Elsevier

Measurement

Volume 176, May 2021, 109170
Measurement

Deformation behavior monitoring of a tunnel in its temporary shoring demolishing process using optical fiber sensing technology

https://doi.org/10.1016/j.measurement.2021.109170Get rights and content

Highlights

  • A novel curvature algorithm is proposed for displacement monitoring.

  • The performance of a tunnel during its shoring demolition process of is obtained.

  • The most dangerous step for tunnel’s temporary shoring demolition is identified.

Abstract

For tunnels with large cross-sections, demolition of the temporary shoring of the tunnel can pose a very serious threat to the safety of the structure; Therefore it is very important to ascertain the deformation behavior of the tunnel in this stage. In this study, a tunnel settlement inversion model, based on the strain measurements of distributed optical fiber, has been proposed and the feasibility of the model has been verified by laboratory tests. Based on field observations, the strain distribution characteristics of the optical fibers and the variations in the mechanical properties of the tunnel were analyzed, taking the demolition process into consideration. The results of the monitoring indicated that demolition of the bottom shoring will cause large and uneven crown settlements; this is because this step will weaken the vertical supporting force at the bottom of the arch. Besides, continuous removal of the bottom shoring of the tunnel can result in large settlement and deformation of the crown, which should be pay attention in this stage. The settlement curve of the tunnel has also shown that the strain distribution of the optical fibers was able to reflect the performance of the tunnel in the demolition process of its temporary shorings.

Introduction

In the construction of subway tunnels, the New Austrian Tunnelling Method (NATM) [6] has been widely adopted in China because of its simple, adaptive and cost-effective technical feasibility. When using the New Austrian Tunnelling Method to excavate a large cross-section tunnel, Temporary shoring is required to stabilize the tunnel that has been excavated. Many researchers have studied the mechanical properties of the temporary shoring: Choi and Shin made an analysis of the stability of a tunnel excavated in a weak rock mass. Based on the analysis results, they proposed a new optimal support patterns for the tunnel [3], Kolymbas proposed a calculation method of the axial force and bending moment of the temporary support in a tunnel [12], Luo et al. studied the deformation rule and mechanical characteristics of temporary shoring in soil tunnel constructed by sequential excavation method [14].

It can be concluded from these studies that the existence of temporary shoring is of great significance to the stability of a tunnel. Support structures such as these usually undergo large deformation and are subjected to complicated forces during the demolition process; therefore this stage is dangerous and can cause accidents [15]. Many researchers have carried out profound and careful research on the demolition of the temporary shoring of tunnels: Zhang et al. systematically studied the mechanical properties and structural responses of the tunnel lining for high-speed railway tunnels excavated in loess ground, and indicated that the lining’ s contact pressure reaches its first peak value when the tunnel’ s shoring is removed [21], Ivanes et al. studied the stress distribution of the support arches after the tunnel’s temporary shoring is removed using finite element method [9]; Ghazvinian et al. analyzed the impact of demolishing temporary shoring over stability of dam diversion tunnel [1]; Zhang made an analysis on deformation of the temporary shoring in its demolition process based on the monitoring data [20]. However, they mainly studied the deformation of an individual section rather than the overall deformation of the tunnel; research on an entire tunnel’s deformation is rare. In addition, for most of the existing means of tunnel engineering monitoring (such as total station), only the behavior of the material at the sensor’ s location can be obtained. Any information relating to the structure’s overall deformation behavior is difficult to obtain. This means that these devices or sensors are unable to obtain the distributed deformation along the whole tunnel. This makes it difficult to study the deformation rule of the whole tunnel in the process of demolishing the temporary shoring.

In recent years, optical fiber sensing monitoring technology, such as the Brillouin optical time domain reflection strain/temperature measurement technology (BOTDR) and the strain/temperature measuring Brillouin optical time-domain analysis technique (BOTDA) have been widely used in structural health monitoring of geotechnical engineering.

The distributed optical fiber sensing system based on BOTDR technology can detect the external temperature or strain by using self-releasing Brillouin scattering effect, which uses a single end incident laser. The optical maser emits continuous light with a certain frequency. Through the external modulation technology, the detection pulse light is formed. The pulse light enters the sensing optical fiber and generates self-releasing Brillouin scattering light signal with certain frequency shift and frequency, and returns to the detection equipment along the sensing optical fiber. The distribution of Brillouin frequency shift along the fiber distance is acquired by the data acquisition system. According to the linear relationship between the Brillouin frequency shift and temperature or strain, the monitoring can be realized [16]. However, the self-releasing Brillouin scattering effect is very weak, and the backscattered light can be easily affected by the sensing distance and has a large attenuation. Therefore, for long-distance sensing, the performance of BOTDR technology is unsatisfactory.

In 1980s, The BOTDA technology based on stimulated Brillouin scattering effect in optical fiber was born, which solved problems of insufficient measurement distance and low spatial resolution of the BOTDR technology. The monitoring system based on the BOTDA adopts the double-ended test method. Two laser beams are incident from both ends of the system, one of the beams directly enters the sensing optical fiber as continuous light, and the other laser beam is converted into pump pulse light after pulse modulation, and enters the optical fiber at the other end. When the pump pulse power reaches a certain threshold, stimulated Brillouin scattering effect occurs in the sensing optical fiber. According to the relationship between the Brillouin frequency shift and temperature or strain, the distribution of Brillouin gain (or loss) spectrum along the optical fiber can be demodulated. Further, the temperature or strain along the optical fiber can be obtained [19].

Many researchers have successfully applied these methods to tunnel monitoring: Mohamad et al. studied a field trial of BOTDR monitoring of a segmental bolted tunnel lining and indicated the maximum compressive strains measured below the tunnel spring line nearest to the excavated tunnel were larger than the maximum tensile strains measured at the tunnel crown [16]; Klar et al. studied the feasibility of automated detection of the tunnel excavation with Brillouin optical time-domain reflectometry and found that the proposed system was capable of detecting even small tunnel with 0.5 mm diameter, 20 mm depth and 10 m length [11]; Moffat et al. proposed a sensor based on Brillouin fiber optic in order to monitor the tunnel wall displacements [17]; Fajkus made an analysis of the highway tunnels monitoring using an optical fiber implemented into primary lining [5]. These studies suggest that optical fiber sensing monitoring technology can be used to take distributed measurements over extended structures. However, the optical-fiber sensing technology can only measure the strain or temperature distribution along the length of the optical fiber. As a result, while using the optical fiber sensing technology, most researchers pay attention to the monitoring of strain fields of the structure. In the process of construction of a tunnel, the deformation (like settlement deformations of tunnels) is more inclined to be the control standard, to evaluate the safety of the structure. Therefore, it is meaningful to study the use of the optical fiber sensing technology to monitor the distributed deformation of structures from a safety standpoint.

Jie et al. derived equations relating the strain measured by optical fibers and the subsidence of sea dykes at each minimum strain sampling section, and calculated the subsidence distribution [7];Based on the deformation regularity of tunnel roofs, Gong-yu et al. proposed three mathematical models, namely a circular arc model, a parabolic model, and a triangular model, to establish the relationships between the axial strain and the vertical deformation of a tunnel [8]; However, the influence of the neutral-axis thickness on the deformation calculation was not considered, which led to an error in the deformation calculation at low span height ratios of the structure. Kim et al. estimated the deformation of a concrete beam by rearranging the formula of the classical beam theory to link the deformation of the beam with strains measured directly using an optical fiber [10]; Sheng et al. presented an improved conjugated beam method for monitoring the deformation distributions of simply-supported and continuous structures [18]. These methods can accurately calculate the deformation of the structure, but only for certain boundary conditions, and the calculation process is complex.

In summary, present studies on deformation rule of a whole tunnel in the demolition process is rare and it is hard to obtain the monitoring data of a whole tunnel with conventional monitoring method. Optical fiber sensing technology can achieve distributed measurement, but the algorithm that transfer the measured strain to the deformation of the structure needs improvement. Based on the optical fiber sensing technology, a model used for calculating the deformation of a tunnel is proposed, and the deformation rule of a whole tunnel in the demolition process has been explored in this paper.

The paper is composed of three main sections: The first section presents the general principle behind DFOS (Distributed Fiber Optic Sensing) strain measurements and a tunnel settlement inversion model used for calculating the deformation of a tunnel based on the DFOS test data; the second section presents the results of the feasibility study of the proposed model; the third section presents the monitoring results of the field trial and the analysis of the tunnel’s performance in the demolition process of the temporary shoring.

Section snippets

The working principle of optical fiber sensing technology

When the axial strain or the temperature of an optical fiber changes, the frequency of the backscattered Brillouin light in the optical fiber will drift relative to the frequency of the injected light pulse, which is shown in Fig. 1(a). The drift of the frequency of the Brillouin scattered light has a linear relation to the axial strain and temperature of the optical fiber, as shown in Fig. 1(b). Fig. 1(b). shows this liner relationship of a common single mode bare optical fiber. By measuring

Experimental research into optical fiber for curvature measurements

According to the tunnel settlement inversion model proposed in the previous section, in order to calculate the settlement of the tunnel, the curvature of each micro segment must first be obtained. In order to validate the reliability of using optical fiber for curvature measurements, a laboratory test was carried out. Optical fiber was glued along a PVC tube with a radius of 10 mm. Aluminum alloy calibration blocks of 0.1 m height, and with a curvature radius of 1 m, 2 m and 3 m, were used to

Site description

The first phase of the new airport line in the Beijing Rail Transit System starts at K35 + 454.232 and ends at K35 + 544.232. The entire length of the line is 90 m with 10.7–12 m of soil cover and its slope is 20‰. The cross-section of the line is horseshoe-shaped, as shown in Fig. 11. The CRD (cross diaphragm) method was used for the excavation; the CRD method represents a shift from excavating one side of the tunnel at a time to excavating both sides alternately. This method can enable the

Strain measurement results

Fig. 18 has shown the measured strain data by the strain sensing optical fiber before the temperature compensation. Fig. 19 has shown the final strain measurements that were recorded in the optical fiber under all six steps presented in Fig. 13; In Fig. 19, temperature compensation had been applied to these results as this can directly reflect the performance of the structure.

For step 1, after the first, third and fifth sections of the bottom shoring had been demolished, the supporting

The settlement curve of the crown

Based on the tunnel settlement inversion model mentioned in Section 2.2, the settlement of the crown could be obtained using the strain curve that was recorded by the optical fiber as shown in Fig. 23. An FEM simulation was carried out to validate the settlement curve (this only refers to the change trend of the settlement curve obtained from the FEM simulation; the values were compared with the total station measurement data). The FEM model has been presented in Fig. 22. The model was meshed

Summary

In this paper, the performance of a tunnel in the demolition process of its temporary shoring has been extensively investigated through DFOS field observations. Based on the field measurements as well as the theoretical analysis, the following conclusions could be drawn:

  • (1)

    The calibration test showed that the displacement curve of the structure can be deduced from the strain curve of the optical fiber through the use of the tunnel settlement inversion model and the displacement obtained from the

CRediT authorship contribution statement

Zi-xiang Li: Conceptualization, Methodology, Writing - original draft, Software. Gong-yu Hou: Supervision. Tao Hu: Data curation. Tian-ci Zhou: Investigation. Hai-lin Xiao: Validation.

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

The authors are grateful for the support of the Central University Major Achievement Transformation Project in Beijing under Grant No. ZDZH20141141301 and the National Natural Science Fund Committee and Shenhua Group Co., Ltd. Jointly Funded Key Projects under Grant No. U1261212, U1361210.

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