MODELING NATURAL STRESSES IN THE ARC SYNCLINE AND COMPARISON WITH IN-SITU MEASUREMENTS

In the Provence coal basin, the HBCM company mines lignite at great de pths. Several measurements of the natural stresses were carried out us ing flat jack and hydraulic fracturing. A strong stress anisotropy and high stresses were observed. To interpret these measurements, large s cale numerical modelling is used. This study presents a comparison bet ween the predicted natural stresses and those measured during field in vestigations. Three models are successively developed and loaded on th e basis of assumptions about current tectonics and historical tectonic s. Calculations lead to a better explantation of the orientations and magnitudes of the principal stresses which are not in contradiction wi th the observation of a NE-SW stress deviation in the vicinity of the area where the measurements were carried out. It appears that the natu ral stresses in the Arc depend mostly on the major geological structur es in comparison to the loading type. The results indicate that there is a relatively good match between predicted and measured stress magni tudes, and orientations. Finally, on the basis of a good estimation of the tectonic loading stages, it is possible to explain the distributi on of the natural stresses in the Arc syncline, even though the magnit udes of the tectonic stresses were unknown. (C) 1997 Elsevier Science Ltd.