Two examples of stress measurements by the H.T.PF. method

The Hydraulic Test on Preexisting Fracture (H.T.P.F.) stress determination method, based on measurements of the normal stress supported by planar fractures with various dip and strike has been applied in two different situations. In the first case the magnitude of the minimum principal stress has been evaluated near the flank of a mountain range, i.e. in a domain where the vertical direction is not a principal direction. In the second example, the variation of the ratio between the minimum horizontal principal stress and the vertical stress has been measured with some accuracy in a salty deposit. It is shown that the mechanical behaviour of this layered material is that of a composite material and that the larger the anhydrite content the larger the deviatoric stress. Consequences for hydraulic fracture propagation are discussed and illustrated by small scale injection tests (a few m 3 ).