The prediction of damage to the rock mass is a very important factor to evaluate the quality of the excavation process in tunneling, so that it would allow the optimization of explosive charges utilized in successive blasting rounds, as well as lowering risks of instability from rock loosening, less support costs and water inflows. Upon developing a mathematical approach to evaluate rock damage from underground blasts, practical applications were accomplished to confirm it, both in tunneling excavations and underground mining. Examples of these studies are described in detail.
The detonation of explosives confined in boreholes generates a large volume of gases at high temperatures (2000 – 5000 °C) and high pressures (10 – 40 GPa). The sudden application of these effects to the cylindrical surface of the hole generates a compressive stress pulse in the rock, which may be a source of damage in the surrounding zone. The dimensions of that zone depend on the size of explosive charge detonated, rock's dynamic strength and density, wave velocity propagation, and vibration velocities transmitted to the rock mass.
When an explosive charge detonate inside a borehole several zones can be distinguished in the surrounding rock:
Zone of crushing,
Zone of radial cracking,
Zone of extension and expansion of fractures and
Elastic Zone, where no cracks are formed.
The damage that may occur in nearby rock happens behind the elastic zone (Fig. 1 left). Excavation of underground openings by rock blasting methods results in fragmentation with m a certain volume that should not exceed the perimeter established in the corresponding design. Deviations of that perimeter from their outside and inside limits are called overbreak and underbreak respectively, with the word backbreak used when overbreak is excessive. The more general concept of EDZ (Excavation Damaged Zone) applies to the fractured and fragmented rock volumes that surround a cavity upon blasting, also called DOW (Damage tothe Opening Wall) by Maerz N.H. et al., 1996 - see Fig. 1. These deviations are normally undesirable because they generate higher costs in the constructive process of the underground opening.
The factors influencing the magnitude of EDZ can conveniently be grouped into two categories, which are rock mass characteristics (geological factors) and explosive (blasting factors) as Table 1 summarizes. The blasting factors normally result from poor blast design and/or execution. Inadequately design of the perimeter part of the blast-round, i.e., countour holes is likely to result in EDZ, but the central part of the blast (cut) may also cause perimeter damage. Even a well-design blast can give poor results if it is not correctly implemented. Particularly important are the accurate location marking and drilling of blast-holes. Much over-break is caused by blast-holes that diverge or converge, and holes that fail to detonate on time and in sequence.