Digital monitoring of rotary-percussive drilling with down-the-hole hammer for profiling weathered granitic ground

Abstract

Rock and geotechnical engineering investigations involve drilling holes in ground with or without retrieving soil and rock samples to construct the subsurface ground profile. On the basis of an actual soil nailing drilling for a slope stability project in Hong Kong, this paper further develops the drilling process monitoring (DPM) method for digitally profiling the subsurface geomaterials of weathered granitic rocks using a compressed airflow driven percussive-rotary drilling machine with down-the-hole (DTH) hammer. Seven transducers are installed on the drilling machine and record the chuck displacement, DTH rotational speed, and five pressures from five compressed airflows in real-time series. The mechanism and operations of the drilling machine are elaborated in detail, which is essential for understanding and evaluating the drilling data. A MATLAB program is developed to automatically filter the recorded drilling data in time series and classify them into different drilling processes in sub-time series. These processes include penetration, push-in with or without rod, pull-back with or without rod, rod-tightening and rod-untightening. The drilling data are further reconstructed to plot the curve of drill-bit depth versus the net drilling time along each of the six drillholes. Each curve is found to contain multiple linear segments with a constant penetration rate, which implies a zone of homogenous geomaterial with different weathering grades. The effect from fluctuation of the applied pressures is evaluated quantitatively. Detailed analyses are presented for accurately assess and verify the underground profiling and strength in weathered granitic rock, which provided the basis of using DPM method to confidently assess drilling measurements to interpret the subsurface profile in real time.