Application of Presplit Blasting Technology in Jiaoshan Iron Mine

Nanshan Iron Mountain iron ore mining company recess is a large open pit mines, with the increase of mining depth, increasing slope stability problems have become increasingly prominent, mainly in the shallow steps by community surrounding rock seriously damaged.
In order to reduce the adverse impact of blasting on surrounding rock and ensure the stability of slope, a large number of pre-splitting blasting tests were carried out in the concave mountain iron ore mine, and a good pre-splitting blasting effect was obtained in the application practice of the boundary blasting.
1 Basic requirements for pre-cracking
The pre-splitting blasting is mainly to reduce the damage of the blasting to the permanent slope by arranging the dense blasthole along the excavation boundary, adopting the uncoupled charge, and pre-cracking to form the pre-crack in the main blasting area to achieve the stability of the maintenance slope. purpose. The following basic requirements should be met after pre-split blasting:
(1) Forming a regular pre-cracked surface with a small amount of over-excavation. Generally, the integrity of the surrounding rock in the reserved area after blasting is measured by the half-wall hole trace rate of the pre-cracked surface. The hard rock half-wall hole mark rate should reach 80% or more. Should be more than 60%, soft rock should reach more than 30% [1].
(2) The primary cracks in the rock mass do not expand, and there are few cracks in the explosion.
(3) The unevenness of the pre-cracked surface is less than ±150mm.
(4) The pre-crack width generated by pre-cracking blasting should not be less than 10mm [2], and the earthquake-reduction rate should be above 40%.
2 The formation mechanism of pre-splitting blasting
At present, there are three main theories for the pre-splitting blasting mechanism, namely stress wave interference theory, theory with high pressure gas as the main function, theory of combined combination of explosive stress wave and high pressure gas, and the theory of joint action is accepted by most people. [3-4]. The joint action theory holds that the tangential tensile stress generated by the blast stress wave from the blasthole to the surrounding propagation exceeds the ultimate tensile strength of the rock, and cracks are generated on the hole wall or even the adjacent blasthole connection line, followed by the gas wedge of the explosive gas. Under the action, the crack further expands and penetrates to form a pre-cracked surface.
Brown's high-speed photography test proves that the cracking of the wall of the blasthole occurs after the detonation wave, while the shock wave is formed, and the crack spreads later, which can be considered as gas generation [3], that is, the pre-crack is due to stress wave and explosion. The gas is formed by the interaction of gas.
3 Pre-splitting blasting parameter calculation
In the practice of pre-cracking blasting, the calculation methods of pre-cracking blasting parameters mainly include engineering analogy method, empirical formula method and field test method. Some scholars also use theoretical calculation method to calculate pre-crack blasting parameters.
3.1 non-coupling coefficient
Pre-cracking blasting usually uses a non-coupling charge structure while using low-power explosives. Therefore, the uncoupling coefficient is a key parameter in pre-splitting blasting, which directly affects the amount of pressure acting on the wall of the hole after the explosion of the explosive. Pressure acting on the wall of the hole during blasting

In the formula, P is the pressure acting on the wall of the hole, MPa; P0 is the initial explosion pressure of the explosive, MPa; Vd is the volume of the drug pack, m3; VD is the volume of the blasthole, m3; r is the adiabatic index.
According to the definition of the uncoupling coefficient, the equation (1) can be transformed into

Where D is the diameter of the blasthole, m; d is the diameter of the packet, m. In order to prevent the excessive pressure on the wall of the blasthole during blasting, the surrounding rock of the hole wall is destroyed. The compressive stress acting on the rock around the hole wall should be less than the ultimate compressive strength [σ] pressure of the rock, combined type (2 )available

3.2 line charge density and charge length
(1) Line charge density qL. Actual line charge density in pre-crack

Where qL is the line charge density, kg/m; ρ is the explosive density, kg/m3.

(2) The length of the charge is l1. When the pre-split blasting adopts the continuous charging structure, the charging length l1 is the same as the blasthole length l; if the interval charging structure is adopted, the diameter of the medicine package should be the equivalent medicine package diameter type, l1 is the charging length, m; l is The length of the blasthole, m.

Defined by the uncoupling coefficient, combined with the formula (5), it can be seen that the pre-splitting blasting adopts the length of the uncoupled charge structure.

Borehole charge

3.3 blast hole spacing
(1) Empirical formula method. Hole spacing given by Langefors [5], Sweden

(2) Theoretical calculation method. The pre-crack spacing is related to the dynamic compressive strength, dynamic tensile strength and blasthole diameter of the rock [6]. In the pre-split blasting, in order to ensure that the wall of the blasthole is not damaged by pressure and form a tensile crack, the wall pressure of the hole should be satisfied.

Where [σ] pressure is the ultimate compressive strength of rock, MPa; [σ] tensile ultimate tensile strength, MPa.

Since the pre-crack formation process is dynamic, the tensile strength of the rock in equation (11) should take the dynamic tensile strength value, which is obtained by equation (11).

In the practice of pre-cracking blasting engineering, the following principles should be followed, that is, on the basis of meeting the engineering needs, the blasthole spacing should be increased as much as possible, and the engineering cost should be reduced [3], and the wall pressure should be replaced by the dynamic compressive strength [σ] pressure of the rock. , the maximum value of the pre-crack blasting hole spacing is:

The dynamic compressive strength and dynamic tensile strength of the rock mass on the site vary greatly. In the actual application process, the static strength index of rock is often used to calculate (13), and it is adjusted by field test to obtain better pre-cracking effect. .
4 engineering applications
4.1 Engineering Geology Overview
The concave mountain iron ore is mainly vanadium-bearing magnetite and imaginary hematite, f=4～7; the surrounding rocks are mainly andesite , tuff , trachyte and diorite, f=4～12, pre-cracking There are two main types of rock in the blasting area:
(1) Suihua pyrite mineralization Huilong diorite porphyrite, gray or gray-green, plaque-like structure, hard texture, joint development, f=8~12.
(2) Coarse rock, strong weathering, loose texture, jointed dense mesh, f=4~8.
In order to ensure the stability of the slope and improve the safety and economy, the research and application of pre-splitting blasting technology with different parameters have been carried out in the concave mountain iron ore.
4.2 Pre-cracking parameters
4.2.1 Hole arrangement and aperture
The role of the pre-crack is to form a pre-crack. It is not advisable to use too large apertures and diameters [10]. The mine adopts TY-370GN hydraulic down-the-hole drilling rig to drill pre-splitting holes along the design boundary line. The diameter of the blasthole is 150mm. The drilling inclination angle is consistent with the slope angle of the step, that is, 60°. The arrangement of the pre-cracking hole is shown in Figure 1.

4.2.2 Blasting parameters

The blasting parameters include the uncoupling coefficient, the line charge density, the charge length, and the blast hole spacing.

(1) No coupling coefficient. According to the above, the coupling coefficient K has a functional relationship with the compressive strength of the rock. The uncoupling coefficient can be calculated according to the formula summarized by the Maanshan Mine Research Institute under different geological conditions.

See Table 1 for the strength index of surrounding rock in pre-splitting blasting area of ​​the concave mountain iron ore mine.

(2) Line charge density and charge length. The pre-splitting blasting of the concave mountain iron ore is based on the interval charge, and the structure is shown in Figure 2.

The 50mm ammonium fry drug roll is bundled on the detonating cord laid along the length of the blasthole. The height of the step is 12m and the hole depth is 14.5~15m. Among them, the damage of the blasting to the slope is weakened, and the pre-cracking hole is ultra deep. .5~1.0m, actual line charge density

The lengths of the pre-crack blasting charges are different in different lithologic surrounding rocks, and the calculation is carried out according to formula (6). The results are shown in Table 2.

(3) Hole spacing. The maximum hole spacing is calculated according to equation (13). The results are shown in Table 2.
4.3 Pre-cracking blasting results

Due to the heterogeneity of the surrounding rock, some local cracks and joints exist in the surrounding rock. The actual pre-crack spacing can be greater than the pre-crack spacing calculated in Table 2. In the early stage of the Aushan Iron Mine, pre-cracking tests of pre-split hole spacing of 1.3-2.0 m were carried out in the Suihua Huichang diorite area, and pre-cracking of pre-cracking hole spacing of 1.2-1.8 m was carried out in the coarse-faced rock area. Blasting test. Among them, the pre-cracking parameters can be used to generate pre-cracks. According to the test results, the parameter range of the pre-splitting blasting in the concave mountain iron ore can achieve better results.

According to the results of different hole distance tests, it can be found that the hole pitch becomes larger and the pre-cracking blasting effect is worse. The hard rock can be appropriately larger than the soft and poor integrity rock. Considering the basic requirements and cost factors of pre-splitting blasting, the pre-cracking blasting parameters finally adopted by the concave mountain iron ore are shown in Table 4.

Through the pre-splitting blasting, the concave surface blasting is basically flat, and the unevenness is generally less than ±25cm. The rock mass structure is less affected by blasting, which improves the stability of the slope. The slope angle and boundary line are both The design requirements are met, and the application effect of pre-cracking blasting is shown in Figure 3.

5 Conclusion

(1) The uncoupled charge structure can reduce the initial pressure acting on the pre-crack wall when the explosive is exploded, and avoid compression damage to the hole wall. The coefficient of non-coupling and the amount of explosives are related to the wall pressure of the hole. The uncoupling coefficient can be calculated according to the ultimate compressive strength of the surrounding rock.

(2) According to the pre-crack formation mechanism, the pre-crack spacing is related to the dynamic compressive strength, dynamic tensile strength and bore diameter of the rock. The maximum value of the pre-split hole spacing can be calculated by the above parameters.
(3) In the hard and intact rock, the range of pre-cracking blasting parameters varies greatly. Under the premise of ensuring the quality of pre-cracking blasting, in order to reduce the cost, large hole spacing can be used to increase the line charge density appropriately.
(4) Pre-split blasting in rock with soft and poor integrity can also achieve better results, but the half-wall hole trace rate is lower, the unevenness is larger than ±25cm, and the practical application should use small hole spacing and reduce Line charge density.
(5) Combining theoretical calculations with field tests, the pre-splitting blasting parameters of the Fushan iron ore in the Suihua gabbro diorite and trachyte are obtained. Through the application of pre-split blasting technology, the damage of the surrounding rock by the production blasting is reduced, the stability of the slope is guaranteed, and the pre-splitting blasting of similar mines has certain reference significance.
references
[1] Wang Xuguang. Blasting design and construction [M]. Beijing: Metallurgy Industry Press, 2014. [2] Zhang Guoping. Open Coal Mine in Chaoyang end wall presplit blasting [J]. Coal Technology, 2014 (3): 91-93.
[3] Chen Qingkai, Zhu Wancheng. Pre-splitting blasting forming mechanism and design method of pre-cracking hole spacing [J]. Journal of Northeastern University: Natural Science Edition, 2011 (7): 1024-1027.
[4] Tang Hai, Liang Kaishui, You Qinfeng. Discussion on the mechanism of pre-splitting blasting and its influencing factors [J]. Blasting, 2010 (3): 41-44.
[5] Yang Tianhai, Longyuan, Gu Wenbin, et al. Glossy, pre-cracking blasting parameters selection and quality measures [J]. Blasting, 2000(1): 11-13.
[6] Liu Weizhou, Zhang Xiliang. Calculation and analysis of reasonable parameters of pre-splitting blasting in Nanshan mining area [J]. Mining Express, 2004 (6): 13-15.
[7] Yuan Kang. Pre-splitting blasting seam and parameter calculation principle [J]. Blasting, 2013 (1): 58-62.
Author: Wang Ming; Maanshan Iron & Steel Group Mining Co., Maanshan Institute of Mining blasting limited liability company;
Chen Nengge ; Ma Steel Group Mining Co., Ltd.;
Article source: "Modern Mining"; 2016.8;
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