Vibrations from blasting

1. Vibrations from blasting Where is blasting used? • Mining • Construction • Demolition • New constructions • Densification Kim André Larsen •Ph.D.Student• Department of Civil Engineering

2. Vibrations from blasting Disturbance from blasting • Vibrations • Air blast (Low frequency air vibration) • Fly rocks Kim André Larsen •Ph.D.Student• Department of Civil Engineering

3. Vibrations from blasting Parameters which affect vibration characteristics • In practically the same ones which influence the results of the blasts. • The surrounding geology and rock mechanics • In homogenous and massive rock masses the vibrations are propagated in all directions, but not in complex geological structures. • Thickness of soil overlaying rock. • Magnitude of the vibrations decrease rapidly with distance increase if soil overburden is present. • Large energy is used up in overcoming friction between particles and in displacing them. Kim André Larsen •Ph.D.Student• Department of Civil Engineering

4. Vibrations from blasting Parameters which affect vibration characteristics • Distance from the blast. The earth acts as a filter through which the lower frequencies pass. At long distances the ground vibrations will have more energy in the low frequency range. • Powder factor • More or less explosive can both give increased vibrations Kim André Larsen •Ph.D.Student• Department of Civil Engineering

5. Vibrations from blasting Parameters which affect vibration characteristics • Delay period • Distance between blast holes • Blast hole diameter • Blast hole depth • Distance from blast hole to free edge • Blast hole inclination Kim André Larsen •Ph.D.Student• Department of Civil Engineering

6. Vibrations from blasting Frequencies from blasting • Blasting in coal mines • Frequency between 4 and 12 Hz is very well propagated. • 90% of the blasts produces frequencies under 20Hz • Construction blasting • Creates higher frequencies with the most often happening at abaout 35-40 Hz Kim André Larsen •Ph.D.Student• Department of Civil Engineering

7. Vibrations from blasting Seismic waves from blasting • Wave front is displaced radially from the point of detonation • Rock blasts usually gives • Rayleigh waves • Love waves • Love waves are faster then Rayleigh (1.73) Kim André Larsen •Ph.D.Student• Department of Civil Engineering

8. Vibrations from blasting Seismic waves from blasting • Love waves: Particle motion is transvers to that of propagation • Rayleigh waves: Eliptical particle orbit, usually a motion that is contrary to the propagation direction • The Rayleigh wave carries about 70-80% of the total engergy, and dominates the surface ground movement at several hundred meters from the blast Kim André Larsen •Ph.D.Student• Department of Civil Engineering

9. Vibrations from blasting Measuring • Blasts with delay gives a complex interaction in both time and space because of the different velocities in which the waves travels. • Measuring must therefore be performed in three directions. Radial, vertical and transversal. Kim André Larsen •Ph.D.Student• Department of Civil Engineering

10. Vibrations from blasting Measuring • Rocks: Sensors is bolted to the rock • Unconsolidated soil: The sensor is buried in a box Kim André Larsen •Ph.D.Student• Department of Civil Engineering

11. Vibrations from blasting Measuring • Type of sensors • Velocity gauge measures the particle velocity • Acceleration seismograph Kim André Larsen •Ph.D.Student• Department of Civil Engineering