The International Information Center for Geotechnical Engineers

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

6.0 Case Studies

Two case studies will be examined, one looking at the use of presplit blasting in a mine and the other comparing presplit blasting with smooth blasting of the final wall faces. The first study was done at the EkatiTM Diamond Mine by Peterson and the second study was done by Hu et al. of high rock slope excavations in China.

6.1. Blast Damage Mechanisms at EkatiTM Mine by Peterson (2001)

The EkatiTM Diamond Mine is located in the Northwest Territory, Canada. The study was done to determine the quality of the final pit wall using the presplit blasting technique. As a part of the study, vibration and gas penetrating equipment were set up and a total of 3 blasts were monitored. The blasts monitored included a production blast, a wall control blast, and a pre-shear blast. For each blast, a summary of vibration and gas penetration data along with assessment of the rock mass was carried out.

Beginning with the production blast, the data obtained from this blast can be used as a reference in comparing the other two blasts because no measures, i.e. presplitting, were used to dampen the affects. The monitoring equipment was set up in the rock behind the last row of blast holes and observations were taken of this remaining rock mass, treating it as a hypothetical final face. Figure 6.1 taken from Peterson (2001) is a post-blast cross-section pointing out the extent of damages. Fragmentation was observed 5-meters behind the last row of blast holes and additional cracks were observed in the intact rock up to 25-meters, with dense cracking observed up to 10-meters. The 25-meter cracks were expected to be a result of displacement along joints. Gas pressure data recorded a pressure drop of negative 67 kPa and the vibration data provided results with a peak of 1730 mm/s.

Figure 6.1

Figure 6.1: Cross Section Showing Varying Degrees of Damage from Production Blasting. Peterson (2001)

The wall control blast was done with a presplit fracture in place prior to the production blast. For data collection and monitoring, two rows of smaller, lighter packed buffer blast holes and then the rows of production blasting were used. Monitoring equipment was placed 5-meters back from the presplit contour. For this blast, damage could be seen in the remaining rock mass, specifically at the corners, and small wedge failures along the crest. The gas pressure data gave a peak value of negative 70kPa and the PPV values measured were considerably lower than the production blast, with one instrument measuring approximately 600 mm/s and the other a little less than 1000 mm/s.

The final blast measurement was done for the actual presplit blast. This blast generated a continuous fracture along the line of blast holes, with some cratering and displacement. Instrumentation was again placed 5-meters behind the presplit row. The peak gas pressure measurement was a positive 67 kPa and the velocities from the presplit blast measured 685 mm/s.

With this data, assessment of damages was done and comparison of production blasting versus wall control (presplit) blasting were made. First assessed was the PPV. While the above PPV values give general decreasing trend of PPV with each blast, as expected, a plot of PPV versus scaled distance was also done for the production blast and wall controlled blast. While this process is complicated and dependent upon statistical analysis, the results from this showed that the PPV values for the wall controlled blast were lower. However, at distances greater than 60-meters, a convergence of data was observed, signifying at larger distances presplitting has little reduction of PPV values. Summary of the data is shown in Figure 6.2.

 Figure 6.2. PPV data

Figure 6.2: PPV Data Comparison. Peterson (2001)

Vibration data was then used to calculate properties of the rock mass and these were, in turn, used to estimate fracture criteria based on PPV measurements and tensile strength of the rock. Based on a production blast, the model estimated new fractures with a radius of 5.4-meters, matching well with the observations from the production blast. The pressure data was also used to estimate damage. The underlying principal for this estimation is that an increase in volume will create a decrease in pressure, or as the rocks or rock joints expand, then the pressure measured will drop. This accounts for the wall-control blast and the production blast, but for the presplit blast, a positive pressure was measured indicating gas penetration from the blast. The greatest damage, though, is correlated with the dilation, and therefore the larger blasts performed in the production and wall control blasts.

Recommendations from these observations were given as well. Peterson concluded that vibrations do cause damage, as noted by the new fractures, but when it comes to instability of the final face, it is dilation and heave that cause the most harm at Ekati. The conclusion, then, is that it is not gas penetration that causes damage necessarily, but the movement of the blocks ahead of the gases. Reducing confinement in the larger blasts and distributing the blast energy over greater areas can reduce the subsequent heave associated, increasing stability of the remaining rock mass. Presplitting is beneficial to this end by allowing movement along the face instead of into it, and as noted in the presplit blast discussion, by distributing the charges over many drilled holes. While presplit blasting was beneficial, to improve stability of the final face requires proper design of all the blasts, especially those directly in front of the presplit face.

6.2. Comparison of Blast-Induced Damage between Presplit and Smooth Blasting of High Rock Slope by Hu et al. (2013)

Advance of computers has allowed for more modeling techniques over the last decade. This is noted by the use of numerical modeling comparing the damages of presplit and smooth blasting for the study done by Hu et al. Although both types of damage control can provide satisfactory results, the mechanisms for which the damage occurs from each are different, and therefore preventing the extent of the damage must be done differently. As Hu et al. states, damage is caused by the final face blasting, whichever method is chosen, and then the accumulation of damage from all other blasts.

In determining the damage, the model had to capture the initial compressive damage and then the ensuing tension damage. This was done by combining the two damages through mathematical relations and then providing them simultaneously in the model. Next, a system was implemented to capture the damage accumulation from each step. To do this, the stresses and strains from the previous step must be remembered by the elements for the next step while the rock section is removed, replicating conditions for an actual blast. Parameters for the explosives were found, as well as the attenuation rate of the rock. For the rock parameters, data from the Xiluodu Dam high rock slope was used, and similar to the Ekati mine, a plot of the measured PPV vs. scaled distance was created.  This was compared to the model constructed  PPV values. The above explanation is a very simplified process as to what was actually done for setting up the model, but for the purpose of the paper, focus will be placed more on the results.

With the models set up, the smooth blasting technique was looked at first. One should expect from the sequence of detonation that there will be little reduction in the cumulative damage from smooth blasting because it is detonated last. Looking at the model data, this proved true. Figure 6.3 taken from Hu et al. shows this with captures of each sequence of the blasting process. In the first production blast, the damage extends into the remaining rock mass and then, the second production blast adds to this with a much greater degree of damage. The buffer blasts do not change the amount of damage noticeably when compared to the second production blast, but a noticeable change of damage along the face is induced from the smooth blast. Noticeable damage from the buffer blasts is not seen due to the reduction of explosives and the buffers still being at great enough of a distance from the face. On the other hand, damage is seen from the smooth blast in a column due to the close proximity and geometry of the blast. From analyzing points in the mesh, it was concluded that damage to the main rock mass was a result of production blasting while the column directly surrounding the smooth blasts saw the greatest amount of damage from the smooth blast itself. Damage from buffer blasting had the least effect on overall damage.

Figure 6.3

Figure 6.3: Damage Process for Each Step of Excavation in Smooth Blasting. Hu et al. (2013)

Presplitting was then analyzed, and in contrast to smooth blasting, cumulative damage should be affected by the presplit blast because it is first in the blast sequence.  Figure 6.4 from Hu et al. shows the reduction of damage spreading to the main mass from the presplit.  The initial presplit blast causes damage along the final face in a column, but this does not extend into the main rock mass as the first production blast did in smooth blasting.  Then, when the production blasts are fired, the extent of damage is halted at the presplit contour.  However, when comparing the zones around the final face, the extent of columnar damage around the presplit face, noted by the red in Sequence I, is about 30% higher than that of the smooth blast.

Figure 6.4

Figure 6.4: Damage Process for Each Step of Excavation in Presplit Blasting. Hu et al. (2013)

The benefits and downsides for each technique must be taken into consideration when designing for a blast. The benefit presplitting provides by reducing the damage to the main rock mass from production blasting should be taken advantage of, but the 30% increase of damage around the presplit face should attempted to be reduced. To accomplish this, Hu et al. suggests a modified presplit where the first production blast is detonated, next the presplit, and then, the final production blast and the buffer holes. This has the supposed effect of limiting the much greater damage seen in the second production blast of smooth blasting while reducing the overburden in the presplit blast, and thus the confinement and ensuing columnar damage.

In comparing the High Rock Slopes study to the Ekati Mine study, it is interesting to note that for the high rock slopes no gas penetration sensors were used to collect data. This was because the objective of the paper was to compare smooth blasting with presplit blasting, but if a similar study were done incorporating gas pressures as well as vibration data into a numerical model, the results for the high rock slopes may have resembled those closer to the Ekati mine, where block heave was the dominating blast damage mechanism. However, the two geologic conditions of the two sites were also different and most definitely affected the studies to a degree and would need to be accounted for when comparing.

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