Transversely isotropic creep behavior of phyllite and its influence on the long-term safety of the secondary lining of tunnels
Introduction
The creep behavior of rocks has a crucial influence on the long-term safety of engineering structures (Feng and Jimenez, 2015). The influence is more complicated for transversely isotropic rocks due to the presence of weak planes (e.g., stratification, bedding, joint, sheet, and foliation). Previous studies have focused mainly on the short-term mechanical behavior of such rocks, including the tensile behavior of sandstone (Roy et al., 2017), slate (Dan, 2011), and gneiss (Dan, 2011) and the compressive behavior of phyllite (Kumar, 2006), gneiss (Karakul et al., 2010), shale (Chen et al., 2018;), sandstone (Khanlari et al., 2015), and basalt (Vishnu et al., 2018). These research results show that the tensile strength, compressive strength, and fracture patterns were affected significantly by weak plane-loading angles. However, limited research has been conducted to study the creep features of transversely isotropic rocks.
Based on the triaxial compressive creep test, Fu et al. (2007) found that the nonlinear creep rate of oil-bearing mudstone increased as the weak plane-loading angles increased. Dubey and Gairola (2008) conducted multi-stage uniaxial compressive creep tests to study the influence of structural anisotropy on the time-dependent deformation of rocksalt with θ = 0°, 45°, and 90°. Their results showed that θ affected the instantaneous, steady, and accelerated strain of rock and that these effects tended to be weaker as the axial loading stress increased. Liu et al. (2015) used a series of one-step, triaxial compressive tests to study the creep behavior of cox argillite with θ = 0° and 90°. In their study, the steady creep rate of rock with θ = 0° was larger than the rate with θ = 90°. Zhang and Rothfuchs (2004) used uniaxial creep and relaxation tests to study the long-term behavior of cox argillite that was buried at different depths, had different water contents, and had different sizes. Their findings indicated that there was no significant scale effect and no evident anisotropic effect on the creep behavior of cox argillite.
For tunnels situated in layered strata, anisotropic squeezing deformation of surrounding rocks and failure of supporting structures are easily to be encountered under the joint action of weak planes and creep effect of rock mass, such as Frejus tunnel in calcschist (Panet, 1996), Sidi Mezghiche tunnel in flysch (Panet, 1996) and drifts of the Underground Research Laboratory in callovo-oxfordian clayston (Carrillo et al., 2016). In these tunnels, the convergence in the direction orthogonal to weak planes were dominant and asymmetric yield or strong support system were adopted to control the anisotropic deformation. During the construction of highways and high-speed railways in western China, a large number of tunnels will cross through phyllite stratum. Phyllite is a low-grade, metamorphic rock with well-developed foliations. Its creep effect may lead to cracking of secondary lining, causing a negative impact on tunnel operation. For example, cracks appeared widely in secondary lining of Xuecheng tunnel situated in national road 317, China, after two years of operation due to the strong creep deformation of phyllite (Wang et al., 2018). Then, the tunnel was closed for maintenance for one year and reinforcement methods, including grouting and applying bolts at arch springing, were implemented to suppress the propagation of cracks. Therefore, investigating the transversely isotropic creep behavior of phyllite is of immense importance and plays a vital role in the design and construction of tunnel supporting structures.
In this paper, we report the results of our study of the time-dependent behavior of phyllite based on the combination of numerical simulation and experimental studies. The failure strength, fracture pattern, and failure process were obtained through a series of uniaxial compressive creep tests of specimens with different θ and different water contents. A new numerical model was developed to describe the entire creep process of transversely isotropic rocks, including the decay, steady, and accelerated creep stages). This numerical approach was used to investigate the failure process in the secondary lining of a tunnel situated in the phyllite stratum. The purpose of our research is to reveal the transversely isotropic creep behavior of phyllite and propose a method to guide the design and construction of geotechnical engineering structures related to layered rock mass.
Section snippets
Geological environment
The altitude of China decreases from the west to the east with a ladder form. The first ladder contains mainly the Tibetan plateau, which has an average altitude of 4000 m. The second ladder lies between the plain and the Tibetan plateau, and it has an average altitude in the range of 1000 to 2000 m. The third ladder spans the entire eastern part, which has an average altitude of 500 m.
In recent years, many highways and railways have been built or planned in the transition zones from the second
Preparation of Specimens
Phyllite specimens were collected from the Zhegu mountain tunnel. The original sizes of the rock blocks were more than 300 × 300 × 300 mm to ensure the integrity of the specimens. The wet drilling method (Fig. 4) was used to obtain specimens with different angles, i.e., θ = 90°, 30°, and 0°. The processed cylindrical specimens had diameters of 50 mm, lengths of 100 mm, an error of ±0.5 mm, and a face parallelism of ±0.02 mm. Table 2 shows the test cases.
Experimental process
Tests were conducted using an apparatus
Stress-strain curve
Fig. 5 shows the creep curves of saturated (Xu et al., 2019) and dry specimens with θ = 90°, 30°, and 0°, respectively. Taking dry specimens as examples, to analyze the creep behavior, first, the creep curves for dry rocks with each loading level (Fig. 6) were derived based on the test results. Then, the detailed creep features of the rocks can be obtained in each loading stage. Fig. 7 shows the creep curves at the first and final loading stages. It shows that, before the final loading stage,
Numerical simulation
Based on the bonded parallel contact (BPC) and the smooth joint contact (SJC) in the particle flow code (PFC) software (Itasca Consulting Group, 2008), researchers have established some numerical approaches that can describe the tensile and compressive behavior of transversely isotropic rocks (Duan et al., 2016; Wang et al., 2017). In BPC, a bond can transmit both the force and moment between adjacent particles, and it will break if external stresses exceed its tensile or shear strength. The
Engineering application
The secondary lining of tunnels in soft layered stratum are likely to be cracked during their long-term operation. Therefore, in this section, the failure process in secondary lining of tunnels situated in the phyllite stratum is investigated through numerical simulation.
Discussion
The best way to validate the numerical simulation is to conduct similarity model tests to identify the process of gradual damage that occurred on the secondary lining. In our previous study (Xu et al., 2020), we studied this process by gradually increasing the stress on the loading plate. This method can approximate the continuous increase of external loads on the lining due to the creep effect of surrounding rock in a model test. Fig. 19 shows that the process by which the test lining cracked
Conclusions
The uniaxial compressive creep test was conducted to study the time-dependent behavior of phyllite. A new numerical model based on discrete element method was proposed to describe the transverse isotropy and three creep stages of rocks, and this numerical approach was used to analyze the failure process of secondary lining in layered stratum. The main conclusions of this study are as follows:
- 1)
Foliation and water content have significant effects on the creep behavior of phyllite. The creep
Declaration of Competing Interest
The authors declare that they have no conflicts of interest.
Acknowledgements
This research was supported by the Key R&D Program of Sichuan Province, China (No. 2019YFG0001), the National Natural Science Foundation of China (No. 52008351), the Postdoctoral Science Foundation of China (No. 2020TQ0250), the China National Railway Group Science and Technology Research Program (No. P2019G038-4) and the Open Foundation of MOE Key Laboratory of Engineering Structures of Heavy Haul Railway (Central South University) (No. 2020JZZ01).
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