Luohe Iron Mine is located in Hefei City, Anhui Province, China. It is a concealed deposit composed of multiple minerals, including largescale and high-phosphate, high-sulfur, vanadium-bearing magnetite, largescale pyrite (containing some ore bodies associated with copper), and largescale anhydrite. The iron ore body is layered and flat lenticular. It is elliptical in plane projection and its bulk is dome-shaped. In the middle of the iron ore body, there is mainly disseminated lean iron ore, while the rich and thick ore is mostly found around it. A map of the geological structure of the mine is shown in Fig. 1.

Based on the shape, dip angle, and thickness of the ore body, as well as other conditions of its occurrence, the majority of the ore bodies north of the II longitudinal exploration line exhibit a vertical open stoping at the deep hole stage and subsequent backfill, and the corners of the ore body exhibit point column, upward stratified filling mining, or medium-deep hole, sublevel open stoping and subsequent backfill. The majority of ore bodies located south of the II longitudinal exploration line were extracted with the use of sublevel open stoping with a medium-deep hole and subsequent backfill, and the corners of the ore body exhibit point column, upper stratified filling mining.

Three filling series were established at the Luohe Mine filling station. Each series was equipped with two boreholes: one for working and one for standby. The borehole in the Quaternary layer had a diameter of φ370 mm and was cased inside a φ273 × 10 mm tubing. Upon borehole reaching the bedrock, the dimension of borehole call for a φ250 mm diameter with no casing. High-grade cement slurry was used for well cementation. After the well was cemented, a bimetallic composite, wear-resistant pipeline was installed in the borehole. The pipeline dimension was φ203 × 26 mm (with a wear-resistant layer of 14 mm). A schematic diagram of the filling borehole structure is shown in Fig. 2. The windstone gate employed a high manganese wear-resistant pipeline with a diameter of φ159 × 12 mm, and the return air roadway and main roadway used ordinary steel pipelines having the same diameter. The branch pipelines of the stope were 6-inch and 4-inch PE pipelines and steel composite pipelines, respectively.

Pipeline plugging accidents have frequently occurred in the #2-1 borehole of Luohe Mine over its several years of operation. These accidents have had a serious impact on mine production. A high-definition camera was used to scan the hole to check for breakage and blockage of the inner wall of the borehole. The location of the damaged section was then recorded. It was found that the sections at 80, 130, 170 and 230 m were seriously damaged. A scanning image of the inner wall of the borehole is shown in Fig. 3. It is evident in Fig. 3 that, under the impact and friction of the slurry, the inner wall of the borehole has broken and become exposed to the bedrock. Dredging only the drilling rig will not solve the root cause of the pipeline blockage.

(1) Electromagnetic flowmeters

An electromagnetic flowmeter from Yuyao Yinhuan Flow Meter Co., Ltd., Ningbo City, China consisting of a sensor and a converter, was used. A flowmeter is an inductive instrument for measuring the volume flow of a conductive medium, as shown in Fig. 4. This study used an electromagnetic flowmeter to monitor the flow of the filling slurry in the borehole, and the data were recorded at one-minute intervals. The main technical parameters are shown in Table 1.

(2) Radioactive density gauge

The radioactive densitometer was manufactured by EcoPhysPribor, Russia, and it was used for the noncontact measurement of the basic parameters, such as the slurry and fluid density, during various flow processes in the pipeline. The device is shown in Fig. 5. The density of filling slurry in the field environment can be continuously monitored by the device, and the resulting density value is output as a signal. Data are recorded at one-minute intervals. The main technical parameters are shown in Table 2.

The mined-out area at Luohe Mine is treated using the total tailings cemented filling technology. The tailings originated from a beneficiation plant. After it was thickened by a deep cone, low-concentration tailings slurry was mixed with cementing material in a mixing barrel and then transported to the mined-out area. The concentration of filling slurry was 68%–69%, and the cement-to-sand ratio was 1:10. The sand produced at the beneficiation plant was classified into two categories: 3 series and 2 series. The concentration of sand in the 3 series was approximately 55%, with a dry sand flow rate of 360 t/h. In the 2 series, the sand concentration was approximately 40%, and the dry sand flow rate was 270 t/h.

The tailings mortar was collected from the bottom of the deep cone thickener and then sampled using the standard quartering method after it was dried. The physical parameters of the total tailings were measured and analyzed using a pycnometer and a Topsizer laser particle size analyzer. The basic physical properties of the total tailings are presented in Table 3.

At a concentration of 68%, the slurry had a bulk density of 1.82 g·cm⁻³. The yield stress and viscosity were measured under different ratio conditions using the Brookfield R/S plus rheometer equipped with a V40-20 slurry rotor. The measured rheological parameters are listed in Table 4. The yield stress of the slurry is within 100 Pa when the mass concentration is between 66% and 68%, which results in better fluidity of the filling slurry.

The drilling depth is approximately 500 m. When the casing is lowered, it is necessary to overcome both its own weight and the screw thread rotation torque. Thus, the chosen casing material was a bainite seamless steel pipeline with high strength and excellent wear resistance. The pipelines were connected using screw threads.

The original bimetallic composite pipeline had an inner diameter of 151 mm. It is recommended to reserve 9.5 to 12.7 mm of space for the cement slurry, according to engineering practice. The inner diameter of the original pipeline must be 25.4 mm larger than the outer diameter of the casing, and the outer diameter of the casing must not exceed 132 mm. Based on the hydraulic formula, the pipeline’s maximum hydrostatic pressure is at its lowest point, and it measures approximately 8.94 MPa. The performance standard for the pipeline material requires the thickness of the pipeline layer that bears pressure to be greater than 6 mm. According to the requirements of mine production, the pipeline must have a service life of at least 5 years. Therefore, the wear-resistant layer of the pipeline should be no less than 5 mm thick. Following this analysis, two types of pipelines with inner diameters of 100 and 80 mm were found to meet the specified conditions. The relevant parameters are presented in Table 5.

The Buckingham formula [23,24] is used to analyze the pipeline resistance under different pipeline diameters, as shown in Eq. (1):

(1) i=16τ03D+32vD2μ where i is the resistance during slurry transportation, Pa/m; τ₀ is the slurry yield stress, Pa; v is the slurry flow speed, m/s; μ is the slurry viscosity, Pa·s.

Fig. 6 shows the change in the pipeline resistance for a pipeline diameter of 100 mm. It is evident in Fig. 6 that at an inner pipeline diameter of 100 mm, slurry concentrations of 66% and 68% are suitable for gravity transportation at a rate of 240 m³/h. When the slurry concentration is 70%, the maximum flow rate for gravity transportation is 120 m³/h.

Fig. 7 illustrates the change in pipeline resistance for an 80 mm diameter pipeline. It can be seen from Fig. 7 that the maximum flow rate for the self-flow transportation of filling slurry of 68% concentration is 120 m³/h within a pipeline having a diameter of 80 mm. The resistance of the pipeline is inversely proportional to its inner diameter. Reducing the diameter of the pipeline results in increased resistance. The gravity of the slurry alone may not be sufficient to overcome this resistance. Therefore, the slurry cannot flow at such a high speed in the vertical pipeline and can only be pumped to replenish energy [25]. If a pipeline having an inner diameter of 80 mm is used, the filling slurry concentration must be reduced to have sufficient power [26,27]. Maintaining the appropriate slurry concentration is crucial to ensure filling quality. It is important to note that improving fluidity by reducing slurry concentration may have a significant impact on stope stability. Furthermore, even if the flow rate is reduced to 80 m³/h, the length of the horizontal pipeline for filling slurry self-flow transportation is only 950 m. The maximum distance of underground horizontal pipelines is approximately 2300 m. However, pipelines having an inner diameter of 80 mm do not meet the requirements for mine production.

The mine utilizes gravitational potential energy to transport the slurry. The distance of gravity transport is directly affected by the amount of pipeline resistance. Based on the analysis above, it is evident that the pipeline resistance is excessive when the pipeline diameter is 80 mm, and thus, this diameter fails to meet the production requirements of a mine. Therefore, this calculation only considers the self-flow transport distance of horizontal pipelines having different flow rates, given a pipeline with a 100 mm diameter. The calculation results are shown in Table 6, and the coverage of self-flow transport is shown in Figs. 8–10. When the filling flow rate is 80 m³/h, the self-flow distance of the horizontal pipeline is 2.27 km, and the stope can be fully self-filled at 455 m underground. When the filling flow rate is 140 m³/h, the self-flow distance of the horizontal pipeline is 1.39 km. At the −455 m stage, self-flow is capable of transporting slurry to most of the stopes, with only a few areas in the southwest corner unable to be covered. When the filling flow rate is 180 m³/h, the horizontal pipeline can support self-flow up to a distance of 0.99 km, allowing approximately half of the stopes at the −455 m stage to be serviced through self-flow.

It is important to acknowledge that there may be a discrepancy between the self-flow transport coverage calculated according to rheology and the value in practice, which could result in a larger self-flow transport coverage. In summary, the casing’s inner diameter is 100 mm, the pipeline’s wall thickness is 13.5 mm, the length of a single pipeline is 6.0 m, and the calculated unilateral gap is 12.0 mm. However, when a well that is 500 m deep is drilled, some deflection during the construction of the original borehole is inevitable. The deflection distance is often much larger than the gap spacing. If the casing becomes bent and stuck during the construction process, all previous efforts may become wasted. Therefore, the scientific scheme of lowering a casing pipeline is the core of borehole repair technology.

The damaged parts of the filling pipeline often exhibit lining uplift, or the lining may peel off. The inner diameter of the borehole may be thinned to varying degrees, which leads to the bending deformation of the pipeline when the casing pipeline is lowered or the inability to withdraw the casing. Deformation may ultimately cause the pipeline to become stuck. In current technology, if there is significant damage to the borehole, a drill pipe is typically used to fully dredge the drill bit to ensure the inner wall is smooth before positioning the casing. However, the gap between the single-sided pipeline wall is only 12 mm and dredging the borehole only ensures that the drill pipeline is unobstructed when the drill pipe is lowered. It could still be blocked or even become stuck when the casing is lowered. To address this issue, a construction scheme is proposed that involves installing a drill bit beneath the casing and lowering the casing into the borehole as if it were a drill pipe. In cases of significant damage, the drill bit can be utilized to rotate and impact obstacles, which significantly increases the likelihood of successful construction.

Due to the threaded connection between the pipelines, there are no redundant connecting parts on the external surface, and the pipeline appears smooth. Insufficient clamping force can cause the pipeline to fall off, which results in the entire borehole being discarded and poses a significant risk. Therefore, to ensure pipeline stability and to prevent it from falling off the well, a combination of pneumatic and manual chucks is used.

It is not possible to determine the damage or the blockage of the filling pipeline after it has been plugged. If the internally damaged fragments are not cleaned completely, the problem of “pipeline sticking” may easily occur during the process of lowering the casing. Therefore, for a less problematic construction, it is necessary to clean the inner walls before lowering the casing. The design of a construction process based on the above considerations is shown in Fig. 11, and an on-site construction drawing is shown in Fig. 12.