Strip mine slope coal mining method based on layered and segmented freezing reinforcement of weak layer

Abstract

The invention discloses a strip mine slope coal mining method based on layered and segmented freezing reinforcement of a weak layer. The mining method comprises the steps that S1, the position of a water-containing weak layer of a mining area is determined, and the mining area is divided into a plurality of construction sections; s2, a water-containing weak layer in the first construction section is manually frozen, a frozen reinforced wall is formed, after the mechanical strength of the frozen reinforced wall meets the design slope stability requirement, surface soil and rock stratum stripping is conducted on the section, and then a coal seam is mined; s3, when the Nth construction section (N is greater than or equal to 1) is mined, manually freezing the water-containing weak layer of the (N + 1) th construction section; after mining of the Nth construction section is completed, waste rocks generated by stripping of the (N + 1) th construction section are used for conducting inner row backfilling and foot pressing on the side slope foot of the Nth construction section; and S4, the step S3 is repeated, and operation of all the construction sections is completed in sequence till construction is completed. According to the mining method, the weak layer of the slope can be actively reinforced essentially, so that safe, efficient and large-scale recovery of the slope coal is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal mining, and particularly relates to an open-pit mine side slope coal mining method based on layered and segmented freezing reinforcement of a weak layer. BACKGROUND

[0002] Open-pit mining is an important part of coal energy supply, and has the advantages of safety, high efficiency, and high recovery rate. With the gradual depletion of shallow resources, the mining depth and slope height of open-pit mines are increasing, and the problem of slope stability is becoming increasingly prominent. At the same time, in the process of open-pit mining, the coal seams located in the final side slope (i.e., "side slope coal") are left in large quantities due to the requirement of slope stability, resulting in significant resource waste. At present, for the mining and recovery of side slope coal, the related technologies propose a throwing blasting-hang bucket shovel non-transportation mining process and a shortwall fully-mechanized mining technology.

[0003] The throwing blasting-hang bucket shovel non-transportation mining process performs strong throwing blasting in the side slope coal area, throws the coal-rock mixture to the pit bottom, and then uses a large hang bucket shovel to dump the heap. Although this method has been successfully applied in certain large mines, the equipment investment is extremely high, and the severe blasting operation causes serious disturbance to the slope rock mass, which easily induces potential landslide risks. At the same time, this process produces a large amount of dust and noise, and the environmental problems are prominent. Moreover, its operation space is parallel to the main mining area, and there is a time-space cross interference, which limits its applicability.

[0004] The shortwall fully-mechanized mining technology moves the shortwall coal mining machine or continuous miner used in underground mining to the open-pit slope platform for operation. This method has high flexibility, is suitable for different sizes of side slope coal, and has a good resource recovery rate. However, its operation space is extremely narrow, the production efficiency is low, the equipment needs to be moved frequently, and the operating cost is high. More importantly, this technology directly mines on the weak layer slope, which seriously damages the original mechanical balance of the slope rock mass and greatly increases the risk of landslide accidents, and the operation safety highly depends on complex real-time monitoring and expensive passive support measures.

[0005] The mining methods in the related technologies all belong to the disturbance mining mode, that is, without effective reinforcement of the slope itself, especially the mechanical weak layer (such as the water-rich argillaceous soft rock layer) therein, direct mining activities are performed, which significantly weakens the slope stability, forces the slope design angle to be maintained at a relatively gentle level (usually 30°-40°), and thus limits the recoverable coal amount. In order to control the landslide risk, high-cost slope monitoring and treatment need to be performed, or the mining needs to be abandoned, resulting in a difficult-to-reconcile contradiction between safety and recovery. Especially in open-pit mines where argillaceous soft rock weak layers are generally present, and the temperature difference between cold and hot climates is large, the freeze-thaw cycle further aggravates the deterioration of the weak layer, making the safe recovery of side slope coal face greater challenges.

[0006] In summary, the side coal recovery process in the relevant technologies has fundamental defects such as poor safety, low economic efficiency, large disturbance, and harsh applicable conditions. Summary of the Invention

[0007] The present invention aims to at least partially solve one of the technical problems in the related art.

[0008] Therefore, embodiments of the present invention propose an open-pit mine sidewall coal mining method based on layered and segmented freezing reinforcement of weak layers, which can proactively reinforce weak layers of slopes in essence, thereby achieving safe, efficient, and large-scale recovery of sidewall coal.

[0009] The open-pit coal mining method based on layered and segmented freezing reinforcement of weak layers according to embodiments of the present invention includes: S1, determine the water-bearing weak strata in the side coal mining area, and divide the mining area into multiple sequentially connected construction sections along the strike; S2, artificially freeze the water-bearing weak layer in the first construction section to form a frozen reinforced wall. After its mechanical strength meets the design slope stability requirements, the topsoil and rock strata are stripped from the section, and then the coal seam is mined. S3. While mining the Nth construction section (N≥1), the water-bearing weak layer of the N+1th construction section is artificially frozen. After the Nth construction section is mined, the waste rock generated from the stripping of the N+1th construction section is used to backfill the slope toe of the Nth construction section. S4. Repeat step S3 to complete the mining, freezing reinforcement and slope toe backfilling operations of all construction sections in sequence until the entire side coal mining area is completed.

[0010] In some embodiments, in step S1, a freezing borehole network is designed based on the distribution of aquifers, wherein the spacing L between adjacent boreholes on the same exploration line is determined by the following formula: L=2R*tanα Where R is the design freezing radius and α is the design borehole inclination angle.

[0011] In some embodiments, in step S1, the artificial freezing is layered freezing, and the designed depth H of the freezing borehole is determined based on the bottom elevation of the target water-bearing weak layer, specifically: H = h1 - h2 + Δh Where h1 is the borehole elevation, h2 is the bottom elevation of the target aquifer, and Δh is the safety margin for over-depth, which is greater than or equal to 2 meters.

[0012] In some embodiments, during layered freezing, the effective freezing depth h of a single hole is set for the target water-bearing weak layer, specifically as follows: h = h3 - h2 + Δh′ Where h3 is the top elevation of the target aquifer, h2 is the bottom elevation, and Δh′ is the upward and downward extension freezing margin, which is greater than or equal to 4 meters.

[0013] In some embodiments, in step S1, when dividing the construction sections, the frozen areas of adjacent construction sections are connected at the joint by means of inclined drilling or overlapping arrangement of freezing holes.

[0014] In some embodiments, in steps S2 and S3, the artificial freezing is dynamically controlled freezing, and the cooling power and freezing cycle are dynamically adjusted based on the real-time feedback data from temperature sensors and displacement monitoring devices installed in the freezing area and slope.

[0015] In some embodiments, in step S3, the inner backfill footing also includes constructing a masonry dam on the backfilled waste rock body, with a drainage pipe pre-embedded in the masonry dam.

[0016] In some embodiments, after the coal seam and rock strata are exposed in each construction section, the exposed coal seam and rock strata are covered and sealed with multiple layers. The sealing layer includes, from the inside out, at least a mud penetration layer, an impermeable isolation membrane, and a compacted protective soil layer.

[0017] In some embodiments, step S2 further includes optimizing the final design slope angle of the mining slope based on the mechanical parameters of the weak layer after freezing and reinforcement, wherein the optimized design slope angle is greater than the original design slope angle under the unreinforced condition.

[0018] The open-pit coal mining method based on layered and segmented freezing reinforcement of weak layers in this invention first uses layered and segmented artificial freezing technology to precisely reinforce the water-bearing weak layers in the slope, transforming them from mechanically weak links into high-strength frozen and reinforced walls, eliminating the main cause of landslides at the source and achieving inherent safety.

[0019] Based on this, the segmented flow operation and internal drainage footing process efficiently couple the freezing, mining and backfilling processes in time and space, greatly shortening the construction period. Furthermore, the waste rock from the subsequent section is used to permanently support the slope of the previous section, realizing the on-site resource recycling of waste, significantly reducing costs and improving efficiency.

[0020] Scientific optimization of the slope design angle based on measured mechanical parameters after freezing allows for steeper slope angles under the same safety factor, thereby significantly increasing recoverable coal reserves and directly translating safety investment into economic benefits. Furthermore, the accompanying multi-layer sealing technology for coal seam outcrops effectively prevents spontaneous combustion of coal seams, completing a comprehensive safety and environmental protection loop. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the internal discharge foot of the side coal segmented mining according to an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the cryogenic drilling design according to an embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of the layered and segmented freezing technology for drilling according to an embodiment of the present invention.

[0024] Figure 4 This is a schematic diagram of the layered coverage of the coal outcrop on the side of the wall according to an embodiment of the present invention.

[0025] Figure 5 This is a schematic diagram of slope optimization according to an embodiment of the present invention.

[0026] Figure label: 11-Aquifer weak layer; 12-Stripping zone; 13-Frozen borehole; 14-Masonry dam; 141-Drainage pipe; 15-Mud seepage layer; 16-Impering membrane; 17-Compacted protective soil layer; 18-Original design slope; 19-Design slope after freezing; 21-Bedrock; 22-Coal seam; 221-B1 coal seam; 222-B3 coal seam; 223-B5 coal seam; 23-Mudstone; 24-Coarse sandstone; 25-Rock strata. Detailed Implementation

[0027] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0028] The following describes, with reference to the accompanying drawings, an open-pit coal mining method based on layered and segmented freezing and reinforcement of weak layers.

[0029] like Figures 1 to 4 As shown, the open-pit coal mining method based on layered and segmented freezing reinforcement of weak layers according to an embodiment of the present invention includes: S1, determine the position of the water-bearing weak layer 11 in the side coal mining area, and divide the mining area into multiple sequentially connected construction sections n along the strike.

[0030] Based on geological survey reports, borehole data, and geophysical data, the location, thickness, and distribution range of weak rock layers (such as argillaceous soft rock) with poor mechanical properties and high water content in the slope profile are accurately identified. Then, based on the effective influence radius of the freezing project, equipment operating efficiency, and slope stability calculations, the entire side slope area to be mined is cut into several independent units of reasonable length along its extension direction (strike), namely construction section n.

[0031] This step forms the basis for all subsequent refined operations. Identifying the weak layer is to pinpoint the target area requiring reinforcement, ensuring precise and effective intervention measures and avoiding resource waste. Segmentation creates the space for introducing a freeze-mine-backfill continuous operation mode to resolve the conflict between continuous long slopes and limited working faces.

[0032] S2, artificially freeze the water-bearing weak layer 11 within the first construction section n1 to form a frozen reinforced wall. After its mechanical strength meets the design slope stability requirements, strip the topsoil from the rock layer in this section. Figure 1 (as shown in stripping zone 12), followed by mining of coal seam 22.

[0033] Within construction section n1, for the identified water-bearing weak layer 11, freezing boreholes 13 are drilled and a low-temperature working medium (such as low-temperature brine) is circulated to freeze the pore water in the weak layer, cementing the soil and rock particles, thereby significantly improving its compressive and shear strength and forming a continuous, high-strength frozen reinforced wall. After verifying that its strength meets the standards through in-situ tests (such as vane shear and wave velocity tests), conventional stripping and coal mining operations are then carried out.

[0034] Before disturbing the slope, its internal mechanically weak zones are modified in advance, transforming weak layers that could potentially trigger landslides into stable structures. This fundamentally changes the operational premise, significantly reducing the safety risks of subsequent stripping and mining activities under reinforced wall protection.

[0035] S3. While mining the Nth construction section (N≥1), the water-bearing weak layer 11 of the N+1 construction section is artificially frozen. After the Nth construction section is mined, the waste rock generated from the stripping of the N+1 construction section is used to backfill the slope toe of the Nth construction section.

[0036] When the equipment is mining coal in section N, the freezing system has already started operating in the adjacent section N+1 to pre-reinforce it. After section N is mined out, a free face is formed at its toe. At this time, the waste rock (soil, rock) generated from section N+1, which is being stripped, is immediately transported directly to the toe of section N for backfilling and compaction, and structures such as masonry dams can be constructed.

[0037] By parallelizing the two time-consuming processes of freezing (long cycle) and coal mining (relatively short cycle) in space, the waiting time between them was avoided, ensuring that the reinforced slope wall always advanced ahead of the mining face, forming a dynamic safety barrier. Utilizing waste rock from subsequent sections to provide anti-sliding force to the preceding sections achieved slope treatment using waste material. This not only saved on waste rock transportation costs but also proactively enhanced the long-term stability of the already mined slopes, resolving the risk of instability after slope exposure following mining.

[0038] S4. Repeat step S3 to complete the mining, freezing reinforcement and slope backfilling of all construction sections n in sequence until the entire side coal mining area is completed. This achieves systematic, standardized and replicable safe and efficient mining of the entire long side coal mining area, ensuring that the entire operation process follows the unified safety logic of pre-reinforcement, re-mining and immediate support from beginning to end.

[0039] It is understandable that mining methods in related technologies involve direct excavation when the slope's mechanical state is unknown or weak. The mining method of this invention, however, proactively raises and locks the slope's mechanical state (especially the strength of the weak layer) to a known, safe threshold or higher before excavation by freezing.

[0040] Spatial resources were created through segmentation, and temporal resources were reorganized through process overlap. Spatially, three parallel subspaces were formed: a freezing preparation zone, a mining operation zone, and a backfilling stabilization zone, ensuring that different types of operations do not interfere with each other. Temporally, the freezing and curing period was hidden within the preceding mining period, achieving time folding. Materially, waste rock was redefined as support material, completing a closed-loop material flow cycle within the system, greatly improving economic efficiency and environmental friendliness.

[0041] Throughout the entire mining process, a freezing reinforcement wall was always forming in front of the slope as an advance support, while a continuously piled-up pressure foot counterweight body served as a follow-up guarantee.

[0042] Therefore, the mining method of this invention fundamentally eliminates the main cause of landslides in weak layers by actively reinforcing the slope, transforming the probabilistic risk of slope instability into a deterministic control of achieving the required strength, thus improving the safety level.

[0043] Because slope stability is guaranteed, the design slope angle can be significantly increased (e.g., from 35° to 65°), so that the amount of coal that can be mined can be significantly increased at the same slope height, the resource recovery rate can be greatly improved, and direct economic benefits can be created.

[0044] For example, precise layered freezing avoids the energy waste of full-section freezing, and internal discharge footing saves huge external transportation costs and land occupation fees for spoil heaps, achieving zero external discharge of waste rock and on-site resource utilization, which meets the requirements of green mining.

[0045] Furthermore, the assembly line operation mode reduces equipment idle time, shortens the overall construction period, and improves overall construction efficiency, making side coal recovery more attractive in terms of economic benefits.

[0046] In some embodiments, such as Figure 2 As shown, in step S1, based on the distribution of the water-bearing weak layer 11, a network of freezing boreholes 13 is designed, wherein the spacing L between adjacent boreholes on the same exploration line is determined by the following formula: L=2R*tanα Where R is the design freezing radius and α is the design borehole inclination angle.

[0047] After the weak layer identification and preliminary segmentation are completed in step S1, the detailed design phase of the freezing project begins. The engineers have the spatial morphology (occurrence, thickness, and depth) of the target aquifer weak layer 11. The freezing process and equipment have been selected, thereby determining the effective design freezing radius (i.e., the radius of the freezing cylinder) for a single borehole under specific geological conditions.

[0048] Based on the slope geometry and the attitude of the weak layer, the optimal borehole design dip angle α was determined so that the borehole could penetrate the target weak layer with the most efficient path.

[0049] Along the slope direction, a series of parallel exploration lines (e.g., KT01, KT02, KT03, etc.) are laid out. On each exploration line, the center distance L between adjacent frozen boreholes 13 (e.g., between ZK11 and ZH12) on that line is calculated using the formula L=2R*tanα.

[0050] Based on the calculated spacing L, the borehole opening position of each borehole is precisely marked on the exploration line, thus forming a borehole network covering the weak layers of the entire construction section. Subsequent drilling, lowering of freezing pipes and fluid supply pipes are carried out according to this network.

[0051] This shift from relying on vague judgments based on individual engineer experience to precise calculations guided by clear mathematical models enhances the reliability and repeatability of the design. Formulas ensure the continuity of the frozen wall and the absence of structurally weak zones, providing a reliable technical foundation for the entire active reinforcement concept.

[0052] The formula provides the maximum permissible hole spacing while ensuring safety (interlocking). This design allows for the use of the fewest possible boreholes while meeting engineering requirements, thereby maximizing cost savings in drilling, pipe freezing, and cooling consumption, and avoiding resource waste caused by overly conservative approaches.

[0053] In some embodiments, such as Figure 3 As shown, in step S1, the artificial freezing is layered freezing. The design depth H of the freezing borehole 13 is determined based on the bottom elevation of the target water-bearing weak layer 11, specifically: H = h1 - h2 + Δh Where h1 is the borehole elevation, h2 is the bottom elevation of the target aquifer stratum 11, and Δh is the safety margin for over-depth, which is greater than or equal to 2 meters.

[0054] It is important to note that Figure 3In the vertical direction, from bottom to top, the layers are bedrock 21, B1 coal seam 221, mudstone 23, B3 coal seam 222, water-bearing weak layer 11, B5 coal seam 223, coarse sandstone 24, mudstone 23, and water-bearing weak layer 11.

[0055] The ground elevation h1 at the borehole location is known (obtained through measurement), and the bottom elevation h2 of the target water-bearing weak layer 11 (such as the key mudstone layer) is accurately obtained through geological exploration.

[0056] Engineers directly apply the formula H = h1 - h2 + Δh for calculation. Here, h1 - h2 calculates the theoretical vertical distance from the surface to the weak stratum floor, ensuring the borehole penetrates the entire target weak stratum. Δh is a pre-set value based on engineering experience and specifications, explicitly stipulated to be no less than 2 meters. The drilling rig drills to the designed depth H, ensuring the final borehole position penetrates the weak stratum floor and extends at least Δh (≥2 meters) below it. Geological survey data may contain errors, and stratigraphic interfaces may fluctuate. A depth exceeding 2 meters ensures effective penetration even if the actual stratum floor is deeper than predicted.

[0057] The freezing process should not only occur within the weak layer, but ideally form a frozen root in the stable strata below its bottom, similar to the anchoring section of an anchor bolt. This can more firmly lock the frozen reinforced wall in the underlying stable rock and soil, preventing the reinforced wall from undergoing overall shear slip along the bottom plate interface, and greatly improving the reliability of the reinforcement.

[0058] By mandating safe ultra-deep drilling (Δh≥2m), the quality risks of insufficient depth leading to suspended or inadequately anchored frozen reinforced walls are eliminated from the design stage. Drilling depth design is simplified from a complex geological problem into a clear arithmetic rule, improving design efficiency and consistency across different projects.

[0059] Furthermore, in stratified freezing, the effective freezing depth h of a single well is set for the target aquifer weak layer 11, specifically as follows: h = h3 - h2 + Δh′ Where h3 is the top elevation of the target aquifer 11, h2 is the bottom elevation, and Δh′ is the upward and downward extension freezing margin, which is greater than or equal to 4 meters.

[0060] Based on the determined drilling depth, the elevation h3 of the top plate of the weak layer is further clarified. The formula h = h3 - h2 + Δh′ is applied. Where h3 - h2 is the actual thickness of the weak layer. Δh′ is the extension freeze allowance, which is specified to be no less than 4 meters. This 4 meters usually refers to an extension of approximately 2 meters above the top plate and below the bottom plate of the weak layer (or allocated according to thermal calculations).

[0061] At the top and bottom boundaries of the weak layer, the temperature may not reach as low as in the core region, and strength growth may be insufficient. Extending the freezing upwards and downwards ensures that every point along the full thickness of the weak layer is within the effective freezing zone where strength growth is sufficient, rather than in a semi-frozen transition zone. Strengthening the weak layer interface significantly reduces the risk of the frozen wall failing prematurely due to end creep or melting during long-term service.

[0062] In some embodiments, in step S1, when dividing the construction sections, the frozen areas of adjacent construction sections are connected at the joint by means of inclined drilling or overlapping arrangement of freezing holes.

[0063] It is important to note that segmented slope construction inevitably creates joints between adjacent sections. If two independent, perpendicular frozen areas are simply placed side-by-side, a vertical cold joint, either unfrozen or weakly frozen, is very likely to form at the joint. This cold joint will become a mechanical weakness in the entire frozen reinforced wall, potentially becoming a sliding surface or seepage channel, thus negating the overall effectiveness of the segmented reinforcement.

[0064] Therefore, at the joint of the frozen area between adjacent construction sections, inclined holes can be drilled. These holes are not drilled vertically downwards, but at a certain angle, pointing towards the adjacent construction section. This allows individual frozen columns to extend laterally in the joint area, effectively interlocking and intertwining with the frozen bodies in the adjacent section, forming a mechanically interlocking structure. Alternatively, the frozen areas of adjacent construction sections can use overlapping freezing holes at the joint, ensuring that even at the joint, the freezing temperature field is continuous and superimposed, preventing low-temperature blind spots.

[0065] In some embodiments, in steps S2 and S3, the artificial freezing is dynamically controlled freezing, and the cooling power and freezing cycle are dynamically adjusted based on the real-time feedback data from temperature sensors and displacement monitoring devices installed in the freezing area and slope.

[0066] Temperature sensors (such as thermocouples and thermistors) are deployed in layers within the frozen borehole 13, the target weak layer, and the surrounding soil and rock mass that may be affected by the frozen wall to monitor the formation and evolution of the temperature field in real time. Displacement gauges, inclinometers, GNSS monitoring points, etc. are deployed on the slope surface and inside (through boreholes) to monitor the minute deformations of the slope during freezing and mining processes in real time.

[0067] Monitoring data is transmitted in real time to the central control room or cloud analysis platform. The measured temperature data is compared with the freezing design model (such as target frozen wall thickness and average temperature) to determine whether the freezing development rate meets the standards, whether it is uniform, and whether there are weak areas. The measured displacement data is compared with the slope stability early warning threshold to evaluate the actual effect of freezing reinforcement and the overall safety status of the slope.

[0068] If monitoring shows that the freezing development rate is slower than expected, or the temperature in a certain area is too high, increase the brine circulation flow rate of the corresponding freezing pipe in that area or decrease the brine temperature (increase power) to accelerate freezing.

[0069] If the frozen wall has already reached the designed thickness and strength, or the slope displacement is extremely stable, the cooling power can be appropriately reduced to maintain the frozen wall and avoid over-freezing.

[0070] If the intensity reaches the target ahead of schedule, the main freeze period can be ended early, and the freeze can be maintained.

[0071] If abnormal situations are encountered (such as the discovery of local hot water flow), the freezing time of that area can be extended.

[0072] The duration of the frozen wall should be precisely controlled according to the subsequent mining progress. Freezing can only be stopped after the backfilling of the footing is completed and the slope is permanently supported.

[0073] Therefore, dynamic control can provide early warnings of potential risks (such as uneven freezing or local non-cohesion) through displacement and temperature anomaly signals, and immediately intervene by automatically or manually adjusting parameters to nip potential problems in the bud. Through precise control of the temperature field, it is possible to ensure uniform temperature and strength development throughout the frozen reinforced wall, especially between different boreholes and in the joint areas, avoiding the formation of local weak points and thus guaranteeing the overall mechanical properties of the reinforced body.

[0074] In some embodiments, such as Figure 1 As shown, in step S3, the inner backfill foot also includes constructing a masonry dam 14 on the backfilled waste rock body, and a drainage pipe 141 is pre-embedded in the masonry dam 14.

[0075] First, the waste rock (mixture of soil and rock) stripped from the N+1 construction section (e.g., the 2nd construction section n2) is transported and dumped to the toe of the slope formed after the Nth construction section (e.g., the 1st construction section n1) is mined. The waste rock is then initially spread and compacted using equipment such as bulldozers to form a loose mass foot stockpile with a certain volume and density.

[0076] The rubble and gravel from the waste rock dumping are mixed with cement and sand to form mortar. One or more dam structures are then constructed on the top and / or outer free face of the primary toe body using a mortar-masonry technique. The dam body typically has a trapezoidal or gravity-type cross-section, and its foundation rests on the toe body or a treated stable foundation.

[0077] The masonry dam 14 is a rigid structure with high shear and compressive strength. It not only transmits the force generated by the compressive load, but its structure itself also provides an active anti-sliding barrier, increasing the lateral restraint force on the slope toe. The masonry dam 14 binds and locks the loose lower toe body into a more integrated composite to a certain extent, preventing the toe body from undergoing internal shear failure or leading edge collapse under slope thrust, thus improving the integrity and effectiveness of the toe structure.

[0078] During the construction of the masonry dam 14, drainage pipes 141 are pre-embedded through the dam body at certain intervals (such as horizontal and vertical intervals). The drainage pipes 141 are usually made of PVC or PE perforated pipes, and the pipes are wrapped with crushed stone or geotextile as a filter layer to prevent fine particles of silt from being lost and causing pipe blockage.

[0079] The pre-embedded drainage pipe network 141 forms an artificial drainage channel system that can quickly draw out and drain the accumulated water inside the toe body and at the toe of the slope behind the dam. Through continuous drainage, the groundwater level and pore water pressure in the toe area and adjacent slope toe are effectively reduced, maintaining the high effective stress state of the rock and soil mass and avoiding adverse hydrostatic pressure and frost heave damage to the masonry dam 14 body caused by water pressure.

[0080] In some embodiments, such as Figure 4 As shown, after the coal seam 22 and rock strata 25 are exposed in each construction section, the exposed coal seam 22 and rock strata 25 are immediately covered and sealed with multiple layers. The sealing layer includes at least a mud penetration layer 15, an anti-seepage isolation membrane 16, and a compacted protective soil layer 17 from the inside to the outside.

[0081] After the coal seam 22 in a certain construction section is mined out and the sidewall (i.e., the outcrop) is fully exposed, work begins immediately (usually within a few hours to a day). First, the loose coal and loose rocks on the surface of the outcrop are cleared to make it relatively flat.

[0082] Layered construction (from inside to outside): The first layer is the mud penetration layer 15. Using existing loess, fly ash (power plant solid waste), and water, a viscous slurry is mixed. A small amount of cement or water glass can be added to enhance adhesion and curing. The slurry is then evenly applied to the surface of the coal seam 22 outcrop using a spraying machine, pumping system, or manual application. Complete coverage is required, and a certain pressure must be applied to allow the slurry to penetrate into the micro-cracks and pores on the coal surface.

[0083] The slurry penetrates the fissures in the coal seam, blocking the capillary channels and pores through which oxygen can enter the coal—an effect that a single surface covering cannot achieve. Macroscopically, this forms a dense covering layer, initially isolating the air and providing a smooth, well-adhesive base layer for the subsequent HDPE membrane.

[0084] The second layer is the impermeable membrane 16. This is a high-density polyethylene (HDPE) membrane or a linear low-density polyethylene (LLDPE) membrane, which has extremely low gas (oxygen) and water vapor permeability. Before the mud layer is completely dry (to facilitate adhesion), the HDPE membrane is laid from top to bottom, tightly adhering to the mud layer. The membranes are sealed together using hot-melt welding or special tape, ensuring the joints are equally airtight. The membrane must completely cover the mud layer and extend a certain distance into the surrounding stable rock strata.

[0085] HDPE membrane has excellent air tightness, making it the most effective barrier to block oxygen molecules. It forms the main waterproof layer of the fire protection system, almost completely isolating coal seam 22 from the outside atmosphere, while preventing surface water infiltration and the escape of gases such as methane from coal seam 22.

[0086] The third layer is a compacted protective soil layer 17. This layer directly utilizes loess or sandy clay free of large, sharp stones, obtained from the stripping of subsequent construction sections. The soil is backfilled in layers on the HDPE membrane, each layer approximately 30-50 cm thick, and thoroughly compacted using compaction machinery (such as a vibratory roller). The compacted thickness is typically no less than 1 meter to provide sufficient mechanical protection and thermal insulation.

[0087] This prevents the HDPE membrane from aging due to UV radiation, being chewed by animals, or being punctured by falling rocks or human activity. The thick soil layer has high thermal inertia, reducing the impact of external temperature changes on the interior. Its weight further compresses the underlying layers, ensuring a long-term tight seal.

[0088] In some embodiments, such as Figure 5 As shown, in step S2, the final design slope angle of the mining slope is optimized based on the mechanical parameters of the weak layer after freezing and reinforcement. The optimized design slope angle is greater than the original design slope angle under the unreinforced condition.

[0089] After freezing is completed in step S2, but before large-scale stripping, the true mechanical properties of the weak layers after freezing are directly obtained through in-situ testing. For example, boreholes are drilled within the frozen wall to conduct in-situ direct shear or vane shear tests, directly measuring the shear strength of the weak layers. Alternatively, parameters such as the elastic modulus and dynamic shear modulus of the frozen body can be indirectly deduced through acoustic or seismic wave testing. Furthermore, based on an established frozen soil mechanical model, strength parameters for different regions can be calculated using monitored temperature field data.

[0090] Using the obtained high-strength parameters after freezing, the original slope stability calculation model (such as Slide software or finite element analysis software using the limit equilibrium method) is updated. In the model, the original weak interlayer is replaced with a frozen reinforcement layer with high cohesion and internal friction angle value.

[0091] Using the original design slope angle A (e.g., 35°) as a benchmark, and ensuring that the safety factor meets or even exceeds the specification requirements, the design slope angle is gradually increased for trial calculations. Through repeated calculations, the maximum stable slope angle B (e.g., 65°) that meets the safety requirements under the new mechanical conditions is found. This angle is the angle of the optimized frozen design slope 19.

[0092] The optimized slope design (new slope line, step parameters) will be used as the final construction drawing. Subsequent topsoil and rock stripping (the later part of step S2) and the entire segment's mining will be carried out according to the designed slope angle after freezing.

[0093] The relationship between slope angle and recoverable coal volume is non-linear. At the same slope height, an increase in slope angle reduces the horizontal width occupied by the slope, and the released triangular area represents the additional recoverable coal volume. For example, for an open-pit mine sidewall stretching several kilometers, optimizing the slope angle from 35° to 65° increases the amount of coal recovered from a single sidewall by tens of thousands of tons.

[0094] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0095] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0096] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0097] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0098] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0099] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. An open-pit mine side slope coal mining method based on hierarchical segment freezing reinforcement of weak layers, characterized in that, The method comprises the following steps: S1, determining the position of the water-bearing weak layer in the coal mining area of the side slope, and dividing the mining area along the strike into a plurality of sequentially connected construction sections; S2, artificially freezing the water-bearing weak layer in the first construction section to form a frozen reinforcement wall, and after the mechanical strength of the wall meets the design requirements of the slope stability, stripping the topsoil and rock layer in the section, and then mining the coal seam; S3, while mining the Nth construction section (N≥1), artificially freezing the water-bearing weak layer of the N+1th construction section; after the Nth construction section is completed, using the waste rock generated by the stripping of the N+1th construction section to internally discharge and backfill the toe of the slope of the Nth construction section; S4, repeating step S3 to complete the mining, freezing reinforcement and toe backfilling of all construction sections in turn until the entire side slope coal mining area is completed.

2. The method for exploiting coal in the side slope of an open-pit mine based on the hierarchical segment freezing reinforcement of the weak layer according to claim 1, characterized in that, In step S1, based on the distribution of the water-bearing weak layer, a freezing borehole network is designed, wherein the spacing L between adjacent boreholes on the same exploration line is determined by the following formula: L=2R*tanα Wherein, R is the design freezing radius, and α is the design inclination angle of the borehole.

3. The method according to claim 2, wherein, In step S1, the artificial freezing is stratified freezing, and the design depth H of the freezing borehole is determined according to the floor elevation of the target water-bearing weak layer, specifically: H=h1-h2+Δh Wherein, h1 is the borehole elevation, h2 is the floor elevation of the target water-bearing weak layer, and Δh is a safety over-depth allowance, which is greater than or equal to 2 meters.

4. The method according to claim 3, wherein, In stratified freezing, the effective freezing depth h of a single hole is set for the target water-bearing weak layer, specifically: h=h3-h2+Δh' Wherein, h3 is the roof elevation of the target water-bearing weak layer, h2 is the floor elevation, and Δh' is the upward and downward expansion freezing allowance, which is greater than or equal to 4 meters.

5. The method for exploiting coal from the side slope of an open-pit mine based on the hierarchical segment freezing reinforcement of a weak layer according to claim 1, characterized in that, In step S1, when dividing the construction sections, the freezing areas of adjacent construction sections are connected by overlapping arrangement of inclined boreholes or freezing holes at the joint.

6. The method for exploiting coal from the side slope of an open-pit mine based on the hierarchical segment freezing reinforcement of a weak layer according to claim 1, characterized in that, In steps S2 and S3, the artificial freezing is dynamic control freezing, and the real-time feedback data of the temperature sensors and displacement monitoring devices arranged in the freezing area and the slope are used to dynamically adjust the refrigeration power and the freezing period.

7. The method for exploiting coal from the side slope of an open-pit mine based on the hierarchical segment freezing and reinforcing the weak layer according to claim 1, characterized in that, In step S3, the internal discharge backfilling toe also includes building a dry masonry dam on the backfilled waste rock body, and the dry masonry dam is pre-buried with a drainage pipe.

8. The method for exploiting coal in the open-pit mine side slope based on the hierarchical segment freezing reinforcing weak layer according to claim 1, characterized in that, After the coal seam and rock layer of each construction section are exposed, the exposed coal seam and rock layer are covered and sealed by multiple layers, and the sealing layer comprises at least a mud permeation layer, a seepage isolation membrane and a compacted protective soil layer from inside to outside.

9. The method for exploiting coal from the side slope of an open-pit mine based on the hierarchical segment freezing and reinforcing the weak layer according to claim 1, characterized in that, In step S2, the final design slope angle of the mining slope is also optimized based on the mechanical parameters of the weak layer after freezing reinforcement, and the optimized design slope angle is greater than the original design slope angle under the un-reinforced condition.

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