2.1 Study Area
As shown in Fig. 1, the Yuanbaoshan Open-pit Coal Mine is located 35 km east of Chifeng City, Inner Mongolia Autonomous Region. The surface water system near the Yuanbaoshan Open-pit Coal Mine has two rivers, Laoha River and Yingjin River, both of which are perennial rivers. Yingjin River is 194.6 km long, with a watershed area of 10,598 km2, a maximum annual flood discharge of 2,650 m3/s, a multi-year average discharge of 4.02×108 m3 and a multi-year average discharge of 12.8 m3/s. The Yingjin River is the largest tributary on the left bank of the Laoha River and joins the Laoha River at Dongbajia. The original course of the Yingjin River ran through the centre of the open-pit mine, dividing the open-pit mine into two parts: north-east and south-west. The Yingjin River was diverted before the mining of the Yuanbaoshan open-pit, and the Yingjin River passed through the north side of the Yuanbaoshan Open-pit Coal Mine. The Laoha River has a total length of 421.8 km, a watershed area of 33067 km2, a maximum annual flood flow of 9840 m3/s, a multi-year average discharge of 4.297×108 m3 and a multi-year average discharge of 13.6 m3/s. The Laoha River flows from southwest to northeast through the southern part of the river valley plain, 3 km from the Yuanbaoshan Open-pit Coal Mine.
Yuanbaoshan Open-pit Coal Mine has been built since 1990, and the mining coal seams are No.5 and No.6 coal. The annual production capacity reaches 10.00 Mt, forming a pit with a length of 3500 m, a width of 3000 m, and a depth of about 195 m. The north slope of the open-pit is the working slope and the south is the temporary working slope, with 6 ~ 7 single bucket excavator stripping steps.
As shown in Fig. 2, the stratigraphy of Yuanbaoshan Open-pit Coal Mine is mainly sandy soil, rounded gravel sand, pebble layer, weathered sandstone, fine sandstone, coal and mudstone. The grain size of round gravel is 50 ~ 150mm, and the permeability coefficient of the loose layer of the Fourth Series is 17 ~ 700m/d. Influenced by the Yingjin River and Laoha River, the main water source of the Yuanbaoshan Open-pit Coal Mine is the Fourth Series aquifer, and receives lateral recharge from the river, accounting for about 95% of the pit drainage.
In the early mining stage of Yuanbaoshan Open-pit Coal Mine, the amount of water drainage reached 500,000 m3/d, and in 2020, the amount of water drainage is still 184,000 m3/d. The existing groundwater pumping and dewatering control is mainly by pumping out a large amount of groundwater, thus lowering the groundwater level to a reasonable position, which leads to a sharp drop in the groundwater level around the open-pit mine, serious waste of groundwater resources and deterioration of the ecological environment. In order to solve the problems of water resources and ecological protection in the production of open-pit coal mine, a continuous lateral water cut-off curtain was constructed between the mine and the river. As shown in Fig. 3, a long, narrow, deep trench was excavated underground using large trench-forming machines such as double-wheel mills or hydraulic grabs on the loose layer of soil between the pit and the outer river. The deep trench was protected by a mud wall during excavation. When the excavation reaches the design depth, the bottom of the trench is cleaned and then concrete is poured into the excavated trench using the conduit method to form a continuous concrete wall. The constructed continuous slurry wall blocked the hydraulic connection between the loose layer and the trench.
The water cut-off curtain at the pit boundary was constructed along the first slope platform of the existing pit in the Yuanbaoshan Open-pit Coal Mine to intercept water outside the pit and minimize the amount of pit drainage. As shown in Fig. 4, the water cut-off curtain extends from the bedrock outcrop at the southern end of the pit to the Yingjin River, with a length of approximately 3.665 km, a width of 0.8 m and a depth of 22 ~ 70 m..
The water level in the loose layer on the outside of the curtain was raised by 8.18 ~ 9.12 m after the construction of the curtain was completed; the water level in the loose layer inside the curtain was lowered by 6.84 m compared to the original water level; the water level difference between the inside and outside of the curtain was as high as 15.02 ~ 15.96 m. The volume of water drainage was reduced by 70,000 m3/d compared to that before the construction of the curtain. This proves that the water cut-off curtain has a significant effect on water cut-off and effectively protects the groundwater resources in the open-pit area. At the same time, it is necessary to further analyse and study whether the increased water level on the outside of the curtain affects the stability and safety of the slope of the open-pit mine.
2.2 Research Methods
A slope stability calculation model was constructed to identify the influence of water cut-off curtain on slope stability and safety in the loose layer of Yuanbaoshan Open-pit Coal Mine. The model is constructed by using the limit equilibrium method and numerical simulation method to study the stability coefficients of the slope of the open-pit coal mine under the condition of water cut-off curtain.
Model Construction
Based on the engineering geological conditions of the Yuanbaoshan Open-pit Coal Mine, the slope profile shown in Fig. 5 is inscribed. The slope profile from top to bottom is composed of Quaternary sandy soil, sand and gravel layer, Neoproterozoic sandstone, Jurassic 5 coal, sandstone, 6 coal, mudstone and sandstone.
The previous parameters of the geotechnical body of Yuanbaoshan Open-pit Coal Mine were studied and analysed. Then, it was compared and analysed with the physical and mechanical indexes of the geotechnical body of open-pit coal mine around Yuanbaoshan Open-pit Coal Mine. Finally, the physical and mechanical parameters of sandy soil, sand and gravel layer, fine sandstone, coal, sandstone and mudstone layers of Yuanbaoshan Open-pit Coal Mine were derived as shown in Table 2.
Table 2
Physical and mechanical parameters of rock and soil mass of slope in Yuanbaoshan open-pit
Formation age | Rock and soil type | moisture content (%) | c(kpa) | φ(°) | E(MPa) | µ | γ(kN/m3) |
Natural | Saturated | Natural | Saturated | Natural | Saturated | Natural | Saturated | Natural | Saturated |
Quaternary system | sandy soil | | 0 | 0 | 37.6 | 23.3 | 10.2 | 11.4 | 0.32 | 0.35 | 22.2 | 24.2 |
Sand and gravel layer | | 8.9–27 | 2.7–8.1 | 31.9–34.8 | 26.1 | 18.8–24.4 | 15.6–20.3 | 0.25 | 0.28 | 18.4 | 24.8 |
Neoproterozoic | fine sandstone | | 120.0 | 33.6 | 25.0 | 19.8 | 550 | 407 | 0.30 | 0.32 | 20.0 | 22.8 |
Jurassic systerm | Coal | 20.1 | 60.0 | 16.2 | 31.0 | 20.2 | 600 | 402 | 0.29 | 0.33 | 13.1 | 17.7 |
fine sandstone | 13 | 173.0 | 60.6 | 27.0 | 21.6 | 880 | 695 | 0.25 | 0.27 | 21.1 | 23.6 |
mudstone | 11.5 | 109.0 | 10.2 | 24.7 | 19.2 | 500 | 380 | 0.35 | 0.39 | 21.1 | 25.7 |
weak layers | 16.5 | 2.2 | 0.6 | 10.9 | 2.3 | 20 | 12 | 0.45 | 0.48 | 18.0 | 21.2 |
The slope profile of the Yuanbaoshan Open-pit Coal Mine, shown in Fig. 5, is simplified into a slope stability calculation model, shown in Fig. 6, with a first-level platform width of 16m and a slope angle of 25°.
Limit equilibrium method
The geotechnical body of the slope of the Yuanbaoshan Open-pit Coal Mine is mainly the Quaternary sand and gravel layer and soft rock, which has the characteristics of bulk structure. The Morgenstern-Price method based on the limit equilibrium theory is used to eliminate the errors in the calculation method by considering all equilibrium conditions and boundary conditions at the same time. The stability of the slope is calculated and analysed using Lizheng Geotechnical Calculation Software.
As shown in Fig. 6, in the systematic calculation and analysis of the slope stability coefficient of Yuanbaoshan Open-pit Coal Mine after the implementation of the water cut-off curtain, in addition to the thickness of the water cut-off curtain itself (0.8m), the difference between the water level inside and outside the curtain and the position of the curtain wall were also considered. Three specific working conditions were taken into account.
(1) Condition 1: Before the construction of the water curtain.
(2) Condition 2: Different water level elevation outside the curtain, ①the water level outside the curtain is raised to 448m, ②the water level outside the curtain is raised to 452m, ③the water level outside the curtain is raised to 462m. The water level line inside the wall is at the bottom of the quaternary system.
(3) Condition 3: the water level outside the curtain is restored + different curtain positions, ①the curtain is located 10m from the foot of the first platform slope; ②the curtain is located 1m from the foot of the first platform slope; ③the curtain is located 15m from the foot of the first platform slope.
Numerical modelling
Midas GTS geotechnical and tunnelling simulation and analysis software was used to create the numerical model of the open-pit slope shown in Fig. 7. According to the requirements of calculation accuracy, the grid size of the whole geometric model of the slope is divided by 1 m and the curtain wall is divided by 0.4 m, and the quadrilateral cell grid pattern is adopted, with a total of 38,203 cells.
The model calculation process adopts the Mohr-Coulomb strength criterion, the maximum number of iterations is set to 15 and the displacement convergence criterion is adopted. The strength reduction method is used to calculate the slope damage criterion and the reduction factor is the slope stability factor. The strength reduction method is used to gradually reduce the cohesion c and internal friction angle φ of the slope geotechnical body during the simulation until the numerical solution does not converge or a plastic penetration region is formed.