Mine environmental restoration and management model

After mineral resources are extracted from underground, the most direct impact on geological environment is to change the original ground stress. All kinds of underground mining areas are widely developed mining subsidence problems, such as subsidence basin, collapse pit, ground cracks and other surface rock and soil body failure phenomena. As of 2015, the total subsidence problem area caused by resource mining in Inner Mongolia Autonomous Region is about 306.37km2, which is concentrated in underground mining coal, ferrous metal, non-ferrous metal and precious metal mining areas, among which the subsidence problem area caused by coal mining is about 305.77km2, accounting for about 99.80% of the total problem area.

1. Definition and analysis of mine environmental restoration and management mode

Mine environmental restoration and governance model takes mine geological environment restoration and governance as the main research object. As can be seen from the definition of mine environmental restoration model, the key contents of the model are shown in Figure 1, including: mine environmental geological problems, mine environmental restoration objectives, mine geological environment background, mine development and utilization conditions and mine geological environment restoration and governance model. As can be seen from Figure 1, mine environmental restoration and treatment is a systematic study of mine geological environment, mine environmental geological problems and mine restoration and treatment technologies. Mine environmental restoration and governance mode is an organic and systematic combination of mine environmental restoration technologies, but the technical composition of restricting and guiding mine environmental restoration and governance mode is closely related to the other four aspects. As shown in Figure 1, the five research contents of mine environmental restoration and governance modes, including mine geological environment background, mine development and utilization conditions, and mine environmental restoration objectives, are correlated with each other. The author divides the correlation into the outer "progressive" relationship and the inner "five elements" relationship.

The contents of the outer "progressive" relationship have progressive research correlation with each other, and this "progressive" relationship is helpful to promote the study of the model of mine environmental restoration and governance. Various contents in the internal "five elements" relationship establish internal multi-directional correlation through the mine environmental restoration data model, and become the research content of the technical composition of the guiding model, the application scope of the restoration model, the verification of the model benefit, and the optimization and improvement of the model. Therefore, the five contents also have the characteristics of "two-way" correlation.

2. Macro construction of mine environmental restoration and governance model

The mode of mine environmental restoration and management is a tool to prevent and solve the contradiction between mine environment and problem restoration. Based on the conditions of mine development and utilization and the geological environment background of mine, it is necessary to grasp the law of problem occurrence and put forward preventive measures on the one hand, and repair and control the existing mine environmental problems on the other hand. Therefore, the model is based on the practice of mine environmental restoration to carry out theoretical research, with both typical mine environmental restoration project cases as the technical research background and specialized research on restoration technology, combining the mine environmental restoration experience in different regions of China with the mine restoration theory. A set of mine environmental restoration model with reasonable technical composition, suitable for mines in different areas and complete system has been formed.

The primary task of mine environmental restoration and management model research is to deeply understand the mine environmental problems. Domestic scholars have many classification schemes for mine environmental problems, such as classification based on mine development stage, classification based on mineral species, classification based on the manifestation and impact results of mine environmental geological problems, and classification based on the characteristics of mine environmental geological problems. In the study of model objects, the author focuses on the comprehensive classification scheme of problem nature, and divides common mine environmental problems into: Ground collapse problem, solid waste problem, liquid waste problem, open-pit slope stability problem, water layer structure damage problem and mine secondary geological disaster problem.

According to the functional requirements of the model, the author divides it into three functional sub-modules: engineering disaster elimination module, ecological restoration module and biological restoration module. The functional module of mine environmental restoration and governance mode is shown in Figure 2, in which the main function of the engineering disaster elimination module is to eliminate or prevent regional disaster hazards. The technical types of the module include disaster elimination and restoration technology and engineering restoration technology, which are mainly used to strengthen and transform surface rock and soil bodies and prevent unsafe hazards such as rock and soil bodies and geological bodies. The main function of the two sub-modules, ecological restoration module and biological restoration module, is to promote the restoration of rock and soil mass and ecological environment, and use ecological restoration technology and biological restoration technology to cooperate with the engineering disaster elimination module to complete the mine environmental restoration, so as to achieve the land restoration and ecological restoration goals of the model.

According to the decomposition of mode function modules, the author sets the code for the engineering disaster elimination module, ecological restoration module and biological restoration module in the order of constituting technologies as follows: Disaster elimination and restoration technology (Ai ~ Di), ecological restoration technology (Ei), biological restoration technology (Bei), the code is conducive to the systematic description of the composition of the model, and the macro construction of the mine environmental restoration management model such as "Ai+Bi+Ci+Di+Ei+Bei" and other organic construction of mine environmental restoration technology.

3. Establishment process of mine environmental restoration and management model

The establishment process of mine environmental restoration and governance model is shown in Figure 3. "Restoration of mine environmental problems" is the first step, which goes through the study of technology and objectives, and constitutes the establishment process of the model in a clockwise cycle. In this process, the restoration and governance technology and problems, and the restoration and governance objectives and problems have mutually feeding effects of benefit feedback, and each step has different influence characteristics on the model.

The restoration of mine environmental problems, namely: the understanding of the restoration object, mainly includes the investigation of mine geological environment, the assessment of geological environmental problems, and the typical treatment project cases. Mine environmental investigation refers to five aspects: mine geological environment investigation, mine water environment investigation, mine ecological environment investigation, mine atmospheric environment investigation and mine space environment investigation. This paper focuses on the geological environment survey of mines, and provides basic data on the scale, characteristics, background and occurrence rules of environmental geological problems to illustrate the establishment process of the model. Mine geological environment assessment is a quantitative and qualitative evaluation carried out on the basis of field investigation, taking into account the complexity of geological environment background and the degree of restoration of mine environmental problems, weighing a number of mine environmental indicators.

The degree of harm and the degree of impact of mining activities on the environment, so it plays a guiding role in the application scope and application object of the restoration model. The collection of mine environmental restoration cases is the basis of the extraction and sublimation model, which is helpful to discuss the benefits and effects of the model. There are great differences in the remediation methods for similar mine environmental problems in different regions. Therefore, the remediation and governance cases become the best test method for the pertinence and universality of the remediation and governance model.

Restoration technology research is subject to the joint constraints of restoration project cases and restoration objectives. Reasonable restoration technology can come from either restoration and governance project cases or local governance experience, and an organic model is formed after being determined and coded according to module functions. Therefore, the model is the optimization and organic combination of restoration and treatment technologies, and the characteristics of the technology restrict a number of characteristics of the model, such as: the applicable conditions of the model, the key technologies of the model, the expected results of the model, etc., which has a great impact on the learning, understanding and promotion of the model by mine users. In addition, the expected benefit of remediation technology, construction technology, economic cost and other aspects restrict the application of remediation model.

The objective of mine environmental restoration is the expected benefits and results of mine environmental restoration proposed on the basis of the understanding of the restoration image and the restoration experience of mine environmental problems and the existing restoration technology. It has a guiding effect on the construction of the model and becomes the basis for judging the rationality of the application scope of the model. Mine environmental restoration goals are divided into geological engineering goals, land restoration goals and ecological restoration goals, which must conform to the government policy, laws and regulations and the actual situation of the mine environment. Mine environmental restoration goals can lead to the establishment of targeted restoration models, and mine environmental restoration governance models can be classified according to different restoration goals.

4. Characteristics of mine environmental restoration and management mode

4.1 Pertinence

 The model study focuses on restoration object, restoration target, restoration technology and other contents, and each content focuses on targeted research. Specifically, the application scope of the restoration mode related to mining subsidence problem takes into account the different performance characteristics of the surface of the problem, such as subsidence basin, collapse pit, collapse trough and ground fracture, etc. The engineering restoration technology of different occurrence forms must correspond to different targeted technologies in order to achieve the benefits of the restoration mode.

4.2 Universality

Mine environmental restoration and management model is a set of systematic and perfect mine environmental restoration theory, which involves many mine environmental geological problems. In other words, the model is applicable to the mines or regions where the vast majority of mine environmental problems occur. Universality means that under the constraints of the scope of application of the mine environmental restoration and treatment model, the model can be used in the mine environmental restoration and treatment project with similar problems. For example, in Shanxi, Shaanxi, Ningxia and other coal mining areas in the Loess Plateau mine secondary geological disaster control project, the hidden landslide in the mining area can be used according to the characteristics of loess landslide, the scope of application for the loess region of the mine secondary geological disaster restoration and management model, and in the model description with similar regional engineering cases to guide the preparation of treatment plans.

4.3 Procedural

Mine environmental restoration and management model can be directly applied to mine environmental restoration projects. According to the scope of application agreed by the model, it is necessary to clearly point out the characteristics of the model's composition technology, key technology and main process, analyze the engineering benefits that can be realized by the model according to existing engineering cases, and ensure that the model is simple and practical. In order to facilitate the government units, mine management departments to promote the mine environment management and prevention work. Therefore, the pattern is process-oriented, and the realization of its benefits benefits from the user's correct grasp of the characteristics of the pattern and the accurate implementation of the repair and governance technology of the constitutive pattern.

5. Optimization of mine environmental restoration and management mode

The model of mine environmental restoration and management is the study of the complex system of mine environment, which is constantly updated and optimized. The optimization of the model is closely related to the response of mine geological environment, the benefit of mine environmental restoration, the objective of mine environmental restoration and the technology of mine environmental restoration. The optimization factors of mine environmental restoration mode are shown in Figure 4.

The model of mine geological environment response can be optimized, because the model must be established on the basis of mine environmental problems and development background. According to the characteristics of geological environment response to different mine environmental problems under many influencing factors, the type and number of problems involved in the mine environmental restoration and governance model can be expanded. In addition, due to the continuous progress and update of mine environmental restoration targets due to national policies, the model should optimize and update the technical composition to achieve new targets as the degree of target requirements changes. The benefit demand of mine environmental restoration is related to the expected effect of the model, which needs to be established on the basis of statistical data such as cost, time and area, which requires the optimization of the key technology and process flow of the model, and the continuous optimization of the model to meet the benefit demand. Finally, with the continuous progress and improvement of mine environmental restoration and management technology with the difficulty of restoration, demand and patent renewal, the model needs to be upgraded and updated to achieve the optimization of model level and benefits.

6. Example of mine environmental restoration mode

Based on the geological environment survey of coal mines in North China and the research content and construction steps of the mine environmental restoration and management model, this paper gives the following example of the restoration and management model of ground collapse problem, as shown in Table 1, and explains the subsidence constructed wetland model.

6.1 Constructed wetland model of subsidence basin

The model "subsidence area filling + digging shallow pad/local subsidence pit backfilling + water barrier layer setting + constructed wetland development" is an example of constructed wetland model for subsidence basin in areas with high diving level and large surface subsidence deformation, and is suitable for flat or hilly areas. The land use goal is to repair topography, restore forest and grass land and build constructed wetland system.

It can be seen that in the implementation process of this model, there are main technologies such as subsidence area filling, deep digging and shallow pad, water barrier layer setting and constructed wetland development. The technical contents and functions are as follows:

1) Subsidence area filling. In view of the existing deformation area of subsidence basin, solid wastes such as gangue and fly ash are used to fill the deformation area, basically repairing the damaged terrain and geomorphology in the area with strong deformation and keeping it consistent with the surrounding area.

2) Dig deep pad shallow/local collapse pit backfill. The surface soil in the area to be repaired is mechanically peeled off, and the deformation area with small deformation is treated with deep digging and shallow pad. The existing collapse pit can be repaired with solid waste backfilling, and the pre-peeled topsoil is repeated after the design elevation is reached.

3) The water barrier layer of "deep digging area" is set. The water barrier layer is set with clay mud or impermeable film, and can be used as constructed wetland development area after the setting work is completed.

4) Development of constructed wetlands. According to the mine meteorological and hydrological conditions, a reasonable combination of wetland plants was selected based on the water accumulation caused by mine ground collapse and the technology of constructed wetland landscape development. In addition, with the mine water treatment technology, the coal mine wastewater purification and wetland landscape design can be combined to build constructed wetlands.

In terms of process flow, the topsoil in the area to be filled in the subsidence basin is firstly removed by mechanical stripping, solid waste such as gangue and fly ash is used to fill the filling area, and the pre-stripped topsoil is re-covered after the design elevation is reached. The uneven area in the affected area of the subsidence basin should be dug deep and shallow, and the water barrier layer should be set in the dug deep area to ensure that the water insulation of the dug deep area and the surrounding environment is intact. After the water barrier is set up, the dug area will be developed as constructed wetland or combined with the water subsidence area for unified development.

From the above description of the model, it can be seen that the topographic and geomorphic restoration of the ground subsidence area is a typical idea for the treatment of ground subsidence. On the basis of not affecting the original landform type, the use of coal gangue, fly ash and other repair

The terrain of the complex area can be repaired, and the restoration can be basically integrated with the original surrounding terrain. In general, because the collapse area is often large, the topographic slowdown is more economical than digging deep and shallow.

6.2 Baorixile mining area treatment project case

6.2.1 Mining area overview

Baolixile Mining area is located in the eastern part of the Chenqi coalfield, under the Chenbarhu Banner of Hulunbuir City. Since its development in the 1970s, it has been mined for nearly four decades. The mining area is wide-open and belongs to the high plain landform. The overall terrain is a gentle slope with a slope of 2° ~ 5° and a relative elevation difference of 120 m. The surface of the mining area is covered by the quaternary loose layer and there is no bedrock exposure. In the interior of the mine area, the strata are simple, mainly Cretaceous Jalainur Group, Cenozoic Quaternary Pleistocene series and Holocene series.

6.2.2 Distribution and cause analysis of ground collapse problems

The corresponding surface position of the main well fields in Baolixile mining area is mostly natural grassland. Due to the obvious surface collapse and deformation caused by mining for many years, a large area of surface collapse occurs along the coal mining face, most of which are subsidence basins with a large number of large subsidence pits inside the basins and strip-shaped fractures developing around the subsidence pits. The main reason for the formation of ground collapse in mining areas is the lack of awareness of mine environmental geological problems in the past. In the late 1990s, the number of small coal mines increased sharply, and large-scale and high-intensity mining caused serious mine ground collapse.

6.2.3 Mode selection and management project

Baolixile Mining area ground collapse control is the ground collapse control of the closed mine. Based on the analysis of the current situation of ground collapse problem in the mining area, considering the geological environment background of the mine, analyzing the terrain, geomorphologic characteristics of the area to be repaired, as well as the land use target and ecological restoration target in the later stage, the above model example 2 can be used for ground collapse control in Baolixile mining area. The improved model applied to the mining area is "dig deep and pad shallow/local collapse pit backfill + constructed wetland development". The mode is used as follows:

1) Dig deep and pad shallow: there are some water-logging areas in the sunken basin of Baolixile mining area. In the treatment project, dig deep and pad shallow in the water-logging area and its surrounding areas to further water storage, which has certain use value, such as drinking water points for livestock or fish ponds. The shallow bedding area is mainly the surface rupture area caused by the deformation of the subsidence basin. The soil in the deep digging area is used to cover the shallow bedding area, basically repair the topography of the shallow bedding area, and provide site preparation for the subsequent development of constructed wetlands.

2) Local collapse pit backfill: The collapse pit developed in the subsidence basin to be repaired in Baolixile mining area is backfilled with mine solid waste and the surface is leveled to keep the topographic relief consistent with the surrounding area. Backfill materials come from Shenbao open-pit dump, mainly gangue and gravel soil, which can not only backfill the collapse pit, but also make good use of mine solid waste. After backfilling the collapse pit, cover its surface with 30cm thick cultivated soil in order to restore vegetation.

3) Development of constructed wetland: The treatment project of Baolixile Mining area retains the ground collapse area with accumulated water, or further digs the surrounding deep collapse pit, and builds the stagnant water depression formed by digging deep and shallow into a drinking point for livestock and a fish pond. As shown in Figure 6, the treatment effect of the subsidence basin in Baolixile Mining area is as follows: In other areas where the restoration target is grassland, after the restoration of the basic terrain, the topsoil and cultivated soil are repeated on the surface, and grass seeds suitable for local growth such as Leymus chinensis, dipweed and artemisia cold are mixed and planted.

6.2.4 Restoring the Effect of the governance Mode

Baolixile Mining area adopts the method of deep excavation and shallow treatment for the ground collapse area with accumulated water, and uses the excavated lake mud as the cultivated soil for the adjacent collapse pit and the shallow area after treatment to cover the soil and grass, so as to rationally utilize the land resources in the ground collapse area, improve the local ecological environment of the collapse pit, make the environment of the problem restoration area more beautiful, and develop the animal husbandry in the mining area. By using the solid waste of the mine dump to backfill the subsidence pit developed in the subsidence basin, the mine solid waste is effectively used and the influence of the subsidence pit is eliminated. After treatment, the ecological environment in the sunken basin is basically restored and integrated with the surrounding environment. The constructed wetland has a good adaptability to the local climate, and can store natural precipitation and provide some water for the animal husbandry and fishery in the mine.