April 07, 2020

Controlled blasting of open pit near the slope

With the downward extension of the open pit, the fixed final slope is getting higher and higher, and the stability of the slope is becoming more and more prominent. Although there are many factors affecting the stability of the slope, the impact of blasting vibration on the direct damage of the slope is also insoluble and neglected. In order to protect the stability of the slope, in addition to taking other effective measures, the blasting of the adjacent slope should be strictly controlled. The main methods of controlling blasting near the slope are: pre-splitting blasting, smooth blasting and buffer blasting.
First, pre-cracking blasting
The so-called pre-cracking blasting is to design a slope boundary line along the open pit mine, drilling a row of densely drilled holes, loading a small amount of explosives in each hole, and detonating before the main blast hole of the mining belt is unexploded, thus exploding a certain Width (generally greater than 1 to 2 cm) and through the pre-cracks of each borehole. Since this pre-crack separates the mining belt from the slope, the seismic wave of the subsequent mining belt blasting is absorbed in the pre-cracking zone and produces a strong reflection, so that the seismic wave intensity transmitted through it is greatly weakened, thereby reducing The seismic effect reduces the damage to the slope rock mass, improves the smoothness of the slope slope, and protects the stability of the slope.
Compared with ordinary production blasting, pre-splitting blasting generally has a damping effect of 40% to 60%. Found 330 engineering and Daye iron ore showed that due to the pre-crack, blasting vibration can be reduced by 50% -82%, the extent of damage can be reduced by 40%, the surface roughness of the pre-split ± 20cm, the degree of retention walls of the hole For: hard rock is greater than 80%, soft rock is greater than 50%.
(1) Trenching pre-splitting blasting
In the open pit mine pit line trenching, along with the trench digging, pre-split blasting is performed to form a fixed side gang. There are two types of pre-split holes: vertical holes and inclined holes. Due to the limitation of drilling conditions near the slope, vertical drilling is mostly used. Figure 1 shows the form of cloth holes for pre-splitting blasting using inclined drilling when Nanshan Iron Mine is digging trenches.

Fig.1 Drilling arrangement of trench pre-splitting blasting
1-pre-cracked hole; 2-assisted hole; 3-buffer hole
(2) Pre-splitting blasting near the slope
The pre-splitting blasting near the slope is mainly used for the slope formed by the expansion, and is generally used when the working line is near the slope of 15-20m. The drilling arrangement of pre-cracking blasting is shown in Figure 2.

Figure 2 blasthole arrangement of pre-splitting blasting near the slope
1-pre-cracked hole; 2-buffer hole; 3-main blasthole; 4-mining boundary line
Most of the main blast holes use millisecond blasting to minimize the maximum amount of charge. In order to ensure the shock absorption effect of the pre-cracking belt, reduce the damage of the main blasting blast to the pre-cracking surface, the pre-cracking hole adjacent to the slope and the auxiliary hole between the pre-cracking hole and the main blasting hole (for hard rock) and the buffer hole Etc., according to the shock blasting arrangement, properly reduce the hole spacing, row spacing, ultra-deep and the amount of charge per hole.
(III) Basic parameter selection
In order to achieve the requirements of pre-cracking, the key is to reasonably determine various basic parameters in pre-cracking. The pre-cracking blasting parameters mainly include: drilling diameter, drilling spacing, line charge density and non-coupling coefficient. Whether these parameters and the charge structure are reasonable is the key to determining the success of pre-cracking.
1, drilling diameter D
When pre-cracking blasting, the diameter of the pre-split hole should generally be smaller, the purpose is to reduce the hole spacing, reduce the charge per hole, and improve the pre-cracking effect. In pre-splitting blasting, a down-the-hole drilling rig and a cone drilling rig with a diameter of 80 to 200 mm are usually used, and a drilling rig with a diameter of 60 to 80 mm is also used for perforating.
2, the hole spacing α pre-splitting blasting hole spacing is relatively small, generally 7 to 15 times the diameter of the hole, that is
α=(7~15)D (1)
Hard rock generally takes a small value, and soft rock takes a large value.
The empirical formula for the pre-crack hole pitch recommended by Maanshan Mine Research Institute is
α=19.4D(K-1) -0.523 (2)
Where α-pre-crack hole spacing, cm;
D-drilling diameter, cm;
K- does not couple the coefficient.
Fig. 3 is a graph showing the relationship between the pitch of holes and the coefficient of non-coupling at different apertures plotted according to equation (2).

Figure 3 Relationship between hole spacing and non-coupling coefficient at different apertures
3. Line charge density qx. It refers to the actual charge per metre of borehole (excluding the amount of drug added at the bottom of the hole, not including the length of the fill).
According to the actual experience of Daye Iron Mine, the line charge density qx can be calculated as follows
(3)
or
(4)
Where qx-line charge density, g/m;
σ Y - ultimate compressive strength of rock, Pa;
--hole spacing, cm;
D-Drilling diameter, mm.
Equation (3) is the calculation formula for the No. 2 rock explosive, and Equation (4) is the calculation formula for the use of the gel explosive.
The result of the above test calculation is the normal line charge density value of the pre-crack. About 1m at the bottom of the borehole, due to the seriousness of the clamping, the density of the line charge should be different according to the lithology, and the amount of charge is appropriately increased, generally increasing by 1 to 2 times.
Table 1 shows the parameters of the pre-splitting blasting recommended by Maanshan Mine Research Institute and some mines.
Table 1 Pre-cracking parameters
Ordinary pre-cracking
Ordinary pre-cracking
Aperture
(mm)
Explosive
Pitch
(cm)
Line charge density
(g/m)
Aperture
(mm)
Explosive
Pitch
(cm)
Line charge density
(g/m)
80
No. 2 rock or ammonium oil
70~150
400~1000
32
No. 2 rock or ammonium oil
30~50
150~250
100
No. 2 rock or ammonium oil
100~160
700~1400
42
No. 2 rock or ammonium oil
40~60
150~300
125
No. 2 rock or ammonium oil
120~210
900~1700
50
No. 2 rock or ammonium oil
50-80
200~350
150
No. 2 rock or ammonium oil
150~250
1100~2000
80
No. 2 rock or ammonium oil
60~100
250 to 500
200
No. 2 rock or ammonium oil
200~250
2200~2600
100
No. 2 rock or ammonium oil
70~120
300~700
250
No. 2 rock or ammonium oil
250~300
2500~3500
4, no coupling coefficient K
The ratio of the diameter of the borehole to the diameter of the pack is called the non-coupling coefficient, which is a very important comprehensive parameter for the pre-splitting blasting. Since the charge is not coupled, a certain annular gap is formed between the drug pack and the wall of the hole. The existence of the annular gap can reduce the initial pressure of the detonation wave, protect the hole wall and prevent excessive pulverization of the surrounding rock; at the same time, it can delay the blasting time and facilitate the expansion of the pre-crack. The size of the non-coupling coefficient is mainly related to the nature of the rock.
Regarding the calculation of the non-coupling coefficient, the calculation formula recommended by Maanshan Mine Research Institute is

(5)

Table 2 shows the relationship between the non-coupling coefficient of some mines in China and the ultimate compressive strength of rock.
It can be seen from Equation 5 and Table 2 that the uncoupling coefficient decreases as the ultimate compressive strength of the rock increases. For hard rock, the non-coupling coefficient should take a small value, and in soft rock, it should take a large value. Mine practice shows that in order to ensure the pre-cracking effect, the non-coupling coefficient should be greater than 2, and the general value range is between 2 and 5.
Table 2 Relationship between non-coupling coefficient and ultimate compressive strength of rock
Mine name
category
Rock type
Rock ultimate compressive strength (MPa)
No coupling coefficient, K
Nanshan Iron Mine
Huilong diorite
Coarse rock
94.1
44.1
3.75
4.3
Daye Iron Mine
Diorite
98~137.2
2 to 3.5
In front of the mountain iron ore
Diorite
Mixed rock
96.2
81.2
3.1
3.5
(4) Charge and detonation
Explosives used in pre-cracking blasting should be explosives with low detonation velocity and good explosive performance. Special explosives in foreign countries have wing sleeve positioning. Because there is no special pre-cracking blasting coil in China, it is difficult to achieve the positioning requirements. Usually, bamboo sheets or thin wood strips are used on the side spacers, so that the medicine rolls are not protected. The wall of the hole is in direct contact. As for the structure of the medicine roll, it is preferable to use a continuous column charge, but since the linear density of the domestic medicine roll exceeds the required line charge density, the form of the interval charge can be used, and the longitudinal line spacing can be used to achieve the required line charge. density. Figure 4 shows the structural form of a continuous columnar charge and a discontinuous charge.

Figure 4 Continuous columnar and intermittent charge structure
A-intermittent charge; b-continuous charge
1-detonating cord; 2-filling; 3-air gap; 4-explosive
The detonation of the pre-split hole generally adopts a detonating cord, so that the explosive in each hole can be simultaneously detonated to ensure the full utilization of the explosive energy.
In order to achieve the purpose of pre-cracking, the pre-cracking hole must be detonated ahead of the main blasthole, and the time difference is determined by the ability to form a pre-cracking zone. Generally, the time difference of the pre-cracking hole leading the main blasthole is 50-120ms, the hard rock takes a small value, and the soft rock takes a large value. The time difference used in the pre-splitting blasting of Daye Iron Mine is 110ms.
In some mines, according to the needs of the project, under certain lithological conditions, the pre-cracking hole leads the main explosion hole several days earlier, and has achieved good results.
(5) Construction technology
In order to obtain a more uniform pre-cracked wall surface, the drilling accuracy must be ensured. The practice of pre-cracking blasting at home and abroad shows that the borehole deviation of the bottom of the hole should not exceed 15-20 cm. The deviation along the direction of the pre-cracking surface can be relaxed, but the deviation of the direction of the vertical pre-cracking surface should be strictly controlled. Only in this way can the wall surface be flattened.
The lower part of the pre-cracked hole usually has cracks around 1 m from the bottom of the hole. If the bottom is also to be protected, the hole depth should be reduced appropriately. In addition, in order to prevent the seismic wave of the mining blast from winding from the end of the pre-cracking line, the pre-crack end should be elongated (60-100) D, which is 60-100 times of the aperture. In order to better protect the slope, it is better to reduce the hole spacing, row spacing (or resistance line) and charge amount for several rows of mining holes adjacent to the pre-crack line.
In short, pre-splitting blasting is an effective measure to protect open-pit mine slopes. Especially for slope areas with poor stability or need to be protected, it is necessary to carefully use pre-splitting blasting. Of course, compared with the normal mining blasting, the pre-splitting blasting has a large amount of drilling and blasting work, complicated construction process and high cost, which is its biggest disadvantage.
Second, smooth blasting
The smooth blasting and pre-splitting blasting near the slope are basically similar. A row of dense parallel boreholes is drilled along the boundary of the slope. A small amount of explosives is placed in the holes, and then detonated after the drilling and detonation in the mining area. The center of the borehole is lined up to form a flat rock wall. Figure 5 is a schematic view of the hole in the Nanshan Iron Mine using smooth blasting to clean the slope.

Figure 5 Smooth blasting drilling arrangement
1-light face; 2-buffer hole; 3-main blast hole
The blasting parameters of the smooth blasting near the slope are: bore diameter, hole spacing, minimum resistance line, uncoupling coefficient and line charge density. In order to obtain a flat rock wall surface, the blasting parameters of the smooth blasting should be determined according to the same principle of pre-splitting blasting. Tables 3 and 4 are the values ​​of the smooth blasting parameters recommended by Langefels, Sweden, and the US Blaster Handbook, respectively.
Table 3 Glossal blasting parameters recommended by Langevs, Sweden
Aperture (mm)
Pitch (m)
Resistance line (m)
Line charge density (kg/m)
30
0.5
0.7
37
0.6
0.9
0.12
44
0.6
0.9
0.17
50
0.8
1.1
0.25
62
1.0
1.3
0.35
75
1.2
1.6
0.5
87
1.4
1.9
0.7
100
1.6
2.1
0.9
125
2.0
2.7
1.4
150
2.4
3.2
2.0
200
3.0
4.0
3.0
Table 4 Glossy blast parameters recommended by the US Blaster Handbook
Aperture (mm)
Pitch (m)
Minimum resistance line (m)
Line charge density (kg/m)
51~64
0.91
1.22
0.12~0.37
79~89
1.22
1.52
0.19~0.74
102~114
1.52
1.83
0.37~1.17
127~140
1.83
2.13
1.17~1.49
152~165
2.13
2.74
1.49~2.33
Note: 1. The smooth blasting parameters vary with the nature and structure of the rock. The parameters listed in the table are average values;
2. When using colloidal explosives, the diameter of the medicine roll shall not be greater than 1/2 of the diameter of the drill hole, that is, the coefficient of non-coupling shall not be less than 2.
It can be seen from the correspondence between the parameters in the above two tables that the results are basically the same.
There are many similarities between smooth blasting and pre-splitting blasting, the fundamental difference being the time of detonation. Since the hole spacing of the smooth blasting can be slightly larger, the perforation blasting work can be slightly less, but the seismic reduction effect is not as good as the pre-splitting blasting. According to the determination of Daye Iron Mine, the smooth blasting effect of the smooth blasting is 17.5%~22.6% lower than that of the multi-row hole blasting, and the pre-cracking blasting can reduce it by more than 40%.
Smooth blasting does not reflect or suppress the explosion stress waves of mining blasting. In order to protect the slope more effectively, smooth blasting is often used in conjunction with buffer blasting.
Third, buffer blasting
Buffer blasting near the slope is to arrange a number of rows of resistance lines along the adjacent slope line and buffer holes that gradually decrease in charge, forming a buffer layer capable of attenuating seismic effects and detonating after normal mining blasting. It is the easiest way to control blasting. In order to make the blasting seismic effect of each row of boreholes in the buffer layer not exceed the blasting vibration of the last row of smooth boreholes, according to the calculation principle of the vibration velocity, the following relationship should be
(6)
Where Q J , Q i - are the total dose of the last row of smooth holes and the total dose of the drill holes in any row of the buffer layer, kg;
R J , R i - are the distances of the last row and any row of holes from the protection point, respectively.
According to the above relationship, the closer to the slope of the hole, the smaller the R i value and the smaller the Q i value. Therefore, the parameters of the last row of buffer holes should be adjusted so that their row spacing and charge amount gradually decrease toward the slope. Figure 6 is a layout diagram of the buffer blasting drilling of Daye Iron Mine.

Figure 6 Drilling and blasting drilling layout of Daye Iron Mine
1~15-number of blasting sections; b i - drilling row spacing
The parameters and dose distribution of the 4 rows of buffer holes in the figure must strictly comply with the requirements of (6).
Table 5 shows the measured values ​​of the shock absorbing effect of the buffer blasting in Daye Iron Mine.
Table 5 Seismic effect of measured buffer blasting in Daye Iron Mine
rock
Program
Number of holes (a)
Total dose (kg)
Number of segments
Maximum period and dose
(kg)
Buffer blasting
Earthquake reduction efficiency
Diorite
Buffering and spacing
38
8030
10
1 1360
17.5%
Diorite
Multi-sequence
38
9150
15
7 890
Diorite
Buffering and multi-segment
38
5540
12
6 630
19.8%
Diorite
Multi-sequence
56
8680
15
3 830
Diorite
Buffering and multi-segment
41
4740
12
11 510
Diorite
Multi-sequence
72
11650
12
6 2050
22.6%
Diorite
Interval
46
6800
6
4 1380
31.2%
Buffer blasting is about 20% lower than the general sequential blasting.
Buffer blasting is most suitable for protecting loose rock slopes. In order to protect the slope more effectively, buffer blasting is often used in conjunction with pre-cracking or smooth blasting. If used in combination with pre-cracking, buffer blasting is initiated after the pre-cracking blast, and the main blasting hole in the mining area is detonated before blasting; if it is used together with smooth blasting, the blasting of the main blasting hole in the mining area is initiated before the blasting of the light surface.
The main advantages of buffer blasting are: simple and cheap; it can be larger than the diameter of the pre-cracked and smooth blasting holes, while the larger diameter is advantageous for accurately arranging the holes and facilitating the drilling of deep holes. Its disadvantages: if it is not used in combination with pre-cracking or smooth blasting, it cannot be used for the section of rapid transition; the backlash of the main blasting of the mining area may sometimes partially or completely destroy the platform to be buffer blasted.

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