Aiming at the problem of surface movement and long-term stability of a work plane of deep well strip mining in Shandong Province, an observation station is set up on the surface of strip mining, and the surface deformation value during strip mining is measured with advanced measuring instruments; on the stable surface of the old mining area, the surface deformation monitoring work is also carried out for new buildings. In addition, the FLAC3D simulation method is used to determine the subsidence factor of different mining depth, mining width, mining length, and mining thickness, and the mathematical model between the subsidence factor and mining depth, mining width, mining length, and mining thickness is established. After the surface of the old goaf is basically stable after strip mining, the high-rise buildings are built. By changing the size of the new buildings and the amount of the load imposed on the surface, the surface deformation is simulated and calculated, and the relationship between the different load positions, load sizes, loading building sizes, and the surface activated deformation is obtained. The measured value of the surface deformation confirms that the load of the new buildings can induce the activation of the old goaf and make the surface generate secondary deformation, but the activated deformation makes the new building within the range of 1, so the new building is safe.
With the expansion of China’s coal mining scale and the rapid development of urbanization, land resources for mining is becoming increasingly scarce. Thus, some important buildings have to be built over the goaf [
The deformation of the new buildings over the goaf is mainly affected by the residual deformation of the surface movement after coal seam mining and the secondary deformation “activated” by the new building load [
The broken rocks in the goaf tend to stabilize after a long period of natural compaction, and there are a large number of holes and cracks in the caving zone and the fractured zone [
Over the years, scholars at home and abroad have done a lot of in-depth researches in this study, and there are many breakthroughs in the researches. Swift explains the relations between overburden strata movement of the internal joint and ground surface subsidence [
In the present study, a case study was carried out on 11021 and 11041 working faces in the east wing strip of a coal mine in the Jining mining area of Shandong Province. Based on the measured data of surface movement and the surface deformation of different mining depth, mining thickness, mining width, and mining length simulated by FLAC3D, the corresponding subsidence factor is obtained, and the regression equations between the subsidence factor and mining depth, mining thickness, mining width, and mining length are established, respectively [
The mine is located in Jining City, Shandong Province, China. The coal seam is nearly horizontal with a mean thickness of 8 m. All panels in this mine are using the fully mechanized top coal caving method.
Panels 11021 and 11041 are selected for this case study. These two panels are 100 m apart at a depth of 1000 m and both are approximately 80 m wide along the dip. Panels 11021 and 11041 are about 800 m long along the strike, respectively. The stratigraphic distribution of the mine is shown in Figure
Stratigraphic distribution of the mine.
In the mining process, the observation station of surface movement was established, the plane coordinates of the observation points were measured by the Network CORS system and the 2-second total station, and the elevation was observed by the Dini03 electronic level produced by the Trimble Company of the US. The observation points of surface movement are arranged along the strike and the trend of the work planes 11021 and 11041, and the arrangement of the observation points is shown in Figure
Map of observation points.
In the work planes 11021 and 11041, work was stopped from March 2012, and from then, the task of the ground surface deformation observation was performed. The observation work was operated for one year after the end of mining process of the work planes, and then it was stopped. Many plane and vertical movement values were observed by using Network CORS, Tianbao Dini03, and other electronic levels according to some technical requirements about the
By using the software of the probability integral method to invert the rock movement parameters, the parameter inversion calculation is carried out for the measured surface movement observation value, and the predicted parameters of the surface movement of the work planes 11021 and 11041 are obtained, as shown in Table
List of improved probability integral method parameters.
Coal rake | Rock movement parameters | ||||
---|---|---|---|---|---|
tan | |||||
3 | 0.14 | 0.26 | 2.1 | 90° | 0.027 |
According to the parameters obtained by inversion, the maximum deformation value at any point under the influence of coal seal mining is obtained. The probability integral method, based on the normal distribution function as the influence function, expresses the surface subsidence basin with the integral formula. The related formulas for calculating the movement and deformation of any point on the surface, respectively, are expressed in the following [ Maximum subsidence value: Maximum dip: Maximum curvature: Maximum value of horizontal movement: Maximum value of horizontal deformation:
In the above formulas,
The influence of different mining depth, mining width, mining length, and mining thickness on the surface movement characteristic in deep well strip mining is quantitatively studied. According to the geological conditions of work planes 11021 and 11041, 16 three-dimensional models are established for different mining conditions, and the surface subsidence values of different models are calculated by numerical simulation, so as to determine the functional relationships between mining depth, mining width, mining length, mining thickness, and the surface subsidence factor in deep well strip mining.
Due to the large depth of work planes 11021 and 11041, the numerical model cannot cover all the rock strata, except for the coal, roof, and floor strata. Figure
16 strata of the numerical model.
The mechanical properties of the coal and rock strata in the FLAC3D numerical model are usually determined by the laboratory experiments.
Numerical simulation results in different mining depth.
Mining depth (m) | Maximum subsidence value (m) | Subsidence factor | Remark |
---|---|---|---|
800 | 1.117 | 0.14 | Mining width 80 m |
900 | 0.961 | 0.12 | |
1000 | 0.719 | 0.09 | |
1100 | 0.558 | 0.07 |
Four calculation models of strip mining simulation are established on the basis of the FLAC3D numerical simulation software towards the mining depths 800 m, 900 m, 1000 m, and 1100 m, and its mining width is 80 m, leaving width is 100 m, and recovery ratio is 45%. The obliquity of the coal seam is calculated horizontally, and the outcomes of simulation are shown in Table
Numerical stimulation results in different mining width.
Mining width (m) | Maximum subsidence value (m) | Subsidence factor | Remark |
---|---|---|---|
80 | 0.719 | 0.09 | Mining length 800 m, mining depth 1000 m |
100 | 1.403 | 0.13 | |
120 | 1.516 | 0.19 | |
140 | 2.064 | 0.26 |
According to the data in Table
Relationship between the subsidence factor and the mining depth.
It can be seen from Figure
Four numerical simulation models are established, respectively, for the mining width of 80 m, 100 m, 120 m, and 140 m. The mining depth of each model is 1000 m, the leaving width is 100 m, and the mining thickness is 8 m. The coal seam dip is calculated horizontally. The simulation results are shown in Table
Numerical stimulation results in different mining length.
Mining length (m) | Maximum subsidence value (m) | Subsidence factor | Remark |
---|---|---|---|
400 | 0.561 | 0.02 | Mining depth 1000 m, mining thickness 8 m, mining width 80 m, leaving width 100 m |
600 | 0.876 | 0.06 | |
800 | 0.719 | 0.09 | |
1000 | 1.197 | 0.13 |
According to the data in Table
Relationship between the subsidence factor and the mining width.
It can be seen from Figure
Towards the strip mining plan, that is to say, the mining depth is 1000 m, the mining thickness is 8 m, the mining width is 80 m, and the leaving width is 100 m, besides, the recovery ratio is 45%, four simulation models are set up. And, the mining length is 400 m, 600 m, 800 m, and 1000 m, respectively. The simulation outcomes of obliquity of the coal seam based on the horizontal calculation are given in Table
Numerical stimulation results in different mining thickness.
Mining thickness (m) | Maximum subsidence value (m) | Subsidence factor | Remark |
---|---|---|---|
2 | 0.402 | 0.2 | Mining depth 1000 m, mining width 80 m, leaving width 100 m |
4 | 0.515 | 0.13 | |
6 | 0.597 | 0.1 | |
8 | 0.719 | 0.09 |
According to the data in Table
Relationship between the subsidence factor and the mining length.
It can be seen from Figure
For the mining plan of “mining depth 1000 m, mining width 80 m, leaving width 100 m, and recovery ratio 45%,” four calculation models are set up according to the mining thickness of 2 m, 4 m, 6 m, and 8 m, respectively. The boundary conditions and rock mass mechanical parameters of each model are consistent with the previous calculation. The numerical simulation results are shown in Table
According to the data in Table
Relationship between the subsidence factor and the mining thickness.
It can be seen from Figure
The goafs that are introduced in the case analysis are composed of panels 11021 and 11041. With the mining depth of 1000 m, the mining width of 80 m, the leaving width of 100 m, and the mining thickness of 8 m, the inclination angles of the coal seam are ranging from degree one to degree two, and the goaf area is approximately 0.25 km2. There are villages, factories, and other buildings on the surface above the goaf. After the surface of the goaf tends to be stable, the mechanical parameters of each layer are weakened to varying degrees.
In order to further study the influence of new buildings on the secondary deformation of the ground surface of the goaf, the rules between different load positions, loading building sizes, load weight, and the surface secondary subsidence values were explored. According to the geological conditions of work planes 11021 and 11041, several three-dimensional models are established, and the corresponding surface secondary subsidence values are solved by using the numerical simulation software. At the same time, the basic shape of the surface deformation is revealed by using the field survey data, which provides data for comparing with the numerical simulation data in this paper.
In order to study the surface secondary deformation characteristics of the building load in different positions of the old goaf, the simulation scheme is as follows. For the model after the surface of the old goaf is basically stable after strip mining, the three-level load with 0.4 MPa, 0.6 MPa, and 0.8 MPa is applied on the marginal area of the open-off cut, medium area of the work plane, medium area of the goaf, and marginal area of the stop line. As shown in Figure
Building load location.
Due to the effect of the building load on the surface above the old goaf, the surface activation will generate new deformation, and the surface secondary subsidence values are shown in Figure
Relationship between load position and surface secondary subsidence.
Taking 0.6 MPa as an example, it shows the relationship between the load position and the surface secondary settlement caused by activation. Under 0.6 MPa, the secondary subsidence value of position 1 is 148.60 mm, position 2 is 131.53 mm, position 3 is 128.29 mm, and position 4 is 136.71 mm. It can be seen that under the same level of load, the relationship between the amount of the surface secondary subsidence value at different load positions is position 1 > position 4 > position 2 > position 3. Under other levels of load, this relationship also holds.
It indicates that the secondary deformation value of the surface activated by the new buildings is different in the different building location. The secondary surface deformation value activated by the new buildings located in the medium area of the goaf is the least, followed by the medium area of the work plane, the marginal area of the open-off cut, and the marginal area of the stop line. As the load level increases, the stability of the middle area of the goaf becomes more obvious. In order to reduce the impact of the secondary deformation of the surface on the new buildings and reduce the settlement range of the building foundation when the buildings are constructed over the old goaf, it is better to choose the medium area of the old goaf as much as possible to avoid the two dangerous areas, the marginal area of the open-off cut and the marginal area of the stop line.
In order to study the influence of the building size on the surface secondary subsidence value, the specific simulation scheme is as follows: nine numerical simulation models with a building size of 40 × 20 m2, 20 × 100 m2, 40 × 60 m2, 40 × 100 m2, 40 × 140 m2, 60 × 100 m2, 80 × 100 m2, 100 × 100 m2, and 120 × 100 m2 are set up with 0.6Mpa. The boundary conditions and rock parameters of each simulation scheme remain unchanged.
After the completion of excavation, 0.6 MPa building load is applied on position 3 for different size of models. Due to the effect of building load on the surface above the goaf, the surface activation will generate new deformation, and its subsidence value is shown in Table
Secondary subsidence value with different building sizes.
Building size (m2) | Secondary subsidence value (mm) |
---|---|
40 × 20 | 68.13 |
20 × 100 | 86.55 |
40 × 60 | 111.11 |
40 × 100 | 128.29 |
40 × 140 | 136.4 |
60 × 100 | 155.99 |
80 × 100 | 174.93 |
100 × 100 | 188.44 |
120 × 100 | 197.84 |
It can be seen from Table
With the regression analysis on the maximum values of secondary deformation obtained by simulation of each scheme, the regression curve between the building size and the value of surface secondary settlement caused by activation is shown in Figure
Fitting curve of building size and maximum surface secondary settlement.
The quadratic polynomial function of the new building size and the maximum value of surface secondary settlement is as follows:
The influence of the width-to-length ratio of new buildings on the secondary deformation of the ground surface of the goaf is further studied. The analysis of the building model selected with its length of 100 meters and different width-to-length ratios shows that the value of “activated” surface subsidence increases as the ratio of the building width to length becomes bigger, as shown in Table
Secondary sink in different building width-to-length ratios.
Size of building (m2) | Width-to-length ratio of building | Value of secondary deformation (mm) |
---|---|---|
20 × 100 | 0.2 | 86.55 |
40 × 100 | 0.4 | 128.29 |
60 × 100 | 0.6 | 155.99 |
80 × 100 | 0.8 | 174.93 |
100 × 100 | 1.0 | 188.44 |
120 × 100 | 1.2 | 197.84 |
Maximum values of secondary deformation obtained by every plan and simulation are analysed by use of the MATLAB software. The curvilinear regression of the value of the secondary deformation and the width-to-length ratio of the building is shown in Figure
Fitting curve of the building width-to-length ratio and maximum secondary settlement.
When the length of the new building is 100 meters, the quadratic polynomial function about the building width-to-length ratio and the secondary settlement of the “activated” surface is
The exclusive e-commerce building with fifteen storeys was initially constructed by Jining on the surface of 11021 and 11041 strip mining work planes in March 2016 on the basis of the construction plan. Comparing the maximum deformation of the building location calculated by the probability integral method with the measured deformation data, it can be known that the surface of the goaf was approaching the maximum subsidence value due to the influence of the surface movement after coal seal mining. Due to the effect of the new building load on the surface of the old goaf, the secondary deformation was caused by the surface activation. In order to ensure the stability of the building foundation, deformation monitoring was carried out in the surface observation points A13, A14, A15, A16, A17, A18, A19, A20, C14, C15, C16, and C17 around the building in the initial construction stage, and the arrangement of the observation points is shown in Figure
Overview of observation points around the building.
The observation began on March 1, 2016. After the first observation, the observation was conducted once for each additional layer until the top is closed. After the construction work is completed, the observation was conducted once every two months. By December 1, 2018, the observation had lasted for two years and nine months, with a total of 13 observations. The curve of the surface secondary settlement caused by activation of the monitoring point is shown in Figure
Monitoring point settlement.
It can be seen from Figure
In this area, the maximum subsidence of the ground surface monitored at the newly built 20-layer buildings in the nonexcavated areas is generally 20 mm to 30 mm, while the maximum subsidence of the 15-floor experience building is 112.3 mm. The result indicates that the effect of the load of the newly built buildings induces the activation of the old goaf and secondary deformation of the ground surface. The maximum value of surface secondary deformation of the goaf is about 4 times than that of the nonexcavated areas.
Due to the small amount of surface movement and no water on the surface in deep well mining, Lidar and RTK technology are combined to collect field data of surface subsidence in mining areas, manual intervention processing is carried out on the acquired point cloud data, and the point cloud data and the generated DEM are used to analyze the surface subsidence deformation of the coal mine; besides, D-InSAR technology and deformation monitoring method of Network CORS based on Beidou can be carried out, and the “third class” elevation measurement accuracy can be achieved by improving the outdoor survey system and implementing a more precise data processing model [
In order to control the surface deformation, protect the surface buildings, and prevent the impact of rock burst, the mining methods such as strip mining and backfill mining are adopted in the underground mining work plane [
After the mining of the underground coal seam, the overlying rock above the goaf will gradually be in a relatively balanced state after a long time of natural compaction. When the new building is located on the surface above the old goaf, under the effect of the building load, it will produce additional stress to the foundation soil and transfer it down according to certain rules, changing the stress state of the fractured rock above the old goaf, making the overlying rock of the goaf produce new deformation and failure [
After the operation of the deep well strip mining, the surface movement and the secondary deformation of the goaf caused by the applied load have a great impact on the surface buildings. This paper analyzes the surface subsidence characteristic of the two work planes No. 11021 and No. 11041 in a Jining mine and the stability of the new building with 15 storeys after the surface of the goaf stabilizes. According to field measured data and FLAC3D numerical simulation data, the mathematical models of subsidence coefficients and “activated” deformation characteristics of the goaf were obtained under different mining conditions.
During deep well strip mining, under the premise of other fixed conditions, the subsidence coefficient decreases with the increase in the mining depth, showing a prior inverse relationship, and the function model expresses
The stability of the new buildings built on the surface in the medium area of the goaf is the best. When the buildings are constructed over the old goaf, we should choose the medium area of the old goaf as much as possible. There is a quadratic function relationship between the size of the new building and the “activated” subsidence of the goaf, and the function model expresses
Based on the measured data and simulation analysis, the relationship between different geological conditions and the surface subsidence factor in deep well strip mining and the rule that how new buildings on the surface of the old goaf influence the secondary deformation are obtained. The conclusions can also be applied to the study of the stability of the building foundation of the goaf in the same geological conditions. It is also used to the construction of the infrastructure in the mining area, the planning of the town in the mining area, and the protection of buildings and the design of the coal seam mining.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The research of this study was sponsored by the Key Research and Development Plan of Shandong Province (2017GSF220010).