Research progress in the mining technology of the slowly inclined, thin to medium thick phosphate rock transition from open-pit to underground mine

a. According to the requirements of the project document, we have done a fully systematic investigation on the geological data and production materials of the inclined medium-thick phosphate orebody of No. 2 pit of Jinning Mine and the gently inclined thin-medium thick phosphate orebody of No. 4 mining area of Kunyang Phosphate Mine, and fully grasped the geological characteristics of the deposit and mining status of the two mining areas.

b. The samples were obtained by density from the inclined medium-thick phosphate orebody of No. 2 pit of Jinning Mine and the gently inclined thin-medium thick phosphate orebody of No. 4 mining area of Kunyang Phosphate Mine, and processed into standard cubes in the laboratory. Triaxial, uniaxial, shear and rheological tests were then conducted to obtain the mechanical properties and deformation characteristics of the orebody and surrounding rock.

c. This paper has used ICT tomography technology to make a systematic analysis of the fracture distribution in ore samples and rock samples, and to study the mechanical relationship between the fracture distribution and the deformation features.

d. We have used the Digital Panoramic Borehole Camera System (DPBCS) to study the distribution of broken rock around the stope, and determine the distribution of the fracture zone and the deformation zone.

e. This paper has done a theoretical research on the different mining techniques and processes of the underground mining of the inclined medium-thick phosphate orebody of No. 2 pit of Jinning Mine and the gently inclined thin-medium thick phosphate orebody of No. 4 mining area of Kunyang Phosphate Mine.

f. The study used the gently inclined thin-medium thick phosphate orebody of No. 4 mining area of Kunyang Phosphate Mine as the engineering background, and used the large similar material simulation experiment to simulate the underground mining process of gently inclined thin-medium thick difficult mining phosphate rock containing soft interlayer in the room, and researched the influence of underground mining on the surrounding rock movement and the breaking rules of underground overlying strata as shown in Figure 2.

Fig. 2

According to the subsidence displacements of the similar material model at different excavation stages (Figure 3), The mining deformation of the surrounding rock underground, and of the overlying strata develops in three stages, namely: 1) small and local deformation, 2) continuous linear increase, and 3) the violent nonlinear collapse of the entire system. Finally, the global subsidence curve is in asymmetrical trough shape and presents a nonlinear failure characteristics.

Fig. 3

g. The study used the gently inclined thin-medium thick phosphate orebody of No. 4 mining area of Kunyang Phosphate Mine as the engineering background and used the large numerical simulation software to build a mathematic model on the underground mining of gently inclined thin-medium thick difficult mining phosphate rock containing soft interlayer. To reveal the failure law of primary rock stress, and the deformation law of surrounding rock, simulating the underground mining process and quantitatively researching the deformation and failure characteristics of the surrounding rock, the moving law of rock under different mining methods.

h. The study took the deep orebody which was in the north mining area of Yunnan Phosphating Group Corporation Limited Jinning Phosphate Mine No. 2 pithead as the engineering background, and used theoretical analysis, indoor similar material simulation experiments, FLAC3D numerical simulation (Figure 4) and other comprehensive research methods. Then from the perspective of engineering mechanics, having systematically researched the deformation characteristics of overlying strata and the activity law of ground pressure during the underground mining process of inclined thin-medium thick phosphate orebody for different dip angles, different mining layers, different mining methods, different mining thicknesses, different roof hardness and different interlayer thicknesses. At the same time, it has analysed the hidden danger and disaster prevention measures in the underground mining process.

Fig. 4

i. Choosing the better mining methods to be suitable for the inclined thin-medium thick phosphate orebody of No. 2 pit of Jinning Mine by studying the degree of damage to rock by different mining methods, then determining the mining methods and mining technologies corresponding to the underground mining of inclined thin-medium thick phosphate orebody.

j. After the open-pit mining is transferred to underground mining, the open-pit end slope, surrounding rock mass and underground mining constitute a compound mining system. During the period of open-pit mining, the rock mass around the end of the open-pit slope and the bottom of pit was disturbed. On this basis, the secondary disturbance is caused by the excavation of underground, and with the advance of excavation space, the dynamic superposition of the secondary disturbance effect influences and changes the state of stress and deformation.

At the same time, the stress redistribution and the deformation and failure change of the rock mass around the open-pit slope and the bottom of the pit after being disturbed also have an important influence on underground mining, making the hazards between the two systems induced each other, thus, a composite dynamic change system is formed.

a. According to the requirements of the project assignment paper, the engineering geological conditions and hydrogeological conditions in the eastern mining area of No. 6 pit of Jinning Phosphorus Mine were systematically investigated. The physical and mechanical properties of the ore and rock body in this area were tested in the laboratory and the physical and mechanical parameters of these were obtained. Meanwhile, theoretically, the mechanical mechanism of the instability and failure of the slope after the mining method is converted from open-pit to underground and its main influencing factors are analysed.

b. According to the specific occurrence conditions of the deep gently inclined medium-thick difficult-mining ore body in the eastern mining area of No. 6 pit of Jinning Phosphorus Mine of Yunnan Phosphate Group, a typical 114 geological exploration line section is selected along with the decline of the ore body. And according to the similar theory, a similar material physical model test in the laboratory is carried out to investigate the stress, deformation and fracture response characteristics and slumping form under the influence of open-pit slope and underground mining face overlying strata mining after mining method of phosphorus mine convert from open-pit to underground. At the same time, according to the results of similar model tests, combined with relevant theories, the mechanical mechanism of the mining failure of the slope rock mass after mining method of it convert from open-pit to underground is analysed. Based on the results of similar simulation model tests, the concept of ‘mining impact factor’ considering the factors such as slope and underground mining excavation disturbance is proposed, and the theoretical calculation formula of the ‘mining disturbance coefficient’ reflecting the mine pressure characteristics such as deformation and failure of slope and underground mining face overlying strata after mining method of phosphorus mine convert from open-pit to underground, which is defined as the ratio of the maximum variation in stress to the initial stress for each excavation in the stope structure and given as follows (Eq. 1): (1) Kmax=|(σi−σ1)maxσ0|□[0,+□) K_{max} = \left| \frac{(\sigma_{i} - \sigma_{1})_{max}}{\sigma_{0}} \right| \square [0,+\square) where K_max is the mining disturbance coefficient, σ_i is the stress of each excavation step (MPa), σ₁ is the stress of the first excavation step (MPa), and σ₁ is the initial stress (MPa).

The subsidence coefficient is the ratio of the maximum subsidence of the overlying strata to the mining thickness of the phosphate rock under the full mining of the stope structure, which reflects the degree of influence the mining disturbance has on the overlying rock (Eq. 2). (2) W=WmaxD W = {{{W_{max}}} \over D} where W is the maximum subsidence of the overlying rock (m), and D is the thickness of the orebody mined (m).

c. According to the similarity theory, the deep gently inclined medium-thick difficult-mining ore body in the eastern mining area of No. 6 pit of Jinning Phosphorus Mine of Yunnan Phosphate Group is used as the engineering background. The indoor bottom friction model simulation test under the conditions of top pillar thickness in different realms (10 m, 20 m, 30 m) and different slope heights (65 m, 108 m) is used to investigate the mining response characteristics, deformation failure mechanism and mode of the slope body above the goaf after mining method of phosphorus mine convert from open-pit to underground. The mining response characteristics, deformation failure mechanism and mode were studied. Meanwhile, the influence of the thickness of the top pillar in the realm and the slope height on the stability of the mining slope was analysed.

d. Using the deep gently inclined medium-thick difficult-mining ore body in the eastern mining area of No. 6 pit of Jinning Phosphorus Mine of Yunnan Phosphate Group as the engineering background, a +2150 m ore excavation geological section was selected along the strike as the model simulation plane. Through the simulation experiment of similar materials, the deformation and fracture law of the roof surrounding rock of the mining face when mining by chamber and pillar method with different structural parameters (Figure 5, bord 10 m, pillar 3 m; Fig. 6, bord 10 m, pillar 5 m; bord 10 m, pillar 8 m) of mining face after mining method of phosphorus mine convert from open-pit to underground was studied and the structural parameters of mining face were optimised. According to the results of the model test, combined with the relevant theory, the mechanical characteristics, instability failure mode and mechanism of the pillar in the mining face were systematically analysed. Finally, using the cusp catastrophe theory, a simplified mechanical model system for the pillars when mining by chamber and pillar method is established. The mechanism and conditions of the sudden failure of the pillars are studied and analysed.

Fig. 5

Fig. 6

e. Based on the Lagrange finite difference method, the FLAC^3D numerical simulation method is used to study the chamber and pillar method mining parameters such as the reasonable thickness of the top pillar of the open-pit mine realm and the waste rock cover layer at the bottom of the open-pit, optimal structure size of underground bord and pillar, and ore mining order after the mining method of the deep gently inclined medium-thick difficult-mining ore body in the eastern mining area of No. 6 pit of Jinning Mine convert from open-pit to underground.

a. Taking the inclined medium-thick phosphorus ore body of No. 2 pit of Jinning Phosphorus Mine as the research object, based on the site investigation, the review of previous research results and theoretical analysis, the theoretical calculation and FLAC^3D numerical simulation tests for mine pressure law during single segmentation mining were carried out. The results show that the numerical simulation results are similar to the theoretical solution. The vertical stress peak caused by the mining of the working face gradually moves away from the working surface with the increase of the burial depth, and the vertical stress concentration factor gradually decreases with the increase of the burial depth. A stress-increasing zone is formed within 13 m of the floor rock stratum in front of the working face, and a stress-decreasing zone is formed in the floor rock stratum below the goaf, and its influence depth can reach 30–40 m. During single segmentation mining, the roof rock stratum is plastically damaged. The plastic zone area of the roof above the working face is the largest, and the plastic zone runs from the working face to the earth surface. Plastic failure also occurred in the stress rise zone below the working face. The damage zone is 5–10 m in front of the working face, and the maximum depth is 12 m.

b. Taking the inclined medium-thick phosphorus ore body of No. 2 pit of Jinning Phosphorus Mine as the research object, based on the site investigation, the review of previous research results and theoretical analysis, the theoretical calculation and FLAC^3D numerical simulation tests for mine pressure law during multiple segmentation mining were carried out. The results show that the stress state of the floor and the surrounding rock of the roadway is a dynamic process with underground mining. When multiple working sites are mined simultaneously, the stability of the surrounding rock of the lower segmentation roadway is affected by the upper segmentation mining. The vertical stress concentration factor and the influence range of the lower segmentation roadway are increased compared with the single segmentation mining.

When multiple working sites are mined simultaneously, the vertical stress of the lower segmentation roadway is affected by the mining. The range of stress concentration and the value of stress concentration factor are increased compared with the single segmentation mining, and the maximum principal stress value is smaller than the tensile strength of rock mass. The rock mass will not have fracture failure, but the roof of the roadway may be partially collapsed. The theoretical solution of the stress increase concentration coefficient during the different segmentation mining simultaneously is smaller than the single segmentation mining, On the one hand, it's because when digging the segmented roadway in the development project, the segmental roadway stress is released in advance. On the other hand, the reason for that is the segmented roadway in the inclined phosphorus rock layer is distributed along with the phosphorus rock layer which causes the distance between the segmented roadways which is relatively long, and the vertical stress attenuation is greater.

c. Taking the inclined medium-thick phosphorus ore body of No. 2 pit of Jinning Phosphorus Mine as the research object, based on the site investigation, the review of previous research results and theoretical analysis, the theoretical calculation and FLAC^3D numerical simulation tests for mine pressure law during mining different layers, in turn, were carried out. The results show that when the hanging side seam is mined, the stress of the roof above the goaf will be released, and the principal stress of the roof is positive, that is, tensile stress appears on the roof. As the mining progresses, the degree of stress release gradually increases, and the maximum principal stress peak of the roof is greater than the tensile strength of the roof rock mass. After the phosphorus ore body is mined to form a goaf, the stress concentration is concentrated at the top and bottom of the interlayer, and the bottom is mainly composed of compressive stress. Due to the geostatic stress, the middle part tends to move to the lower goaf. As the mining progresses, the zone of tensile stress in the interlayer increases continuously, and the stress concentration at the top gradually weakens. The maximum tensile stress generally appears in the top middle part of the lower goaf. At the end of the mining of the phosphate rock layer in the footwall, attention should be paid to the stability of the roof of the lower ore layer. If the stability of the ore layer is poor, there will be occurrence such as falling and wall caving, which are prone to accidents.

After the mining of the upper ore layer, a large-scale plastic failure zone appeared in the roof of the goaf, and the top pillar of the hanging side phosphorus seam mined by the sectional shrinkage stoping-caving method collapsed and then fell. In the lower mining layer, the location and area of the plastic zone continuously change dynamically as the mining progresses. After the early shallow-hole shrinkage mining in the lower ore layer, the newly added plastic zone mainly appears on the top and bottom of the roof of the hanging side seam, and there is no plastic failure in the interlayer. After the later mining of the lower ore layer, the plastic zones appear in the top middle and top parts of the goaf. After further mining, the plastic zone is further expanded, mainly concentrated in the middle and top of the goaf. Because the dolomite of the Meicun formation is hard and complete, the rock strength is large, and there is no plastic zone on the bottom plate.

d. Taking the deep inclined medium-thick phosphorus ore body in the northern mining area of No. 2 pit of Jinning Phosphorus Mine of Yunnan Phosphorus Group as the research object. Through the site investigation, theoretical calculation, FLAC^3D numerical simulation and other comprehensive research methods, a systematic study on the rock mass displacement law of single segmentation mining, simultaneous mining of different segmentations and mining of different ore layers were conducted. The results show that with the increasing depth of underground mining, the mining impact range and the displacement value of the roof continuously change dynamically. The displacement variation law is analysed from three aspects: roof, interlayer and surface. After the phosphate ore body is mined to form a goaf, the upper part of the goaf becomes a pressure relief zone, and the roof appears in different degrees of subsidence. The maximum sinking area is located in the upper-middle part of the goaf and dynamically moves down as the mining level continues to advance. As the mining continues downwards, the vertical distance of the goaf continues to increase, and the amount of subsidence of the roof of the stope gradually increases.

When the hanging side seam is mined, a bottom drum phenomenon occurs on the bottom plate. When the lower phosphorus rock layer is mined, the interlayer rock mass of the bottom drum sinks in the direction of the bottom plate goaf. The position of the maximum displacement value is gradually moved downwards from the point at the first mining of the lower plate, and the maximum values are 0.68 m and 0.92 m, respectively. Influenced by the mining of the phosphorus rock layer on the upper and lower plates, the maximum and influence range of surface subsidence increases. The position of the maximum displacement value is gradually moved from the middle of the goaf to the mining direction, and the maximum value is increased from 0.11 m to 0.40 m. The mining impact range also increases with the excavation, increasing from 7.2 m to 22.7 m.

a. Taking the multiple cracks gently inclined slope in the east mining area of No. 6 pit of Jinning Phosphate Mine containing soft interlayer as the research object, through some comprehensive research methods such as site investigation, laboratory test, in-situ test, numerical calculation of rigid body limit equilibrium method and engineering analogy,. there has been a systematic analysis of engineering overview, geological survey and exploration in the early stage, stability analysis, the treatment scheme and the monitoring measurement plan of phosphate rock high slope. The results showed that the rock of slope surface had 3 groups of main structural planes, two groups of them are jointed planes, the other one is bedding plane. Jointed planes were mainly concentrated in 220 ∼ 265° ∠70 ∼ 78°and 305 ∼ 354°∠82 ∼ 89°, bedding plane was mainly concentrated in the range of 60 ∼ 80°∠20 ∼ 30°landslide boundary was mainly controlled by these two groups of joints, and the bottom plane slid along the bedding plane. The three groups of structural planes cut the rock into blocks, the strength of potential sliding plane would be reduced under the heavy rainfall and blasting load, and it would easily form the pulling type, multistage sliding pull crack type failure under the condition of the perimeter rock cut by the crack. Currently, the most possible potential sliding surface was white (yellow) phosphorus-containing clay rock in the second member of Meishu Village Group (∈1m2), the buried depth of this layer was about 5–20 m, and the thickness was uneven. It generally showed increasing thickness from north to south, and this layer was the main control plane of the middle-shallow layer landslide in the slope.

If the slope adopts a single-slope type, as long as the excavation did not cut the rock layer in the process of continuing mining for 100 m, the strata had no obvious shear outlet, the possibility of sliding pull crack type failure was small. But with the increase of rheological displacement and the gravity function of slope, the rock layer would generate shear slip along the weak interlayer or bedding, the middle and lower rock layer may have slight uplift along the free face. When the bending and the uplift raise to a certain stage, the slope had the possibility of buckling or bending failure.

The slope was almost impossible to generate deep slip, as long as the deep bedding structural plane was not cut off to form a free face during the excavation process, but the slope still could undergo middle layer sliding and shallow sliding. The possibility of of occurrence of middle layer sliding was small for the slopes of I–I’ and III–III’ sections, so the shallow sliding needed to be reinforced. However, the possibility of occurrence of middle layer sliding was large for the slope body of II–II’ section, and it similarly needed to be reinforced to prevent slope failure. Currently, many places of shallow landslides were in the critical state or partial slip state, if it does not consider the weakening phenomenon of rock parameters with water and time, but consider the reduced situation of mechanical parameters with long-term creep after the landslides slid, the landslides would continue to slide, so it was necessary to strengthen monitoring and partial reinforcement treatment.

According to the characteristics of bedding high slope of phosphate rock, making a feasibility and economic contrast of several schemes such as anti-slide pile and anchor cable (rod) reinforcement, this paper thought that it could take several measures such as anchor cable (rod) combined reinforcement, slope cutting, lay down the anti-seepage geomembrane and gunite in this slope treatment project. To effectively predict the trend of slope deformation, at the same time, made the slope normal construction and provided a basis for dynamic design. All-round and multi-angle monitoring and arranging of surface displacement, deep displacement and supporting structure stress are carried out during the slope construction period. As for the construction period and heavy rain period, the shallow displacement was initially determined once a day, the deep displacement was once every two days, anchor cable monitoring was once a week, crack monitoring was once every two days; it was usually once a week for normal observation, and the threshold was 3–5 mm/day for early-warming standard. Besides, as for the frequency of monitoring and early-warning standards, it was necessary to adjust the frequency and warning threshold combined with specific engineering conditions.

b. Taking the oversize high-steep slope of Jianshan Phosphate Mine as the research object, combining site investigation and indoor analysis, qualitative analysis and quantitative analysis, engineering geology and rock mechanics, theoretical analysis and laboratory test, this paper emphasised the technical conditions of open-pit mining and the analysis of mechanical properties of slope rock mass, and paid attention to the study of deformation failure mechanism and instability mode of slope excavation, we have done a systematic research on the stability and control technology of the slope, which was excavated layer by layer in Jianshan Phosphate Mine. The main research results are as follows: Based on field investigation, theoretical analysis, experimental research and simulation calculation, put forward the ‘plane mixed-mode’ of deformation failure of the slope, which was excavated layer by layer in Jianshan Phosphate Mine, the shape of slip surface was linear type in the upper and circular arc type in the lower. By analysing the corresponding engineering geological conditions, we obtained that changes of occurrence in the deep ore body were the main reasons for the cause of the instability of slope excavation, and due to the above reason and the cutting of dominant structural plane to the slope rock, it is easy to form unstable landslides.

c. Based on the results of laboratory tests, results of field rock shear tests, back analysis of slope geotechnical parameters, we have determined the macroscopic rock mechanics parameters of slope rock in the study area. Then we used the analysis method of catastrophe theory, studying the slope disaster mechanism, the necessary and sufficient conditions for slope instability were pointed out, and the stability of slope excavation has been verified. Besides, we used FLAC3D numerical calculation software to establish a 3D geomechanics model and analysed the dynamic evolution process of slope excavation in Jianshan Phosphate Mine, and the simulation results showed that the slope would be unstable if we continue to extend mining under the existing mining technology, and the potential slip instability of slope was a slip-compression-bending failure. Based on the occurrence characteristics of deep orebody, the study has compared anchor cable reinforcement to slope cutting and press stand, and put forward that the comprehensive technical measure for the stability of extended mining was slope cutting and press stand.

d. With the implementation of research results, it could effectively solve the problem of stability of deep mining in the high-steep slope of Jianshan Phosphate Mine, and it could add 800,000 tons of deep orebody in the eastern mining area of Jianshan Phosphate Mine. If the unit price was 100 yuan per ton, and the mining cost increased by 4 yuan per ton due to the governance cost (The total amount of orebody was 7.5 million tons during the period of 2011.07–2013.10), it would bring a direct economic benefit of 50 million yuan in total.

Using the traditional caving method and the empty field method to mine the gently inclined thin to medium-thick phosphate ore body with 3–5 m thick soft interlayer, there are problems of high ore depletion rate and large loss rate, and it is difficult to guarantee safety.

Using the traditional filling method to mine the gently inclined thin to medium thick phosphate ore body with 3–5 m thick soft interlayer, it can solve the problem of high ore depletion rate, large loss rate and safety, but due to the lower grade of phosphate rock and the increase of impurity content, the economic value is lower and cannot withstand the mining cost of traditional filling method. And the production capacity of the filling method cannot meet the requirements of large-scale mining.

Overall transition of open-pit mining, underground mining, open pit and underground combined mining, isolation roof thickness, influences separately and each other on blasting vibration on open pit and underground mining, and the relationship between slope stability and stability of underground roadway, surrounding rock and stope need to perform quantitative analysis to establish a system balance. The experience of existing metal mines cannot be copied and applied because of the different occurrence states and conditions of the ore body.

It is necessary to find a filling material that is relatively inexpensive and easy to localise in the mine. From the perspective of the two large systems in the open and underground, the engineering practice of open-pit and underground mining with overall benefit and safety optimisation is achieved through the low-cost filling method to solve the mining problem of gently inclined thin to medium thick phosphate deposit with a soft interlayer.

It is imperative to accelerate the development progress of complete sets of equipment suitable for large-scale filling system technology, and there should be focus on automation, mechanisation and high efficiency so that the failure rate of filling mining can be reduced, and the filling scale can reach 2–3 million square metrer per year to adapt to capacity requirements after the open-pit mines are converted to underground mining.

It is important to strengthen the links between universities, enterprises, design institutes, professional technology and engineering companies, and carry out joint research in three stages of open-pit, underground and open-pit mining. From the four aspects of theoretical research, practical application research, engineering experiment and verification research, and industrialisation, we can gradually solve the overall transition of open-pit mining, underground mining, open pit and underground combined mining in the process of mining method convert from open-pit to underground of gently inclined phosphate deposits with soft interlayer; isolation roof thickness; the influences separately and each other on blasting vibration on open pit and underground mining; slope stability and stability of underground roadway, surrounding rock and stope and other issues.