A research into the failure mechanism around horizontal boreholes excavated in sedimentary rocks

Tokai University*

Tono mine is one of the uranium mines of Japan and it has been the subject of extensive research regarding the nuclear waste disposal in rock. The top formation of the mine consists of sedimentary rocks ranging from sandstone to siltstone. An exploration adit was excavated by using the NATM, and this adit therefore was named as the NATM adit. At an extension of this adit, a series of horizontal boreholes were excavated for measuring in-situ stresses. The failure was observed in these boreholes, particularly at their crown and sidewalls. Following these observations some new boreholes were drilled into sedimentary rocks to investigate the causes and mechanism of failure of these non-supported boreholes. It is experimentally known that the rock exhibits some anomalous behaviour before the ultimate rupture, such as seismic electric signals, high acoustic emission activity. Furthermore, the behaviour of sedimentary rocks greatly depends upon their water content and some swelling, shrinkage, slaking may occur depending upon the environmental conditions. To investigate the coupled behaviour of sedimentary rocks in relation to clarifying the in-situ observations in the adit a research program was initiated, which consists of (1)Experiments (2)In-situ multi-parameter monitoring, and (3)Numerical studies. Experiments were concerned with, physical, electrical and mechanical properties of sedimentary rocks and their water-content migration characteristics. In-situ multi-parameter real-time monitoring involved electrical potential variations, acoustic emission activity, deformation, temperature and humidity variations in the boreholes and in the adit, which may be indicators for the possible failure around the boreholes. Numerical-studies were concerned with the conventional elasto-plastic stability analysis and also coupled analyses of water-content migration and stress field. The experimental studies clearly indicated that rocks around the boreholes exhibit time dependent ...