Softening and instability evolution of strip coal pillar under water immersion in goaf

Abstract

The softening and instability of the coal pillar caused by water immersion is the primary factor contributing to water inrush in the goaf of the same layer. Similarly, the softening and instability of underground reservoir dam bodies due to water immersion is a critical factor leading to reservoir water failure and dam breakage. Investigating the dynamic process of coal pillar softening and instability under water immersion conditions is of paramount importance for the prevention and control of mine water hazards in mining areas and the safety assessment of underground reservoirs. This study uses the mining of the A ore strip as an engineering background, conducts theoretical analyses and laboratory tests on coal pillars subjected to varying soaking durations, and examines the macroscopic and microscopic damage and deterioration processes of coal pillars under soaking conditions from the perspectives of mechanical properties, hydrochemical characteristics, and microstructural characterization. The experimental results indicate that within 0 to 40 days of soaking, the uniaxial compressive strength of the coal pillar decreases by 50%, the pH value of the solution shifts from weakly acidic to weakly alkaline, multiple ion concentrations undergo significant changes, mineral compositions alter, and porosity exhibits a pronounced changing trend. Based on these research findings, this paper explores the physicochemical coupling mechanism of internal structural evolution and damage deterioration in soaked coal pillars, elucidating the entire process of damage-deterioration-instability. The dynamic process of water-immersed coal pillars was studied and visualized using similar materials with water-blocking additives, revealing their spatiotemporal evolution laws. Through mechanical derivations and laboratory test results, this paper identifies the key factor influencing the instability of the overlying rock mass above the coal pillar roof, termed the “effective support width,” and investigates the mechanical deterioration mechanisms of water-immersed coal pillars. Finally, based on theoretical research and indoor experiments, considering the dual effects of stress and water pressure, the calculation formula for the effective support width of water-immersed coal pillars was derived, clarifying the evolution mechanism of “damage-deterioration-instability” in the overburden rock mass above the coal pillar roof. This study discusses the dynamic process of coal pillar softening-instability under water immersion conditions in goaf areas, which holds significant implications for the retention of waterproof coal pillars in coal mines, safe mining practices, and the protection and recycling of coal pillars. Additionally, research into the damage-deterioration-instability of coal pillars provides valuable guidance for the construction of underground reservoirs and the determination of appropriate widths for coal pillar dam bodies.

Introduction

Strip mining is a common mining mode of coal mine, and the coal pillar is an important condition to ensure the safety of mining. According to the data, there were 100 serious accidents in China’s coal mines^1,2, accounting for 8.53% of the total number of coal mine accidents. According to the analysis of accident types^3,4, water damage in goaf is the most typical factor^{5, 6–7}, and water inrush induced by the protective coal pillar left over from strip mining in the same coal seam becomes the main threat under the immersion, softening and dynamic coupling⁸.Water resources in mining areas are scarce. In order to solve the problem of how to use water resources efficiently, many scholars have conducted safety evaluation and research on underground reservoirs^9,10. Research on the stability of protective coal pillar is a key issue to reveal water inrush in goaf and evaluate the safety of dam body, and the maintenance of protective coal pillar is an important factor to control the movement of overlying rock and the settlement of the surface^{11, 12–13}. Based on the immersion test, Yao Qiangling et al. disclosed the damage mechanism of the coal pillar caused by water content^{14, 15–16}. Shi Weigang et al. derived the formula for determining the width of the water-proof coal pillar through theory¹⁷. Li Jianhua et al. discussed the influence of water pressure in the goaf on the strength of the coal pillar dam via the immersion experiment¹⁸. Zhang Kai et al. utilized numerical simulation to reveal the types and intensities of water-rock interaction at different stages. Wang Lujun and Wang Xuebin et al.employed laboratory tests to analyze the coal pillar damage caused by the cracks in the dam body from discrete to macro cracks to penetration^{19, 20, 21–22}. Wang Fangtian et al. investigated the dynamic damage evolution characteristics of the coal pillar dam by numerical simulation²³.

The goaf area of coal mines is interconnected with the regional groundwater system, jointly constituting the occurrence and migration environment of groundwater in the mining area. Under the influence of mesospheric medium conditions and water-rock coupling effects, the unique environmental characteristics of the goaf, along with the protective coal pillar, surrounding rock, and waterproof coal pillar, lead to processes of damage, instability, and seepage failure. Concerning the changes in stability and anti-failure capability of the coal pillar under long-term water accumulation and immersion, as well as the physical and mechanical evolution mechanisms during the instability and failure of the overlying rock layer, current fundamental research remains relatively inadequate. Especially in the context of underground reservoir projects, a systematic quantitative understanding of the relationship between the instability of submerged coal pillars and the potential damage to underground reservoir dam structures has not yet been established. This lack of comprehensive analysis hinders the ability to conduct both qualitative and quantitative assessments of underground reservoir safety. It is therefore imperative to address these pressing scientific challenges.

Many scholars have achieved certain research findings. However, the majority of these results center on the mechanical testing of the immersed coal pillar, the permeability and moisture content of coal rock samples. There are scarce studies on the repeated immersion - instability - immersion process of the retained protected coal pillar, the deterioration of the bearing capacity of the coal pillar, and the assessment of the water-blocking (bearing) capacity of the retained protected coal pillar.

Based on the coal pillar softening overburden failure and evolution project of a certain mine, this paper selects the carboniferous and Permian coal seam as the research object. The aim is to deeply analyze the macro and micro damage and deterioration evolution process of the submerged coal pillar through research methods such as constructing the softening and instability model of the submerged coal pillar, conducting mechanical tests and microscopic tests, and carrying out similar simulation tests. Reveal the influence law of the effective support width on the overbearing capacity during the softening process of the coal pillar. The research results will provide theoretical basis and parameter calculation methods for the quantitative evaluation of the “damage” caused by the immersion of water-blocking coal pillars, fill the gap in the study of the relationship between the instability of water-blocking coal pillars and the damage of underground reservoir DAMS, offer key technical support for the stability assessment of goed-out areas and the safety guarantee of underground reservoir DAMS, and help coal mining areas achieve the dual goals of safe mining and efficient utilization of water resources.

Study area conditions and experimental methods

This paper takes a mine as an instance. The geological structure of the mine is straightforward and it is influenced by the quaternary unconsolidated aquifer and the Ordovician limestone karst aquifer. The coal seam is exploited by shaft single level, and the coal mining approach is integrated mechanized coal mining technology with longwall backward direction. The No.3 coal seam is mainly mined with a coal thickness of 2.6 m. The floor elevation of the coal seam ranges from − 415 to -630 m, with an average mining depth of 550 m. The overlying quaternary layer is approximately 300 m thick, and the bedrock thickness is about 250 m. Strip mining is adopted in the mining area. The mining width of the working face is 50 m and the remaining width is 100 m. The schematic diagram of the working face is presented in Fig. 1. Since the mining area was put into operation, a certain scale of goaf has been formed, some of which are situated in low-lying areas, and there is water accumulation in the goaf area with poor drainage. The normal water inflow of the mine is 353m³/h, and the maximum water inflow is 589.8m³/h.

Fig. 1 [Images not available. See PDF.]

Working face position. (a) Vertical direction, (b) horizontal direction.

Softening and instability stress theory of immersed coal pillar

Mechanical model of immersed coal pillar

Under the action of water immersion in the goaf, the coal pillar will be subjected to the joint action of water osmotic pressure in the goaf and mine pressure in the roof. Based on one side of the goaf and combined with the limit equilibrium theory, a mechanical model of the flooded coal pillar is built^{6,9,12,17,24,25}. According to scholars’ research and laboratory mechanical test analysis, the failure zone of coal pillar consists of three parts: elastic zone, shaping softening zone and shaping rheological zone^26,27.

1) The strength expression of coal pillar in the elastic region is:

1

In the formula, is the maximum and minimum principal stress of coal body, MPa; is the uniaxial compressive strength of the elastic stage, MPa; is the stress coefficient; is the effective internal friction Angle, (°).

2) Molding and softening stage Softening modulus is used to characterize the softening degree of coal pillar^9,12. With the double action of water pressure and roof overburden pressure, the internal mechanical strength, internal friction Angle and cohesion of coal pillar decrease, so the strength expression of coal pillar in the molding and softening area is as follows:

2

In the formula, is the uniaxial compressive strength of coal pillar at the softening stage, MPa; is the softening modulus of coal pillar, MPa ( values in reference^14,17,24).

3) The strength expression of coal pillar in the plastic flow stage is as follows:

3

In the formula, is the residual strength of the coal pillar, MPa (Fig. 2).

Fig. 2 [Images not available. See PDF.]

Mechanical theoretical mode.

Calculation of effective width of coal pillar

Considering that the coal pillar is pressed by water pressure, overburden pressure and periodic pressure, the loading condition of coal pillar is complicated. Therefore, based on the assumption that the coal pillar is uniform and continuous with isotropy and the permeation water pressure is uniform, the effective thickness of the coal pillar under the action of permeation water pressure and overburden pressure is considered in this section (Fig. 3)²⁸. The force balance equation of coal pillar is as follows²⁹:

4

In the formula, is the remaining thickness of the coal pillarm; is the horizontal stress of coal pillar, m; is the friction coefficient of the coal pillar, .

According to the studies of previous scholars, the strength of coal pillar in the failure region is much greater than the stress in the x direction, so formula (3) becomes formula (5), and formula (6) is obtained by replacing formula (5) with (4) through linear algebra, and the boundary conditions are substituted: When , (p is the water pressure of goaf infiltration), when , the formula of horizontal stress and vertical stress in the failure area is shown in formula (7).

5

6

7

Fig. 3 [Images not available. See PDF.]

Stress state of coal pillar.

According to scholars’ research^16,30, it is found that the softening curve of coal pillar is close to the linear formula (8):

8

In the formula, is the strain gradient in the plastic region; is the inelastic width of coal pillar, m; x is the distance between the edge point of the coal pillar and the strain point, m.

Substitute formula (3) into (8) to obtain the strength of coal pillar in the plastic zone:

9

The stress at the junction between the failure zone and the plastic zone is continuous. Therefore, the abrupt point is consistent with the distance from the edge and the distance from the failure region, and formula (5) is substituted into formula (9) to change into formula (10), and formula (9) is combined with the coal pillar balance equation, and the stress in the plastic region is formula (11): According to scholars’ research^10,13,17,18, the results show that the stress above the failure area and the plastic area is , and k₁ is the vertical stress concentration coefficient on one side of the coal pillar; H is the buried depth of coal seam, m; is the bulk weight of overlying rock, KN/m³. Equation (12) can be obtained by substituting the stress value and Eq. (10) with Eq. (11), and the calculated result is equal to Eq. (13). According to Fig. 2, the stress concentration coefficient on the other side is k₂, and the calculation formula is (14).

10

11

12

13

14

The effective supporting width of coal pillar is:

15

In the formula, is the effective support width, m; is the left shaping and softening area, m; is the softening area of the plastic zone on the right, m. Assuming that the water pressure on both sides is the same and the moisture content reaches the maximum, the friction coefficient is set to 0.53, the coal thickness is set to 2.6 m, the buried depth is set to 550 m, the water pressure is set to 0.32 MPa, and other related parameters are set according to the mine data and the research of previous scholars^9,22. The calculated total softening distance of 2.6 m coal pillar and is 9.34 m and 10.77 m respectively.

Mechanical testing of water-immersed coal columns: analysis of moisture content

The coal seam is influenced by the disruption of mining activities, which can generate or augment cracks. The stability of the coal pillar will vary under the inundation of goaf water. The influence of the effective support width on the softening process of the coal pillar is analyzed, and a similar simulation test is devised. To guarantee the resemblance between the simulation and the actual circumstances, the coal pillar strength test was conducted under the water immersion condition to obtain the softening curve of the coal pillar.

The coal samples and water samples utilized in this experiment were sourced exclusively from the No. 3 coal seam of Mine A. To minimize potential testing errors, large coal samples without apparent fractures, joints, or other structural defects were carefully selected. Subsequently, coal column processing and experimentation were conducted in strict compliance with the ISRM standards.

Fig. 4 [Images not available. See PDF.]

Test procedure for evaluating the compressive strength of water-immersed coal samples.

Firstly, the coal samples were dried for 8 h and subsequently sealed with cling film to prevent moisture absorption. The samples were divided into 9 groups, each consisting of 3 samples. The coal column samples in the first group were directly subjected to compressive strength testing. The remaining 8 groups were immersed in water (pH = 5.8) for varying durations of 5, 10, 15 days, and so forth. Once the soaking requirements were fulfilled, the coal samples were sequentially subjected to compressive strength tests, as illustrated in Fig. 4. The results of the compressive tests, along with the moisture content data, are summarized in Table 1.

Table 1. Test results of uniaxial compression test.

Soaking time /d	Coal sample size /mm	Mean failure load /KN	Average compressive strength /MPa	MoistureContent /%
0	50.1 × 49.6 × 98.7	9.72	4.042	0
5	49.8 × 49.6 × 99.2	8.58	3.551	6.38
10	49.7 × 49.5 × 98.7	7.46	3.075	7.75
15	48.9 × 49.5 × 97.6	6.49	2.658	8.51
20	49.3 × 48.7 × 98.7	5.77	2.383	9.05
25	49.5 × 49.2 × 98.5	5.02	2.087	9.46
30	49.2 × 48.9 × 97.3	5.05	2.023	9.79
35	48.7 × 49.7 × 97.3	5.02	2.017	10.07
40	48.9 × 49.5 × 98.3	5.01	2.015	10.31

It can be observed from Fig. 4; Table 1 that the initial compressive strength of the coal pillar is 4.042 MPa. After soaking for 30 days, the compressive strength decreases to 2.023 MPa, while the moisture content increases from 0 to 9.79%. The trend of the moisture content slope initially exhibits a rapid increase followed by a gradual increase. Upon soaking for approximately 25 days, the compressive strength stabilizes. At 30 days of soaking, the compressive strength reduces to 50% of its initial value, with the moisture content reaching approximately 10%, indicating a softening coefficient of 0.50. After 40 days of soaking, the compressive strength remains at approximately 49.8% of the initial strength, corresponding to a softening coefficient of approximately 0.50 and a moisture content of approximately 10%. These results suggest that the coal pillar has softened and reached a stable state under prolonged soaking conditions.

Microscopic and chemical analysis of water-immersed coal columns

(1) Test on the composition of coal pillars.

By applying the Stokes sedimentation theorem in fluid mechanics, samples with particle sizes smaller than 10 micrometers and smaller than 2 micrometers were respectively collected from the water content of the water-immersed coal column and the coal column used in the mechanical test. In conjunction with specific X-ray diffraction patterns, the composition and changes of the water-immersed coal column under various soaking durations were analyzed.

(2) CT Scan Porosity Test.

A water-saturated coal column was scanned using a desktop micron-level CT scanner. By employing X-ray fluoroscopy images and computer-based three-dimensional digital reconstruction technology, a detailed digital three-dimensional characterization of the sample’s microstructure was achieved. The procedure involved preheating the scanner to ensure optimal performance. Subsequently, the cabin door of the X-micron CT scanner was opened, and the cylindrical core sample (with a diameter of 3 cm and a height of 2 cm), obtained via a standard drilling rig during the preparation process, was carefully placed on the sample stage. The cabin door was then closed securely. Upon completion of the CT scan, the acquired data were reconstructed and imported into specialized image analysis software for further evaluation. The extracted three-dimensional features of the samples are presented in Fig. 5.

Fig. 5 [Images not available. See PDF.]

CT scanning procedure for water-saturated coal samples.

(3) Water chemical test.

The aqueous solution utilized in this experiment was identical to that employed in the coal sample soaking test. Three coal samples were subjected to soaking in an acidic aqueous solution (pH = 5.8) for varying durations, specifically 5 days, 10 days, and 15 days. A total of eight water samples were collected. The concentrations of conventional ions, including , , , , , , , and , in the aqueous solution were analyzed using an ICP-720ES inductively coupled plasma optical emission spectrometer. Additionally, the pH value of the soaking solution was measured using a pH30 meter.

Similar simulation of mechanical strength softening of coal pillar

According to the water immersion softening formula for coal columns (Eq. 16), a similarity-based simulation study of coal columns was performed. Based on the original materials presented in this paper, a water-blocking material (Vaseline) was incorporated to investigate the compressive strength of similar coal column simulations under water immersion conditions. The testing procedure is illustrated in Fig. 6, with the objective of identifying analogous simulated materials that replicate the softening behavior of coal columns. The soaking duration was adjusted according to the similarity ratio derived from the similarity simulation methodology. Two sets of simulations were conducted for each group of specimens, and the averaged simulation results are summarized in Table 2.

Fig. 6 [Images not available. See PDF.]

Softening process of specimen immersed coal pillar. (a) Test specimen loading diagram, (b) coal pillar compression diagram.

16

Table 2. Test results of similar simulation uniaxial compression test.

Water-resisting material	Specimen size(d*h)	Specimen soaking time	The average crushing pressure of the specimen(KN)
			0 h	1 h	2 h	3 h	4 h	5 h
5%	100.24*49.74	Specimen 1	0.112	0.068	0.061	0.045	0.044	0.045
	100.12*49.87	Specimen 2	0.142	0.064	0.066	0.053	0.050	0.039
	99.72*49.50	average value	0.127	0.066	0.0635	0.049	0.047	0.042
8%	99.84*50.49	Specimen 1	0.342	0.175	0.096	0.094	0.099	0.130
	100.42*50.87	Specimen 2	0.205	0.192	0.132	0.125	0.115	0.112
	99.48*50.46	average value	0.2735	0.1835	0.114	0.1095	0.107	0.121

In the formula (16), is the compressive strength of the original rock, MPa; T is the soaking time of primary rock, h; is the intensity similarity ratio, is the time similarity ratio is a constant, MPa.

According to the existing research results^14,15,17,18, 5% and 8% water-blocking materials were selected to study the mechanical strength changes of materials with different proportions of water-blocking properties. According to Fig. 7, it is analyzed that the strength of the sample after adding 5% water-blocking agent decreases exponentially with the immersion time, and the strength softening process accords with the law of gradual decay of coal pillar with the immersion time in practice. According to the curve fitting degree analysis, the fitting degree R² of 5% water inhibitor is greater than 0.95, indicating that the curve fitting degree is good, and it can be used as a coal pillar softening simulation material.

Fig. 7 [Images not available. See PDF.]

Curve of compressive strength change of water-blocking additive specimens with different proportions with immersion time. (a) 5% water blocking additive specimen, (b) 8% water blocking additive specimen.

Similar simulation experiments and numerical simulation test designs were conducted to validate the theoretical analysis

The experiment was carried out on a two-dimensional analog platform, the size of which was length × width × height = 2000 mm×300 mm×1800 mm.

Fig. 8 [Images not available. See PDF.]

Similar simulation schematic diagram. (a) Stratigraphic map, (b) schematic diagram of similar simulation model, (c) monitoring equipment.

According to the similarity theorem and relevant similarity criteria, in conjunction with the actual conditions of the mining area, a similar simulation model was constructed with a geometric similarity ratio of 1:150 and a dynamic similarity ratio of 1:1.6 (see Table 3). A specific water pressure was applied to the aquifer through channels arranged on both sides of the channel steel and monitored using water pressure instruments. Monitoring points were strategically placed in the goaf and strip areas to investigate the activation mechanism of coal and rock columns under water immersion conditions. The arrangement of these monitoring points is illustrated in Fig. 8. Following the completion of the model construction, the water-richness of the aquifer was simulated by utilizing transparent plates on both sides of the channel steel and expansion glue.

Based on the primary mechanical property parameters of the coal mine rock strata and adhering to prior proportioning experience, river sand was selected as the aggregate, gypsum and calcium carbonate as the cementing materials, and mica sheets as the stratified materials for simulating the rock strata. Given that water accumulation in the mined-out area may alter the mechanical properties of the similar materials, appropriate water-blocking additives must be incorporated into the cementing material. In light of the softening behavior of the coal column strength over time determined through previous proportion tests, vaseline was proposed as the water-blocking additive. The specific main mechanical parameters, along with the stratification and proportioning of the models, are detailed in Table 3.

Table 3. Similar simulation proportioning material.

Serial Serial number	Stratum name	Thickness of stratum /m	Compressive strength /MPa	Unit weight /(t/m3)	Material weight /kg
					River sand	Plaster	Calcium carbonate	Water blocking additive	Water
10	mudstone	14.1	31.6	2.65	82.04	1.64	6.56	4.51	9.02
9	Fine sandstone	34.5	67.8	2.57	200.73	10.04	10.04	11.04	22.08
8	mudstone	22.2	31.6	2.65	129.16	2.58	10.33	7.10	14.21
7	Medium fine sandstone	17.4	64.8	2.55	101.24	5.06	5.06	5.57	11.14
6	mudstone	33.6	31.6	2.65	195.49	3.91	15.64	10.75	21.50
5	siltstone	10.2	64.0	2.60	59.35	2.97	2.97	3.26	6.53
4	mudstone	7.8	31.6	2.65	45.38	0.91	3.63	2.50	4.99
3	Medium fine sandstone	42	64.8	2.55	244.36	12.22	12.22	13.44	26.88
2	Coal seam	2.6	18.5	1.40	15.13	0.45	1.06	0.83	1.66
1	Fine sandstone	22.5	67.8	2.57	130.91	6.55	6.55	7.20	14.40

This experiment focuses on strain observation, which is conducted using the ISM-CONTR-VG5-2DB series non-contact video strain and displacement precision measurement system (as shown in Fig. 8). Once the model has been dried and set, waterproof observation points are arranged on the model surface according to the measurement point layout diagram (as shown in Fig. 8). These points can be continuously monitored by the photogrammetry monitoring system throughout the entire process of the analogous model strip mining and water immersion experiments in goaf areas. A total of three survey lines are established within the rock strata, with a sparser distribution in the upper part and a denser distribution in the lower part. The vertical distances from each survey line to the coal seam, from top to bottom, are 774 mm, 400 mm, and 200 mm, respectively, and they are designated as Line A, Line B, and Line C.

The specific procedure of the model experiment is described as follows:

(1) Select materials and process specimens according to the performance requirements, adhering to the corresponding simulation material ratio specifications.

(2) Install channel steel on the front and rear sides of the model frame. Lay the model layer by layer from bottom to top based on the dimensions of each layer, sprinkle mica powder between layers, and compact the simulated materials in each layer. Additionally, install stress or strain sensors within the laid rock layers as per the experimental design.

(3) After the model is constructed, it should be naturally maintained for 3 to 5 days. Subsequently, remove the channel steel and allow the model to dry naturally under good ventilation conditions.

(4) Once the model is fully dried and set, strip mining should be conducted in accordance with the experimental requirements.

(5) After completing the strip mining, seal the model and inject water into the goaf. Observe and record the activation and movement of overlying rocks under the influence of accumulated water in the goaf.

Analysis of simulation results of instability of immersed coal pillar

The evolutionary process of mechanical parameters in water-immersed coal columns

As can be observed from Fig. 4, there is a clear relationship between the tensile strength, moisture content, and soaking time of the coal sample after being submerged in water. Overall, as the soaking time increases, the tensile strength exhibits a gradually decreasing trend. Notably, the downward trend is more pronounced during the first 25 days of soaking. Conversely, the moisture content demonstrates a gradually increasing trend, with a more significant upward trend observed within the first 5 days of soaking. Based on the analysis presented in Fig. 4, it is evident that the original compressive strength of the coal column is 4.042 MPa. After 25 h of soaking, the compressive strength decays to 2.087 MPa, which corresponds to 48.4% of the tensile strength of the dried coal sample specimen. From day 25 to day 40, the variation in the compressive strength of the water-immersed coal column is 0.072 MPa. Additionally, the moisture content of the coal column changes by 9.46% after 25 days, while the moisture content change value for the water-immersed coal column from day 25 to day 40 is 0.85%. These results indicate that at this stage, the coal pillar has reached its mechanical attenuation value, and the moisture content has stabilized. Based on the mechanical parameters of the water-immersed coal column and the temporal trend of moisture content changes, relevant formulas were fitted and are presented in Eqs. (17) and (18).

17

18

Analysis of the reasons for the significant reduction in compressive strength of water-immersed coal samples: Within the water-immersed coal particles, the presence of water acts as a lubricant, diminishing the inter-particle interactions and thereby leading to a substantial decrease in compressive strength. This suggests that the instability of the mechanical properties of water-immersed coal masses is closely associated with the moisture content of the coal sample³¹.

The evolutionary process of water-saturated coal pillar minerals

Quantitative analysis of the variations in mineral composition revealed a dynamic transformation process, as depicted in Fig. 9. The coal sample primarily consists of quartz, dolomite, clay minerals (kaolin and illite), calcite, and pyrite. Upon increasing the soaking time, distinct trends were observed: quartz exhibited an initial decrease followed by an increase; clay minerals demonstrated an initial increase followed by a decrease; calcite showed a gradual decline; and pyrite displayed a steady increase. These changes can be attributed to the chemical reactions between calcite and dolomite with hydrogen ions ( ) in the solution, as detailed in Eq. 19. Simultaneously, carboxyl and phenolic groups within the coal sample adsorb water, generating weak acids that accelerate the dissolution of calcite and dolomite. Quartz, being chemically stable, generally resists acid-induced reactions. However, prolonged soaking induces alterations in the solution’s pH, leading to partial dissolution of quartz and a subsequent reduction in its content. Clay minerals are susceptible to ion exchange and hydration-induced expansion, which initially enhances their abundance (as shown in Eq. 20). As the solution transitions from a weakly acidic to a weakly alkaline environment, clay minerals react with hydroxide ions ( ), thereby reducing their overall content, as illustrated in Eq. 21.

19

20

21

Fig. 9 [Images not available. See PDF.]

Alterations in the chemical composition of water-imbibed coal columns.

Based on the aforementioned analysis, it is evident that the coal pillar immersed in the soaking solution undergoes a series of physical and chemical reactions as the composition of the soaking solution changes. Under the influence of water, processes such as disintegration, fragmentation, and consolidation occur within the coal pillar’s constituents, leading to the redistribution and enlargement of internal pores. This phenomenon significantly affects the mechanical properties of the coal pillar. Furthermore, this finding suggests that the softening of the coal pillar is closely associated with alterations in its mineral composition.

The chemical evolution process of water in water-immersed coal columns

Based on the water chemical analysis, Fig. 10 illustrates the variation of values with soaking time. The value increased from an initial of 5.8 to 7.47, transitioning from weakly acidic to weakly basic conditions. To analyze the reasons for this change, it is observed that as the soaking time progresses, hydrogen ions in the solution participate in reactions such as ion exchange and hydrolysis. This phenomenon aligns with the alteration in mineral composition of the coal column after soaking. From the analysis of the relationship between soaking time and values, it is evident that after 20 days of soaking, the values increase in the solution slows down compared to the earlier stages, indicating a weakening and stabilization of the reactive capacity of the solution components. Furthermore, it should be noted that variations in values can exert a certain degree of “degradative” influence on the coal pillar.

The concentrations of the cations and in the solution exhibited an initial decrease followed by an increase. The trends of and generally increased over time; however, the variation in content was minimal. The anion and demonstrated an overall upward trend, with showing relatively stable levels. In contrast, and exhibited a decreasing trend. Based on the prior analysis of mineral composition, calcite was identified within the coal pillar. During the leaching process, the dissolution of calcite resulted in an increase in the concentrations of and . Pyrite, which contains trace amounts of sulfur, underwent oxidation and dissolved in the solution, forming . Consequently, the dominant anions in the solution shifted from and to . This phenomenon aligns with the compositional changes observed in the water-soaked coal pillar as previously discussed.

Fig. 10 [Images not available. See PDF.]

Alterations in the chemical composition of water-saturated coal columns. (a) Ph value, (b) variations in the concentrations of major chemical ions.

The evolutionary process of pores in water-immersed coal columns

The variations in porosity and permeability within the water-immersed coal column were systematically analyzed using CT scanning. As shown in Fig. 11, the porosity and permeability of the sample reached their maximum values after a soaking period of 5 days. Specifically, the porosity increased from 9.03 to 11.16% (a change of 2.13%) during the first 5 days, followed by an increase of 3.59% from day 5 to day 10. From day 10 to day 30, the porosity exhibited minimal variation, changing by only 0.56–0.54%, and from day 30 to day 40, the change was further reduced to 0.08–0.12%. The rate of porosity change during the initial 10-day period was significantly higher compared to subsequent stages, indicating that intense physical and chemical reactions occurred within the coal pillar during this time, leading to a marked increase in internal pore volume. In contrast, during the final 10 days, the internal reactions within the coal pillar stabilized, resulting in little to no change in porosity. The trend of permeability variation closely mirrored that of porosity.

Fig. 11 [Images not available. See PDF.]

Investigation of alterations in the pore structure of water-immersed coal columns.

The failure and evolutionary process of coal pillar overburden

Based on the results of similar simulation experiments, it is evident that the coal pillar protected by the retained strip (hereafter referred to as the coal pillar) undergoes three distinct stages, as illustrated in Fig. 12. According to the analysis presented in Fig. 12, as the soaking time of the coal pillar increases, the coal pillars adjacent to the strip goaf gradually soften. The initial softening and instability of the coal pillar occur, leading to shear failure. Subsequently, further softening of the coal pillar causes shearing and collapse of the overlying rock strata. The fracture zone within the goaf develops upward toward the protective coal pillars on both sides. The increased span of the collapse zone results in elevated pressure on the rock strata, thereby promoting the upward development of the fracture zone induced by coal seam mining. With prolonged soaking time, continuous softening of the coal pillar leads to secondary softening and instability, with a greater degree of softening compared to the previous stage. Fractures in the overlying rock develop alternately in horizontal and vertical directions, causing the failure range of the overlying rock to expand progressively. This expansion further enlarges the goaf space, leading to the fracture of the simply supported masonry beam. Over time, the coal pillars on both sides soften gradually due to water flow erosion, culminating in external-to-internal shear damage of the strip coal pillar. The fractures in the overlying rock of the coal pillar intersect, exacerbating the damage range and inducing surface settlement.

Fig. 12 [Images not available. See PDF.]

Similar simulation process for softening the coal pillar. (a) The first softening and instability of the coal pillar, (b) the second softening and instability of the coal pillar, (c) the third softening and instability of the coal pillar.

The strain variations of the softened overburden in coal pillars were monitored using the contact video strain-displacement precision measurement system. Based on the monitoring data, Fig. 13 was generated. Analysis indicates that the overlying strata are continuously exposed to water saturation in the gob area, leading to repeated cycles of damage, deterioration, and reactivation in the coal (rock) pillar. The damage is primarily concentrated in the strip mining working face and above the softened coal pillar, with its extent significantly expanding both horizontally and vertically.

Under water immersion, the coal pillar exhibits a “damage-deterioration-activation” phenomenon. In terms of spatial distribution and temporal evolution, the following patterns are observed:

From a temporal perspective, as the coal pillar softens, the overlying strata undergo uneven settlement. The movement of the overlying rock in the goaf area displays a wavy pattern, with pronounced settlement characteristics of the coal pillar. Spatially, transverse upper fissures and vertical fissures develop alternately, forming a network-like fracture structure. This fracture network propagates continuously upward from a lower position, and the volume of the goaf space gradually increases. Upon the first softening of the coal pillar, the wavy subsidence of the overlying rocks on both sides of the strip goaf became more evident. The maximum subsidence values on both sides of the strip goaf increased to 0.32 m and 0.27 m, respectively. The settlement above the coal pillar was measured at 0.23 m. The subsidence of the overlying rocks far from the goaf formed an overall subsidence basin, with a maximum subsidence value of 0.23 m, indicating that the coal pillar supported the load of the overlying strata. During the second softening of the coal pillar, the wavy subsidence above the goaf became more pronounced. The maximum subsidence values on both sides of the strip goaf increased to 0.38 m and 0.32 m, respectively. The settlement above the coal pillar was measured at 0.25 m. The subsidence of the overlying rocks far from the goaf presented an overall subsidence basin, with a gradual increase in subsidence. The maximum subsidence value of the subsidence basin reached 0.25 m. This indicates that the coal pillar continued to soften, leading to a decrease in internal friction angle and cohesion, which caused instability and settlement of the overlying strata. Upon the third softening of the coal pillar, the wavy subsidence of the overlying strata on both sides of the strip goaf became even more significant. The maximum subsidence values on both sides of the strip goaf increased to 0.50 m and 0.43 m, respectively. The settlement above the coal pillar was measured at 0.37 m. The overlying rocks far from the goaf subsided, forming an overall subsidence basin with a significant increase in subsidence. The maximum subsidence value of the subsidence basin reached 0.37 m. At this stage, the coal pillar experienced shear failure, continued softening, and induced overall settlement of the overlying strata. In summary, the movement and deformation of the overlying strata near the goaf area exhibit a wavy subsidence basin, with a subsidence basin directly above the strip goaf and overall subsidence above the strip coal pillar. The movement and deformation of the overlying strata far from the goaf area present an overall subsidence basin. The subsidence basin is centered on the coal pillar, with greater subsidence in the middle and less on both sides.

During the mining-induced overburden movement, water immersion leads to damage in the coal pillar, initiating an instability process characterized by the sequence of “articulated rock beam → primary fracture → secondary fracture” within the key stratum. The lithological differences between the upper and lower sections of this key layer, along with variations in fracture propagation paths, result in significantly uneven subsidence among Line C and Lines A and B. Specifically, the subsidence observed along Line C is markedly increased due to the combined effects of “lag superposition” during secondary fracturing and the “deformability” of the overlying weak rock formations. Concurrently, the degradation of the coal pillar integrity may lead to dual disturbances in monitoring, including failure of monitoring points and data distortion, particularly in flash spot monitoring systems.

Fig. 13 [Images not available. See PDF.]

Monitoring of overlying rock settlement following coal pillar softening. (a) Buckling strain diagram for the first softening of coal pillar, (b) buckling strain diagram for the second softening of coal pillar, (c) buckling strain diagram for the third softening of coal pillar.

In order to further investigate the evolution of the coal pillar during its instability phase, specific measurement lines and points were selected for detailed data analysis (as shown in Fig. 14). The results indicate that under water immersion conditions, the coal pillar undergoes gradual softening, leading to instability. Consequently, the water-soaked coal pillar loses its ability to support the overlying rock strata, causing the load to transfer to both sides of the gob area. Simultaneously, stress concentration occurs in the coal pillars on either side. As soaking time increases, the mechanical strength of the coal pillar progressively weakens until it eventually fails and fractures, thereby inducing settlement of the overlying rock strata. The stepwise settlements of the overlying rock are measured as 0.032 m, 0.068 m, and 0.142 m, respectively. In summary, the coal pillars in the water-soaked zone exhibit a gradual and sequential failure pattern from the outer regions toward the inner regions, with each fracture event causing corresponding stepwise settlement of the overlying rock.

Fig. 14 [Images not available. See PDF.]

Settlement curve of monitoring points.

Softening and instability of the strip coal pillar induced by water immersion

According to the theoretical formula calculations, as well as the results from indoor tests and similar simulation experiments, it has been determined that the effective support width of the coal pillar is influenced by the moisture content, soaking duration, and water pressure within the goaf. Through theoretical analysis, it can be concluded that the instability of the coal pillar is associated with its shear-softening behavior. The evolutionary characteristics of the softening failure of the coal pillar are illustrated in Fig. 15.

During the coal pillar soaking process, as depicted in Fig. 15 (a), it undergoes ion exchange, chemical reactions, and pyrite oxidation. Initially, the coal pillar experiences shear failure. Water infiltrates from the goaf into the coal pillar, leading to the initial soaking of the lower portion of the coal pillar. As soaking time progresses, porosity and permeability increase, thereby expanding the soaking area of the coal pillar in this region and further inducing its softening. Under the combined influence of water pressure and repeated disturbances, the coal pillar undergoes degradation, softening, fracturing, and instability.

Through comprehensive analysis and research on the composition, water chemical characteristics, and microscopic pore structure of the coal column, it was observed that during the immersion process, mineral components undergo physical and chemical reactions with and ions in the solution, leading to fluctuations in pH values. These reactions result in a gradual reduction of both compressive and tensile strengths of the coal column, ultimately causing its failure under hydro-mechanical stress. Further investigation into the deterioration mechanism reveals that calcite within the coal pillar reacts with ions, disrupting the framework structure composed of rock-forming minerals. Simultaneously, and ions are exchanged by , increasing their concentration in the solution. However, as the ion exchange progresses, the exchange capacity diminishes over time. Additionally, sulfides within the coal react with ions, dissolving in water and elevating the concentration of ions. Under the influence of both chemical and physical reactions, inter-particle pores in the coal column become interconnected, significantly developing the fracture network. Micro-fractures form, propagate, expand, and communicate with each other, intensifying the development of the fracture grid and thereby compromising the structural integrity of the coal column. As soaking duration and area increase, clay minerals (kaolinite and illite) in the coal pillar swell upon contact with water, and particle migration re-closes fractures, reducing porosity and permeability. With rising pH levels, ions in the solution participate in reactions, reopening previously sealed fissures. This process facilitates continued fracture development until they become interconnected.

During the water-coal reaction process, after the pores and internal structure of the coal sample reach the saturation stage, hydrogen ( ) and hydroxide ( ) fully react to achieve stability, gradually causing the pH value of the soaking solution to tend toward neutrality or alkalinity. Over time, the water absorption rate, mineral composition, and pore structure of the coal pillar stabilize, leading to a gradual deceleration in the propagation rate of fractures. Due to the expansion of fractures within the coal pillar, the local framework structure of the coal weakens, and the interaction between water and mineral components continuously alters the strength and deformation characteristics of the coal sample. The coal column undergoes long-term water immersion and deterioration, resulting in continuous expansion and development of its internal pore and fracture structures. Changes in mineral composition and solution environment lead to a softening phenomenon in the mechanical properties of the coal. Ultimately, this results in the deterioration, instability, and failure of the coal column.

The aforementioned analysis demonstrates that the deterioration, softening, instability, and failure of the coal column occur progressively from the exterior to the interior, with the lower portion initiating first. The seepage volume at the lower part exceeds that at the upper part, as illustrated in Fig. 15(b). Under the influence of water seepage in the goaf, the elastic modulus, cohesion, and internal friction angle of the coal pillar decrease, reducing the effective support load width of the coal pillar. Shear failure of the coal pillar continues to develop upward, further softening the coal pillar while increasing the softening distance and decreasing the effective support width, as depicted in Fig. 15(c).

As the soaking time of the coal pillar progresses, under the combined effects of permeable water pressure and roof load, accumulated water infiltrates toward the top interface and both sides of the coal pillar along coal seam pores or fractures. At a certain stage of soaking, the effective support width of the coal pillar becomes insufficient to sustain the overlying load, leading to uneven stress distribution at the top interface of the coal pillar. This causes reverse fractures at the top interface and subsequent settlement of the overlying rock layer, as shown in Fig. 15(d). Under the synergistic effects of osmotic water pressure, softening of the immersed coal column, and creep, accumulated water further penetrates into the deeper regions of the coal column. With time, the softening and deterioration intensity of the water-immersed coal and rock continue to increase. The coal pillar exhibits softening and deterioration damage, with the damaged area extending deeper into the coal pillar. This reduces the effective support width, making it insufficient to bear the top load, ultimately causing the overall collapse of the overburden rock, as illustrated in Fig. 15(e). This further elucidates the mechanism of coal pillar failure from the microscale to the macroscale.

The entire process of coal pillar degradation, from initial damage to eventual instability, is dynamically governed by multiple interacting factors. This process can be categorized into three distinct stages: the initial damage stage, the deterioration development stage, and the structural instability stage. In the initial damage stage, external shear stress and seepage field characteristics lead to water infiltration in the goaf area, resulting in the first instances of damage and saturation at the lower portion of the coal pillar. During the deterioration development stage, chemical interactions between the coal pillar minerals and water occur, including processes such as calcite dissolution, ion exchange, and pyrite oxidation. Simultaneously, pores and fissures undergo cyclic opening and closing influenced by variations in environmental parameters, such as solution pH. In the structural instability stage, under the combined influence of osmotic water pressure and roof loading, the mechanical properties of the coal pillar progressively deteriorate, leading to a continuous reduction in its effective supporting capacity. The damaged zone expands gradually from the outer regions toward the interior. When the remaining effective support width is no longer sufficient to withstand the overburden load, structural failure ensues, potentially triggering collapse of the overlying strata. Overall, this progressive failure mechanism represents a dynamic evolutionary process driven by the coupling of multiple physical and chemical fields, including stress, seepage, chemical reactions, and damage.

Fig. 15 [Images not available. See PDF.]

The dynamic evolutionary process of coal pillar softening and instability. (a) Local microscopic changes of coal pillar, (b) shear failure of coal pillar, (c) water softening of coal pillar-collapse, (d) pillar softening-creep, (e) overall instability of the coal pillar.

Discussion

Taking Mine A of the North China Xing coalfield as a case study, this paper conducts research through laboratory testing and theoretical derivations. It analyzes the mechanism of deterioration, softening, and instability of strip coal pillars in goaf areas, as well as the microscopic changes in water-soaked coal pillars. The study achieves the visualization of softening deformation in strip coal pillars and establishes guidelines for setting effective coal pillars for strip mining, providing significant guidance for practical applications in strip mining. The deterioration and instability of water-immersed coal pillars result from the combined effects of multiple factors, with complex reaction degrees and mechanisms. This paper provides an initial exploration of this intricate process.

The preliminary research and experimental findings presented in this paper indicate that the degradation of coal pillars is associated with the physical properties of coal and the pH of the soaking solution. From the perspectives of mechanical characteristics, hydrochemical properties, and microscopic analysis, it is demonstrated that coal pillars are influenced by multiple factors in the soaking environment. These factors include: the intrinsic physical properties of the coal pillar, such as its composition and the development of primary pores and fractures; soaking duration, where prolonged exposure leads to increasingly interconnected fractures, thereby reducing mechanical strength until reaching a maximum softening point beyond which no further changes occur; and the hydrochemical characteristics of the soaking solution, wherein varying solutions induce differing extents of physicochemical reactions, fracture propagation, and mechanical deterioration of the coal pillar^30,32,33.

In this experiment, the microscopic analysis of the fracture connectivity in water-immersed coal columns was conducted solely based on porosity and permeability. Future studies should incorporate additional perspectives, such as pore volume, specific surface area, pore size distribution, and surface fractal dimension of coal columns, to comprehensively evaluate the connectivity between pore throats and pores. Furthermore, a deeper investigation into the quantitative and qualitative multi-scale transformation characterization linking the microscopic pore structure with macroscopic mechanical properties is warranted.

The coal pillar utilized in this experiment was sourced from the Carboniferous-Permian coal mine A of the North China type. The origin of the coal pillar was relatively uniform but exhibited certain randomness and limitations. Future research should focus on investigating the deterioration of coal pillars under varying degrees of water accumulation and diverse water accumulation environmental conditions in other North China type coalfields, thereby establishing the relationship between mechanical deterioration damage of water-immersed coal pillars and pore characteristics.

This paper primarily investigates the entire process of coal pillar damage, deterioration, and instability. Future research can be extended in the following directions: First, further investigation into the modification of coal pillar reinforcement materials is recommended³⁴. By leveraging the resource utilization of industrial solid waste and advanced composite material modification technologies, novel support materials with enhanced water resistance and anti-deterioration properties can be developed, thereby improving the durability of coal pillars against water erosion. Second, it is essential to deepen the understanding of the synergistic mechanism between the coal pillar and the support system³⁵. Integrating the newly proposed method for determining hydraulic support resistance with the progressive deterioration of the coal pillar can facilitate the establishment of a precise calculation model that considers the dynamic attenuation of mechanical parameters within the coal pillar. Third, future studies should focus on the dynamic response of the coal pillar-support system under multi-field coupling conditions. A comprehensive consideration of the spatio-temporal evolution of multiple factors—including stress, seepage, and chemical interactions—is necessary to construct an integrated theoretical and technical framework—from material performance optimization to engineering support design. This would provide a more robust foundation for the safe and efficient extraction of coal resources.

Conclusion

This paper investigates the damage and deterioration mechanisms of submerged coal pillars. By integrating laboratory experiments, similarity simulations, and theoretical analyses, it systematically elucidates the entire process of coal pillar degradation and instability, achieving the following innovative outcomes:

(1) Coal samples were collected from A Coal Mine in the North China Coalfield, and coal pillar specimens were immersed in raw ore mine water. The experimental results indicated that the composition of the coal pillars and the pH value of the soaking solution are closely related to the degree of deterioration. Through laboratory testing, changes in the chemical composition, water chemistry, and porosity of the coal pillars over time were identified. The damage and deterioration mechanism of the soaked coal pillars was summarized into three distinct stages: the initial damage stage, the deterioration development stage, and the structural instability stage.

(2) The indoor test results of the coal column indicate that the uniaxial compressive strength decreased from 4.042 MPa to 2.015 MPa over a period of 0 to 40 days, representing a 50% reduction. The pH value shifted from weakly acidic to weakly alkaline. In the solution, the concentration of and ions increased by 8.67 mg/L, the concentration of ions increased by 6.39 mg/L, while the concentrations of and ions decreased by 13.38 mg/L. Additionally, the concentration of ions increased by 14.96 mg/L. During the soaking process in mine water, the mineral composition of the coal pillar underwent significant changes. The water absorption and expansion behavior of clay minerals, as well as the “damage-deterioration” phenomenon observed in the later stages of the coal sample, were markedly reduced. The porosity exhibited a trend of first increasing, then decreasing, and subsequently increasing again. This further suggests that the “deterioration-damage” of the coal pillar is closely associated with its mineral composition and value. These findings elucidate that the microstructural evolution and fracture development of the coal pillar contribute to changes in its macroscopic characteristics to a certain extent.

(3) By incorporating appropriate water-blocking additives into analogous materials and investigating the dynamic process of deterioration, softening, and instability in water-immersed coal pillars, the issues of water accumulation and coal pillar softening were effectively addressed. Furthermore, the dynamic visualization of the softening and instability processes in water-immersed coal pillars was successfully achieved. The results of similar simulation experiments reveal the following patterns regarding spatial distribution and temporal evolution: In terms of temporal evolution, as the coal pillar undergoes softening, the overlying rock experiences uneven settlement. The movement of the overlying rock in the goaf area exhibits a wavy pattern, and the settlement characteristics of the protective coal pillar are pronounced. Spatially, transverse upper fissures and vertical fissures develop alternately, forming a network structure of fractures. This fracture network progressively extends upward from a lower position, and the volume of the goaf space also demonstrates a gradually increasing trend. Following water accumulation in the goaf area, the coal pillars on both sides of the strip goaf are further weakened by factors such as water pressure, overburden pressure, and water-induced softening. This leads to shear failure of the coal pillars from the exterior toward the interior, resulting in longitudinal and transverse fractures in the protective coal pillars. Due to the reduction in effective support width, the stress in the overlying rock is redistributed, causing further uneven settlement of the overlying rock.

(4) Based on theoretical research that considers the dual effects of overlying rock and water pressure, a calculation formula for the effective support width of water-immersed coal pillars was derived. Through this theoretical formula, the factors influencing the effective support width of coal pillars were systematically analyzed. The coefficient of friction was set to 0.53, the coal seam thickness to 2.6 m, the burial depth to 550 m, and the water pressure to 0.32 MPa, among other related parameter values. When the maximum distances for the complete softening of the 2.6 m coal pillar are 9.34 m and 10.77 m, respectively, the coal pillar undergoes sequential softening from the outer regions inward and from the lower sections upward, leading to a continuous decrease in the effective support width. This ultimately results in the overall instability of the overlying rock structure. Furthermore, the physical and chemical evolution mechanism of “deterioration-instability” for water-soaked coal pillars in goaf areas was clarified. The characterization of internal pore structures within the mineral composition of coal pillars under weakly acidic conditions was revealed, and the dynamic process of damage-deterioration-instability of coal pillars was explored from both macroscopic and microscopic perspectives.

(5) This study focuses exclusively on the A mine of the Carboniferous-Permian coal series in the North China type. Future research should investigate the deterioration of coal pillars under varying degrees of water accumulation and diverse hydrological environmental conditions, with the objective of elucidating the relationship between mechanical degradation damage in submerged coal pillars and pore structure evolution.

Author contributions

Author Contributions Statement Xu Wang and Min Cao were primarily responsible for drafting the manuscript. Shangxian Yin and Huiqing Lian contributed to the development of writing ideas and methodologies. Enke Hou conducted the scientific review of the paper. Qixing Li and Xiangxue Xia finalized the layout of the figures and tables. Tao Yan , Sihai Yi, and Haorui Wang provided technical support for the experimental section. All authors reviewed the manuscript.

Funding

Financial support by the National Key R&D program of China (No. 2024YFC3013802); Hebei Natural Science Foundation Project (No. D2025508011); The Fundamental Research Funds for the Central Universities (No. 3142025007) are gratefully acknowledged.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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