The International Information Center for Geotechnical Engineers

Summary of Surface Blasting and Damages with Analysis of Two Mitigation Techniques – Presplit and Smooth Blasting

2.0 - Mechanics of Rock Blasting

The basic principle behind rock blasting is the release of energy from a chemical compound, the explosive, in the form of expansive gases and heat inside a hole drilled into a relatively concentrated part of the rock mass.  This process is fairly complicated but can be broken into three main components: initial pressure buildup, wave transmission and air blast (USACE 1972).  The rapid expansions of gases in the hole can create pressures reaching a maximum of 100,000 atmospheres and occurs within milliseconds (USACE 1972). 

The immediate area surrounding the drilled hole undergoes crushing from the initial release of gases to a distance approximately equal to the initial radius of the drill hole (Langefors 1978).  In this vicinity, the compressive forces from the gas pressure are larger than the compressive strength of the material, thus crushing.  However, beyond this area, compressive forces are reduced below the compressive strength of the rock.  The crushed area is marked in Figure 2.1, taken from the USACE (1972) blasting manual. 

Despite the fact that the wave no longer results in failure of the rock through direct compression, the rock will still fracture. This failure type can be seen as the radial cracks extending in Figure 2.1 perpendicular to the circumference of the drill hole. As the compressive wave moves outward in a concentric ring, hoop stresses, as termed by the USACE (1972) manual, or tangential stresses, as Langefors (1978) refers to them, create tensile stress on the rock, noted as σT in Figure 2.1. Unconfined tensile strength of a rock is much lower than that of compression. Since the wave can maintain the minimum intensity needed for tension over a greater distance, even as the intensity falls well below that needed for crushing, the radial cracks extend farther than the crushed zone. To gauge the order of magnitude of this distance, a blast hole with a diameter of 40mm can have a range from tens of centimeters to a meter for radial cracking (Langefors 1978).

 Figure 2.1 - Blast Fracturing Zones Figure 2.1: Zones of Fracturing and Deformation. Adapted from USACE (1972)

Another failure mode present in a blast is the rebounding of the compression wave upon hitting a free face. As the wave hits a free face it is rebounded back, reversing the direction of the wave while the tail end of the same wave is still pushing forward, putting the rock in tension (USACE 1972). This creates spalling upon the free face. Free faces can be any pre-existing joint or bedding plane within the rock mass, fractures created by the neighboring blast, or the opening face. The concept of rebound is important when considering blast damages because it can be used as a means to reduce any further fracturing to a final face, which will be covered in greater detail later. Although fracturing and crushing are limited to the zones above, the shockwave produced by the explosion propagates outward in the form of primary waves and shear waves for thousands of feet.

Finally, the large build-up of gases must be expelled in what is the slowest of events to take place during a blast. The previous processes created the conditions needed for the expanding gases to actually break the rock (Langefors 1978). The expelling of the gases pushes the free face outward and expands the radial cracks, creating tension and further failure. If the blast does as intended, the cracks will extend to the free face and the burden will be propelled outwards completely. Ensuring that this entire process occurs is heavily dependent upon the spacing of the blast holes, bench height, and the thickness of the burden, among other factors. These will be covered further in Section 4.

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