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The modified Doorstopper stress measurement technique consists in recording the recovered strains caused by stress relief at the bottom of a borehole drilled in a rock mass and converting the relieved strains into stresses by using the deformability parameters at the measuring point. This conversion is usually based on the assumption of a linear elastic behaviour. Hence, when plastic strains are induced at or near the borehole bottom prior to or during stress relief, the stress tensor calculated by assuming an elastic behaviour can be wrong. Plastic strains develop at the borehole bottom or in its vicinity, causing local damage. Core discing is probably the most visible manifestation of core failure, however, even if no visible signs of damage are present, plastic strains can be induced at or near the borehole bottom, affecting the recovered strains and the calculated stresses. Using Flac3D, the stress relief process for different 3D in situ stress states in an elasto-plastic cohesion softening friction hardening model is simulated in order to determine if the non-elastic behaviour can be detected from the strain recovery curves. Errors on the far-field stress state obtained by assuming an elastic behaviour are calculated. Finally, means of minimizing core damage in the presence of high in situ stresses are briefly explored and discussed. |
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