Abstract:

In order to investigate the beneficial effects of the combined drilling process of down-the-hole hammer and impact drilling, in this paper, the rock fragmentation process between rock and impact hammer head before and after the arrangement of small drill holes and under different hole diameters was investigated based on the HJC （Holmquist-Johnson-Concrete） constitutive model and the intrusion analysis method in the numerical simulation of LS-DYNA. At the same time, the rock fragmentation efficiency between the impact hammer drilling alone and the combined drilling process was quantitatively investigated in conjunction with the trial drilling test on site of the Jinyu Bridge in Quanzhou. The simulation results show that, with a constant number of small boreholes, as the diameter of the small boreholes drilled in the rock mass increases, the volume fraction of damaged rock mass increases. The increase in the diameter of the small boreholes reduces the energy required to break the unit volume of rock, which is beneficial for improving the fragmentation effect of the rock mass. The efficiency of energy transfer is related to the contact area between the impact hammer head and the rock. Under a constant number of small boreholes, an increase in the diameter of the small boreholes enhances the efficiency of energy transfer. However, beyond a certain value, an increase in the diameter of the small boreholes becomes unfavorable for the effective transfer of energy. During the process of the impact hammer impacting on the rock formation, the stress wave spreads out to the deeper part of the rock with time and gradually forms five  elongated fissures radiating from the center to the periphery. In addition, the results of on-site trial drilling tests show that the drilling efficiency of the combined drilling process can be improved by 54.47% on average compared with impact drilling alone. Comprehensive analysis shows that the implementation of the combined drilling process of down-the-hole hammer and impact drilling rig is conducive to improving the efficiency of rock crushing on site.

Key words: filled pile, impact drilling, down-the-hole hammer, LS-DYNA numerical simulation, rock fragmentation efficiency