This report provides details of the vibrations caused by the explosive excavation of rock for the Penmaenbach tunnel which forms part of the A55 north Wales coast road improvement scheme. In particular comparison is made between predictions from blasting trials (by the transport and road research laboratory) and the vibrations actually measured during construction. The 'upper bound' vibration prediction established during the site investigation was proven an accurate working guide. However, variations in the pyrotechnic delay elements of the longer delay detonators caused vibration levels to be generally lower than expected and this effect is investigated and explained. The analysis and presentation of the vibration is discussed and the advantages of multiple regression techniques are apparent when data are not reasonably characterised by traditional 'square' or 'cube root' scaling methods. The factors which influence the input and propagation of energy from explosive sources are discussed in the context of conditions prevailing during construction works in general. The frequency distribution of the vibration is presented for various ranges during the trial, tunnelling and pre-splitting rounds. The frequency domain analysis showed that, at locations within 50 m of the charges, frequencies well in excess of 200 Hz were present and the implications of this finding on the specification of site measuring equipment are given. (Author/publisher)
Abstract