Research on the Settlement Patterns of Tunnel-Surrounding Rock Under Groundwater Conditions

Abstract

In the current excavation of water diversion tunnels, significant challenges such as water inrush, rockburst, and large deformations continue to persist. Consequently, research on the stability of the surrounding rock after tunnel excavation is of great scientific importance. To address the impact of groundwater on tunnel-surrounding rock deformation under complex geological conditions, this study utilizes a combination of field monitoring and numerical simulation. Specifically, the research simulates the excavation process under both groundwater and non-groundwater conditions. Systematically, it analyzes the deformation patterns of tunnel-surrounding rock under groundwater conditions. The findings reveal the following: (1) Settlement and excavation mileage exhibit a clear trend of “steep decline, gradual decline, stable,” both stabilizing around 35 m after the excavation reaches the monitoring cross-section. Changes in groundwater levels, whether rising or falling, can either accelerate or delay the point at which settlement stability is achieved. (2) The numerical simulation settlement curves closely align with the field monitoring curves, with simulated settlement values slightly exceeding the monitored results. However, the error rate between the two remains below 20%, indicating the reliability of the method. (3) Groundwater significantly impacts water-sensitive strata such as loess and sandstone, with maximum settlement values at the tunnel vault and ground surface under groundwater conditions being 2 to 3 times those observed in the absence of groundwater. This study provides a scientific basis for optimizing tunnel design and construction processes. Future research should focus on refining the numerical simulation model, by incorporating additional monitoring data for validation and enhancing the safety of tunnel construction.

1. Introduction

With the rapid development of infrastructure construction in China, the safety issues of tunnel projects under complex geological conditions have increasingly come to the forefront [1,2,3,4]. Groundwater, as a critical factor influencing the stability of tunnel-surrounding rock [5,6], has a complex distribution in strata and is highly unpredictable [7,8]. The interaction mechanisms between groundwater and surrounding rock have long been a key research focus in rock mechanics and engineering [9,10]. Groundwater significantly impacts tunnel-surrounding rock by softening the rock, reducing rock mass strength, and altering the stress state of the surrounding rock, leading to deformation and failure [11,12,13].

During tunnel excavation, groundwater can induce deformation and failure of tunnel-surrounding rock, resulting in severe engineering accidents [14,15]. Groundwater, through seepage, alters pore pressure in the surrounding rock, thereby influencing its mechanical properties and increasing the risk of rock deformation or collapse [16,17]. The seepage of groundwater can cause variations in the temperature distribution of surrounding rock, further affecting its mechanical performance and tunnel stability [18]. Chemical reactions between groundwater and surrounding rock, including dissolution and chemical erosion, can alter rock chemistry, affecting its mechanical properties and reducing its load-bearing capacity and deformation resistance [19,20].

Additionally, tunnel excavation also impacts the groundwater system [21]. Tunnel excavation disturbs the original geological structure, altering the flow paths and permeable channels of groundwater. This can lead to an increase or change in groundwater seepage, affecting tunnel stability and negatively impacting the surrounding ecological environment [22]. During excavation, the decline in groundwater levels may lead to further unloading of the surrounding rock, exacerbating its deformation and instability [5,23]. Additionally, tunnel excavation may increase the risk of groundwater inrush. This not only threatens construction safety but may also damage the surrounding geological environment [24].

To address the complexity of groundwater development, scholars worldwide primarily use two research methods: field monitoring and numerical simulation. Numerical simulation offers advantages such as being unrestricted by site working conditions and space, providing repeatability and adjustability, making it suitable for analyzing extreme conditions [25,26,27]. Huang et al. [28] conducted numerical simulations to investigate the influence of groundwater on the stress distribution of tunnel-surrounding rock, demonstrating that groundwater significantly reduces rock shear strength and increases deformation risks. Qiu et al. [29] combined indoor experiments with field monitoring to demonstrate that water inrush significantly reduces the shear strength and elastic modulus of loess, increasing its compressibility. Field monitoring offers high accuracy, real-time observation capabilities, and the ability to validate numerical simulation feasibility, making it applicable in most scenarios. Gao et al. [30] combined field monitoring data with numerical simulation technology to conduct a detailed analysis of settlement characteristics under groundwater influence, proposing a dual-effect mechanism of groundwater on rock deformation. Wang et al. [31] combined indoor experiments with field monitoring to study the impact of groundwater on tunnel rock permeability, showing that groundwater seepage further accelerates rock softening effects. Tan et al. [32] conducted field monitoring to analyze the deformation patterns of deep shafts at different construction stages, proving that groundwater pressure and rock mechanical properties are the primary factors influencing shaft deformation.

However, current research has certain limitations, primarily in the following aspects: Firstly, most studies focus on the mechanical effects of single factors on rock deformation, while research on tunnel deformation mechanisms under complex geological conditions involving groundwater–rock interactions remains insufficient [33]. Secondly, while numerical simulation technology has been widely applied in the stability analysis of tunnel rock, its results often require integration with field monitoring data to effectively validate model reliability [34]. Therefore, systematically studying the deformation patterns of tunnel-surrounding rock under groundwater influence, through the integration of field monitoring and numerical simulation, remains a key research focus in rock mechanics and engineering.

Given the complexity of interactions between groundwater and surrounding rock stability during tunnel excavation, understanding the mechanisms by which groundwater influences the stability of surrounding rock remains a significant challenge. Based on this, the current study focuses on the Tunnel No. 1 of the Total Canal of the Xiaolangdi North Bank Irrigation Project as the research focus. Using ground surface and vault settlement monitoring data, we investigate the settlement patterns of the rock mass after tunnel excavation. Furthermore, numerical simulations are used to analyze the mechanisms that groundwater uses to influence rock mass deformation. This research aims to provide theoretical support for rock mass stability analysis and reinforcement design in similar engineering contexts, offering scientific guidance for ensuring the safety and stability of tunnel constructions [35,36].

2. Engineering Background

This study is based on the Tunnel No. 1 of the Total Canal of the Xiaolangdi North Bank Irrigation Project. The study section is located in Jiyuan City, Henan Province. The tunnel inlet elevation is 228.12 m, and the outlet elevation is 229.75 m. The groundwater levels are 2.8 m above the base slab. The geological strata in the study section are complex. The upper cover stratum consists of loess, while the lower strata consist of mudstone sandstone, with local interlayers of mudstone. The tunnel site has characteristics such as shallow burial depth and a complex surrounding environment. The geological conditions are special, resulting in poor cave formation conditions. The geological profile of the tunnel site is shown in Figure 1. The tunnel cross-section features a circular arch with a straight wall, buried to a depth of approximately 20 m and with a diameter of 2.16 m. The tunnel lining structure consists of sprayed C25 concrete, B20@1000 systematic anchor bolts, A6@200 reinforcing mesh, national standard I14 W-beam steel arches, and C20 reinforced concrete lining throughout the cross-section. The spacing between the steel arches is 1 m. The tunnel body support is shown in Figure 2.

During construction, a collapse accident occurred. The collapse material mainly originated from the top and side rock of the face and the loess stratum between the arch vault and the previously sprayed cement-soil. The collapse caused significant ground settlement over the tunnel crown, with a collapse pit diameter of approximately 30 m.

3. Research Methods

3.1. Monitoring Scheme

A visual deformation monitor (RU-B03-S100) was installed near the collapse pit. Ten monitoring points were evenly distributed along the horizontal center line, each equipped with targets and spaced 5 m apart, numbered sequentially from Point 1 to Point 10. By monitoring the target displacement, the overall motion or displacement of the measured object is recorded, and the data are uploaded to the cloud platform in real time. This study focuses on analyzing Points 1 to 5 in detail. The monitoring instrument and point distribution are illustrated in Figure 3.

3.2. Model and Parameters

The tunnel excavation process was numerically simulated using FLAC 3D 6.0 software. The model dimensions were 50 m × 50 m × 130 m (X × Z × Y). In order to avoid the influence of boundary effects, the tunnel diameter of 3~5 times is selected in the model range. The tunnel was positioned 30 m above the bottom boundary and 25 m away from both left and right boundaries. The tunnel has a clear height of 2.16 m. The excavation advance in the simulation was 130 m. The geological strata comprised five layers: an upper loess stratum and alternating mudstone, sandstone, and calcareous sandstone strata. The Mohr–Coulomb constitutive model was used for the analysis. The computational parameters for each stratum were determined based on rock mechanics test results and field monitoring data, as shown in

Table 1. Physical and mechanical properties of various strata.

		Density (g/cm3)	Elastic Modulus (Pa)	Poisson	Cohesion (Pa)	Friction (°)
Loess	Saturated	1.86	2.79 × 107	0.2	1.36 × 104	21
	Unsaturated	1.86	8.43 × 106	0.2	7.48 × 103	23
Shale	Saturated	2.37	5.75 × 107	0.26	1.39 × 105	29
	Unsaturated	2.37	1.74 × 107	0.26	7.65 × 104	31
Sandstone	Saturated	2.46	6.93 × 108	0.27	6.62 × 105	37
	Unsaturated	2.46	2.09 × 108	0.27	3.64 × 105	39
Argillaceous Sandstone	Saturated	2.18	1.94 × 108	0.26	4.82 × 105	35
	Unsaturated	2.18	5.87 × 107	0.26	2.65 × 105	37
Calcareous Sandstone	Saturated	2.51	1.33 × 109	0.27	7.08 × 105	38
	Unsaturated	2.51	4.02 × 108	0.27	3.9 × 105	40

. Two sets of parameters were used to account for both groundwater and non-groundwater conditions.

The calculation process implements the full-face excavation method, where support is installed after each 1 m excavation, a Null element is assigned to the excavation area to simulate a excavation process, repeated 130 times to simulate the completion of excavation. After each 1 m excavation, the support structure is immediately installed. Anchor bolts are modeled using cable elements, steel arches with beam elements, the lining structure is simplified as shell elements, and the rebar mesh is combined with sprayed concrete layers [37,38]. A schematic representation of the support structure and the 3D model is provided in Figure 4. To facilitate comparative analysis, a monitoring cross-section at Y = 40~60 m was selected, corresponding to actual monitoring points labeled as Target 1~5. Figure 5 illustrates the monitoring point layout.

Based on the influence of groundwater conditions on tunnel excavation projects, this study first implements numerical simulation analysis under groundwater action to validate the field monitoring data. Furthermore, simulations were performed under non-water conditions to systematically analyze the mechanisms by which groundwater affects the tunnel excavation process. The groundwater level parameter is set at Z = 35 m, which is higher than the tunnel vault elevation, as shown in Figure 6.

3.3. Model Verification

The analysis of maximum settlements at the vault and ground surface, as shown in

Table 2. Comparison of simulated and monitored settlements at various monitoring cross-sections.

Cross-Section Position	Vault Settlement (mm)			Surface Settlement (mm)
	Monitored	Simulated	Error Rate	Monitored	Simulated	Error Rate
Y = 40 m (Target 1)	25.24	25.5	1.03%	9.23	9.73	5.42%
Y = 45 m (Target 2)	25.71	25.8	0.35%	10.38	10.87	4.72%
Y = 50 m (Target 3)	25.73	25.83	0.39%	11.54	11.94	3.47%
Y = 55 m (Target 4)	25.19	25.91	2.86%	12.23	12.9	5.48%
Y = 60 m (Target 5)	25.66	25.96	1.17%	13.24	13.8	4.23%

, after each cross-section stabilizes, leads to the following conclusions: the measured settlements are slightly lower than simulated values, and both increase with excavation depth. Generally, vault settlements are approximately 2~3 times the ground surface settlements. The calculated error rates for vault settlements range from 0.35% to 2.86%, and those for ground surface settlements range from 3.47% to 5.48%. According to relevant studies [39,40], the permissible error rate between measured and simulated values is typically ±20%, indicating that the current model is viable.

4. Results and Discussion

4.1. Settlement Monitoring Analysis

Field monitoring data were collected and analyzed to generate comparative curves illustrating the settlements of the tunnel vault and the ground surface across various cross-sections.

As illustrated in Figure 7, the ground surface and vault settlement curves for the five monitoring cross-sections initially decline rapidly, then decline gradually, and ultimately stabilize once the excavation distance reaches 35 m. This trend is primarily attributed to stress redistribution within the soil mass. During early excavation stages, soil stress is rapidly released, resulting in a high settlement rate. As excavation progresses, stress gradually transfers to the surrounding soil, establishing a new equilibrium and decreasing the settlement rate. Furthermore, as the excavation face moves away from the monitoring cross-sections, the disturbance zone decreases, and the soil strengthens through consolidation and compaction, naturally slowing the settlement rate. Consequently, groundwater conditions significantly affect both settlement and the time needed to reach a stable state. If the water level rises, the effective soil stress decreases, accelerating plastic deformation and increasing the initial settlement rate, causing the stabilization point to occur earlier (e.g., at 30 m). Conversely, if the groundwater level drops, the soil skeleton strengthens, delaying plastic deformation development and pushing the stabilization point further (e.g., at 40 m) [41].

The vault settlement rates and values are significantly higher than those of the ground surface, and the difference in settlements increases gradually with excavation progress. Monitoring data indicate that the maximum ground surface settlements for Targets 1 to 5 are 9.23 mm, 10.38 mm, 11.54 mm, 12.23 mm, and 13.24 mm, respectively, while the maximum vault settlements are 25.24 mm, 25.71 mm, 25.73 mm, 25.19 mm, and 25.66 mm, respectively. The settlement differences increase gradually during excavation, reaching maximum values of 16.01 mm, 13.33 mm, 13.19 mm, 12.56 mm, and 14.42 mm. Both ground surface and vault settlements tend to stabilize once excavation reaches 35 m.

4.2. Settlement Simulation Analysis

4.2.1. Displacement Cloud Images Analysis

Using the FLAC3D software, Z-directional displacement cloud map slices with and without groundwater were generated and then analyzed through comparison.

The surrounding rock’s Z-directional displacement cloud image with groundwater influence is clearly presented in the upper section of Figure 8. The maximum vault settlements within Y = 40~60 m are 2.55 cm, 2.58 cm, 2.583 cm, 2.591 cm, and 2.596 cm. The maximum difference in vault settlements among cross-sections is 0.64 mm. The maximum ground surface settlements within Y = 40~60 m are 9.73 mm, 10.87 mm, 11.94 mm, 1.29 cm, and 1.38 cm. The maximum difference in ground surface settlements among cross-sections is 4.07 mm. The surrounding rock’s Z-directional displacement cloud image without groundwater influence is clearly presented in the lower section of Figure 8. This figure effectively illustrates the displacement pattern of the rock in the absence of groundwater action. Within the range Y = 40~60 m, the maximum vault settlements are 7.67 mm, 7.68 mm, 7.79 mm, 7.85 mm, and 7.91 mm. The maximum difference in vault settlements among monitoring cross-sections is 0.24 mm. Similarly, the maximum ground surface settlements within the same range are 5.77 mm, 5.78 mm, 5.89 mm, 5.95 mm, and 6.01 mm, with a maximum difference of 0.24 mm among cross-sections.

By comparing settlement data under groundwater influence and without groundwater influence, it is observed that the maximum vault settlements under groundwater influence are 3.28 times greater than those without groundwater influence, while the maximum ground surface settlements are 1.8 times greater. When groundwater is the primary influencing factor, both tunnel vault and ground surface settlements show a significant upward trend due to the softening of strata lithology upon water contact. The results indicate that groundwater can amplify tunnel-surrounding rock deformation by modifying soil mechanical parameters (e.g., cohesion and internal friction angle) and seepage pressure. This effect is particularly pronounced in weaker geological formations, significantly influencing settlement development.

4.2.2. Displacement Curves Analysis

During the excavation process, settlement data at the crown and ground surface of each monitoring cross-section were collected. Graphs were then generated to visually analyze the settlement patterns. Subsequently, a comparative analysis was performed between the settlement conditions under both scenarios with and without groundwater influence.

As illustrated in Figure 9, the settlement rates of the vault and ground surface at all monitoring cross-sections demonstrate consistent trend characteristics. After tunnel excavation, the vault rock mass is in an unsupported state, resulting in a substantial increase in vault settlement rates. The ground surface settlement rates are lower than the vault settlement rates due to time-space effects. As the support structure becomes effective, the settlement rates gradually decline. Eventually, with soil stress redistribution and soil consolidation and compaction, the settlements stabilize. The simulation processes in the absence of and under groundwater influence exhibit consistent trend characteristics, as shown in Figure 10. Both simulation processes demonstrate a settlement evolution pattern characterized by “steep decline—gradual decline—stable”. However, settlement values in the absence of groundwater influence are significantly lower than those under groundwater influence.

4.3. Groundwater Action Mechanism Analysis

Based on the aforementioned analysis, groundwater significantly impacts the stability of tunnel rock masses, primarily due to the complex interactions between groundwater and the surrounding rock mass [42]. For weaker and less stable strata with low conductivity, such as loess and mudstone, the absence of groundwater leads to higher soil strength and stability, characterized by uniform stress distribution, reduced pore water pressure in the surrounding rock mass, and a direct relationship between ground surface settlement and vault settlement, with minimal interrelation between them. In such scenarios, settlements predominantly result from unloading effects and surrounding rock deformation due to excavation [43]. Conversely, groundwater presence alters stress distribution within strata via permeation [44], softening the surrounding rock mass [45], thereby reducing its strength and load-bearing capacity and causing structural failure and significantly increased settlements. Furthermore, groundwater raises pore water pressure in the surrounding rock mass [46], leading to greater vault settlements and subsequently destabilizing the ground surface, thereby intensifying ground surface settlements. Thus, groundwater primarily influences ground surface and vault settlements through four mechanisms: (1) permeation effects: groundwater alters stress distribution in surface soil and rock mass via permeation, softening soil and causing structural failure; (2) water pressure changes: groundwater pressure variations directly affect rock mass and soil stress states, inducing deformation and settlements; (3) flow and erosion: groundwater flow may erode surface soil and rock mass, leading to soil loss and structural degradation [47]; (4) water–soil interaction: groundwater alters soil physical properties through interaction, reducing its strength and stability [48].

Figure 11 illustrates a schematic diagram depicting the mechanisms by which groundwater influences surface settlement and vault settlement. The figure primarily focuses on the direct effects of groundwater on both surface and vault settlement, as well as their interdependent relationships.

Accordingly, the presence of groundwater significantly influences the characteristics of ground settlement and crown settlement during tunnel excavation. When groundwater is present within the tunnel’s excavation zone, both the ground settlement and crown settlement exhibit notable increases, with an interrelated relationship between them. Conversely, when the groundwater level lies outside the tunnel’s excavation zone, the magnitude of the settlement for both the ground and the crown is relatively small, and the interaction between them is weaker. The mechanism of groundwater action is complex, primarily operating through multiple physical processes, such as permeation, water pressure changes, and flow characteristics, which collectively affect the deformation of the ground and crown. In engineering practice, it is essential to comprehensively consider hydrogeological conditions and tunnel excavation methods. By implementing appropriate groundwater control measures, the influence of groundwater on ground and crown settlement can be effectively minimized.

5. Conclusions

This study addresses the issue of groundwater’s influence on tunnel-surrounding rock deformation under complex geological conditions, through the combination of field monitoring and numerical simulation. A comparative analysis of settlement curves and numerical values under groundwater and non-groundwater conditions reveals the deformation patterns of tunnel-surrounding rock. This analysis shows that groundwater significantly impacts surrounding rock deformation. The following main conclusions are drawn:

(1)

The settlement rates after tunnel excavation demonstrate a clear “steep decline—gradual decline—stable” evolution trend, stabilizing around 35 m once excavation reaches the monitoring cross-sections. Rising groundwater levels reduce the effective stress in the soil, accelerating plastic deformation and resulting in an earlier stabilization point. Conversely, falling groundwater levels strengthen the soil skeleton, delaying plastic deformation development and resulting in a later stabilization point.

(2)

The numerical simulation settlement curves closely align with the field monitoring curves, both exhibiting a three-phase trend of “steep, gradual, stable”. The error rates for vault settlements range from 0.35% to 2.86%, and those for ground surface settlements range from 3.47% to 5.48%. Both error rates fall below 20%, indicating the simulation’s reliability and its good correlation with the monitoring results.

(3)

Groundwater significantly influences the deformation patterns of tunnel-surrounding rock, particularly in strata such as loess and mudstone, which soften upon contact with water. Deformation quantities in these strata are significantly greater under groundwater influence than in conditions without groundwater. Under groundwater influence, the maximum vault settlement displacement is 3.28 times greater than that without groundwater influence, and the maximum ground surface settlement displacement is 1.8 times greater. This indicates that groundwater significantly amplifies rock deformation, warranting special attention in similar engineering projects.