularly required by law enforcement authorities to control potential unrest inside the community or between them, such as in June 2010 when violent clashes in Osh took place. this kind of organization certainly matches the interests of countries of origin and receiving country -Russia.

In this example we can see working a quite specific post-soviet model of co-operation and co-production in the sphere of migration policy, suitable for nearly all the "players", since it meets the search both by the Russian state and the migrants' countries of origin of a control and regulation of the work force circulation through the creation of more institutionalized bodies. The closed relations between the States and the life-paths of many of the actors who did a Soviet career -real children of the "Affirmative action empire", especially in the security apparatus is undoubtedly a enabling element for this particular social and administrative construction.

It is unclear to what extent it is suitable for the migrants themselves, though the organisation claims to act also as a kind of trade union, negotiating decent -though under Russian workers rates-wages with the employers. It will be definitely more difficult to find this kind of models in Western countries, though state apparatus could be willing to build such.

In spite of their comfortable position, it remains doubtful that these organisations could be instrumental in the reality of the migrations flows and procedures, and observation and interviews with ordinary migrants show that there is a big gap between their self-legitimating strategies through economic interest and/or political ambitions ; their legitimization from above by State authorities : the very few regular contacts they happen to have with ordinary migrants who mostly leave quite isolated, who often come through very locally settled middlemen or by family ties.

c) Looking for Kazakhs diaspora: "oni zhe mestnye !"

Investigating migration issues in a bordering region with Kazakhstan can't let apart the question of Kazakh diaspora and Kazakh migration. Our findings in this matter go all in the same direction though they come more from indirect observations and in contrast with the other (real?) migrant communities : Kazakhs are so intimately part of the regional history, landscape and political life7 that their position can't be compared at all to their neighbouring citizens from other Central Asian states (Naumova & Larina, 2006, Savin, During the 1990s, the region indeed witnessed important immigration/repatriation flow from Kazakhstan, with Kazakhs (already a significant minority in the region), Russians and other nationalities -Germans… -joining or rejoining Russian territory. Despite this quite tense immediate Post-Soviet period, close ties remained at both political and economic level. In the 2000s, the situation stabilized and the flourishing economy of Kazakhstan even led to some reverse movements, back to Kazakhstan... Actually, would they be living in Omsk oblast for decades or having crossed the border a few months ago (and often having acquired Russian citizenship), Kazakhs are mostly considered to be "locals" (mestnye) or "theirs" (svoi) and not "labour migrants". Rules and procedures at the UFMS don't apply to them (or no more than to Russian citizens from an other region), and they are very often compared to Siberian Tatars as to their position and weight in the local society. (own Mosque, slightly different from the Tatar one...). As we have seen before, they are invited as a diaspora to have a look and a say over the regional migration policy ; they are not noticed nor arrested by the police and don't appear in local mass media for causing any kind of public disorder nor they are required in the migrant quota by employers.

Though quite closed, Kazakh community in Omsk appears to be integrated in the sense of administrative status and perception by the local population. They are doing quite well in business, and are indeed perceived in the region as a full component of the regional multinational population, that comprises also Germans and and Siberian Tatars, less likely Chechens and Armenians, even less likely Tajiks or Uzbeks. This observation can be an element in arguing that the Soviet style hierarchy of nations and peoples is being reshaped by the contemporary administrative and economic context

Conclusion:

The use of diaspora in this regional context shows a mix of perceptions that can resort to Soviet-style national policies and national minorities' representations that tends to further distance labour migrants from the former neighbours or svoi they could have expected to be, going to Russia for work.

Not only old recipes but also new attempts are done to shape a new pattern of relationship between Russia and its former southern Republics: thus, colonial type labour force management is combined with economic pragmatism serving market oriented objectives while the remaining strong willingness for social and political control over the workforce makes difficult to speak of a "liberal" model. In fact, the (post)-colonial scheme is at the same time softened and strengthened by the strong legacy of the Soviet constructs.

Could "post" be analytically removed to leave alone:

-Soviet style strongly anchored type of labour management, the migrants representing the ideal achievement of workforce management and of more global social control that has never not really been achieved in the Soviet economy ? The biographic profile of many of the employers interviewed (former military men or policemen having doing part of their career in the Soviet times) has certainly an influence on this conclusion.

-colonial style perceptions, attitudes and practices that are little constrained by State discourse and public opinion general mood. This (post)-colonial attitudes that could be compared -though with a certain "time lag", to some Western European countries experience.

Put together, those two characteristics could lead more surely than the strict importation of the post colonial theory

Not so routine job if taking into account the big scandal that led to the dismissal and arrest of the Head of UFMS in

on corruption charges implying the Chinese diaspora

Interviews and documents consulted, July 2010.

See also the more recent creation of the National Cultural Autonomies[START_REF] Yu | Nacional'no-kulturnye avtonomii :statistiki i kommentarii[END_REF].

, but born in Tajikistan and "ethnic" Kyrgyz, the organisation is oriented mostly 4 Interview, July 2010. 5And also a real blessing for the field researcher ! 6 The Head of the center insists on the fact that he is also a deputy president of the Kyrgyz diaspora and was at the time of the interview (July 2010) creating the Society of Tajikistan Friendship…

Kazakhs and Russians in Omsk region have a long history of relationship since the very beginning of the first Cossack settlement in 1716 the Governor himself, L. Polejaev, has worked in Karaganda for a long time and only came back to Omsk in 1987 to become first Party secretary in the region. Several documents exist already for many years, facilitating trade, border cattle husbandry, border crossing, customs exemptions etc.

, to look more in depth into the ways history of State formation and contemporary contexts join to ensure complex forms of political and social domination[START_REF] Béatrice | Anatomie politique de la domination[END_REF].


Ph.D Zhen-Shun Hong
email: zshong@seu.edu.cn

ABSTRACT

The intercept of the log(1+e) -logσ v ´ straight line is introduced to describe the effect of the starting point on the compressibility of natural and reconstituted clays. It is found that when the effective stress exceeds the remoulded yield stress, the compression behaviour of reconstituted clays is controlled solely by the water content at the remoulded yield stress and the liquid limit. Comparison of the compression behaviour of natural and reconstituted clays indicates that their difference in compressibility is caused by soil structure and the difference in water content at the compression starting point. The compression behaviour of natural clays can be classified into three regimes: 1) the pre-yield regime characterised by small compressibility with soil structure restraining the deformation up to the consolidation yield stress; 2) the transitional regime characterised by a gradual loss of soil structure when the effective stress is between the consolidation yield stress and the transitional stress; and 3) the post-transitional regime characterised by the same change law in compression behaviour as reconstituted clays when the effective stress is higher than the transitional stress. For the investigated clays, the transitional stress is 1.0-3.5 times the consolidation yield stress. The compression index varies solely with the void ratio at an effective stress of 1.0 kPa for both natural clays in post-transitional regime and reconstituted clays when the effective stress exceeds the remoulded yield stress, and when compressed in such cases the compression curves of both natural clays and reconstituted clays can be normalised well to a unique line using the void index. KEYWORDS: clays; compressibility; soil structure; laboratory tests NOTATION σ t ' = transitional stress σ yr ' = remoulded yield stress σ v ' = effective vertical stress σ vy ´ = consolidation yield stress C c = (e 100 -e 1000 ) C c * = (e * 100 -e * 1000 ) C CL = compression index of natural clays in log(1+e) -logσ v ' plot C CLR = compression index of reconstituted clays in log(1+e) -logσ v ' plot e = void ratio e 1 = void ratio at σ v ' = 1 kPa e 100 = void ratio of natural clays at σ v ' = 100 kPa e 1000 = void ratio of natural clays at σ v ' = 1000 kPa e * 100 = void ratio of reconstituted clays at σ v ' = 100 kPa e * 1000 = void ratio of reconstituted clays at σ v ' = 1000 kPa e L = void ratio at liquid limit e yr = void ratio at remoulded yield stress e vy = void ratio at consolidation yield stress EICL = extended intrinsic compression line ICL = intrinsic compression line I v = void index, equal to (e -e * 100 ) / (e * 100 -e * 1000 ) I vn = void index for natural clays, equal to (e -e 100 ) / (e 100 -e 1000 ) pre-yield regime = effective vertical stress lower than consolidation yield stress post-transitional regime = effective vertical stress higher than transitional stress post-yield regime = effective vertical stress higher than consolidation yield stress r = coefficient of correlation transitional regime = effective stress higher than consolidation yield stress but lower than transitional stress w L = liquid limit w n = natural water content w yr = water content at remoulded yield stress

INTRODUCTION

Natural clays generally have a compression curve (in terms of void ratio e versus effective vertical stress σ v ' in the semi-logarithmic plane) lying above that of reconstituted clays owing to the effect of soil structure (e.g. [START_REF] Leroueil | Behavior of destructured natural clays[END_REF][START_REF] Leroueil | Discussion on "Correlations between index tests and the properties of remolded clays[END_REF][START_REF] Locat | The origin of structuration of the Grande-Baleine marine sediments, Québec, Canada[END_REF][START_REF] Leroueil | The general and congruent effects of structure in natural soils and weak rocks[END_REF][START_REF] Schmertmann | The mechanical aging of soils[END_REF]Cotecchia & Chandler, 1997;[START_REF] Hong | On compression characteristics of Ariake clays[END_REF]. It is common practice to refer to reconstituted clays when assessing the effect of soil structure on the mechanical behaviour of natural sedimentary clays (e.g. [START_REF] Skempton | The sensitivity of clays[END_REF][START_REF] Houston | Property interrelationships in sensitive clays[END_REF][START_REF] Nagaraj | A critical reappraisal of compression index equation[END_REF]Hight et al., 1987;[START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF][START_REF] Liu | Virgin compression of structured soils[END_REF][START_REF] Hong | On compression characteristics of Ariake clays[END_REF][START_REF] Chandler | Clay sediments in depositional basin: the Geotechnical Cycle[END_REF]Cotecchia & Chandler, 2000;[START_REF] Liu | Modelling the destructuring of soils during virgin compression[END_REF]. It has been recognised that the soil structure restrains the deformation of natural clays under effective vertical stress up to the consolidation yield stress, consequently resulting in the low compressibility of clays until the stress level exceeds the consolidation yield stress (e.g. [START_REF] Butterfield | A natural compression law for soils[END_REF][START_REF] Holtz | Lessons from oedometer tests on high quality samples[END_REF][START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF]. The difference of void ratio between natural clays and reconstituted clays at the same stress level often increases with increasing consolidation stress up to the consolidation yield stress, but decreases when the applied stress level is larger than the consolidation yield stress (e.g. [START_REF] Liu | Virgin compression of structured soils[END_REF][START_REF] Hong | On compression characteristics of Ariake clays[END_REF][START_REF] Liu | Modelling the destructuring of soils during virgin compression[END_REF]. [START_REF] Leroueil | Discussion on "Correlations between index tests and the properties of remolded clays[END_REF] classified the compressive behaviour of natural clays into two states: the intact state as it occurs in natural deposits; and the destructured state referring to the breakdown of the original clay structure when submitted to volumetric or shear deformations. The former is related to elastic deformation whereas the latter is related to elasto-plastic deformation.

The main parameters affecting the compressibility of natural clays in the destructured state are not well understood. It should be noted that the void ratio or the water content at the consolidation yield stress for natural clays is different from that of reconstituted clays. Hence, the difference in compressive behaviour between destructured natural clays and the reconstituted clays cannot be directly attributed to soil structure without considering the effect of the starting state on compressibility. As a result, the following questions arise: 1) what is the fundamental mechanism responsible for the compressive behaviour of natural clays at the destructured state defined by [START_REF] Leroueil | Discussion on "Correlations between index tests and the properties of remolded clays[END_REF], i.e., when the stress level is greater than the consolidation yield stress? 2) how does the soil structure of natural clays degrade with increasing stress when the stress level reaches and exceeds the consolidation yield stress? 3) what is the fundamental mechanism which causes the difference in compressibility between natural clays and reconstituted clays?

With these questions in mind, changes in compressibility are first discussed in this paper based on oedometer test data for reconstituted clays having different initial water contents and different liquid limits, in an attempt to clarify the fundamental mechanism responsible for the compressive behaviour of reconstituted clays. Then, the compression behaviour of natural clays is investigated, and compared with that of reconstituted clays in order to understand the role of soil structure during consolidation when the effective stress is greater than the consolidation yield stress.

Finally, the normalised compression behaviour of natural clays is analysed and compared with that of reconstituted clays in order to better understand the mechanism responsible for the difference in compression behaviour between natural clays and reconstituted clays. CONSOLIDATION Hong et al. (2010) performed 42 oedometer tests to investigate the compression behaviour of three reconstituted clays (Lianyungang clay; Baimahu clay; and Kemen clay) with a wide range of initial water contents, as shown in Table 1. The test results illustrate an inverse "S" shape of the e -logσ v ' compression curves, as typically shown in Figure 1, similar to those of many natural clays. This similarity in shape of the e -logσ v ' compression curves between natural clays and reconstituted clays indicates that there is a particular stress for reconstituted clays which is similar to the consolidation yield stress for natural clays. As for the compression behaviour of natural clays, the compression curves of reconstituted clays can be divided into two lines which intersect at a particular stress. Small compressibility occurs when the effective stress is lower than this particular stress, which acts in a similar role to the consolidation yield stress of natural clays. [START_REF] Mitchell | Fundamentals of soil behavior (2 nd edition)[END_REF] called this particular stress the 'pore water suction', and indicated that the liquid limit for all fine-grained soils corresponds to about 6.0 kPa of pore water suction and about 1.7-2.0 kPa of shearing resistance. [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF] termed the particular stress as suction pressure. Herein, it is termed as the remoulded yield stress σ yr ´ to avoid confusion with descriptions of the behaviour of unsaturated soils.

FUNDAMENTAL MECHANISM FOR CHANGE IN COMPRESSBILITY OF RECONSTITUTED CLAYS DURING

As typically shown in Figure 1, the compressibility of a reconstituted clay increases dramatically when the effective stress is higher than the remoulded yield stress. For a given reconstituted clay, the compressibility of a reconstituted clay is affected by the starting point (σ yr ´, e yr ) of the compression line whenever the effective vertical stress is higher than the remoulded yield stress. The e yr is the void ratio at the remoulded yield stress σ yr ´. Hence, the compression index C c * depends on the values of the remoulded yield stress and the void ratio at the remoulded yield stress. Note that the compression index of reconstituted clays is defined as [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF]:

C c * = (e * 100 -e * 1000 )
(1) where e * 100 and e * 1000 are the void ratios of the reconstituted clays at the effective vertical stresses σ v ´ of 100 kPa and 1000 kPa, respectively. The starting point (σ yr ´, e yr ) for reconstituted clays is similar to the starting point (σ vy ´, e vy ) of the post-yield line for natural clays when the effective stress is greater than the consolidation yield stress. Note that the terms of σ vy ´, e vy represent the consolidation yield stress and the void ratio at the consolidation yield stress, respectively.

To investigate the effect of the starting point (σ yr ´, e yr ) on the compressibility of reconstituted clays at effective vertical stress levels higher than the remoulded yield stress, the oedometer test data for Lianyungang clay, Baimahu clay, and Kemen clay reported by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF] Interestingly, it coincides with the order of decreasing liquid limits (see Table 1).

These results indicate that the compression index C c * is greatly affected by the parameters e yr , σ yr ´ and the liquid limit w L . Hence, an index is needed to consider the combined effects of e yr , σ yr ´, and w L on the compression behaviour of reconstituted clays with different starting points (σ yr ´, e yr ) and different liquid limits.

It has been well documented that the e -logσ v ' compression curves with an inverse "S" shape for many natural clays can be represented by two straight lines in a plot of log(1+e) or ln(1+e) against logσ v ´ or lnσ v ´ (e.g. Hashiguchi & Ueno, 1977;[START_REF] Butterfield | A natural compression law for soils[END_REF][START_REF] Onitsuka | Interpretation of oedometer test data for natural clays[END_REF][START_REF] Sridharan | Discussion on "interpretation of oedometer test data for natural clays[END_REF][START_REF] Hong | A method of correcting yield stress and compression index of Ariake clays for sample disturbance[END_REF][START_REF] Hong | Void ratio-suction behavior of remolded Ariake clays[END_REF]. [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF] illustrated that the bilogarithmic method can be also used to represent well the inversely S-shaped e -logσ v ' compression curves of reconstituted clays by two straight lines. The stress at the intersection point of the two straight lines is the consolidation yield stress and the remoulded yield stress for natural clays and reconstituted clays, respectively. Hence, the following equations can be obtained for expressing the compression curves of natural clays in the post-yield regime and those of reconstituted clays for the effective vertical stress higher than the remoulded yield stress:

log(1+e) = log(1+e 1 ) -C CLR ×logσ v ´ (for reconstituted clays) (2) log(1+e) = log(1+e 1 ) -C CL ×logσ v ´ (for natural clays) (3)
The parameter e 1 is the void ratio at an effective vertical stress of 1 kPa, and C CL and C CLR represent the compression index of natural clays and reconstituted clays in the log(1+e) -logσ v ´ plot, respectively.

It is indicated in Equations ( 2) and (3) that the intercept of the straight post-yield line of a natural clay or the straight line of a reconstituted clay for the effective vertical stress higher than the remoulded yield stress corresponds to log(1+e 1 ), and is simply represented by the parameter e 1 . Figure 4 schematically shows the concept of e 1 for a given reconstituted clay with different initial water contents. As reported by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF], the starting point (σ yr ´, e yr ) varies with the initial water content. The higher the initial water content, the greater is the void ratio at the remoulded yield stress, the lower is the remoulded yield stress. It can be observed in Figure 4 that a unique value of e 1 can be expected for a given position of (σ yr ´, e yr ). That is, for a given clay with a certain liquid limit, the difference in initial water content will result in different positions of (σ yr ´, e yr ), and in turn different values of e 1 . In the same fashion, reconstituted clays with different liquid limits but at the same initial water content still have different starting points of the compression lines with the effective vertical stress higher than the remoulded yield stress, consequently resulting in different values of e 1 . It should be mentioned that the parameters e yr , σ yr ´, and liquid limit are not disassociated each other; they are related with the following equation as described by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF]:

e yr /e L = w yr /w L = 2/(σ yr ´)0.42 (4)

where the remoulded yield stress σ yr ´ is expressed in kPa; the parameter e L represents the void ratio at liquid limit; the parameter w yr is the water content at the remoulded yield stress. Equation ( 4) indicates that given a certain value of one parameter among e yr , σ yr ´ and e L , the other two parameters vary unambiguously. That is, the reconstituted clays having different liquid limits or different initial water contents certainly have different positions (σ yr ´, e yr ), and thus different values of e 1 . Hence, the void ratio at 1 kPa (e 1 ) can reflect the combined effects of e yr , σ yr ´, and e L on the compression behaviour of reconstituted clays. In this study, the index e 1 is used for comparison of the compressibility of various reconstituted clays at different initial water contents.

Figure 5 shows the relationship between C c * and e 1 for the three reconstituted clays based on the oedometer test data reported by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF]. It is interesting to note that the C c * increases solely with increase in e 1 for the three clays at different initial water contents and with different liquid limits. This confirms that the index e 1 can reflect the effect of the starting state (σ yr ´, e yr ) on the compressibility of various reconstituted clays at different initial water contents when the effective vertical stress is higher than the remoulded yield stress. For further investigating the effects of the starting points of the compression lines at the effective vertical stress higher than the remoulded yield stress, the test data for other reconstituted clays compiled from the available literature shown in Table 1 were used to establish the relationship between C c * and e 1 . The liquid limits of the reconstituted clays cover a wide range from 37% to 220%. All the data were obtained from multi-stage loading oedometer tests. As discussed by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF], the log(1+e) -logσ v ´ plot is appropriate for interpreting the oedometer test data for Lianyungang clay, Baimahu clay and Kemen clay by two straight lines. These three clays were tested using a modified oedometer apparatus with the first applied vertical stress as low as 0.5 kPa. The other reconstituted clays listed in Table 1 from the available literature were tested in conventional oedometers with a first vertical stress generally higher than the remoulded yield stress. The oedometer test data for these reconstituted clays can be interpreted by a straight line in the log(1+e) -logσ v ´ plot, as typically shown in Figure 6. The correlation coefficients for the 90 reconstituted clays, r, reach as high as 0.97-1.00, most being larger than 0.99. Figure 7 shows the relationship between C c * and e 1 for all the 90 reconstituted clays investigated, including data of the three clays reported by [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF]. It is interesting to note that a unique relationship is obtained with the data reported by independent researchers. A regression analysis gives the following equation with a correlation coefficient r of 0.99. C c * = 0.183e 1 -0.0021(e 1 ) 2 (5) The unique relationship of C c * versus e 1 indicates that when the effective vertical stress is higher than the remoulded yield stress, the compression index C c * of a reconstituted clay depends solely on the starting state, and is a function of the void ratio at the remoulded yield stress and the liquid limit.

On the other hand, [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF] reported that the e-logσ v ´ compression curves of various reconstituted clays with different initial water contents can be normalised well with respect to the void index I v when the effective vertical stress is higher than the remoulded yield stress; whereas it is not the case at the effective vertical stress being smaller than the remoulded yield stress, in which the compression curves lie on the left of the normalised line, and the I v -logσ v ´ curve with a higher initial water content lies above that with a lower initial water content, as typically shown in Figure 8.

The void index I v is defined by [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] 2010), which can be expressed as:

I v = 3.0 -1.87 logσ v ´ + 0.179 (logσ v
´)2 (7) where σ v ´ is expressed in kPa. The EICL extends the intrinsic compression line (ICL) of [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] to wider ranges for both of the initial water content and the effective vertical stress: 0.7 -2.0 times the liquid limit and as low as 1.5 kPa, respectively. It was demonstrated that when the effective vertical stress is higher than 25 kPa, the EICL coincides with the ICL suggested by [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] which is expressed by Equation ( 8).

I v = 2.45 -1.285 logσ v ´ + 0.015 (logσ v ´)3 (8) where σ v ´ is expressed in kPa. Within the range of effective stress from 1.5 kPa to about 10 kPa, the EICL lies a little above the ICL [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF]. Note that [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] suggested that ICL should be used only for effective vertical stress ranging from 10 kPa to 4000 kPa. All the 90 reconstituted clays have a normalised behaviour of compression curves with respect to the void index only if the effective vertical stress is higher than the remoulded yield stress, the same as Figure 8 of this study and Figure 13 in [START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF].

COMPARISON OF COMPRESSBILITY BETWEEN NATURAL CLAYS AND RECONSTITUTED CLAYS

It has been well known that the compression behaviour of natural clays differs from that of remoulded clays, as schematically shown in Figure 9. Although the undisturbed and the remoulded specimens have the same initial water content (i.e. natural water content w n ), the consolidation yield stress of the undisturbed specimen is higher than the remoulded yield stress due to the soil structure formed during the depositional and the post-depositional processes. Hence, the compression starting point of the post-yield line for the undisturbed specimen differs from that of the remoulded specimen at the remoulded yield stress. That is, the water content or the void ratio of a natural clay at the consolidation yield stress is greater than that of the reconstituted clay at the same stress. As illustrated in the previous section, the compressibility of reconstituted clay is greatly affected by the water content at the starting point. The difference in water content at the starting point between natural clays and remoulded clays should be an important factor responsible for the difference in compressibility. In the same fashion as in the analysis for the compression behaviour of reconstituted clays, the index e 1 is also adopted to analyse the difference in compressibility between natural clays and reconstituted clays. For this purpose, the relationships between the compression index and the parameter e 1 for various natural clays should be established, and then compared with Equation (5) established based on reconstituted clays to clarify the role of water content on the difference in compressibility between reconstituted clays and natural clays. The data for the natural clays investigated here includes Ariake clays and other clays compiled from the available literature. For comparison purposes, the compression index C c of natural clays is defined as:

C c = (e 100 -e 1000 ) ( 9) where e 100 and e 1000 are the void ratios of natural clays at effective vertical stresses σ v ´ of 100 kPa and 1000 kPa, respectively.

The oedometer test data for natural Ariake clays were disposed here to investigate the compressibility of natural clays, as shown in Table 2. These data were used by [START_REF] Hong | Evaluation of sample quality of sensitive clay using intrinsic compression concept[END_REF] for quantitatively evaluating the degrees of sample disturbance. The clays have a wide spectrum of liquid limits, ranging from 34% to 140%. These data can be also interpreted well by two straight lines in the log(1+e) -logσ v ´ plot, as typically shown in Figure 10. The relationship between C c and e 1 for all the investigated 119 Ariake specimens listed in Table 2 is shown in Figure 11. The relationship from Equation ( 5) for reconstituted clays is also plotted in the same figure for comparison. It is interesting to note that the parameter C c has a unique relationship with the parameter e 1 for all the natural Ariake clays investigated; more importantly, the relationship between C c and e 1 for natural Ariake clays is consistent with the relationship of C c * versus e 1 for reconstituted clays. This indicates that the compression index C c for natural Ariake clays should be the same as the compression index C c * for reconstituted clays, if they have the same value of e 1 . In other words, the difference in compressibility between undisturbed and remoulded specimens of a given natural Ariake clay is caused by the difference in the starting point as schematically shown in Figure 9. Consequently, the compressibility of the natural Ariake clays investigated depends solely on the void ratio or the water content at the consolidation yield stress and the liquid limit, when the applied stress reaches and exceeds the consolidation yield stress. Such a result suggests that once the effective vertical stress overcomes the consolidation yield stress, which is the result of complex geological processes involving deposition environment, consolidation, erosion, thixotropic hardening, leaching, weathering etc. as described by [START_REF] Leroueil | Discussion on "Correlations between index tests and the properties of remolded clays[END_REF], the natural Ariake clays investigated have the same change law in compression behavior as the corresponding reconstituted clays.

In order to further compare the compressibility of other natural clays with reconstituted clays, the oedometer test data of other natural clays were compiled from the available literature, as shown in Table 3. Similar to the reconstituted clays listed in Table 1 and the natural Ariake clays in Table 2, all the data were obtained from multi-stage loading oedometer tests. The liquid limits range from 34% to 122%, and the natural water contents vary from 41% to 120%. As aforementioned, the index e 1 can be used to assess the compressibility of natural clays when they have a linear log(1+e) -logσ v ´ line in the post-yield regime. However, there are two types of compression curves for the natural clays listed in Table 3, as typically shown in Figure 12. The oedometer test data for the natural clays of Type one can be interpreted by two straight lines in the log(1+e) -logσ v ´ plot, like the natural Ariake clays listed in Table 2. But the compression curves for the natural clays of Type two have a transitional drop zone just after the consolidation yield stress is exceeded. That is, there are three zones in the log(1+e) -logσ v ´ plot. The range of stress for the middle zone varies from the consolidation yield stress to the transitional stress σ t '. The transitional stress is defined as the stress responsible for the starting point of the third straight line defining for the behaviour of Type two clays. Its value is about 1.4-3.5 times the consolidation yield stress for the investigated clays, as shown in Figure 13. The behaviour of Type one clays may be considered as a special case of the behaviour of Type two clays, in which the range of stress from the consolidation yield stress to the transitional stress is extremely narrow. The above phenomenon implies that the shape of compression curves and the degradation of the soil structure associated with the irreversible deformation for various natural clays may depend on the specific mechanism responsible for the presence of soil structure. The existence of the transitional regime implies the gradual change in microstructure during consolidation associated with the relatively large deformation in the transitional regime. The absence of the transitional regime indicates a dramatic change in microstructure at the vicinity of the consolidation yield stress associated with little change in deformation, as illustrated by [START_REF] Hong | Experimental study of macro and micro-behavior of natural diatomite[END_REF]. Several researchers have investigated the change in microstructure of soils during consolidation (e.g. Collins & McGown, 1974;Delage & Lefebvre, 1984;Griffiths & Joshi, 1989;[START_REF] Tanaka | A microstructural investigation of Osaka Bay clay: the impact of microfossils on its mechanical behaviour[END_REF][START_REF] Hong | Experimental study of macro and micro-behavior of natural diatomite[END_REF]. However, more studies on changes in microscopic level during consolidation are needed in the future to better understand the mechanisms responsible for the existence of the transitional regime in some natural clays and for absence in others.

In the analysis of the natural clays of Type two, the value of e 1 is obtained by extending the third straight line after the transitional stress up to the effective vertical stress of 1 kPa. Figure 14 gives the relationship between C c and e 1 for the natural clays listed in Table 3, including both Type one and Type two clays. It can be seen that the relationship for natural clays of Type one in the post-yield regime is consistent with that of Type two clays when the effective vertical stress is higher than the transitional stress. It is encouraging to note that the relationship for natural clays listed in Table 3 is almost identical to that by Equation ( 5) established based on reconstituted clays, even though the test data were compiled from literature reported by independent researchers.

The above analysis illustrates that the compression behaviour of natural clays can be classified into three regimes: 1) the pre-yield regime with small compressibility when the effective stress is lower than the consolidation yield stress; 2) the transitional regime with gradual loss of soil structure when the effective stress is higher than the consolidation yield stress but lower than the transitional stress; and 3) the post-transitional regime when the effective stress is higher than the transitional stress, in which natural clays have the same change law in compression behaviour as reconstituted clays. In some cases, the transitional zone can be ignored when the transitional stress is close to the consolidation yield stress.

Note that the three regimes mentioned above for natural soils are important for both theoretical development and practice. When modelling the compression behaviour of natural clays, it is clear that the three regimes should be accounted for by adopting different equations; in practice, much attention should be paid on the transitional regime because this regime corresponds to an unstable soil state and the compression behaviour is characterised by a 'collapse' nature.

NORMALISED BEHAVIOUR OF NATURAL CLAYS

For normalising the compression behaviour of natural clays, the void index I vn is introduced with e 100 and C c instead of e 100 * and C c * in Equation ( 6), expressed as follows.

I vn = (e -e 100 ) / (e 100 -e 1000 ) = (e -e 100 ) / C c (10) It should be pointed out that unlike the remoulded yield stress of reconstituted clays, the consolidation yield stress of Type one clay or the transitional stress of Type two clay may be greater than 100 kPa. In such a case, e 100 is situated in the pre-yield regime or in the transitional regime. Consequently, it can no longer be used for calculating the void index to normalise the compression curves. To tackle this problem, the values of e 100 are determined by extrapolating the linear post-transitional bilogarithmic line.

The compression curves of natural Ariake clays shown in Figure 10 were normalised with the void index I vn defined in Equation ( 10), as shown in Figure 15. It can be seen that the relationship between the void index I vn and the effective vertical stress for the six natural Ariake clays become a unique line when the effective vertical stress is higher than the consolidation yield stress; whereas the oedometer test data in the pre-yield regime cannot be normalised. Such normalised behaviour of natural Ariake clays is similar to that of various reconstituted clays at different initial water contents. Figure 16 shows the normalised values of I vn in the post-yield regime for all the natural Ariake clays investigated. The intrinsic compression line (ICL) and the extended intrinsic compression line (EICL) are also shown in the same figure for comparisons. It can be seen that the oedometer test data of 119 natural Ariake clays can be normalised with the void index defined in Equation ( 10), when the data is in the post-yield regime. It is interesting to note that when the effective vertical stress is higher than 40 kPa, the normalised behaviour of natural Ariake clays can be expressed well by the intrinsic compression line (ICL) which was proposed by [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] based on the test data of reconstituted clays. However, the experimental data of natural Ariake clays lie a little above the ICL, and closer to the EICL when the effective vertical stress is lower than 20 kPa.

The oedometer test data of Figure 12 were re-plotted in the form of the void index I vn versus logσ v ´, as shown in Figure 17. It can be seen that the clays of Type one can be normalised well with the void index I vn when the effective vertical stress is higher than the consolidation yield stress. This normalised behaviour is the same as that of the natural Ariake clays (see Figure 15). However, for Type two clays, only if the effective vertical stress is higher than the transitional stress, can the compression curves be normalised with the void index I vn . Within the range of stress level from the consolidation yield stress to the transitional stress, the values of I vn for Type two clays lie above the normalised line, and gradually converge toward the ICL when the effective stress approaches the transitional stress. The compression curves normalised with respect to I vn for Type one clays listed in Table 3 in the post-yield regime are also compared with the ICL (see Figure 18), showing that they are identical. Figure 19 gives the normalised oedometer test data for Type two clays listed in Table 3 with the effective vertical stress higher than the transitional stress. It can be seen that the normalised data are also consistent with the intrinsic compression line (ICL). The above results indicate that the compression curves of natural clays can be normalised well with regard to the void index defined by Equation ( 10) when the effective vertical stress is higher than the transitional stress; whereas the compression curves of natural clays in the pre-yield regime and in the transitional regime cannot be normalised with the void index.

CONCLUSIONS

When the effective stress is greater than the remoulded yield stress for reconstituted clays or larger than the transitional stress for natural clays, the extrapolated void ratio at an effective stress of 1.0 kPa (e 1 ) can be used as a powerful index for reflecting the combined effects of both the initial water content at the starting point and the liquid limit on compressibility, and the compression index increases uniquely with increase in e 1 for all the investigated reconstituted and natural clays having a wide range of liquid limits. The unique relationship can be expressed as C c * = 0.183e 1 -0.0021(e 1 ) 2 .

The compressibility of a reconstituted clay depends on three parameters: the remoulded yield stress, the void ratio at the remoulded yield stress and the liquid limit. When the effective stress is lower than the remoulded yield stress, the compressibility is mainly controlled by the remoulded yield stress which restrains the deformation in the same fashion as the consolidation yield stress of natural clays. When the effective stress is larger than the remoulded yield stress, the compressibility of a reconstituted clay depends solely on the void ratio at the remoulded yield stress and the liquid limit.

The compression behaviour of a natural clay can be classified into three regimes: the pre-yield regime characterised by small compressibility up to the consolidation yield stress with soil structure restraining the deformation; the transitional regime with gradual loss of soil structure when the effective stress is between the consolidation yield stress and the transitional stress; and the post-transitional regime characterised by the same change law in compression behaviour as a reconstituted clay when the effective stress is higher than the transitional stress. In some cases, the transitional zone can be ignored if the transitional stress is close to the consolidation yield stress.

The compression curves of both natural clays in the transitional regime and reconstituted clays when the effective stress is larger than the remoulded yield stress can be normalised well with the void index, consistent with the normalised compression line (ICL) for reconstituted clays suggested by [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF].

List of Tables and Figures

Table 1. Sources of reconstituted clays Table 2. Physical indices of natural Ariake clays Table 3. Sources of natural clays from literature Liquid limit=202%, by [START_REF] Cerato | Determining intrinsic compressibility of fine-grained soils[END_REF] Liquid limit=158%, by [START_REF] Nagaraj | Rationalization of Skempton's compressibility equation[END_REF] Liquid limit=101%, by [START_REF] Cerato | Determining intrinsic compressibility of fine-grained soils[END_REF] Liquid limit= 85%, by [START_REF] Hight | Characterization of the Bothkennar clay: an overview[END_REF] Liquid limit= 61%, by Hong et al. (2009) Liquid limit= 37%, by [START_REF] Sridharan | Compressibility behavior of remolded, fine-grained soils and correlation with index properties[END_REF] Fig

  The normalised compression curve for reconstituted clays at different initial water contents and with different liquid limits is designated as the extended intrinsic compression line (EICL) byHong et al. (

Fig. 1 .

 1 Fig. 1. Typical e -logσ v ' compression curves of reconstituted clays at different initial water contents based on Hong et al. (2010) Fig. 2. Relationship between C c * and e yr for three reconstituted clays at different initial water contents Fig. 3. Relationship between C c * and σ yr ´ for three reconstituted clays at different initial water contents Fig. 4. Effect of starting points on compressibility for a given clay at different initial water contents Fig. 5. Relationship between C c * and e 1 for three reconstituted clays at different initial water contents Fig. 6. Typical linear log(1+e)-logσ v ´ lines of reconstituted clays at σ v ´ > σ yr ´ Fig. 7. Relationship of C c * versus e 1 for all 90 reconstituted clays Fig. 8. Typical I v -logσ v ' curves of reconstituted clays at different initial water contents Fig. 9. Comparison of the starting points between undisturbed and remoulded specimens of natural clays Fig. 10. Typical compression curves of natural Ariake clays in log(1+e)-logσ v ´ plot Fig. 11. Relationship between C c and e 1 for 119 natural Ariake clays Fig. 12. Typical compression curves of two types of natural clays listed inTable 3 Fig. 13. Relationship between transitional stress and consolidation yield stress for natural clays of Type two listed in Table 3 Fig. 14. Relationship between C c and e 1 for the two types of natural clays listed in Table 3 Fig. 15. Typical normalized behaviour of natural Ariake clays with void index I vn Fig. 16. Normalised line of 119 natural Ariake clays in post-yield regime Fig. 17. Typical normalised behaviour with void index I vn for two types of natural clays listed in Table 3 Fig. 18. Normalised behaviour with void index I vn for natural clays of Type one in post-yield regime Fig. 19. Normalised behaviour with void index I vn for natural clays of Type two with effective vertical stresses larger than transitional stresses

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .

 1234 Fig.1. Typical e -logσ v ' compression curves of reconstituted clays at different initial water contents based on[START_REF] Hong | Compression behaviour of reconstituted soils at high initial water contents[END_REF]

Fig. 8 .Fig. 11 .

 811 Fig. 6. Typical linear log(1+e)-logσ v ´ lines of reconstituted clays at σ v ´ > σ yr ´

Fig. 13 .

 13 Fig. 13. Relationship between transitional stress and consolidation yield stress for natural clays of Type two listed in Table3

Fig. 14 .

 14 Fig. 14. Relationship between C c and e 1 for the two types of natural clays listed in Table 3

Fig. 18 .Fig. 19 .

 1819 Fig. 16. Normalised line of 119 natural Ariake clays in post-yield regime

  increases with increase in void ratio at the remoulded yield stress e yr , and decreases with increase in the remoulded yield stress σ yr ´. It can be also found that at the same value of e yr or at the same value of σ yr ´, the parameter C c * decreases in the order of Baimahu clay, Lianyungang clay and Kemen clay.

	compression index C c
		were considered here. The relationships of C c	* versus
	e yr and C c	* versus σ yr ´ are given in Figures 2 and 3 respectively. For a given clay, the

*

Table 3

 3 Fig. 13. Relationship between transitional stress and consolidation yield stress for natural clays of Type two listed in Table3Fig. 14. Relationship between C c and e 1 for the two types of natural clays listed in Table 3 Fig. 15. Typical normalized behaviour of natural Ariake clays with void index I vn Fig. 16. Normalised line of 119 natural Ariake clays in post-yield regime Fig. 17. Typical normalised behaviour with void index I vn for two types of natural clays listed in Table 3Fig. 18. Normalised behaviour with void index I vn for natural clays of Type one in post-yield regime Fig. 19. Normalised behaviour with void index I vn for natural clays of Type two with effective vertical stresses larger than transitional stresses

Table 1 .

 1 Sources of reconstituted clays

	Soils	Initial water content Liquid limit Plastic limit Density of soil particles Source of data
		(%)	(%)	(%)	(g/cm 3 )
	Calcium montmorillonite Clay	-	189-220	31-35	2.80	Mesri & Olson (1971)
	Sail Clay	158	158	-	2.83	Nagaraj & Srinivasa (1983)
	Black Cotton Clay	84-97	84-97	-	2.65-2.73	Nagaraj & Srinivasa (1983); Nagaraj et al. (1993)
	Red Soil	39-50	37-50	18-27	2.63-2.70	Nagaraj & Srinivasa (1983); Nagaraj et al. (1993); Sridharan & Nagaraj (2000)
	Brown Soil	58-62	59-62	-	2.65	Nagaraj et al. (1993); Sridharan & Nagaraj (2000)
	Bentonite Clay	300	300	70	2.80	Nagaraj & Srinivasa (1983)
	Bothkennar Clay	65-85	67-85	24-41	2.64-2.80	Hight et al. (1992); Allman & Atkinson (1992)
	Silty Soil	39	39	30	2.65	Sridharan & Nagaraj (2000)
	Cochin Clay	56	56	38	2.61	Sridharan & Nagaraj (2000)
	Illitic Soil	73	73	52	2.58	Sridharan & Nagaraj (2000)
	BC Soil	74	74	36	2.70	Sridharan & Nagaraj (2000)
	Kaolinite Clay	42-74	42-59	26-45	2.64-2.68	Sridharan & Nagaraj (2000); Cerato & Lutenegger (2004)
	Attapulgite Clay	202-354	202	108	2.83	Cerato & Lutenegger (2004)
	Atchafalaya Clay	101-177	101	35	2.80	Cerato & Lutenegger (2004)
	Boston Blue Clay	45-79	45	23	2.80	Cerato & Lutenegger (2004)
	Lianyungang Clay	50-146	74	33	2.71	Hong et al. (2010)
	Baimahu Clay	64-180	91	38	2.65	Hong et al. (2010)
	Kemen Clay	43-122	61	30	2.67	Hong et al. (2010)

Table 2 .

 2 Physical indices of natural Ariake clays

	Locations of Ariake clays	Depth	Natural water content Liquid limit Plastic limit Density of soil particles	Source of data
		(m)	(%)	(%)	(%)	(g/cm 3 )
	Ashikari	2-38	30-124	34-100	17-41	2.48-2.67	Hong and Han (2007)
	Hama	1-18	71-168	39-129	23-53	2.37-2.66
	Ariake	2-40	31-145	66-140	20-47	2.58-2.68
	Fukudomi	2-14	32-149	44-103	21-49	2.51-2.67

ACKNOWLEDGEMENTS

This study is supported by the National Natural Science Foundation of China (Grant No. 41172240; Grant No. 50878050). The authors are grateful to Dr Jie Yin, Mr Xia Bian, and Mr Qinghua Wang for their help during the preparation of the manuscript.

The Quarterly Journal of Engineering Geology and Hydrology 33, No. 1, 7-39. Chung, S. G., Nagendra Prasad, K., Nagaraj, T. S., Chung, J. G. & Jo, K. Y. (2004).

Assessment of compressibility behavior of naturally cemented soft clays. Marine Georesources and Geotechnology 22, No.1, 1-20. Collins, K. & McGown, A. (1974). The form and function of microfabric features in a variety of natural soils. Géotechnique 24, No. 2, 223-254. Cotecchia, F. & Chandler, R. J. (1997). The influence of structure on the pre-failure behaviour of a natural clay. Géotechnique 47, No. 3, 523-544. Cotecchia, F. & Chandler, R. J. (2000). A general framework for the mechanical behaviour of clays. Géotechnique 50, No. 4, 431-447. Delage, P. & Lefebvre, G. (1984). Study of the structure of a sensitive Champlain clay and of its evolution during consolidation. Canadian Geotechnical Journal 21, No. 1, 21-35. Feng, T. W. (1991). Compressibility and permeability of natural soft clays and surcharging to reduce settlement. PhD thesis, University of Illinois at Urbana-Champaign. Giao, P. H., Dung, N. T. & Long, P. V. (2008). Integrated geotechnical-geophysical investigation of soft clay at a coastal site in the Mekong Delta for oil and gas infrastructure development. Can. Geotech. J. 45, No. 11, 1514-1524. Griffiths, F. J. & Joshi, R. C. (1989). Change in pore size distribution due to consolidation of clays. Géotechnique 39, No. 1, 159-167. Hashiguchi, K. & Ueno, M. (1977). Elasto-plastic constitutive laws of granular materials. IX Int. Conf. Soil Mech. Found. Engrg., Tokyo: Balkema, Rotterdam, Specialty Session 9, pp. 73-82. Hight, D. W., Jardine, R. J. & Gens, A. (1987). The behaviour of soft clays. Chapter 2, Embankment on soft clays, Athens: Public Works Research Centre of Greece, pp. 33-158.


M Wan

B Chen

Y G Chen

Y J Cui

J Wang

Temperature Effects on the Unsaturated Permeability of the Densely Compacted GMZ01 Bentonite under Confined Conditions

Keywords: GMZ bentonite, nuclear waste repository, temperature, water-retention property, unsaturated permeability

In this study, temperature controlled soil-water retention tests and unsaturated hydraulic conductivity tests for densely compacted Gaomiaozi bentonite -GMZ01 (dry density of 1.70 Mg/m 3 ) were performed under confined conditions. Relevant soil-water retention curves (SWRCs) and unsaturated hydraulic conductivities of GMZ01 at temperatures of 40°C and 60°C were obtained. Based on these results as well as the previously obtained results at 20°C, the influence of temperature on water-retention properties and unsaturated hydraulic conductivity of the densely compacted Gaomiaozi bentonite were investigated. It was observed that: (i) water retention capacity decreases as temperature increases, and the influence of temperature depends on suction; (ii) for all the temperatures tested, the unsaturated hydraulic conductivity decreases slightly in the initial stage of hydration; the value of the hydraulic conductivity becomes constant as hydration progresses and finally, the permeability increases rapidly with suction decreases as saturation is approached; (iii) under confined conditions, the hydraulic conductivity increases as temperature increases, at a decreasing rate with temperature rise. It was also observed that the influence of temperature on the hydraulic conductivity is quite suction-dependent. At high suctions (s > 60 MPa), the temperature effect is mainly due to its influence on water viscosity; by contrast, in the range of low suctions (s < 60 MPa), the temperature effect is related to both the water viscosity and the macro-pores closing phenomenon that is supposed to be temperature dependent.

Introduction

In a conceptual multi-barrier disposal radioactive waste repository (Figure 1), significant Temperature-Hydraulic-Mechanical (THM) phenomena take place in the engineered barrier and in the near field due to the combined actions of heating and hydration (Sanchez et al, 2004). The hydraulic property of the compacted bentonite used as engineered barrier material is one of the key properties for the design of such a disposal system. This explains the large number of studies that have been performed in this area: [START_REF] Dixon | The hydraulic conductivity of dense clay soils[END_REF], [START_REF] Nachabe | Estimating hydraulic conductivity for models of Soils with Macropores[END_REF] and [START_REF] Liu | An investigation of the physical properties of clayey materials used in nuclear waste disposal at great depth[END_REF] tested the permeability of saturated compacted bentonites and analyzed the related influencing factors; [START_REF] Villar | Caracterización termo-hidro-mecánica de una bentonita de Cabo de Gata[END_REF][START_REF] Villar | Thermo-hydro-mechanical characterisation of a bentonite from Cabo de Gata[END_REF] and [START_REF] Komine | Simplified evaluation on hydraulic conductivities of sand-bentonite mixture backfill[END_REF] reported different empirical relations between dry density and saturated permeability of compacted benonite; [START_REF] Komine | Simplified evaluation on hydraulic conductivities of sand-bentonite mixture backfill[END_REF] and [START_REF] He | Calculation of saturated hydraulic conductivity of bentonite[END_REF] predicted the saturated permeability of bentonite based on the changes in porosity. For the unsaturated bentonite, after an investigation to the unsaturated permeability of the mixture of the Kunigel V1 bentonite and Hostun sand under confined conditions, [START_REF] Loiseau | Transferts d'eau et couplages hydromécaniques dans les barrières ouvragées[END_REF] found that for suction lower than 23MPa, the unsaturated permeability increases with suction decrease, while for suction higher than 23MPa, the unsaturated permeability decreases as suction decreases. Under both confined conditions and unconfined conditions, [START_REF] Cui | Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant-volume and free-swell conditions[END_REF] tested the unsaturated permeability of the mixture of Kunigel-V1 bentonite/Hostun sand based on the instantaneous profile method, and found that as suction decreases, the unsaturated permeability decreases to a certain value and then turns to increase. [START_REF] Cho | The temperature effects on hydraulic conductivity of compacted bentonite[END_REF] reported that the influence of temperature on the permeability of bentonite is mainly because the intrinsic permeability, viscosity and density of water are influenced by temperature. Changes in viscosity of water with temperature have been found to be the most important mechanism [START_REF] Towhata | Volume change of clays induced by heating as observed in consolidation tests[END_REF][START_REF] Cho | Influence of temperature elevation on the sealing performance of the potential buffer material for a high-level radioactive waste repository[END_REF][START_REF] Villar | Influence of temperature on the hydro-mechanical behaviour of a compacted bentonite[END_REF].

GMZ bentonite has been selected as the potential buffer/backfill material for the construction of the engineered barrier in the Chinese deep geological disposal program for high level radioactive nuclear waste, thanks to its high montmorillonite content, high cation exchange capacity (CEC), large specific surface and other desirable properties [START_REF] Liu | An investigation of the physical properties of clayey materials used in nuclear waste disposal at great depth[END_REF]. Studies on the mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior of the GMZ bentonite have been conducted over years (Ye et al., 2010b). The investigation of the hydraulic properties of the GMZ bentonite has been the gravity center of the recent studies. [START_REF] Liu | An investigation of the physical properties of clayey materials used in nuclear waste disposal at great depth[END_REF] tested the saturated permeability and analyzed the related influencing factors of the compacted GMZ bentonite. Using the instantaneous profile method, Ye et al. (2010a) tested the unsaturated permeability of the densely compacted specimen, with a dry density of 1.7Mg/m 3 , under confined (constant-volume) conditions. Results

show that the unsaturated hydraulic conductivity of the compacted bentonite changes from 1.13×10 -13 m/s to 8.41×10 -15 m/s (gravimetric water content from 12% to 28%) and it is not solely function of suction. While under unconfined (free-swelling) conditions, the unsaturated hydraulic conductivity of the Gaomiaozi bentonite is in a larger range of 1.0×10 -12 -1.0×10 -15 m/s. Based on the Kozeny-Carmen semi-empirical function, [START_REF] Niu | The Equations of Unsaturated Permeability Considering the Micro-structure[END_REF] proposed a semi-empirical equation for the calculation of the unsaturated permeability of the GMZ bentonite with the consideration of micro-structural changes.

As far as the influence of temperature effect is concerned, Ye et al. (2009b) reported that the water retention capacity of the highly-compacted GMZ bentonite and bentonite-based mixtures decreases as the temperature increases, regardless of the confining conditions.

In this paper, the soil-water retention curves (SWRCs) of the densely compacted Gaomiaozi bentonite (GMZ01) under confined conditions and at various temperatures (20°C, 40°C and 60°C) are presented. Based on the results obtained, the unsaturated permeability of the GMZ01 is investigated by performing infiltration tests under controlled temperature.

Materials

The Gaomiaozi deposit is located in the northern Chinese Inner Mongolia autonomous region, 300 km northwest from Beijing (Ye et al., 2009a(Ye et al., , 2010b)). Some basic properties of the GMZ01 bentonite tested in this paper are listed in Table 1, which indicates that the GMZ01 bentonite has high cation exchange capacity and high adsorption ability.

Experimental Methods

The instantaneous profile method has been adopted in this study. This method was successfully used by many researchers to determine the unsaturated hydraulic conductivity of geomaterials (Daniel, 1982;Richards and[START_REF] Richards | Capillary conductivity values from moisture yield and tension measurements on soil columns[END_REF][START_REF] Hamilton | Composition of nonthermal ions in the Jovian magnetosphere[END_REF][START_REF] Watson | An instantaneous profile method for determining the hydraulic conductivity of unsaturated porous materials[END_REF][START_REF] Meerdink | Unsaturated hydraulic conductivity of two compacted barrier soils[END_REF][START_REF] Fujimaki | A flux-controlled steady-state evaporation method for determining unsaturated hydraulic conductivity at low matric pressure head values[END_REF][START_REF] Cui | Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant-volume and free-swell conditions[END_REF]Ye et al., 2010a). As an unsteady-state method, it can be used either in the laboratory or in situ [START_REF] Benson | Measuring unsaturated hydraulic conductivity in the laboratory and field[END_REF].

In order to apply this method to determine the unsaturated permeability of the GMZ01 bentonite at different temperatures, on the one hand, the SWRCs of this soil should be determined at relevant temperatures, and on the other hand, the corresponding suction profiles should be determined by performing infiltration test at different temperatures with suction monitoring. For a given temperature, the hydraulic gradient was determined using the suction profile; the water flux was determined using the water content profile; the hydraulic conductivity was then calculated based on the generalized Darcy's law. The detailed calculation procedure can be found in Ye et al. (2009a).

Determination of SWRCs

Suction control

The vapour equilibrium technique (for high suctions) and osmotic technique (for low suctions) were employed for suction control in this study. At high suctions, the experimental setup used was described by [START_REF] Ye | Measurement of soil suction in laboratory and soil-water characteristics of Shanghai soft soil[END_REF], as shown in Fig. 2. Note that the vapor equilibrium technique was employed by number of researchers for controlling total suction in unsaturated soil tests [START_REF] Bernier | Suction-controlled experiments on Boom clay[END_REF][START_REF] Blatz | A system for controlled suction in triaxial tests[END_REF][START_REF] Lloret | Mechanical behaviour of heavily compacted bentonite under high suction changes[END_REF][START_REF] Chen | Soil-water characteristic curves of Gaomiaozi bentonite[END_REF].

In this study, the confined GMZ01 specimen was placed in a desiccator and the water vapour above a saturated salt solution was circulated to provide the desired suction to the specimen. Saturated salt solutions and their corresponding suctions imposed at 20, 40 and 60°C are shown in Table 3 ( [START_REF] Tang | Controlling suction by the vapour equilibrium technique at different temperatures and its application in determining the water retention properties of MX80 clay[END_REF].

For low suctions, the osmotic technique was used and the corresponding setup is shown in Fig 3 [START_REF] Delage | Suction controlled testing of non saturated soils with an osmotic consolidometer[END_REF][START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted unsaturated clay[END_REF]. Note that [START_REF] Tang | Calibration of the osmotic technique of controlling suction with respect to temperature using a miniature tensiometer[END_REF] studied the temperature effect on the calibration curve of PEG solutions and found that this effect is insignificant. Thus, in this study, the osmotic technique was employed without temperature correction.

Apparatus

Custom-designed stainless steel cells with small holes in two ends (Fig. 2, Ye, 2009a) were employed for water retention test under confined conditions. The holes were designed as channels for moisture exchange between the specimen in the cell and the circulating air (or PEG solution) around it.

For the temperature control, the setups were placed in ovens (Fig 3 and Fig 4), which have temperature controlled to an accuracy of ±0.1°C. Note that temperatures of 20, 40 and 60°C were selected as the testing temperatures in this study.

Specimen preparation

The GMZ01 bentonite powder was compacted into a thin cylindrical specimen, which has a final dimension of 20 mm in diameter and 6 mm in height. For the compaction, a press was used and the compaction was carried out at a velocity of 0.1 mm/min. The final dry density and water content of the compacted specimen were 1.70g/cm 3 and 10.65%, respectively.

Infiltration test

The schematic layout of the temperature controlled infiltration test is shown in Fig. 5. A custom-designed cylinder (Ye et al., 2009a(Ye et al., , 2010a) ) is put in an oven with temperature controlled to an accuracy of ±0.1°C. The resistive relative humidity (RH) sensors [START_REF] Cui | Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant-volume and free-swell conditions[END_REF] were used to monitor the RH changes. Note that the same type of sensor was used by Ye et al. (2009aYe et al. ( , 2010a)). It can be seen from Fig. 5 that the sensors were installed every 30 mm along the length of the cell (4 sensors) with a fifth sensor in the upper base plate of the cell. As the sensors measure the air relative humidity, no direct contact with soil specimen was allowed. For this reason, a small cavity was bored in the soil for each transducer. This cavity had a dimension allowing introducing the transducer cap: a porous stone of 2 mm thick and 5 mm in diameter. This porous stone separated the transducer from the soil sample and allowed the air humidity transfer from the specimen to the transducer (Ye et al., 2009a).

The distilled water was used in the infiltration test. The water absorbed by the specimen can be quantified by calculating the water volume change in the left marked glass pipe, which can be compensated by water from the right tube, in the U-shaped system outside the oven. Two drops of silicone oil were added into the left pipe to prevent water evaporation. A soft tube was used for connecting the U-shaped system to the inlet of the specimen in order to warm up the water to current testing temperature before absorption. The humidity and temperature changes were recorded by the data logging system.

A double-piston mould was used for the compaction of the specimen [START_REF] Cui | Yielding and plastic behaviour of an unsaturated compacted silt[END_REF].

The powder of the GMZ01 bentonite was compacted in 5 layers. After the first layer (30 mm) was compacted and the surface of specimen was carefully scarified for the integrity of the specimen, the equal parts of the GMZ01 powder were added from two ends of the mould and then compacted to two 15 mm sub-layers. This procedure was repeated for the other 3 layers. The compaction was conducted at a speed of 0.1 mm/min. The specimen has a final height of 150 mm, a dry density of 1.70 Mg/m 3 , a suction about 90 MPa for 40°C temperature and 100MPa for 60°C temperature, and a degree of saturation around 0.49 for 40°C temperature and 0.41 for 60°C temperature.

The unsaturated permeability test on the GMZ01 bentonite at 20°C was previously measured and reported by [START_REF] Ye | Advances on the knowledge of the buffer/backfill properties of heavily compacted GMZ bentonite[END_REF] and thus only the infiltration tests at temperatures of 40°C and 60°C were performed in this study.

Results and discussion

SWRCs

The SWRCs of the highly-compacted GMZ01 specimen following wetting path at different temperatures (20°C, 40°C and 60°C) under confined conditions are shown in Fig. 5. Based on these results, an equation can be proposed to describe the water retention curves of the densely compacted GMZ01 bentonite (1.7 Mg/m 3 ): Intra-aggregate water moves into macro-pores (inter-aggregates pores) with temperature increase (Ye et al, 2009a). This process decreases the suction in the macro-pore level. As the suction is controlled, water flows out from the macro-pores, leading to a decrease of water retention capacity. At low suctions, capillary effect plays a decisive role in the water retention capacity. Increase of temperature causes changes in surface tension, which results in decrease of water content under constant suction conditions.

In order to quantitatively assess the influence of temperature on the water retention capacity of the bentonite under different suctions, a ratio k T is defined as follows:

% 100 1 2 1 × - = T T T T w w w k (3)
where w T1 and w T2 are water content at temperature T1 and T2 respectively for the same suction.

The relationship between the ratio k T and suction for the GMZ01 bentonite at 40°C and 60°C are given in Fig. 7. It can be observed that the effect of temperature on the water retention capacity is closely related to suction, particularly in the range from 30 to 60 MPa. This effect reaches a maximum at a suction around 40 MPa.

Unsaturated permeability

Test at 40°C

The relative humidity changes with hydration time in the infiltration test at 40°C are shown in Fig. 8. Based on the SWRCs measured at 40°C (see Fig. 6), the development of suction with hydration time can be obtained. Note that the conversion from relative humidity to suction was done using the Kelvin's law. Fig 8 indicates that, for the relative humidity sensor located 3 cm from the hydration water inlet at the bottom of the specimen, suction decreases rapidly in the first 200 h of hydration and then decreases much more slowly. For suction measured at 6 cm, it begins to decrease rapidly after 100 h hydration and gradually decreases after 800 h hydration. As it is relatively far from the water inlet, suctions measured at 12 cm and 15 cm from the bottom of the specimen start to decrease rapidly after 200 and 300 h of hydration, respectively. The slope of the curve of suction versus time decreases as the distance from the inlet increases. The test was stopped after about 1670 h hydration, when the sensor at 3 cm distance from the inlet indicated that zero suction (100% relative humidity) was achieved at this height.

The relationship between the unsaturated hydraulic conductivity and suction is shown in Fig. 9. It can be observed that at 40°C temperature, the unsaturated hydraulic conductivity of the GMZ01 with a dry density of 1.7 Mg/m 3 is on the whole between 1.64×10 -13 m/s and 1.34×10 -14 m/s. During the initial stages of hydration, the hydraulic conductivity gradually decreases with suction decrease, and the hydraulic conductivity reaches the minimum value of 1.34×10 -14 m/s when the suction drops to 45 MPa; the hydraulic hydraulic conductivity keeps steady in the range of suction from 20 MPa to 60MPa; but when suction drops to a level lower than 20 MPa, the unsaturated hydraulic conductivity increases rapidly and reaches 1×10 -13 m/s.

Test at 60°C

The unsaturated hydraulic conductivity of the confined GMZ01 determined at 60°C is shown in When the soil suction is decreased from the initial value (about 80 MPa) to zero, the hydraulic conductivity first decreases from 2×10 -14 m/s to 7×10 -15 m/s and then increases to 1×10 -13 m/s, which is close to the saturated hydraulic conductivity. As in the first stage, water transfer is primarily governed by the network of large pores and these large pores are progressively decreasing in quantity and in size, resulting in hydraulic conductivity decreases. After completion of this large-pore clogging by gel creation, a normal conductivity increase with suction decrease is observed (Ye et al., 2009a).

Influence of temperature on the unsaturated hydraulic conductivity

To further assess the influence of temperature on the unsaturated permeability of the highly compacted GMZ01 bentonite, the unsaturated hydraulic conductivity of the confined specimen at 20°C (Ye et al, 2009a) are compared to those measured at 40°C and 60°C (Fig. 11). It can be seen that under confined conditions, the unsaturated hydraulic conductivity of the highly compacted GMZ01 bentonite increases with temperature rise. Moreover, the rate of change also decreases as temperature increases. The temperature effect becomes more significant at higher suctions (above 20 MPa). In the range of lower suctions (less than 20 MPa), it is observed that the lower the suction the less the temperature effect. The possible explanation is that for lower suctions the moisture absorbed by the bentonite is mainly associated with microstructure changes and the temperature effect on the microstructure is not significant.

The influence of temperature on the hydraulic conductivity is mainly related to the influence of temperature on the water viscosity and the pore structure of the bentonite. To remove the influence of temperature on water viscosity, the relative hydraulic conductivity is introduced to allow for a better analysis of the influence of temperature on hydraulic conductivity. Relationships between the relative permeability and degree of saturation (Sr) of the confined GMZ01 at 40°C and 60°C are given in Fig. 12. It can be observed that when Sr is higher than 0.57, the hydraulic conductivity at 60°C is similar to that observed at 40°C. This means that in this range of degree of saturation the influence of temperature on permeability is mainly due to the influence on water viscosity. On the contrary, when Sr is lower than 0.57, the relative permeability at 40°C is found higher than that at 60°C. Interestingly, this threshold corresponds to a suction of 60 MPa, and from Figs 9, 10 and 11 it can be observed that when s > 60 MPa the hydraulic conductivity decreases with suction decrease. As mentioned above, in this suction range hydration leads to progressive macro-pores closing thus to a decrease in hydraulic conductivity. This macro-pore closing process can be assumed to be more significant at higher temperature because of softer clay aggregates and lower water viscosity, explaining a lower hydraulic conductivity at 60°C than at 40°C. As the relative hydraulic conductivity has been found independent of temperature when Sr > 0.57 (Fig. 12), it can be supposed that the macro-closing process ended when Sr > 0.57; in other words, the influence of temperature on pore structure became insignificant in this range.

It is also important to note that the obtained results could be affected by the possible density gradient along the specimen as identified by [START_REF] Dixon | Two large-scale sealing tests conducted at Atomic Energy of Canada's underground research laboratory: the buffer-container experiment and the isothermal test[END_REF] and [START_REF] Villar | Behaviour of a bentonite barrier in the laboratory: experimental results up to 8 years and numerical simulation[END_REF]. This density gradient can be formed owing to the expansion of the hydrated bentonite that intrudes into the drier area under the effect of swelling pressure. If it occurs, the computation of degree of saturation without considering this gradient is not correct and the water retention curve considered is also inappropriate.

In other words, the simultaneous profile method meets its limitation. Because in this study, no specific analyses were conducted after the infiltration tests, this phenomenon can not be verified. Further studies will be performed to investigate this aspect.

Conclusions

The SWRCs of the highly compacted GMZ01 confined specimens on wetting path and at different temperatures (20°C, 40°C and 60°C) show that the water retention capacity decreases as temperature increases; and the influence of temperature depends on suction. The ratio k T can be used to quantitatively describe the influence of temperature on water retention capacity of bentonite at different suctions.

Under confined conditions and at 40°C temperature, the unsaturated hydraulic conductivity of the GMZ01 bentonite at a dry density of 1.7Mg/m 3 is between 1.64×10 -13 m/s and 1.34×10 -14 m/s. At 60°C temperature, the value is slightly lower, between 1.19×10 -13 m/s and 1.79×10 -14 m/s.

For all the temperatures considered, the unsaturated hydraulic conductivity decreases slightly in the first stage of hydration. The value of the hydraulic conductivity becomes constant as hydration progresses. Finally, the hydraulic conductivity increases rapidly with suction decreases when saturation is approached. This phenomenon may be explained by the changes in the soil microstructure.

Under confined conditions, the hydraulic conductivity increases as temperature increases, at a rate that decreases with temperature rise. Also, the influence of temperature on the hydraulic conductivity is quite suction-dependant. At high suctions (s > 60 MPa) or low degrees of saturation (Sr < 0.57), the temperature effect is mainly due to its influence on water viscosity; on the contrary, in the range of low suctions (s < 60 MPa) or high degrees of saturation (Sr > 0.57), the temperature effect is related to both the water viscosity and the macro-pores closing phenomenon that is supposed to be temperature dependent. Note that further studies are needed to investigate the possible dry density gradient effect on the hydraulic conductivity determined based on the simultaneous profile method.

  Fig.6indicates that, the water retention capacity decreases as temperature increases and the degree of the temperature influence depends on suction. This phenomenon can be analyzed separately at low and high suctions. At high suctions (> 4 MPa), changes of clay fabric and fluid in intra-aggregate spaces play a significant role in water retention capacity of GMZ bentonite.

Fig. 10 .

 10 Fig.10. It can be seen that the values are generally between 1.79×10 -14 m/s and 1.19×10 -13 m/s. As the infiltration of water progresses, suction drops from 80 MPa to 55 MPa, while the unsaturated hydraulic conductivity decreases slightly. With suction reduction from 55 MPa to 20 MPa, the hydraulic conductivity remains almost constant despite of the suction changes. For suction lower than 20 MPa, the hydraulic conductivity rapidly increases with decreasing suction and reaches a final value of 1×10 -13 m/s.

Fig. 1 .Fig. 8 .Fig. 9 .Fig. 10 .Fig. 11 .Fig. 12 .

 189101112 Fig.1. Schematic view of a high level nuclear waste repository(Sanchez, 2004)

Table 1

 1

	Basic Properties of GMZ01 bentonite
	Property	Description
	Specific gravity of soil	2.66
	pH	8.68-9.86
	Liquid limit (%)	276
	Plastic limit (%)	37
	Total specific surface
	area/	570
	(m 2 •g -1 )
	Cation exchange
	capacity/	0.773 0
	(mmol•g -1 )
	Main exchanged	Na + (0.433 6), Ca 2+ (0.291
	cation/	4), Mg 2+ (0.123 3),
	(mmol•g -1 )	K + (0.025 1)
		Montmorillonite(75.4%),
	Main minerals	quartz (11.7%), feldspar (4.3%),
		cristobalite (7.3%)

Table 2

 2 Salt solution and corresponding suction at different temperatures (MPa)(Tang 2005)

	Salt solution	20°C	40°C	60°C
	LiCl 2	309.0	-	340
	MgCl 2	150.0	162.4	187.7
	K 2 CO 3	113.0	122.0	144.8
	Mg(NO 3 ) 2	82.0	103.1	139
	NaNO 2	57.0	-
	NaNO 3	39.0	49.5	61.6
	NaCl	38.0	40.6	44.2
	(NH 4 ) 2 SO 4	24.9	32.2
	KCl	21.0	27.8	33.4
	ZnSO 4	12.6	-
	KNO 3	9.0	-
	K 2 SO 4	4.2	5.1	5.5

ACKNOWLEDGEMENTS

The authors are grateful to the National Natural Science Foundation of China (Projects No. 41030748, No.40772180 and No.40728003), Kwang-Hua Fund for College of Civil Engineering at Tongji University, China Atomic Energy Authority (Project [2007]831), and Shanghai municipality (Leading Academic Discipline Project -B308).