In-situ experiments on an excavation disturbed zone induced by mechanical excavation in Neogene sedimentary rock at Tono mine, central Japan

Abstract

The excavation disturbed zone (EDZ) is defined as the rock zone where rock properties and conditions have been changed due to the processes induced by excavation, such as fracturing, stress redistribution and desaturation. Based on our results of the in situ blasting excavation disturbance experiment at the Tono mines, the rock mass around the drift behaved elastically and the excavation damage induced by blasting was considered to be a primary factor in causing a change in rock properties. The excavation damage was considered to depend on the excavation method. So we performed an excavation disturbance experiment with mechanical excavation using a boom header in the Neogene sedimentary soft rock to compare the rock property change and width of the EDZ induced by blasting. Geological observation, rock mass displacement and vibration measurement were performed during the drift excavation. Seismic refraction and tomographic surveys, borehole expansion test and hydraulic test were performed before and after the drift excavation. A low seismic velocity layer of the EDZ with a width of ca. 0.3m and a seismic velocity of 65–70% of the intact rock was detected by a seismic refraction survey along the drift wall. Deformability and hydraulic conductivity were not changed in the rock mass up to 0.3m from the drift wall. The change in rock properties and width of the EDZ was shown to depend on the excavation method. In this geological setting, the mechanical excavation method is more effective in limiting the extent of the EDZ and change in rock mass properties.