Effects of the Quaternary aquifer water pressure on fault activation leading to roof water hazards

Xingyu Miao 1,2

Wenping Li 1,2✉ Emailwenpingli65@cumt.edu.cn

Qiqing Wang 1,2

Guojie Ma 1,2

Yuchu Liu 1,2

1 School of Resources and Geosciences China University of Mining and Technology 221116 Xuzhou China

2 Institute of Mine Water Hazards Prevention and Controlling Technology University of Mining and Technology 221116 Xuzhou China, China

Xingyu Miao^1,2, Wenping Li^1,2, Qiqing Wang^1,2, Guojie Ma^1,2, Yuchu Liu^1,2

1 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China.

2 Institute of Mine Water Hazards Prevention and Controlling Technology, China University of Mining and Technology, Xuzhou 221116, China.

Correspondence author: Wenping Li, wenpingli65@cumt.edu.cn.

Abstract

Fault activation is a common phenomenon during coal seam mining in proximity to faults. Fault activation significantly exacerbates the risk of roof water hazards through multiple mechanisms. To better understand the mechanism of roof water hazards caused by fault activation, numerical simulation and physical experiments were employed. By coupling FLAC3D with PFC3D, it was demonstrated that fault activation occurs in a phased manner, essentially involving the processes of shear failure and fracture propagation. During coal mining, fault activation induces stress concentration at fault ends and a substantial increase in the height of the water-conducting fracture zone. Furthermore, the degree of fault activation positively correlates with the advance distance; more active faults exhibit greater maximum subsidence per unit advance distance. Physical experiments demonstrate that fault activation exhibits a negative correlation with the pore water pressure required based on fault dip angle and width. These findings reveal that considering the influence of Quaternary aquifer water pressure on fault activation provides a theoretical basis and valuable insights for preventing and controlling roof water hazards during coal mining operations near faults.

Keywords

Fault activation

Roof water hazard

Coupling FLAC3D with PFC3D

Quaternary aquifer

Introduction

As coal mining intensity increases and mine depths continue to extend, the impact of water hazards on coal extraction grows increasingly significant (Hu and Zhao 2021; Li et al. 2019; Zhou et al. 2022). Among these, roof water hazards represent one of the most common types encountered in Chinese coal mining operations, frequently causing roof water inrush accidents (Wu 2014; Zeng et al. 2024). Statistics indicate that approximately 80% of water inrush incidents are associated with fault activation induced by mining activities (Yi et al. 2020; Yan et al. 2023). During coal extraction, whether faults become active is closely tied to engineering geological conditions (Gui and Lin 2016; Liu et al. 2020; Schuster et al. 2023). Therefore, studying the mechanisms of fault activation within the context of engineering backgrounds holds significant importance for the prevention and control of mine water hazards.

In recent decades, mine water hazards caused by fault activation have resulted in significant economic losses and casualties (Cai et al. 2019; Tian et al. 2021). Consequently, numerous scholars have conducted extensive physical and numerical simulations to investigate the mechanisms of fault activation. For instance, Zhu et al. (2022) employed COMSOL Multiphysics numerical modeling to investigate fault reactivation characteristics and stress evolution patterns. Sun et al. (2024) utilized FLAC3D numerical simulation software and similar material simulation tests to establish a stress-displacement response model for faults under mining conditions. Xia et al. (2022) studied the mining-induced reactivation mechanism triggered by the sublevel caving method. Zhu et al. (2023) revealed the mechanism and evolution patterns of delayed water inrush triggered by fault activation. Chen et al. (2023) established a model for rock mass instability and fracturing caused by fault activation under mining conditions. Shan et al. (2023) developed a mechanical model considering fault slip induced by principal stress unloading. Sun et al. (2023) proposed a theoretical safety factor to evaluate fault activation during coal mining. Yan (2024) et al. investigated the reactivation of critical stress faults triggered by fluid injection. Guo et al. (2023) investigated the driving forces of fault activation and factors influencing its barrier effect on rock mass movement under mining conditions. Zhang et al. (2022) examined the influence mechanisms of different geological parameters on small fault activation in deep mining workings. Song and Liang et al. (2021) studied the impact of fault tangential stress variations on fault activation. These studies collectively hold significant implications for the prevention and control of mine water hazards. However, fault reactivation is not solely attributable to coal mining activities but results from multiple interacting factors. None of the aforementioned studies considered the influence of hydrostatic pressure from Quaternary aquifers on fault reactivation, despite this pressure playing a crucial role in promoting such reactivation.

Therefore, a numerical model was established by coupling FLAC3D and PFC3D, and the effects of water pressure on faults were further investigated through physical experiments. The findings of this research can provide reference value for future mine water hazards caused by fault activation.

Engineering Background

The 815 working face at Xinhu Coal Mine is a large-scale mining face within the Huaibei Mining Group, producing 3 Mt of high-quality coking coal annually. As shown in Fig. 1a, Xinhu Coal Mine is located within Bozhou City, Anhui Province, China. The working face has a designed strike length of 2144 m, a dip width of 230 m, and an area of 493120 m². The corresponding ground elevation ranges from + 29.7 m to + 31.8 m, while the coal seam floor elevation spans from − 598.2 m to -865.5 m. The face employs fully mechanized top coal caving mining technology. The coal seam in the 815 working face is stably preserved, but its structure is complex. The 815 working face jointly mines the 8₁ and 8₂ coal seams. The 8 coal seam has a total thickness ranging from 5.18 to 8.92 meters, with an average thickness of 7.17 meters. Multiple faults developed within the 815 working face, all being normal faults, with F16 being a high-angle normal fault. As shown in Fig. 1b, the mining direction of the 815 working face progressed from the upper panel to the lower panel, with a total excavation length of 310m. According to on-site geological survey data, this roof water inrush incident was caused by the activation of the F16 fault in the 815 working face. The water source of this sudden water inrush primarily originated from the Quaternary aquifer, causing extensive damage to the 815 working face and resulting in the suspension of production at the mine.

Fig. 1

Geographic location map of the 815 working face

Method

Numerical simulation method

PFC3D-FLAC3D coupled modeling approach

In this study, a discrete-continuous coupling method based on boundary control walls was employed to address the problem by coupling FLAC3D-PFC3D. During the coupled analysis, FLAC3D simulates the mechanical behavior of the medium within the continuous domain at the macroscopic level. Meanwhile, PFC3D focuses on simulating the mechanical behavior of the medium within the discrete domain from a microscopic perspective. The mutual coupling interaction between the two primarily occurs at the contact boundary between the continuous and discrete domains (Tizpa et al. 2023; Wang et al. 2025). As shown in Fig. 2, during this process, computational data between different domains is efficiently and accurately transmitted and exchanged via the Socket I/O interface, ensuring the coordinated operation of the entire coupled system and seamless data interaction.

Fig. 2

Principles of FLAC-PFC coupling calculation

During the coupling process, when nodes or zones in FLAC3D are subjected to forces, velocities, and displacements are calculated based on the equations of motion. Particle states are updated, and wall positions are determined (Wu et al. 2022). Subsequently, particles, walls, and regions synchronously update velocities and displacements, with data transferred between the two models. This process repeats in subsequent cycles, recalculating contact forces between walls and particles at their new positions (Bai et al. 2022). As illustrated by the Eq. (1), this method facilitates the transfer of information that includes coordinates, velocities, and contact forces between the two models, thereby achieving coupling (Liu et al. 2025).

$\left\{ {\begin{array}{*{20}{l}} {\sum {{F_{i,j}}} ={F_j}} \\ {\sum {{R_{i,y}}} {F_{i,x}}= - {M_z}} \\ {\sum {{R_{i,z}}} {F_{i,x}}= - {M_y}} \\ {\sum {{R_{i,y}}} {F_{i,z}} - {R_{i,z}}{F_{i,y}}={M_x}} \\ {\sum {{R_{i,z}}} {F_{i,z}}=0} \end{array}} \right.$

The model employed in this study within PFC3D is linear. As shown in Fig. 3, its contact forces comprise linear and damping components. The linear component provides elastic behavior and friction, while the damping component provides viscous behavior. The Eq. (2) is as follows:

${F_c}={F^l}+{F^d}$

(2)

Fig. 3

Linear contact model

FLAC3D employs the Mohr-Coulomb model, whose criteria are expressed by principal stresses 𝜎₁, 𝜎₂, and 𝜎₃—the three components of the generalized stress vector in this model. The principal stresses must satisfy the following Eq. (3):

${\sigma _1} \leqslant {\sigma _2} \leqslant {\sigma _3}$

In FLAC3D, compressive stress is negative, and the failure criterion employed is a composite Mohr-Coulomb criterion with tension cutoff. As shown in Fig. 4, the region from point A to point B is defined by the Mohr-Coulomb failure criterion, while the section from B to C follows the tensile failure criterion. Point D represents the maximum tensile strength of the material. The condition Eq. (4) satisfied by various parameters is as follows:

$\left\{ {\begin{array}{*{20}{l}} {{f^s}= - {\sigma _1}+{\sigma _3}{N_\phi } - 2c\sqrt {{N_\phi }} } \\ {{f^t}={\sigma _3} - {\sigma ^t}} \\ {{N_\phi }=\frac{{1+\sin (\phi )}}{{1 - \sin (\phi )}}} \\ {\sigma _{{max}}^{t}=\frac{c}{{\tan \phi }}} \end{array}} \right.$

Fig. 4

FLAC3D Mohr-Coulomb failure criterion

Implementation of PFC3D-FLAC3D coupled numerical modeling

To achieve coupling between FLAC3D and PFC3D, the PFC module from FLAC3D version 6.0 was utilized. In this study, PFC3D was employed to simulate the Quaternary aquifer, while FLAC3D modeled bedrock strata. To optimize computational efficiency, the model domain was defined as 700m in length, 5m in width, and 200m in height. The model comprised 59079 particles and 38233 zones configured with the Mohr-Coulomb model. Subsequently, the Quaternary aquifer was modeled as a void space, with particles distributed using a linear contact model configuration. As shown in Fig. 5, this model configuration effectively investigates the role of Quaternary aquifers in triggering fault activation during coal mining operations.

Fig. 5

Initial configuration of the FLAC3D-PFC3D coupling numerical simulation

Model setup

Based on the geological conditions of Xinhu Coal Mine, a uniform equivalent vertical load of 8 MPa is applied to the top boundary of the coupled model to represent the vertical stress from the overlying strata. The bottom boundary is velocity-constrained and fixed. The water pressure in the Quaternary unconsolidated aquifer is set to 4 MPa (Liu et al. 2021; Yu et al. 2024). Coal seam excavation commenced at X = 100 meters, proceeding in 10 steps of 25 meters each. Following the fault crossing, excavation continued in 3 steps of 20 meters each. Particle density was set at 2000 kg/m³, with a damping coefficient of 0.7 and porosity of 0.4. The fault simulated the normal fault F16 in the 815 working face, featuring a dip angle of 78° and a displacement of 7.5 m. As shown in Table 1, physical and mechanical parameters for rock and coal strata were derived from actual field geological exploration data, while fault parameters were based on prior experience (Xin et al. 2023; Liu et al. 2025; Zhang et al. 2025).

Table 1

Rock physical and mechanical parameters used in the Simulation of the fault hanging wall
rock formation	Thickness (m)	density (kg/m³)	Elastic modulus (GPa)	cohesion (MPa)	internal friction angle (°)	tensile strength (MPa)
quaternary aquifer	20	2000	0.37	0.03	22.00	0.08
weathering zone	15	2000	0.37	0.03	22.00	0.08
mudstone	45	2630	4.79	5.45	28.90	2.29
sandstone	13	2570	10.04	7.60	36.40	3.90
coal	2	1400	2.40	1.28	36.70	0.40
mudstone	13	2390	13.69	5.20	32.19	2.60
sandstone	5	2435	11.32	4.60	35.10	2.80
fine sandstone	5	2583	9.57	5.91	36.60	3.30
mudstone	19	2390	13.69	5.20	32.19	2.60
sandstone	5	2500	10.93	6.12	36.00	3.01
fine sandstone	32	2680	11.58	6.03	37.21	3.52
coal	8	1400	2.40	1.28	36.70	0.40
sandstone	7	2483	10.49	6.03	35.28	2.88
mudstone	11	2630	4.79	5.45	28.90	2.29
fault	165	1000	0.17	0.40	28.0	0.18

Physical experiment method

Principles of physical experiment

While coal seams are mined, the hydraulic pressure within the Quaternary aquifer is not constant. When the hydraulic gradient reaches a specific value, changes in pore water pressure alter the stability of rock and soil masses, potentially triggering water hazards. According to the principle of effective stress, higher pore water pressure reduces effective stress, diminishing fault stability and increasing susceptibility to fault activation. The Eq. (5) is as follows:

$\sigma ^{\prime}=\sigma - u$

Experimental design

To further investigate the effects of hydrostatic pressure in Quaternary aquifers on different faults, the soil samples used in this experiment were loose soils from the Quaternary aquifer extracted via drilling at Xinhu Coal Mine. This experiment primarily examined the impacts of varying fault widths and dip angles. The specific experimental plan is detailed in Table 2:

Table 2

Experimental design plan
Experiment number	Fault height (cm)	Soil layer thickness (cm)	Fault width (cm)	Fault dip angle (°)
1	15	20	1	60
2	15	20	1	75
3	15	20	1	90
4	15	20	2	60
5	15	20	2	75
6	15	20	2	90
7	15	20	3	60
8	15	20	3	75
9	15	20	3	90

Experimental procedure

As shown in Fig. 6, this experimental procedure primarily consists of three steps: First, install the experimental apparatus by mounting the pore water pressure sensor on the monitoring port of the tank and connecting the strain gauge to verify the seal integrity of the apparatus. Next, lay the soil layers: fill the fault zone to a height of 15 cm and the soil layer to a height of 20 cm according to the calculated mass, then add water and allow it to settle. Finally, initiate the experimental gas pressure increase until water breakthrough and sand collapse occur, recording the experimental data.

Fig. 6

Experimental device diagram

Results and discussion

Numerical simulation results and discussion

As coal seam mining progresses, stress redistribution in surrounding rock strata may activate faults. Numerical simulation is essential for investigating the mechanisms of fault activation. This simulation primarily analyzes the zones in FLAC3D from three aspects: the development of the plastic zone, vertical stress, and the evolution of vertical displacement.

As shown in Fig. 7, fault activation does not occur simultaneously across the entire structure but proceeds in distinct phases. The activation process can be summarized as follows: upper activation phase (a), near-coal seam activation phase (b), and overall activation phase (c). Specifically, during the advance of the working face, shear failure first occurs in the upper part of the fault. Subsequently, as coal mining approaches the fault zone, failure develops near the coal seam. Finally, the newly generated failure continues to develop, connecting with previously established failures, leading to the overall activation of the fault. Figure 7c shows that the plastic zone has already developed into the weathering zone. At this point, due to the combined effects of overlying rock pressure, fault activation, and water pressure, the fractures ultimately connect to the Quaternary loose aquifer, causing water outburst and sand collapse.

Fig. 7

Developmental process of the plastic zone

As shown in Fig. 8, throughout the entire advance process, the coal face and the surrounding rock at the cut-off point primarily underwent shear failure, with the overall distribution of the plastic zone forming a saddle shape. The height of the water-conducting fracture zone within the roof strata exhibits a pattern of initial increase followed by a decrease as the working face advances. When the working face advances to 250 meters, the height of the water-conducting fracture zone significantly increases due to the overall activation of the fault. Upon reaching the footwall of the normal fault, although the base of the weathered zone undergoes shear failure, the increased coal seam height leads to a reduction in the height of the water-conducting fracture zone.

Fig. 8

Height of the water-conducting fracture zone at different advancing distances of the working face

As shown in Fig. 9, a parabolic tensile stress zone forms within the roof. Stress concentration occurs both in front of the cutting face and behind the working face, with maximum compressive stress near these two locations. This results from the macro stress arch spanning the goaf area, whose arch feet are precisely supported at these points—areas where stress is highest near the arch feet. Following coal seam excavation, the fully decompressed strata above the goaf release stress, transferring pressure toward the unexploited zone. Consequently, a high-stress arch forms on the outer periphery of the low-stress zone. Overall, regardless of the advanced distance of the working face, a stress distribution pattern emerges where stress is low in the center and high at both ends.

During the advancement of the working face, stress concentration occurs both ahead of the cutting edge and behind the working face, forming a stress distribution pattern that is lower in the middle and higher at both ends. During coal seam mining, a tensile stress zone appears and continuously expands above the fault, confirming that the fault activation begins from its upper section. Figure 9c shows a distinct stress concentration near the lower part of the fault, with compressive stress significantly increasing. Fault activation generates stress concentration zones at the ends near the fault zone, further indicating that faults near the coal seam have begun to activate.

Fig. 9

Vertical stress distribution at different advancing distances of the working face

As shown in Fig. 10, after coal extraction, the direct roof strata in the goaf undergo downward displacement, bending, and collapse under the combined effects of self-weight stress, overlying strata, and hydrostatic pressure. The displacement contour map of the overlying strata roughly resembles an arch bridge. As the working face advances, the resulting displacement and deformation of the overlying strata gradually propagate toward the surface. During mining operations, the influence zone of mining activities continuously expands, with the greatest subsidence occurring beneath the immediate roof of the coal seam.

Fig. 10

Vertical displacement diagram at different advancing distances of the working face

As shown in Fig. 11, the maximum surface subsidence increases progressively with the advancement of the working face. As the hanging wall approaches the fault during advancement, the subsidence per unit distance increases significantly. After coal mining reaches the footwall, the maximum subsidence per unit distance exceeds that of the hanging wall, indicating that the fault activation level in the footwall is more pronounced than in the hanging wall. The final numerical simulation result of 2.92 m closely matches the monitored data of 3.07 m.

Fig. 11

Maximum subsidence at different advancing distances of the working face

Physical experimental results and discussion

During the experiment, changes in pore water pressure at different locations were monitored, which holds significant importance for studying water outbursts. Simultaneously, this research provides valuable insights for mine water hazard early warning systems. Three monitoring points were established within the apparatus: CH1, CH2, and CH3, arranged from top to bottom. The experimental results are as follows:

Fig. 12

Pore water pressure curves at different monitoring points

As shown in Fig. 12, the pore water pressure throughout the experiment generally follows a “decrease-increase-decrease” trend. Water volume at each monitoring point continuously loses and gains. When losses exceed gains, pore water pressure decreases; conversely, it increases. After the channel opened, pressure relief occurred at the model's base, creating an instantaneous hydraulic drop funnel that rapidly decreased pore water pressure. As the mixture loss stabilized, flow velocity increased continuously, intensifying hydrodynamic forces and causing pore water pressure to rise to a peak. Finally, due to substantial mixture loss, pore water pressure decreased again, ultimately stabilizing near the zero level.

Under different conditions, the trend of pore water pressure variation remains consistent. However, as the fault width decreases, the peak elevation of pore water pressure at each monitoring point occurs later, and the peak magnitude increases. Conversely, as the fault dip angle decreases, the peak elevation of pore water pressure at each monitoring point occurs earlier, and the peak magnitude also increases.

Overall, smaller fault widths and smaller fault dips require higher pore water pressures to activate the fault. The increase in water pressure alters the local stress field, reducing the effective stress. This makes it easier for the shear stress to overcome the effective stress, leading to the opening of fractures and the occurrence of water outbursts.

Conclusions

This study employs numerical simulations based on the coupling of FLAC3D and PFC3D to investigate the influence of the Quaternary aquifer water pressure on fault activation leading to roof water hazards. Subsequently, through physical experiments, the effects of hydrostatic pressure in the Quaternary aquifer on water damage occurrence at faults of varying widths and inclinations are investigated. The detailed conclusions are summarized as follows:

1) During coal seam mining, fault activation is primarily influenced by mining activity, water pressure, and overburden pressure. The essence of fault activation is the process of shear failure and fracture propagation. Fault activation does not occur uniformly across the entire structure but progresses in stages, with fractures ultimately propagating throughout the entire fault zone.

2) During coal seam mining, fault activation causes stress concentration at fault ends and a significant increase in the height of the water-conducting fracture zone. Furthermore, as fault activity intensifies during coal mining, the maximum subsidence per unit advance distance increases.

3) The width and dip angle of faults exhibit a negative correlation with peak pore water pressure. According to the principle of effective stress proposed by Karl Terzaghi, when total stress remains constant, higher pore water pressure results in lower effective stress. Shear stress more readily overcomes effective stress, thereby activating faults and ultimately causing fractures to open, leading to water hazards.

4) The findings of this study reveal the mechanism of fault activation under conditions involving hydrostatic pressure from the Quaternary aquifer. This contributes to future coal mining operations by providing a basis and approach for preventing and controlling roof water hazards when faults exist in the vicinity.

Author Contribution

Xingyu Miao: Writing–Original Draft, Methodology, Conceptualization, Software; Wenping Li: Writing–Reviewing, Supervision, Funding Acquisition, Formal Analysis; Qiqing Wang: Investigation, Editing, Supervision; Guojie Ma: Data Curation. Yuchu Liu: Supervision. All authors reviewed the manuscript.

Funding

The research was jointly supported by the National Natural Science Foundation of China (42372316).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Conflicts of interest

The authors declare no competing interests.

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Yes

Experiment number	Fault height (cm)	Soil layer thickness (cm)	Fault width (cm)	Fault dip angle (°)
1	15	20	1	60
2	15	20	1	75
3	15	20	1	90
4	15	20	2	60
5	15	20	2	75
6	15	20	2	90
7	15	20	3	60
8	15	20	3	75
9	15	20	3	90

Experiment number	Fault height (cm)	Soil layer thickness (cm)	Fault width (cm)	Fault dip angle (°)
1	15	20	1	60
2	15	20	1	75
3	15	20	1	90
4	15	20	2	60
5	15	20	2	75
6	15	20	2	90
7	15	20	3	60
8	15	20	3	75
9	15	20	3	90

Experiment number	Fault height (cm)	Soil layer thickness (cm)	Fault width (cm)	Fault dip angle (°)
1	15	20	1	60
2	15	20	1	75
3	15	20	1	90
4	15	20	2	60
5	15	20	2	75
6	15	20	2	90
7	15	20	3	60
8	15	20	3	75
9	15	20	3	90