Drilling Parameters in the Evaluation of Rock Mass Quality

Abstract

The production of a geotechnical model-based on-site investigation and surface mapping is particularly challenging for tunnels due to the depth at which the excavation is performed. Besides the lithology and the weathering grade, the main factors influencing the underground excavation are related to the rock mass structure, namely joints and major faults. The knowledge in advance of the conditions to be found during the excavation usually saves time and money and increases safety. In the present work, the excavation of two large tunnels was studied, aiming to obtain correlations between the rock mass properties and the parameters obtained by the drilling machine ahead of the excavation front. The study included the geological mapping of the excavation surfaces, the determination of the intact rock strength and the record of the drilling parameters, namely the penetration rate, feeder pressure, pressure in the hammer, rotation pressure and strike pressure. The variability of the results was analysed and the Pearson coefficient $\left| {\text{r}} \right|$ established for each parameter in order to find the most accurate correlations. It was concluded that, for deep structures, monitoring these parameters constitutes an essential complement to the previous geotechnical site investigation, contributing to a better definition of immediate support measures to apply along the excavation works.