The strata movement caused by underground mining is a process advancing with time, so the strata movement mechanism caused by mining is different in different stages. According to the investigation data of surface monitoring and macroscopic damage, the strata movement and deformation in the west area of Chengchao Iron Mine can be divided into two stages: before and after the appearance of surface collapse pits. The first stage is the stage of surface collapse, which caused by the overburden above the goaf is destroyed and extended to the surface; the second stage is the expanding stage of collapse area after the collapse pit appears.
In order to analyze the mechanism of strata movement in the west area of Chengchao Iron Mine, we can establish a typical geological model of Chengchao Iron Mine by combining engineering geological profile of the mining area perpendicular to orebody and the field investigation of joints (Fig. 25).
4.1 The stage of the overburden failure in goaf extends to surface collapse
In the initial stage of underground mining, with mining development, the goaf is gradually expanded. At this time, a small amount of rock in the upper part of the goaf will collapse under the action of gravity, and then the rock at the top of the goaf will form an arch under the action of horizontal tectonic stress, forming the supporting vault of the goaf. The supporting vault can ensure the stability of the upper rock mass temporarily, and the surface will not produce obvious deformation and failure.
With the continuous expansion of goaf in plane and depth, the pressure arch effect becomes more and more obvious, and the arch span becomes larger and larger. However, the span of the arch must adapt to the properties of the arch materials. When the span increases to a certain extent and the rock mass at the top of the goaf can not form a supporting vault, the rock mass at the top will become unstable and will gradually collapse into the goaf. After a large enough goaf is formed by underground mining, the collapse will gradually increase and eventually transfer to the surface, forming a collapse pit on the surface. The specific process is generally shown in Fig. 26. According to Fig. 22, the caving process of the western roof also conforms to the general law.
The main factors that affect the rate of overburden collapse are as follows:
(1) Scope and height of goaf: As of April 2006, the cumulative vertical height of mining in the west area of Chengchao Iron Mine is about 52.5m, the average mining depth of orebody is 412.5m, and the ratio of mining depth to cumulative vertical height is 7.9. Bai YR and others[35-36] analyzed the law of surface subsidence and strata movement caused by underground mining in the eastern area of Chengchao Iron Mine and considered that when the ratio of mining depth to mining cumulative vertical height is less than 17, strata movement will affect the surface; otherwise, it will not affect the surface. According to Bai YR's conclusion, the mining thickness ratio in the west area of Chengchao Iron Mine has already reached the influence range in 2006. Therefore, it is reasonable that the west area of Chengchao Iron Mine collapsed suddenly in April 2006.
(2) Quality of overlying strata: At this stage, if the rock mass quality at the top of the goaf is not good enough, the collapse may develop rapidly. The main lithology of goaf overburden in the west area is gypsum, diorite and marble, and the lithology quality is average. The average karstification rate of marble is 1.30% from 0 to - 150m elevation. The investigation of rock mass structural planes shows that there are a large number of joints in gypsum, diorite and marble, and the overall orientation of the joints is disordered and the shape is irregular. There are many types of overlying strata in the western goaf, which naturally forms the soft layer of rock strata contact surface with steep dip Angle. Thus it can be seen that the overall quality of overlying rock mass in the western goaf is poor.
(3) Groundwater drainage: On the one hand, in the process of underground mining, the goaf cuts off the direct connection between overburden water system to water system in the rock mass below, which increases the effective stress in the overlying strata; on the other hand, due to the groundwater drainage, the hydrodynamic pressure in the overlying strata increases.
(4) Horizontal tectonic stress in the mining area: There is a large horizontal tectonic stress in Chengchao Iron Mine. After the formation of goaf, the horizontal tectonic stress will be concentrated in the overlying strata, which will promote the collapse of overburden with disordered joint distribution.
In addition, the horizontal tectonic stress increases the abutment pressure of surrounding rock on both sides of the goaf and increases with the increase of the scope of the goaf. At this time, the mechanism of the maximum horizontal stress on the surrounding rock can be simplified to the form of fixed pillars at both ends, which is called the "rock pillar" failure mode of surrounding rock, The "rock pillar" of surrounding rock is composed of one structure or several structures. Its mechanical model and bending moment diagram are shown in Fig. 27.
Before the overlying strata fracture and collapse, the overlying strata and rock pillars only have bending deformation, so the deformation transferred to the surface is very small, and there is no crack on the surface. According to the mechanical model of rock pillar, the "pillar" around goaf becomes the bearing body of horizontal tectonic stress in surrounding rock. With the increase of the height of goaf, the horizontal stress on the "pillar" becomes larger and larger, and then it begins to deform and produce tensile stress on the "pillar". When the tensile stress in the "pillar" reaches the allowable tensile strength or the horizontal stress reaches the shear strength, the "pillar" begins to collapse(Fig. 28). After the "pillar" near the goaf is destroyed, another "pillar" in the outer layer begins to bear the horizontal tectonic stress in the surrounding rock. The damaged "pillar" and the roof caving rock can protect the "pillar" in the outer layer and lead to the reduction or disappearance of the "pillar" effect. It can be seen that the " pillar " effect caused by the large horizontal tectonic stress in the mining area increases the scope of underground goaf and is also one of the reasons for large surface movement and subsidence.
In a word, the appearance of surface collapse in the west area of Chengchao Iron Mine is the inevitable result because of underground mining, and the collapse range is affected by horizontal tectonic stress and overburden inclination, which exceeds the prediction in footwall and end.
4.2 The outward expansion stage of collapse area
When the surface collapse pit appears, the surface collapse deformation begins to enter the expansion stage of collapse range. After the collapse area is formed, horizontal tectonic stress release will lead to tensile deformation of the surrounding rock of the collapse pit and tensional fracture to the collapse area. The rock with tensional fracture will collapse under the action of gravity. In this paper, the mechanical mechanism of cantilever beam is used to analyze the deformation of footwall rock mass in the west area of Chengchao Iron Mine.
The NE and NWW trending steep joints in the footwall granite of the Western District play a major role in the deformation and cracking of the rock mass. The joint cuts the footwall into a cantilever beam model. The stress diagram of the first cantilever beam closest to the collapse failure area is shown in Fig. 29.
Before the collapse of the surface, the surrounding rock arm of the overburden caving range is in force balance state in the horizontal direction (as shown in Fig. 29 (a)). There will be no horizontal deformation. After the collapse pit appears on the surface and enters the second deformation stage, the rock in the collapse area becomes a nearly granular structure due to caving. The horizontal stress exerted on the surrounding rock arm begins to decrease (as shown in Fig. 29 (b)). Because of the unbalanced force in the horizontal direction, the rock arms begin to tilt to the goaf. After the rock arm closest to the collapse area tilt to a certain extent, the horizontal stress on the outer rock arm also began to decrease, so that the outer rock arm also begin to tilt to the goaf due to the unbalanced force. Therefore, the range of surface deformation is gradually extended. When the horizontal toppling deformation of the rock arm reaches a certain degree, and the internal tensile stress reaches the allowable tensile strength or the horizontal stress reaches the shear strength, the rock arm will break (as shown in Fig. 29 (c)).
Several factors affect the deformation range of surface subsidence at this stage:
(1) Scope of goafs: After the surface collapse, the scope of underground goafs continues to expand in depth and plane, which makes the scope of surface collapse continue to expand. The rock arms in the expanding range of goafs will produce vertical subsidence under the action of gravity, and their outer rock arm will continue to transfer deformation outward due to unbalanced force. It can be seen that the range of goafs, especially the range of plane expansion, play a decisive role in the range of surface movement and subsidence.
(2) Characteristics of surrounding rock: There are four groups of structural planes in the footwall granite of the orebody in the western area, which are NE, NWW, NNW and NEE. The structural plane with steep dip angle cuts the footwall granite into steep rock pillar, so there is the above mode of the bending failure transfer of cantilever beam of surrounding rock makes the surface collapse and deformation. The occurrence of structural plane in surrounding rock also determines the distribution characteristics of surface fractures.
(3) Tectonic stress in the mining area: there is a large horizontal tectonic stress field in the mining area. The maximum principal stress along the orebody strike direction is only 1.28 times of the gravity, and the horizontal stress perpendicular to the orebody strike direction is 0.48 times of the vertical stress. The horizontal stress is the driving force of surrounding rock deformation and has an important impact on the surface movement and subsidence.
In the first stage of strata movement mentioned above, the gradual increase of underground goaf scope in the horizontal and vertical direction will cause the surface deformation and collapse. But the surface deformation is still in a certain range and has not been transferred to the range far away from the goafs, as shown in Fig. 30 (a). At this stage, underground mining is the main reason of surface deformation, and gravity is the main driving force of surface collapse.
After the strata move into the second stage, under the action of the release and compression of the horizontal tectonic stress, i.e., the maximum principal stress GX, which is parallel to the orebody strike, the rock mass around the collapse pit, which is divided into columnar shape by faults and joints, begins to produce arch expansion deformation towards the goaf and collapse pit. At the same time, the release of horizontal tectonic stress GY, which is perpendicular to the orebody strike, causes the rock mass to produce unloading tensile fracture deformation in the collapse direction. The combined action of the two directions of tectonic stress makes the columnar surrounding rock collapse (as shown in Fig. 30 (b)). So far, the extension of the surface deformation range has entered the tectonic stress leading stage. After the surface deformation enters into the tectonic stress leading stage, the surface deformation in the relevant areas within the deformation range is mainly horizontal movement, and the vertical displacement is relatively small. However, the dip angle of columnar structure rock mass in this area decreases with time, and the ratio of surface vertical displacement to horizontal displacement begins to increase. From the spatial point of view, the closer to the goaf, the larger the ratio of vertical displacement to horizontal displacement, and vice versa. Thus, the funnel-shaped deformation curve is formed on the surface.
4.3 Page and line numbers
According to the above analysis of strata movement mechanism, based on the deformation state of rock mass, the strata are divided into six areas, as shown in Fig. 31.
(1) Vertical subsidence area: The area is located above the goaf, which is mainly formed in the first stage of strata movement and expands with the expansion of underground goaf.
(2) Toppling slip area: The area is the range of collapse and fracture of rock pillar around the collapse pit after the formation of vertical subsidence area. In this area, after the cantilever beams around the vertical collapse area topple to a certain extent and break, the rock pillars begin to slide to the goafs. Therefore, the deep rock mass in this area should have obvious slip surface, and its deformation mode is sliding and toppling, and staggered open cracks are formed on the surface. Some areas of powder ore storage yard and mining area highway belong to this area. No matter the horizontal displacement or vertical displacement of the surface in this area is large, they can reach several meters. There is usually a large width crack or obvious fracture in this area, as shown in Fig. 32 and Fig. 33.
(3) Toppling area: Because of the toppling and sliding of the front rock pillars, the rock pillars in this area also toppled, but their interiors were not broken. Therefore, the deformation mode of rock mass in this area is mainly toppling, and the area forms open cracks on the surface, which is the main area of surface crack expansion. Part of the transport tunnel belongs to this area. The surface deformation in this area is large, which can reach tens of centimeters. The surface deformation is mainly horizontal displacement, and the ratio of horizontal displacement to vertical displacement is much larger than this value of the toppling slip area. There are usually wide through cracks in the toppling area, such as the cracks in the transport tunnel in Fig. 34. The outer boundary of the area corresponds to the boundary of the surface subsidence range.
(4) Deformation area: Due to horizontal stress release, the rock mass in this area also has a certain toppling deformation. However, due to the small deformation, only small cracks are formed on the ground. In this area, the surface deformation is mainly horizontal deformation, the ratio of horizontal displacement to vertical displacement is further increased. The deformation area is also the production area of cracks, with a certain number of microcracks. The outer boundary of the area corresponds to the boundary of the surface movement range.
(5) Deformation accumulation area: The area is located at the outermost edge of the influence range of surface deformation. Although the rock mass in this area is also affected by the underground goafs, the influence is very small, and the horizontal displacement and vertical displacement of the rock mass deformation are not obvious. There is no obvious surface feature either in the deep or on the surface.
(6) Undisturbed area: Because the rock mass outside the deformation area is not affected by underground mining, it can be called undisturbed area.
The six areas of deep rock mass will continue to expand downward and around with mining development.