Shafts have a key role as access, transportation, and ventilation routes in mining, tunneling, and underground construction. In addition, the high-level radioactive waste disposal planned in Japan will require the shafts that are deeper than several hundred meters. In recent years the raise boring method is widely employed for shaft excavation in limestone quarries in Japan; however, this method possibly encounters serious troubles in fractured or weathered rock masses, such as collapse of shaft walls, stoppage of excavation, and breakage of tools. These troubles lead to an extension of construction period and an escalation of budget. Therefore, it is essential to understand the rock mass conditions around a shaft before or during excavation. In this study, an estimation method of rock strength from the excavation data of a tunnel boring machine (TBM) was applied to the shaft excavation with a raise boring machine (RBM) in a limestone quarry. Rock strengths were estimated from the thrust force and cutting depth and from the torque and cutting depth during shaft excavation with the RBM; the two strengths show a similar trend from the bottom of the shaft to the surface. In addition, the depth at the low rock strength was coincident with that at argillaceous or cracked shaft walls. The ratio of the two estimated strengths is probably an important index that alerts collapse of shaft walls and stoppage of excavation. This study validated the applicability of the estimation method of rock strength to the raise boring method.
Shafts have a key role as access, transportation, and ventilation routes in mining, tunneling, and underground construction. In addition, the high-level radioactive waste disposal planned in Japan will require the shafts that are deeper than several hundred meters. These vertical or inclined shafts are excavated with the drilling and blasting (D & B) method or the mechanical method, in a similar way to horizontal tunnel excavation. The D & B method is suitable for hard rock breakage and has been used for shaft excavation; however, the operation is non-continuous, and problems on safety, noise, and vibration possibly occur. The mechanical methods include the raise boring, the down reaming, the boxhole boring, and the shaft boring modified from the horizontal tunnel boring (Bilgin et al., 2014). As described in the next section, in recent years the raise boring method is widely employed in limestone quarries in Japan. This method requires no explosives and no rock supports under ideal rock mass conditions, and hence realizes safe and rapid shaft excavation. In contrast, excavation in fractured or weathered rock masses may cause serious troubles such as collapse of shaft walls, stoppage of excavation, and breakage of tools. Therefore, it is essential to understand the rock mass conditions around a shaft before or during excavation with the raise boring method. However, few reports have been published on the mechanisms and the excavation data of the raise boring method (Shaterpour-Mamaghani and Bilgin, 2016; Shaterpour-Mamaghani et al., 2016).
