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Article

Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam

by
Ersheng Zha
1,
Mingbo Chi
1,*,
Zhiguo Cao
2,
Baoyang Wu
2,
Jianjun Hu
3 and
Yan Zhu
1,*
1
China Academy of Safety Science and Technology, Beijing 100012, China
2
State Key Laboratory of Water Resource Protection and Utilization in Coal Mining, Beijing 102299, China
3
State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering, Shenzhen University, Shenzhen 518060, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(14), 8012; https://doi.org/10.3390/app15148012
Submission received: 15 June 2025 / Revised: 6 July 2025 / Accepted: 14 July 2025 / Published: 18 July 2025
(This article belongs to the Section Civil Engineering)
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Abstract

This study conducted uniaxial creep tests on coal samples under both natural and water-saturated conditions for durations of about 180 days per sample to study the stability of coal pillar dams of the Daliuta Coal Mine underground reservoir. Combined with synchronized acoustic emission (AE) monitoring, the research systematically revealed the time-dependent deformation mechanisms and damage evolution laws of coal under prolonged water immersion and natural conditions. The results indicate that water-immersed coal exhibits a unique negative creep phenomenon at the initial stage, with the strain rate down to −0.00086%/d, attributed to non-uniform pore compaction and elastic rebound effects. During the steady-state creep phase, the creep rates under water-immersed and natural conditions were comparable. However, water immersion led to an 11.4% attenuation in elastic modulus, decreasing from 2300 MPa to 2037 MPa. Water immersion would also suppress AE activity, leading to the average daily AE events of 128, which is only 25% of that under natural conditions. In the accelerating creep stage, the AE event rate surged abruptly, validating its potential as an early warning indicator for coal pillar instability. Based on the identified long-term strength of the coal sample, it is recommended to maintain operational loads below the threshold of 9 MPa. This research provides crucial theoretical foundations and experimental data for optimizing the design and safety monitoring of coal pillar dams in CMURs.

1. Introduction

Coal dominates energy production and consumption in China. However, coal mining leads to groundwater contamination and depletion, with approximately 60% of mine water remaining underutilized annually [1]. This contradiction is particularly pronounced in western China’s major coal-producing regions in north-western China, which contribute 70% of the nation’s coal output yet possess only 6.7% of the country’s water resources [2]. To address the strategic needs of Western economic development and national priorities, Academician Gu Dazhao pioneered a key technological framework centered around the principle of “diversion, storage, and utilization”, which encompasses the design, construction, operation, and safety monitoring of coal mine underground reservoirs (CMURs) [2]. The technology utilizes goafs as storage spaces and constructs underground reservoirs by connecting artificial dams with coal pillar dams. Implemented for the first time in the Shendong Mining Area, 35 CMURs have been established [3], ensuring the synergistic utilization of coal resources and mine water in western China.
CMURs consist of two main structural components as the main water barriers: coal pillar dams and artificial dams. Coal pillar dams serve as the primary water-retaining structures. These dams undergo stress loading–unloading adjustments during mining disturbances and are subjected to coupled influences from multiple factors during long-term operation. These factors include in situ geological conditions, ground stress, mine pressure, and dynamic disturbances such as earthquakes or mining tremors. Consequently, the stability of coal pillar dams and surrounding coal–rock masses has become a critical determinant for the long-term safety of CMURs. Existing research has extensively investigated the hydro-mechanical coupling behavior of coal pillar dams and coal–rock masses in CMURs. Studies have examined their mechanical responses, failure characteristics, and fracturing patterns under conditions of different moisture contents [4,5], water immersion pressures [6], and dry–wet cycles [7]. Further research has explored the deformation behavior of coal pillar dams under long-term water immersion [7] and established failure criteria for coal–rock masses accounting for moisture content effects [4]. Concurrently, by numerical modeling [8], physical simulation with similar materials [9], and theoretical modeling, scholars have investigated how mining-induced disturbances affect coal pillar mechanics and failure mechanisms. For example, Wu Qiang et al. [10] integrated the pressure arch theory with traditional safety factor design principles based on the mechanical properties of yielded coal pillars, thereby proposing a design method for room and pillar water-preserved mining pillars.
As the primary load-bearing structure of CMURs, coal pillar dams may experience disasters such as reservoir leakage and support failure under prolonged high in situ stress conditions. Furthermore, the mechanical properties of coal and rock masses undergo further degradation due to long-term immersion in reservoir water. Conducting long-term creep tests under single-stage loading is essential for understanding the long-term mechanical behavior of coal pillar dams and analyzing their stability characteristics. Existing research has systematically investigated the long-term creep behavior of rocks under single-stage loading. For example, Itô and Sasajima [11] conducted a decade-long field creep test on granite and gabbro, revealing the significant influence of temperature fluctuations on long-term deformation. Bérest et al. [12] performed a 650-day creep test on halite, documenting an extremely slow creep rate of 7 × 10−13. Okubo et al. [13] executed a 12-year creep experiment on water-saturated tuff, establishing relationships between creep strain, strain rate, and time. Lyu et al. [14] carried out an 875-day salt rock creep test, uncovering differential initial creep characteristics under varying stress levels. Additionally, as a typical anisotropic soft rock, coal has attracted considerable academic attention regarding its pronounced time-dependent characteristics. Zhou et al. [15,16,17] investigated the creep mechanical behavior of coal–rock under varying temperatures and pore pressures, establishing a creep mechanics model that accounts for coupled temperature–pore pressure effects in deep coal–rock masses. Furthermore, Liu et al. [18] examined the time-dependent creep mechanical behavior of rock masses following deep coal mining. However, critical knowledge gaps persist regarding single-stage coal-specific time-dependent fracture evolution under prolonged water immersion, particularly in CMUR operational contexts where coal pillars experience decades of saturation. The absence of acoustic emission (AE)-monitored long-term creep data for water-immersed coal impedes the accurate prediction of dam service life.
In summary, while significant progress has been made in research on coal pillar dams in CMURs and single-stage long-term creep experiments of rocks, there remains a notable gap in systematic studies concerning the time-dependent mechanical behavior and acoustic emission fracture patterns of coal pillar dams under prolonged water immersion. To address this research gap, this study conducts uniaxial single-stage long-term creep tests integrated with acoustic emission (AE) monitoring on water-immersed coal–rock specimens from the Daliuta CMUR project. The investigation analyzes the influence of long-term water immersion on the mechanical behavior of coal and rock masses and their pre-failure characteristics. This research establishes a methodological framework and provides experimental data to support the long-term mechanical analysis and stability assessment of CMURs in the Shendong Mining Area.

2. Methodology

2.1. Selection of Long-Term Loading Stress

As the primary underground load-bearing structure within CMURs, coal pillar dams feature sections perpetually submerged below water level, as shown in Figure 1. The reservoir water level fluctuates between 3 and 4 m through operational adjustments based on intake and discharge volumes rather than maintaining a constant height. Without specialized waterproof treatment, one side of these coal pillar dams remains under sustained water immersion. To evaluate their long-term stability, laboratory and field tests must be carried out to establish the relationship between the in situ stress states of coal pillar dams and their long-term strength and determine appropriate stress levels for designing long-term creep mechanical tests so as to investigate time-dependent mechanical response patterns during the sustained loading of coal pillar dams.
According to field borehole tests conducted by previous research among CMURs in the Shendong Mining Area [19], the in situ stress states of coal pillar dams in the regional CMURs range from 0 to 12 MPa. The testing results obtained by previous research [20] (Figure 2) on samples from the Daliuta CMUR showed that the uniaxial compressive strength (UCS) of Daliuta coal specimens under non-immersed conditions is approximately 15.5 MPa, while the UCS exhibits a distinct declining trend with increasing immersion duration.
Based on the mechanical characteristic curves under uniaxial conditions [20] and representative characteristic curves [21] for coal at Daliuta CMUR, it is acknowledged that the long-term strength of Daliuta coal pillar dams at failure is approximately 60% of its short-term strength (≈9 MPa). Considering the actual in situ stress conditions and detrimental factors affecting dam stability, the experimental load was set at 10 MPa (65% of short-term strength). Acoustic emission (AE) monitoring was implemented to capture and analyze time-dependent mechanical responses during prolonged creep deformation and the microcrack propagation process within the coal.

2.2. Testing Apparatus and Procedure

The creep specimens were obtained from Daliuta Coal Mine in the Shendong Mining Area (Yulin, China). Testing was performed using the long-term uniaxial creep testing machine with a maximum axial load capacity of 600 kN and specimen dimensions of φ50–100 mm × H100–200 mm. The constant-weight loading system was applied, ensuring constant stress during deformation. The concurrent acoustic emission testing was carried out by the PCI-II acoustic testing system manufactured by Physical Acoustics Company (Princeton Junction, NJ, USA), which has an 18-bit AD A/D converter and a 1 kHz−3 MHz bandwidth frequency range and can continuously record waveforms at 10 MS/s. The AE detection threshold is set at 24 dB to avoid the environmental noise affecting the effectiveness of the data.
To simulate the prolonged field immersion conditions of coal pillar dams, the test setup incorporated a water supply-regulation system with a reservoir chamber integrated into the loading base (Figure 3). This system maintained continuous water saturation throughout the creep deformation. Concurrently, the microcrack propagation dynamics during creep were monitored using the PCI-II Acoustic Emission System with synchronized data acquisition.
To investigate the long-term water immersion effects on the mechanical behavior of coal pillar dams, uniaxial creep tests were conducted on two standard coal specimens, one under both natural conditions and one under water-immersed conditions at a constant 10 MPa loading level over approximately six months. Coal specimens (φ50 mm × H100 mm) were drilled perpendicular to bedding from Daliuta CMUR coal seams. All samples met the ISRM standards for parallelism (±0.05 mm) and flatness (±0.02 mm). Water-immersed samples underwent vacuum saturation (48 h) to achieve full saturation. The experimental protocols were executed as follows:
Water-Immersed Sample: The experiment is carried out under continuous water immersion; the applied axial load level is set as 65% UCS (10 MPa). At 171.5 days, the axial load was stepwise loaded by one level (5% UCS) every 8 h. When the load level reached 100% UCS, the specimen completely failed under this load level.
Natural Sample: The specimen was not water-immersed. The axial load level is set as 65% UCS (10 MPa) and was tested for 213 days under this load level.

3. Results and Discussion

3.1. Time-Dependent Axial Strain

3.1.1. Water-Immersed Sample

The time–axial strain curve of the long-term immersed specimen is illustrated in Figure 4a, with a total loading duration of 174 days. The test results indicate that the creep curve of water-immersed coal–rock comprises three distinct stages:
Stage I: Transient and primary creep stage (0–40 d). The specimen rapidly entered the primary creep stage followed by immediate deformation stabilization. This transient creep stage was exceptionally brief (≈1 d). During early steady creep (0–30 d), the strain decreased progressively, meaning that the strain at 5.927 d is 0.491%, while at 34.074 d it is 0.4668%. The strain variation rate is calculated as −0.00086%/d.
Stage II: Steady-state creep stage (40–130 d). After Stage I creep completion, the strain–time curves transitioned from minor fluctuations to gradual growth. The overall curve exhibited steady creep deformation characteristics. The average strain rate is 3.04 × 10−6/d, indicating stable development of the sample.
Stage III: Accelerated creep stage (130–173 d). Following Stage II, the strain–time curves showed a sustained increase after initial rapid growth. The average creep strain rate is 0.00234%/d. At 171.5 days, axial load was increased incrementally at about 5% UCS ≈ 0.8 MPa per step every 8 h to show the accelerating creep process until rock failure (Figure 4b).
Analyzing the unique phenomenon of the long-term creep deformation process of coal under water immersion conditions, negative creep phenomenon occurred in the first stage of coal–rock. Negative creep is a property in which the axial strain of a material decreases with time. Negative creep is more common in metal and alloys under high-temperature and low-stress conditions [22,23,24,25]. The cause of this phenomenon is that the internal energy reduction in the material is greater than the external stress and is closely related to the initial strain and composition of the material [26]. In terms of rock materials, Tang [27] and Wang [28] et al. have observed the negative creep phenomenon in rock. It is believed that the negative creep phenomenon is induced by the fact that only part of the pores and voids of the structural rock are compacted under the low-stress condition during creep, and the other parts of the pores and voids are gradually compacted during the subsequent creep process, which causes the non-uniform deformation of the rock mass. The stress release phenomenon during this period is equivalent to the stress reduction, and the skeleton of rock produced elastic rebound, which is an abnormal phenomenon.
As Stage I creep approached completion (35–40 d), the coal sample exhibited a rapid increase in time-dependent deformation, indicating partial creep deformation compensation. During this period, uniform creep deformation occurred, with continuous growth transitioning into Stage II. At the onset of Stage III creep deformation, a minor accelerated deformation emerged, attributed to fragmented creep damage. Existing research demonstrates that rock failure under uniaxial compression primarily occurs through end-zone compression-induced tensile cracking [29]. Following prolonged creep–water immersion weakening, the localized disintegration of end-zone coal–rock material may occur [30], causing abrupt strain acceleration and propelling the specimen toward the accelerating creep stage.

3.1.2. Natural Sample

The long-term creep deformation results of the coal sample under natural conditions are shown in Figure 5. The results indicate that compared to water-immersed coal–rock, this specimen under natural conditions experienced a longer primary creep stage before entering the steady-state creep stage. The primary creep stage lasted approximately 36 days, with a strain rate of about 0.0127%/d, after which it entered the steady-state creep stage. During this stage, minor strain fluctuations occurred, but the overall strain variation was minimal, with a strain rate of approximately 3.0 × 10−6/d.
Comparing the long-term creep behavior between natural and immersed conditions, it was revealed that higher strain magnitudes are needed in natural condition coal samples for entering the steady-state creep stage. This phenomenon arises from two factors: On the one hand, as heterogeneous geological materials, coal specimens inherently exhibit strength variations. On the other hand, natural specimens lack bound water interactions. Being typical clay minerals, dehydration shrinkage generates contraction stresses that trigger microcrack network propagation. Previous studies using CT scanning [30,31,32] have investigated meso-structural differences between natural and saturated coal samples, finding increased mesopore proportions in drier specimens. These pores create stress concentration points that become prioritized failure paths under compression, significantly amplifying deformation. Concurrent research [33] demonstrates higher internal friction coefficients in drier coal specimens, accelerating elastic strain energy accumulation during loading. When reaching critical energy thresholds, abrupt energy releases manifest as “rockburst” failures, exhibiting stepwise strain jumps. In contrast, water-saturated specimens exhibit smoother deformation curves due to water’s viscous damping effect, which moderates energy release. This fundamentally explains the more pronounced fluctuation characteristics observed during creep experiments for natural specimens.

3.2. Instantaneous Loading Modulus

During loading, to investigate the time-dependent damage characteristics of water-immersed coal–rock under long-term creep deformation, we measured the loading modulus via instantaneous unloading–reloading cycles. Since unloading/reloading beyond the rock’s long-term strength may induce damage [34], the unloading magnitude was controlled to minimize impacts on time-dependent mechanical properties. Specifically, after sustained loading, specimens were instantaneously unloaded by 1 MPa and then reloaded to original stress levels while monitoring modulus evolution.
Figure 6 shows the loading modulus variation curves of rock during loading/unloading for natural and water-immersed coal samples. During the primary creep stage, the loading modulus increased for both, but the mechanisms differed: For the natural coal sample, in the early loading stage, pre-existing microcracks within the coal sample gradually closed under sustained load, increasing the contact area between particles and enhancing frictional resistance, thereby improving resistance to deformation [35,36]. This reflects the strain hysteresis of viscoelastic deformation, which gradually equilibrates over time, exhibiting hardening characteristics [37]. For water-immersed coal samples, during the primary creep stage, water’s lubricating effect further promotes the re-interlocking of fracture surfaces after slippage, accelerating crack closure [38].
During the steady-state creep stage, the loading modulus of both water-immersed and natural coal samples remained within a certain range. Comparing the steady-state creep stage, the loading modulus of the natural sample was larger than that of the water-immersed sample. The loading modulus of the natural sample was approximately 2300 MPa, while that of the water-immersed sample was approximately 2037 MPa. This indicates that for the Daliuta Coal Mine in the Shendong Mining Area, long-term water immersion reduces the mechanical performance of coal–rock by approximately 11.4%.

3.3. Acoustic Emission Characteristics

3.3.1. Water-Immersed Sample

During long-term creep testing, acoustic emission (AE) monitoring was synchronously employed to measure fracture patterns during coal–rock creep. Due to a prolonged test duration, only AE data from the initial 67 days were compiled, as shown in Figure 7. A total of 8508 AE events were recorded in this period. The analysis reveals pronounced AE activity during the initial testing phase (transient creep stage), with instantaneous AE ring-down counts reaching 1734 times per sec and AE energy peaking simultaneously. During the strain reduction phase (1–30 d), both AE ring-down counts and energy gradually increased over time. Conversely, the strain increase phase (30–40 d) exhibited minimal AE activity, indicating that compaction hardening between bedding planes dominated deformation without new fracturing. Steady-state creep displayed insignificant AE signals.
Investigating AE ring-down count and energy evolution, it is revealed that in the transient creep stage (1 d), the closure of pre-existing fractures and stress concentration induced high-frequency AE signals [39]. Throughout the strain reduction phase (1–30 d), water-induced non-uniform weakening triggered localized fracturing, progressively increasing AE activity. During the strain hardening phase (30–40 d), internal compensation mechanisms suppressed AE generation as specimens underwent structural reorganization into steady deformation. Subsequent AE events displayed low-energy characteristics, signaling stabilized deformation progression.

3.3.2. Natural Sample

To investigate the full compression failure process of natural coal–rock under a single stress level, the natural coal sample was selected for creep acoustic emission testing and compared with the water-immersed sample. The results are shown in Figure 8. During the entire loading process, 11,499 AE events were recorded. Pronounced AE activity occurred in the initial testing phase (transient stage). Upon entering the steady-state creep stage, both AE ring-down counts and energy levels were low, indicating weak acoustic emission. During rapid load increase, AE activity intensified significantly.
Comparing the AE characteristics between natural and immersed conditions, common patterns exist in both conditions: AE signals peaked during transient creep, diminished in steady-state creep, and then reactivated when approaching accelerated creep. Notably, AE energy shifts markedly increased before accelerated creep. This validates AE feature metrics as indicators for identifying creep stages and providing early warning for accelerated creep in coal–rock pillars.
However, crucial differences exist between two conditions. The water-immersed sample exhibited lower AE event rates of 127 events per day, compared with 500 events per day for natural samples. It should be noted that these reported event rates represent the directly measured experimental values during the testing period. Statistical analyses to quantify uncertainty and variability, such as calculating confidence intervals, represent an important focus for our subsequent investigations. This divergence arises from water-filled pores and fractures reducing the stress concentration at crack tips. Prolonged immersion also induces non-uniform expansion, where internal damage releases energy gradually rather than through sudden AE events [40]. Furthermore, the heterogeneous deterioration from water–rock interactions triggers non-uniform deformation during long-term creep, necessitating the continuous monitoring of dam deformation and targeted engineering interventions at Daliuta CMURs.

4. Conclusions

Based on the Daliuta CMUR engineering project, this study collected coal samples from the reservoir area and conducted long-term creep mechanical behavior tests for about 180 days per specimen under both natural and water-immersed conditions. We analyzed differences in creep behavior, mechanical strength parameters, and microfracture evolution between natural and saturated coal–rock specimens, providing fundamental mechanical parameters and analytical foundations for long-term stability assessments of CMURs. Further validation, including in situ stress field monitoring and simulation, depth-dependent mechanical, seepage, and AE signature across diverse coal mining regions in Xinjiang and Shanxi, is suggested. Additionally, the generalized coal creep model considering the long-term water immersion effect is one of our further future research targets. The key conclusions are summarized below:
(1)
Physical–chemical water effects significantly alter early-stage deformation mechanisms. During the initial phase (0–40 days) for immersed specimens, negative creep occurred with strain rates as low as −0.00086%/d. This phenomenon correlates with non-uniform pore compaction and elastic rebound. Water infiltration promoted the continuous low-stress compaction of partially unclosed pores, triggering localized stress release and skeletal elastic recovery, thereby causing anomalous strain retraction, which is a behavior unobserved in natural condition specimens.
(2)
Water immersion exerts continuous deterioration effects on coal–rock throughout long-term creep. Although steady-state creep rates under both natural and saturated conditions were similar (≈3.0 × 10−6/d), instantaneous unloading tests revealed a 11.4% reduction in loading modulus for saturated specimens (2037 MPa) compared to natural samples (2300 MPa). This discrepancy stems from water molecules softening mineral particles. Concurrently, due to water filling and saturation effects, the daily AE event rate under immersion condition (127 events/day) was significantly lower than under natural conditions (500 events/day), indicating that water immersion suppresses AE generation and alters damage accumulation patterns.
(3)
For Daliuta CMUR, it is recommended that actual dam loads be controlled below 9 MPa to prevent accelerated creep. Although negative creep in water-saturated coal pillars temporarily mitigates deformation, continuous elastic modulus degradation necessitates higher safety redundancy in design. Microseismic monitoring should supplement real-time stability observations, and sudden energy surges or sustained event rate increases should be treated as accelerated creep precursors, which requires emergency engineering measures.

Author Contributions

Conceptualization, E.Z. and M.C.; methodology, E.Z.; validation, E.Z. and Y.Z.; formal analysis, E.Z. and B.W.; data curation, Z.C. and J.H.; writing—original draft preparation, E.Z.; writing—review and editing, M.C.; visualization, Z.C. and B.W.; supervision, M.C. and Z.C.; funding acquisition, E.Z. and M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Deep Earth Probe and Mineral Resources Exploration-National Science and Technology Major Project (No. 2024ZD1004505), Beijing Natural Science Foundation (No. 8254049), the National Natural Science Foundation of China (No. 52374139), and the Fundamental Research Funds for China Academy of Safety Science and Technology (No. 2024JBKY07 and No. 2025JBKY05).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions of the study are included in the article, and further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of the CMUR and structure.
Figure 1. Schematic diagram of the CMUR and structure.
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Figure 2. UCS of Daliuta coal samples at different immersion times [20].
Figure 2. UCS of Daliuta coal samples at different immersion times [20].
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Figure 3. Schematic of the uniaxial creep testing apparatus for water-immersed coal sample.
Figure 3. Schematic of the uniaxial creep testing apparatus for water-immersed coal sample.
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Figure 4. Long-term creep deformation characteristics of coal under water-immersed conditions.
Figure 4. Long-term creep deformation characteristics of coal under water-immersed conditions.
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Figure 5. Long-term creep deformation characteristics of coal under natural conditions.
Figure 5. Long-term creep deformation characteristics of coal under natural conditions.
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Figure 6. Loading modulus variation curves under both natural and water-immersed conditions.
Figure 6. Loading modulus variation curves under both natural and water-immersed conditions.
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Figure 7. Creep acoustic emission features of water-immersed coal sample.
Figure 7. Creep acoustic emission features of water-immersed coal sample.
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Figure 8. Creep acoustic emission characteristics of natural coal–rock specimens.
Figure 8. Creep acoustic emission characteristics of natural coal–rock specimens.
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MDPI and ACS Style

Zha, E.; Chi, M.; Cao, Z.; Wu, B.; Hu, J.; Zhu, Y. Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam. Appl. Sci. 2025, 15, 8012. https://doi.org/10.3390/app15148012

AMA Style

Zha E, Chi M, Cao Z, Wu B, Hu J, Zhu Y. Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam. Applied Sciences. 2025; 15(14):8012. https://doi.org/10.3390/app15148012

Chicago/Turabian Style

Zha, Ersheng, Mingbo Chi, Zhiguo Cao, Baoyang Wu, Jianjun Hu, and Yan Zhu. 2025. "Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam" Applied Sciences 15, no. 14: 8012. https://doi.org/10.3390/app15148012

APA Style

Zha, E., Chi, M., Cao, Z., Wu, B., Hu, J., & Zhu, Y. (2025). Long-Term Creep Mechanical and Acoustic Emission Characteristics of Water-Immersed Coal Pillar Dam. Applied Sciences, 15(14), 8012. https://doi.org/10.3390/app15148012

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