Japan Atomic Energy Agency has developed a method for accurate evaluation of the distributions of actual in situ stress state at any point using limited results of in situ stress measurements from surface based investigations. We assumed that the actual stress components are formed by a combination of overburden pressure and plate tectonic force, and constructed two types of model, three-dimensional finite element and boundary element models that considered the presence of geological heterogeneities, such as variations in rock type and faults. Afterwards, as a validation of these models, we applied this method to in situ stress state evaluation for some locations around the Mizunami Underground Research Laboratory (MIU).
In the design of underground structures, it is important to incorporate information on the in situ stress field not only for design purposes, but also for the safe construction and operation of the underground structure. Generally, the scale of the structure will determine the stress measurement needs for quantitative understanding of the in situ stress field. However, obtaining sufficient measurements can be very difficult because of the limitations such as budget, schedule and size of underground construction. In the case of a high level waste repository that may occupy several cubic kilometers of underground space, understanding of the in situ stress field is important to minimize development of excavation disturbed zones around tunnels and shafts. Japan Atomic Energy Agency (JAEA) has been investigating the development of a methodology to estimate the in situ stress field based on limited stress measurements. In this paper, the numerical simulation methods developed to consider horizontal stress due to plate tectonics are introduced and the application of the method using in situ stress data from the MIzunami Underground research laboratory (MIU) are presented.