This study intends to assess the damage in the rock mass promoted by stress as long with rock blasting during an underground excavation opening. In order to achieve this objective, it was conducted the geotechnical rock mass characterization by core logging, representing data before the gallery development, and in situ mapping, performed directly at excavations. This characterization was held in two ore drives of the Cuiabá mine, an underground operation. In order to quantify the rock mass deterioration, it was calculated correlation factors between core logging and in situ mapping by using mean values of the GSI (Q') system for three lithologies: the Banded Iron Formation (BIF) and two types of schist (X2CL and XG). The results have confirmed the reduction of rock mass quality after the excavation opening. Moreover, it has demonstrated a higher depreciation in the X2CL and BIF lithologies, mostly caused by the stress fracturing formation.
The rock mass characterization is a critical step in order to design a geomechanical model as long with dimension support for underground excavations. So, it is strongly recommended the proper classification of any rock mass before the excavation development.
This characterization can be performed by two manners: in situ mapping and geotechnical logging from boreholes recovered by the drilling process, which is essential when it is not possible to map. Both methodologies can be based on collecting parameters from Q system (Barton et al. 1974) that can be used as inputs to define the Geological Strength Index - GSI (Marinos & Hoek, 2000).
Kaiser (2006) affirms that the rock mass behavior at deep underground excavations is usually different if compared with the previous conditions. This difference is promoted by the redistribution of stresses around underground opening, which is more favorable at hard rocks. In addition, the drill and blasting operation also cause damage to the excavation limits. This technique is used during the gallery development and the explosive energy affects the rock mass, resulting in fragmented blocks. According to Silva & Silva (2006), the shock waves generated after blasting cause damage to the remaining rock.