Accurate determination of drilling parameters in time series for estimate of rock strengths

Geotechnical engineering usually produces drillholes in the ground for investigation and construction. Drilling is a rock-breaking process by applying normal (thrust) and shear (torque) force from the drill bit to the rock below the bit. These rock-breaking data can be obtained by digital monitoring and recording the drilling parameters through an instrumented drilling machine. However, there is no mature and standard method to determine rock strength properties (such as unconfined compressive strength, UCS, or tensile strength) from real-time monitored drilling parameter (such as thrust force, torque, rotation speed, drilling speed and specific energy). This paper presents a complete procedure to accurately determine each drilling parameter. More importantly, the specific energy develops nonlinearly with change of the thrust force, which is related to the UCS and tensile strength of the rock. This finding provides an insight into determining the UCS and tensile strength of the rock based on real-time monitored drilling parameters. In addition, novel test setups are demonstrated to determine the thrust force and torque from hydraulics pressures and rotation speeds. These setups can significantly reduce the sophisticated instrumentation cost for drilling monitoring studies. Three type rocks including granite, limestone and sandstone are used for the testing. The findings from this study provide supporting theories to upgrade drilling monitoring technique to a standard geotechnical testing method.