Rock stress measurements and the state of stress at an open-pit limestone mine in Japan

Abstract

A series of rock stress measurements is performed to estimate the states of local paleostress and present (in situ) stress at the Torigata open-pit limestone mine in Japan using the calcite strain gauge (CSG) technique and the compact conical-ended borehole overcoring (CCBO) technique, respectively. A set of back and forward numerical analyses is then carried out to evaluate the states of regional and local present stresses and the mine-induced rock slope stability using a three-dimensional finite element model and the state of local present stress measured by the CCBO technique. The results of rock stress measurements from this and other studies suggest that the maximum principal stress direction in the study area has rotated counterclockwise from the southeast (NW–SE trend) to the northeast (NE–SW trend) since the late Cretaceous period up to the present time. The results of the back analysis show that the directions of the horizontal principal regional tectonic stresses obtained by the back analysis are in good agreement with those of the horizontal principal local in situ stresses measured by the CCBO technique. However, the horizontal regional tectonic stresses are more compressive than the horizontal local in situ stresses. It results from the fact that the horizontal regional stress due to gravity is not considered in the back-analyzed horizontal regional tectonic stress, but it is included in the local in situ stress measured by the CCBO technique. The results of the forward analysis show that the local stress obtained by the forward analysis, especially its horizontal components, is in good agreement with the horizontal local in situ stress measured by the CCBO technique, and that the magnitude of the vertical normal stress increases more rapidly than those of the horizontal normal stresses with depth. As a result, the ratio of the horizontal normal stress to the vertical normal stress is largest at the current excavation level (ground surface) and decreases with depth. It means that the stress field within the mine-induced rock slope is strongly affected by the horizontal components of the local in situ stress.