Predicting powder factor in mixed-face condition: development of a correlation based on investigations in a tunnel through basaltic flows

Several approaches are available for estimating the powder factor in tunnel and bench blasting. These approaches consider those rock mass properties which are the most significant parameters in a rock-explosive interaction. However, they provide no information on the effects of mixed or multiple geological formations (mixed face) on the powder factor. Values for rock mass properties and the powder factor are collected along a length of 1075 m in the link tunnel of the Koyna Hydro-electric Project (Stage IV) in Maharashtra, India, which passes entirely through a mixed face and intersects more than one formation at a single location. A critical analysis of these data led to the development of a correlation for predicting the powder factor in the tunnel, considering uniaxial compressive strength, Bartonʹs rock mass quality (Q), the thickness of each formation, and the effects of interformational contact surfaces.