Catastrophe Information Characteristics and Prevention Measures of Water Inrush in Tunnel Approaching Fault with Different Water Pressure

Abstract

In order to ensure the safety of the tunnel approaching the fault and prevent water inrush disasters, and then take reasonable protective measures, a fault-tunnel-surrounding rock is established by using a three-dimensional (3D) discrete element numerical analysis method, which takes into account the fluid-structure coupling effect. Based on the method of control variables, the catastrophe information characteristics of displacement and water pressure of the surrounding rock of the tunnel face and the corresponding characteristics of changes before the occurrence of water inrush disasters were studied under different fault water pressures during the excavation of the tunnel approaching the water-rich fault. The results show that, during excavation at the same step, displacement and its magnitude in the surrounding rock escalate as fault water pressure increases. The maximum pressure of the water in the surrounding rock is also constantly increasing. As tunnel excavation progresses, at constant fault water pressure, longer excavation distances result in greater axial displacement of the surrounding rock mass and increased water pressure at corresponding positions within the surrounding rock, leading to higher magnitude increases. As excavation proceeds, the displacement and water pressure in the surrounding rock and the increase of its amplitude continue to increase. Pre-reinforcement grouting techniques and pipe umbrella support systems that are very effective protective measures can be determined by a comprehensive approach integrating advanced geological forecasting methods, real-time water pressure detection, and the analysis of stress-strain and seepage pressure field variations in the surrounding rock mass.

1. Introduction

As the “14th Five-Year Plan” outline was proposed and the “Belt and Road” strategy advanced in China, in order to meet the needs of social development and economic growth, the number of water conservancy, transportation, and other tunnel projects in China has been increasing annually. The selection of tunnel location is increasingly constrained by a variety of factors such as geographic location and construction environment [1,2,3,4]. Therefore, tunnel construction under complex geological conditions has received increasing attention. Water-bearing faults are a common adverse geological condition encountered in tunnel construction. Due to the typical presence of fault breccia, fractured rock, mylonite, and fault gouge within these faults, the rock mass is loose and broken, and often exhibits characteristics of high stress and high-water pressure. Combined with improper construction management, the construction process is highly susceptible to sudden water and mud inrush disasters, posing a significant threat to tunnel construction and personnel safety. Consequently, conducting research on sudden water inrush disasters at the tunnel working face in faulted zones is of urgent importance in order to provide theoretical guidance for the construction of tunnel projects in fault zones.

The anti-inrush body is the rock mass between the tunnel face and the fault that can block the water flow from the fault into the tunnel. The instability criteria of the protective layer at the tunnel working face and the minimum safe thickness of the anti-inrush body are important aspects in the study of the mechanism of sudden water inrush in tunnels near faults. In recent years, some scholars have conducted beneficial explorations and achieved fruitful results. Based on the total potential energy principle, Song et al. [5] established a mutation model for the instability of the surrounding rock of a deeply buried tunnel passing through a fault fracture zone, and derived the mechanical criteria for instability. Zhang et al. [6] conceptualized the tunnel-karst system, analyzed the mechanism and mode of sudden water inrush in faults, and derived a formula for calculating the minimum safe thickness of the rock mass in the fault section of a karst tunnel, which was verified by engineering examples. Based on the theory of simple silos and the characteristics of the stress distribution of the strata, Xie et al. [7] calculated the deformation characteristics of the tunnel under the action of longitudinal loads in the fractured zone, and analyzed the effects of burial depth, fracture zone width, and inclination on the longitudinal deformation and stress of the tunnel. Based on the theory of silo and the limit equilibrium method, Meng et al. [8] derived a formula for calculating the ground stress received by the anti-burst rock mass, obtained the mechanical criteria for sudden water and mud inrush in the fractured zone of water-bearing faults, and analyzed the effect of the width of the fault fractured zone on the minimum safe thickness of the anti-burst rock mass. Xu et al. [9] adopted the principle of slice method and the rock strength to realize a semi-quantitative analysis method for calculating the minimum safe thickness of rock against water inrush, and studied the semi-analytical solution of the minimum safe thickness of rock against water inrush in filled karst caves. Guo et al. [10] used fracture mechanics and hydraulic theory to analyze the lag effect and expansion effect of sudden water inrush at the tunnel working face, believing that the sudden water inrush at the tunnel working face is caused by the decrease of the critical water pressure for hydraulic fracturing due to the disturbance of excavation. Savvides et al. [11] investigated clayey originated rock properties through machine learning. They established a formula for calculating the safe thickness of the rock wall at the tunnel working face based on the critical water pressure. The above research results have strongly promoted the research on the surrounding rock failure and sudden water inrush mechanism in tunnels near water-bearing faults, and greatly advanced the progress of the key theories on the mechanism and prevention of sudden water inrush disasters in tunnels approaching fault.

The instability of the protective layer at the tunnel working face and the determination of the minimum safe thickness of the anti-burst body are the fundamental basis for establishing the theoretical criteria for the instability and sudden water inrush at the tunnel working face near water-bearing faults [12,13,14,15,16]. This can also provide the basis for implementing targeted comprehensive prevention and control measures to manage the occurrence of sudden water inrush disasters. In this regard, Wang et al. [17] developed a three-dimensional geological model experimental system and conducted experimental studies on the evolution of displacement, seepage pressure, and the mass of the outburst material during the process of sudden water and mud inrush in tunnels under the influence of the fault fracture zone. Liu et al. [18] independently developed an indoor experimental system that considers mass migration, and conducted permeability and erosion-induced sudden water and mud inrush experiments under different Talbot indices, investigating the influence of particle loss on the porosity, permeability, and water stability of completely weathered granite. Wang et al. [19] used a self-developed experimental system to conduct sudden water and mud inrush experiments in the fault fracture zone under different water pressure, clay content, and sample dry density conditions, analyzing the variation patterns of the gushing water volume, porosity, and permeability during the process of sudden water and mud inrush. Geng et al. [20] used a numerical simulation method to investigate the influence of fault dip angle on the stability of tunnel surrounding rock under orthogonal and parallel faults, revealing the influence law of fault dip angle on the tunnel’s sudden water and mud inrush. Xue et al. [21] established a fault zone and tunnel model using FLAC3D 3.00 and simulated the excavation of the Xiamen Haicang Tunnel, and analyzed the characteristics of surrounding rock stress and strain under different support methods. The aforementioned studies have achieved a series of important research results, greatly promoting the research on the mechanism of sudden water inrush in tunnels near water-bearing faults and the technological progress in the prevention and control of sudden water inrush disasters in fault tunnels. However, most scholars in the study of the influence of faults on the law of sudden water inrush in tunnel surrounding rock typically consider the fault activation and the formation of water-conducting channels under the influence of tunnel excavation at a single water pressure strength [22,23,24,25,26]. There is less research on the influence of fault water pressure on the law of tunnel sudden water inrush. In the actual construction process, the geological environment around the tunnel is highly complex, and the water pressure acting on the surrounding rock is not uniform. The mechanism of sudden water inrush in fault zones and the corresponding prevention and control countermeasures make it difficult to reach a consensus. In view of this, in order to further study the influence of fault water pressure on tunnel construction, this paper takes the instability and sudden water inrush of the tunnel working face near water-bearing faults as the research object, and adopts the three-dimensional discrete element method to analyze the evolution law of the displacement field and seepage pressure field under different fault water pressures during the process of sudden water inrush at the tunnel working face. In addition, the prevention and control measures for sudden water inrush disasters in tunnels near water-bearing faults are also explored. The research results can provide certain guidance for the prevention and control of sudden water inrush disasters in tunnels approaching faults.

2. Numerical Computation Considering Fluid-Solid Coupling

The built-in fracture seepage module of 3DEC was employed to investigate the evolution of water inrush channels in the tunnel working face near water-bearing faults. The variation of fracture water pressure and block displacement during the process of fracture connectivity were analyzed.

2.1. Discrete Element Computational Model and Mesh Division

As shown in Figure 1, a fault-tunnel-surrounding rock computational model is established ahead of the tunnel face. Selecting the center of the tunnel as the coordinate origin, the cross-section of tunnel is configured as a straight wall semi-circular arch with dimensions mirroring those of the Xianglushan Tunnel in central Yunnan, standing at 6 m in height and spanning 6.5 m. The model edge length method is 60 m, 40 m, and 40 m, respectively. The bottom interface of the model is set as a fixed interface, while the other interfaces are set as free interfaces. The model initializes with the overburden rock mass weight corresponding to a burial depth of 500 m, serving as the initial geostress on the upper boundary. The water pressure increased from 1 MPa to 2.5 MPa, and a simulation experiment was conducted every 0.5 MPa for tunnel excavation under five different working conditions. Figure 1 shows the tunnel excavation approaching the rich-water fault model. The mechanical properties of the of surrounding rock of tunnel and joints are detailed in

Table 1. Mechanical parameters of the rock masses (modified from Gu et al. [27]).

	Volumetric Weight γ (kN/m³)	Bulk Modulus K (GPa)	Shear Modulus G (GPa)	Poisson’s Ratio ν	Cohesion c (MPa)	Internal Friction Angle φ (°)
Rock mass	2000	1.44	19.4	0.25	0.35	28.6
Fault fracture zone	1800	0.48	6.4	0.25	0.12	28.6

and

Table 2. Mechanical parameters of the joint (modified from Gu et al. [27]).

	Normal Stiffness kn (GPa)	Shear Stiffness ks (GPa)	Internal Friction Angle φ (°)	Cohesive c (MPa)	Tensile Strength σb (MPa)
Joint	18.6	6.2	30	0.5	0.45

, respectively.

The 3DEC 5.20 software implements a range of elastoplastic constitutive relationships. For the block constitutive model, the Mohr–Coulomb failure criterion is employed, effectively capturing both elastic and plastic deformations within the rock mass. This model integrates fluid-structure interaction through a coupled approach, wherein the fluid pressure acting on the fault is dynamically updated as the excavation progresses. The interaction between the fluid and the solid matrix is governed by the effective stress principle, ensuring that changes in water pressure directly influence the stress state within the surrounding rock and faults. The joint constitutive model uses contact elasticity and plasticity under Coulomb slip failure, with shear and tensile responses determined by cohesion, tensile strength, and residual friction values.

Additionally, 3DEC features a robust Discrete Fracture Network (DFN) generation capability. The DFN module treats embedded fracture groups in the rock mass as discrete, finite-planar fractures, with each fracture approximated as circular. The geometry of these fractures in the DFN model is influenced by independent statistical distributions of various parameters, including fracture size (radius), orientation, and location. The principles of model mesh discretization are illustrated in Figure 2. This detailed methodological approach enhances the model’s accuracy in simulating the complexities of fluid-structure interactions, particularly in the context of varying water pressures in fault zones.

2.2. Numerical Computation Process and Scheme

This study primarily investigates the tunnel inrush process and its underlying mechanisms in fault rupture zones. The tunnel excavation process was subjected to a certain degree of simplification, with the full-face excavation method adopted. The excavation steps were set at 16 m, 4 m, 2 m, and 2 m, respectively. When the excavation reached a distance of 4 m, the step length was reduced to 1 m until the failure of the anti-inrush body at the tunnel working face.

The numerical analysis procedure is as follows: (1) Solving for the initial state. Use the command INSITU to specify the ground pressure and pore water pressure. The “set mech on flow off” command is used to activate the mechanical process while deactivating the fluid process. The fluid bulk modulus is set to 0. After a certain number of computational steps, the model reaches a state of mechanical equilibrium. The above steps ensure that the stress solution is obtained without changing the fluid pressure. Once mechanical equilibrium is achieved, the “set mech off flow on” command is used to activate the fluid process while deactivating the mechanical process. The fluid bulk modulus is then set to its actual value, and the model is allowed to reach a new equilibrium state obtaining the initial steady-state pressure. At this point, the initialization equilibrium stress and steady-state stress of the model can be obtained separately. (2) Obtain a coupled solution. The “set mech on flow on” command is used to simultaneously activate both the mechanical and fluid processes. In this case, the model can run a certain number of fluid steps concurrently with a certain number of mechanical steps, as defined by the “set nmec number” and “set ngw number” commands, respectively. In this case, 3DEC can run a series of flow steps while running a certain mechanical step size. The final attainment of a steady state is determined by the maximum unbalanced force being below a given threshold.

3. Simulation Results and Analysis

3.1. Discussion: Evolution Characteristics of Displacement Field under Different Water Pressures

The present study analyzes the minimum safe thickness under four different water pressure scenarios within the fault rupture zone, assuming constant parameters such as a burial depth of 500 m, a fault dip angle of 70°, and a fault rupture zone width of 30 m. The water pressures considered are increasing from 1 MPa to 2.5 MPa. The failure condition is defined as a sudden increase in the maximum unbalanced stress, leading to uncontrolled and continuously increasing displacements at the tunnel face. This ultimately results in the ejection of rock blocks and the occurrence of a tunnel inrush. The evolution of the displacement field of the anti-inrush body at the tunnel working face during the excavation process is shown in Figure 3. The numbers in different colors in the figure represent the magnitude of the displacements.

The displacement contour plots shown in Figure 3 represent the displacement states of the model at the point of equilibrium following each excavation stage. From the observations in Figure 3, it is evident that under the same fault water pressure conditions, the magnitude of displacements within the anti-inrush body increased significantly after the fourth excavation stage. This indicates that as the tunnel progresses towards the water-bearing fault, the displacements of the anti-inrush body at the tunnel working face, which were initially induced solely by the unloading due to excavation, gradually transition to being influenced by both the unloading and the water pressure from the upstream water-bearing fault. Consequently, the impact of the upstream water-bearing fault on the stability of the anti-inrush body becomes increasingly pronounced. Under the water pressures of 1 MPa and 1.5 MPa, the displacement contour plots after the sixth excavation stage depict the displacement patterns at the point of evident instability and failure of the tunnel working face. However, for the 2 MPa and 2.5 MPa water pressure, the instability and failure of the tunnel working face was observed after the fifth excavation stage. The minimum required thicknesses of the anti-inrush body at the tunnel working face were determined to be 3–4 m for the water pressures of 1 MPa and 1.5 MPa, and 4–5 m for the 2MPa and 2.5 MPa water pressure. Furthermore, it was observed that under the same excavation stage, the maximum lateral displacement of the anti-inrush body at the tunnel working face gradually increased with the increase in fault water pressure. These findings suggest that as the fault water pressure increases, the minimum safe thickness of the anti-inrush body required for the tunnel to traverse the water-bearing fault increases.

Figure 4 presents the displacement variation curves at the center of the tunnel working face. As shown in Figure 4a, the trends of displacement changes at the tunnel working face center are generally consistent across different water pressures as the excavation progresses. As the tunnel advances towards the water-bearing fault, the lateral displacement response at the tunnel working face becomes increasingly pronounced, and the growth rate remains relatively consistent. From Figure 4b, it can be observed that, at the same coordinate location, the displacement response continuously increases with the rise in water pressure. Moreover, this change becomes more evident as the tunnel working face approaches the fracture zone of the fault. As the water pressure increases, the displacement is more susceptible to sudden changes when the thickness between the tunnel working face and the fault remains the same, indicating a higher risk of sudden water inrush.

3.2. Discussion: Evolution Characteristics of Seepage Field under Different Water Pressures

During the excavation of the tunnel approaching the rich-water fault, the change of water pressure inside the anti-inrush body is illustrated in Figure 5, where the numbers in different colors represent the water pressure. As can be observed from Figure 5, continued excavation does not lead to significant seepage through the face protective layer, suggesting that the excavation disturbance has a limited impact on the seepage stability of the face protective layer. As the tunnel advances and the distance between the tunnel working face and the water-bearing fault decreases, the influence of the excavation disturbance becomes more pronounced. When small-scale water inrush occurs at the lower part of the tunnel working face under the seepage pressure, this can be considered a precursor to the potential for sudden water inrush, with the face protective layer thickness approaching the minimum safe thickness. Eventually, the protective rock layer at the tunnel working face is unable to withstand the water pressure from the overlying fault, leading to an overall instability and failure. This results in a sudden change in the inrush flow rate, the formation of a stable sudden water inrush channel within the face protective layer, and a high-volume, high-pressure water inrush event. Furthermore, as shown in Figure 5, when the distance between the tunnel working face and the fracture zone of the water-bearing fault is the same, the maximum lateral extrusion displacement of the tunnel working face increases with the continuous rise in fault water pressure, indicating a higher susceptibility of the tunnel’s protective system to sudden water inrush failure.

To observe the evolution of the displacement field and seepage field of the face protective layer during tunnel excavation near a water-bearing fault, with water pressure changes as the object of study, 8 monitoring points were set up at the center and edges of the tunnel working face. This focuses on the water pressure changes at the monitoring point located at the center of the tunnel working face. Figure 6 illustrates the changes in water pressure during the excavation process of tunnel working face under different water pressure conditions, at the points with coordinates (6, 0, 0), (7, 0, 0), and (8, 0, 0), corresponding to curves 10, 11, and 12, respectively.

As shown in Figure 6, under different fault water pressures, the changing trend of water pressure at the same monitoring point during tunnel working face excavation is essentially the same. Prior to any failure, the water pressure first exhibits an increasing trend, and then suddenly decreases after reaching a relatively stable state. The model calculation shows that up to approximately 20,000 steps, the face protective layer is minimally affected by water pressure, remaining in a relatively stable state. However, at around 27,000 steps, fault water seeps through cracks and enters the face protective layer. As a result, the excavation face experiences the combined impacts of excavation progress and crack water pressure, resulting in a gradual decrease in water pressure at the monitoring point. By the time the calculation reaches approximately 50,000 steps, the tunnel working face has experienced a sudden water and mud burst failure, with a large volume of water gushing out, causing the water pressure at the monitoring point to approach zero.

3.3. Prevention Measures for Water Inrush Disasters in the Tunnel Approaching the Fault

The information on the evolutionary process of water inrush disasters in fault fracture zones is crucial for guiding the safe construction of tunnels. Without such information, the research would be merely “castles in the air”. Disaster prevention measures based on unfavorable geological information serve as a bridge connecting the catastrophe information characteristics of water inrush disaster and actual construction situation of the tunnel. The prevention measures for water inrush disaster should be formulated based on the development scale of the fault in front of the tunnel working face. Different levels of water inrush require different prevention measures. The higher the water pressure of the fault, the higher the level of the water inrush will happen in the tunnel, the higher level preventive measures need to be implemented. Currently, the main control measures for water inrush in tunnel fault fracture zones include support, drainage, and sealing measures. Drainage is primarily achieved through “remote drainage decompression” to reduce groundwater pressure, while sealing is mainly accomplished through “near-end grouting” to strengthen the fault fracture zone and improve its strength.

This paper summarizes the prevention and control strategies for water inrush disasters in tunnels near water-rich faults during the construction period. Based on a comprehensive advanced geological prediction method, combined with advanced water pressure detection and the laws of surrounding rock displacement and seepage pressure changes in tunnels near water-rich faults, the application of advanced grouting technology and pipe-roof support measures are used to establish water inrush disaster prevention and control measures, as shown in Figure 7.

The comprehensive prediction approach combines advanced geological forecasting, ahead-of-face water pressure detection, and the laws governing surrounding rock displacement and seepage pressure changes in tunnels near water-rich faults. The comprehensive advanced geological forecasting method utilizes a combination of seismic, transient electromagnetic, ground-penetrating radar, and pre-drilling techniques to predict the conditions of the surrounding rock in front of the tunnel working face near the fault. The catastrophe information characteristics of surrounding rock in tunnels working face near water-rich faults indicate that both the displacement field and the seepage field will undergo corresponding changes prior to the occurrence of water and sand inrush disasters triggered by the nearby fault. By integrating the precursory information characteristics, the results of field monitoring and advanced geological forecasting, and the laws governing surrounding rock displacement and seepage pressure changes. The potential for water inrush disasters in tunnels near water-rich faults can be predicted and verified.

Timely decision-making and the implementation of suitable engineering measures can often prevent water inrush disasters in tunnels near water-rich faults, thus avoiding economic losses and project delays. Common preventive measures include pre-grouting reinforcement and pre-pipe roof support. Pre-grouting reinforcement densifies the rock mass, enhances the overall integrity of the rock mass, allowing a range of rock layers to bear the load together, thereby increasing its resistance to bending and shear forces. Injecting grout into rock layers via small guide tubes improves the mechanical properties of the surrounding rock. The pre-pipe roof support technique creates an integrated load-bearing structure through borehole grouting and pipe roof installation, effectively supporting and reinforcing the surrounding rock on both sides of the tunnel, and preventing collapses. This technique involves inserting steel pipes into pre-drilled holes along the tunnel excavation profile and grouting inside the pipes, combined with robust steel frame supports, forming a pre-support system for rock with poor self-stability. By predicting the potential for water inrush disasters, appropriate pre-grouting and pipe roof support parameters can be selected to ensure safe tunnel excavation while minimizing excess control and material waste.

4. Conclusions

(1)

When the water pressure in the fractured zone of the water-rich fault ahead of the tunnel increased from 1 MPa to 2.5 MPa, the minimum required thickness of the anti-inrush body in the tunnel working face gradually increased. As the fault water pressure elevated, the displacement within the same excavation step and over the same computational time period continued to increase. Furthermore, as the tunnel working face approached closer to the water-rich fault, the lateral displacement changes at various points of the anti-inrush body became more pronounced. This indicated that the anti-inrush body was increasingly susceptible to water inrush failure, necessitating a greater minimum safe thickness.

(2)

As the water pressure in the fractured zone of the water-rich fault ahead of the tunnel working face increased from 1 MPa to 2.5 MPa, a similar trend of changing water pressure was observed at the same monitoring points despite the advancing tunnel excavation. Within the same excavation step and over the same computational time period, the maximum water pressure within the anti-inrush body also continued to increase. Furthermore, as the tunnel working face approached closer to the water-rich fault, the changes in water pressure at various points of the anti-inrush body became more pronounced.

(3)

The disaster prevention and control strategy of tunnel approaching the water-rich fault water should be based on the comprehensive overrun geological forecast method, the results of overrun water pressure detection and the displacement field, seepage pressure field change rule of the surrounding rock when tunnel approaching the water-rich fault water, so as to determine the reasonable overrun grouting technology and pipe shed support construction parameters, to develop a reasonable tunnel approaching water-rich fault water disaster prevention and control measures.