Research on the Water Inrush Mechanism and Grouting Reinforcement of a Weathered Trough in a Submarine Tunnel

Abstract

Based on the structural and geological characteristics of the F1 weathering trough of a submarine tunnel and its spatial relationship with the cavern, a simplified calculation model of the weathering trough water inrush was established, and the formation, development process and influencing factors of the water inrush channel in the water-resistant rock layer were carried out by a numerical simulation of particle flow. It shows that the integrity and stability of the critical water-resistant rock mass is the key to preventing water inrush, and the identification and positioning of the water inrush channel is the basis for the grouting reinforcement design of the weathering groove of the submarine tunnel. Based on above research results, the F1 weathering trough was blocked and reinforced by the composite grouting method, and the engineering reinforcement effect was good.

1. Preface

At present, there are few studies on the mechanism of water inrush in tunnels at the international level. Most of them have analyzed the mechanical mechanism of water inrush in submarine tunnels based on the theory of rock mass limit equilibrium, and given the formula of critical water pressure for water inrush in submarine tunnels [1,2,3,4,5,6,7,8,9,10,11,12]. With the development of computing technology, numerical simulation has been more and more widely used in the field of rock seepage mechanics. For the seepage-damage coupling mechanism, it generally needs to be in FLAC, UDEC and other commercial programs or based on elastoplastic mechanics, fracture mechanics and damage. The numerical model of mechanics theory introduces media fracture and damage judgment criteria, embeds the permeability-damage evolution equation describing the media failure expansion zone, and studies the seepage-damage coupling behavior in the process of hydraulic fracturing or water inrush [13,14,15,16]. In this article, a microscopic simulation of the dynamic process of water inrush from rock masses is conducted in a coupled environment, and a method to study the theory of water inrush in submarine tunnels from a microscopic perspective in the future is given.

According to the structural geological characteristics of the F1 weathering trough of a submarine tunnel and its positional relationship with the tunnel, the tunnel water inrush geological model was established, and the formation process of the water inrush channel in the water-resistant rock layer and the influencing factors of water inrush were analyzed through the numerical simulation of particle flow. Preliminary prediction and positioning of the subsea tunnel crossing the weathering groove water inrush channel are the basis for the grouting reinforcement design of the undersea tunnel crossing the weathering groove.

2. Overview of the Undersea Tunnel Project

The YK8+326~YK8+460 section of the F1 weathering trough on the right line of a submarine tunnel is 134 m long, with a sea depth of 10 m to 18 m, and a roof covering layer thickness of 30 to 35 m. It is dominated by strongly weathered gray-yellow granodiorite, which contains brown-yellow bands, interspersed with monzonite veins and partially white waxy kaolin. According to the TSP detection results and the geological data of the ultra-long exploration hole, the weathering trough is roughly narrow on the left and wide on the right, with a thin upper part and a lower thickness similar to a horizontal horn, as shown in Figure 1.

The groundwater of the F1 weathered deep trench is mainly recharged by the vertical infiltration of seawater, and it is a fully and strongly weathered granite formation [17]. Fully and strongly weathered granite formations have low strength and almost no self-stabilization ability below the groundwater level. They are very sensitive to water penetration and are prone to seepage damage.

According to the probe test, the maximum water pressure of the F1 weathered deep groove is 0.3–0.5 MPa, and the maximum water inflow of a single hole is 50 m³/h. The influencing factors of water inrush in submarine tunnels mainly include the water source, water pressure, water barrier, geological structure and tunnel construction behavior. The geometric shape of the longitudinal section of the tunnel is equivalent to a rectangle according to the condition of an equal area. According to the spatial relationship between the F1 weathering groove structure and the longitudinal section of the tunnel, when the weathering groove is located obliquely above the tunnel, the longitudinal upper and lateral cross water inrush model is established (Figure 2). When the weathering trough is located obliquely below the tunnel, it establishes a longitudinal downward and lateral cross water inrush model.

In the simulation process, the rock formation between the tunnel and the F1 weathering trough is regarded as a relative water-resistant rock layer, and the magnitude of the risk of water inrush in the tunnel as the thickness of the relative water-resistant rock layer changes is simulated quantitatively. The basic assumptions are as follows:

(1) Considering the influence of seawater pressure, the static pressure of seawater is considered according to its own weight; uniformly distributed load is applied on the surface of the tunnel model, and the water pressure of 18 m water depth is considered.

(2) The F1 weathering tank is a saturated soil containing pressurized water, which has good water permeability and is connected to seawater, and the pressurized water pressure is 0.5 MPa.

(3) Boundary conditions: the lower part of the model is fixed, the left and right boundaries restrict horizontal movement, the side and bottom edges are impervious boundaries and the tunnel excavation boundary is pervious.

(4) Through stress release, the dynamic unloading effect of the surrounding rock of the tunnel is simulated.

Due to space limitations, this article only shows the longitudinal upper and lateral cross water inrush model and calculation results (the same below).

3. Establishment of a Mesoscopic Model of Water Inrush Channel

Based on our understanding of the water inrush characteristics of the weathering trough of the above-mentioned subsea tunnel, this section analyzes and discusses the formation process of the water inrush channel of the F1 weathering trough, the key parts of the water inrush and the influencing factors of the water inrush through the numerical simulation of the particle flow.

3.1. Calculation Model

According to Figure 2, a calculation model of the longitudinal cross-section water inrush mode is set up (Figure 3), and the model size is width × height (b × h) = 50 m × 65 m. The width of the weathering trough is 8 m and the inclination angle is 60°; the tunnel excavation simulates the excavation of the upper step and the size is 13 m × 7 m. It includes 17,666 particles and 15,543 domains.

3.2. Calculation Parameters

There is a process of trial and error for obtaining appropriate meso-mechanical parameters from macro-mechanical parameters, because there is no theory to support the correlation between the two sets of mechanical parameters [8]. In view of the different computational mechanical parameters of the tunnel surrounding the rock and weathering trough rock mass, considering the calculation amount and calculation accuracy requirements, the parameters of soil particles and fluid domains can be found in the literature [18,19,20].

3.3. Simulation Scheme and Process

In numerical calculations, to reveal the process and mechanism of water inrush in weathered channel pipelines caused by changes in the thickness of impermeable rock columns during tunnel excavation, two calculation models of longitudinal cross-section staggered water inrush modes were constructed. The simulation process is as follows:

(1) The pore water pressure inside the F1 weathering groove is stable at 0.5 MPa before tunnel excavation, and the soil inside the weathering groove has good permeability and is connected to seawater. The permeability of the impermeable surrounding rock between the weathering groove and the excavation tunnel is poor, and the water pressure and surrounding rock in the weathering groove are in a mechanical equilibrium state.

(2) Based on the variation of the thickness of the impermeable rock column, the variation patterns of simulated tunnel surrounding rock stress release (dynamic unloading effect), maximum displacement of tunnel excavation face surrounding rock and diffusion range of weathering groove water pressure under various water inrush modes are analyzed.

4. Analysis of the Law of Water Inrush from the Weathering Trough

Due to the large number of calculation schemes and limited space, only the calculation results of the water-resistant rock layer thickness of 5 m under the longitudinal and lateral cross water inrush mode are used as an explanation.

4.1. The Law of Water Inrush in the Upper Lateral Cross Mode

(1) Penetration process of waterproof surrounding rock fissures during tunnel excavation. Figure 4 shows the distribution of cracks in the surrounding rock and the propagation process of the surrounding rock cracks as the tunnel excavation step increases when the thickness of the surrounding rock is 5 m under the action of 0.5 MPa-confined water in the weathering trough. The red part is the crack in the surrounding rock. Formed and penetrated, the arrow in the box indicates the direction of excavation of the tunnel face.

Before the excavation of the submarine tunnel, under the action of the confined water in the weathering groove, the water pressure of the weathering groove increased the compactness of the surrounding rock mass. The soil inside the weathering groove has good permeability, and some soil has hydraulic fracturing; the surrounding rock permeability is poor, and the water pressure of the weathering trough and the surrounding rock are in a mechanical equilibrium state. With the excavation of the tunnel, the surrounding rock stress is initially released (stress release 5%), the water pressure balance in the weathering trough is disturbed and some soils split and crack; the tunnel excavation causes stress concentration, resulting in the vault and the surrounding area of the arch foot. There are cracks in the local surrounding rock, but the disturbance range is small at this time.

As the surrounding rock stress of the tunnel continued to gradually release (10% stress release), cracks appeared in the water rock mass between the weathering groove and the tunnel, and the cracks in the local area of the tunnel excavation surface and in the weathering groove continued to increase, but the overall scope did not change much. When the stress release of the surrounding rock of the tunnel reaches a certain condition (stress release 20%), the fracture area near the vault and the arch foot of the tunnel excavation surface increases sharply, as does the splitting of the surrounding rock caused by the excavation and the splitting caused by the hydraulic fracturing in the weathering channel. The cracks move towards each other, and the surrounding rock near the tunnel vault and arch foot interact with the cracks caused by the confined water in the wind flower trough.

Therefore, in the process of tunnel excavation, special attention should be paid to the rupture of the vault and the surrounding rock of the arch foot. These are the key parts of the submarine tunnel’s pressurized water weathering trough for a water inrush disaster.

(2) Changing process of displacement field during tunnel excavation

Figure 5 shows the distribution form and change process of the particle displacement field with the continuous increase of tunnel excavation steps under the action of 0.5 MPa of pressurized water in the weathering trough.

In the initial stage of stress release of the surrounding rock of the tunnel, the displacement of the face of the tunnel excavation gradually increases, and the area where the rock mass is broken and loosened gradually increases. As the stress of the surrounding rock of the tunnel continues to be released (10–20% stress release), the hydraulic fracturing area of the soil in the weathering trough expands sharply, resulting in a hollow area, and some particles are lost.

When the critical thickness of the isolation rock pillar is penetrated at the arch foot, the displacement of the surrounding rock at the face of the tunnel excavation suddenly doubles, and the damage range of the surrounding rock expands sharply. The particles flowed sharply into the excavation surface (Figure 5).

4.2. Analysis of Water Inrush Law

It can be seen from Figure 6 and Figure 7 that due to the different spatial cross-arrangement of the tunnel axial direction and the weathering groove axial space, the location and range of cracks and failures in the isolated rock pillar between the weathering groove and the excavated tunnel are different; however, under the conditions of a subsea tunnel, most of the key parts of the confined water weathering groove for water inrush are located near the tunnel vault and arch toe. During tunnel excavation, the initiation, connection, expansion and penetration of micro-cracks in the surrounding rock under the action of the high water pressure gradient of the weathering trough are mechanisms of the formation process of the water inrush channel of the F1 weathering trough. During the tunnel excavation process, the stress concentration caused by the excavation will cause the crack damage zone in the surrounding rock. When the damage zone communicates with the crack produced by the weathering trough water pressure, it will inevitably cause a water inrush and mud inrush accident.

The key to tunnel construction is to maintain the stability of the surrounding rock in front of the work; that is, to take advance pre-reinforcement measures for bad geological bodies in the front before excavation to a critical distance. The thickness of the critical water-resistant surrounding rock under the geological conditions of the F1 weathering trough is about 5 m, and its advanced curtain grouting reinforcement design should be implemented at least 5 m in advance of the poor geological body to implement full-section or partial-section pre-grouting reinforcement.

5. Engineering Case Analysis

For the subsea tunnel crossing the F1 weathered trough project, advanced curtain grouting reinforcement is an effective auxiliary construction measure. The reinforcement range shall be fully or partially reinforced with pre-grouting at least 5 m in advance before the arrival of the bad geological body.

5.1. Grouting Reinforcement Plan

Combined with the formation conditions of the F1 weathering trough, a composite grouting construction plan is formulated:

(1) First, the new grouting material, Malisan N, is used for the water shutoff design on the water-rich fissure weathered granite formation on the lower step; the grouting method adopts the penetration grouting reinforcement scheme, and the designed maximum grouting pressure is 2 MPa (Figure 8). The purpose is to resolidify the fractured rock mass, strengthen the surrounding rock and, at the same time, block and reduce the seepage water, quickly and effectively block the water gushing fissure, reduce the formation permeability coefficient and solve the permeability problem of the surrounding rock.

(2) Then, on the upper step, full-face advanced curtain grouting is the main method for the dense and low-permeability fully and strongly weathered granite anhydrous strata. The grouting method adopts a split grouting reinforcement scheme, and the maximum grouting pressure is designed to be 4 MPa. A critical-crossing pulp vein skeleton is formed. The formed pulp vein squeezes the soil and strengthens the soil with the function of the pulp vein skeleton. At the same time, it has the functions of infiltration, filling and compaction, and solves the problems of the strength and stability of the surrounding rock.

5.2. Analysis of Reinforcement Effect

After the excavation of the grouting wall is broken, the tunnel face is dry and basically water-free, and the grouting veins in the soil can be clearly seen after the excavation. The stability of the tunnel face has been significantly improved compared with that before grouting. With the cooperation of the large pipe shed, the collapse of the soil is effectively controlled and the excavation can be carried out smoothly (Figure 9).

After the initial support is completed, the surrounding rock deformation is very small, the maximum rate of vault sinking and peripheral convergence is less than 5 mm/d, the surrounding rock deformation is small and the stability is good. The length of this grouting cycle is 40 m and the cavern is successfully excavated to 35 m, leaving 5 m as the grouting wall for the next cycle; the composite grouting reinforcement of the F1 weathering trough has achieved good results.

6. Conclusions

(1) During the excavation process of the submarine tunnel, the stress concentration caused by the excavation would cause the surrounding rock to produce fissure damage areas. When the damage area communicates with the fissures generated by the weathering trough water pressure, it will inevitably cause water inrush and mud inrush accidents. It is also the mechanism of the formation of the water inrush channel in the weathering tank.

(2) Ensuring the integrity and stability of the critical water-resistant rock mass is the key to preventing water inrush. Preliminary prediction and positioning of the subsea tunnel crossing the weathering groove water inrush channel is the basis of the subsea tunnel crossing the weathering groove grouting reinforcement design; the thickness of the critical water-resistant surrounding rock under the geological conditions of the F1 weathering groove is about 5 m, and it should be before the arrival of the bad geological body. Grouting reinforcement should be implemented at least 5 m in advance.

(3) Based on a certain degree of model simplification, this paper only analyzes the law of water inrush in the axial direction of the tunnel, which is an attempt of particle flow in the study of water inrush in submarine tunnels. The real submarine tunnel water inrush is spatially distributed, and the impact of the weathering groove on the critical water pressure of the water inrush and the critical thickness of the surrounding rock cannot be ignored, which needs to be discussed in future studies.