Development of a new classification system for assessing of rock mass drillability index (RDi)

The drilling process and its results are affected by various parameters of the rock material and rock mass. The effects of rock material have been emphasized in various studies; however lack of perfect knowledge of rock mass structural parameters may lead to unpredictable results. This paper presents a new classification system for specifying the rock mass drillability index (RDi). For this purpose, six parameters of the rock mass, including texture and grain size, Mohs hardness, uniaxial compressive strength (UCS), joint spacing, joint filling (aperture) and joint dipping have been investigated by physical modeling and rated. Physical modeling in particular has been used for investigating the effects of joint characteristics on drilling rate. In the proposed RDi system, each rock mass is assigned a rating from 7 to 100, with a higher rating corresponding greater ease of drilling. Based on the RDi rating, the drilling rate may be classified into five modes: slow, slow-medium, medium, medium-fast, and fast.