Constructing a water-blocking layer in the dump sites of an open-pit coal mine is of great significance to improve the water retention capacity of the reclamation area. The permeability law of the water-blocking layer is costly to be analyzed and researched by means of field tests. In this study, the soil-rock mixture samples similar to the water-blocking layer were prepared, and the rock block proportion (RBP) and hydraulic pressure were adopted as test variables to conduct a permeability test, which provided theoretical support for on-site construction. The results show that when the hydraulic pressure is less than the confining pressure, the permeability increases at a steady rate as the rock block proportion increases. When the hydraulic pressure is close to or equal to the confining pressure, penetrating cracks are easily formed between the soil-rock interfaces of the sample with a higher rock block proportion, resulting in a rapid increase in permeability. With the increase of hydraulic pressure, the migration of the internal fine particles in the sample with a rock block proportion of 40% or less leads to the partial cracks closure, which gradually reduces the permeability. The internal structure of the sample with a rock block proportion more than 40% experiences a process in which the permeability decreases with the crack closure to a significant increase due to the penetrating crack formation. In summary, the soil-rock mixture with about 30% rock block proportion is characterized by suitable permeability and stability, which guarantees the construction cost on-site at the same time. In addition, increasing the rolling times of the truck can increase the compaction of the water-blocking layer and reduce the permeability. The roughness and gradation of the rock blocks can improve the permeability and stability of the water-blocking layer.
The new open-pit coal mine reclamation process in China divides the topsoil adopted for ecological restoration from top to bottom into humus, aquifers, and water-blocking layer, as shown in Figure
Distribution of reclamation strata in the dump sites.
Water-blocking layer.
Soil-rock mixture exists in many natural geological and artificial civil engineering projects [
Zhou et al. [
The construction of the water-blocking layer of open-pit coal mines in China has only been proposed in recent years, so there is currently little research on the water-blocking layer of open-pit coal mine dump sites. This study simulated the environment of the water-blocking layer to prepare soil-rock mixture samples and tested the effects of rock block proportion and hydraulic pressure on permeability. By comparing with other scholars’ researches, the analysis carried out from materials and microstructure draws the conclusions which are beneficial to ensure a balance between permeability and construction difficulty.
In the context of water-blocking layer of open-pit coal mine dump site, clay, a by-product produced during the coal mining process, contains rocks of different sizes and shapes mixed in it, which is impossible to avoid. In order to simulate the similar engineering situation, the clay (0∼1 mm) collected on-site and sandstone were adopted to prepare soil-rock mixture samples. The materials are dried with a drying box before sample preparation. Sandstone shown in Figure
Sieved soil and sandstone: (a) 0∼1 mm; (b) 1∼2 mm; (c) 2∼5 mm; (d) 5∼8 mm; (e) 8∼10 mm.
The test set contains four kinds of samples with rock block proportions (weight proportions) of 30%, 40%, 50%, and 60% (the samples are called A30, A40, A50, and A60 in this paper). Considering that the compaction of the water-blocking layer is carried out by repeated rollings of the truck, the compaction method of the samples was chosen to adopt a consolidometer to compact all the samples under the same conditions. According to the results of Wan-Jun [
Particle gradation of soil-rock mixture.
Sample preparation process with consolidometer. (a) Filling. (b) Compacting. (c) Removing the sample.
For the permeability test equipment, the British GDS triaxial test machine shown in Figure
GDS triaxial test machine.
GDS structural diagram. (1) Permeable stone with filter paper; (2) cell pressure chamber; (3) back pressure; (4) base pressure; (5) axial force.
Permeability test process is conducted as follows: (1) Install the sample wrapped with rubber film and permeable stone into the testing machine to ensure the tightness of the sample. (2) Fill the cell pressure chamber with water, pay attention to the change in permeability, ensuring the sample is still sealed. (3) Apply 50 N axial pressure to ensure close contact between the sample and the machine. (4) Apply a 300 kPa confining pressure, and then conduct the permeability test.
Each sample with different rock block proportions was tested for permeability at 100 kPa, 150 kPa, 200 kPa, 250 kPa, and 300 kPa hydraulic pressures. The hydraulic pressure
Rock block proportion and hydraulic pressure are the two common variables to study the permeability of soil-rock mixtures [ Rock block proportion: the clay in the water-blocking layer is excavated by using an excavator in the open-pit mining area without blasting. Therefore, the rock block proportion of the clay obtained by excavation can be reduced through a more detailed excavation process, such as using a small bucket capacity excavator or a frontloader. Hydraulic pressure: the purpose of the water-blocking layer construction is to ensure that the water required for plant growth is not easy to lose. Therefore, hydraulic erosion is one of the main sources of damage. Because the compaction degree and uniformity in the on-site field must be lower than those in the laboratory samples under the same compaction strength, a relatively large hydraulic pressure was selected to observe the change of permeability, which can predict the damage of the water-blocking layer on-site.
From the test results reflected in Figure The numerical simulation of Chen [ From Figures
Variation curve of rock block proportion and permeability. (a)
To sum up, first, during the construction of the water-blocking layer, the rolling times of the truck should be increased to improve the compaction. Second, no more than 30% rock block proportion on-site can better avoid the increase in permeability caused by the increase in hydraulic pressure. At the same time, a certain rock block proportion for engineering can effectively reduce the construction costs.
As shown in Figure Combined with the analysis in Section One of the reasons that the soil-rock interface is the weak face of the soil-rock mixture is that the fine particles and the surface of the rock blocks do not fit well due to the fractal shape, resulting in larger pores. Wang’s study [
Curve of hydraulic pressure and permeability of soil-rock mixtures: (a) RBP = 30%; (b) RBP = 40%; (c) RBP = 50%; (d) RBP = 60%; (e) summary.
Schematic diagram of internal structure failure (RBP = 50%): (a) initial; (b) compacted; (c) penetrating crack formation.
Rock blocks comparison: (a) this study; (b) Wang’s study.
Comparison of rock block parameters.
Rock block | Size (mm) | Type | Surface |
---|---|---|---|
Current study | 1–10 | Sandstone | Rough |
Wang’s study | 2–5 | Marble | Smooth |
As the most vulnerable part of the internal structure of the soil-rock mixture, the cracks affect the resistance to hydraulic pressure. According to Wang’s research [
At the same time, the relatively rough surface of the rock blocks increases the friction between the materials and reduces the effect of hydraulic pressure, making the soil-rock interface difficult to be damaged and finally improving the stability of the internal structure.
Similar to test conditions, the water-blocking layer is laid above the dump site which acts like a permeable stone hindering the migration of fine-grained clay. The following conclusions can be made from the analysis: (1) Hydraulic pressure changes the internal structure through the migration of fine particles in the soil-rock mixture. The 30% rock block proportion can better prevent the formation of penetrating crack with the increase of hydraulic pressure. (2) The roughness and gradation of rock blocks affect the permeability and stability of the water-blocking layer.
In this study, the soil-rock mixture samples with conditions similar to the water-blocking layer of an open-pit coal mine were prepared to conduct permeability tests to study the permeability under different rock block proportions and hydraulic pressure. The following conclusions are drawn from this study: Under the method of compaction by the truck, the rock block proportions will reduce the degree of compaction and increase the permeability. Therefore, during the construction of the water-blocking layer, the number of rolling times of the truck should be increased as much as possible to improve the compaction. With the increase of rock block proportions, the shortening of the distance between rock blocks makes it easy to form penetrating cracks at the soil-rock interface, resulting in a great increase in permeability. The migration of fine particles under hydraulic pressure changes the internal structure of the soil-rock mixture. With the increase of hydraulic pressure, the internal structure of the sample with a rock block proportion more than 40% experienced a process in which the permeability decreases with the crack closure to a significant increase due to the penetrating crack formation. The soil-rock mixture with no more than 30% rock block proportion is characterized by suitable permeability and stability. At the same time, a certain rock block proportion for engineering can effectively reduce the construction costs. The roughness of the rock blocks can increase the friction between the soil and the rock and improve the stability of the soil-rock mixture. At the same time, reasonable gradation can make the soil-rock mixture form a more dense internal structure and reduce its permeability.
Permeability is an important index for evaluating the performance of water-blocking layer. In addition to the rock block proportions and hydraulic pressure, the shear failure caused by the uneven settlement of the dump site makes the shear resistance an important factor affecting the permeability. Rock block helps us to improve the shear resistance of the soil-rock mixture. Therefore, a comprehensive consideration of the effects of rock block grading and rock block proportions on the shear strength and permeability can provide a more comprehensive theoretical guidance for the stability of the water-blocking layer in an open-pit coal mine, and it is also the direction for further research.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this article.
This study was supported by the Independent Research Project of State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (SKLCRSM18X001).