Practice of controlled blasting technology for open pit slope

Northwest iron ore is one of a large open-air iron mine. The height of the mining stage is 12m, and the mining to the final boundary position is every 2 stages, and the height of the section is 24m; the stage slope angle is 65°. The lithology of iron ore is dominated by dolomite and iron ore layers, which are dense and solid, and the bedding and joints are relatively developed. The slanting core and the bottom are slate and mica rocks, which constitute the top and bottom of the ore-bearing stratum. The rock bedding is developed and the mechanical properties are poor. The fractures in the area are developed, and the tectonic fractures and weathering fissures are relatively developed, especially in the shallow part. The lithology of the mining area is complex and varies greatly. Open-pit mining destroys the original stratigraphic structure. There are folds and fault structures and small fracture zones in the mining area, which reduces the stability of the ore and rock and may cause the instability of the stope slope. In order to ensure the construction quality and stability of the slope, pre-splitting blasting technology is adopted according to the experience of large-scale open pit slopes at home and abroad.
1 blasting design
1.1 Blasting plan
Pre-splitting blasting is to rupture a certain width of crack in the designed slope contour before blasting in the protection zone, weakening the propagation of seismic waves to the slope during blasting in the main body and blocking the cracks developing outside the slope to weaken the main blasting. For the vibration hazard of the surrounding protected objects and the damage to the slope rock mass and its stability, the main blasting hole forms a smooth slope along the pre-cracking surface.
According to the construction experience of pre-cracking blasting and the geological conditions of the open-pit iron mine, the construction plan is made: a row of pre-cracking holes is drilled along the design slope top line in the boundary area, the drilling dip angle and the stage slope angle are consistent; Through the vertical main blasting hole, loose blasting is used; in order to reduce the impact of blasting on the adjacent slope, reduce the damage to the fixed side gang, and set a row of inclined buffer holes between the main blast hole and the pre-split hole [1]; After the parallel section of the field, the height of the step is 24m. The pre-split hole is perforated once, and the buffer hole and the main explosion hole are single-stage blasting. The arrangement of the blasthole is shown in Figure 1. Before the blasting construction, through a large number of on-site tests, the blasting effect and the pre-splitting blasting effect are tested and analyzed. According to the characteristics of rock mass water and joint fissure development, the pre-splitting blasting and buffer blasting parameters are designed and adjusted, and according to the stope The actual situation is constantly adjusted and optimized.

1.2 Pre-cracking parameters
1.2.1 Drilling diameter
By comparing the 150 and 120mm aperture perforation blasting schemes, the rock mass is large in most areas of the upper part of the mine, and the water content is large. The larger the diameter of the inclined hole, the lower the hole formation rate. When the hole diameter is 120mm, the efficiency of wearing the inclined hole is high, the hole formation rate and the blasting effect are good, so the pre-cracking hole diameter is 120mm.
1.2.2 blasthole length
The depth of the blasthole is determined by the elevation of the site. In order to prevent the blasting stress wave and the blasting rupture in the main blasting area from reaching the reserved area, the pre-cracking hole must be super deep. The pre-cracking hole depth L1 is determined by the following formula:

In the formula, L1 is the length of the pre-cracking blasthole, m; H is the height of the stepped step, m; α is the designed slope angle, 65°; h1 is the blasthole ultra-deep, 0.5-1.0 m. Calculate that L1 = 27.5 ~ 28.5m, take 28m.
1.2.3 Pitch
In pre-splitting blasting, the size of the blasthole spacing directly affects the width and flatness of the pre-crack. According to experience, the pre-cracking distance between the medium and upper rock masses is

Where a is the bore spacing, m; d is the bore diameter, 120 mm. Calculate the hole spacing a = 1 ~ 1.4m, the hard rock takes a large value, and the soft weathered rock takes a small value.
1.2.4 Line Charge Density
The line charge density of the pre-crack is one of the most important parameters in pre-cracking. The density of the line charge is too large, destroying the wall of the pre-cracked hole and the surrounding rock, and the half-wall porosity is difficult to meet the requirements; the line charge density is insufficient, and the pre-crack is difficult to form [2]. The charge density of the open-air deep-hole pre-cracking blasting line is determined by the following formula:

In the formula, q1 is the charge density of the blasthole line, kg/m; σ is the compressive strength of the rock, MPa; the other symbols have the same meaning as before.
According to the empirical values ​​of mines at home and abroad and the specific conditions of the mine, the line charge density is generally 0.50~0.75kg/m.
1.2.5 blockage length

Reasonable blockage length and good plugging quality play an important role in improving the blasting effect and improving the energy utilization rate of the explosive. The length of the blockage should be compatible with the amount of explosives. The blockage is too short, the amount of drug is too much, and the pre-cracked orifice is easy to form a funnel shape. If the blockage is too long and the amount of the drug is too small, it is difficult to form a complete pre-crack. According to experience, the length of the blockage is generally 12 to 20 times.
The diameter of the blasthole [3] is adjusted according to lithology. Before filling, use the explosive wrapper or woven bag to put it into the lower part of the blockage section, and then fill it with the drilled rock powder. Keep the charge section spaced from the air to prevent the filled rock powder from falling into the charge site.
1.2.6 Charge structure
The radial uncoupled charge significantly reduces the contact between the column and the wall of the hole, minimizing the breakage of the rock around the hole wall by the blasting, and at the same time, part of the shock wave energy acts on the rock of the adjacent hole wall, causing it to be in a tension state, resulting in An obvious pre-crack. The open-hole deep-hole pre-splitting blasting pre-split hole charge coupling coefficient is generally 2~4, the pre-split hole diameter of the mine is 120mm, the pre-cracking column diameter is about 45mm, and the radial uncoupling coefficient is 2.7. The axially-spaced segmented charging technique is used in the axial direction, and the pre-cracking drug column is taped to the detonating cord at intervals. As the depth of the blasthole increases, the initial stress of the rock mass generally increases, and the clip at the bottom of the blasthole is more obvious. In order to overcome the problem of making a large hole at the bottom of the hole, the bottom is a charging reinforcement section, and the added dose is evenly spread to the length of the bottom of the hole of 1 to 2 m. The upper part is the weakening section of the charge, and a non-charged section is set at a certain distance from the surface. The length of the non-charged section is related to the formation condition, the pre-crack hole diameter and the charge amount, and the charge amount near the top of the blockage section is It is 1/3 to 1/2 of the design value. The medicine column tied to the detonating cord can be tied to the bamboo piece, processed outside the hole, and manually sent into the hole. The charge structure is shown in Figure 2.

The pre-cracking blasting parameters are adjusted according to the nature of the rock. Table 1 shows the main lithology of the mine and the corresponding pre-splitting blasting parameters.

1.3 buffer hole and main blast parameters

The buffer hole and the main blast hole are both 150mm. Considering the specific requirements and conditions of the project, the rock is mainly slate fractured rock, slate and dolomite. The consumption of unit explosives is 0.4-0.7kg/ M3, adjusted according to the lithological soft and hard conditions. In order to obtain a high-quality excavation profile, the buffer hole is set as a tilted hole, the angle is larger than the pre-cracked hole, and the buffer hole charge amount is about 1/2 to 2/3 of the normal hole charge amount [4]. According to the position of the drilling space, a row of 7.5m deep buffer auxiliary holes is arranged in front of the buffer hole. Table 2 shows the buffer holes, auxiliary holes and main blast parameters corresponding to the main lithology of the mine.

1.4 detonation network and detonation method
Delayed blasting network using detonating cord and plastic detonating tube. The pre-cracking column in the hole is connected by a double-guided detonating cord, and the outer connecting detonating cord is simultaneously detonated.
In order to ensure the pre-cracking effect, the pre-cracking hole leads the main blasting hole to detonate at least 100ms [5], and the hole extension period generally adopts 5 non-electrical millisecond detonating tube detonators.
The main blast hole and the buffer hole are double-expanded non-conducting squib detonators to realize the delay in the hole, and the front row exceeds the front and rear rows 1 to 2 to detonate. The detonator of the detonator in the hole is connected to the detonating cord outside the hole, and the detonating tube is connected to the end of the detonating cord at the end of the detonating cord, and is connected with the pre-cracking main detonating cord and is detonated by the non-electrical millisecond detonating tube detonator. The network connection is shown in Figure 3.

2 on-site construction technical measures
According to the lithology of different regions, the main technical measures are as follows:
(1) According to the proposed parameter calculation formula, calculate the reasonable parameter range of different lithologies, and then select the reasonable parameters from the actual situation of the side gang. In the application, the parameters are adjusted according to the actual situation on site. Because the Dongbang South rock mass belongs to the multi-fracture soft rock and the fractured rock mass, the rock layer contains water, and the hole spacing and the line charge density are appropriately adjusted.
(2) The charge density is appropriately adjusted according to the actual hole distance and the surrounding blasthole condition. In the construction, if the actual hole spacing is greater than the design hole distance, the amount of the drug should be increased appropriately; the amount of the drug should be appropriately increased on both sides of the unpored hole.
3 application effects
According to the pre-splitting blasting test and theoretical analysis, the pre-splitting blasting construction was carried out in the southeast, north and southwest of the stope. The southeastern stop of the stope is dominated by slate broken rocks, and the rock has large water content. Pre-splitting blasting is carried out in the fractured water-bearing rock mass. The half-wall hole trace rate is low, reaching about 60%, but it still achieves the protection slope stability. For the purpose of sex, there is no umbrella rock and pumice on the slope, the unevenness is less than 20cm, the blasting effect is shown in Figure 4(a); the north side of the stope is mainly slate, the rock is relatively intact, and the water content is small, and the half wall porosity rate More than 80%, the slope unevenness is less than 20cm, the blasting effect is shown in Figure 4(b); the southwestern gang is dominated by dolomite, the rock hardness coefficient is large, the integrity is good, and the half-wall hole mark rate is over 90%. The unevenness is less than 20cm, and the pre-cracking effect is better, as shown in Figure 4(c).

4 Conclusion
The three pre-split blastings form a through-crack between the pre-split holes, and the cracks are flat, which effectively reduces the vibration of the slope under the premise of ensuring the blasting quality, and achieves the purpose of protecting the slope. Through the pre-crack blasting construction, it is found that the more complete and uniform the rock, the better the pre-splitting blasting. Pre-splitting blasting is carried out in a hard rock formation, and the blasting parameters are easy to grasp, and a good pre-cracking effect is easily obtained. When pre-split blasting is carried out in multi-fracture soft rock and broken rock mass, the hole pitch should be reduced accordingly, and the amount of charge can be appropriately reduced to achieve the purpose of protecting the slope.
references
[1] Lou Xiaoming, Lai Hongyuan, Tang Xiaojun. Zijin Gold copper mine helped by high and steep slope blasting shock absorption study [J]. Blasting, 2011, 28(2): 56-59.
[2] Wang Xuguang. Blasting design and construction [M]. Beijing: Metallurgical Industry Press, 2013.
[3] Fox for the people. Heidaigou open coal mines Presplitting Blasting optimization design and implementation [J]. Journal of North China Institute of Science and Technology, 2012, 9(2): 26-29.
[4] Shen Lijin. Application of presplit blasting technology in open slope [J]. Nonferrous Metals: Mine Section, 2004, 56(3): 28-29.
[5] Wei Xuezhong. Application of presplit blasting technology in slope protection engineering of Dafeng open pit mine [J]. Opencast Mining Technology, 2014(12): 11-12.
Author: strong division, Cui Zhengrong; Sinosteel Maanshan Institute of Mining Research Co., Ltd., State Key Laboratory of Metal Mine Safety and Health;
Article source: "Modern Mines"; 2016.4;
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