More information

More information

During a blast, or when operating heavy construction equipment, energy is released. This energy is dispersed in all directions; some of it, in the form of seismic waves (elastic waves). The propagation velocity of the seismic waves ranges from hundreds of meters per second in the ground to thousands of meters per second in rocks. Seismic waves disperse in a spherical manner in all directions, where the center of the sphere is the point of energy excitation at the energy source. 

As the seismic waves propagate, they cause vibrations that are measured in four ways: frequency, displacement, velocity and acceleration. In order to determine risk level to structures, one usually refers to the particle velocity of the vibration that is created at the structure's foundations or on the ground, near its base. The order of magnitude of this velocity is millimeters per second to centimeters per second. 

Apart from the level of energy produced by the energy source, the ground properties significantly influence the level of potential damage to a structure. Vibration measurement involves the positioning of seismographs, including a sensor called a "geophone". Geophones are tri-axial sensors; i.e., they are sensitive to vibrations in three directions in an orthogonal coordinates system. 

What is the scaling law? 

The scaling law provides a prediction equation of the vibration levels as a function of the distance from the point at which the energy is released, and as a function of the amount of energy released. 

 The released energy is dependent on the type of the energy source. In a blast, the level of energy released is dependent on the maximal charge of explosives per delay. In dynamic compaction, for example, there is a dependency on the tamper mass, the drop height and the number of drops. 

 The scaling law (merely terminology, and not a law in the legal sense of the word) is an equation describing the velocity of a particle as a function of the scaled distance (=SD). The scaled distance is defined as the effective distance that is dependent on the level of energy released. 

•  In a blast, the SD will be defined as the ratio between the distance from the point of the blast to the point of measurement and the maximum charge per delay. 

•  In dynamic compaction the SD will be defined as the ratio between the distance to the potential energy (the tamper mass and the drop height). 

•  The scaling law equation is obtained by carrying out experiments with a controlled energy release in various distances and energy level. In a blast – carrying out a number of trial blasts, at varying distances and using different amounts of charge per delay. In dynamic compaction – by carrying out trial drops from different heights, etc. 

•  The implementation of a linear regression analysis or power approximation on the data produce the scaling law equation. 
•  In a blast – The scaling law equation provides us with another equation, the charge per delay equation, which determines the maximum charge per delay as a function of the distance between the structure and the blast site. The equation allows optimizing the blasts and the quarrying work, without harming nearby structures as a result of the side effects of the blast. 
•  The calculation of the scaling law is also important for proper planning of work involving heavy construction equipment, in the vicinity of structures. The Site Law equation enables us to calculate the safe operation distances between the energy source and structures, and thus prevent damage. 

•  The scaling law is adapted to the topography, geology and geophysics of each particular work site. 

Air blast  

Blasting produces energy. Solids become hot gasses in a fraction of a second, expanding rapidly and producing force on the surrounding rock. Approximately 90% of the energy released in a blast breaks rock; most of the energy required to break the molecular bonds in the rock. The remaining energy – about 10%, is mostly converted into seismic energy, and also into pressure waves - air blast. Ashock wave is not acoustic noise, despite the fact that it is mentioned in noise regulations. It is not heard; it occurs in frequencies that are lower than the frequency that are audible to the human ear, however it can be felt as a result of a light vibration of glass panes in windows, in pictures, etc. (tremors are caused by air blast and not by the ground vibrations). Air blast is a sudden, sharp change in air pressure, and quickly dissipate and diminish. There are various types of overpressure air blast, which may be caused at various stages of the blast. 

In extreme situations, air blast may cause damage, such as the breakage or cracking of windowpanes. In any case, when dealing with the amounts of explosives used in quarrying and in boreholes (as opposed to blasts on the surface), air blast cannot cause damage to structures. Air blast can, though, harm human beings (and animals), when the intensities are much higher than permitted by regulations. 

Air blast propagate at a velocity of approximately 330 meter/second through air – significantly slower than the propagation velocity of the seismic wave, which is greater by almost one order of magnitude. Consequently, the effect of the vibrations will be felt first, and the effects of the air blast afterwards – at a certain delay (dependent on the distance). Air blast are strongly influenced by atmospheric conditions such as cloud cover, temperature, wind velocity and direction; as well as by topographic conditions. 

Air blast is a side effect, which, when high intensity is involved, comprises a serious problem. It can be minimized and prevented from reaching a level that would pose a risk. Air blast is measured using L type microphones; the dynamic range they operate in renders them particularly suitable for measuring air blast. The microphones are connected to vibration measurement device as a separate channel.