Introduction

Mine grouting curtain is the process of forming a curtain wall of a certain size and range through grouting at the groundwater runoff channel, artificially changing the hydrogeological conditions and water inlet boundary to intercept or block groundwater, thereby achieving the dual purpose of safe mining of mineral resources and controlling the source of groundwater in aquifers1,2. In China, since the construction of the first mining area diversion curtain at Qingshanquan Coal Mine in the 1960s, the grouting curtain diversion technology has been widely used for water resource protection and mining resource recovery in mining areas after years of development3. It has been applied in mines such as Dahongshan Iron Mine, Shuikoushan Lead Zinc Mine, Dongguashan Copper Mine, Wuzhuang Iron Mine, Laixin Iron Mine, Jigang Zhangmatun Iron Mine, and Zhongguan Iron Mine, achieving good economic and social benefits4.

The grouting curtain wall is hidden underground and has typical characteristics of concealed engineering, which leads to significant blindness in the selection of the ___location, wall size, grouting materials, grouting parameters, and grouting effect inspection of the curtain wall construction. Currently, the interception rate of many mine curtain walls is generally low. Many domestic scholars have also conducted in-depth research and exploration. Chen et al.5 assessed the acoustic velocity increase of rock masses as a result of consolidation grouting in different geological conditions, such as fault sites, weathered areas and excavation-induced damage zones, and established an empirical relationship between the acoustic velocity of rock masses before and after consolidation grouting. In this reference6, a computational fluid dynamics (CFD) simulation approach of dam foundation grouting based on a 3D fracture network model is proposed. Fan et al.7 proposes a hybrid fuzzy comprehensive evaluation method to assess curtain grouting efficiency by considering both the permeability and tightness of a grout curtain. A three-level hierarchical model of curtain grouting efficiency assessment is established that considers permeability and tightness as criteria and Permeability (LU), Rock Quality Designation (RQD) and the Fracture Filled Rate (FFR) as alternatives. In this reference8, To improve the grouting reinforcement effect of fractured rock mass, the theory of self-stress grouting reinforcement was put forward for the first time, and the self-stress grouting material was developed. Jones et al.9 back-analyse and verify the ground treatment by the SPI from water pressure tests conducted in primary production grout boreholes, at the De Hoop Dam in South Africa. Wang et al.10 analyzes and studies the selection of single row hole grouting curtain parameters and grouting construction results for the Zhongguan Iron Mine grouting curtain project using methods such as unit permeability change curve, dispersion analysis, inspection hole inspection result analysis, and observation hole water level observation data; Dong et al.11 analyzed the grouting curtain techniques used in Zhangjiamao well field in northern Shaanxi, including dual position bidirectional drainage grouting, full section zoning grouting of burnt rock, and real-time inspection of anti-seepage and interception effects; Han12 proposes a comprehensive analysis method for weak areas of curtains based on hydrogeological analysis of curtain areas, analysis of curtain drilling data, water release tests, transient electromagnetic detection, and connectivity tests; Literature13,14,15 evaluates the construction quality of curtain walls using methods such as grouting information analysis and geophysical detection after the completion of construction; Lu16 analyzed the characteristics, classification, influencing factors, and current application status of grouting curtain technology in large water mines. It also introduced successful cases of grouting curtain in recent years, as well as grouting curtain monitoring and inspection techniques; Han17 analyzed the successful implementation of the “fishbone type” drilling modified clay slurry grouting curtain test project in a deep buried mine in Yunnan province.

The above-mentioned research mainly focuses on curtain walls with shallow burial depth or small scale, lacking in-depth analysis and characterization of the microscopic characteristics of the injected strata. For high water pressure strata with large burial depth, the small pores and cracks in the rock layers play an important role in the instability and water inrush of the rock layers18,19. In the field of rock mechanics, scholars such as Feng Zijun20, Kang Zhiqin21, Yang Renshu22, and Gong Weili23 have used micro-CT scanning technology to study the microstructure characteristics and geometric parameters of coal rock fractures. Therefore, using micro-CT scanning technology to analyze the development characteristics, distribution characteristics, and geometric parameters of rock pores and fractures in the injection layer can provide a basis for selecting injection materials and designing injection parameters from a microscopic perspective. In addition, traditional ground vertical drilling or underground inclined drilling and grouting processes are used for existing curtain walls, with large drilling depth, high cost, low exposure of pore and crack rates, and small effective grouting sections24,25. With the continuous increase in mining depth and intensity, the scale of curtain wall construction has increased, and the internal and external water pressure difference borne by the wall has increased. The traditional drilling construction cost caused by long-distance curtains has also increased. However, how to reduce drilling footage, improve the filling rate of fine pores and cracks in grouting curtain, and ensure the minimum safe thickness of curtain walls are the key issues that need to be urgently solved for large curtain walls.

The author used a combination of vertical drilling and directional drilling to construct a large-scale curtain interception project for the first time in a certain mine. The micro-CT scanning and hydrogeological structural system analysis methods were used to study the macro and micro characteristics of the injected rock mass, and to analyze the injection conditions of the curtain wall construction strata; By using theoretical analysis, numerical calculation, and data analysis methods, combined with the conditions of the injected formation, a systematic study was conducted on the construction position of the curtain wall, the minimum safe thickness of the wall, the grouting pressure, the layout of grouting boreholes combining vertical and horizontal directional drilling, the grouting materials and their applicability. The grouting effect was also verified by drilling core and water discharge tests.

Project overview

Zhuxianzhuang coal mining is located in the northern section of the Sudong syncline, surrounded by mostly Carboniferous and Ordovician limestone (Fig. 1). The northeastern part of the mine field is distributed with Jurassic conglomerate aquifer, which is the fifth aquifer (group) of the mining area (commonly known as the “five aquifer”). The gravel in this layer is mainly composed of limestone and a small amount of sandstone and metamorphic rock, with calcium cementation as the main component, followed by mudstone and sandy cementation, karst development, and is the fifth aquifer of the mine (commonly known as the “five aquifer”). The five and four strata, as well as the coal bearing strata, are in unconformable contact and belong to the foothill alluvial sedimentary facies. The gravel composition is mainly composed of limestone fragments, with a diameter of 0.2–7 cm and poor sorting. The cement is purple red mudstone. And the northern part of the mine field is affected by the Taqiao fault, making the lower part of the “Wuhan” an isolated block, with a coverage area of about 9 km2, of which 2.8 km2 is within the mine field (as shown in Fig. 2).

Fig. 1
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Study the administrative position where the coal mine is located.

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Fig. 2
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Schematic diagram of the “five-inclusive” stratigraphic relationship and hydraulic linkage.

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The “Five Containing” strata are extensively overlaid on the coal bearing strata, Taiyuan formation limestone aquifer and Ordovician aquifers in an angular unconformable contact relationship, with karst development and strong hydraulic connections with Taiyuan formation limestone aquifer, Ordovician and other aquifers (as shown in Fig. 3). According to statistics, the total reserves of coal resources in the 8th coal seam under the “Fifth Aquifer” are about 18 million tons, and the proven recoverable reserves are about 10 million tons. During the coal seam mining process, the underground water in the “Fifth Aquifer” and the underground water in the Taiyuan formation limestone aquifer and Ordovician limestone aquifer that participate in the supply can enter the mine through various water channels, causing water damage and leading to the stagnation of a large amount of coal resources in the area. The gray and ash aquifers in this area are important sources of domestic and industrial water, and the use of conventional aquifer drainage and depressurization for coal mining in this area will inevitably result in a significant waste of water resources. Therefore, considering the difficulty and cost of construction, water resource protection, and safe mining of coal resources, the design adopts the method of “five aquifer” grouting curtain and local dewatering mining to ensure the maximum protection of aquifer water resources while ensuring the safe mining of coal resources. So as to control the formation of mine water disasters and the waste of groundwater resources in aquifers.

Fig. 3
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Structural relationship profile of each aquifer.

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Microscopic characteristics of injected strata

Principle and equipment of microscopic CT technology

The imaging principle of CT technology26 is that when X-rays pass through a certain thickness of material, their intensity will experience a certain degree of attenuation (as shown in Fig. 4), and the attenuation law can be expressed as:

$$I = I_{0} e^{ - \mu L}$$
(1)
Fig. 4
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Schematic diagram of nanoVoxel-4000 CT.

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Among them, I0 is the incident X-ray intensity, I is the outgoing X-ray intensity, L is the material thickness, and μ is the linear attenuation coefficient of the material, which is related to factors such as material density and relative atomic mass of constituent elements. After X-rays penetrate the rock core, the detector receives the X-rays that have passed through the material, converts them into visible light, converts them into electrical signals through photoelectric conversion, and then converts them into digital signals through an analog/digital converter before inputting them into a computer for processing. This experiment uses the nanoVoxel-4000 series X-ray 3D microscope, with a maximum accuracy of 500 nm. The dual detector design scheme can significantly improve the magnification and accuracy of imaging.

Test materials and methods

Scanning experiments were conducted on grouted rock cores using micro-CT technology. The test samples were taken from the rock layers in the grouting area, with a lithology of breccia, a height of 16 cm, a diameter of 5.3 cm, and a CT scanning resolution of about 30 μm.

Experimental results

Rock porosity and fracture rate

By performing micro-CT scanning on the rock sample, its material structure data is obtained. Visualization software is used to reconstruct and analyze the scanning data, obtaining X, Y, and Z direction slice images. The gray values of different substances such as pores, fractures, and minerals in the slice images are statistically analyzed. By setting the gray value threshold of the pore and fracture properties of the sample, the slice images are screened for pores and fractures, thereby obtaining the proportion of fractures and pores in a single slice image and the entire tested rock sample. Figure 4 shows the three-dimensional reconstruction and X, Y, Z direction slice images of the breccia sample.

By analyzing the grayscale values of breccia slice images, the development characteristics of pores, cracks, and minerals on CT scan slices of rock masses can be characterized. As shown in Fig. 5, larger scale cracks and pores in the rock mass can be clearly observed, but smaller cracks in the grayscale image are difficult to directly observe. Therefore, a reasonable method needs to be selected for segmentation. Due to the significant difference in grayscale values between the pores and cracks in the sample and the rock mass, threshold segmentation method is used for processing. Extract breccia slices and set segmentation thresholds based on grayscale differences to extract pores and cracks from the sample (as shown in Fig. 6). After extracting the fractured pores, it was found that the volume of pores and fractures accounted for 0.62% of the total volume of a single cross-sectional sample.

Fig. 5
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The cutaway of breccia: (a) XY direction slice of breccia, (b) XZ direction slice of breccia, (c) YZ direction slice of breccia.

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Fig. 6
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Breccia slice native crack rendering.

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By using visualization software, the number of pixels occupied by each section crack can be calculated, and the number of pixels in each slice sample can also be calculated. The ratio of the two is the crack rate of the slice, which can be used to calculate the crack rate of each layer in the Z direction and observe the characteristics of the change in crack rate. As shown in Fig. 7, the fracture rate mainly varies between 0.25% and 2.8%. The fracture rate of most slices is mainly distributed between 0.25% and 0.5%, accounting for about 73.8%, while the rest is concentrated between 0.5% and 2.8%, accounting for about 26.2%.

Fig. 7
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Breccia rock layer-by-layer fracture rate curve.

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Geometric parameters of rock fractures and pores

Calculate the sphericity of the extracted pores and fractures, generally defining those with sphericity less than 0.3 as fractures and those with sphericity greater than 0.3 as pores. Use this definition to screen the pores and fractures, as shown in Fig. 8, where the fractures are red and the pores are blue, and the fracture volume accounts for 0.24% of the sample volume, and the pore volume accounts for 0.38% of the sample volume.

Fig. 8
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Three-dimensional display of breccia pores and fissures: (a) 3D display of pores and fractures, (b) 3D display of pores, (c) 3D display of fractures.

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To further obtain the geometric parameters of pores and fractures, the extracted breccia pores and fractures are further labeled and screened. Due to the irregularity of pores, their size is described using equivalent diameter, which is equivalent to a regular sphere27. The calculation formula for equivalent diameter (Deq) is:

$$D_{eq} = \sqrt[3]{{6 \times {\text{V/}}\pi }}$$
(2)

The volume of a single pore (V) was calculated using image post-processing software based on the number of pixels and resolution (as shown in Fig. 9). The three-dimensional display of the conglomerate was sieved to obtain the proportion of pores in different equivalent diameter ranges, as shown in Table 1. Through statistical data on the number of equivalent diameters of pores, it can be seen that pores with equivalent diameters ≤ 200 μm are the main ones; However, in terms of volume fraction, the equivalent diameter is mainly between 200 μm < Deq ≤ 500 μm, reaching as high as 32.27%.

Fig. 9
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Three-dimensional display of breccia pores and fissures: (a) 3D display of crack markers, (b) 3D display of pore markers.

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Table 1 Number of pores and volume fraction of different equivalent diameter intervals of breccia.
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Extract and screen rock fractures, perform quantitative statistics on marked fractures, and calculate the volume, area, length, and width of fractures. Volume and area are calculated based on the number of pixels and resolution, while length and width are the maximum and minimum values of the Fret diameter, respectively. Fret diameter28 is a commonly used parameter to describe the size of irregular particles, defined as the diameter in any direction passing through the center of the particle. Cracks can be classified into closed cracks (< 0.2 mm), slightly open cracks (0.2–1.0 mm), medium open cracks (1.0–5.0 mm), and wide open cracks (5.0–10.0 mm) according to their width; Ultra wide tensile fractures (> 10 mm)29, while the largest proportion of volume in breccia fractures is medium tensile fractures, with an average of 2009.61 μm, followed by wide tensile fractures, with an average of 7601.96 μm.

Seepage simulation

Considering the influence of gravity, numerical simulations of low-pressure water seepage under unidirectional and multi-directional pore structures in micro rock masses were carried out. The evolution laws of important parameters such as pressure, velocity, and mass flow rate of rock mass pore seepage at the micro scale were obtained, providing a basis for the selection of grouting parameters and numerical analysis. Using the seepage simulation module in AVIZO FIRE software, based on Darcy’s law, simulate and calculate the flow of fluid in the sample. By setting boundary conditions, input and output pressures, and fluid viscosity, simulate and calculate the permeability of the sample. The specific calculation principle is as follows30:

$$Q = K\Delta PA/(\mu L)$$
(3)

Among them, Q: The flow rate of fluid through rocks per unit time, m3/s; A: Liquid passing through the cross-sectional area of rocks, m2; μ: The viscosity of liquids, Pa·s; L: The length of the rock, m; ΔP: The pressure difference before and after the liquid passes through the rock, Pa; K: Absolute penetration rate, m2.

Taking breccia as an example, set the input pressure to 0.13 MPa, output pressure to 0.1 MPa, and viscosity to 0.001 Pa·s. Due to the fact that the entire sample crack is not connected, the connectivity of the locally cut portion of the original crack was determined to be connected in the Z direction. The AVIZO FIRE software was used to simulate and analyze its permeability. The left and right boundaries of the model are no flow boundaries, and the upper and lower boundaries are constant head boundaries. And the permeability was measured to be 4.071 × 10−12 m2 (Fig. 10). Figure 8 is a schematic diagram of Z-direction seepage simulation in breccia rock.

Fig. 10
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Seepage simulation of breccia in the Z-axis direction.

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Macroscopic characteristics of the injected strata

Analyzing the hydrogeological structural characteristics and their supplementary, radial, and drainage features of the injected strata is the basis for designing curtain walls and the key to ensuring that curtain walls can effectively intercept groundwater. The author analyzes the development characteristics and hydrogeological conditions of the “Five Containing” strata in the construction area of curtain walls from a macro perspective, providing support and basis for curtain wall design.

Characteristics of thickness variation in the “Fifth Aquifer” of the injected strata

The variation in thickness of the “Fifth Aquifer” in the mining area is mainly influenced by the morphology of the ancient bedrock surface and later erosion. According to the drilling data of this mine, the overall thickness of the mining area is thick in the middle and thin around, gradually thinning and tapering towards the deeper and eastern parts. The erosion surface is roughly parallel to the 8th coal seam, leaning northeast with an inclination angle of 15–25°. The thickness of the conglomerate in the mining area is 0–100.6 m, with an average thickness of 55 m, and the floor elevation is −212.85–345.39 m, with an average of − 242.96 m. According to the contour map of the “Fifth Aquifer” strata (as shown in Fig. 11), from hole 84–19 through hole I-11 to the west of the line connecting holes 2–4, the overlying sandstone of the “Fifth Aquifer” is eroded to the north, forming direct contact between the “Fifth Aquifer” and the “Four Inclusions”. In this zone, the thickness of the “Fifth Aquifer” is 0–60 m, which is the most closely connected zone between the “Four Inclusions” and the “Fifth Aquifer”; From the cross-section, the thickness of the “Fifth Aquifer” increases from south to north.

Fig. 11
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Thickness contour map of “five bearing” strata.

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Characteristics of karst development in the “Fifth Aquifer” of injected strata

In the “Five Containing” conglomerate aquifer, the gravel is mainly composed of limestone, cemented with calcium and mud, and karst is relatively developed but uneven. According to the statistical analysis of 32 drilling data in the “Fifth Aquifer” coverage area, the types of karst development include caves, dissolved pores, and fractures. The maximum diameter of the cave exposed by drilling holes (I ~ I3) is 16 m, and caves with a diameter of 0.2–1.0 m are generally the most common. The degree of karst development is closely related to lithology, fault structures, and burial depth. From a lithological perspective, the middle and lower parts of the strata are mainly composed of gray conglomerate, with limestone as the main component and a corresponding decrease in cement. The limestone conglomerate has a large size and increased density, with karst development; The second is the direct contact zone between the “four inclusions” and “Fifth Aquifer”, which forms weathering fissures and caves due to weathering and dissolution; At the same time, there is rock fragmentation near the fault structure, such as the development of karst on both sides of the F25 fault, resulting in severe drilling leakage near the fault layer. According to drilling data statistics, the development rate of shallow karst below − 350 m is 8.82–13.12%, while the development rate of deep karst below − 350 m is significantly reduced, with a karst development rate of only 0.83–4.5% (see Table 2); Table 3 shows the statistical table of karst visibility rate in boreholes, which gradually decreases with increasing depth; According to the statistics of drilled rock caves (see Table 4), at an elevation of − 350 m, the size of shallow karst development is relatively large, ranging from 1 to 16 m, the depth of karst development at an elevation of − 350 m is below 1m, which reflects the vertical heterogeneity of karst development from a lateral perspective. According to the statistics of water leakage data in the “Fifth Aquifer” conglomerate layer of boreholes (see Table 5), most of the leakage points in boreholes are above − 350 m, indicating that the permeability of the “Fifth Aquifer” formation above − 350 m is relatively better than that below − 350 m.

Table 2 Statistical table of karst rate of “Fifth Aquifer” stratigraphic line.
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Table 3 The statistical table of karst drilling rate in “Fifth Aquifer” formation is presented.
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Table 4 Statistical table of hole diameter in “five contained” strata.
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Table 5 Statistical table for leakage of “Five Containing” Conglomerate.
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The migration law of grouting slurry in the curtain of the aquifer on the top plate

Numerical simulation technology has flexibility and intuitiveness, and can continuously display the migration process of slurry, which can be used to reveal the flow law of slurry in conglomerate pores and fractures. The grouting materials used in on-site grouting curtain projects mostly use cement slurry, usually a cement slurry with a water cement ratio of 1:1. In the numerical simulation of the article, a slurry with a water cement ratio of 1:1 was selected to study the transport law of the slurry. The physical properties of the slurry with a water cement ratio of 1:1 were obtained through indoor experimental testing, with a viscosity of 0.0972 Pa·s, a density of 1460 kg/m3, and a solidification rate of 75.9%.

Control equation for slurry transport

Grouting curtain is essentially the process of displacing groundwater in the fissures of conglomerate pores with grout, and the flow of grout in the fissures belongs to the category of two-phase flow. From the continuity equation and motion equation of slurry and groundwater31, the governing equation for the two-phase flow of slurry and groundwater can be obtained as follows:

$$\frac{\partial (\varphi \rho )}{{\partial t}} + \nabla \cdot \left( { - \rho \frac{k}{\mu }\nabla p} \right) = 0$$
(4)
$$\rho = s_{w} \rho_{w} + s_{s} \rho_{s}$$
(5)
$$\frac{1}{\mu } = s_{w} \frac{{k_{wr} }}{{\mu_{w} }} + s_{s} \frac{{k_{sr} }}{{\mu_{s} }}$$
(6)
$$k_{sr} = \frac{{k_{s} }}{k},\;k_{wr} = \frac{{k_{w} }}{k}$$
(7)
$$s = s_{w} + s_{s} = 1$$
(8)

In the formula, is the porosity of the grouted rock mass The densities of the slurry and groundwater are respectively The volume fraction of slurry and groundwater is 1, as they completely fill the entire pore and fissure The relative permeability of slurry and groundwater depends on the volume fraction function The effective permeability or relative permeability of slurry and groundwater; The absolute permeability of the medium The dynamic viscosity of the slurry and groundwater respectively. The density and dynamic viscosity values of the slurry are obtained from slurry performance testing experiments.

The equation for crack seepage is:

$$\nabla [d_{f} \rho ( - \frac{k}{\mu }\nabla p)] = 0$$
(9)

In the formula, \(d_{f}\) is the width of the crack.

Numerical model and parameter selection

According to the distribution characteristics of conglomerate pores and fractures in CT scan results, it can be concluded that the internal structure of conglomerate is mainly dominated by pore structure, supplemented by fracture structure. Therefore, the rock mass in the grouting layer can be simplified into two types: porous media and pore single fracture media. In this paper, COMSOL Multiphysics numerical software is used to establish pore media models and pore single fracture media models respectively, and study the slurry transport laws of the two different media models.

Pore medium model

Based on the above analysis, the conglomerate aquifer in the grouting layer is considered as the pore medium, and a 30 m × 30 m numerical model of the pore medium is established. One grouting hole is arranged, and the grouting hole opening is set as a constant pressure boundary with a diameter of 152 mm. The upper and lower boundaries of the model are non flowing boundaries, and the left and right boundaries are constant head boundaries. The mesh division and boundary conditions of the numerical model are shown in Fig. 12.

Fig. 12
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Numerical calculation model of porous media.

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Pore single fracture medium model

Consider the conglomerate aquifer in the grouting layer as a pore single fracture medium, establish a numerical model of pore single fracture medium with a size of 40 m × 30 m, arrange one grouting hole and one single fracture, with a length of 20 m and a fracture opening of 5 mm. The grouting hole opening is set as a constant pressure boundary, and the grouting hole diameter is 152 mm. The upper and lower boundaries of the model are non flowing boundaries, and the left and right boundaries are constant head boundaries. The mesh division and boundary conditions of the numerical model are shown in Fig. 13.

Fig. 13
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Numerical calculation model of porous single fracture medium.

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Calculate parameters and operating conditions

According to the on-site data, the characteristic hydrological parameters of the aquifer in the grouting layer are q = 0.326–6.84 m/d. The average value of the hydrological parameter q is taken to define the permeability of the porous medium as k = 4.071 × 10−12 m2. In the physical and mechanical property testing experiment of conglomerate, the porosity was found to be 18.5%, so the porosity of the porous medium was taken as 15% in the numerical model. According to the range of on-site grouting pressure, the grouting pressure p in the numerical model is taken as 4, 5, and 6 MPa, respectively. Based on the burial depth of the conglomerate aquifer in the grouting layer, the static water pressure p0 of the model is taken as 2.0 and 3.0 MPa. The numerical model calculation parameters and operating conditions are shown in Tables 6 and 7.

Table 6 Calculating parameters of numerical model.
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Table 7 Numerical model calculation conditions.
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According to the above calculation parameter table and calculation condition table, study the migration law of slurry in two media models separately.

Numerical simulation results and analysis

The numerical simulation results use volume fraction to characterize the transport law of slurry and groundwater in the medium. According to the calculation condition table, the transport characteristics of slurry under six different conditions were simulated in both media models.

Numerical simulation results of porous media

The migration patterns of slurry under different working conditions are similar. In this paper, the migration patterns of slurry under injection pressure of 5 MPa and static water pressure of 2 MPa are selected as examples to analyze the migration patterns of slurry in porous media. t represents injection time, and r represents the diffusion radius of slurry in porous media, as shown in Fig. 14.

Fig. 14
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Grouting pressure 5 MPa grout migration pattern.

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From Fig. 14, it can be seen that the migration pattern of the slurry in the porous medium is clearly axisymmetric, with the red part representing the slurry and the blue part representing groundwater. In the initial stage of grouting, the slurry diffuses in a circular shape around the pore medium from the grouting hole. As the grouting time increases, the diffusion distance of the slurry gradually increases. When the grouting reaches a certain level, the diffusion of the slurry gradually stabilizes. Under this condition, the final stable diffusion radius of the slurry is 10.25 m, and the diffusion distance is 20.5 m. If the grouting time continues to increase at this time, the diffusion range of the slurry is limited, the difficulty of grouting increases, and the grouting effect is not good.

To further analyze the influence of different grouting pressures and static water pressures on the diffusion range of the slurry, the calculation results for different working conditions in the article were used to establish the curves of the diffusion radius of the slurry under different grouting pressures as a function of grouting time, as shown in Fig. 15.

Fig. 15
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Grout diffusion radius-grouting time curve: (a) Grouting pressure 4 MPa, (b) grouting pressure 5 MPa, (c) grouting pressure 6 MPa.

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From Fig. 15, it can be seen that the variation curves of the diffusion radius of the grout with grouting time under three different grouting pressures are similar. As the grouting time increases, the diffusion radius of the slurry gradually increases, but the growth rate gradually slows down and eventually stabilizes. According to the statistical analysis of the numerical calculation results, the diffusion radius of the slurry after stable diffusion under different operating conditions is 10.12–10.44 m. The higher the grouting pressure, the shorter the time required for the diffusion of the slurry to reach stability. This is because the solidification characteristics of the slurry occur earlier under higher grouting pressure. In addition, the greater the hydrostatic pressure, the longer it takes for the slurry to reach a stable diffusion state. This is because the greater the hydrostatic pressure, the stronger its ability to hinder the slurry, and the longer it takes for the slurry to reach a stable state.

Numerical simulation results of porous single fracture media

Similarly, the article selects the migration pattern of slurry under injection pressure of 5 MPa and static water pressure of 2 MPa as an example to analyze the migration law of slurry in porous single fracture media, as shown in Fig. 16.

Fig. 16
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Grouting pressure 5 MPa grout migration pattern.

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From Fig. 16, it can be seen that the migration pattern of the slurry in the porous single fracture medium is symmetrical along the direction of the fracture. As the grouting time increases, the diffusion range of the slurry gradually expands. Due to the existence of cracks, during the entire grouting process, the slurry first enters the crack channel through the grouting hole at the initial moment, and then diffuses into the pore medium through the crack channel. This is because the resistance experienced by the slurry when flowing through the crack channel is much smaller than that experienced in the pore medium, that is, the slurry will always diffuse in the direction of lower flow resistance. The slurry exhibits significant longitudinal infiltration and diffusion in the porous medium, and as the grouting time increases, the longitudinal diffusion distance gradually increases and eventually stabilizes. When the grouting pressure is constant, the time required for the slurry to diffuse the same distance increases with the increase of static water pressure, indicating that static water pressure has a certain impact on the diffusion distance of the slurry; Under the same hydrostatic pressure, as the grouting pressure increases, the time required for the slurry to diffuse to the same position becomes shorter. In addition, compared to the diffusion of slurry in non fractured porous media, its diffusion in porous single fractured media is easier, and the time required for diffusion over the same distance is shorter.

In order to provide a clearer description of the variation of the migration distance of the slurry with grouting time, the curves of the longitudinal and transverse migration distances of the slurry with grouting time under different grouting pressures are presented. The longitudinal diffusion distance represents the distance along the y-axis from the grouting hole, as shown in Fig. 17.

Fig. 17
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Variation of slurry diffusion distance with time: (a) Grouting pressure 4 MPa, (b) grouting pressure 5 MPa, (c) grouting pressure 6 MPa.

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Figure 17 shows the variation of the diffusion distance of the slurry with time under three different grouting pressures. From the analysis of the figure, it can be concluded that under the same working condition, the lateral diffusion distance of the slurry is much greater than the longitudinal diffusion distance. This is because the existence of cracks makes the slurry preferentially diffuse along the direction of the cracks. The longitudinal and transverse diffusion distances of the slurry gradually increase with the increase of grouting time, but when the grouting time reaches a certain level, the growth rate of the diffusion distance of the slurry gradually decreases, that is, the diffusion distance of the slurry tends to stabilize. Under different grouting pressures and hydrostatic pressures, the longitudinal diffusion range of the slurry is 10.05–10.23 m, and the transverse diffusion range of the slurry is 16.3–16.98 m. When the grouting pressure is low, the change in static water pressure has a significant impact on the length of grouting time. For example, when the grouting pressure is 4 MPa, the time difference for the same distance of slurry diffusion under two different static water pressures is 30 h. However, when the grouting pressure is high, the time difference for the same distance of diffusion under different static water pressures is 5 h. This indicates that when the grouting pressure is high, the change in static water pressure has a smaller impact on grouting time.

Curtain water retention schemes based on macroscopic and microscopic characteristics of rock layers

Location of curtain wall construction

To ensure the efficiency and reliability of curtain wall construction, curtain walls should be avoided from being built in locations with large-scale dynamic water supply conditions, in order to increase the flow path of dynamic water and reduce the hydraulic slope, thereby reducing the impact of large-scale dynamic water on curtain walls. The Ordovician limestone aquifer is a strong aquifer with excellent water yield and sufficient dynamic water supply. Therefore, the layout of the curtain wall should be as far away from the exposed line of the Ordovician limestone aquifer as possible. At the same time, in order to reduce costs and shorten the length of the curtain wall, the curtain wall should be as close as possible to the 10th coal outcrop line without affecting the normal mining of the 8th coal seam, and the area of the “five bearing” strata on the dewatering side of the curtain wall should be reduced. Therefore, it is selected between the coal seam 10 outcrop line and the Taiyuan Formation Limestone Aquifer roof interface or the coal seam 8 outcrop line, and to avoid disturbance to the wall caused by the mining of the coal seam 8, the average distance from the coal seam 8 outcrop line is ensured to be no less than 150 m.

The curtain wall needs to achieve the function of interception, intercepting groundwater from various directions such as the top, bottom, left, and right of the wall, that is, blocking the main runoff channel to prevent bypass supply. Therefore, the position of the curtain section should be selected in the continuous and stable thickness section of the “five aquifer” layer to ensure that the top and bottom of the wall are in a stable relative aquitard. According to the geological structure, the top of the curtain wall is located in the relatively water-resistant section of the “Five Containing” sandstone, and the bottom is located in the relatively water-resistant section of the interbedded sandstone and mudstone in the coal bearing strata, thus forming a “bounded top and bottom” closed water blocking wall to prevent the flow around the top and bottom of the wall. In addition, considering the geological structural conditions, curtain walls should avoid fault arrangements as much as possible, especially for large faults cut by the “Fifth Aquifer”. If it cannot be avoided, drilling holes should be increased at the fault, grouting volume should be increased, and the hydraulic connection between the fault and the “Fifth Aquifer” should be cut off to ensure the stability of the wall.

Therefore, in order to prevent groundwater from flowing around, the upper and lower panels of the curtain wall are expanded outward into the relative aquitard for 15 m, with an average of 60 m; The curtain wall cuts off the boundary of the “Five Containing” strata on both sides, with a designed distribution length of approximately 3.13 km (as shown in Fig. 18).

Fig. 18
figure 18

Plane layout of curtain wall position.

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Selection of drilling techniques for curtain wall construction

Flexible selection of drilling construction technology based on the height and thickness of the “Five Containing” strata. Due to the requirement of a large burial depth in the “double opening” directional inclined section during directional drilling, conventional vertical drilling is used for the development of the “five bearing” strata at shallow positions below − 300 m; The development of the “Wuhan” formation is carried out at a depth of − 300 to − 400 m using ordinary directional drilling- Horizontal holes are used below 400 m.

According to the construction technology and layout position, the curtain wall is divided into two parts: the northern wall and the eastern wall. The walls of the northern line are all located in the shallow section (including the bottom plate, which is basically above − 350 m), and can be divided into conventional straight holes and shallow bedding holes according to the type of drilling. The straight hole section is located at an elevation of approximately − 340 m above the direct contact zone of the fourth and fifth sections, as well as the bottom plate of the fifth section. The shallow bedding section is located near the endpoint of the northern line. There is an aquitard between the fourth and fifth aquifers, and the top and bottom plates of the fifth aquifer are located between − 350 to − 380 m. The eastern wall can be divided into shallow and deep sections. The shallow section (buried at a depth of − 380 m or more) is located on the south side of the eastern line and uses shallow bedding holes; The deep section (buried below − 380 m) adopts horizontal holes, and the design and construction section of the curtain wall is shown in Fig. 19.

Fig. 19
figure 19

Design and construction section of curtain wall.

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Curtain wall thickness and drilling spacing size

The purpose of constructing a curtain wall is to block the groundwater runoff channel and intercept groundwater from all directions on one side of the wall. Due to the certain burial depth of the suspended curtain wall, a certain amount of water head pressure is borne on one side of the wall, especially at the bottom of the curtain wall. Therefore, in order to ensure the long-term effectiveness of the curtain wall, it is necessary to determine and analyze the thickness of the curtain wall. According to the "Code for Grouting curtain in Mines", the thickness of curtain walls in soluble rock formations should not be less than 10 m, but no calculation expression for curtain wall thickness is provided. According to literature32, the thickness of the curtain wall can be estimated using Eq. (1):

$$\delta \ge \frac{{\text{H - h}}}{{J_{\varphi } \left( {1 - {{{\text{K}}_{\varphi } } \mathord{\left/ {\vphantom {{{\text{K}}_{\varphi } } {{\text{K}}_{0} }}} \right. \kern-0pt} {{\text{K}}_{0} }}} \right)}}$$
(10)

Among them, δ is the thickness of the curtain wall (m), H is the water head borne upstream of the curtain wall (m), and h is the water head borne downstream of the curtain wall (m), Allow seepage gradient for the curtain wall, and determine the permeability coefficient of the curtain wall (m/s), The permeability coefficient of the curtain layer.

The construction goal of the curtain wall is to achieve 85% of the interception effect of the curtain wall, which can be considered as reducing the permeability of the curtain layer by 85%. Therefore, it can be known as 0.15; The curtain wall is filled with slurry stones in the geological cracks, so according to literature2, it is selected as 60; The maximum height difference of the curtain wall is 400 m. By substituting the above parameters into Eq. (1), the minimum thickness of the curtain wall can be calculated to be 7.84 m.

When the minimum thickness of the curtain wall is determined, the reasonable setting of the drilling spacing is the key to ensuring the minimum thickness of the curtain wall and achieving the construction purpose of the curtain wall. Due to the uneven development of pores, fractures, caves, etc. in the injected strata, and the large spacing between boreholes, it is easy to cause the disconnected pores, fractures, caves, etc. in the strata between boreholes to be unable to be filled with slurry, resulting in gaps; The drilling spacing is too small, the construction cost is too high, and the cycle is too long. The relevant literature uses the diffusion distance of the slurry to calculate the spacing between boreholes. The author believes that the spacing between boreholes should mainly consider the connectivity of the fractures in the grouting layer. The diffusion distance of the slurry is the distance that the slurry moves under the high-pressure push of the grouting pump. When the pore and fracture connectivity is good, the diffusion of the slurry can continue to move along the connected fractures until the push pressure is less than the sum of static water pressure and pipe wall friction. Therefore, under a certain rated working capacity of the grouting pump, the diffusion distance of the slurry is determined by the length of the fracture extension.

Therefore, in order to ensure the minimum safe thickness of the curtain wall, considering construction costs and efficiency, the drilling spacing is set to be twice the minimum safe thickness of the curtain wall. The numerical simulation results show that the diffusion range of slurry in porous media is 10.12–10.44 m, the longitudinal diffusion range of slurry in porous-single fractured media is 10.05–10.23 m, and the transverse diffusion range is 16.3–16.98 m under different working conditions. The rock mass medium type of grouting horizon is mainly pore medium. According to the range of grouting pressure and numerical calculation results, it is proposed that the grouting hole spacing in the curtain wall grouting project is 20 m, two rows of staggered holes are arranged, and the effective width is about 40 m. The spacing between straight holes is 20 m, with two rows of staggered holes and one inspection hole set every 80 m; Horizontal boreholes are arranged side by side, with staggered horizontal sections. The distance between the branch holes of each horizontal hole is 20 m, and each group of holes is equipped with one inspection and reinforcement hole extending to the opposite end (as shown in Fig. 20).

Fig. 20
figure 20

Plane layout of vertical and Directional Borehole spacing.

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Applicability of grouting materials and slurry proportions

The ratio of grouting materials and grout is a key parameter for ensuring the construction quality of curtain walls and controlling the grouting process. The geometric parameters of pore and fracture development in different injection layers are different, and the particle size distribution of grouting materials is also different. Therefore, the two are mutually selected and matched during the grouting process; The physical and mechanical properties of different slurry ratios are different, and analyzing their applicability is of great significance for ensuring the strength of curtain walls. Due to the large scale of the curtain wall construction and the large amount of grouting, based on environmental protection and cost considerations, cement and fly ash are mainly used as grouting materials.

Injectability of grouting materials

By using a laser particle size analyzer to analyze the particle size of the experimental Portland cement and fly ash, the particle size distribution characteristics of the two materials can be obtained (as shown in Fig. 21).

Fig. 21
figure 21

Material particle size distribution.

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From Fig. 20, it can be seen that the content of 3–32 μm particles in composite Portland cement accounts for about 62.55%; The particle size of fly ash is mainly concentrated in the range of 10–250 μm, accounting for about 88%. In the general grouting process of fractured rock masses, the opening of rock fractures should be greater than three times the maximum particle diameter of the grouting material, that is, b ≥ 3D, which means that the opening of fractures for cement particle grouting should be greater than 96 μm, and the opening of fractures for fly ash particle grouting should be greater than 750 μm. Based on the equivalent diameter range of pores in Table 1 and the development of fractures, smaller cement particles can meet the filling of small-sized pores and fractures, and can fill pores with a volume proportion of 86.77%, as well as micro tension, medium tension, wide tension and other fractures; Fly ash particles are relatively large and can fill some pores and cracks, but cannot fill small-sized pores and cracks. The pore filling volume is about 23.56%, and the cracks can fill medium and wide tension cracks, as well as a small number of micro tension cracks. Therefore, when meeting the requirements for crack opening, pure water slurry should be used to fill the small cracks during the pressure grouting stage in rock formations with small crack opening. For karst developed holes, grouting can be carried out by adding larger particles of fly ash for non pressure filling and grouting, reducing the amount of cement and lowering the grouting cost.

Applicability of slurry mix ratio

To effectively seal and reinforce the injected rock layer, the grouting stone body should meet the maximum water pressure requirements of the curtain wall. According to the water pressure difference between the inner and outer walls of the curtain wall and its burial depth, the maximum water pressure that the wall can withstand is 4 MPa. According to the requirements of curtain grouting in karst fracture developed strata in the Zhuxianzhuang mining area, the uniaxial compressive strength, flexural strength and other parameters of the slurry were measured respectively for Portland cement single slurry with different water–solid ratio and cement fly ash slurry with different fly ash content, and the performance research of grouting materials suitable for various structures of the coal mine strata was carried out. The experimental design adopts the international standard orthogonal experimental design method, which can measure the slurry performance parameters efficiently, quickly, and comprehensively. In the test, the influence of slurry water solid ratio, fly ash content, and other factors on the slurry performance was specifically considered. The main test design is as follows: for Portland cement single slurry, the water-to-solid ratio is the most important factor affecting the slurry material performance. Therefore, five water solid ratios of 3:1, 2:1, 1:1, 0.8:1, and 0.6:1 were designed to conduct a comprehensive test. Five groups of tests were designed to measure the performance parameters of cement slurry materials with different water–solid ratios. For the performance test of cement fly ash slurry, the content of fly ash has a significant impact on the performance of cement fly ash slurry. Therefore, a 2-factor 5-level orthogonal test is designed, a total of 25 groups of tests, mainly to study the influence of the water–solid ratio of slurry and the content of fly ash on the performance of slurry. The test factors and levels are shown in the table below (Table 8).

Table 8 Orthogonal experimental factors and levels of cement—fly ash grout.
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Therefore, the author focuses on cement slurries with water to solid ratios of 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, and 3.0; The physical and mechanical properties of cement fly ash slurry with water to solid ratios of 0.6, 0.8, 1.0, 2.0, 3.0, and fly ash contents of 20%, 30%, 40%, 50%, and 60% were tested for flexural strength and compressive strength (Figs. 22 and 23). Therefore, the 28 day compressive strength of slurry stones with different water solid ratio should meet the strength requirements of the curtain wall under the maximum water pressure.

Fig. 22
figure 22

Strength curves of cement slurry stones with different water solid ratios: (a) compressive strength, (b) flexural strength.

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Fig. 23
figure 23

Strength curve of cement fly ash slurry with different water solid ratio: (a) compressive strength, (b) flexural strength.

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The “Five Containments” curtain grouting project in Zhu Xian Zhuang Coal Mine primarily aims to grout the 5th aquifer, 4th aquiclude, and 4th aquifer, thereby forming a water-blocking curtain to prevent water inrush accidents during coal seam mining. However, the curtain wall covers a wide range and is large, and the 5th aquifer is cemented with gravel, so cracks and karst are developed. Therefore, according to the particle size analysis of cement-based grouting materials, the strength and physical and mechanical properties experiments of cement slurry and cement-fly ash slurry under different mixing ratios, the applicability of cement slurry and cement-fly ash slurry in the curtain grouting process is analyzed and discussed, to find cement-based slurry with low cost and good effect suitable for curtain grouting engineering of rock mass.

Strength analysis of slurry stone body

To ensure that the stone body formed by previous grouting will not be washed away during subsequent grouting, the compressive strength of the stone body formed by grouting at the age of 7 days should be not less than 1 MPa. In addition, the final grouting pressure standard of the curtain grouting project is: the final grouting pressure of the third water-resisting layer and the fourth aquifer section in the vertical drilling section is 2–3 MPa, the final grouting pressure of the fifth aquifer section is 5 MPa, and the final grouting pressure of the horizontal drilling section is 5 MPa. After the curtain wall is completed, it needs to meet the tolerance requirements of 4 MPa water pressure difference. Therefore, the strength of the stone body formed by grouting at the age of 28 days should meet the requirements of grouting end pressure and curtain wall strength. Tables 9 and 10 were obtained by screening the strength properties of stone bodies formed by cement slurry and cement-fly ash slurry.

Table 9 Mix proportion of cement slurry meeting the strength performance conditions of grouting curtain in Zhu Xian Zhuang Coal Mine.
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Table 10 Cement-fly ash slurry mixture ratio meeting the strength performance conditions of grouting curtain in Zhuxianzhuang Coal Mine.
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Applicability analysis of injected medium

For the larger holes with karst development, fly ash with larger particles can be mixed for pressure less filling and grouting, and the water–solid ratio with a higher stone rate can be selected to improve the filling efficiency, reduce the amount of cement and reduce the grouting cost. Therefore, the cement-fly ash slurry is screened under the condition that the strength performance of the grouting stone body and the stone rate is more than 75%. See Table 11 for details.

Table 11 Mix proportion of cement-fly ash slurry meeting stone rate and strength performance conditions.
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Grouting pressure

Grouting pressure is the driving force behind the diffusion of grout, overcoming flow resistance, and displacing groundwater. It is an important lever and control factor to ensure the effectiveness of grouting. Therefore, the reasonable selection of grouting pressure plays a key role in the construction quality of curtain walls. The author uses numerical analysis methods based on the construction conditions of curtain walls to analyze and calculate the grouting pressure during curtain wall construction, in order to obtain a reasonable grouting pressure that ensures the construction size of curtain walls.

From Fig. 17, it can be seen that the longitudinal and transverse diffusion distances of the slurry gradually increase with the increase of grouting time, and the grouting tends to stabilize. The longitudinal diffusion range of the slurry tends to stabilize at about 10 m, and the transverse direction tends to stabilize at about 16 m, which can ensure the minimum safe thickness of the curtain wall construction. However, the growth rate of the horizontal and vertical diffusion distance of the grout increases with the increase of grouting pressure. Therefore, in order to improve grouting efficiency, combined with the setting time of the grout, a grouting termination pressure of 5–6 MPa can be used. During this period, porous jump grouting and intermittent grouting processes can be used according to the pressure change process.

Evaluation of grouting curtain effect

The inspection of grouting effect is an important link to ensure the quality of curtain wall construction. Therefore, the author analyzes the cement content of rock debris in the inspection hole, the physical and mechanical properties of the slurry stone body, and the changes in the water level inside and outside the curtain wall during the water discharge test to obtain the diffusion of the slurry, the strength of the stone body, and the interception effect. This comprehensively evaluates the construction effect of the curtain wall to ensure that the construction quality of the curtain wall meets the design standards.

Inspection of slurry diffusion situation

Each group of horizontal drilling holes is equipped with one inspection and reinforcement hole extending to the opposite end. By checking the composition of rock debris and drilling fluid leakage in the reinforcement hole, analyzing the proportion of cement stone bodies and the filling rate of pores and cracks, the diffusion of slurry and grouting effect within a 20 m range between horizontal branch holes can be determined, and secondary reinforcement can be carried out according to the situation to ensure the diffusion range of slurry.

Stone body strength

In the design of curtain wall construction, inspection boreholes are drilled every 80 m in the vertical section. The inspection boreholes are generally constructed after the completion of the grouting hole construction in the early stage, playing a role in inspection and reinforcement. Therefore, by inspecting the drilling holes during construction, some of the grout stone bodies formed in the early grouting holes can be exposed. By conducting indoor physical and mechanical property tests on the stone bodies, the grouting effect can be analyzed and judged (as shown in Fig. 24). The experimental results show that the unconfined uniaxial compressive strength of the stone body in the dry state is 21.3 MPa, and in the saturated state it is 11.2 MPa. As shown in Fig. 24, the unconfined compressive strength failure state of the stone specimen in both dry and saturated states is shown. The saturated state exhibits plastic deformation compared to the dry state, indicating that water has a significant reducing effect on its physical and mechanical properties.

Fig. 24
figure 24

Drying and Saturated Unconstrained Compressive Strength Test of Stones: (a) Dry sample test photo of stone body, (b) saturated sample test photo of stone body.

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The construction of this curtain requires the wall to resist a minimum water pressure of 4 MPa, and the strength of the stone body in the water holding state is 11.2 MPa, fully meeting the requirements of water pressure difference strength. Moreover, the stone body sample is formed under large cavities and solidified under low-pressure grouting filling. For smaller cracks and pores, the slurry is squeezed under high-pressure conditions, and the spacing between cement slurry particles decreases under pressure, causing water molecules to precipitate. Therefore, the strength of the stone body in the rock cracks and pores is higher than that of the stone body solidified under non pressure conditions, and it can fully resist the water pressure difference inside and outside the wall. Therefore, it can be judged that the wall strength effect formed by grouting curtain is good.

Verification of interception effect

The ultimate goal of constructing a curtain wall is to intercept and block the runoff channels of the aquifer. Therefore, the interception effect of the wall is a key indicator to test the success of the curtain wall construction, and the pumping and discharging water test is the most direct and effective method to test the interception effect25.

The water release test adopts the method of underground water release and water level observation of boreholes on the well. During the water release period, the water level elevation of the “five contents” observation holes inside and outside the curtain wall is monitored and recorded. By comparing the changes in the water level of the “five contents” measurement holes inside and outside the wall, the curtain interception effect of the wall can be compared. As shown in Fig. 25, the variation curve of the water level of the “five aquifers” observation holes inside and outside the curtain wall during the water release period can be seen. It can be seen that there is a significant difference in the water level of the “five aquifers” observation holes inside the wall during the water release period, and the water level of the observation holes inside the wall drops significantly. The water level of the observation holes outside the wall is basically stable, and the water level of the observation holes inside the wall remains in the lowered position after the water release is completed, without recovery, indicating that the curtain wall has cut off the supply of the “five aquifers” and other aquifers to the inner part of the wall, playing a good curtain.

Fig. 25
figure 25

Water level variation curve of “five contained” observation hole during discharge.

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In addition, the designed interception rate of the curtain wall is 85%, and the residual water volume of groundwater from outside the wall to inside the wall is 300 m3/h. During the water release test stage, the residual water volume of the curtain wall was less than 170 m3/h. According to the water release test results of the mine in 1993, when the depth was reduced by about 100m, the water release volume of the “Fifth Aquifer” was about 700 m3/h, and when the water level dropped to − 230 m, the depth was about 180 m. Based on this, it can be calculated that the water volume before the construction of the “Fifth Aquifer” wall in this stage was about 1260 m3/h. Therefore, the actual interception rate when the “Fifth Aquifer” water level dropped to − 230 m is not less than 86.51%. According to the numerical simulation used in reference33 to predict and calculate the water volume and interception rate when the water level of the “Fifth Aquifer” drops to − 350 m, it can be concluded that the interception rate when the water level of the “Fifth Aquifer” drops to − 350 m is 95.26%. Overall, it can be concluded that the curtain wall has a high interception rate and good effect.

Conclusion

  1. (1)

    Scanning and analyzing the core of injected stratum by micro-CT scanning, it is found that the equivalent diameter of the breccia aquifer is mainly ≤ 200 μm in number, and the equivalent diameter is mainly ≤ 200 μm in volume, up to 32.27%; Among the fractures, the largest volume proportion is middle tension fracture, with an average of 2009.61 μm, followed by wide tension fracture, with an average of 7601.96 μm; The permeability is 4.071 × 10−12 m2;

  2. (2)

    According to the calculation results of two numerical models: porous media and porous single fracture media, the diffusion range of slurry in porous media increases with the increase of grouting time, but the growth rate gradually slows down. The diffusion range of slurry after stable migration under different working conditions is 10.12 ~ 10.44 m; In the pore single fracture, the migration pattern of the slurry is symmetrical along the direction of the fracture. Due to the existence of the fracture, the slurry enters the fracture channel first through the injection hole at the initial moment, and then diffuses into the pore medium through the fracture channel;

  3. (3)

    Based on the particle size distribution characteristics of grouting materials and the microscopic characteristics of the injected medium, it is found that pure water slurry should be used for cracks with small openings. For karst developed pores, fly ash can be added for non pressure filling and grouting. Cement slurry with a mix ratio of 0.6, 0.7, 0.8, 0.9, cement fly ash slurry with a water to solid ratio of 2.0, 1.0, 0.8, 0.6, and corresponding fly ash content of 20%, 20%, 20%, and 30% can meet the construction requirements;

  4. (4)

    According to theoretical calculation and numerical analysis, the minimum safe thickness of curtain grouting is 7.84 m, the drilling spacing is 20 m, and two rows of staggered holes are used for straight holes, and one inspection hole is set every 80 m; Horizontal drilling holes are arranged side by side, and horizontal sections are staggered, and the upper and lower spacing of branch holes of each horizontal hole is 20 m;

  5. (5)

    The construction quality of the curtain wall was analyzed and inspected through drilling and water discharge tests. The saturated compressive strength of the slurry stone body was found to be 11.2 MPa, and the water level difference between the inside and outside of the wall reached 140 m in 7 days. The interception rate was not less than 86.51%, and the interception effect of the wall was significant, meeting the design goals and requirements.