Estimation of optimal drilling efficiency and rock strength by using controllable drilling parameters in rotary non-percussive drilling

Abstract

Fundamental rock-drilling studies are aimed at optimizing the drilling efficiency by identifying the optimal drilling conditions and rock drillability. In this study, a field-drilling test is performed using a rotary non-percussive drilling machine equipped with a new drilling-monitoring system to investigate the effects of drilling parameters on the specific energy (SE) and rate of penetration (ROP). During the drilling test, different thrusts (8.5–70 kN) and rotation speeds (40–400 r/min) are applied on a predrilled borehole re-injected with pure concrete. The magnitudes of the thrust, torque, ROP, and rotation speed are measured simultaneously by the new drilling-monitoring system to calculate the changes in SE. A modified SE model, which only includes a few controlled drilling parameters, is then proposed to determine the optimal drilling conditions. The results first show that torque has a linear relationship with thrust but shows no obvious changes with respect to the rotation speed. According to the energy balance principle in rock drilling and the relationship between SE and ROP, the optimal drilling efficiency with the maximum attainable ROP is then innovatively identified around the lower point of the second reduction in SE. In conclusion, the slope of the linear relationship between the thrust and penetration rate per revolution is only related to the rock properties and can be used to define the strength parameters of stratification by using the measured drilling parameters.