Numerical Investigation on Crack Propagation and Fatigue Life Estimation of Shield Lining under Train Vibration Load

Abstract

Dynamic loads such as the train vibration load usually act on the shield tunnel lining in the long term, which could make the initial flaws in shield segment propagate and gradually weaken the robustness of the tunnel structure. In this paper, a three-dimensional numerical model of shield tunnel lining structure with the initial defect is built to study its dynamic reaction and fatigue crack propagation under the train vibration load. Furthermore, the damage to intact shield segment caused by train vibration load is studied by employing the rain-flow counting method and the Miner damage theory, and a rational fatigue life estimation for the concrete shield tunnel lining is finally made. Results show that crack propagation is influenced by both the train speed and train axle, the higher the train speed, the longer the final crack, and train axle has a larger influence than train speed on the crack propagation in shield tunnel segment. The shield tunnel lining structure of Nanjing Metro Line 5 can meet the demand of working for a hundred years under the current working conditions.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

As one kind of the public transportation, subway has played a more and more important role in the contemporary society nowadays, especially in the modern metropolises. The main structure of the subway system is the so-called shield tunnel lining structure, which usually consists of plenty of concrete shield segments. The shield segments are manufactured in the factory and transported to the construction site in a long distance, and they are usually assembled one by one in situ; thus, some initial flaws such as cracks may come into being in these shield segments. Furthermore, the lining structure of the shield tunnel is under train vibration load day and night, making these flaws propagate ceaselessly. All of these produce a large threat to the shield tunnel system. Moreover, even if the shield tunnel lining is intact without any defect, it has a significant and realistic value to make a rational life estimation of the shield tunnel lining.

During the past several decades, numerous numerical and experimental research studies have been conducted on shield tunnel structures. The Railway Engineering Society of the United Kingdom [1] studied the vertical wheel-rail force of the train by both theoretical analysis and experiment in 1970s. Newton and Clark [2] compared the Timoshenko beam model with the Euler beam model and concluded that the predicted wheel-rail forces of the train using these two models are close to each other, while the Euler beam model is much simpler than the other one. Yang and Hung [3–5] proposed a simplified 2.5D finite element-infinite element coupling model, using finite element to simulate the near field and infinite element to simulate the far field, and obtained quite good results. Sheng et al. [6, 7] proposed a semispace numerical method for a circular tunnel based on the wavenumber finite and boundary element method. Degrande et al. [8] developed a finite element-boundary element coupling method to calculate subway vibration and proposed a highly efficient numerical model. Forrest and Hunt [9, 10] proposed a pipe-in-pipe model to study the train-induced ground vibration in underground tunnel, and the computational efficiency of this model is rather good. Seleznev et al. [11, 12] developed a method to study an elastic half-space containing a cylindrical tunnel. Gardien and Stuit [13] proposed a model consisting of three submodels to analyse the tunnel vibration induced by train and to reduce the numerical burden using finite element analysis. Other research studies on tunnel vibration included the application of Green’s functions [14] and three-dimensional finite element simulations of long tunnels in half-space [15–17]. For the studies of practical projects in engineering, Lai et al. [18] conducted a study concerned with the assessment of the vibration induced by the passage of commuter trains running in a tunnel placed underground of the city of Rome, using both the numerical simulation and field monitoring. Huang et al. [19] studied the dynamic characteristics of the tunnel invert for high-speed railways. Connolly et al. [20] analysed 1500 records of ground vibration from 17 high-speed railway stations in Europe and found that the soil property was the main factor which had an influence on the ground vibration. Yi et al. [21] and Wei et al. [22] studied the dynamic reactions of Guangzhou Metro Line by both the indoor model tests and numerical simulations. Besides, a series of numerical simulations on damage and fatigue analysis of some representative structures in civil engineering have been conducted [23–27], and lots of useful results have been obtained for practical engineering.

The shield tunnel lining segment is usually precast with concrete in factories. Concrete is a kind of man-made material which is brittle and sometimes fragile. Lots of studies have been conducted on the fracture and fatigue properties of concrete in the past century [28–32]. Besides, Shah [33] acquired the fracture parameters for the concrete fracture model using the three-point bending test. Kazemi [34] proposed the process zone model and applied it to the fracture analysis of concrete structures. Karihaloo and Nallathambi [35] found that the fracture toughness of concrete was not determined by the shape or dimension of the specimen but by the size effect model with mixed variables. Xu et al. [36, 37] proposed the double-K fracture criterion for concrete, which has been widely used for the fracture analysis of concrete structures since then. By employing the double-K fracture criterion of concrete, Tian et al. [38, 39] conducted prototype load tests and three-dimensional numerical simulations to investigate the fracture property and failure model of concrete shield segments.

In this paper, a three-dimensional numerical model of the shield tunnel lining structure based on Nanjing Metro Line 5 is built in ABAQUS to study the structure reaction and fatigue crack growth under train vibration load. Besides, damage to the shield segment induced by the train vibration load is also calculated by employing the Miner damage theory and S-N fatigue law of concrete, and the fatigue life of the concrete shield lining is rationally estimated.

2. Numerical Model and Train Vibration Load

2.1. Numerical Model of the Shield Lining

The numerical model of the shield tunnel lining is built according to the real shield tunnel of Nanjing Metro Line 5 in China. Thickness of the segment is 350 mm, and the inner diameter of the tunnel lining is 5500 mm. The intact lining ring consists of six components, which include one sealing-roof segment, two adjacent segments, and three standard segments. The joints of the adjacent segments are connected by M30 bolts. The cross section of the tunnel lining ring is illustrated in Figure 1, in which A denotes the standard segment, B denotes the adjacent segment, and K denotes the sealing-roof segment. Due to the complexity of rebars in the concrete shield tunnel segments, they are not considered in the current numerical simulation.

[figure omitted; refer to PDF]

The numerical models of the shield tunnel are built in FEM software ABAQUS, which are shown in Figures 2–4. Note that there are circumferential bolts and longitudinal bolts in the numerical model of the shield tunnel lining, as shown in Figure 4, and they are “tied” with the concrete in the numerical simulation.

[figures omitted; refer to PDF]

[figure omitted; refer to PDF]

The time-history curve of the stress in the standard segment at the bottom is analysed using the rain-flow counting method, and the amplitude, average value, and loading cycles of the stress are finally obtained, which is illustrated in Figure 12.

[figure omitted; refer to PDF]

The damage to the shield tunnel segment caused by train vibration load for one time can be obtained by the above stress analysis, as in Figure 12, using the Miner damage theory and S-N equation of the concrete, which is estimated as 2.42 × 10⁻⁸, and its corresponding fatigue life lgN is about 7.62.

The train of Nanjing Metro Line 5 departs 129 times a day and consists of 6 carriages. So, the cyclic time of the train vibration load for 100 years is about N₁₀₀ = 6 × 129 × 365 × 100 = 28251000, and its corresponding lgN₁₀₀ = 7.451 < 7.62. Therefore, the shield tunnel lining structure of Nanjing Metro Line 5 can meet the demand of working for a hundred years under such working conditions.

5. Conclusions

In this paper, a three-dimensional numerical model of the shield tunnel lining structure is built to investigate the structure reaction and fatigue crack growth under train vibration load. Furthermore, damage of the shield segment caused by train vibration load is studied by employing the Miner damage theory and S-N fatigue law of the concrete, and thus a rational fatigue life estimation for the concrete shield tunnel lining can be finally made. The following conclusions can be drawn:

(1) Crack propagation shows the overall same path under different train speeds, while the final crack lengths are a little different. The higher the train speed, the longer the final crack.

(2) Train axle has a larger influence than train speed on the crack propagation. The final crack under a heavier train axle is so long that it almost cuts through the shield segment, while the final crack under a lighter train axle is so short and it only develops to a local crack.

(3) The damage of the shield segment caused by train vibration load can be determined by the stress analysis using the rain-flow counting method, and the results show that the shield tunnel lining structure of Nanjing Metro Line 5 can meet the demand of working for a hundred years under the current working conditions.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (grant no. 51808114) and Natural Science Foundation of Jiangsu Province (grant no. BK20170670).

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