Modelling natural stresses in the arc syncline and comparison with in situ measurements

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