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IN-SITU CHARACTERIZATION OF THE BEHAVIOUR OF DEEP CLAY LAYERS

Objective

THE OBJECTIVES OF THIS PROJECT ARE :
- TO COMPLETE GEOMECHANICAL INVESTIGATIONS IN THE BOOM DEEP CLAY FORMATION, UNDER NATURAL CONDITIONS AND AFTER HEATING,
- TO DEVELOP LABORATORY AND IN-SITU METHODS TO STUDY DEEP CLAY LAYERS, AND TO COMPARE THE RESULTS OBTAINED BY THESE TWO APPROACHS.

THIS WORK FORMS A BASIS FOR DIMENSIONING THE STORAGE FACILITIES IN DEEP CLAYS. THE INTERESTING POINT IS TO STUDY THE TIME-LAG BEHAVIOUR OF THIS MATERIAL, AT AMBIENT AND HIGH TEMPERATURE.

THE SUB-CONTRACTORS OF ANDRA FOR THIS PROJECT ARE :
- BRGM (BUREAU DE RECHERCHES GEOLOGIQUES ET MINIERES) FOR THE SET UP, EXECUTION AND INTERPRETATION OF IN-SITU TEST,
- LMS (LABORATOIRE DE MECANIQUE DES SOLIDES DE L'ECOLE POLYTECHNIQUE) FOR THE LABORATORY THICK TUBE TEST AND FOR THE CONCEPTION OF BOREHOLE PROBES IN COOPERATION WITH SEDITECH-MAZIER AND BRGM,
SEDITECH-MAZIER FOR THE DESIGN AND FABRICATION OF BOREHOLE PROBES.

EXPERIMENTS ARE PERFORMED IN THE U.R.L. OF CEN/SCK AT MOL (BELGIUM).
The time dependent behaviour of deep clays was studied, in the laboratory or in situ, by means of tests of similar geometry, in order to get easy comparisions and to study scale effect. Cylindrical geometry was chosen as it resembles in situ works (tunnels, galleries) more closely.

A new test on the hollow cylinder was carried out. The experimental system permits experiments in which 3 loading parameters may be controlled independently. Different types of experiments can therefore be conducted to study various aspects of the mechanical behaviour of rocks.

In the particular case of Boom clay, the following conclusions were reached: time dependent behaviour is the major aspect of the mechanical response of the material to loading;water content influences both short and long term mechanical resistance of the clay. Consequently it was possible to obtain experimentally the short and long term convergence curves for the Boom clay.

In situ creep or relaxation dilatometer tests were carried out using new techniques and loading paths. It was shown that the time dependent convergence of boreholes can reach significant values, and is dependent on the direction of the borehole. The initial state of the stress was therefore concluded to be anisotropic.

The proposed constitutive model appears to be very suitable to explain the behaviour of the Boom clay, in view of the experimental results. In particular, it is shown that the scale effect is low for Boom clay.
1. EXPERIMENTS AT AMBIENT TEMPERATURE.
1.1. LONG TERM BOREHOLE DILATOMETRIC TESTS. THREE TESTS ARE CONSIDERED. TWO IN HORIZONTAL HOLES AND ONE IN A VERTICAL HOLE. SURVEYS OF EXPERIMENTS ARE STILL CARRIED OUT.
1.1.1. MEASUREMENTS.
1.1.2. INTERPRETATION.
1.2. LABORATORY TESTS ON THICK TUBE SAMPLES OF CLAY.
1.2.1. EXPERIMENTS.
1.2.2. INTERPRETATION.

2. EXPERIMENTS AT HIGH TEMPERATURE. DESIGN OF APPARATUS LED TO CONSTRUCTION OF THE FIRST PROBE IN 1988.
2.1. APPARATUS FABRICATION AND TEST IN LABORATORY. PRELIMINARY TESTS ON COMPONENTS OF THE PROBE ARE STILL UNDERTAKEN.

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Coordinator

NATIONAL RADIOACTIVE WASTE MANAGEMENT AGENCY OF FRANCE
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Address
Rue Jean Monnet 1-7
92298 CHATENAY MALABRY
France

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