Repair method and repair structure for mine concave landform remodeling

Abstract

The invention provides a restoration method and a restoration structure for mine concave landform remodeling, and relates to the technical field of industrial byproduct ardealite reutilization, the restoration method comprises the following steps: A, carrying out pretreatment on a concave mining pit, including clearing, foundation compaction and slope trimming work, and arranging a water intercepting and draining ditch and a water collecting tank outside the concave mining pit; b, a bottom functional layer is constructed, specifically, a gravel layer is laid at the bottom of the pretreated mining pit to serve as a foundation drainage layer, then an impermeable layer is laid on the foundation drainage layer, and finally a gypsum material is adopted to conduct layered rolling on the impermeable layer to form a foundation stabilizing layer; c, on the foundation stabilizing layer, dihydrate gypsum is used as filler to be filled to the designed elevation in a layered mode, and a gypsum filling layer is formed; d, a top functional layer is constructed on the gypsum filling layer, specifically, a gypsum base layer is laid firstly, then a waterproof sealing layer is laid on the gypsum base layer, and finally planting ditches are reserved in the waterproof sealing layer and covered with growth soil to serve as a vegetation recovery layer. According to the invention, industrial solid wastes can be consumed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of recycling of industrial by-products phosphogypsum, and in particular to a repair method and structure for remodeling of concave mine topography. BACKGROUND

[0002] With the rapid development of the mining industry in China, the ecological environment has been seriously damaged by years of mining in domestic mines, and due to the uneven distribution of different types of ores, targeted mining of the required ores has caused many different types of damaged topography, among which the damage caused by "concave mining pits" to the ecological environment is particularly significant, mainly including vegetation damage, soil erosion and pollution, and increased risk of geological disasters.

[0003] The by-product hemihydrate phosphogypsum from the semi-wet process phosphoric acid system in the phosphorus chemical industry has potential cementitious activity, and after activation and modification, it can be prepared into a gypsum-based ecological repair material, which can quickly set and harden when stirred with water, has the advantage of high strength, and can be applied to waste mine ecological repair filling, or paste filling, underground filling, etc.

[0004] Traditional ecological repair materials containing phosphogypsum are mainly backfilled by single dumping, without systematic structural design in the abandoned mining pit according to the characteristics of the "repair material", and the non-standard backfilling process may cause secondary disasters such as ground subsidence, landslides, mudslides, and ecological imbalance. Therefore, there is an urgent need for a repair method that can not only solve the problem of phosphogypsum solid waste disposal, but also effectively avoid the instability of the backfilling body and secondary ecological damage. SUMMARY

[0005] The present application provides a repair method and structure for remodeling of concave mine topography, which takes into account the characteristics of phosphogypsum and the natural advantages of the "concave mining pit" surrounded by mountains on four sides, and provides a repair method and structure for remodeling of concave mine topography, which can accommodate a large amount of low-strength gypsum-based ecological repair material while ensuring safety and stability.

[0006] The present application provides the following technical solutions to achieve the above-mentioned purposes: A repair method and structure for remodeling of concave mine topography, comprising the following steps: A, pretreatment of the concave mining pit, including cleaning, foundation compaction and slope finishing work, and setting a drainage ditch and a water collection tank outside the concave mining pit; B, constructing a bottom functional layer, including laying a gravel layer as a foundation drainage layer at the bottom of the pretreated mining pit, then laying a impermeable layer on the foundation drainage layer, and finally forming a foundation stable layer by layering and rolling the gypsum on the impermeable layer; C, filling the foundation stable layer with dihydrate gypsum as filler to the designed elevation to form a gypsum filling layer; D. Constructing a top functional layer on the gypsum filling layer, the top functional layer comprising a gypsum base layer laid on the gypsum filling layer, then a waterproof sealing layer laid on the gypsum base layer, and finally a vegetation restoration layer formed by reserving a planting ditch and covering growth soil on the waterproof sealing layer.

[0007] Further, the step A removing comprises removing dangerous rocks, floating soil, sundries, garbage and contaminated soil in the concave mining pit; and the step B slope trimming comprises cleaning the soil slope around the concave mining pit into a ladder shape.

[0008] Further, the step B further comprises spraying a concrete layer on the surface of the slope of the concave mining pit, and covering a gypsum protection layer on the concrete layer as a karst water-resisting layer.

[0009] Further, the step B further comprises spraying a concrete layer on the surface of the slope of the concave mining pit, and covering a gypsum protection layer on the concrete layer as a karst water-resisting layer.

[0010] Further, the step B further comprises spraying a concrete layer on the surface of the slope of the concave mining pit, and covering a gypsum protection layer on the concrete layer as a karst water-resisting layer.

[0011] Further, the step C gypsum filling layer uses the hemihydrate gypsum with a bearing capacity of not less than 8% after compaction, and is filled in layers with a thickness of not more than 50 cm.

[0012] Further, the step D gypsum base layer has a thickness of not less than 1 m and is made of gypsum with a compressive strength of not less than 3 MPa; the waterproof sealing layer has a thickness of 5-30 mm; and the growth soil has a thickness of 50-200 cm.

[0013] Further, the waterproof sealing layer is made of asphalt slurry or waterproof mortar.

[0014] A repair structure for remodeling a concave landform of a mine, comprising a concave mining pit, a bottom functional layer arranged on the foundation of the concave mining pit, a gypsum filling layer arranged on the bottom functional layer, a top functional layer arranged on the gypsum filling layer, a concrete layer arranged around the gypsum filling layer and in contact with the concave mining pit, and a gypsum protection layer arranged on the contact surface between the concrete layer and the gypsum filling layer; the bottom functional layer comprises a foundation drainage layer, an anti-seepage layer and a foundation stabilizing layer arranged in sequence from bottom to top; and the top functional layer comprises a gypsum base layer, a waterproof sealing layer and a vegetation restoration layer arranged in sequence from bottom to top.

[0015] Further, the vegetation restoration layer is planted with shallow-rooted plants.

[0016] Compared with the prior art, the present application has the following beneficial effects: 1. The present application uses phosphogypsum, a by-product of phosphorus chemical industry, as the main repair material. This not only solves the problem of new environment destruction caused by the mining of traditional backfill materials (such as guest soil) and the difficulty of obtaining resources, but also avoids the risk of heavy metal and organic pollution that may be brought by construction waste backfill. It can large-scale absorb industrial solid waste, which meets the environmental protection requirements.

[0017] 2. The present application designs a "gradient function layer system" according to the characteristics of phosphogypsum as a hardening cementing material. The system effectively blocks the erosion of groundwater to the gypsum filling body by setting the foundation drainage layer, impermeable layer at the bottom and karst water-resisting layer at the slope, ensuring the long-term stability of the main structure. The design of the foundation stabilizing layer and the top water-resisting layer (including the gypsum base layer) provides a homogeneous high-strength foundation, which can effectively disperse the load, prevent uneven settlement and structure cracking, and eliminate safety hazards such as collapse and landslide. Compared with traditional soil and rock filling, the use of dihydrate gypsum as the main filler has higher bearing capacity and smaller swelling rate, and the formed reformed topography structure is more compact and stable.

[0018] 3. The present application effectively restores the ecology and improves the land value. The present application sets a vegetation restoration layer at the top of the reformed topography, and according to the characteristics of the gypsum soil, shallow-rooted crops are scientifically selected for planting. This not only restores the vegetation cover in the mining area and integrates into the local ecosystem, but also brings certain economic benefits. The repaired land structure is stable and has strong usability, which is not limited to planting, but can also be used for factory building or roadbed construction, etc., significantly improving the comprehensive utilization rate of the land.

[0019] 4. The present application effectively controls the leaching of pollutants inside the filling body through the complete multi-layer protection system, especially the water-resisting and impermeable design at the top and bottom. The water quality of the surface runoff formed by the final closure can reach the third class standard of surface water, avoiding secondary pollution to the surrounding water environment. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a structural schematic view of the present application; The figure legend: 1- concave pit; 2- gypsum filling layer; 3- concrete layer; 4- gypsum protection layer; 5- foundation drainage layer; 6- impermeable layer; 7- foundation stabilizing layer; 8- gypsum base layer; 9- waterproof seal layer; 10- vegetation restoration layer DETAILED DESCRIPTION

[0021] For the purpose of promoting the understanding of the present application, the application will be described in further detail below with reference to the attached drawings; in these drawings, there are shown preferred embodiments of the application; however, the application can be practiced in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will convey the subtleties and nuances of the present application to those skilled in the art.

[0022] It should be noted that the terms "vertical", "horizontal", "upper", "lower", "left", "right", and similar terms used herein are for the purpose of illustration only and are not intended to be limiting.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the use of the terms "and / or" herein is intended to include the meaning of "either or both" and "any and all combinations of one or more of the associated listed items.

[0024] A method for repairing a concave landform of a mine, comprising the following steps: A. Pre-treating the concave mine, including removing, compaction of the foundation, and slope trimming, and setting a drainage ditch and a water collecting pool outside the concave mine; B. Building a bottom functional layer, including laying a gravel layer as a foundation drainage layer at the bottom of the pre-treated mine, then laying an anti-seepage layer on the foundation drainage layer, and finally forming a foundation stable layer by layering and rolling the gypsum material on the anti-seepage layer; C. On the foundation stable layer, using dihydrate gypsum as a filler to form a gypsum filling layer by layering and filling to the design elevation; D. On the gypsum filling layer, building a top functional layer, including laying a gypsum base layer first, then laying a waterproof sealing layer on the gypsum base layer, and finally reserving a planting ditch and covering the growing soil as a vegetation recovery layer on the waterproof sealing layer; The step A of removing includes treating dangerous rocks, floating soil, sundries, and garbage and contaminated soil at the bottom in the concave mine; the slope trimming includes cleaning the soil slope around the periphery into a stepped shape; The thickness of the gravel layer in the step B is 20-50 cm; this layer mainly functions to block the underground water from entering the upper gypsum filling body through capillary siphon action, so as to avoid affecting the strength and stability of the filling body; the anti-seepage layer uses a clay anti-seepage layer or an HDPE geomembrane anti-seepage layer to prevent water from penetrating; the foundation stable layer is 1.0-3.0 meters and is leveled and stabilized by using gypsum with a compressive strength of 3.0-5.0 MPa, so as to slow down the bottom damage caused by stress concentration and uneven stress; The step B further comprises spraying concrete on the slope surface of the pit, and then covering a gypsum protection layer as a karst water-resisting layer; The concrete has a thickness of 5-15 cm, and the gypsum protection layer has a thickness of not less than 75 cm and is covered by gypsum with a compressive strength of not less than 3.0 MPa; In the step C, the dihydrate gypsum of the gypsum filling layer has a bearing capacity of not less than 8%, and is filled in layers with a thickness of not more than 50 cm; the dihydrate gypsum is selected to absorb a large amount of low-strength phosphogypsum by-products; In the step D, the gypsum base layer has a thickness of not less than 1 m and is made of gypsum with a compressive strength of not less than 3 MPa; the gypsum base layer is used to disperse the surface load, protect the gypsum filling layer, and maintain the stability of the top structure; the waterproof seal layer has a thickness of 5-30 mm; the waterproof seal layer is used to prevent rainwater from eroding the gypsum base layer and the gypsum filling layer below; the growth soil has a thickness of 50-200 cm; and the waterproof seal layer is made of asphalt slurry or waterproof mortar; The present application provides a repair structure for remodeling a mine concave topography, which comprises a pit 1, a bottom functional layer arranged on the foundation of the pit 1, a gypsum filling layer 2 arranged on the bottom functional layer, a top functional layer arranged on the gypsum filling layer, a concrete layer 3 arranged around the gypsum filling layer and on the contact surface of the pit, and a gypsum protection layer 4 arranged on the contact surface of the concrete layer 3 and the gypsum filling layer; the bottom functional layer comprises a foundation drainage layer 5, an anti-seepage layer 6 and a foundation stabilization layer 7 arranged in sequence from bottom to top; the top functional layer comprises a gypsum base layer 8, a waterproof seal layer 9 and a vegetation restoration layer 10 arranged in sequence from bottom to top; shallow-rooted plants are planted on the vegetation restoration layer; the shallow-rooted plants include azalea, rose and garlic; The mine concave topography is an irregular concave valley topography formed after serious over-mining of a mine, and the repair system specifically comprises a gradient functional layer system composed of a foundation drainage layer, an anti-seepage layer, a karst water-resisting layer, a foundation stabilization layer, a gypsum filling layer, a top water-resisting layer and a vegetation restoration layer arranged in sequence from bottom to top on the bottom of the pit after pretreatment of the pit, setting of a water collecting pool and a drainage ditch outside the pit, and setting of the water collecting pool and the drainage ditch outside the pit.

[0025] 1. The gypsum-based ecological restoration material is prepared from by-product phosphogypsum of a phosphorus chemical industry enterprise, and is divided into a gypsum cementing material and a dihydrate gypsum filler. The gypsum cementing material has cementing properties, and main components are CaSO4·1 / 2H2O; the dihydrate gypsum filler mainly contains CaSO4·2H2O, and leaching toxicity meets backfill requirements in the Technical Code for Pollution Control of Phosphogypsum Utilization and Harmless Storage (HJ 1415-2025).

[0026] 2. The mine abandoned pit pretreatment method is to clean up side slope dangerous rock, floating soil, branches, and bottom garbage, contaminated soil, and to compact the foundation, and to clean up the surrounding soil slope in a stepped manner.

[0027] The abandoned mine is often damaged by vegetation and soil exposure after years of mining, making the soil more susceptible to rainwater erosion, leading to serious soil erosion. At the same time, the waste rock and slag produced in the process of mining contain harmful substances such as heavy metals, which may enter the soil with rainwater, causing soil pollution. The waste rock, slag soil layer and side slope dangerous rock in the abandoned pit need to be cleaned up.

[0028] 3. The foundation drainage layer is arranged at the bottom of the pretreated pit, which is a gravel layer with a thickness of 20-50 cm. To prevent the occurrence of capillary siphon phenomenon in the gypsum backfill body, a gravel drainage layer needs to be set at the bottom of the filling body. Under the action of the high void of the gravel layer, the generation of capillary siphon phenomenon in the gypsum backfill body can be blocked. However, the capillary siphon effect has the following hidden dangers: (1) Strength change: Capillary siphon effect affects the distribution and dynamic balance of water in the heap, and further affects the strength development of the backfill body. Under the action of capillary effect, uneven distribution of water may lead to local strength deficiency or uneven strength of the backfill body, especially the damage to the air-hardening gypsum material is most prominent.

[0029] (2) Stability influence: The migration of water and solute may change the internal structure and mechanical properties of the gypsum backfill body, affecting its overall stability. For example, excessive water in local areas may cause phosphogypsum to soften and reduce in strength, thereby causing problems such as settlement and collapse of the heap.

[0030] 4. The impermeable layer is arranged on the foundation drainage layer, which is mainly a clay impermeable layer or an HDPE geomembrane impermeable layer.

[0031] 5. The karst water-resisting layer is arranged on the surface of the pretreated pit slope, which is composed of a concrete spraying layer and a gypsum protection layer. The concrete spraying layer is arranged on the surface of the pit slope with a thickness of 5-15 cm; the gypsum protection layer is arranged on the surface of the concrete spraying layer and is prepared by filling the gypsum cementing material, with a compressive strength of ≥3.00 MPa and a thickness of ≥75 cm.

[0032] Abandoned mining pit is a concave valley, and the surrounding area of the concave area is a plain or a mountain. In the annual rainfall, the surface water of the surrounding area penetrates downward, and the karst water-resisting layer can separate different aquifers, so as to weaken or even cut off the hydraulic connection between the aquifers, so that the underground water in each aquifer exists relatively independently.

[0033] However, as a hard-setting cementitious material, the percolation of the groundwater in the karst area and the soft soil area around the abandoned mining pit will change the internal structure and mechanical properties of the gypsum backfill body, and affect the overall stability thereof.

[0034] 6. The foundation stabilizing layer is formed by the gypsum cementitious material through layer-by-layer rolling, has a compressive strength of 3.0-5.0 MPa, a thickness of 1.00-3.00 m, and a strength fluctuation that is not suitable for being too large.

[0035] The foundation stabilizing layer mainly provides a high-strength homogeneous foundation and protects the bottom impermeable layer. During the repair process, the bottom of the abandoned mining pit is compacted by a road roller and then laid with clay or HDPE geomembrane. Since the clay is a powder material and has no cementitious properties, as the height of the gypsum backfill increases, the settlement of the inhomogeneous foundation will be different, that is, the settlement deformation of the high-strength foundation area is small, the low-strength foundation area forms a splitting phenomenon, and local settlement will occur, which will damage the bottom impermeable function layer.

[0036] To prevent this phenomenon, the foundation stabilizing layer is formed by the gypsum cementitious material through layer-by-layer rolling, has a uniform and consistent strength, can provide a homogeneous foundation, and avoid uneven settlement to damage the impermeable layer.

[0037] And has a certain impermeability, meets the impermeability requirements of the first general industrial solid waste landfill, and can be used as an impermeable layer.

[0038] 7. The gypsum filling layer is arranged on the foundation stabilizing layer, has a single-layer filling thickness of ≤50 cm, a compaction degree of ≥90%, and a mechanical bearing ratio of ≥8%, and is cyclically and reciprocally filled to a set elevation.

[0039] The gypsum filling layer is a concave main filling layer of the abandoned mining pit. By virtue of the advantages of the concave mining pit, the surrounding mountains, the concave middle part, and the bottom foundation stabilizing layer, the gypsum filling layer is filled with relatively low-strength gypsum dihydrate, which has the advantage of being able to accommodate a large amount of low-strength gypsum dihydrate. According to the JTG F10-2006 Highway Subgrade Construction Technical Specification, the mechanical bearing ratio of the gypsum filling layer meets the requirement of the highest grade highway, that is, the mechanical bearing ratio is ≥8%.

[0040] This experiment was conducted according to JTG E40-2007, the standard for testing geotechnical engineering for highways. A compound of gypsum cementitious material and dihydrate gypsum was used. CBR (carrying ratio) specimens were formed at the optimum moisture content and 90% compaction. After soaking in water for 4 days, the bearing ratio and expansion ratio were as follows: The data above shows that when using traditional earth and rock filling, with a gravel content of 29% and a compaction degree of 92%, the bearing capacity of 2.5mm is 6.98% and the expansion ratio is 2.567%; when the compaction degree is 95%, the bearing capacity of 2.5mm is 8.51% and the expansion ratio is 1.585%.

[0041] When dihydrate gypsum was used for filling, with a compaction degree of 90%, the 2.5mm bearing capacity ratio was 20.22%, and the expansion ratio was 0.190%. It can be seen that compared with traditional soil and rock filling, dihydrate gypsum has the characteristics of high bearing capacity ratio and low expansion rate.

[0042] 8. The top waterproof layer is located above the gypsum fill layer and consists of a gypsum base layer and a waterproof seal layer. The waterproof seal layer is 5-30mm thick; the gypsum base layer is located below the waterproof seal layer, with a compressive strength ≥3MPa and a compacted thickness ≥1.00m.

[0043] After the gypsum fill layer is completed and reaches the design elevation, a waterproof layer is designed on its surface. The top waterproof layer consists of a gypsum base layer and an asphalt slurry seal layer or a waterproof mortar seal layer.

[0044] 1) The functions of gypsum base layer are as follows: The base course can disperse concentrated loads. When surface loads are transferred to the base course, the structural characteristics of the base course material allow the load stress to be diffused to a certain extent in the horizontal direction. That is, when the gypsum base course is subjected to load, the granular structure inside will be compressed and rearranged, thereby distributing the load stress over a larger area and reducing the destructive effect of local stress concentration on the roadbed.

[0045] Maintaining the stability of the top vegetation layer structure requires a base layer material with a certain degree of stability to ensure the stability of the entire vegetation layer structure. For example, when the surface is subjected to repeated vehicle loads or affected by environmental factors (such as temperature changes, rainwater soaking, etc.), a stable base layer can effectively prevent the overall deformation of the surface structure, such as preventing longitudinal cracks, transverse cracks, or wave-like bulges and other defects.

[0046] 2) The functions of asphalt slurry seal or waterproof mortar seal are as follows: The stabilizing effect on the underlying gypsum layer: It prevents the gypsum fill layer from being eroded by rainwater. For example, in rainy areas, without a waterproof layer such as an asphalt slurry seal or a waterproof mortar seal, rainwater will directly penetrate into the foundation gypsum fill layer, softening the gypsum soil and reducing its bearing capacity. An asphalt slurry seal or a waterproof mortar seal has a certain degree of water resistance, preventing rainwater from quickly penetrating into the foundation gypsum fill layer.

[0047] 9. The vegetation restoration layer is located above the top waterproof layer and consists of pre-reserved planting trenches and growing soil. The growing soil is 50-200cm thick and covered with a mesh net to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives. Its main characteristic is that it is a shallow-rooted vegetation.

[0048] After years of mining, abandoned pits have resulted in the felling of large numbers of trees, a sharp decline in vegetation cover, and exacerbated soil erosion and reduced soil fertility. The destruction of vegetation also deprives animals of habitats and food sources, leading to a reduction in biodiversity.

[0049] Taking advantage of the characteristics of gypsum-based ecological restoration materials, planting trenches are constructed on top of the gypsum fill. The planting trenches are then filled with soil, and the surrounding platform is backfilled with compacted soil to a thickness of 50-150cm. Geotextile is then used to cover the trenches to prevent rainwater erosion.

[0050] Meanwhile, to prevent the top impermeable layer and the stabilizing layer from being damaged by the penetration of vegetation roots and stems, and to maximize the economic benefits of vegetation planting, the vegetation restoration layer is planted with shrubs, herbaceous plants and crops such as azalea, rose, pomegranate, red maple, nandina, osmanthus, scallion, garlic and chives, and its main feature is that it is shallow-rooted vegetation.

[0051] Example 1. A method for reshaping and restoring concave landforms in mines, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0052] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a layer of geotextile with a layer of HDPE geomembrane sandwiched between two layers of geotextile as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer. The concrete is 10cm thick and made with gypsum slag cement. The gypsum protective layer is formed by compacting gypsum cementitious material with a micro-roller as the filling elevation rises. It has a width of 1.00m, a compaction degree of 90.10%, and a compressive strength of 4.07MPa after 7 days of natural curing.

[0053] The foundation drainage layer prepared by crushed stone can effectively block the capillary siphon effect of gypsum fill as a breathing material on groundwater at the bottom of the pit, and can avoid the local strength reduction and stability deterioration of air-hardened gypsum fill due to the capillary effect of groundwater.

[0054] The concrete spraying layer is prepared using gypsum slag cement, which effectively prevents the erosion of traditional cement structures by sulfates in gypsum, avoiding damage to the concrete structure. The gypsum protective layer is formed by compacting gypsum cementitious material as the filling elevation rises. During construction near the edge of the slope, it is difficult to control the compaction degree at a high level, and compaction can only be achieved using micro-machinery. However, even at a low compaction degree, gypsum cementitious material still possesses high strength after hardening, effectively supporting the concrete spraying layer against lateral displacement deformation from the slope and preventing cracking. Dihydrate gypsum itself has low cementitious activity and is an inert filler; to ensure compaction, it is extremely easy to damage the concrete spraying layer during construction. Therefore, gypsum cementitious material is used to establish the gypsum protective layer.

[0055] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. This layer is constructed by layering and compacting gypsum cementitious material, with a cumulative compacted thickness of 1.5m and a compaction degree of 90.54%. After natural curing, its 7-day average compressive strength is 4.10MPa. Above the foundation stabilization layer is a gypsum fill layer, constructed using dihydrate gypsum as the raw material. Each layer has a compacted thickness of 45cm, a compaction degree of 93%, and a mechanical bearing capacity ratio of 25.69%. This layering is repeated until the top of the pit reaches the set elevation. Above the gypsum fill layer is the top waterproof layer, which consists of a gypsum base layer and an asphalt slurry seal layer. The asphalt slurry seal layer is 7mm thick. The gypsum base layer, located below the asphalt slurry seal layer, is formed by layering and compacting gypsum cementitious material, achieving a compaction degree of 90%. The gypsum base layer has a compressive strength of 4.67MPa and a compacted thickness of 2.00m.

[0056] The function of a gypsum base layer is to disperse concentrated loads from surface mechanical activities. Specifically, the granular structure within the gypsum base layer undergoes mutual compression and rearrangement under load, thus distributing the load stress over a larger area and mitigating the destructive effect of localized stress concentration on the gypsum fill layer. Simultaneously, the gypsum base layer effectively prevents overall deformation of the surface structure, such as preventing the formation of longitudinal and transverse cracks, thereby protecting the waterproofing function of the asphalt slurry seal layer and ensuring the stability of the vegetation layer structure.

[0057] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base layer. It should be covered with biodegradable mesh to prevent rainwater erosion. The vegetation restoration layer should be planted with shrubs, herbs, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0058] Example 2. A method for reshaping and restoring concave landforms in mines, comprising the following steps: 1) The depression pit is pretreated, including the removal of dangerous rocks, slag layers and accumulated water, followed by compaction of the foundation, and the setting of a water-blocking wall and drainage ditch at the edge of the backfill area at the top of the depression pit.

[0059] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1), with a particle size of 20-40mm and a compacted thickness of 30cm, to obtain the foundation drainage layer. The upper part of the foundation drainage layer is a seepage-proof layer, specifically prepared by layered compaction of clay, with a gravel content ≤15%, a cumulative compacted thickness of 80cm, and a permeability coefficient of 5.97*10. -6 cm / s. Simultaneously, a karst seepage-proof layer is constructed on the entire slope of the pretreated pit obtained in step 1), specifically consisting of a concrete spraying layer and a gypsum protective layer; the concrete thickness is 10cm, and it is prepared using gypsum slag cement; the gypsum cementitious material of the gypsum protective layer is compacted and filled as the filling elevation rises, with a compaction degree of 90.33%, a width of 1.00m, and a compressive strength of 3.57MPa after natural curing for 7 days.

[0060] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. The foundation stabilization layer is made of gypsum cementitious material laid in layers and compacted, with a cumulative compacted thickness of 1.5m and a compaction degree of 93%. After natural curing, its 7-day average compressive strength is 7.2MPa and its permeability coefficient is 3.68*10 -6cm / s. The foundation stabilization layer is topped with a gypsum fill layer, constructed from dihydrate gypsum. Each layer has a compacted thickness of 45cm, a compaction degree of 94%, and a mechanical bearing capacity of 23.45%. This layer is cyclically filled to the set elevation at the top of the pit. Above the gypsum fill layer is a top waterproof layer, composed of a gypsum base layer and an asphalt slurry seal. The asphalt slurry seal is 7mm thick. The gypsum base layer, located below the asphalt slurry seal, is formed by layered compaction of gypsum cementitious material, achieving a compaction degree of 94.67%. The gypsum base layer has a compressive strength of 7.21MPa, a compacted thickness of 2.00m, and a permeability coefficient of 4.21*10. -6 cm / s.

[0061] The difference from steps 2) and 3) in Example 1 is that the compacted clay layer thickness is increased to 80cm, the HDPE geomembrane is omitted, and to ensure seepage prevention, the compaction and compressive strength of the foundation stabilization layer are increased, giving the foundation stabilization layer a certain seepage prevention function. This meets the requirement in the "Technical Specification for Pollution Control of Utilization and Harmless Storage of Phosphogypsum" (HJ 1415—2025) that the saturated permeability coefficient of the foundation layer should not exceed 1.0×10⁻⁶. -5 The speed requirement is cm / s, and the thickness is not less than 0.75 m.

[0062] A vegetation restoration layer is constructed above the top waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150 cm above the plaster base layer surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by its shallow root system.

[0063] Example 3. A method for reshaping and restoring concave landforms in mines, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0064] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a two-layer geotextile and one-layer geomembrane layer as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer. The concrete is 10cm thick and made with gypsum slag cement. The gypsum protective layer is made by gradually compacting and rolling gypsum cement in layers as the filling elevation rises, with a width of 3.00m, a compaction degree of 94.67%, and a compressive strength of 7.21MPa.

[0065] The difference from step 2) in Example 1 is that: gypsum cementitious material is used to gradually compact and roll the gypsum protective layer in layers as the filling elevation rises. Increasing the thickness of the gypsum protective layer further facilitates compaction by the road roller. The prepared gypsum protective layer, even with a lower degree of compaction, possesses high strength and is more resistant to creep deformation from the slope and underground infiltration and erosion by rainwater.

[0066] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. This layer is constructed by layering and compacting gypsum cementitious material, with a cumulative compacted thickness of 1.5m and a compaction degree of 88%. After natural curing, its 7-day average compressive strength is 3.5MPa. Above the foundation stabilization layer is a gypsum fill layer, constructed using dihydrate gypsum as the raw material. Each layer has a compacted thickness of 45cm, a compaction degree of 94%, and a mechanical bearing capacity ratio of 23.45%. This layer is cyclically filled to the set elevation at the top of the pit. Above the gypsum fill layer is the top waterproof layer, which consists of a gypsum base layer and an asphalt slurry seal layer. The asphalt slurry seal layer is 7mm thick. The gypsum base layer is placed below the asphalt slurry seal layer and is formed by layered compaction of gypsum cementitious material. The gypsum base layer has a compressive strength of 7.21MPa, a compacted thickness of 2.00m, and a permeability coefficient of 4.21*10⁻⁶. -6 cm / s.

[0067] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0068] Example 4. A method for reshaping and restoring concave landforms in mines, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0069] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is then laid on top of the foundation drainage layer, followed by a two-layer geotextile and one-layer geomembrane layer as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer. The concrete is 10cm thick and made of gypsum slag cement. The gypsum protective layer uses gypsum cementitious material and is filled as the filling elevation rises, with a width of 1.00m, a compaction degree of 90.94%, and a compressive strength of 4.21MPa after 7 days of natural curing.

[0070] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. This layer is constructed by layering and compacting gypsum cementitious material, with a cumulative compacted thickness of 1.5m and a compaction degree of 88%. After natural curing, its 7-day average compressive strength is 3.5MPa. Above the foundation stabilization layer is a gypsum fill layer, constructed using dihydrate gypsum as the raw material. Each layer has a compacted thickness of 45cm, a compaction degree of 94%, and a mechanical bearing capacity ratio of 23.45%. This layering is repeated until the top of the pit reaches the set elevation. Above the gypsum fill layer is the top waterproof layer, which consists of a gypsum base layer and a waterproof mortar seal layer. The waterproof mortar seal layer is 20mm thick. The gypsum base layer is placed below the asphalt slurry seal layer and is formed by layering and compacting gypsum cementitious material, achieving a compaction degree of 90%. The gypsum base layer has a compressive strength of 4.67MPa and a compacted thickness of 2.00m.

[0071] The difference from step 2) in Example 1 is that a 20mm thick waterproof mortar seal layer is used instead of an asphalt slurry seal layer, which has higher strength and is more resistant to load damage from the ground surface.

[0072] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0073] Comparative Example 1. A method for reshaping and restoring a concave landform in a mine, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0074] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a layer of geotextile with a layer of HDPE geomembrane sandwiched between two layers of geotextile as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer; the concrete thickness is 10cm and is prepared using ordinary Portland cement.

[0075] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. This layer is constructed by layering and compacting gypsum cementitious material, with a cumulative compacted thickness of 1.5m and a compaction degree of 90.54%. After natural curing, its 7-day average compressive strength is 4.10MPa. Above the foundation stabilization layer is a gypsum fill layer, constructed using dihydrate gypsum as the raw material. Each layer has a compacted thickness of 45cm, a compaction degree of 93%, and a mechanical bearing capacity ratio of 25.15%. This layer is cyclically filled to the set elevation at the top of the pit. Above the gypsum fill layer is the top waterproof layer, which consists of a gypsum base layer and an asphalt slurry seal. The asphalt slurry seal is 7mm thick. The gypsum base layer, located below the asphalt slurry seal, is formed by layering and compacting gypsum cementitious material, achieving a compaction degree of 90%. The gypsum base layer has a compressive strength of 4.67MPa and a compacted thickness of 2.00m.

[0076] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0077] Compared to Example 1, the main difference lies in step 2), where ordinary silicate cement is used to prepare fine sand concrete for the karst anti-seepage layer, and no gypsum protective layer is installed. During construction, the gypsum fill is easily damaged by factors such as surface water pressure and slope creep deformation. Simultaneously, rainwater seepage pressure enters along the cracks at the edge of the soil pit slope, promoting the slow-setting sulfate erosion of the fine sand concrete made of ordinary silicate cement by the gypsum fill, ultimately hollowing out the interior of the gypsum fill and causing natural disasters such as cracks and collapses in the backfill landscape.

[0078] Comparative Example 2. A method for reshaping and restoring concave landforms in mines, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0079] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a layer of geotextile with a layer of HDPE geomembrane sandwiched between two layers of geotextile as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer. The concrete is 10cm thick and made with gypsum slag cement. The gypsum protective layer is formed by manually compacting gypsum cementitious material as the filling elevation rises using a micro-roller. It has a width of 1.00m, a compaction degree of 90.10%, and a compressive strength of 4.07MPa after 7 days of natural curing.

[0080] 3) The upper part of the impermeable layer obtained in step 2) is a gypsum fill layer (without a foundation stabilization layer), constructed using dihydrate gypsum as the raw material. The single-layer compacted thickness is 45cm, the compaction degree is 93%, and the mechanical bearing capacity ratio is 25.15%. This layer is cyclically filled in layers until the set elevation at the top of the mining pit. Above the gypsum fill layer is a top waterproof layer, composed of a gypsum base layer and an asphalt slurry seal layer. The asphalt slurry seal layer is 7mm thick. The gypsum base layer is placed below the asphalt slurry seal layer and is formed by layered compaction of gypsum cementitious material, achieving a compaction degree of 90%. The gypsum base layer has a compressive strength of 4.67MPa and a compacted thickness of 2.00m.

[0081] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0082] Compared to Example 1, the main difference is that a foundation stabilization layer was not set in step 3). During the filling process, the gypsum fill is compacted in layers and is dihydrate gypsum, which does not have cementing properties and has a mechanical bearing capacity of 25.15%. As the backfill elevation of the fill layers increases, the bottom HDPE geomembrane and crushed stone layer will experience local subsidence due to uneven foundation stress, which will lead to the cracking of the bottom impermeable layer. Finally, under the action of groundwater flow, the local water content of the low-strength dihydrate gypsum fill at the bottom will increase, thereby eroding and hollowing out the dihydrate gypsum fill, causing natural disasters such as cracks and collapses in the backfill landform.

[0083] Comparative Example 3. A method for reshaping and restoring a concave landform in a mine, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0084] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a layer of geotextile with a layer of HDPE geomembrane sandwiched between two layers of geotextile as an impermeable layer. The clay has a gravel content ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. Simultaneously, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spray layer and a gypsum protective layer. The concrete is 10cm thick and made with gypsum slag cement. The gypsum protective layer is formed by manually compacting gypsum cementitious material as the filling elevation rises using a micro-roller. It has a width of 1.00m, a compaction degree of 90.10%, and a compressive strength of 4.07MPa after 7 days of natural curing.

[0085] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer. The foundation stabilization layer is made of gypsum cementitious material laid in layers and compacted, with a cumulative compacted thickness of 1.5m and a compaction degree of 90.54%. After natural curing, its 7-day average compressive strength is 4.10MPa. The upper part of the foundation stabilization layer is a gypsum fill layer, which is filled with dihydrate gypsum as raw material. Its single-layer compacted thickness is 45cm, the compaction degree is 93%, and the mechanical bearing capacity ratio is 25.15%. It is filled in layers in a cyclic manner to the set elevation of the top of the pit. (No top waterproof layer is set).

[0086] 4) Construct a vegetation restoration layer on top of the gypsum base layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches in the gypsum base layer, which are then backfilled with clay to a thickness of 50-150cm above the gypsum base layer surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0087] Compared to Example 1, the main difference is that a top waterproof layer was not set in step 3), meaning that soil was directly covered and planted on the surface of the gypsum dihydrate fill. After the landform reshaping is completed, during the long-term planting process by farmers, surface activities such as agricultural machinery tilling will occur, loosening the soil. Under rainwater soaking, a large amount of low-strength gypsum dihydrate at the bottom will gradually loosen and turn into mud. When heavy rain comes, it will be washed away outside the fill area, causing soil erosion and environmental pollution.

[0088] Meanwhile, gypsum dihydrate fill has a weak resistance to concentrated stress. Under intense surface activity, it is prone to local settlement and deformation, which eventually leads to cracks. The structure of the fill is unstable, which has a significant impact on safety and is not conducive to the comprehensive utilization of land for various purposes. For example, in addition to being unsuitable for planting, it is also unsuitable for factory construction and roadbed construction.

[0089] Comparative Example 4. A method for reshaping and restoring a concave landform in a mine, comprising the following steps: 1) The sunken pit is pretreated, including the removal of unstable rocks, slag layers, and accumulated water, followed by compaction of the foundation. A water-retaining wall and a drainage ditch are then installed at the edge of the backfill area at the top of the sunken pit.

[0090] 2) Lay crushed stone at the bottom of the pretreated pit obtained in step 1) as a foundation drainage layer. The crushed stone particle size is 20-40mm and the thickness is 30cm. Clay is laid on top of the foundation drainage layer, followed by a layer of geotextile and a geomembrane (two layers of geotextile with a layer of HDPE geomembrane in between) as an impermeable layer. The clay has a gravel content of ≤15%, a compacted thickness of 30cm, and the HDPE geomembrane thickness is 2.0mm. At the same time, a karst impermeable layer is constructed on the entire slope surface of the pretreated pit obtained in step 1), specifically composed of a concrete spraying layer and a gypsum protective layer. The concrete is 10cm thick and made of gypsum slag cement. The gypsum protective layer is made by manually compacting gypsum cementitious material with a micro roller as the filling elevation rises. The width is 1.00m, the compaction degree is 90.10%, and the compressive strength is 4.07MPa after 7 days of natural curing.

[0091] 3) The upper part of the impermeable layer obtained in step 2) is the foundation stabilization layer, which is made of gypsum cementitious material laid in layers and compacted, with a cumulative compacted thickness of 1.5m and a compaction degree of 90.54%. After natural curing, its 7-day average compressive strength is 4.10MPa. The upper part of the foundation stabilization layer is a gypsum fill layer, which is filled with dihydrate gypsum as raw material. Its single-layer compacted thickness is 45cm, the compaction degree is 93%, and the mechanical bearing capacity ratio is 25.15%. It is filled in layers in a cyclic manner until the set elevation of the top of the pit is reached. The upper part of the gypsum fill layer is the top waterproof layer, which is composed of asphalt slurry seal layer with a thickness of 7mm (without gypsum base layer).

[0092] 4) Construct a vegetation restoration layer on top of the waterproof layer obtained in step 3). The vegetation restoration layer requires pre-reserved planting trenches on the plaster base layer, which are then backfilled with clay to a thickness of 50-150cm above the plaster base surface and covered with mesh to prevent rainwater erosion. The vegetation restoration layer is planted with shrubs, herbaceous plants, and crops such as azaleas, roses, pomegranates, red maples, nandina, osmanthus, scallions, garlic, and chives, characterized by shallow root systems.

[0093] Compared to Example 1, the main difference lies in step 4), where a gypsum base layer is installed instead of a base layer; that is, an asphalt waterproofing layer is directly installed on top of the bottom gypsum fill layer. Concentrated loads from surface mechanical activities will cause deformation of the surface structure, such as the generation of longitudinal and transverse cracks, which will destroy the waterproofing effect of the asphalt slurry seal layer. Rainwater will then seep down through the cracks, damaging the gypsum fill layer.

[0094] The restored landforms formed in Examples 1-4 and Comparative Examples 1-4 were used to collect surface erosion data one year after soil cover by setting up gentle slopes, collecting ditches, and collecting ponds. The characteristic pollutants tested are as follows: The leachate prepared from gypsum-based materials according to HJ557-2010 "Solid Waste - Leaching Toxicity Leaching Method" meets the requirements for characteristic pollutant concentrations of phosphogypsum when used for ecological restoration backfilling in open-pit abandoned mines, as specified in GB-T 32124-2024 "Specifications for Treatment and Disposal of Phosphogypsum" and HJ 1415-2025 "Technical Specifications for Pollution Control in the Utilization and Harmless Storage of Phosphogypsum". However, the concentrations of characteristic pollutants in the leaching and flushing water generated during the backfilling process of gypsum-based materials are difficult to meet the Class III surface water standard, and in most cases, they cannot be discharged and need to be collected for use. In contrast, the ecological restoration system designed with the functional gradient layer described in Examples 1-4 and Comparative Examples 1-2 achieves Class III surface water standards for surface flushing water after backfilling, while in Comparative Examples 3-4, the surface flushing water fails to meet the Class III surface water standard after backfilling due to damage to the impermeable layer.

[0095] Obviously, the above description is only a part of the embodiments of the present invention, and not all of the embodiments. The above embodiments are not intended to limit the present invention, and various modifications and variations can be made to the present invention by those skilled in the art. Any combination, modification, equivalent substitution, improvement, and all other embodiments that can be made by those skilled in the art within the spirit and principles of the present invention should be within the protection scope of the present invention.

Claims

1. A method for reshaping and restoring concave landforms in mines, characterized in that, Includes the following steps: A. Pre-treatment of the sunken mining pit, including clearing, foundation compaction and slope trimming, as well as setting up intercepting drainage ditches and collection pools outside the sunken mining pit; B. Construct the bottom functional layer, including laying a crushed stone layer as a foundation drainage layer at the bottom of the pretreated pit, then laying an impermeable layer on the foundation drainage layer, and finally using gypsum to form a foundation stabilization layer by layering and compacting on the impermeable layer. C. On the foundation stable layer, gypsum dihydrate is used as filler and layered to the design elevation to form a gypsum fill layer. D. On the gypsum fill layer, construct the top functional layer, which includes a gypsum base layer laid on the gypsum fill layer, followed by a waterproof seal layer laid on the gypsum base layer, and finally, on the waterproof seal layer, reserve planting trenches and cover them with growing soil to form a vegetation restoration layer.

2. The repair method according to claim 1, characterized in that: Step A, removal, includes treating dangerous rocks, loose soil, debris, and garbage and contaminated soil at the bottom of the sunken pit; slope trimming includes clearing the surrounding soil slopes into a stepped shape.

3. The repair method according to claim 1, characterized in that: In step B, the thickness of the crushed stone layer is 20-50 cm; the seepage prevention layer is a clay seepage prevention layer or an HDPE geomembrane seepage prevention layer to prevent water penetration; the thickness of the foundation stabilization layer is 1.0-3.0 meters and the material is gypsum material with a compressive strength of 3.0-5.0 MPa, which is formed by layering and rolling to level and stabilize the foundation and reduce the bottom damage caused by stress concentration and uneven stress.

4. The repair method according to claim 1, characterized in that: Step B further includes spraying a concrete layer onto the slope surface of the sunken mining pit, and covering the concrete layer with a gypsum protective layer as a karst waterproofing layer.

5. The repair method according to claim 4, characterized in that: The concrete layer is made of gypsum slag cement concrete and has a thickness of 5-15 cm. The gypsum protective layer has a thickness of not less than 75 cm and is made of gypsum with a compressive strength of not less than 3.0 MPa.

6. The repair method according to claim 1, characterized in that: In step C, the dihydrate gypsum used for the gypsum filling layer has a compaction bearing capacity of not less than 8%, and is filled in layers with a thickness of not more than 50 cm.

7. The repair method according to claim 1, characterized in that: In step D, the gypsum base layer is at least 1 meter thick and made of gypsum with a compressive strength of at least 3 MPa; the waterproof seal layer is 5-30 mm thick; and the growing soil is 50-200 cm thick.

8. The repair method according to claim 7, characterized in that: The waterproof sealant is made of asphalt slurry or waterproof mortar.

9. A restoration structure for reshaping concave landforms in mines, completed by the restoration method described in any one of claims 1 to 8, comprising a concave mining pit (1), characterized in that: A bottom functional layer is provided on the foundation of the sunken pit (1), a gypsum filling layer (2) is provided on the bottom functional layer, a top functional layer is provided on the gypsum filling layer, a concrete layer (3) is provided around the gypsum filling layer and on the contact surface with the sunken pit, and a gypsum protective layer (4) is provided on the contact surface between the concrete layer (3) and the gypsum filling layer; the bottom functional layer includes a foundation drainage layer (5), a seepage prevention layer (6) and a foundation stabilization layer (7) arranged sequentially from bottom to top; the top functional layer includes a gypsum base layer (8), a waterproof sealing layer (9) and a vegetation restoration layer (10) arranged sequentially from bottom to top.

10. The repair structure according to claim 1, characterized in that: The vegetation restoration layer is planted with shallow-rooted plants.

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