Complete in situ stress determination in an argillite sedimentary formation

Andra has conducted a very comprehensive investigation on the in situ stress field within an argillite formation interbeded between two stiffer limestone formations, for the development of its underground research laboratory. A series of hydraulic tests has been conducted in boreholes located in the vicinity of the future site. Borehole and shaft convergence measurements were also carried out to analyse the mechanical behaviour of the rock. Throughout the stress measurement campaign, several borehole images have shown breakouts or induced fractures, that have been analysed in terms of in situ stress field, both in the argillite and in the limestone layers. All these reliable data highlight the high dependency of the state of stress on the rheology of the formation. A high ratio between the two horizontal principal stress components – ranging from 1.3 to 2.0 – has been observed in the limestones, whereas the maximum horizontal principal stress component has been found to be close to the lithostatic stress in the clay formation at the main level of the laboratory; i.e. at the 490 m depth.