Real-time rock strength determination based on rock drillability index and drilling specific energy: an experimental study

The geological condition of surrounding rock is essential for supporting design and disaster prevention of roadways in the underground coal mines. The main point of this research is to compare two methods, namely rock drillability index and drilling specific energy, of assessing the geological conditions of surrounding rocks through theoretical analysis and drilling tests. Firstly, this study suggested from a theoretical point of view that the use of drilling specific energy to assess rock strength at low penetration velocity has limitations and pointed out the superiority of rock drillability index. Subsequently, the Measurement While drilling Test System (MWDTS) is introduced and used to perform a series of drilling tests. Then, based on the monitored drilling parameters, the rock drillability index and drilling specific energy are calculated respectively, and feasibility of these two methods is compared. The experiment results indicate that although the drilling parameters vary greatly during the different drilling tests, the rock drillability index can reflect the rock strength well. However, the fluctuation and error of drilling specific energy at the low penetration velocities are considerable and cannot be used to assess rock strength. Besides, it was found that the error of using the rock drillability index to assess rock strength is smaller than that of drilling specific energy. The above results show that the application scope and the stability of the rock drillability index are better than the drilling specific energy in real-time assessment of rock strength.