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Ground Vibrations and Air Blasts: Causes, Effects and Abatement. Rupture Radius Zones. Illustration of Seismic and Air Waves. Areas of Concern when Blasting. Transmission of Seismic Waves. The transmission of seismic waves is affected by Travel distance Ground attenuation
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Ground Vibrations and Air Blasts: Causes, Effects and Abatement.
Transmission of Seismic Waves • The transmission of seismic waves is affected by • Travel distance • Ground attenuation • Ground characteristics • Geology, discontinuities in rock • Wave type • Frequency • Angle of incidence • Source strength • Spherical spreading and • Elastic properties of the medium
Attenuation of Ground Motion Levels The total energy of the ground motion wave generated in the rock around a blast varies directly with the weight of charge detonated. As the ground motion propagates outward from a blast, the volume of rock subjected to the compression wave increases. Since the energy in the ground shock is distributed over successively greater volume of rock, the peak ground motion levels must decrease.
Scatter Since the rock masses are inhomogeneous, ground motion waves travel through strata of different acoustic impedance. Scattering of the ground motion waves, initiated at boundary discontinuities by reflection, lower the peak vibration levels. High frequencies are selectively attenuated while some lower frequencies are added to the ground vibrations. The presence of joints, fractures, faults and shear zones in the path of a ground motion wave also act to scatter the peak vibrations. Some of the lateral components of ground motions are lost as the wave travels a discontinuity. The degree of re-direction and dissipation of a ground motion wave is related to the nature and frequency of structural discontinuities in rock. The mechanics of ground motion attenuation attributed to rock properties tend to produce a ground motion wave train characteristic of the rock along the path of transmission. Thus, by determining site attenuation factors, the peak levels of ground motions resulting from future blast at that site can be predicted.
Structure Response Due to Ground Vibrations The cracking potential of blast vibrations can be discussed best in terms of the response of the structures through passing vibrations. Structures consist of many components and two of the most important are - Wall and Super structure skeletons. Super structure response is measured by a transducer attached to the corner of the structure while Wall response is measured in the middle of a wall. Super structure motions are those associated with the racking i.e. shearing and torsional distortion of the frame.
Contd. Mid wall motions are associated with the bending of the wall. They are primarily responsible for window rattling, picture frame tilting and the jiggling of the dishes. Generally, cracking from blast occurs where excessive stress and strain is produced within the plains of the walls or between the wall and its corners. Wall and super structure continue to vibrate freely even after the passage of ground motions. Wall motions tend to be larger in amplitude than super structure motions and tend to occur at higher frequencies during free vibrations than those of the super structure. Natural frequencies of walls range from 12 to 20 Hz and those of super structure range from 4 to 12 Hz.
Conclusions Vibrations below 25 Hz can excite high levels of mid wall motions (typically 4 times) and generate most of the secondary noise, rattling and other annoyances. Frequencies below 10 Hz are most serious for potential damage from structure racking. They produce large ground displacements and high level of strain. They also couple very efficiently into structures.
