Analysis and determination of the stress field at the Olkiluoto site

Abstract

Abstract Hydraulic, overcoring and LVDT stress measurements have been conducted for characterizing the in situ stress state at the spent nuclear fuel repository at Olkiluoto site, located in Western Finland. The effect of foliation on the stress measurement data was investigated. The analysis of the hydraulic data conducted in foliated rock types shows that the hydraulically-induced fracture propagates along a foliation plane rather than developing along the true maximum horizontal stress direction as dictated by the theory of hydraulic fracturing. An integration of the stress measurement data considered to be reliable was conducted for establishing the most likely stress field and confidence intervals for the large (site) and small scales. At the depth of the planned repository, the stress magnitudes obtained for the two scales were found to be similar, thus concluding that the confidence in the derived stresses is high. The mean magnitudes of the major, intermediate and minor principal stresses are 22, 18, and 11 MPa, respectively. The estimated orientation of the major principal stress (WNW-ESE) agrees with the regional stress state.