The performance of drilling process is significantly influenced by intact rock and rock mass parameters. However, taking into account all these parameters for estimation of rock drillability is not an easy task. The main purpose of this study is to provide a practical convenient model based on rock mass characteristics and geological sampling from blast holes and operational factors. For this purpose, an empirical formula was developed using linear multiple regression to predict penetration rate in Sarcheshmeh copper mine in Iran. Seven parameters of rock material and rock mass including uniaxial compressive strength (UCS) of rock material, Schmidt hammer hardness value, quartz content, fragment size (d80), alteration, joint dipping as well as two operational parameters of rotary drill including bit rotational speed and thrust were taken into consideration for estimation of newly developed Specific Rock Mass Drillability (SRMD) index. Multiple linear regression analyses were used to develop a new equation for predicting the penetration rate of rotary drills. Comparison of measured SRMD with those predicted by multi-linear regression model showed good agreement with correlation coefficients of 0.82. This highlights the potential of multivariate regression model of rock mass characteristics in rotary drill performance prediction. However, the relationships obtained in this analysis should be considered valid only for geological settings similar to those of Sarcheshmeh copper mine.

1. Introduction

An accurate prediction of blast hole drilling rate helps to make more efficient the planning of drilling operations in a mine [1]. The drillability of rocks depends on many factors, in brief controllable and uncontrollable parameters. Bit type and diameter, rotational speed, thrust, blow frequency and flushing are the controllable parameters. On the other hand the parameters such as rock properties and geological conditions are the uncontrollable parameters. Many investigators have been tried to correlate drillability and various mechanical rock properties. Pfleider and Blake [2] concluded that a rough correlation exists between penetration rate and size range of cuttings, i.e. the higher the penetration rate, the coarser the particle size. Maurer [3] studied crater formation under an indenter and identified three distinct phases for the brittle rocks including crushing of surface irregularities and elastic deformation, extension of crushing zone beneath an indenter, formation of chips. Fish [4] developed a model for rotary drills with penetration rate directly proportional with thrust and inversely proportional with uniaxial compressive strength. Singh [5] showed that compressive strength is not directly related to the drilling rate of a drag bit. Selim and Bruce [6] developed a penetration rate model for percussive drilling using stepwise linear regression analysis. The model is a function of the drill power and the physical properties of the rocks penetrated. Clark [7] stated that drilling strength is mainly dependent on hardness and triaxial strength of rock. Rabia [8] determined surface area and rock impact hardness number from the results of percussive drill cuttings that failed to give correlation with the drill variables. Howart and Rowland [9] correlated rock texture with rock strength and drillability.

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