Failure Analysis and Preventive Measures for Filling Drilling in Deep Mines

With the gradual depletion of shallow surface mineral resources, deep well mining has become the development direction of future mines. The high ground stress in the deep stope may lead to problems such as rockburst, stope collapse, roadway deformation, etc., threatening safe production. Filling mining method is an effective method for safe and efficient mining of deep wells.
Filling the borehole is the only way for the filling mortar to be transported from the surface preparation station to the downhole working surface. It is also the key throat project of the entire filling system. Once it is blocked, the function is invalid, and its recovery is difficult and costly.
What's more, the mine filling system has been in operation for many years, filling borehole blockages and pipeline damage accidents frequently, and multiple boreholes have been constructed near the filling station. Due to limited terrain conditions, it faces the dilemma of no-hole punching and normal to the mine. Production has a great impact. According to the actual production data of a mine [1], the average service life of the filling borehole is generally 40-600,000 m3 of mortar flow. Through the analysis of the causes of failure of the filling borehole, the technical measures to prevent the failure of the borehole are proposed from various aspects, which provides a reference for the smooth operation of the filling system of the mining enterprise.
1 filling the drilling structure
In general, the cross-sectional structure of the filled borehole has four forms depending on the degree of stability of the rock formation through which the borehole passes (Fig. 1). When the drilled hole passes through the intact rock formation and the weak structural surface is small, the bare borehole can be directly used as the filling pipe (Fig. 1(d)); when the shallow surface Quaternary topsoil or rock stratum is broken, it must be installed. Wall casing (Fig. 1(a), Fig. 1(b)), when the mortar flow rate is small, the casing wall can be used directly as a filling pipe (Fig. 1(b)); when the mortar flow is large, in order to extend the hole For the service life, the filling pipe can be installed in the casing. If necessary, the filling pipe can be replaced in time, and the drilling hole can be reused (Fig. 1(a)); when the rock formation condition is good and the mortar flow rate is large, it can be in the drilling hole. Install the filling tube (Fig. 1(c)).

2 Failure analysis of filling borehole
According to the different cross-sectional structure of the borehole, the failure mode of the filling borehole mainly has blockage and wear.
2.1 clogging failure
There are two types of filling borehole plugging, including no pipe bare hole blockage and borehole pipe blockage. The main causes of blockage include the following:

(1) The thicker and thicker residues in the borehole or pipeline are not removed. Before the next transport, the mortar is consolidated, falling off to form a large block, mechanically interlocking with each other, or adhering to the inner wall of the borehole or pipe, resulting in mortar Blocked.
(2) Poor on-site management, foreign materials such as scrap steel ropes and repaired pieces accidentally enter the conveying passage, causing blockage.
(3) The quality of the filling body is inferior, the coarse aggregate gradation and the mortar concentration are unreasonable, and the mixing is uneven, resulting in blockage.
(4) the inner pipe within the borehole materials (e.g., bimetallic composite or ceramic composite lined pipe) massive exfoliation, to lay a trap channel.
2.2 wear failure
In the deep well filling system of gravity flow, the power of the mortar flow is derived from the gravitational potential energy of the vertical pipe section. According to the relationship between the potential energy of gravity and the resistance loss along the pipeline, the state of the slurry pipe is divided into full pipe flow and non-full pipe flow [2].
Full pipe flow is an ideal mode of mortar transportation. The static head provided by the system height difference is exactly balanced with the loss along the pipeline. It is assumed that the system height difference is h, the pipeline horizontal length is L, the mortar bulk density and its The hydraulic gradient during pipe transfer is γm and i, respectively.

At this time, the mortar flows continuously and stably in the pipe, and the pipe wall is in a state of uniform wear.

The non-full pipe flow means that in the process of mortar self-flow, the gravitational potential energy of the system is higher than the energy required to overcome the resistance along the pipeline, that is, γmh>i(h+L), and the remaining energy is converted into kinetic energy according to the law of conservation of energy. As a result, the mortar is accelerated to flow, causing a negative pressure in the pipe to generate an air column phenomenon. The generation of the air column comes from the air entering the pipeline at the discharge port of the open-circuit filling system, and on the other hand, the moisture in the mortar is vaporized. The air column is divided into three regions [3], namely the cavitation zone, the air zone and the water jump zone, as shown in Figure 2.

In the cavitation zone, the mortar falls freely, the conveying speed and pressure are continuously increased, and the normal or oblique impact force is generated on the inner wall of the pipe or the borehole. A mixture of water vapor and air appears in the air zone, and the mortar continues to accelerate in this area until it reaches a maximum. The water jump zone is the transition zone from the non-full pipe flow to the full pipe flow. The mortar transitions from the turbulent flow to the slow change flow, and the local flow velocity decreases sharply. According to the kinetic energy theorem, the flow gradient of the water jump zone is large, and the mortar consumes a large amount. The energy transforms into a huge impact on the tube wall. Therefore, for vertical pipes or boreholes, the wear of pipes or boreholes is most severe at the interface of the transition zone to the full pipe zone.
In the air column, the mortar falls freely under the action of gravity, and the high-speed flowing mortar washes away the pipe or the bore wall. After entering the area from the surface discharge port, the mortar will accelerate continuously, and the continuous flow will become a discontinuous flow. The place where the mortar has a large contact area with the inner wall of the pipe or the borehole will wear faster, forming a gully wear surface and finally grinding through the local pipeline. [4] (Fig. 3), the inner wall of the borehole is uneven, and the clogging is easy to block, which causes the fluid state of the mortar to be disordered and the uneven wear is aggravated; when the mortar enters the full pipe area, the flow is basically stabilized in the form of a continuous body. The effective contact area with the inner wall of the pipe or borehole is approximately equal to the surface area of ​​the latter, and the wear is even and even.

In summary, in the deep well filling system, the filling mortar is often in a non-full pipe flow state in the vertical channel due to the large height difference between the ground filling port of the surface filling station and the downhole filling working face. According to different structural forms of filling boreholes, if the bare holes of the pipeline are blocked or worn, it will directly lead to the failure of the borehole; if the pipeline inside the borehole or the pipeline inside the casing is blocked or worn, if the pipeline is not replaced in time, the borehole will be scrapped. . Therefore, the non-full pipe flow conveying mode is the most common form of self-flow conveying of the filling body, and is also the main cause of pipe wear and drilling failure. In addition, since the filling system is a systematic project in which the surface and the underground, vertical and horizontal sections interact, the blockage and wear of the horizontal section pipes may also cause the filling of the filling holes to fail.
3 filling hole failure prevention measures
3.1 closed circuit full pipe conveying
For deep well filling pipeline conveying systems, closed-circuit full-tube flow mode should be adopted to minimize the entry of air into the pipeline and avoid the formation of air columns in the filling borehole or vertical pipeline, causing the impact wear of the mortar on the borehole or pipeline. Some mines use a funnel open feed port. This design makes it easy to mix air into the mortar to form an air column. Further optimization is recommended. A lead-zinc ore in Yunnan was stirred tank filling port and the vertical conveying pipe drilling using a direct connection to form a closed transport mode, to meet the full bobbin delivery state of slurry in the pipeline. At the same time, a pressure relief three-way valve is installed on the connecting section pipe to facilitate the drilling and discharging pressure during the blockage to reduce equipment damage.
3.2 Pipeline layout optimization
The reasonable arrangement of the filling pipeline can make the filling double line meet the deep well self-flow tube transmission, and not to generate a large residual potential energy head, change the pressure distribution of the filling system, and reduce the erosion wear of the mortar on the pipeline.
Since the effective head provided by the vertical pipe section is proportional to the height of the mortar column, in order to reduce the effective head of the system, it can be achieved by reducing the height of the mortar column. Therefore, for mines with deep mining depth, vertical drilling can be constructed in sections, and horizontal sections are used for transitions; horizontal pipelines are folded back through the upper middle section of the roadway to appropriately increase the resistance along the horizontal pipeline and consume the flowing mortar. Excess energy, or set up a sand storage tank at the appropriate position in the well to release energy. After the slurry is released, it will enter the next stage filling pipeline and continue to be transported to the working surface.
3.3 small diameter increase resistance and wear reduction

When the mortar bulk density and horizontal pipe length are fixed, the hydraulic gradient of the pipe directly affects the height of the air column in the vertical pipe [5]. Studies have shown that the hydraulic gradient of filling pipelines is related to factors such as pipe diameter, pipe length, flow rate and concentration [6]. Therefore, under the premise that the mortar flow rate and concentration should not be greatly adjusted and the length of the pipeline should not be greatly increased, it is feasible and feasible to appropriately reduce the pipe diameter and increase the hydraulic gradient of the pipeline. In the deep well filling system, the small diameter pipe can be used locally to increase the resistance loss along the system, so that the excess energy of the mortar can be effectively consumed to achieve the purpose of increasing the resistance and reducing the grinding [7]. When the vertical pipe diameter is smaller than the horizontal pipe diameter, the former pipe flow resistance is greater than the latter, the system static pressure is mainly consumed on the vertical pipe, that is, the low pressure full pipe conveying mode is formed; when the vertical pipe diameter is larger than the horizontal pipe diameter, the former pipe flow resistance Smaller, the system static pressure is mainly consumed on the horizontal pipe, that is, the high pressure full pipe conveying mode.
The production practice shows that the horizontal pipeline wears when the low-pressure full pipe is transported, the vertical pipeline wears large and is easy to block; when the high-pressure full pipe is transported, the vertical section pipe wears little, and the drilling service life is long, but the horizontal pipeline bears large pressure and wears fast. From the perspective of preventing the failure of filling boreholes, it is relatively easy to consider the replacement of horizontal pipelines after wear, and it is recommended to fill deep wells.
The mine can use the wear-resistant small pipe diameter to increase the resistance in the horizontal pipeline, so that the vertical section pipeline can withstand less pressure, reduce the degree of wear and prolong the service life of the drill. The use of a Copper Ore Reducing the drag horizontal pipe section (FIG. 4), the borehole failures significantly improved.

3.4 Damaged hole repair
The main technical idea of ​​damaged borehole repair is to open up the plugged hole or replace the damaged pipe to quickly restore the drilling function. It is usually costly, difficult, and slow to re-perforate or lower the pipeline after the failure of the drill hole. A nickel mine in Gansu proposed a permanent repair drilling technology [8], that is, constructing a large-diameter borehole and installing a casing with an inner diameter larger than the outer diameter of the recommended filling pipe by 50 to 60 mm, and installing a vertical pipe in the casing. It is not coupled with the casing, and only the pipe is fixed at the hole of the hole and the bottom of the hole, and the middle part is self-stabilized by the rigidity of the pipe itself.
When the pipe in the borehole is damaged, remove it in time, quickly replace the pipe, and restore the borehole filling. This technology is very effective in the application of the mine. However, for deep mines, it is not enough to fix the pipeline only at the upper and lower ends of the borehole due to the long borehole. The pipeline may be uneven due to the multi-directionality of the mortar during the mortar transport. The impact of impact force to close the water hammer, severe vibration, or even break, will inevitably lead to frequent pipe replacement, failing to reflect the technical advantages. Therefore, it is necessary to consider strengthening the false coupling contact between the middle part of the long vertical pipe and the casing. For example, the flexible material (sponge, rubber, etc.) is used to wrap the filling pipe, so that the uncoupling gap with the casing is minimized, and the pipe is damaged. Quickly change when.
3.5 Other measures
In the construction process of deep mine mine filling and drilling, due to poor equipment characteristics, low technical level of operators, and uneven soft and hard formations, deflection will inevitably occur. Practice has shown that the smaller the slope of the borehole, the slower the wear. Therefore, when constructing the filling hole, reliable orientation measures should be taken and the inclination should be strengthened during the construction process.
And correcting the deviation to minimize the deflection of the borehole.
Filling pipelines are an important factor in ensuring the function of drilling. High-strength, wear-resistant pipe materials should be used as much as possible. For example, high-strength surfacing and wear-resistant pipes with lining alloys should be selected as filling pipes in boreholes, although the initial investment may be more Large, but its long service life can effectively prevent drilling failure. In addition, the lining ceramic composite pipe should not be used as the main pipe in the borehole. Although the ceramics have higher rigidity, the brittleness is larger, and it is easy to bend and rupture during the installation process. When the filling is completed, large blocks fall off and block the pipe, which causes the drilling to fail. At present, there is a super wear-resistant composite pipe on the market, which is an inner layer of a modified polyolefin wear-resistant special material (wear-resistant elastomer), an ordinary polyethylene is an outer layer, and a composite die is extruded. Because the pipe of the material has the dual characteristics of wear resistance and lining prevention, and the cost is low, the installation is light, the joint is small, and the inner sealing property is good, and may become a better choice for the tailings filling pipeline of the mine in the future.

The mortar has a solid-liquid unevenness in the borehole pipeline, and the multi-directional impact ejection of the bulk during the rapid flow process causes serious wear on the pipeline. Therefore, the preparation of quality mortar is also an effective method to prevent drilling failure. The coarse and fine aggregate ratio should be precisely controlled, the mixing is even and sufficient, the conveying is continuous and smooth, and the control mortar is in compliance with the full pipe flow state after entering the drilling pipe. At present, many filling mines have introduced DCS (Digital Control System), which can automatically batch and digitally monitor precise feeding. The process interface is simple and clear, and the operation is convenient.
4 Conclusion
(1) Based on the failure mode of filling borehole, the analysis of borehole blockage is mainly caused by the problem of filling operation management and mortar quality control; the pipe wear in the borehole is mainly caused by the non-full pipe transport of the mortar, and the air column is the result of the non-full pipe. The main reason for the flow.

(2) Closed pipe full pipe transportation can effectively reduce the height of the air column in the borehole and reduce the residual potential energy of the mortar flow. Vertical drilling of deep well filling systems can be arranged in sections, or buffer tanks can be placed at appropriate locations to release mortar head energy.

(3) The local variable diameter measures are adopted to make the filling system increase the resistance properly, which can increase the hydraulic gradient of the pipeline and improve the flow state of the mortar in the borehole. The filling pipe in the drilling casing is wrapped with flexible material and is pseudo-coupled to meet the requirements for rapid replacement of damaged pipes, and the drilling can be reused.

(4) Strengthening the control of drilling construction process, scientific selection of filling pipelines, and reliable quality of mortar preparation are also effective methods to prevent drilling failure.

references
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Article source: "Modern Mining"; 2016.10;
Author: Bear promising; Changsha Nonferrous Metallurgy Design and Research Institute; Deep Mine Safety and Efficiency Mining Technology Engineering Research Center of Hunan Province;
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