The International Information Center for Geotechnical Engineers

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

5.0 - Damages and Mitigations

Brief examples have already been given as to what can be considered damage. Damages to the rock mass stem from the mechanics of a blast, mainly the production of waves and the work of expanding gases. Overbreak, excavation stability, and release of load are common types of damage. It should be noted, though, that in the case of mines and other slopes, deep seated failures are not typically attributed to blasting, but shallower failures are of concern (Hoek 2007).  In general, the affect blasting can have on the rock mass is shown in the Hoek-Brown strength criteria.  Equation 5.1 is the modified Hoek-Brown criterion for rock masses and Equation 5.2 is the equation for the material constant mb, or the constant describing the rock mass (Hoek 2002).  Within Equation 5.2 is the parameter D, disturbance factor, which for an undisturbed rock mass is equal to zero, while a bad blast can have a D value of 1, significantly lowering the strength (Hoek 2002).  This gives a numerical validation of how blast damage is typically taken into account for slope stability.

Equation 5.1: Hoek-Brown Failure Criterion. Hoek (2002)

Equation 5.2: Material Constant. Hoek (2002)

Overbreak can be described as the spread of the effects of a blast beyond the intended blast zone, or the propagation of cracks into the solid rock remaining. This is limited to approximately the burden distance and one can expect about an 80% reduction in rock strength at this distance (Peterson 2001). A greater distance and extent of damage, though, can be expected if planes of weakness are extensive in the rock mass.

Similar to overbreak is excavation stability. The vibrations coming from a blast can create movement along existing joints, and in so doing, weaken the bonds between rock blocks, up to hundreds and thousands of feet away from the charge (Peterson 2001).  Frequency of the wave is also important because a rock mass may experience more movement if joint spacing is equal to frequency. Figure 5.1. provides an example of how waves can create movement along joints with the coupling effect of frequency (Peterson 2001). In addition to vibrations, movement from the expansion of the rock mass from gases and pressure changes are equally likely to affect shear strength along the jointsBy reducing the shear strength of joints from movement and heave, a future excavation can create planar and wedge failures in the rock mass.

Figure 5.1: Wave and Frequency Effect on Joints. Peterson (2001)

Release of load damage is the process of spalling described in the mechanics of a blast.  Hoek (2007) provides a great analogy of this mechanism.  It is similar to dropping a steel plate on several rubber mats and letting the mats rebound.  Upon rebound, the steel plate is thrown upwards and the rubber mats separate as they, too, accelerate upward. This separation is the cause of tension failure in rock.  Release of load can form vertical cracks up to 55 meters beyond new faces and can cause instability of the rock face as a result of the cracks (Hoek 2007).

Limitation of these damages is vital to project life spans and safety.  The greatest reduction in damage, but by no means the easiest or most effective, would be to mechanically excavate the rock, eliminating the use of explosives altogether.  This has been done effectively in underground tunneling for many projects and in surface mining, specifically the Bougainville copper mine in Papua New Guinea (Hoek 2007).  Mechanical excavation simply cannot be used all the time because of factors ranging from cost and scale to strength of the intact rock.  Therefore, blasting is employed but this comes at the cost of potentially more damage.

5.1. Vibrations

The P and S waves generated from the blast propagate outwards until they eventually attenuate below values of any significance.  These waves are measured as peak particle velocities per second (PPV) either in mm or feet; the amplitude of the wave depends on two main factors among many, distance from the blast and weight of the explosives.  Relations have been made between PPV and strains in rock, and from this, the likelihood of new fractures forming or existing fractures slipping (Peterson 2001).   Thus, knowing how much the waves attenuate in rock and at what distance vibrations will be of concern is imperative in being able to gauge damage.  Monitoring devices are employed to measure this in real time and allow for calibration of blasting design.

Table 5.1 taken from Peterson (2001) offers a brief guideline as to the ranges vibrations should be kept at for typical mining operations to limit rock damage.  Keeping in mind that the amount of explosives detonated is one of two main parameters determining amplitude of vibrations, one can then see the importance of detonation sequence beyond ensuring even fragmentation.  A time period of 8-ms or 9-ms between detonations of charges is considered as a typical threshold around which charges are considered separate explosions (Dick 1982).  In addition to delay between charges, lower charge weights can be accomplished by smaller diameters and closer spacing, as is the case for blasting a final face.  Next, distance is taken into account.  The main charges in a blast, as discussed for smooth blasting and presplit blasting later, are drilled at a prescribed distance from the final wall face to allow for a dissipation of wave energy.

Other parameters affecting vibration have already been mentioned in Section 4. Excessive overburden and stemming can cause over-confinement of the charges, redirecting the blast energy into the rock instead of outwards. Besides poor design of the burden, excessive overburden can be a result of not allowing the previous row in multiple-row blasting to clear before detonation of the next row. This can be remedied by greater delay between rows. Similarly, charges placed in deep subdrilling cause over-confinement and will result in excessive vibration (Dick 1982).

 Rock Quality Threshold Limits Excavation in poor quality rock 200-600 mm/s  (0.66 – 1.97 ft/s) Excavatioon in good quality rock 600-2000 mm/s  (1.97 – 6.56 ft/s) Excavation with unfavorable jointing and potential for unstable blocks along walls 100 – 600 mm/s  (0.33 – 1.96 ft/s)

Table 5.1: Thresholds of PPV for Rock Quality. Peterson (2001)

5.2. Smooth (Cushion) Blasting

By reducing the weight of individual charges and decreasing the spacing for the final face, smooth blasting allows for less damage to the remaining rock.  The main objective of smooth blasting is to leave a small berm of rock between the final face and the main blast, and then blast the berm with tightly spaced, lightly loaded blast holes.  This forms a continuous crack between holes, giving an evenly contoured final face while keeping large blasts at great enough of a distance to limit damaging vibrations.

In general, a smooth blast will have dimensions with the hole spacing being less than the burden, as mentioned above. The height of a smooth blast is limited by accuracy of drilling, so typically, if a bench is over 60-ft, the blast is separated into two lifts, reducing the overall height and compounding error from drilling (USACE 1972).  Cushion blasting is a form of smooth blasting that takes advantage of the effect  air space has on reducing overall crack density, thus reducing damage to the remaining rock wall (USACE 1972).  For current reference and future reference, Figure 5.2 shows a typical blast design and sequence of smooth blasting and presplit blasting (Hu et al. 2013).

Figure 5.2: Excavation Sequence of Two Methods a) Smooth Blasting and b) Presplit Blasting. Hu et al. (2013)

5.3. Presplit Blasting

A similar form of protective blasting is presplit blasting. Presplitting has the intention of forming a free face barrier, between rock to be removed and rock that is too remain, by detonating a row of lightly packed charges prior to the main blasts along the final contour face.For a comparison with regular blasting, Figure 5.3 is a good representation where the presplit face is on the left and the more broken face of regular blasting is on the right (Hoek 2007). In an ideal presplit, a continuous fracture will form between each blast hole creating a final contour while minimal crushing and radial cracks occur, leaving half of the drill hole still visible and very little breaking of the final face, as seen in Figure 5.3.

One common design among many of a presplit row is a spacing of 24-inches center to center with 3-inch diameters (USACE 1972). Height of the drill hole for presplitting is similar to smooth blasting because deviation may result in the holes not forming a continual fracture. A common range for bench height is between 25-feet and 40-feet (USACE 1972).

Figure 5.3: Comparison between Presplit and Regular Blasting. Hoek (2007)

5.4. Mechanics of Smooth and Presplit Blasting

In order to better understand how smooth and presplit blasting affect the final face, one needs to know the mechanisms utlized.  Some of this was previously explained in the crack formation, but this can be defined further.  Model studies have shown that a charge in contact with the wall will create a region of denser cracking versus a charge with space between the wall and the charge.  The charge with space will form cracking of lower density but the cracks will have with the same length as the charge in contact with the wall (Langefors 1978).  Since only a single crack is needed to extend from one blast hole to the next, obviously creating more cracks than necessary will create a more damaged rock face.  Model studies have also shown that overcharging will have a similar effect of forming dense cracking (Langefors 1978). Again, because the objective is a single, continuous crack , the charge should be given space between the wall of the hole and be of smaller size to reduce overall cracking while still creating a continuous fracture.

Aiding in the process of a crack formation from hole to hole is the distribution of stresses.  A circular opening in elastic material put under tension will experience the largest stresses at the two points nearest and farthest from the imposed stress (Langefors 1978).  These two points guide the direction of the stress and explains why a blast, imposing tensional stress, will have cracks tending in a fairly straight line during presplitting and smooth blasting.  Finally, the design also affects how a fracture forms.  A ratio of spacing to burden below 0.8 is preferred because the crack formed will be straighter compared to a ratio above 0.8.   Delay between blasts is similar to the burden ration in that a shorter delay between blasts will create a more linear fracture line.  Simultaneous detonation will create the straightest face, while shot by shot blasting will form an erratic face. Typically a short delay time is used with results somewhere in between simultaneous and shot by shot blasting (Langefors 1978).