Analysis on the crown convergence deformation of surrounding rock for double-shield TBM tunnel based on advance borehole monitoring and inversion analysis

https://doi.org/10.1016/j.tust.2020.103513Get rights and content

Highlights

  • A convergence deformation prediction method for double-shield tunnels is proposed.

  • Deformation is monitored by advance borehole and a measuring tube with FBG sensors.

  • Constitutive parameters are obtained by inversion of deformation monitoring data.

  • Errors caused by reference point displacement are corrected by numerical model.

Abstract

The surrounding rock deformation during tunnelling is difficult to be measured or calculated accurately, especially in tunnels constructed by double-shield TBM. A method for prediction of the crown convergence deformation of surrounding rock based on advance borehole monitoring and numerical inversion analysis is proposed. Combined with the field deformation monitoring and numerical simulation, the final crown convergence deformation of surrounding rock is obtained. When TBM advances to the test section, the horizontal directional drilling machine is used to drill an advance borehole at the top of tunnel and 1.6 m away from the tunnel face, and the measuring tube with fiber Bragg grating (FBG) sensors has been installed in the borehole to conduct the deformation monitoring. Then, based on the numerical simulation with the viscoplastic creep constitutive model, the final crown deformation of surrounding rock during TBM tunneling process has been predicted, in which the parameters of surrounding rock are obtained through the inversion analysis of deformation data and the error caused by the displacement of reference point has been corrected. The surrounding rock crown convergence deformation was also estimated by using a scaled instrument in the grout hole of lining segment and is approximately the same with the above predicted value, which show that the proposed method can well analyze the surrounding rock crown convergence deformation during tunneling for double-shield TBM tunnels.

Introduction

Double-shield TBM (tunnel boring machine) is more and more widely used in deep tunnel engineering based on its enhanced safety and efficiency and small disturbance to surrounding rock (Hasanpour et al., 2018, Yu et al., 2020). After the excavation and unloading of the surrounding rock in deep-buried tunnel, the initial triaxial stress state changes to biaxial stress state, resulting in stress redistribution and stress concentration, and the surrounding rock converges to the center of the tunnel, which is easy to cause TBM jamming. Convergence deformation can directly reflect the deformation characteristics of surrounding rock, and the monitoring of convergence deformation is the simplest, direct and most widely used one among various monitoring methods (Mezger et al., 2013, Kontogianni and Stiros, 2002). In a cross section of tunnel, the deformation over the tunnel crown is usually the largest, and the TBM jamming is generally caused when the surrounding rock over the TBM squeezes the shield (Wu et al., 2018, Liu et al., 2016). Therefore, the analysis on the crown convergence deformation of surrounding rock is the most important. However, it is difficult to carry out in-situ monitoring in the tunnels constructed by double-shield TBM. Effectively obtaining the crown convergence deformation of surrounding rock is of great significance for analyzing the relationship between deformation value, deformation rate, time and advance distance, etc., which is useful for preventing TBM jamming and ensuring safe construction.

For the analysis of the surrounding rock convergence deformation, different scholars have carried out many corresponding studies. The research methods can be summarized into two categories: monitoring measurment method and numerical analysis method.

In terms of the monitoring measurement method, different researchers have put forward all kind of monitoring methods, and based on different physical principles, many types of sensors and measuring systems have been manufactured (Ariznavarreta-Fernández et al., 2016, Li et al., 2015, Simeoni and Zanei, 2009). However, various monitoring methods above are mostly used in tunnels constructed by non-shield method. For the double-shield TBM tunnel, the surrounding rock is obstructed by shield and lining segments throughout the construction process, so it is difficult to monitor the convergence deformation of surrounding rock (Liu et al., 2020). For the deformation monitoring in tunnels constructed by double-shield TBM, several attempts have been carried out in recently years. Farrokh and Rostami (2009) have measured the convergence deformation at the shield tail by using the scaled instrument through the reserved grout hole of lining segments. However, it can only measure the convergence deformation of some points roughly and cannot get the deformation law during tunneling. In the construction process Jinping-Ⅱ hydropower station in China, the transverse tunnel between the diversion tunnels is used to excavate the adit, and the displacement monitoring of the diversion tunnel is conducted by arranging fiber Bragg grating and multi-point displacement meter in the adit (Zhang et al., 2014, Chu et al., 2014). However, the monitoring method above requires an adit vertical to axis of tunnel, which is not suitable for the general situation and the construction of adits will greatly increase the cost of the project. Overall, effectively and economically monitoring of the surrounding rock deformation is still a challenge in double-shield TBM.

With the development of the computer technology and numerical simulation methods, three-dimensional numerical method has become an important means for the calculation and analysis of engineering problems based on its low cost and high efficiency. Many scholars have applied numerical methods to the study of tunnel deformation and stability analysis and obtained many research achievements (Sakcali and Yavuz, 2019, Guan et al., 2018, Nomikos et al., 2011, Pellet et al., 2009). The accuracy of numerical calculation depends on the selection of constitutive model and its parameters. For the constitutive model, the classical elastoplastic model or its improved model are often used to simulate the deformation characteristics of rock mass (Ring and Comulada, 2018, Ninić et al., 2020). However, the deformation of rock mass with deep overburden will show obvious time effect due to it is under high geo-stress. Therefore, it is necessary to adopt the constitutive model considering the rheology of surrounding rock. After determining the proper constitutive model, the setting of parameter values will become the main influencing factor of calculation accuracy. However, it is very difficult to select the parameters of surrounding rock because the composition of surrounding rock is more complex than other materials, and its mechanical properties are affected by rock mass structure and geological condition. Currently, the main way to solve this problem is to carry out inversion analysis based on the monitoring information in engineering site (Yang, 1996). Through inversion analysis, the equivalent geotechnical parameters can be obtained for specific numerical model, which can reflect the mechanical properties of the rock mass near the excavation area. Generally, inversion analysis is mainly based on the actual displacement data. Therefore, for shield tunnel, it is necessary to propose an effective monitoring method for the surrounding rock deformation. Based on the displacement monitoring data, the constitutive parameters can be obtained by inversion analysis, and then the numerical model can be used to analyze the surrounding rock deformation in the whole construction process.

In this paper, a method for prediction of the surrounding rock crown convergence deformation in double-shield TBM tunnel based on advance borehole monitoring and numerical inversion analysis is proposed. A measuring tube based on the sensing principle of fiber Bragg grating (FBG) was designed. When TBM advances to the test section, the horizontal directional drilling machine is used to drill an advance monitoring borehole at the top of tunnel and 1.6 m away from the tunnel face. And the measuring tube based on the quasi-distributed technology of FBG sensors is installed in the advance borehole to monitoring the surrounding rock deformation during tunneling. Then a comprehensive numerical model considering the process of TBM excavation is established, and the time-dependent behavior of surrounding rock is simulated using the viscoplastic creep model based on the internal variable thermodynamics theory. The constitutive parameters of surrounding rock was obtained by numerical inversion analysis of the deformation data obtained by advance borehole monitoring. And the error caused by the displacement of reference point was corrected by numerical simulation. The actual crown convergence deformation of surrounding rock has also estimated by grout hole and scaled instrument and estimated deformation is used to verify the predicted surrounding rock crown convergence deformation by the method proposed. The overall flow chart of this study is shown in Fig. 1.

Section snippets

Project descriptions

DXL tunnel is a highway tunnel in southwest China, and the total length of the tunnel is about 4.78 km, as shown in Fig. 2. There are abundant mountains along the tunnel, with an elevation of 3547–3566 m, and the maximum overburden thickness of tunnel is about 820 m. The construction length by double-shield TBM is about 4489 m, accounting for 94% of the total length of the tunnel, and the excavation diameter is 9.13 m. Table 1 lists the TBM geometry parameters and related technical parameters.

Measuring principle of fiber Bragg grating

Based on the sensing principle of fiber Bragg grating (FBG) and the quasi-distributed technology of FBG sensors, a measuring tube was designed to monitoring the convergence deformation of surrounding rock. Fiber Bragg grating uses the photosensitive properties of fiber materials to form a spatial phase grating in the fiber core, so as to change and control the propagation behavior of light signals in the fiber core (Hill and Meltz, 1997). The refractive index of FBG shows a fixed periodic

Inversion analysis based on advance borehole monitoring data

In order to analyze the convergence deformation of surrounding rock during TBM excavation process, viscoplastic creep model based on the internal variable thermodynamics theory is used to simulate the time-dependent behavior of surrounding rock. And the creep model is implemented in FLAC3D based on a user-defined model. Additionally, the construction process of double-shield TBM is elaborately simulated. Based on the inversion analysis of the monitoring data of the advance borehole, the

Verification of the predicted results based on estimated value

For the convenience of grouting operation, there are reserved grout holes on the lining segments. Firstly, the gap between surrounding rock and segment lining after deformation can be measured by using a scaled instrument in the grout hole of lining segment. Then, the crown deformation of surrounding rock can be estimated considering the overcut of TBM. The estimation principle is shown in Fig. 15. At the top of the tunnel, a scaled instrument is extended into the grout hole at a certain angle α

Conclusion

The convergence deformation can directly reflect the deformation characteristics of surrounding rock, and guide the design and construction of a tunnel. However, it is difficult to monitor the surrounding rock deformation due to the obstruction of shield and lining segments for tunnels constructed by double-shield TBM. In this research, a method for prediction of the surrounding rock crown convergence deformation based on advance borehole monitoring and numerical inversion analysis is proposed

CRediT authorship contribution statement

Chaoyi Li: Conceptualization, Investigation, Resources, Visualization, Writing - original draft, Project administration. Shaokang Hou: Conceptualization, Formal analysis, Data curation, Writing - original draft. Yaoru Liu: Conceptualization, Methodology, Project administration, Writing - review & editing, Software, Funding acquisition. Pengxiang Qin: Conceptualization, Methodology, Project administration, Validation. Feng Jin: Conceptualization, Methodology, Supervision, Writing - review &

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China with Grant No. 41941019 and the State Key Laboratory of Hydroscience and Engineering with Grant No. 2019-KY-03.

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