0 Preface

The preliminary design is a necessary procedure for the basic construction of open pit mines. It is a comprehensive design based on the overall design of the mining area, the feasibility study report of the open pit mine, and the report of the mine field. It is the next step of safety facility design, construction drawing design and construction organization. The main purpose of design and construction project management is to determine important technical and economic issues such as design capability and site selection, development of transportation mode, mining procedures, main production processes and equipment, safety measures, etc. The design quality and technical level are directly related to the whole. Safety, technical and economic benefits of mine construction and production processes.
At the beginning of graduation, the author was engaged in production design work in a large open pit mine in Inner Mongolia. Later, he was responsible for open pit mine consultation and survey design work in the design institute. During the project survey design, especially the preliminary design process, he deeply realized the design institute's analysis of several key issues. There are some optimisations in the solution and the design scheme is quite different from the mine construction and production implementation scheme. Therefore, the following specific problems, their solutions and ideas are proposed for reference.
1 proposed key issues
1.1 The impact of water
Water (especially groundwater) is one of the key factors affecting the stability of slopes. According to the information currently available, more than 90% of open pit landslides occur in the rainy season or directly related to groundwater, which fully demonstrates the water Groundwater is an important factor affecting the stability of open pit slopes. The side effects of slope groundwater on slope stability are mainly reflected in two aspects: one is to change the stress state inside the slope body, for example, the water level rises to increase the dynamic and hydrostatic pressure in the slope; the second is to reduce the rock mass Strength, especially the shear strength of weak structural surfaces in rock mass.
After the pre-draining project with water slopes, the adverse effects of groundwater on the stability of the slope can be greatly reduced. Limited by the depth of engineering design, the slope stability calculation in the feasibility study stage is based on the undiluted water slope. The preliminary design directly refers to the conclusion of the feasibility study report. Only a few consider the favorable factors of the groundwater level drop after pre-draining. However, another favorable factor is rarely considered, that is, the pre-draining causes the strength of the slope rock mass to increase due to the decrease in water content.
1.2 High slope selection angle selection and improper contour design
With the expansion of mining scale and the improvement of equipment technology, open pit mines have shown a trend of increasing mining depth. At the same time, in order to fully recover resources and reduce production costs, the slopes are showing a trend of increasing steepness. High-steep slopes can save costs and create benefits for enterprises, and also bring great challenges to the safety of stope operations and prevent landslides. The importance of reasonable and stable open-pit mine slope angle and contour design and research under the new situation It is also becoming more and more prominent.
1.3 Problems in developing transportation system design
From a certain point of view, the essence of open pit mining is material removal. The rational layout of the development of the transportation system in the preliminary design directly affects the calculation of the total transportation distance and the capacity and quantity of transportation equipment. Common problems in practical work are: due to space constraints, stripping and mining transportation systems often occur mixed flow and crossover, but from the perspective of safe production and avoidance of interference, stripping and mining transportation systems should be separated as much as possible; There is still room for optimization in the way the ground is connected.
1.4 Dumping volume calculation and height selection
Due to long-term and large-scale development, the current ore fields with good domestic resource reserves have been basically put into operation, and the infrastructure and production stripping of new projects have increased significantly. The area of ​​excavation and the amount of excavation at the time of production are obvious. The increase has directly led to the project construction land exceeding the standard, the most prominent of which is the over-standard area of ​​the dumping site.
2 solution discussion
2.1 Calculation of soft rock slope stability with pre-drainage engineering
Domestic experts have carried out a number of studies on the relationship between soft rock moisture content and rock mass strength, and have achieved considerable research results. Lin Hongzhou and Li Guangxin studied silty clay, etc., pointing out that no matter what kind of soil, when the water content is low, the cohesive force increases with the increase of water content. After reaching a certain value, the cohesive force increases with the water content. Reduced, and the internal friction angle is negatively correlated with the soil moisture content. Liu Jifeng and Lu Mingshi proposed that the inflection point of the function of cohesion and water content occurred at 17.5%. Yan Linchang and other studies have obtained the effect of the increase of water content on the cohesion force than the internal friction angle, and the relationship between cohesion, internal friction angle and soil moisture content is proposed by statistics.
According to the above, the impact of groundwater on the soft rock slope with pre-drainage engineering should be considered comprehensively. After the slope dewatering measures are taken, the water head and soil moisture content of the slope are reduced, the slope strength is increased, the load is reduced, and the stability coefficients of the overall slope and the local danger zone are improved. However, since the actual dewatering effect is difficult to predict accurately, the slope stability with the change of the strength of the weak layer with the change of water content should be carried out in the preliminary design. The slope stability analysis of the strength with the change of water content should be carried out, and the owner is recommended to conduct special research.
2.2 High slope selection angle selection and slope contour design
In the preliminary design, the sloping angles of the high and large slopes are calculated and determined according to different lithologies. The usual calculation results show that the same lithology section is a straight line, and the overall slope contour is a broken line. However, for open rock mines with a particularly thick upper rock or loose layer, the use of a single sloping angle for the same lithology of soft rock or loose layer may result in landslides.
For example, Inner Surface Mine victory inoperative Slope strata of Quaternary Tertiary loose bed and coal strata, belonging to soft rock slope, wherein the fourth region-based distribution, thickness of 12.3 ~ 23.5m, The average is 18.7m; the third system is locally distributed, with a thickness of 0~16m; the whole distribution of coal seams is distributed. The preliminary design determined that the slope angle of the Tertiary and above strata is 18°, the coal stratum is 24°, the slope height is 150 m, and the overall slope angle is 22.79°. When the mining depth reaches 60-80m, a series of landslides occur along the bottom of the Tertiary in the upper slope. Through the analysis of the landslide mechanism, it is found that the slope of the lower part of the Tertiary is too large and affected by water, and the sliding force exceeds the anti-sliding force, which is the main cause of landslide. After adopting engineering measures such as encrypting the ground to dry the well, cutting the upper part, and pouring the lower part of the stone, the landslide is effectively contained. Therefore, the complex shape slope should be calculated and analyzed according to its contour shape. The single slop angle should be used cautiously. It is recommended to design the soft rock slope according to the “upper steep and slow” or concave slope contour.
2.3 Exploring the optimization design of transportation system
In order to reduce the amount of infrastructure stripping and the initial stripping ratio, the open pit design is designed to use a larger working angle, and the layout of the transportation system is compact, thus ignoring the problems caused by the mixed flow and crossover of the vehicle.
In the process of project construction and production, due to adverse factors such as changes in geological conditions and production organization, it is difficult to reach the design angle of the work slope angle, thus creating conditions for avoiding mixed flow and crossover of the stripping and transportation system. When designing the work to help the slope angle, the designer should not only consider the depth and length of the stope, the number of layers, but also consider various influencing factors such as geological changes and production organization, and establish a reasonable and feasible working angle. On this basis, Optimize the design of the development transportation system to avoid mixing and crossover. According to the author's experience, the theoretical calculation of the horizontal open-pit mine with a working angle of about 12° is used. The initial design work reduces the slope angle by 1°, which is beneficial to the development of the optimal layout of the transportation system, and is also in line with the actual situation on site.
In the preliminary design, the first ramp of the dumping site is designed as a half-deck and a half-deck. In the project construction and production process, the safety distance between the soil field and the mining site is generally used. The form of the side fill embankment, as shown in Figure 1, has the advantage of shortening the horizontal transport distance of one ramp length, reducing transportation costs, reducing the number of dumps on the dump, and reducing the volume loss of the dump.

2.4 Land Conservancy Land Saving Measures and Height Selection
Based on years of experience, the main ways to save land in dumps are as follows:
(1) Priority is given to the inner row. The inner row can shorten the transportation distance and reduce the transportation cost, and can save the external dumping area. The goaf in the stable floor and suitable for the inner row should be preferentially arranged inside;
(2) Joint arrangement of internal and external dumping sites, connecting the internal and external dumping sites when conditions permit, reducing the volume loss caused by grading, and saving land on a large scale;

(3) The total abandonment height of the soil dumping site is used. For example, the “Construction Land Use Index of Coal Industry Project: Part of Auxiliary Facilities of Open-pit Mine and Open-pit Mining Area” implemented by open-pit coal mine project stipulates that “the total height of dumping site shall not be less than 86m. determine";

(4) Increase the final slope angle of the dumping site. The main measure is to compress the final transport platform width of the dumping site on the premise of ensuring safety.
The abandonment height is the key to saving land in the dumping site. It is determined by the hydrogeological conditions of the abandoned site, the nature of the abandoned rock and rock, the disposal equipment and the transportation requirements. The land waste is too low, and the following problems exist: First, for the excavation soil where the exfoliated material is mostly loose and the basal lithology is poor, the sloping slope is prone to occur when the abandonment height is too large and the slope is too steep; the second is the effect of excessive altitude on the future ecological link management and restoration work. Larger, and easily lead to soil erosion and even geological disasters.
3 conclusions
In this paper, the influence of groundwater on the stability of slope is analyzed, and the research results of the relationship between the moisture content of soft rock and the strength of rock mass are summarized. It is proposed that the calculation of slope stability should consider the increase of rock mass strength caused by the decrease of water content of slope. The actual landslide mechanism analysis, the high-slope segmentation design is proposed, and the concave contour is preferred. Through the work experience, it is proposed to moderately reduce the working slope angle to facilitate the development of transportation system optimization, and adopt the embankment ramp to save the transportation distance; From the perspective of land, four ways to increase the volume of the dump are proposed and their key issues.
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