At the Damang mine, conventional blasting is achieved by filling the blastholes with explosives (charge and detonator) to the required column and stemming with rock chippings in order to produce an effective blast. After, detonation is initiated, thus causing the chemical energy of the explosive to be released, and the compact explosive turning into a glowing gas with enormous pressure. To break up a large body of hard rock, charges are placed in a series of holes drilled so that as holes closest to the exposed surface are fired. The blasts create new exposed faces at the appropriate distances from the next set of holes in which charge firing is slightly delayed. Holes are fired in a predetermined order, within a millisecond. The mine also employs conventional blasting in the production blasting.

The process of drilling is carried out by compressed air powered drill machines. All holes are drilled based on specified blast design which includes hole depth of 9 m plus a subdrill of 1 m, 115 mm hole diameter, spacing and burden of 3.8 m × 3.8 m respectively and hole angle with respect to the batter face angle. The drilled holes are loaded with emulsion at an explosive column of 7 m. The holes are then filled and covered with the stemming materials to confine gases to enhanced fragmentation and to reduce the fly rocks. The hole diameters applied are 115 mm. Figure 1 shows how blast hole is charged using the conventional blasting technique. Figure 2 shows the applications of the conventional blasting technique at the Damang mine.

Figure 1. Blast hole charging using the conventional blasting technique ([4]).

At the Damang mine, the double saver plugs technique is employed where 2 saver plugs are placed at the bottom of the blasthole, thus reducing the charge column and eventually translating to a reduced amount of explosives. The saver plugs are used to create a bottom deck which result in reductions in flyrock, vibration, drilling costs, and the volume of explosives used. It creates a void at the bottom of the blast hole that when the hole is initiated, a pressure pulse will be directed into the void beneath the saver plugs followed by the high pressure gas front. The high pressure is sufficient to increase fragmentation. Despite the reduction in explosives consumption, fragmentation consequently improves as a result. This is because, the saver plugs are designed such that the blast energy is focused downwards upon detonation of explosives ([4]). Hence, much energy is generated downwards to cause higher fragmentation. Figure 3 shows the samples of saver plugs used at the Damang mine.

In order to ascertain the most efficient blasting technique, the Damang mine employed both the conventional blasting technique and the double saver plugs technique within the same pit over a period. Then during such period, a comparative analysis will be conducted to ascertain the best technique amongst the 2 techniques.

Figure 3. Samples of saver plugs used at the Damang Mine.

Abosso Goldfields Limited (AGL), Damang mine is located in the South-Western part of Ghana about 300 km by road from Accra, the capital town of Ghana. The mine is positioned at Latitude 5˚11'N and Longitude 1˚57'W. The mine is located 30 km North of the town of Tarkwa. The Damang concession covers a total area of 23,666 ha.

The mine experiences a tropical climate with an average annual rainfall of 2030 mm. The area is characterised by two distinct rainy seasons from March to July and September to November. Temperatures are high throughout the year and range from 23˚C to 28˚C. Although there may be minor disruptions to operations during the wet season, there is no operating or long-term constraint on production due to climate ([13]). The vegetation in the area is a mixture of tropical rain forest and semi-deciduous forest. Due to subsidence farming by the local population, the area has undergone massive deforestation and has altered the vegetation in the environments of the mine to secondary forest, shrub and cleared land. For this reason, there is no primary forest on the concession. The principal crops which cause this alteration include cocoa, yam, coffee, plantain, etc. ([4]).

The mineral orebodies of Damang are found within the Tarkwaian sediments, which constitute a significant part of the stratigraphy of the Ashanti belt in south-west Ghana. The Ashanti belt is a structure broadly synclinal that strikes to the north-east, consisting of sediments of the lower Proterozoic and underlying volcanics and metavolcanics and metasediments of the Birimian system. The Tarkwaian unconformably overlies the Birimian and is characterised by lower intensity metamorphism and the predominance of coarse-grained, immature sedimentary units ([4]). The Damang Gold Mine exploits both the oxide and the fresh hydrothermal mineralisation beyond all paleoplacer mineralisation. The hydrothermal mineralisation is found in the Tarkwaian sediments and is the only deposit of its kind located on the eastern side of the Ashanti belt in the south-west of Ghana ([4]).

The Damang mine employs the Open pit mining method for mining both the oxide and hard rock materials. The mining cycle involves drilling, blasting, loading and hauling. The mine employs the conventional shovel-truck system in the load and haul operations. The mining schedule is set up to blast 9 m and 6 m benches and excavate in 3 m flitches. Contractor equipment availability, drilling and blasting procedures, and spatial compliance to design and adhere to geotechnical standards are all important considerations in the mine. Road construction and maintenance, bench preparation, dust suppression, and erosion control are all supported by ancillary equipment such as bulldozers, graders, water trucks, service vehicles, and wheel loaders. Damang has a progressive reclamation strategy in which inactive regions are selected for rehabilitation by contouring and topsoil placement, seeding, planting, and fertilizing ([3]).

The data collection period for this study spans from May, 2022 to November, 2022 at Abosso Goldfields Limited. Both primary and secondary data were obtained in the study. Primary data consisted of the spatial coordinates of blast holes and post-blast pit floor surfaces acquired using the Trimble R10 RTK GPS. Secondary data were obtained from the mine’s Drilling and Blasting database and included blast design parameters (burden, spacing, bench height, hole depth, stemming length, and explosive column length), the number of drilled blast holes, explosive loading records, unit costs of explosives, and saver plug procurement costs. During the data collection stage, the number of drilled blast holes and their associated geometric design parameters were compiled for each blast event. The surveyed blast-hole data were used to determine the total number of holes per blast, which formed the basis for estimating explosive consumption and overall blasting costs. Table 1 shows the blast geometric parameters used for the conventional blasting technique and the double saver plugs technique at the Huni pit cutback. Table 2 shows the explosives parameters used for the conventional blasting technique and the double saver plugs technique at the Huni pit cutback.

Table 1. Blast geometric parameters used at Huni pit cutback.

Table 2. Explosive parameters used at Huni pit cutback.

The study conducted 2 forms of analysis, that is the pit floor analysis and the cost analysis. Cost analysis was performed using Microsoft Excel by comparing the total blasting costs of the conventional and double saver plug blasting methods, considering explosive consumption and saver plug costs.

The pit floor analysis was conducted using Surpac Version 2021 by generating 3-dimensional surface models from the RTK GPS survey data and comparing the resulting post-blast pit floor profiles to evaluate floor evenness and blasting performance under the two blasting techniques. Survey pick-ups were done at the blast areas after mining. The survey data was then processed using the Digital Terrain Model (DTM) created using Geovia Surpac 2021 software and saved. After that, contours of 0.1 m intervals were created, and the results reclassified based on the pit floor analysis requirements. At the Damang mine, the requirement for pit floor analysis was in the range of ±0.3 m. After the modelling was done to obtain the high, good and low pit floors, the Microsoft excel was used employed in the rating of the floors. A total of 3-blasts within the Huni pit were used for the pit floor analysis for both the conventional blasting technique and the double saver plugs technique. The floor analysis was done for all the 3 blasts in each case because all the blasts were within the same reduced level in each case.

The procedure for pit floor analysis using the Surpac software was:

1) Create boundary of the blast from the survey data (as-built or as-mined);

2) Use the boundary string to cut the area of interest from the survey data and save both the boundary and the floor area file separately;

3) Create Digital Terrain Model (DTM) using the floor file and save;

4) Use the saved Digital Terrain Model (DTM) file to create contours of 0.1 m interval and save;

5) Use ‘file tools’ on the Surpac toolbars to classify the contour file by string numbers;

6) After the classification, digitize the respective boundaries of the strings showing high, good and low areas;

7) Use the segment properties to determine the areas of the respective boundaries; and

8) Finally, use the Microsoft excel to do the rating of the floors.

Results for the conventional blasting technique for the 3 blasts were combined because all the shots were adjoining shots and were on the same reduced level. Thus the floor analysis was combined also since all the shots occurred on the same reduced level. There was a comparison of the resultant floor with the ±0.3 m standard floor ranges. The reduced floor for which the blast was taken was 900 m, indicating that the 3 m flitch taken will lead the reduced level to 903 m with the allowable ±0.3 m floor ranges. For the rating of the quality of the floor, reduced levels (z) beyond 903.3 m were classified as High (indicating higher exceedance from the standard). The reduced floor between the 903.3 m to 902.7 m were classified as Good (indicating a good blast falling with the planned ±0.3 m floor ranges). The reduced floor that was below 902.7 m was classified as Low (indicating below the planned ±0.3 m floor ranges). Results for the conventional blasting technique indicated that out of the 1256.6 m² total area blasted, 65.2 m² representing 5% of the pit floor was within the High range, 1191.4 m² representing 95% of the pit floor was Good and none of the pit floor was below the required range of ±0.3 m. Figure 4 shows the results for the floor analysis. Table 3 shows the results for the pit floor analysis for the conventional blasting technique at Huni pit cutback.

Table 3. Pit floor analysis for conventional method at Huni pit cutback.

Note: Where Z represents the reduced level.

Figure 4.Digital terrain model and contour for conventional blasting technique.

Results for the double saver plugs blasting technique for the 3 blasts were combined because all the shots were adjoining shots and were on the same reduced level. Thus the floor analysis was combined also since all the shots occurred on the same reduced level. There was a comparison of the resultant floor with the ±0.3 m standard floor ranges. Results for the double saver plugs blasting technique indicated that out of the 1252.7 m² total area blasted, 29.9 m² representing 2% of the pit floor was within the High range, 1222.8 m² representing 98% of the pit floor was Good and none of the pit floor was below the required range of ±0.3 m. Figure 5 shows the results for the floor analysis. Table 4 shows the results for the pit floor analysis for the double saver plugs blasting technique at Huni pit cutback.

Table 4. Pit floor analysis for double saver plug technique at Huni pit cutback.

Note: Where Z represents the reduced level.

Figure 5. Digital terrain model and contour for double saver plug technique.

Results for the pit floor analysis for the conventional blasting technique and the double saver plugs technique indicated that the double saver plugs technique outperformed the conventional blasting technique. Whereas about 98% of the floor using the double saver plugs technique was Good, 95% of the floor using the conventional blasting technique was Good. The floor that was High beyond the 903.3 elevation was about 2% in the case of the double saver plugs technique whereas that of the conventional blasting technique was 5%. In all, none of the techniques gave resulted in a Low outcome, that is elevations below 902.7 m. Therefore, it can be seen that both techniques resulted in good floor conditions but the double saver plug technique gives a better floor condition than the conventional blasting technique.

The cost analysis was aimed at evaluating the cost of each blast based on the techniques and also to evaluate the cost-effective technique amongst the 2 techniques. The analysis was done on 3 sets of blast each for the conventional and saver plug blasting techniques having the same number of holes. The parameters used for the blasts have been outlined below:

1) Blast Pattern = 3.8 m × 3.8 m;

2) Density of Explosives = 1.15 g/cc (or 1150 kg/m³);

3) Explosive Column (conventional method) = 7 m;

4) Explosive Column (double saver plug ie. two saver plugs in a hole) = 5.5 m;

5) Depth of hole = 9 m + 1 m (subdrill);

6) Diameter of hole = 115 mm or 0.115 m;

7) Stemming Height = 3 m;

8) Total length of saver plug = 1.5 m (0.75 m each);

9) Unit Cost of Saver Plug = $4.00;

10) Density (explosive) = mass (explosive) / volume (explosive);

11) Determining volume of explosive per 1 m drill hole depth;

12) Volume (Explosive) = πr²h, since drill hole is cylindrical in shape;

13) Volume (Explosive) = π (0.115/2)² × 1 = 0.01038689071 m³;

14) Therefore, the volume of explosives per 1 m drill hole depth of 115 mm (0.115 m) hole diameter is 0.01038689071 m³;

15) Determining mass of explosives;

16) Mass (Explosives) = Density (Explosives) × Volume (Explosives);

17) Mass (Explosives) = 1150 kg/m³ × 0.01038689071 m³ = 11.9449 kg; and

18) Mass of explosives per 1 m drill hole depth is 11.9449 kg.

The conventional blasting technique was used for 3 blasts at different dates having the blast numbers as Shot 912_903 m RL number 7, Shot 912_903 m RL number 18 and Shot 912_903 m RL number 33 respectively. Since the entire hole length is loaded with explosives in the case of the conventional blasting technique, the charge length for each hole was 7 m. The quantity of explosive is determined by multiplying the number of holes by the charge length and the mass of explosives per 1 m length of hole depth. Table 5 shows the cost analysis for the 3 blasts for the conventional blasting technique.

In order to ascertain the cost analysis for the double saver plugs technique, a total of 3 blasts at different dates having the blast numbers as Shot 912_903 m RL numbers 42 and 50, Shot 912_903 m RL number 49 and 50b and Shot 912_903 m RL number 51 respectively were used. In the double saver plugs technique, 2 saver plugs of length 0.75 m each are placed at the bottom of the holes. Thus the length of the hole to be charged will be reduced by 1.5 m, thereby making the charge length 5.5 m. The quantity of explosive is determined by multiplying the number of holes by the charge length and the mass of explosives per 1 m length of hole depth. Table 6 shows the cost analysis for the 3 blasts for the double saver plugs blasting technique.

Table 5. Cost analysis for the 3 blasts for the conventional blasting technique.

Table 6. Cost analysis for the 3 blasts for the double saver plugs blasting technique.

From Tables 5-7, it was observed that the cost for the respective blasts for the conventional blasting technique was higher than that of the double saver plugs technique. This is because of the reduced explosive columns in the double saver plugs technique. In total, the cost of the 3 blasts for the conventional blasting technique was $44,683.48 for the 334 number of holes whereas the total cost for the 3 blasts for the double saver plugs technique was $37,780.45 for the same number of holes. This shows an amount of $6903.03 difference in the 2 blasting techniques. Table 7 and Figure 6 show the comparative coat analysis for the 2 techniques for the 3 different set of blasts.

Table 7. Comparative cost analysis for the conventional blasting technique and the double saver plugs technique.

Figure 6. Comparative cost analysis for the conventional blasting technique and the double saver plugs technique.

It can therefore be deduced that the higher the number of holes blasted the greater the revenue generated by using the saver plug.

Also, Saver plug Assuming,

Average Number of holes = (114 + 130 + 90)/3 = 334/3 = 111.333 per day

The average number of holes blasted per day is 111.333 holes

Amount saved per hole using saver plug = total final amount saved from saver plug/ total number of holes blasted

Amount saved per hole using saver plug = (2356.124 + 2686.808 + 1860.098)/ (114 + 130 + 90)

Amount saved per hole using saver plug = 6903.030/334 = 20.667

This implies,

Amount saved by using saver plugs per day = 111.333 × 20.667

Amount saved by using saver plugs per day = $2300.919 per day

If the mine saves $ 2300.919 per day by using saver plugs, then

For 7 days (1 week) assuming blasting occurs every day of the week

$2300.919 × 7 = $16,106.433 per week

For 4 weeks (1 month)

$16,106.433 × 4 = $ 64,425.732 per month

For 12 months (1 year)

$64,425.732 × 12 = $773,108.784 per year

This implies that the Damang mine likely save $ 773,108.784 a year using double saver plugs at a hole diameter of 115 mm based on the observed average number of holes blasted for this study. Table 8 shows assumptions on amount saved by using double saver plug.

Table 8. Assumptions on amount saved by using saver plug.