Influence de l'heterogeneite et de l'endommagement de la roche dans l'interpretation des mesures de contraintes in situ.

摘要

One of the objectives of the current research is to expose the influence of the heterogeneity of the rock material on the interpretation of in situ stress measurements carried out with the modified Doorstopper technique. Generally, in stress measurements, homogeneity of the rock material is a widely made hypothesis that is completely in contradiction with the heterogeneous nature of rock itself. If taken in consideration in situ stress determination, the heterogeneity could lead to the application of the measurement technique to a wider range of materials. In fact, the current method of interpretation of the modified Doorstopper technique accounts for heterogeneity indirectly, based on laboratory experiments and extrapolation of numerical studies in 2D. Still, tridimensional models have not yet been studied to validate this method of interpretation and allow the complete understanding of the influence of a real heterogeneity, which is should be modelled in 3D.;The other part of this work deals with the analysis of the effects of the damage of the rock during the measurements of in situ stresses performed with the modified Doorstopper technique. This phenomenon happens in highly-stressed rock masses and is expressed by failures which could seriously affect the quality and the validity of the measurements, due to the stress redistribution at the bottom borehole. Through tridimensional numerical simulations with FLAC3D, we were able to reach the objectives that we had set.;On the one hand, eight randomized heterogeneous models were created and the results of the analyses were compared within them and with a homogeneous model. Along with the current "equivalent homogeneous anisotropic" method of interpretation, three new alternatives were used to determine the elastic parameters of the rock material. The findings indicate that the modified Doorstopper technique could be applied successfully in random heterogeneous media that are constituted by two phases, disregarding the size, shape and arrangement of the aggregates. As all four ways could be used to calculate the in situ stresses, they will add more quality and precision to the technique because the verification of the calculation could be made. Moreover, the three new approaches show that the number of different laboratory tests could be reduced: the Brazilian test may not be necessary since the biaxial compression and/or the uniaxial compression could be sufficient.;On the other hand, situations where rock damage is occurring at the bottom of the borehole, in the core and on the inner wall of the borehole were successfully modelled with numerical methods. By using a particular law of strain softening, it was possible to study the rock damage and its effects on the interpretation of measurements carried out with the modified Doorstopper technique. The results of the sixteen analyses run in both in elastic and elastoplastic modes suggest the following: (1) In the conditions that were studied and under the given parameters of strength and deformability, the occurrence of rock damage and the type of failures are linked to the orientation of the borehole axes in regard to the directions of principal in situ stresses. (2) The modified Doorstopper technique is applicable in situations in which disking is taking place because the stress redistribution is negligible at the bottom of the borehole before the beginning of coring. (3) For all other cases of rock damage, the current modified Doorstopper technique will yield erroneous results. In order to reduce or prevent rock damage, solutions that could be applied as an attempt to lessen those errors include the careful orientation of the borehole axes relatively to the principal stresses' orientations or the use of high pressure fluid during the boring of the hole and the measurement.