EXPERIMENTAL STUDY OF THE MECHANICAL BEHAVIOUR OF ARGILLACEOUS ROCK.

Objective

CHANGES IN THE MECHANICAL BEHAVIOUR OF CLAYS ACCORDING TO THE TEMPERATURE ARE NOT YET WELL KNOWN. IF, FOR THE SHORT TERM BEHAVIOUR STUDIES, ONE CAN ADMIT THAT THERE IS NO DRAINAGE EFFECT AND THAT A TEMPERATURE RISE LEADS, AT THE SHORT TERM, TO A CLAY VOLUME INCREASE, THIS CANNOT BE STATED CONCERNING THE LONG TERM, WHERE THE DRAINAGE EFFECT MUST BE TAKEN INTO ACCOUNT.
RECENT STUDIES HAVE SHOWN THAT, THANKS TO THEIR TEXTURE REARRANGEMENT, SATURATED REMOULDED POROUS SOILS, REPLACED IN A NORMAL CONSOLIDATED STATE, EXHIBIT, IN THE LONG TERM, A VOLUME DECREASE, WITH, AS CONSEQUENCES, SETTLING EFFECTS AND CRACKINGS.
IT IS POSSIBLE THAT THE LONG TERM RESPONSE OF THE CLAY AT SUCH THERMAL PROMPTING IS A FONCTION OF THEIR OVER CONSOLIDATION DEGREE : NORMAL CONSOLIDATED CLAYS WILL DECREASE, IN FACT, IN VOLUME UNDER THERMAL SOLLICITATION BUT THIS COULD BE DIFFERENT FOR THE OVER CONSOLIDATED CLAYS. THE AIM OF THIS STUDY IS TO BRING CLEAR AND QUANTITATIVE EXPERIMENTAL ANSWERS TO THESE QUESTIONS BOTH WITH A THEORETICAL INTERPRETATION OF THESE PHENOMENA.
THIS RESEARCH WILL BE PERFORMED WITH SCIENTIFIC SUPPORT OF THE CENTRE DE GEOLOGIE DE L'INGENIEUR (ARMINES-ECOLE DES MINES DE PARIS).
The long term effects of temperature variations on the volume and the texture of clay soils, as function of their initial petrophysical and petrographical characteristics and their preconsolidation state, were studied. From the experimental point of view, this study is based on the observation of the volumetrical deformation of samples subjected to thermal loading within 20 and 110 C temperature range with periodical measurements of their permeability in an oedometric cell. A complete textural study (mercury porosimetry and observations under a scanning electron microscope) before and after the experiment allows a continuous appreciation of the evolution of the texture.

A preliminary bibliographical review has shown that the evolution of clay characteristics and their uniaxial volumic strain under different temperature loadings may exhibit an expansive or compactive behaviour due to temperature increase. Most of the time, the behaviour is compactive under the preconsolidation pressure value. Some of the parameters such as water content, consolidation state, plasticity, mineralogy and time play a major part. These parameters have been criteria for the choice of 4 clays for the experimental phase.

The experimental device used was a classical oedometric cell with specific modifications due to the very long term tests at high temperatures.
The main results were;
a compressibility increase between 20 and 110 C;
a creep module evolution with temperature in the case of a loading at this temperature;
a noteworthy creep showing the importance of the time in the strain measurement;
an analogy between mechanical consolidation and thermal consolidation;
an highly irreversible behaviour during a cooling phase;
a modification of the structure material due to the temperature, but different and less important than modifications due to mechanical stresses.
The intrinsic permeability appears to be practically independent of the imposed thermal variations in the range of 20 to 110 C.
1. CHOICE OF THE SITES, AND SAMPLING OF CLAY
THE SELECTION WILL BE MADE FIRST CONSIDERING THAT THE SITES ANSWER TO SOME TEXTURAL GRANULOMETRIC CRITERIA (CLAYS SILTS) AND MECHANICAL CRITERIA (NORMALLY OR OVER CONSOLIDATED CLAYS).
AFTER THAT, WE WILL TAKE THESE MATERIALS UNDER PREDETERMINED CONDITIONS TO AVOID REMOLDING AND OVERCONSOLIDATION DUE TO SURFICIAL DESSICATION.
WE WILL DETERMINE THEN, IN A PRECISE WAY, THEIR INITIAL OVERCONSOLIDATION DEGREE AND THEIR TEXTURE, CONSIDERING THAT THESE TWO POINTS ARE ESSENTIAL FOR THIS STUDY OF THE BEHAVIOUR OF CLAYS TOWARDS THERMAL SOLLICITATIONS.

2. RESPONSES STUDY OF THE SELECTED MATERIAL TOWARDS THE THERMAL MECHANICAL SOLLICITATIONS
THESE RESPONSES ARE MAINLY TEXTURAL REARRANGEMENTS WHICH WILL BE STUDIED IN THE LABORATORY BY PRECISE TEXTURE IDENTIFICATIONS AND BY COMPARISONS BETWEEN INITIAL AND FINAL STATE.

2.1 BASIC PETROGRAPHIC, MINERALOGIC AND TEXTURAL IDENTIFICATION.
DIFFERENT PROCEDURES WILL BE USED : X RAY DIFFRACTOMETRY ANALYSES, ADSORPTION TESTS WITH METHYLENE BLUE, ATTERBERG LIMITS, POROSIMETRY, OBSERVATIONS WITH SCANNING ELECTRON MICROSCOPE, AND PERMEABILITIES.
2.2 DETERMINATION OF THE OVERCONSOLIDATION DEGREE
2.3 CONTROL THANKS TO BLANK TESTS OF THE THERMAL BEHAVIOUR OF THE EXPERIMENTAL APPARATUS USED
THE AIM WILL BE TO WELL SEPARE, IN THE EXPERIMENTAL RESULTS, THE INFORMATION CONCERNING THE CLAY GEOMECHANICAL BEHAVIOUR FROM ARTEFACTS DUE TO THE THERMAL BEHAVIOUR OF THE TESTS APPARATUS ITSELF.
2.4 CREEP TESTS IN OEDOMETER CELLS AND PERMEABILITY MEASUREMENTS
AN AXIAL COUNTERPRESSURE WILL BE APPLIED SO AS TO BE ABLE TO SCAN THE TEMPERATURE RANGE BETWEEN 20 CELSIUS DEGREES AND 120 CELSIUS DEGREES WITHOUT PORE WATER LOSS. THE STRESS WILL BE EQUIVALENT TO THE ONE IN SITU, AND WILL NOT BE LESS THAN 0.01 MPA.
IN CASE OF SWELLING DURING THE SAMPLE SATURATION, THE APPLIED STRESS WILL BE EQUIVALENT TO THE SWELLING PRESSURE. THE THERMAL PROMPTING APPLIED TO EACH SELECTED CLAY SAMPLE WILL CORRESPOND TO THE 4 FOLLOWING TEMPERATURES : 20, 50, 80 AND 120 CELSIUS DEGREES. DIFFERENT TEMPERATURE RISES WILL BE APPLIED, FAVOURING THE EXPLORATION OF PARAMETERS LINKED TO THE THERMAL PROMPTING AND TO THE CREEP :
A) DIRECT TEMPERATURE RISE (CREEP STUDY AT MIND TERM DURING 3 MONTHS, TESTS WITH THE FOUR MENTIONED TEMPERATURES).
B) DIRECT TEMPERATURE RISE, (CREEP STUDY AT LONG TERM DURING 9 MONTHS, TESTS AT 20 CELSIUS DEGREES AND 80 CELSIUS DEGREES).
C) PROGRESSIVE TEMPERATURE RISE AT 20 CELSIUS AND 120 CELSIUS DEGREES (FOR EACH TEMPERATURE LEVEL, CREEP STUDY AT MID TERM FOR 3 MONTHS).
D) PROGRESSIVE TEMPERATURE RISE AND THEN PROGRESSIVE DECREASE, CREEP STUDY DURING 3 MONTHS (FOR EACH TEMPERATURE LEVEL) : 1 TEST FROM 20 TO 50 CELSIUS DEGREES AND THEN FROM 50 TO 20 CELSIUS DEGREES; 1 TEST FROM 20 TO 80 CELSIUS DEGREES AND THEN FROM 80 TO 20 CELSIUS DEGREES; 1 TEST FROM 20 TO 120 CELSIUS DEGREES AND THEN FROM 120 TO 20 CELSIUS DEGREES.
A CERTAIN NUMBER OF THESE TESTS WILL BE REPEATED (TWO OR THREE TIMES) SO AS TO ASCERTAIN THE REPRODUCIBILITY OF THESE RESULTS.

TEXTURAL STUDY
THIS STUDY WILL BE DONE BY APPLYING THE MERCURY POROSIMETRY TESTS, SCANNING ELECTRON MICROSCOPE, PERMEABILITY TESTS FOR EACH SAMPLE (INITIAL AND FINAL STATE). THE COMPARED ANALYSIS OF THE WHOLE RESULTS WILL CERTAINLY GIVE A CONTRIBUTION TO THE THERMAL MECHANICAL BEHAVIOUR KNOWLEDGE RELATED TO THE TEXTURE AND OVERCONSOLIDATION DEGREE.
THE BASIC EQUIPMENT FOR THE TEXTURAL STUDY AND FOR THE MINERALOGICAL, PETROLOGICAL IDENTIFICATIONS AND CLASSICAL GEOTECHNICAL TESTS IS AVAILABLE. NEVERTHELESS, FOR THE THERMOMECHANICALTESTS IT WILL BE NECESSARY TO ADAPT EXISTING DEVICES.

Fields of science

Programme(s)

FP1-RADWASTOM 3C - Research and development programme (Euratom) on the management and storage of radioactive waste, 1985-1989

Topic(s)

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Call for proposal

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Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Commissariat à l'Energie Atomique (CEA)

EU contribution

No data

Address

Centre d'Études de Fontenay-aux-Roses
92265 Fontenay-aux-Roses
France

Total cost