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STUDY ON FRACTURING AND MICROFISSURATION OF GRANITE

Objective

DIGGING OF DRIFTS OR TUNNELS WITH EXPLOSIVES OR TUNNELLING MACHINES AND BORING HOLES INDUCE IN MASSIVE ROCKS (GRANITE) TWO KINDS OF STRESSES: THE FIRST ONE ARE STRESS WAVES AND THE SECOND ONE ARE VARIATIONS OF THE NATURAL STRESSES INDUCED BY THE UNDERGROUND OPENINGS.
EXPLOSIVES LOADS INDUCE CRACKS OF TENS OF CENTIMETRES, OR MICROCRACKS WITH THICKENESSES OF SOME MICRONS. DEPENDING ON THE VALUE OF NATURAL STRESSES AND ALSO ON THE SHAPE OF OPENINGS, SLABING OF WALLS AND BOREHOLES OR SPALLING OF CORES MAY OCCUR. SUCH FRACTURES MODIFY THE MECHANICAL CHARACTERISTICS OF THE MEDIUM AND CREATE NEW FLOW PATHS FOR THE UNDERGROUND WATERS.
FEW STUDIES HAVE BEEN MADE IN ORDER TO EVALUATE THESE POSSIBLE EFFECTS.
OUR PURPOSE IS TO DETERMINE THE EXTENSION OF FRACTURES AROUND MINE OPENINGS AND TO TRY TO QUANTIFY FRACTURING OR MICROFACTURING OF CORES VERSUS LITHOSTATIC PRESSURES.

1. SAMPLING. FRACTURES AND MICROFRACTURES WILL BE STUDIED ON SAMPLES OF GRANITE.
1.1. SAMPLES FROM 100 M DEPTH. STUDY OF EXPLOSIVE INDUCED FRACTURES IN DRIFT WALLS AND OF STRESS RELEASE IN ZONES FAR FROM EXPLOSIVE MINING; RELEASE OF STRESS WILL BE STUDIED BY OVERCORING.
1.2. SAMPLES FROM SURFACE TO 1000 METERS DEPTH (AURIAT BOREHOLE).

2. PREPARATION OF ROCK SAMPLES. SUCCESSIVE IMPREGNATIONS WILL BE MADE WITH DIFFERENT DYES IN ORDER TO IDENTIFY THE FRACTURES INDUCED BY EACH OPERATION.

3. STUDY OF FRACTURES. DIFFERENT SIZES WILL BE CONSIDERED:
. THICKNESSES FROM 10-3 TO 10-6 M
. THICKNESSES OR PORES LESS THAN 10-6 M.
SEVERAL METHODS WILL BE TESTED AND THE BEST ONES USED FOR THE STUDY.
3.1. VOLUME STUDY:
. OPTICAL EXAMENS ON THIN SECTIONS CUT FOLLOWING THREE DIRECTIONS
. ELECTRON MICROSCOPE
. CASTAING MICRO PROBE
. POROSIMETRY TESTS
3.2. SURFACE STUDY. FISSURES WILL BE OPENED AND THE TWO SURFACES LIMITING THE VOIDS WILL BE STUDIED (RUGOSITY).

4. INTERPRETATION. IT WILL BE A TENTATIVE TO FIND A RELATION BETWEEN THE VARIATION OF STRESSES APPLIED TO THE SAMPLE AND THE OBSERVED MECHANICAL EFFECTS OF FRACTURING.

Topic(s)

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Call for proposal

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Coordinator

Commissariat à l'Energie Atomique (CEA)
EU contribution
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Address
Centre d'Études de Fontenay-aux-Roses
92265 Fontenay-aux-Roses
France

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