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Hydroelectric simulation of the phreatic water response of mining cracked soil based on microbial solidification

  • Ying Gao ORCID logo , Mohd Ashraf Mohamad Ismail ORCID logo EMAIL logo , Tao Li ORCID logo EMAIL logo , Bo Li and Jiarui Zhang
Published/Copyright: February 5, 2025
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Abstract

Coal mining in ecologically fragile areas results in the failure of aquiclude layers and the loss of surface water bodies. Herein, research was conducted on the microbial solidification of cracked soils and the corresponding response of the ecological water table. A simulation of mining-induced cracked soils was performed via microbial solidification. The mechanical and hydrological properties of cracked soil samples repaired with different filling materials were compared via unconfined compressive strength and falling head permeability tests. Hydraulic-electric similarity modeling techniques were employed to evaluate the effectiveness of microbial solidification in the aquiclude layers. After low-temperature acclimation, Bacillus megaterium adapted to the geological environment of the study area, exhibiting a high viable cell density. When the cracked soil was filled with a 1:1 ratio of aeolian sand to clay particles, the microbially remediated soil demonstrated optimal mechanical and hydraulic properties. Hydraulic-electric similarity numerical simulations revealed that the ecological water table at the coalface remained within a reasonable range following microbial solidification, suggesting that microbial solidification achieved water-preserving coal mining. These findings provide a reference for restoring aquiclude layers damaged by coal mining.

1 Introduction

Currently, China’s main coal mining areas are concentrated in ecologically fragile areas (Shanxi, Shaanxi, Ningxia, Inner Mongolia, Xinjiang, Guizhou, and other areas), causing damage to aquiclude layers [1], leading to the loss of surface ecological water [2], and further exacerbating environmental degradation in these regions [3]. Research has been conducted on repairing cracked soil and reconstructing aquifers [4], including natural restoration [5,6], mechanical repair [7], and chemical grouting [8]. However, owing to issues such as cost, environmental protection, and grouting technology, large-scale application is not yet possible.

Microbially induced calcium carbonate precipitation (MICP), as an emerging environmentally friendly geotechnical restoration technology, is currently widely used in studies on seabed solidification, concrete crack repair, loose sand stabilization, ancient building crack restoration, island reef soil reinforcement, and desertification prevention [9,10,11,12]. Soil stabilization is a key area of interest [13]. MICP technology utilizes bacterial metabolic pathways to naturally produce nontoxic calcite minerals that bind soil particles, thereby increasing the stiffness, strength, water retention, and fertility of the soil [14,15]. Zhang et al. [1] explored the impact of coal mining on surface ecology, particularly soil moisture distribution and vegetation, and proposed the use of crack-filling restoration and shrub planting to reduce soil moisture loss and restore the structure of mined soils. Ji et al. [16] investigated the restoration of soil structures in loess-aeolian sand mining areas and reported that fungal communities of the Ascomycota phylum promoted the recovery of the original soil structure. Yao et al. [17] employed MICP to stabilize aeolian sand, in which the dry density, permeability, and water retention of the sand were significantly improved. MICP binds soil particles, fills soil pores, and alters the particle arrangement and pore size distribution, solidifying the soil and changing its microstructure [18]. Current microbial solidification experiments focus mainly on stabilizing loose soils, with few studies on the stabilization of filled cracked soils caused by mining activities.

There are many soil stabilization techniques [19,20], but combinations of multiple methods, particularly for the filling of cracks in coal mining areas, have rarely been reported. Therefore, the ecologically fragile Ningxiaota coal mine was used as a case study, the characteristics of fractured soil were simulated, and MICP remediation experiments were conducted on the fractured soil. This research reveals the changes in the physical, chemical, and mechanical parameters of soil after solidification, providing a reference for microbial mineralization restoration in ecologically fragile areas.

The research process (Figure 1) is divided into four parts: (a) field investigation (the depth and width of cracks and the filling characteristics) and sampling (the moisture content and preparation of cracked soil samples); (b) microbial acclimatization (strain selection and low-temperature acclimation); (c) the unconfined compressive strength (UCS) and permeability of the solidified cracked soils; and (d) hydraulic-electric similarity simulation experiments to evaluate the effectiveness of microbial solidification in repairing aquiclude layers disturbed by coal mining.

Figure 1 
               Research process chart.
Figure 1

Research process chart.

2 Background of microbial solidification in coal-mining crack soils

The study area is located in the ecologically fragile Ningtiaota coal mine in northern Shaanxi Province, with Kaokaowusu Gou as the boundary, aeolian sand landforms in the south, and the loess mount landform in the north. According to previous research, the aquiclude of this mine is generally thick (40–120 m), but the thickness of the bedrock is relatively limited. Coal-mining water-conducting cracks are guided through the bedrock, and the aquiclude is damaged to different degrees. Therefore, the study of microbial curing of the aquiclude was carried out, and the background analysis of solidification was as follows.

2.1 Geological background of coal mining

The geological background of this study area was the Ningtiaota coal mine in the Shennan mining area, Yulin City, Shaanxi Province, China. The 2–2 coal is first mined in the southern area of the Ningtiaota Coal Mine, and its area location and mining geological background are shown in Figure 2. The mining thickness of the 2–2 coal is approximately 4 m, and the average thickness of the overlying strata is 198.4 m. There is no significant geological structure in the study area, and the dip angle of the strata is approximately 1°. The overlying strata of 2–2 coal can be generalized into four main hydrogeological layers from bottom to top: the relative aquifuge of the bedrock of the Yan’an Formation of the Jurassic (J2y, averaging approximately 77.9 m), the weathered bedrock aquifer of the Zhiluo Formation of the Jurassic (J 2z , averaging approximately 19.5 m), the aquifuge of loess (the Q2 Lishi loess and N2 Baode loess, averaging approximately 91 m), and the sand layer of the submersible aquifer (the Q3 Salawusu Formation aquifer and the Q4 Quaternary aeolian sand aquifer, averaging approximately 10 m). Sandy submerged aquifers play an important role in supporting surface ecology, but the cracks caused by coal mining lead to a decrease in the level of sandy aquifers. In this process, the thicker loess aquifuge in the study area is particularly important for seepage. Therefore, the use of MICP technology to repair the damaged loess aquitard caused by coal mining is of great significance.

Figure 2 
                  Location of the study area and geologic histograms.
Figure 2

Location of the study area and geologic histograms.

2.2 Characteristics of coal mining crack soil development

Huang Qingxiang reported that cracks formed by coal mining can be categorized into “upward cracks” and “downward cracks” [6,21,22]. The two types of cracks are believed to be directly related, and the development directions of “upward cracks” and “downward cracks” were found to be the same by the microresistivity scanning images of the boreholes [8], as shown in Figure 3 (with a difference of 3–6° in the crack direction). It is difficult to observe “upward cleavage” directly, and the “downward cleavage” feature was observed on the surface in this study, which represents the characteristics of the cleavage soil induced by coal mining.

Figure 3 
                  Comparison of scanning electron microscopy and ground observations of coal mining cracks.
Figure 3

Comparison of scanning electron microscopy and ground observations of coal mining cracks.

To fully simulate the solidification law of shallow soil cracks under in situ conditions, field geological investigations and onsite measurements were carried out on the soil cracks in the loess-covered area in the northern part of the study area. Combined with the existing research results, the following conclusions regarding the characteristics of the soil bodies in coal-mining cracks in the study area were drawn.

2.2.1 Geometric features

The depth and width of four groups of representative soil cracks during coal mining were observed [6,8,22], as shown in Figures 4 and 5 (the coal mining working line is 10 m away from the crack at the time 0 point, followed by 1 observation every 2 days and consecutive observations for 10 days). The width and depth of the soil cracks were affected by the mining process, exhibiting a trend of first increasing, then decreasing, and finally stabilizing at 0.085 and 0.1775 m, respectively.

Figure 4 
                     Dynamic change in the soil crack depth.
Figure 4

Dynamic change in the soil crack depth.

Figure 5 
                     Dynamic change in the soil crack width.
Figure 5

Dynamic change in the soil crack width.

2.2.2 Characteristics of filling

After the stable period of coal mining collapse, the restoration of mining crack soil is concentrated. Field geological investigation and field measurement were performed on the filling characteristics of the coal mining face after the completion of 1a, as listed in Table 1. The fill in the crack consists of aeolian sand and loess, with a filling density between 1.44 and 1.51 g/cm3 and a filling rate between 9.4 and 100%.

Table 1

Characteristics of crack soil filling in the study area

Landforms Filling Average filling rate (%) Average filling density (g/cm3)
Aeolian sand Aeolian sand and loess 100 1.51
Loess ridge Aeolian sand and loess 9.4 1.44

2.3 Geological environment characteristics of microbial remediation

To fully simulate the microbial remediation of coal-mining cracked soils in the in situ geologic environment, the geologic environment characteristics of the cracked soils were analyzed, and the experimental conditions were set on the basis of the geologic environment characteristics.

2.3.1 Temperature characteristics

The temperature conditions are critical for microbial work. The temperature difference between day and night in spring and autumn in Yulin City is significant, which requires temperature adjustments to simulate the in situ characteristics. The experimental temperature for microbial remediation of soil cracks was manually adjusted via a biochemical incubator, and the temperature ephemeral curve of daily adjustment is displayed in Figure 6.

Figure 6 
                     Temperature curve of the soil restoration process.
Figure 6

Temperature curve of the soil restoration process.

2.3.2 Characterization of the soil moisture content

The moisture content of the cracked soil was tested via onsite sampling. The sampling and testing process was as follows: after the completion of the 1a coal mining, ten groups of soil samples (five groups were soil samples far from the crack area, and the other five groups were soil samples near the crack area) were obtained within a depth range of 0–2 m on the surface of the coal mining subsidence area. The natural moisture content of the soil samples was tested, as listed in Table 2. The average moisture contents of the soils away from the cracks (Y1 to Y5) and near the cracks were 23.3 and 16.7%, respectively. The natural moisture content of the soil near the cracks (Y6–Y10) generally decreased due to evaporation. The moisture content variance was large, and the degree of dispersion was high. Therefore, the initial moisture content of the soil samples used in the experiments in this study was 16.7%.

Table 2

Test results of the moisture content of the soil samples in the mining subsidence area

Distance to crack (m) No. Moisture content (%) Average moisture content (%) Variance
5.4 Y1 26.5 23.3 3.4024
5.0 Y2 22.6
4.4 Y3 24.0
3.7 Y4 21.5
3.1 Y5 21.7
0.3 J1 14.8 16.7 3.4264
0.3 J2 17.6
0.4 J3 16.9
0.1 J4 14.5
0.6 J5 19.5

3 Preparation of experimental materials

3.1 Bacterial strain culture and adaptability test

Among the species tested thus far, anaerobic fermenters, anaerobic respiratory bacteria, and specialized aerobic bacteria may be suitable for geotechnical applications. Notably, parthenogenetic anaerobes and microaerobes are the most suitable microorganisms for soil bioconjugation [23]. Northern Shaanxi is perennially cold, and the study area is close to the Maowusu Desert, with an average annual temperature of 7.3°C and an annual precipitation of 365 mL, which is a typical arid/semiarid area [24]. Bacillus megaterium (purchased from the China General Microbiological Culture Collection Center, CGMCC strain No. CGMCC1.16094) was selected for this experiment, which is a heat-resistant and thermophilic bacterium and a parthenogenetic anaerobe. The strain is commonly used in the production of organic fertilizers for crops and the treatment of polluted water and is an environmentally friendly microorganism [25]. Compared with other strains, B. megaterium grows and reproduces faster at low temperatures (10 and 15°C) and has greater urease activity. Therefore, B. megaterium is more suitable for low-temperature environments [26,27].

Inoculation refers to the original bacterial mixture from which a small amount of natural selection of microorganisms was pumped into the new medium The purchased strains were inoculated three times (Figure 7), and each time was used in the temperature curve shown in Figure 6 for cultivation. A ultra violet‒visible spectrophotometer was used to determine the absorbance value of the bacterial fluid samples at a wavelength of 600 nm (OD600 value) every 13 days. The OD600 value reflects the activity of microorganisms, with a positive correlation between the two. In addition, the conductivity of the mixture of bacterial mixture and urea was employed to determine the urease activity of B. megaterium. The larger the value is, the better the activity of the bacterium. As shown in Figure 8, after three rounds of inoculation, the local adaptability of the microorganisms increased, the density of the active bacteria increased substantially, and the bacteria could be used to restore mining cracked soil. Compared with the literature [28], the peak of the OD600 was pushed back and decreased under the fluctuating temperature conditions, but the stabilization period of the bacterial activity was longer, i.e., the microbial action was more durable. In summary, all subsequent microbial curing experiments were carried out 7 days after inoculation with three generations of microorganisms, and then the soil characterization test was performed after solidification for 8 days.

Figure 7 
                  Microbial inoculation.
Figure 7

Microbial inoculation.

Figure 8 
                  Observation results of the microbial OD600 and urease activity in different generations.
Figure 8

Observation results of the microbial OD600 and urease activity in different generations.

3.2 Preparation of cracked soil samples

Synthesizing the characteristics of the cracked soil induced by coal mining in Section 2.2, the geometry of the cracked soil samples for this indoor experiment was set as follows: the width of the crack was 1 cm, the depth of the crack was 2.1 cm (2.1 times the width), and the crack was pointed out downwards (soil cracks made via 3D-printed thin slices). The filler inside the crack was aeolian sand and loess with different mixing ratios. The bacterial solution and cement solution were injected into the cracked soil. The process is shown in Figure 9.

Figure 9 
                  Cracked soil sample preparation process.
Figure 9

Cracked soil sample preparation process.

Based on the crack characteristics of the soil described in Section 2.2, cracked soil samples for the uniaxial compressive strength test and falling head permeability test were prepared, and the moisture content of the soil samples was determined to be 16.7%. The main soil samples were those of the Lishi Formation and Baode Formation. In addition, aeolian sand and loess particles with ratios of 2:1, 1:1, and 1:2 (both taken from the study area and dried and sieved) served as the fillers for the cracks. The crack was filled with three layers of well-mixed filling materials and compacted to control the density of 1.9–2.1 g/cm3 after compaction. The sample groups were subjected to microbial solidification (Table 3), with three samples in each group and a total of 36 solidified remediation soil samples. The comparison group without microbial solidification was prepared with 36 blank soil samples corresponding to the solidified samples in Table 3 one by one.

Table 3

Preparation of cracked soil samples

Grouping Soil type Filling sand-to-soil ratio Test items
1 Baode laterite 2:1 Uniaxial compressive strength
2 Baode laterite 1:1 Uniaxial compressive strength
3 Baode laterite 1:2 Uniaxial compressive strength
4 Baode laterite 2:1 Falling head permeability
5 Baode laterite 1:1 Falling head permeability
6 Baode laterite 1:2 Falling head permeability
7 Lishi loess 2:1 Uniaxial compressive strength
8 Lishi loess 1:1 Uniaxial compressive strength
9 Lishi loess 1:2 Uniaxial compressive strength
10 Lishi loess 2:1 Falling head permeability
11 Lishi loess 1:1 Falling head permeability
12 Lishi loess 1:2 Falling head permeability

4 Experimental methods

4.1 Uniaxial compression test

The third-generation bacterial liquid and cementing liquid (calcium chloride and urea concentrations of 0.1 mol/L) cultivated for 7 days were injected into the cracked soil samples described in Section 3.2 at a volume ratio of 1:1.5, and the samples were prepared according to the fabrication steps in Figure 9. The maintenance was carried out according to the temperature ephemeral curve in Figure 6. After 8 days of maintenance, the uniaxial compressive strength test was conducted using an electric lime soil unconfined compression tester (YYW-II), and the loading rate was controlled at 1 mm/min.

4.2 Falling head permeability test

The third-generation bacterial liquid and cementing liquid (calcium chloride and urea concentrations of 0.1 mol/L) cultivated for 7 days were injected into the cracked soil samples described in Section 2.2 at a volume ratio of 1:1.5, and the samples were generated according to the fabrication steps in Figure 9. The maintenance was performed according to the temperature ephemeral curve in Figure 6. After 8 days of maintenance, the permeability test was carried out using a South-55 falling head permeameter with an initial head height of 100 mm. The changes in the water head and time were recorded, and the test was repeated 5 times.

4.3 Similar numerical simulation tests for hydropower

4.3.1 Principles of similar simulation

4.3.1.1 Hydropower similarity principle

The simulation of hydropower selection is similar to simulation technology (with the current field used to simulate the groundwater seepage field) but different from traditional physical circuit simulation. The virtual circuit model in MATLAB for hydropower is similar to the simulation. One of the main simulation principles is that the seepage of water in the soil conforms to Darcy’s law and Laplace’s equation. Moreover, the current flows in the conducting medium according to Ohm’s law and Laplace’s equation. Owing to the similarity of the equations, the current field can be used to simulate the groundwater seepage field [29,30]. Similar equations for the seepage field and electric field are shown in equation (1). Figure 10 shows the difference cell of the seepage field and the R‒C cell of the circuit field.

(1) V = α H R x x = β Δ x K x x Δ y Δ z R y y = β Δ y K y y Δ x Δ z R z z = β Δ z K z z Δ x Δ y C = γ s Δ x Δ y Δ z I = α β Q t m = β γ t ,

where V denotes the voltage of the circuit field, H is the head of the seepage field, and R xx and R yy represent the horizontal resistance of the circuit field. The x direction is the direction of 0–6 in Figure 13, and the y direction is 0–2. K xx and K yy refer to the horizontal permeability coefficient of the seepage field, R zz represents the vertical resistance of the circuit field, and the z direction is the direction of 0–3 in Figure 13. K zz indicates the vertical permeability coefficient of the seepage field; C represents the circuit of the circuit field, s is the water storage rate of the seepage field containing the water barrier, I denotes the current of the circuit field, Q signifies the flow rate of the seepage field, t m is the time of the circuit field, and t is the time of the seepage field. ɑ, β, and γ represent the three similarity coefficients, where ɑ = 1, β = 10−2, and γ = 10−4. Δx, Δy, and Δz are the differences in spacing in the three directions.

Figure 10 
                        Hydropower similarity simulation unit. Hydropower similarity simulation unit. (a) the difference cell of the seepage field; and (b) the R‒C cell of the circuit field.
Figure 10

Hydropower similarity simulation unit. Hydropower similarity simulation unit. (a) the difference cell of the seepage field; and (b) the R‒C cell of the circuit field.

Figure 11 
                        Simulation flow of synchronous operation.
Figure 11

Simulation flow of synchronous operation.

Compared with traditional geotechnical and hydrological simulation techniques, this technique can realize additional hydrological boundary conditions and simulate the seepage field under the evolution of the hydrological parameters of an aquifer under mining disturbance. Thus, hydroelectric similarity simulations are particularly suited for hydrologic dynamics where aquifer and aquiclude conditions undergo mutual changes. The principle of specifically simulating the ecological water level response of coal mining and MICP restoration is as follows.

4.3.1.2 Mining similarity principle

To simulate the hydrological dynamic response of coal mining disturbances and MICP restoration of the water barrier accurately, the principles of synchronous operation of geotechnical simulation and circuit simulation were adopted. The synchronous simulation process is shown in Figure 11. (1) Based on the original hydrogeological data, Simulink software was used to construct the natural circuit field model before coal mining and simulate the natural diving level. (2) According to the original engineering geological data, the geotechnical model was constructed via UDEC software, and the coal mining crack field was simulated. (3) The coal mining crack field was transplanted into the circuit field (adjusting parameters such as resistance and capacitance in the range of the crack field to 0) and the inversion was simulated via Simulink software to obtain the coal mining water level. (4) On the basis of the experimental law of MICP restoration, the resistance parameters of the recovery area were adjusted using Simulink software, and then the water levels of different areas were reversed after recovery.

Figure 12 
                        Characteristic change curves for different fillings.
Figure 12

Characteristic change curves for different fillings.

4.3.2 Construction of a simulation prototype

The microbial solidification of cracked soil aims to improve the strength and waterproofness of the soil simultaneously, thus effectively reducing water leakage in coal mining subsidence areas. Combined with the measured uniaxial compressive strength and permeability coefficient data (Figure 12), a 1:1 mass ratio of filler sand to soil can account for both strength and hydrological properties, which is suitable for coal mining cracked soil remediation. Therefore, the following similar simulation tests were carried out under this ratio to study the microbial remediation of the coal mining face.

Figure 13 
                     Circuit model based on water‒electricity similarity.
Figure 13

Circuit model based on water‒electricity similarity.

This simulation took the typical hydrogeological conditions of the Ningtiaota coal mine in northern Shaanxi as the model background (Tables 4 and 5). Combined with the hydroelectric similarity rules, the circuit and geotechnical models are shown in Figures 1314. Among them, the water level of the sand layer (ecological diving level) is the target level for water conservation and coal mining. The underlying loess and laterite constitute the aquiclude, which is the target for restoration in this study. Coal mining destroys bedrock, and it inevitably causes the sand layer water level to fall. In this simulation, 100 m of coal pillars was left at each end without mining.

Table 4

Hydrogeological parameters of typical aquifers in the study area

No. Stratum Layer thickness (m) Permeability coefficient (m/day) Water storage rate (1/m) Hydrological boundary conditions
1 Salawusu Formation 10 3.88 1.7 × 10−4 Fixed water levels 198 m and 195 m for Nos. 1 and 41
2 Lishi loess 30 0.017 1.3 × 10−3
3 Baode laterite 60 0.0016 2.6 × 10−3
4 Weathered bedrock 20 0.0838 4 × 10−5 fixed water levels 183 and 180 m for Nos. 1 and 41
5 Bedrock 80 0.001 3 × 10−3
Table 5

Electric field parameters corresponding to typical hydrogeological conditions in the study area

No. Stratum Voltage (V) Horizontal resistance (Ω) Vertical resistance (Ω) Capacitance/10−6F
1 Salawusu Formation Fixed voltages 198 and 195 V for Nos. 1 and 41 0.0026 0.0026 1
2 Lishi loess 0.59 0.59 13
3 Baode laterite 6.25 26
4 Weathered bedrock Fixed voltage 183 and 180 V for Nos. 1 and 41 0.12 0.12 0.4
5 Bedrock 10 10 30
Figure 14 
                     Synchronized constructed geotechnical mode.
Figure 14

Synchronized constructed geotechnical mode.

5 Comprehensive analysis

5.1 Analysis of the uniaxial compressive strength results

Currently, the application of MICP is focused primarily on loose soils [31,32], with limited studies available on the stabilization of cracked soils. According to structural surface control theory, the key factor influencing the improvement in UCS in cracked soils lies in the filling materials within the cracks and the effectiveness of their interface solidification [33,34]. Kulanthaivel et al. [32] reported that MICP is more effective at enhancing the mechanical strength of coarse-grained soils than fine-grained soils. Hataf and Jamali [35] further demonstrated that the UCS of treated soils significantly increased when fine grains were incorporated. Therefore, the ratio of coarse particles (sand) to fine particles (soil) in the filling materials of cracked soils is a critical parameter that requires further investigation. The results of the UCS tests on cracked soils are shown in Figure 15.

Figure 15 
                  Uniaxial compressive strength of microbial solidification with different filling ratios: (a) Baode laterite and (b) Lishi loess.
Figure 15

Uniaxial compressive strength of microbial solidification with different filling ratios: (a) Baode laterite and (b) Lishi loess.

Effectiveness of microbial mineralization: MICP significantly improved the UCS of cracked soils. Compared with untreated samples, samples solidified by MICP showed substantial increases in UCS. The UCS of cracked red soils from the Baode Formation and cracked loess from the Lishi Formation increased by 202–329 kPa and 206–325 kPa, respectively.

Improvement in mechanical uniformity: Before solidification, the standard deviations of the UCS for cracked red soils from the Baode Formation and cracked loess from the Lishi Formation were 10.95 and 24.51 kPa, respectively. After microbial solidification, these values decreased to 10.08 and 4.52 kPa, respectively. Cracked soils exhibited poor mechanical uniformity before MICP treatment, but MICP significantly enhanced their uniformity.

Influence of the particle size ratio: The ratio of coarse to fine particles in the crack-filling materials had a considerable effect on MICP solidification. A higher proportion of sand in the filling material led to an increase in overall strength, although the improvement is relatively modest, with the red soil of the Baode Formation showing a maximum increase of 49 kPa and the loess of the Lishi Formation showing a maximum increase of 60 kPa.

Mechanism of strength enhancement: The mechanical strength after microbial solidification was significantly improved due to the naturally low UCS of cracked soils. MICP-induced calcite minerals effectively bonded loose particles into a solid mass, forming a cohesive structure with the surrounding soils. As shown in Figure 16, the cluster formations between the sand particles consisted of calcite, indicating a clear cementation effect. Furthermore, given the same amount of cementation, the particle size influenced the mechanical strength of the samples, with an increase in the sand content leading to a slight improvement in strength.

Figure 16 
                  Magnified micrograph of fillings.
Figure 16

Magnified micrograph of fillings.

5.2 Analysis of the falling head permeability results

Currently, research on MICP primarily focuses on improving the impermeability of single-type soil [36,37]. In contrast, cracked soil, once filled, represents a mixed-type soil, and studies on the permeability changes of such mixed soil bodies after MICP treatment are lacking. According to Darcy’s law, the porosity characteristics of the filling material in cracks are the key controlling factor for permeability changes in mixed soils. Therefore, the ratio of coarse particles (sand) to fine particles (soil) in the filling material is a critical parameter that requires further investigation to increase the impermeability of mixed soil bodies through MICP solidification. The permeability coefficient test results for cracked soil bodies are shown in Figure 17, and the findings can be summarized as follows:

Figure 17 
                  Permeability coefficient of microbial solidification with different filling ratios: (a) Baode laterite and (b) Lishi loess.
Figure 17

Permeability coefficient of microbial solidification with different filling ratios: (a) Baode laterite and (b) Lishi loess.

Significant reduction in permeability after MICP treatment: Compared with that of the untreated samples, the permeability coefficient of the microbially treated cracked soil decreased significantly, by more than one order of magnitude.

Improved hydraulic uniformity posttreatment: Before solidification, the standard deviations of the permeability coefficient of the cracked Baode Formation red soil and cracked Lishi Formation loess were 0.63 and 1.67, respectively. After MICP treatment, the standard deviation decreased to 0.15 for the Baode Formation and 0.64 for the Lishi Formation, which indicated that the hydraulic uniformity of cracked soils was poor prior to MICP treatment but significantly improved after solidification.

Effect of the sand content on the permeability: As the sand content in the filling material increased, the reduction in permeability became less pronounced, with changes limited to within one order of magnitude. However, when the sand-to-soil ratio changed from 2:1 to 1:1, the permeability significantly decreased by 59.80–66.50%.

Impact of calcite precipitation on permeability: Cheng et al. [38] reported that the permeability of all samples decreased as the amount of CaCO3 precipitation increased. In cracked soil, calcite precipitation from MICP solidification blocked cracks and pores, significantly enhancing impermeability. Additionally, the inherent porosity of the filling material played a direct role in this result.

5.3 Results of hydroelectric similarity simulation

Currently, Visual Modflow is the primary software used to simulate the impact of mining on water levels. Fan et al. [39] used this software to study the effects of coal mining on groundwater and spring points. Visual Modflow, a hydrological modeling tool, treats the mining area as an abstraction activity, making it challenging to simulate changes in the hydrological characteristics of the strata. In this study, a hydroelectric analogy simulation method was used, which allowed for the modeling of hydrological characteristics across different strata. The simulation results and analysis are as follows:

5.3.1 Water level change after coal mining

With coal mining, the crack field in the overburden was translated into an electrical circuit field (Figure 18, where positions 1–6 on the right correspond to the Salawusu Formation, Lishi Formation loess, Baode Formation red soil, weathered bedrock, bedrock, and the 2-2# coal seam, respectively). The simulation results of water level changes in the Salawusu Formation (a sandy layer aquifer) are depicted in Figure 19. Due to the periodic pressure caused by coal mining, only key points of water level change were presented. As shown in Figure 19, the water level decreased to varying degrees after coal mining, with a maximum drawdown of 2.4 m. The cracks caused by coal mining facilitated the drainage of groundwater, resulting in overflow within the Salawusu Formation above the soil layer. This finding aligns with the results of Fan et al. [40].

Figure 18 
                     Coal-mining crack field transplanted to the circuit field.
Figure 18

Coal-mining crack field transplanted to the circuit field.

Figure 19 
                     Simulation results of coal mining dive level changes.
Figure 19

Simulation results of coal mining dive level changes.

5.3.2 Water level recovery after microbial solidification

To evaluate the effectiveness of microbial solidification in preserving groundwater, the strata within the crack zone were solidified based on laboratory experimental results. This process reduced the permeability coefficient by more than one order of magnitude, effectively retaining some hydraulic resistance. The simulation results are shown in Figure 20, where the water level drawdown was successfully controlled, with a maximum drawdown of only 0.5 m. According to the ecological water level theory [4042], a 0.5-m drawdown remains within the ecological viability range, indicating that microbial solidification can effectively restore the aquiclude and protect the overlying ecological groundwater. This simulation demonstrated that microbial solidification had a significant effect on controlling overflow, similar to the findings of Li et al. [43], who proposed aquiclude reconstruction.

Figure 20 
                     Results of the numerical simulation of the water level in the Salawusu Formation.
Figure 20

Results of the numerical simulation of the water level in the Salawusu Formation.

6 Conclusion

  • The depth of the soil cracks produced by coal mining in the study area is 2.1 times greater than the width. The filling material consists of aeolian sand and loess, with a filling density between 1.44 and 1.51 g/cm3 and a filling rate between 9.4 and 100%. Under these characteristics, microbial solidification of filling materials had a better application foundation.

  • Through three rounds of low-temperature domestication and inoculation, the B. megaterium used in the experiment can adapt to the environment of the region where the experiment was carried out, remain active for a long period of time at their peak, and have a high density of viable bacteria.

  • The uniaxial compressive strength and permeability coefficient of the cracked soil were tested at different filling ratios, with an optimal performance at the 1:1 sand-soil mass ratio.

  • The numerical simulation of the hydropower similarity of the microbial remediation of the quarried crack soil layer reveals that the maximum depth of the ecological submergence level after microbial restoration was only 0.5 m. Microbial remediation technology can effectively repair the aquiclude and then protect the overlying ecological submergence level.

Acknowledgments

The authors gratefully acknowledge the financial support and thank Limin Fan and Liqiang Ma Professor for assisting in conducting this research.

  1. Funding information: This article was funded by the National Natural Science Foundation of China ( 42162022), Guizhou Provincial Science and Technology Project, QKHJC-ZK [2022]-YB529 and QKHJC-ZK [2023]-YB445, and the Project of Liupanshui Science and Technology, 52020-2023-0-2-14.

  2. Author contributions: Analyzed the data and wrote the paper Y.G.; investigation, JR.Z.; funding acquisition, B.L.; performed the model experiments, T.L.; corrected the grammar errors and provided some suggestions for improving the manuscript, M.A. All authors have read and agreed to the published version of the manuscript.

  3. Conflict of interest: The authors declare no conflicts of interest.

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Received: 2024-04-27
Revised: 2024-10-08
Accepted: 2024-10-16
Published Online: 2025-02-05

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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