Effects of microfabric on drillability of rocks

Quantifying the rock microfabric is one of the most important research interests and challenges in the field of rock drilling and excavation. In this study, the relationship between quantitative parameters of the rock microfabric and the rate of top-hammer drilling was studied. For this purpose, a drilling operation was performed on five soft rock samples and three hard rock samples and drilling rates were recorded. Thin sections from the rock samples were prepared and micrographs of the sections were reviewed using AutoCAD software and digital boundaries of the grains were clarified and drawn. After preparation and filtering of the digitized thin sections, four basic properties of each grain, including maximum diameter, minimum diameter, area and perimeter, were calculated with the help of the software. These parameters were then analyzed statistically and used to obtain six indexes including equivalent diameter, grain compactness, shape factor, aspect ratio, interlocking index and grain size homogeneity index. Finally, the microfabric and drilling rate data were subjected to regression analysis and the significance of the relationships between the grain parameter values and drilling rate were explored. Drilling rate was found to increase with increasing equivalent diameter and shape factor. In contrast, drilling rate decreased with increasing grain compactness, aspect ratio, grain size homogeneity and grain interlocking index. Grain compactness showed the most significant relationship (R2 = 0.8) with drilling rate among the studied parameters.