Blasting is a powerful excavation method in terms of both production efficiency and economical costs, but its high environmental impact due to noise, vibrations, and potential damage to surrounding structures may limit its extensive application. The use of explosives as excavation tool is usually ruled by the national codes of practice, in which tolerable limits for induced motion are given for the different structure classes. In order to predict the vibrations induced in the ground at a given distance from the blast centre, attenuation laws, derived from either in situ measurements or analytical solutions of simple elastic wave propagation problems, are adopted. As the attenuation laws usually refer to homogeneous continua, they may not be adequate as a predictive tool for complex geological sites. In such cases, numerical analysis provides a valuable alternative, as the whole propagation history of stress waves could be simulated in principle, irrespective of the geological complexity of the specific site. To describe the progressive effects of underground blasting on the surrounding site, a finite element approach is presented. The explosion energy is translated in a time history of pressure at the boundary of the blast hole. Cracking of the nearby rock mass is modelled according to a cohesive crack model, while elastic behaviour is assumed for the non cracked rock mass and soil deposits. Propagation of stress waves from the blast hole is simulated by a time domain 3-D finite element analysis, which is able to provide the time history of all the relevant quantities describing the motion at any given distance. The numerical results can be post processed in order to derive attenuation laws for the most relevant quantities to which the codes of practice usually refer to, i.e., peak particle velocity and principal frequency of the vibration. The model is energy-conserving, thus the energy supplied by the explosive is correctly partitioned into fracture energy of the rock mass close to the blast hole, elastic energy providing the stress wave propagation and kinetic energy of the fragmented rock blocks. Numerical simulations of two literature case-histories are presented, and the numerical results are compared to the available experimental data. Experimental peak particle velocity could be captured remarkably. Principal frequencies for the rock mass could be reproduced as well. In layered sites, the ratio between the stiffness of the different media where stress waves propagate seems to play a key role in the determination of principal frequencies, while less influence is observed on peak particle velocities.
Blasting is a powerful excavat …
AUTORI: Jommi C., Pandolfi A. RIG ANNO: 2007 NUMERO: 2 Numero di pagina: 77