Jean-Lucien Sanchez

The minor detainees in the house of supervised education of the prisons of Fresnes, 1929-1958

Keywords: detainees, minors, house of supervised education, prisons of Fresnes, evaluation, observation, France, judge, justice, pupils

The house of supervised education of the prisons of Fresnes opened in 1929. It actually corresponded to a prison district reserved for minor detainees. But in 1930, a technique of observation and evaluation was developed to organize an orientation of the minors to other penitentiary institutions. They were evaluated by a team consisting of a teacher, a doctor, a social worker and a psychiatrist. The goal was to create a file on the minor and then transmit it to a juvenile judge. Thus, the judge could then make a decision based on the information provided by this file and could decide whether to entrust the minor to his family, or to entrust him to a society of patronage, or to entrust him to another house of supervised education until his age of majority. This penitentiary technique invented in Fresnes constituted an important innovation in the French prison history. Since it was first intended for minor detainees, it was later extended to major detainees and Fresnes became a prison specialized in the evaluation of detainees. Indeed, the National Observation Center of Fresnes opened in 1951 and became national evaluation center in 2010. And it still works today.

From 1929, minor boys were incarcerated in the house of supervised education (maison d'éducation surveillée) of the prisons of Fresnes, in France1 . Then, following the creation in 1945 of the Department of Supervised Education at the Ministry of Justice (Direction de l'Education surveillée), it became in 1958 a special supervised education observation center (centre spécial d'observation de l'éducation surveillée) until 1979, when it finally closed. The law of August 5th, 1850 on the education and the patronage of the young detainees organized the conditions of imprisonment of the minor detainees in France [START_REF] Yvorel | L'enfermement des mineurs de justice au XIXème siècle, d'après le compte général de la justice criminelle[END_REF]. Could be imprisoned minors of both sexes detained for crimes, offenses, contraventions of tax laws; or by paternal correction (correction paternelle); and minor acquitted under Article 66 of the Penal Code as having acted indiscriminately but not entrusted to their parents. Minors sentenced to imprisonment for more than six months and less than two years should serve their sentences in a penal colony (colonie pénitentiaire), as well as those acquitted under Article 66 of the Penal Code as having acted indiscriminately but not entrusted to their parents (until their age of majority, i.e. 21 years old). Those sentenced to more than two years of imprisonment or who have been declared insubordinate in a penal colony should be sent to a correctional colony (colonie correctionnelle), where the regime was harder. Girls detained by paternal correction, those under the age of sixteen who were condemned to imprisonment for any length of time, and those who were acquitted as having acted indiscriminately but not entrusted to their parents were imprisoned in "prison houses" (maisons pénitentiaires). Finally, because of the law of March 24, 1921, vagrant minors over the age of 18 were liable to imprisonment for 3 to 6 months and those aged 13 to 16 could be entrusted to their parents, an institution, an individual, a school of reform or a school of preservation, or a penal or correctional colony [START_REF] Golliard | Dépénaliser le vagabondage ? L'impact relatif du décret-loi d'octobre 1935[END_REF]. However, by a decree of October 30, 1935, vagrant minors should be entrusted to individuals, charitable institutions or Public Assistance. But a circular of the Minister of Justice of March 27th, 1936, organized a transitional period during which, due to a lack of competent institutions with adequate reception conditions, vagrant minors continued to be entrusted to the Prison Administration. Article 2 of the law of August 5, 1850, organized the establishment of a separate district in the prisons assigned to minor detainees of all categories. In Fresnes prisons, a correctional education cell block (quartier cellulaire d'éducation correctionnelle) for girls was set up in the special district of women in 1902, renamed school of preservation (école de préservation) in 1927 (YVOREL, 2007: 59). Then a house of supervised education for boys was set up in 1929 in a part of the third division of the great district of men (grand quartier des hommes). These two institutions were established by a decree on December 31, 1927, which transformed the penal colonies into houses of supervised education for boys and schools of preservation for girls. This decree also transformed correctional colonies into "correctional houses" (maisons de correction), settlers (colons) into "pupils" and supervisors into "monitors". But this reform, as noticed [START_REF] Yvorel | L'influence des réformes de l'administration pénitentiaire sur la vie quotidienne des colons[END_REF], is only a "reform on paper: the words changes but the content remains the same" and the regime of the houses of supervised education and the schools of preservation continued to be as hard as it was in penal and correctional colonies. Minors incarcerated in the house of supervised education of the prisons of Fresnes were in fact subject to a prison regime, because the majority of them were mainly defendant (prévenus), i.e. detainees waiting for their trial, or passengers (passagers), i.e. detainees waiting to be transferred to another house of monitored education. However, from 1930, boys were the object of an important innovation in the field of penitentiary evaluation and observation with the creation of the first "triage center" (centre de triage) (YVOREL, 2007: 63): In the house of supervised education of Fresnes, where children stay for only a few weeks, there can be no question of correcting their morale or amending them in a complete way; it is only possible to do a triage. I was happy to see how conscientious it was done2 .

The time that minors defendant spent in Fresnes was in fact entirely dedicated to the preparation of a file drawn up by the prison staff and then transmitted to the juvenile judge (juge pour enfant). The judge used this file to determine his judgment. This article presents the history of the house of supervised education of the prisons of Fresnes and the development of the first observation and evaluation penitentiary procedure for detainees created by the French Prison Administration, in 1930. This article is mainly based on the consultation of two sources of archives. The archives of the General Inspection of Administrative Services (GIAS, Inspection générale des services administratifs) that are kept in the French National Archives (on the site of Pierrefitte-sur-Seine) and classified under reference number F/1a/4510 to F/1a/4614. This inspection was placed under the responsibility of the Ministry of the Interior (i.e. Home Office) and should inspect all institutions belonging to this Ministry, including prisons. Because the Prison Administration was placed under the responsibility of the Ministry of the Interior until 1911, then it was placed under the responsibility of the Ministry of Justice. But as the inspection of the Ministry of Justice was created only in 1958, the GIAS continued to inspect prisons until that date. The inspectors wrote reports which were then transmitted to the Minister of Justice. Reports on prisons in the department of Seine (1917Seine ( -1939) ) are classified under reference number F/1a/4577 and F/1a/4578 (1920)(1921)(1922)(1923)(1924)(1925)(1926)(1927)(1928)(1929)(1930)(1931)(1932)(1933)(1934)(1935)(1936)(1937)(1938)(1939). The archives of the house of supervised education of the prisons of Fresnes are kept in the Departmental Archives of Val-de-Marne (Archives départementales du Val-de-Marne). The Departmental Archives of Val-de-Marne keep the penal files of the minor detainees which is classified under reference number 2Y5 670 (1936), 2Y5 694 (1937) and 511W 378 to 511W 400 (1947-1957).

THE DAILY FUNCTIONING OF THE HOUSE OF SUPERVISED EDUCATION AND THE SCHOOL OF PRESERVATION

The house of supervised education, also known as the "minor boys quarter" (quartier des garçons mineurs) or "minor district" (quartier des mineurs), was set up in part of the third division of the great district of men of the prisons of Fresnes. But it remained totally separated from the detention of the adults. It was placed under the authority of the assistant director of the prison (sous-directeur) and the pupils were supervised by a staff consisting of a first master, a master and 22 monitors. They were incarcerated in 254 cells (or "rooms") spread over four floors. But 14 cells were used for the general service of the prison, leaving only 240 cells usable. This number was very insufficient and caused situations of chronic overcrowding (as in 1937 where 307 pupils were incarcerated). The ground floor was reserved for syphilitics, "dangerous" pupils and those who were to be placed under special surveillance. On the first floor were the minors who were the subject of a provisional placement order for vagrancy (ordonnance de placement provisoire pour vagabondage). Defendant and "appellants" (i.e. those who appealed their court decision) occupied the second and third floors. On the fourth and last floor were imprisoned, with some defendant, the pupils who went to work every day in Paris in the buildings of the Ministry of Justice. The "passengers" (i.e. those who were waiting for their transfer to a provincial house of supervised education) were distributed among the different floors according to the possibilities of accommodation. In 1935, their distribution was as follows: When he arrived in Fresnes, the minor first encountered the prison clerk (greffe). He was then cleaning himself, putting on a uniform and he was locked in a cell. These cells were divided into seven sections organized as follows:

Table1. Distribution of pupils according to their status

Status

Table2. Distribution of boys in sections according to their ages and convictions

Sections

Ages of the pupils and reasons of their convictions 1st section Pupils between the ages of 13 and 15 arrested for offenses against property or against persons 2nd section Pupils between the ages of 13 and 15 arrested for vagrancy 3rd section Pupils between the ages of 16 and 17 arrested for offenses against property or against persons 4th section Pupils between the ages of 16 and 17 arrested for vagrancy 5th section Pupils between the ages of 17 and 18 arrested for offenses against property or against persons 6th section Pupils between the ages of 17 and 18 arrested for vagrancy 7th section Pupils arrested for incident to probation Source: Seine, Prisons of Fresnes, report to the Minister of Justice (December 12, 1936), National Archives F1a 4577

The school of preservation, also known as the "minor district", was located in the special district of women of the Fresnes prisons. It was placed under the authority of the assistant director of the prison and there were 156 places. The staff consisted of a first mistress, a mistress clerk and eleven instructors. In 1938, the distribution of the minors was as follows: Thus, the boys' schedule allowed them much more outdoor activities than girls because they had 6 hours a day in the summer and 5 hours a day in the winter to go for walks, play games or practice sports in the playgrounds. These physical activities, however, were about their only occupation of the day, outside of meals and a few hours passed in their cell dedicated to reading and writing their mail. The girls had only two and a half hours of exercise in the summer and one and a half hour in the winter, the rest of their time was dedicated to work (which was limited to the manufacture of bouquets of artificial flowers). But this regulatory timetable was absolutely not respected because of a constant lack of staff. There were on average only seven monitors each day while the number of pupils was, for example, 273 in 1938! This situation did not allow them to be able to enjoy their time outside and, in fact, the boys spent most of their day inactive in their cells (because they were subject to a high unemployment) and the girls spent most of their day working in their cells. The Fresnes prison regime was a cellular confinement system and the detainees were not allowed to speak. This prison was actually built according to the precepts of the law of June 5, 1875 on the departmental prisons which recommended separating individually day and night the defendant and to subject to an individual imprisonment the detainees condemned to an imprisonment of one year and one day and beyond. Thus, the minors, like the majors, were subjected to a cellular regime and were placed in common only twice a day: during the physical exercises and during the conferences. Three sports courts for the pupils were set up in three walking yards of the prison, as noticed [START_REF] Yvorel | Les enfants de l'ombre : la vie quotidienne des jeunes détenus au XX e siècle en France métropolitaine[END_REF] in referring to a report written in 1932 by a teacher of Fresnes, Robert Meurillon: "Children aged 13 to 18 cannot spend a whole day without doing anything physically, also three sports fields were created in three courts located behind the house of supervised education: a soccer field, a basketball field, an athletic field […] (p. 64)." But even during conferences and sports sessions, conversations between minors were strictly forbidden. And the rest of the time, they were alone in their cell (except in case of overcrowding). This isolation was intended, according to the Inspector General of Administrative Services, to allow a better observation: I will not insist on the general advantages of this isolation during the short period that pupils spend in Fresnes, but it is interesting to note from the point of view of the psychological study of children, that only in front of their teachers […], without a reason to "show off", they will let themselves go to their true nature, they will be themselves and will more easily abandon themselves to confidences3 .

Table3. Distribution of girls according to their status

Boys were subject to high unemployment. In 1936, barely thirty of them worked in the assembly and painting wooden toys and they earned according to their skills between 4 to 5 francs. Five others were employed as gardeners and five others were employed in the general service of the prison at the salary of 5 francs a day. The others could not work and therefore could not buy food supplements in the canteen (unless if their family could give them money). The girls of the school of preservation were all employed in manufacture artificial flowers and also earned between 4 and 5 francs a day. Five were employed in the general service of the prison, where they earned 5 francs a day. The pupils' diet was the same as that of adults with supplements of sausage, figs, pâté, cheese, chocolate or jam. It was composed in the morning of a black coffee, at 9 o'clock on Mondays, Wednesdays and Fridays, a lean broth (bouillon maigre) and pâté, Tuesdays and Saturdays, a fatty broth (bouillon gras), with beef and figs. At the nine o'clock meal, a quarter of wine was distributed, and at four o'clock was distributed on Mondays, Wednesdays and Saturdays, a stew of potatoes, Tuesdays, beans, Thursdays and Sundays, rice. To this ordinary diet were added all the supplements that the pupil had bought at the canteen and was distributed once a day, at noon. The food was good but the inspectors discovered many irregularities. For example, pupils were entitled from 1936 to a daily meat dish. But it was boiled beef that was first used to make a pot-au-feu: The religious service was celebrated three times on Sundays: once for adult men, once for the pupils of the house of supervised education and once for the adult women and the pupils of the school of preservation. But the chapel was cellular. So the pupils took place in an individual box and could not communicate with their neighbors. The girls received every day a conference given by the teacher. These lectures were accompanied by "light conferences" (conférences lumineuses) and music discs, and covered various topics such as: "the dangers of tobacco, the Mediterranean region, discipline, railways, Mont Saint-Michel, new China, the sugar industry, Scotland [...]." But these conferences were mostly limited, in fact, to listening music discs, playing ball games in the yard, reading children's books or lessons in child care, as evidenced by their schedule: The teacher read in particular the Holidays of Suzette which did not correspond to the age of the audience: "This pink library literature, addressing girls who are mostly prostitutes is ridiculous! Same observation for Chalamet's Handbook of Home Education made for ten-year-old girls5 ." Illiterates' girls were entitled to a literacy class three times a week from 3 pm to 4 pm. For the boys, it was impossible to organize conferences on their premises, because of their disposition. But they received every day the visit of the assistant director of the prison who had an interview with each of them in order to: "make him speak, to probe his degree of perversion, to be personally aware of his possibilities of amendment6 ." Minors could be subject to various punishments such as solitary confinement in the disciplinary ward, deprivations of food, physical exercises and conference, loss of employment and fines. Cell punishments were performed in the adults' disciplinary ward. For boys, 552 punishments were pronounced in 1937 for:

Table6. Reasons for boys' punishment

Reasons of punishments Number of punishments

Offenses against the rule of silence 144

Open Of these 102 punishments, 55 consisted of days of isolation in the disciplinary ward:

-1 punishment of 8 days of isolation; -1 punishment of 6 days of isolation; -26 punishments of 4 days of isolation; -3 punishments of 3 days of isolation; -24 punishments of 2 days of isolation.

THE PROCEDURE FOR OBSERVING AND ASSESSING MINORS

As soon as he arrived in Fresnes, the minor defendant was assessed according to three modes: psychological, social and medical. The time spent by the minor in pre-trial detention was essentially used to create his file, which was then transmitted to the judge. This evaluation was carried out by prison staff (teacher and doctor); by a social service; and by psychiatrists. "The teacher is responsible for provoking confidences7 ." From the first day of his arrival at Fresnes, the pupil was questioned by the teacher, and this first information was written in a special register. The teacher continued his visits to the pupil every day and completed this register by specifying indications collected on his health, his conduct, his visits, etc. Ultimately, it would become a complete summary of his observation. After this first interrogation, the teacher gave the minor a notebook asking him "to tell the story of his life, to search into his childhood memories, to confess his mistakes, and to expose his hopes for the future8 ." The pupil had 24 hours to write his story. Then, when he finished, the teacher pointed out the passages he considered interesting (in blue on the document), and he then noted his personal appreciation of the attitude and character of the pupil. The teacher continued to note every day the observations he made on the pupil from the physical and mental points of view, after his daily visit. Also included on this file were "[…] various information provided by the masters and instructors, written or received letters, reading preferences (library books or subscriptions, with the permission of the director, to certain sports or amusing journals), work attendance or laziness, attention to conferences, trends recognized during physical exercises and games9 ." The day after his arrival, the minor was examined by a doctor who questioned him about his parents' health, his medical history, and so on, and he was subject to a special diet if necessary. Every Wednesday, pupils who arrived the previous week were subjected to an anti-venereal medical consultation. A blood test was done to detect tuberculosis and syphilitics. Then a new blood test was done four weeks later. Finally, a medical notebook was given to the minor where the result of his serology was indicated. This notebook was drawn up under the circular of January 28th, 1931, which stipulated that: "For any minor suffering from syphilis, gonorrhea or tuberculosis, a medical notebook shall be drawn up before the minor is sent to a corrective institution." Any minor with syphilis received a "control and treatment card" on the same model as the one used in the clinics. This card was intended to allow the minor to be able to continue his treatment upon his release, without it being possible to be able to determine the place where he started it. The pupils imprisoned in Fresnes by order of a judge (placés sous mandat de dépôt) were the subject of a social investigation carried out by the delegates of three associations: the Moral Support to Youth on Trial (Aide morale à la jeunesse traduite en justice); the Safeguarding of the Adolescence (Sauvegarde de l'adolescence); and the Social Godmothers (Marraines sociales). This investigation concerned "the family, heredity, material situation, health, morality, education and character of the child" and the delegate completed his investigation by making a proposal of orientation of the minor. These investigations were ordered by the judge and about fifteen francs was allocated to the associations to cover their expenses. The investigation was transmitted to the Fresnes staff about one month after the minor entered the prison. The pupils were then subjected to a medico-psychological examination carried out daily by a team of psychiatrists. But before this examination, the team received the notebook written by the pupil, the special register written by the teacher, the medical notebook and the social investigation. The result of the psychiatric examination was recorded on a sheet with the mentions: general information, biological status, psycho-moral status and proposal of orientation of the minor. In addition to this psychiatric service, a psychiatrist was permanently assigned from 1936 to Fresnes. He went there every Monday and as often as his service required. He examined the pupils from two points of view: from the strictly medical point of view, that is to say he observed and detected the "abnormal"; and from the "penitentiary" point of view, that is to say he selected and directed the pupils to various establishments. The "abnormal" defendant pupil were reported to him by the prison staff and by the expert doctors who could wish to have the result of an observation before writing their expertise or by one of the psychiatrists responsible for the medical-psychological examination and who thought it was necessary to carry out a more in-depth study. Copies of the social investigation and the result of the psychiatric examination were sent to the judge and the president of the juvenile court. Finally, when the date of the minor's trial was fixed, the director of the prisons of Fresnes transmitted to the president of the court a notice indicating the degree of amendment of the pupil and his rehabilitation capabilities. The judge could decide to entrust the minor to his family. In this case, one of his parents should to pick him up in Fresnes. The purpose was to allow the director to talk to the parent and provide advice to him following the observations made on the pupil during his stay in the prison. The judge could also decide to entrust the pupil to a society of patronage. Every Thursday, the pupils entrusted to a Parisian patronage society were taken to the Paris court where a delegate came to pick them up. If the minor was entrusted to a provincial patronage society, the delegate should come to Fresnes to pick him up. The patronage societies received a copy of the social investigation and a copy of the medico-psychological examination of the minor. The judge could finally decide to entrust him until his majority to a house of supervised education or a school of preservation. While awaiting his transfer, the pupil's file was then constituted and the Prison Administration was looking for the establishment which could suit him the best because of his age, his health and his dispositions. Thus, upon the arrival of a minor in another house of supervised education, the director could read his file and know his background and the impression he produced on the staff who evaluated him at Fresnes. While the prison of Fresnes has no longer minor detainees today, the experiment that was conducted there from 1930 was the beginning of a specialization of this prison in the observation and evaluation of detainees. This penitentiary technique based on a "knowledge-power" (savoir-pouvoir) (FOUCAULT, 1975: 32) was first created in Fresnes and then spread to all the other houses of supervised education in France. It was then extended to the major prisoners with the opening in Fresnes of a "triage center" which became a year later National Orientation Center (NOC) [START_REF] Derasse | Observer pour orienter et évaluer. Le CNO-CNE de Fresnes de 1950 à 2010[END_REF]. The creation of the NOC was intended to allow the evaluation and the orientation of adult detainees sentenced to long prison terms. Every six weeks, the center received groups of 60 to 90 detainees (CONSEIL SUPERIEUR DE L' ADMINISTRATION PENITENTIAIRE, 1952: 132). They were held in cells where they had to work and they were observed by several specialists of the prison of Fresnes. They were first subjected to a medical examination completed by radiographic and serological examinations. Then they were subjected to a psychiatric examination and the report included several items: hereditary history, medical history, education, professional activities, marriage and offspring and level of intelligence. The psychiatrist should then make proposals on the most appropriate prison regime to apply to the detainee and on his chances of social reintegration. The detainee was finally subjected to a psychotechnical examination to find out what type of work could be suitable for him. At the end of the session, a commission composed of the director of the NOC, doctors and psychotechnicians examined the detainee's file and chose the best prison where he could serve his sentence. From 1985, the NOC became a National Observation Center and continued to assess detainees before their incarceration but also during the execution of their prison sentence. Finally, following the law of 25 February 2008 on the detention of security

Fig. 1

 1 Fig.1 Division of the great district of men. Gallica. 1913

  , Prisons of Fresnes, report to the Minister of Justice(March 26, 1935), National Archives F1a 4577

Fig 2 .

 2 Fig 2. Outdoor activities of the boys. Geza Vandor, Musée national de l'Éducation. 1937

Fig. 3 .

 3 Fig. 3. Girl making artificial flowers in her cell. Geza Vandor, Musée national de l'Éducation. 1937

Fig. 4 .

 4 Fig. 4. Outdoor activities of the girls. Geza Vandor, Musée national de l'Éducation. 1937

Fig. 5 .

 5 Fig. 5. Boy working in a cell. Geza Vandor, Musée national de l'Éducation. 1937

Fig. 6 .

 6 Fig. 6. Kitchen. Agence Meurisse, Gallica. 1913

Fig. 7 .

 7 Fig. 7. School chapel. Agence Rol, Gallica. 1913

Fig. 8 .

 8 Fig. 8. Arrival interview of the pupil Raymond C. Departmental Archives of Val-de-Marne, 2Y5 694. 1935

Fig. 9 .

 9 Fig. 9. Raymond C. pupil's notebook. Departmental Archives of Val-de-Marne, 2Y5 694. 1935

Fig. 10 .

 10 Fig. 10. Medical notebook of pupil Auguste M. Departmental Archives of Val-de-Marne, 2Y5 670. 1934

Fig. 11 .

 11 Fig. 11. Social investigation of the pupil Jacques D. Archives of Val-de-Marne, 2Y5 670. 1934

Fig 12 .

 12 Fig 12. Medico-psychological examination of the pupil Georges L. Archives of Val-de-Marne, 2Y5 694. 1934

Fig. 13 .

 13 Fig. 13. Record of the pupil Gabriel C. Archives ofVal-de-Marne, 2Y5 670. 1936

Fig. 14 .

 14 Fig. 14. Pupils breaking rocks at the house of supervised education of Saint-Hilaire. Henri Manuel, National School of Prison Administration. 1930

Table5. Schedule of the girls

	Wednesday (17:00 to 18:00)	Baby care
	Thursday (17:00 to 18:00)	Music discs
	Friday (17:00 to 18:00)	Chat (causerie)
	Saturday (17:00 to 18:00)	Baby care
	Sunday (17:00 to 18:00)	Music discs
	Source: Seine, Prisons of Fresnes, report to the Minister of Justice (December 29, 1938), National Archives F1a 4577
	Schedule	Activities
	Monday (17:00 to 18:00)	Housework education
	Tuesday (17:00 to 18:00)	Reading

The prisons of Fresnes formed three parts: the great district of men; the special district of women; and the central infirmary of the prisons of the Seine.

Seine, prisons of Fresnes, report to the Minister of Justice(March 26, 1935), National Archives F1a 4577.

Seine, Prisons of Fresnes, report to the Minister of Justice (December 29, 1938), National Archives F1a

The meat dish, to which the pupils are now entitled every day except Friday, invariably consists of boiled beef. There is meat only on feast days (Christmas, July 14). Adolescents in the midst of physical development, many of whom are anemic, need more nutritious meat than beef presented to them throughout the year, after serving as a pot-au-feu 4 .The regulation also required the provision of fresh fish every Friday. But the inspector noted that the fresh fish was limited to two sardines in oil! Finally, a snack should have been served to the pupils at 4 pm. But it was impossible because of the meal schedules in Fresnes: lunch was served at 9 am and dinner at 4 pm. So this snack was added to the evening meal, in the form of a dessert. The only problem was that this dessert was often made of liverwurst! The girls' meals were often served cold because rations were transported by wagon in the open air from the kitchen to the school of preservation, which was about 200 meters away.4 Seine, prisons of Fresnes, report to the Minister of Justice (December 29, 1938), National Archives F1a 4577.

Seine, prisons of Fresnes, report to the Minister of Justice (December 29, 1938), National Archives F1a 4577.

Seine, prisons of Fresnes, report to the Minister of Justice (December 29, 1938), National Archives F1a 4577.

Seine, prisons of Fresnes, report to the Minister of Justice (December 12, 1936), National Archives F1a 4577.

Seine, prisons of Fresnes, report to the Minister of Justice (December 12, 1936), National Archives F1a 4577.

Seine, prisons of Fresnes, report to the Minister of Justice (December 12, 1936), National Archives F1a 4577.


M Bouassida

P De Buhant

L Dormieuxt

Bearing capacity of a foundation resting on a soil reinforced by a group of columns

Keywords: bearing capacity, design, failure, foot ings/foundations, ground improvement, reinforced soils

A new design method for a foundation on a soil reinforced by columns is described. A lower bound of the bearing capacity is determined within the framework of the yield design theory. It takes into account the three-dimensional nature of the problem and is applicable to a wide range of geometries. A parametric study on the improve ment of the bearing capacity as a function of the proportion of reinforcement, and on the strength characteristics, is presented. A complete analytical solution is given for the strength of a composite cell subjected to a triaxial loading, which provides an insight into the reinforcement mechanism.

INTRODUCTION

As a soil improvement method, the columns reinforcement technique has three purposes: to reduce the settlement of highly compressible soils such as soft clays, to accelerate the stage of primary consolidation, and to enhance the bearing capacity. This Paper is concerned with the last of these.

Common materials for columns are well graded clean sands, gravels or stones, hence the term stone columns [START_REF] Aboshi | The composer: a method to improve charac teristics of soft clays by inclusion of large diameter sand columns[END_REF][START_REF] Soyez | Methodes de dimensionnement des colonnes ballastees[END_REF]. However, the reduction of settlements rather than the bearing capacity of stone column reinforced foundations is generally the primary consideration when designing such structures [START_REF] Balaam | Effect of stone column yield on settlement of ngfd foundations in stabilized clay[END_REF][START_REF] Schweiger | Numerical analysis of stone column supported foundations[END_REF]. As far as bearing capacity is concerned, a remarkable per formance can be obtained by mixing the native soil with a low percentage of lime (the lime Le present article propose uoe methode de dimeo sionoement d'une foundation sur un sol renforce par colonoes, basee sur Ia theorie du calcul a Ia rupture. U ne borne ioferieure de Ia capacite por tante est obteoue eo tenant compte de Ia nature tridimeosioooelle de l'ouvrage. Elle est applicable pour uoe grande variete de geometries. On preseote uoe etude parametrique sur l'amelioration de Ia capacite portaote en fooction de Ia proportion de renforcement et des caracteristiques du materiau renfor�ant et du materiau initial. On s'ioteresse en outre a Ia resistance eo compression d'une eprou vette composite soumise a uo chargemeot triaxial classique, pour laquelle une solution complete est fournie sous forme aoalytique. Elle apporte un eclairage interessaot sur le mecanisme du renforce ment par coloooes. columns technique, Broms & Boman, 19 7 9). The strength of the column material can be described by a Mohr-Coulomb criterion (with appropriate friction angle and cohesion) while the native soil is modelled as a purely cohesive material.

Design calculations concerning the bearing capacity of such reinforced soils have mainly been presented for a trench-like reinforcement under the plane strain assumption [START_REF] Madhav | Strip footing on weak clay stabilized with granular trench or pile[END_REF][START_REF] Pruchnicki | Application de Ia theorie de l'homogeneisation aux colonnes bal lastees[END_REF] and for an isolated column in an axisymmetric condition [START_REF] Hughes | Reinforcing of soft cohesive soils with stone columns[END_REF][START_REF] Brauns | Die Anfangstraglast von Schottersiiu len im bindingen Untergrund[END_REF]. Few attempts have been made to deal with the case of a soil reinforced by a group of columns.

In the first part of the Paper, the strength of a composite cylindrical cell subjected to triaxial test conditions is investigated within the rigorous theoretical framework of the yield design theory [START_REF] Chen | Limit analysis and soil plasticity[END_REF]Salen�on, 1983Salen�on, , 1990)). The solution of this auxiliary problem provides an interesting insight into the way in which the native soil is actually reinforced by the introduction of columns. The second part is devoted to the case of a group of columns. A lower-bound solution for the bearing capacity of a foundation resting on reinforced soil is derived on the basis of the previously determined solution, and a quantita tive analysis of the gain of strength due to the reinforcement is developed.

COMPOSITE CELL SUBJECTED TO THE TRIAXIAL TEST

Consider a cylindrical composite cell ((J of height h made up of a circular reinforcement column surrounded by the native soil. The shape of the cross-section of the cell by any plane z = Z0 is not specified. To illustrate this, a square-shaped cell is shown in Fig. 1. The reinforcement column occupies the domain r < a and is composed of a frictional-cohesive material whose strength is described by a Mohr-Coulomb criterion f1(a) = a1(1sin cp)-a3(1 +sin cp)

-2CC cos cp � 0 (1)
where a 1 and a 3 are the major and minor prin cipal components respectively of the stress tensor a. Compressive stresses are taken as positive.

The domain outside the column (r > a) is occupied by the native soil, which is a purely cohesive material. Its strength is described by a Tresca criterion

(2)

Both criteria are expressed in terms of total stresses and are therefore relevant for a short term analysis. The ratio of the cross-sectional area na2 of the reinforcing column to the total cross-sectional area S of the cell is denoted by l'f, the proportion of the reinforcement. This composite cell is subjected to classical tri axial test conditions. The cell is in contact with two smooth and rigid plates located in the planes z = 0 and z = h. The lower plate is kept fixed while the upper one is given a vertical downward translation motion of velocity V. The force exerted by the upper plate on the cell is denoted by Q = -Qe. (Q > 0 when the vector Q is orient ed downwards). A uniform confining pressure pi s acting on the lateral surface, which is referred to as (}((j L. Thus, the loading of the cell is character-

Q = i=h a •• dS (3c) (3d)
The convention of summation over repeated sub scripts is adopted in equations ( 3a) and (3b) with i orj = x, y, z.

(b) U KA with V u. = 0 for z = 0; u. = -V for z = h (4)

Generally, a given load (Q, p) is said to be safe (in the sense of limit analysis or yield design) if it can be equilibrated by a stress field that complies with the strength criterion everywhere in the cell. The aim of this section is to determine the domain K of safe loads (Q, p) restricted to the region defined by Q � 0 and p � 0. Practically, this involves determining, for a given pressure p, the maximum value of Q such that (Q, p) is a safe load. This value is referred to as Q + (p)

Q + (p) = max[Q > 0, (Q, p) E K ]
The set of points (Q + (p), p) for p > 0 defines the boundary of K (Fig. 2). Q+(p) can be determined by a combination of the so-called static and kine matic approaches, which are now described.

Lower-bound estimate of K: static approach

The static approach simply involves imple menting this definition of K. Thus, in order to p prove that a given load (Q, p) belongs to K, it is necessary to exhibit a stress field a that is SA with (Q, p) (i.e. that satisfies equation ( 3)) and complies with the strength criteria (equation (1) in the domain r < a and equation (2) in the remain ing part of the cell (a < r)).

Static approach using a piece-wise constant stress field. A first static approach to K can be achieved very simply on the basis of a stress field a taking a constant value a • in the native soil and another constant value a < in the column, so that the equilibrium equation ( 3a) is automatically satisfied. The stress tensors a • and a < are chosen in order to satisfy equations (3b) and (3c), as well as (5)

The continuity of the stress vector acting on the interface of the native soil and the column implies the continuity of the following components (in cylindrical co-ordinates)

A solution to the requirements given is where KP = (1 +sin q>)/(1 -s in q>) denotes the usual coefficient of passive stress.

The vertical force equilibrated by the stress field a defined in equation ( 7) is therefore, from equation (3d

) i = h G zz dS = S[p(l -'1 + KP '1 ) + 2C(l -'1 ) + 2cc11.J KP] (8)
which provides a first lower-bound estimate for

Q + (p) Q + (p)/S � [p(l-'1 + K p l'/) + 2C(l -'1 ) + 2cc11.J KP] (9)
Improvement of the static approach. i= h G zz dS = S(p(l -'1 + I'JKp) + 2Cc '1 .J KP

+ 2C{l + 1'/[(KP-l) ln(x) ( 12
)
where x denotes the ratio c/a. Thus, a lower bound estimate of Q+(p) is obtained for any value

of c E [a, b], i.e. for any x E [1, b/a] Q : (p) � p(1-'1 + IJKp) + 2Cc '1 .JK P + 2C{1 + q[(K P -1) ln (x) -!(1 + x2)]} (13)
The maximum value of the • right-hand side of equation ( 13) is achieved for x = .j(K P -1). Thus, if 2 � K P � 1 + (b/a)2, the best (i.e. the greatest) lower bound of Q+(p) that can be obtained from this improved static approach is

2 � K p � 1 + GY Q+(p) >-p(1-+ K) + 2CC IK s ""' '1 '1 p '1-y p + C[2(1 -'1 ) + IJg(<p)] ( 14
)
where g(<p) is the following function of the friction angle <p g(<p) = (K P-1)

In (K P -1)-(K P -2) (15)
For K P � 2, the difference � = 11 Cg (<p) between the lower-bound estimates of Q + (p) given in equations ( 9) and ( 14) is a positive number. In other words the second static approach improves the first one as soon as <p � 19•5°, which is always satisfied in the case of a column made up of a purely frictional material, where the friction angle is usually of the order of 35° (stone columns). The second condition, K P � 1 + (b/a)2, will also be achieved for the usual values of <p, a and b encountered in practice. It is assumed below that this condition is always satisfied.

If K P � 2 (<p � 19•5°), which could correspond
to the case of lime columns, the best lower-bound estimate of Q+(p) provided by equation ( 13) is obtained for x = 1. For this value of x, the present static approach reduces to equation ( 9). It is now established by means of a kinematic approach that equation ( 14) when 2 � K P � 1 + (b/a)2 and equation ( 9) when K P � 2 give the exact limit load that can be sustained by the composite cell.

Upper-bound estimate by means of the kinematic approach

For any kinematically admissible velocity field U (see equation ( 4)), the kinematic theorem [START_REF] Chen | Limit analysis and soil plasticity[END_REF]Salen�on, 1983Salen�on, , 1990) provides a necessary condition of stability in the form of the inequality

0 � Q � Q+(p)=> "��' ••• (U) � "'f", •• (U) (16)
where "��' •• iU) and "'f", •• (U) denote respectively the work of the external forces and the maximum resisting work developed within the composite cell in the velocity field U under consideration.

The external forces acting on the cell are the force Q exerted by the upper plate and the lateral pressure p. Then

"'I' •• ,( U) = Q V + l pn • U dS Ja'lL ( 17
)
where n is the inward unit normal to o'th. For a continuous velocity field U, the maximum resisting work is defined by the integral of a volume density denoted by n(d) which is a func tion of the strain rate tensor d associated with U

"'f", •• (U) = L n(d) dQ (18)
For a given strength criterion f(a), n(d) is defined as

n(d) =max ( -uiidii• f(a) � 0) a ( 19
)
and may be interpreted as the maximum dissi pated energy per unit volume. Referring to the eigenvalues of the strain rate tensor d as d; (i = 1, 2, 3), the expressions of n(d) are (Salen�on, 1983(Salen�on, , 1990) ) (a) Tresca material (equation ( 2))

n (d) = {c( � 1 d ;l ) if � d; = 0 (20a) + oo otherwise (b) Mohr-Coulomb material (equation (1)) (d) = {_E__ (I d;) if I d; � sin <p D dd n tan <p ; ; ; + oo otherwise (20b)
Introducing equations ( 17) and ( 18) into equation ( 16), gives the general expression of the upper bound provided by any velocity field U

Q+(p) � � [I n(d) dQ + l -p n • Ud s ]
Ja'lL (a) ifr<a

n(d) = (KP-1)Cc/tan cp(V /h) (23a) (b) if r >a if KP � 2: n(d) = 2C(V/h)
if KP;;;. 2: r < a.J(KP-1):

n(d) = c[1 + (;y(KP-1)JV/h)
a.J(KP-1) < r:

n(d) = 2C(Vjh) (23b)
For the calculation of "11/" , •• (U), two cases have once again to be considered, according to the value of KP (a) KP � 2

"11/" , •• (U) = S[217CC.JKP + 2C(1-17)]V (24a) (b) 2 � Kp � 1 + (�Y 'if/" res( U) = S {217Cc .J KP + C[2(1 -11 ) + '1g(cp)]} v (24b)
where g(cp) is the function introduced in equation ( 14). The (resisting) work of the confining pressure in the velocity field defined in equation ( 22) can be easily derived from 27) and ( 28) suggests that the strength of the composite cell as a whole is equal to that which would be obtained by considering the same cell composed of an equivalent frictional and cohesive material defined by

f -Ue, • n dS = f div(Ue,) dO JV�L J� which leads to = f U'(r) + U( r ) dil (
(a) if KP � 2 KP • q = tan 1 2 4 + 2 = 1 -11 + 17Kp (29) (1t cp • q ) } ceq= .JKP • q [Cc11 .JKP + C(1 -17)] (b) if 2 � Kp � 1 + (�Y K eq = tan2( � + cp • q ) p 4 2 = 1-17 + 17KP + C[(1 -11 ) + hg(cp)]} (30)
In both cases, the equivalent cohesion c•q thus defined appears to be a function of the propor tion of reinforcement 17, and of the strength char acteristics C, cc and cp, whereas the equivalent

1•0 ,... .---------------------, 0•9 • •• • • • • • •• • ••• • • - •• •- • -• • • •• • • • •• ••• • • ••• • •• • • • . .. .
... .. _ o-a � •� •-• �-�. -• - ---- ---- --• -•--•-•-- ---- - - - -------- - --------------•• •••o•:-2 <..> - - --
Equations ( 29) and (30) show that the equivalen t cohesion is a linear ly increasing function of cc;c C«JfC = a( '7 , cpXCc/C) + P('7, cp) (32) turns out to be smaller than that of the original soil and decreases significantly for increasing values of the surface ratio '7 • Indeed, in this case, the reinforcement effect is ent irel y due to the fric tional prop erti es of the column material and can be quantitatively measured by the equivalent fric tion angle q>eq_ Likewise, Fig. 5 presents the variation of the equivalent friction angle q>eq as a function of lfJ for several values of the surface ratio ranging from 0•05 to 0• 3. q>eq is, as expected, an incre asing func tion of both '1 and q> which reaches 19° for '7 = 0•3 and q> = 38°.

BEARING CAPACITY OF A FOUNDATION

The problem of the: bearing capacity of a smooth and rigid footing on a soil layer n of limited depth d which occupies the region -d � z � 0 (Fig. 6) is now considered. This soil has been reinforced by a group of vertical columns in the region located beneath the foun dation. The area .91 of contact between the footing and the soil layer is located in the plane z = 0. The strength of the column material and that of the native soil are again described by equations (l) and (2) respectively. In this section, a truly three-dimensional stat ic approach is app lied to determine a lower-bound estimate for the bearing capacity Q -r. It is then shown how the same result can be obtained from very simple reasoning making use of the notion of equivalent material already intr oduced.

Three-dimensional static approach to Q + In order to achieve a result that could be applied to a wide range of practical situations, t he approach is not restricted to a particular sh ape of the footing or to a particular distribution of the columns below the foundation. However, for the sake of simplicity , all columns are supposed to A given vertical load Q applied to the soil by means of the footing is safe (i.e. Q:::; ; Q+) in the sense of yield design if it can be equilibrated by a stress field a that satisfies the conditions

Uxz = Uyz = 0 } (d) J(.ol) Uzz dS = Q Uxz = Uyz = Uzz = 0 (z = 0)-(d)
in the columns: f1(a):::; ; 0 in the native soil: f 2 (a):::; ; 0 (33)

In the construction of such a stress field, the result already obtained is used.

Static approach using a piece-wise constant stress fi eld. Around each column i a circular cylinder denoted re; of radius c = xa is defined, where x is a fixed parameter chosen in the interval [1 , D/2a] so that two different cylinders rei and rei cannot intersect each other (Fig. 7).

Improvement of the static approach in the case

In the region outside such cylinders rei, the stress field is as given in equation (35a). Inside each of them the construction of the stress field is the same as already given where 2C is substituted for pin equations (lOa) and (11) to ensure the contin uity of the stress vector across the boundary of rei. Thus

Interpretation based on the concept of equivalent material

The previous result may be interpreted based on the concept of equivalent material and related notions of equivalent cohesion c•q and friction angle tp•q, as introduced with reference to the composite cell. Indeed, adopting a heuristic point of view, one could be tempted to substitute the equivalent material thus defined for the heter ogeneous reinforced soil, so that the solution of the initial bearing capacity problem (Fig. 8(a)) would reduce to that of the equivalent problem shown in Fig. 8(b).

Proceeding further with this kind of reasoning, a simple static approach can be implemented on the latter problem by considering the following stress field Qu'? = 2d(CKP •q + c•q .jKP •q )

(42)

which could be interpreted as a lower-bound esti mate for the equivalent bearing capacity problem. Now, referring to equations ( 29) and ( 30), it appears immediately that Qu'? is identical with the lower-bound estimate Q16 given for the initial problem by equations ( 36) and (39) for KP ::::; 2 and KP ;;:, 2 respectively.

However, the concept of equivalent material should be handled very cautiously as it bears no significance apart from the particular situation where the reinforced soil is submitted to a triaxial loading (axisymmetrical about the direction of reinforcement). It does not apply, for example, to the description of the overall strength properties of the reinforced soil when subjected to off-axis

I 0 \Ill C'q; I I : i ! i ! i ! i ! i ! i ! i ! l l l l •J<c . � • • • • • "' • • •"' : , : ) . • . A' fi: [ i � • ... . . : . � ; • : • • : ' "" • . • . • • • � . • � • " • • • . • • • • • • • • • • • : . : : . • • • • i • • • : .. • .
: . . : • . . • . , .: . : • . .

• . : . • . • : . • forces. To attempt to implement a static approach on the equivalent problem, using stress fields with inclined principal directions, would lead to completely misleading conclusions. The idea of homogenization, which is only partially conveyed in the notion of equivalent material, can be rigorously formulated (de Buhan & Salen9on, 1990). It relies on the assumption that the reinforcing inclusions are regularly dis tributed throughout the soil mass, and that the spacing between two such successive inclusions can be considered small enough when compared with a characteristic length of the problem, such as the width of the footing. Such a theoretical approach has already been successfully applied in the geotechnical field to the design of reinforced earth structures (de Buhan, Mangiavacchi, Nova, Pellegrini & Salen9on, 1989). Contrary to what might be suggested by the preceding notions of equivalent cohesion and friction angle, it provides clear evidence of the anisotropic strength proper ties of the reinforced soil as a homogeneous material.

Quantitative assessment of the bearing capacity improvement

Although the exact value of the bearing capac ity remains unknown, and only lower-bound esti mates Qu(17) given by equations ( 36) and ( 39

R = 1 + � [(K p -2) + �c .j KP J (b) for KP ;;:, 2 (43) '1 [ C' J '1 R = 1 + l (KP-2) + C .jKP + 4 g(tp)
where g(<p) is as given in equation ( 15). As is apparent from this equation, the reinforcement �;�;�;�;�;�;�;�;�; � i i t� l � i [ �� " " " ����'0:'\t:..���\ Reinforcement by stone columns. The reinfor cing material is a compacted sand or gravel with a high friction angle (<p = 35°-40°) and practically no cohesion (CC/C = 0). The enhancement of the bearing capacity of a foundation lying on such a reinforced soil remains limited. Thus, for '1 = 0•2, the reinforcing factor can barely reach the value 1• 5. This theoretical result confirms the idea that increasing the bearing capacity is not the prime objective of this kind of reinforcement.

Reinforcement by lime columns. The constituent material is obtained by mixing a small proportion of lime with the in situ clay, thus increasing the strength of the latter by a factor of up to 20. Although significantly lower than in the case of the stone column material, a friction angle can also be measured. As can be seen from Fig. 9(b) (for '1 = 0•2), the bearing capacity can easily be increased threefold even if a zero friction angle is assumed.

CONCLUSIONS

The improvement of the bearing capacity of a foundation soil expected from its reinforcement by columns has been investigated in two stages. First, a triaxial loading test on a representative volume comprising a single column of reinforce ment was simulated. The strength properties of such a composite cell expressed in terms of com pressive force and confining pressure have been determined exactly through the combined imple mentation of the yield design static and kinematic approaches. Such a study paves the way for a possible experimental validation of the theoretical predictions.

Making use of the optimal stress field exhibited in the static solution of the composite cell model, a general lower-bound estimate for the bearing -------------------------------, (b) 40,--------------------------------30 capacity of a foundation-raft lying on a soil rein forced by a group of columns was then deter mined in the form of readily usable analytical expressions. The main interest of this estimate lies in the fact that it provides a reliable assessment for the three-dimensional bearing capacity problem, regardless of the shape of the founda tion and the geometrical arrangement of the columns. It is a convenient engineering design tool, allowing the efficiency of different technical solutions to be compared in terms of bearing capacity improvement. Moreover, it can be shown that the range of applicability of the lower-bound solution developed in this Paper (equations ( 36) and ( 39)) can be extended to take gravitational forces into account, and to the case where the strength properties of the column material are expressed in terms of effective stresses. However, it remains to be seen whether this lower-bound solution is sufficiently close to the exact value of the bearing capacity. This could be determined by implementing upper bound kinematic solutions for the problem, pro vided that the shape of the foundation and the exact distribution of the columns were specified, in which case a homogenization method should probably be used.

�20 10 R � 11 . • • • •• • • • • ••••••• • • •• • • ••••• ••••• •• ••• • • • • •• ••• • •• •• •• • • -. •• •. • • • • ••••• • •• • • •• • --••

  Fig. I. Composite cell under triaxial loading

Fig. 2 .

 2 Fig. 2. Domain K of safe loads (p, Q)

  a<r a" • = G ss 5 = P} u zz ' = p + 2C other G;i = 0 r<a a,/= a8/ = p } uzz " = KP p + 2Cc.JKP other a u = 0

Fig. 3 .

 3 Fig. 3. Three-zone axisymmetrical stress field

  now a velocity field of the form U = U(r)e� -(V/h)ze,, where the radial velocity U(r) is (a) for r < a (in the column) U(r) = trK P (� ) (22a) (b) for r > a (native soil) U( r ) = t[r + :2 (Kp-1)](�) (22b) This velocity field is kinematically admissible with V and remains continuous across the inter face r = a. The density n(d) can be computed from equation (20)

  Introducing these values of "11/" , •• (U) and equation (26) into equation (21), gives two different expres sions of an upper-bound estimate for Q +(p) according to the value of KP . It can easily be verified that each of them is equal to the corre sponding lower-bound estimate given by equa tion (14) or equation (9). Finally, a combination of a static and a kinematic approach have yielded the exact value of Q+(p), i.e. KP � 2 Q : (p) = p(1-'1 + Kp 17) + 2C(1 -17) + 2Cc11.JKP 2 � Kp � 1 + (�Y Q+(p) = p(1 -+ K ) + 2Cc .JK s of a homogeneous cell composed of a frictional and cohesive material (friction angle cp* and cohesion C*), the limit load Q * +(p) can readily be derived from the strength criterion (equation (1)), where cp and cc must be replaced by cp* and C* (28) with KP * = (1 + sin cp*)/(1 -sin cp*), which appears to be a linear function of the confining pressure p. A simple identification of equations (

  ............ ...... ...... ............ . ...

Fig. 4 .

 4 Fig. 4. Equivalent cohesion C.,. for ce = 0 plotted against friction angle tp

Fig. 4 pFig. 5 .

 45 Fig.4p resents the variation of the ordinate P('7, cp) at th e origin (cc;c = 0) for '1 varying between 0•05 and 0•3. P('7, cp) can be i nt erpreted as th e equivalen t cohesion in the case of a purely fric tional column materia). This equivaJent cohesion

Fig. 6 .

 6 Fig. 6. Rigid footing on a soil reinforced by a group of columns

  Consider first the stress field defined as follows (a) outside the domain located below the area Uxx = Uyy = 2C; Uzz = 0; other U; j = 0 (34) (b) below the area (d) in the native soil Uxx = Uyy = 2C; Uzz = 4C; other U; j = 0 (35a) in the columns Uxx = Uyy = 2C; Uzz = Kp2C + 2C\jKP; other u ii = 0 (35b) The continuity of the stress vector and the com patibility of a with the strength criteria are obvi ously satisfied. It can be easily verified that the stress field a defined above is statically admissible with the force Qu given by This equation provides a first lower-bound esti mate of Q+ /d which offers the advantage of being valid without restricting the geometrical characteristics or the strength parameters.

  KP ;::,: 2. Consider the following cases.

  (a) Outside the domain located below the area (d), the stress field is the same as in equation (34).

( b )

 b Below the area (d), let D be the minimal dis tance between the axes of two columns.

0 Fig. 7 .

 07 Fig. 7. Zoning of the mass of soil below the foundation

  (a) within the zone composed of the equivalent material (Jxx = (Jyy = 2C; } (40) u = 2CK eq + 2C eq I K eq zz p field obviously complies with the strength criteria of the original soil and of the equivalent material in each of the two zones. It equilibrates the force

  ) are available, it seems reasonable to assess the bearing capacity improvement due to the reinforcement by means of the ratio R = Qu( 'I )/Qu(O), where Qu(O) represents the value of the lower bound obtained for the unreinforced structure (Qu(O) = 4dC) (a) for KP::::; 2

Fig. 8 .

 8 Fig. 8. Bearing capacity problem: (a) initial; (b) equivalent

Fig. 9 .

 9 Fig. 9. Contour lines of the bearing capacity enhance ment factor R: (a) 1J = 0•1, (b) 1J = 0•2; (c) 1J = 0•3

NOTATIONa

  radius of the column d surface of the foundation C cohesion of the native soil c• cohesion of the reinforcing material cohesion of the equivalent material c•• D minimal distance between the axes of two columns K domain of safe loads (Q, p) applied to the com posite cell K P coefficient of passive stress of the reinforcing material of the reinforcement ({I friction angle of the reinforcing material ({I•• friction angle of the equivalent material

/ /


A Valinataj Omran

G Busco

S Dozias

C Grillon

J-M Pouvesle

E Robert

Distribution and penetration of reactive oxygen and nitrogen species through a tissue phantom after Plasma Gun treatment

Keywords: Plasma Gun, RONS, tissue phantom, target gap, nozzle shape

The transport and distribution phenomena of reactive oxygen and nitrogen species (RONS) into biological tissue following non-thermal plasma treatment have received much attention. The aim of this study is to test the efficacy of Plasma Gun (PG) on transporting the reactive species in an agar composed tissue phantom. RONS are generated on the agar gel by the plasma treatment and they continue to spread in the depth after plasma exposure. The amount of RONS after passing through the tissue phantom are strongly depending on discharge parameters such as distance to the target and capillary shape.

Introduction

Atmospheric pressure plasma jets have attracted a considerable attention for numerous applications in medicine and biology. The biomedical effects of plasma are strongly linked to the plasma generated RONS. An important issue in application of cold atmospheric plasma for tissue treatment concerned the penetration of RONS into biological tissue [1]. Because of the unique properties (soft and hydrated nature), RONS distribution and penetration can vary from tissue to tissue. While human skin is able to resist the penetration of many molecules, smaller molecules and species can pass through the corneal layer of the epidermis [2].

The tissue properties such as roughness and conductivity can influence the plasma treatment. Also, the gap distance (i.e., distance between the tip of the capillary and the target) may lead to variation of the plasma characteristics such as concentration of reactive species and gas temperature which can in turn change the effectiveness, distribution and penetration of the plasma on/in the tissue.

In the present work, we have focused on the RONS transportation and distribution in an agar tissue phantom after PG treatment. This agar model is very significantly different from real biological tissue but it can allow to get some precious information on transport of RONS in water containing media. Our results clearly and visually indicate that RONS are accumulated in the tissue phantom after the plasma treatment, and that continue to diffuse over and across the tissue. The influence of different nozzle shapes and gap distances on the distribution and penetration of RONS is studied.

Material and Method

Figure 1 shows a schematic of the experimental setup and two different nozzles. The plasma studied in this work is Plasma Gun (PG) composed of a coaxial dielectric barrier discharge reactor (Fig. 1) [3]. A 2 cm long powered electrode is set inside the 12 cm Pyrex capillary, 4 mm inner diameter and 6 mm outer diameter. In straight tube (Fig. 1 (a)), the inner diameter of the nozzle is 4 mm but in a tapered one (Fig. 1 (b)), the inner diameter at the end of the tube is 1.5 mm.

The helium flow rate through the reactor capillary is 0.5 slm. In this work, the PG device is driven by μs duration voltage pulses (positive polarity, repeated at 2 kHz). The applied voltage is measured using a high-voltage probe (Tektronix P6015A) and the discharge current on the agar target is measured using a 100 Ω shunt resistor. Also, discharge power is measured from V-Q Lissajous figures obtained using a 1.5 nF capacitor connected to the grounded electrode in series.

Preparation of agar target

The agar target is prepared by dissolving 1.5% agar powder in physiological saline solution. The solution is heated on a conventional heating magnetic stirrer until all the powder is dissolved. Agar gels of 2 mm thickness are prepared by pouring 4,4 mL of solution into an 53 mm diameter polystyrene Petri dish. The agar is stored at 4ºC for 12 hours before use. Square sections of approximately 15 x 15 mm were cut using a scalpel and removed with a lab spatula. These agar squares, used for RONS penetration experiments, were laid on the wells of a 96 well-plate overfilled with the recipient medium. In addition, the agar gel with KI-starch is used to evaluate the spatial distribution of oxidation reactions induced by PG. The agar target is prepared by dissolving 1.5% agar powder, 0.3% potassium iodide and 0.5% starch ((C6H10O5) n) in physiological saline solution. The KIstarch gel reagent can detect hydroxyl radicals (OH), atomic oxygen (O), O3, H2O2, and HO2 radicals.

H2O2 measurement in PBS solution

To measure the concentration of hydrogen peroxide passed through the gel into PBS after PG treatment, the Amplex™ Red assay kit was used. The Amplex red molecule is converted in the highly fluorescent resorufin in presence of H2O2 and peroxidase (HRP). For fluorescence intensity measurement, we used a microplate reader spectrofluorometer (Victor 3V, PerkinElmer), excitation wavelength at 531/25 nm and emission wavelength of 605/10 nm .

Not treated (NT) and helium treated gels were used as controls. Fifteen minutes after each treatment, the agar gel was removed and 50 µl of PBS containing diffused ROS were collected and transferred into individual wells of a 96well plate. The working solution of 100-μM Amplex red reagent and 0.2-U/mL HRP were prepared, following the manufacturer's protocol. Then, 50 μL of the working solution was mixed with the same volume of the loaded sample. The 96-well plate was incubated in the dark at room temperature for 30 min and then read with the microplate reader.

RONS measurement in PBS solution:

The RONS concentrations passing from the agar gel target to the PBS are measured using Dihydrohodamine 123 (DHR, Thermo Fisher Scientific). DHR is a nonselective reactant, reacting with H2O2 and peroxynitrite (ONOO -). It may react with other derivatives of molecular oxygen, including 1 O2 . RONS are detected by the formation of fluorescent Rhodamine123 after the reaction with nonfluorescent DHR. In this experiment, we used DHR 123 5µM in PBS. The solution (420 µl) was distributed in each well of a 96 well plate and the 15mm15mm agar gel targets with 2 mm thickness were placed over the top of the well so that the film contacted the PBS inside the well. Fifteen minutes after the treatment, the agar gel was removed and 200µl of the PBS containing the fluorescent probe were collected and transferred in a new 96 well plate. Fluorescence was measured using the microplate reader spectrofluorometer, excitation at 485nm and emission at 535nm.

Results

The waveforms of applied voltage and current of the plasma jet are presented in Fig. 2(a); it can be found that the peak applied voltage is 9 kV and the frequency is 2 kHz. Since the discharge power is applied in several short current pulses it is not a trivial task to calculate the power dissipated in the discharge. The method commonly employed for obtaining the discharge power is based on the charge-voltage Lissajous figure. The area of the closed Q-V loop for one period of the applied voltage is equal to the energy dissipated per one cycle. The mean discharge power is then simply the energy per cycle multiplied by the working frequency. A Q-V Lissajous figure of the plasma jet is shown in Fig. 2 (b) for tapered tube. It is obtained for applied voltage amplitude of 9 kV, 2 kHz signal frequency, gas flow of 0.5 slm and 10 mm, 15 mm and 20 mm distance between the tapered nozzle and the substrate. At these operating conditions, the area of the Lissajous figure for 10 mm, 15 mm and 20 mm gaps shown in Fig. 2 are 45 µJ, 32 µJ and 25 µJ giving a power of 90 mW, 64 mW and 50 mW, respectively. As expected, the amount of charge transferred to the agar target during a discharge cycle markedly decreased with increasing target gap. In addition, about the same value have been measured for straight tube. We used a KI-starch gel to show the oxidation reactions on the tissue surface [4]. As shown in Fig. 3, the color of the surface of the KI-starch gel changed from colorless to purple after plasma treatment. As expected, for different plasma conditions, an area with a strong purple color is observed in the center of the dish, which indicates that contact of the plasma with the tissue surface induces oxidation reactions caused by the generated ROS.

Images of the treated KI-starch gel dish were exported and processed in ImageJ (Fig. 4). Real dimension of the 8-Bit grey scale images were assigned in mm units using the set scale function. The intensity and the width of the purple spot were measured by tracing a straight line, passing through the center of the treated area. Data from each plot profile was exported in Origin software for analysis.

Concerning the plasma/gap distance (Fig. 3 and4), the central strong purple area exhibits a bigger radial expansion and a decrease in intensity when the gap is increased from 10 to 20mm. At small gap, the central point on the agar gel surface shows a less pronounced purple color. In a first approximation, we may think that this could be due to the hollow shape distribution of the ROS in the plasma plume [5], but this is not totally consistent with the diffusion of the ROS within the agar medium elsewhere. Additional study must be conducted to study the potential effect of ablated material in that area. Figures 3 and4 also show the strong influence of the nozzle shape on the ROS distribution. The concentration of the reactive species that penetrate into the agar gel is of particular interest for a better understanding of the transport of RONS in the biological tissue. The concentration of H2O2 and RONS in PBS after penetration through the agar film under the different gaps and different nozzles are shown in Fig. 5. It is found that the concentration of H2O2 and RONS decreases with the increase in the gap to 20 mm. The treatment distance affects the energy deposition on the target and also the concentration of the RONS in the PBS. However, the change in concentrations of RONS is not in proportion to the energy deposition. It seems that the contribution of the humidity in production of the RONS is more than the charge deposition. A closer treatment distances result in a greater level of evaporation creating a more humid atmosphere that may affect the RONS concentration. In addition, significant higher concentrations are obtained for plasma treatment with tapered nozzle than the case of straight nozzle. In tapered tube, the He gas flow produces a more localized point of contact with the agar film, together with a higher gas velocity, which might result in a localized increase in RONS density at the gas-surface interface that may further facilitate the transport of RONS into the agar film. Such enhanced RONS penetration was measured to be more pronounced using DHR probe than with the H2O2 selective Amplex red assay.

Conclusions

The knowledge of plasma-tissue interaction is important for the development of cold plasma technology for biology, medicine and food industry. In this work, we used a tissue phantom using an agar gel that acts as a physical target and barrier to investigate how the RONS produced by a PG can penetrate through the tissue. The concentration of RONS produced by a PG distributing on the surface and penetrating through the agar in different nozzle to target distance and nozzle shape were measured. As previously shown by other teams, we confirm that RONS have the ability to diffuse through few millimeter thickness of water containing medium. The produced RONS concentration is strongly linked to the application conditions such as distance to the target and shape of the plasma applicator inducing different conditions of air mixing with the helium flow. This emphasize on the fact that diagnostics of plasma plume and long lived species production must be performed in the closest condition to the real biological ones.

These results shows the potential of water containing tissue phantoms in determining an illustrative basis for specifying optimal and/or safe plasma conditions and provide consumer confidence in utilizing cold atmospheric plasma in medicine and cosmetics.

Acknowledgments

This work was supported by Cosmetosciences, a global training and research program dedicated to the cosmetic industry, located in the heart of the Cosmetic Valley, this program led by University of Orléans is funded by the Région Centre-Val de Loire, France

Fig. 1 .

 1 Fig. 1. Schematic diagram of the experimental set-up for a) straight nozzle and b) tapered nozzle c) RONS delivery through agar gel.

Figure 1 (

 1 Figure 1 (c) shows the setup which can allow to visualize the penetration of RONS through the tissue phantom. A tissue phantom, made of a 2 mm thick agar gel, is placed on top of a 96-well plate; each well is filled with phosphate buffered saline (PBS, pH 7.4). The plasma plume produced by the cold atmospheric PG contacts the top of the agar surface. Then, the concentrations of RONS generated in

Fig. 2 .

 2 Fig. 2. a) Voltage pulse applied on the electrode and current time evolution measured across the target with a 100 Ω resistor for 10 mm target gap b) Q-V plots measured at different target gaps for tapered tube.

Fig. 3 .

 3 Fig. 3. Photographs of the KI-starch gel after plasma-jet treatment for 120 s with different nozzles and plasmatreatment distances.

Fig. 4 .

 4 Fig. 4. Relative comparison of ROS concentration distributions on the KI-starch gel with different plasma treatment distances for a) straight nozzle and b) tapered nozzle.

Fig. 5 .

 5 Fig. 5. The concentrations of (a) H2O2 and (b) RONS of the treated PBS samples after penetration through the agar gel under the different target gaps for tapered and straight tubes. Vp: 9 kV, f: 2 kHz, He flow: 0.5 slm.