The long-term safety of a deeply buried soft rock tunnel lining under inside-to-outside seepage conditions

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Abstract

For deeply-buried long tunnels, which are constructed in environments with high stress and soft rock, stability is definitely an important engineering problem. In this study, the buried depth of the chlorite schist section of a headrace tunnel in the Jinping II Hydropower Station was between 1550 and 1850 m. In this project, rheological problem, the wetting-induced softening problem and inside-to-outside seepage problem were all prominent, which posed serious threats to the long-term stability of the tunnel lining. This study detailed inspected the engineering geological conditions of the chlorite schist sections, the extrusion deformations of the surrounding rock mass following the excavation, and the surrounding rock support and reinforcement after the expansion excavations. In addition, the characteristics of rheological mechanical and wetting-induced softening for the chlorite schist were determined through laboratory testing and field monitoring. The rheological mechanical characteristics of the chlorite schist were not obvious when the stress was low. However, they were quite obvious when the stress was high. Based on the results, the rheological mechanical behaviors of the chlorite schist were described by a visco-elastoplastic rheological model (CVISC). Then, this study verified the rationality of the existing reinforcement scheme and obtained the final deformation stability time of the surrounding rock through a numerical simulation of the support reinforcement and the secondary lining during the operational period. Furthermore, the safety of the lining structure during the operational period was evaluated. These results may potentially play an important role in the guidance of future engineering designs and construction and may potentially be used as a reference for the support designs of similar deeply buried soft rock projects.

Introduction

Soft rock has the features of low strength, large deformations, serious wetting-induced softening, and obvious rheological effects (Liao et al., 2006, Sharifzadeh et al., 2013). In headrace tunnels, being excavated in deeply-buried soft rocks, the problem of deformation during the excavation and installation of supports is quite serious. And these problems along with their controlling factors will undoubtedly pose major threats to long-term safety of the tunnel lining during its operational period. For example: ① The long-term rheological deformation of the surrounding rock, which is under high stress, will constantly increase the lining pressure; ② During its operational period, the long-term inside-to-outside seepage will cause rock softening which can reduce the strength and deformation modulus, and leads to cracking and instability of the lining; ③ The cracking of the lining will speed up the seepage and softening of the surrounding rock (Tang and Tang, 2012, Zhao et al., 2015, Zhang et al., 2016). As a result, a vicious cycle will occur. For example, the chlorite schist of the Jinping II Hydropower Station exhibited the characteristic of a typical soft rock. After the excavation and support of the upper section, extrusion deformations of the surrounding rock appeared within a short time and became worse as further time elapsed. If these problems are not addressed during the construction period, the maintenance cost and power generation loss will become incalculable when cracks and damages occur to the lining due to the extrusion deformations of the surrounding rock. Therefore, stability of tunnels in deeply-buried soft rock and tunnel lining both are key controlling factors in the long-term safety of deep tunnel engineering projects.

Many researchers have carried out in-depth studies regarding soft rock engineering. By considering mechanical properties of weak rocks, Yoshida et al. (1997) calculated and analyzed the weakening of mudstone due to absorption of water and its influence on mechanical behavior of surrounding rock mass in tunnels. Mechanical response of surrounding rock mass during the excavation of rounded tunnel in viscoelastic media under hydrostatic pressure was studied by Fahimifar et al. (2010). Sulem et al. (1987) calculated and analyzed the rheological mechanical laws of the surrounding rock in rounded soft rock tunnels using an empirical formula. Guan et al. (2008) used Burger-deterioration rheological model to calculate and analyze the surrounding rock deformation in a mountainous tunnel. Pellet et al. (2009) adopted a numerical simulation method to calculate the rheological mechanical behaviors of the damage zones in surrounding rock mass following the excavations of underground galleries. By considering support in soft rock tunnels, Sakurai (1978) deeply examined stresses and support structures during the excavation of tunnels. Carranza and Diederichs (2009) calculated and analyzed the mechanical behavior of wire mesh and shotcrete composite lining in rounded tunnels. Creep characteristics of galleries in soft surrounding rock mass with bolt-grout supports were studied by Lian et al. (2008) by numerical simulation. Pellet (2009) studied the contacts between the tunnel lining in viscoelastic media and the surrounding rock within easily damaged areas. He et al. (2015) applied the material point method to evaluate the safety of excessive deformations in tunnels.

In deeply-buried soft rock tunnel engineering, it is necessary to consider following three factors: rheology of the rock mass, inside-to-outside seepage (Zarei et al., 2012, Zarei et al., 2013) and wetting induced softening. These factors play a vital role in the safety of tunnels in deeply-buried soft rock masses because of the complex physical and mechanical properties of soft rocks, unfavorable geological environment and engineering conditions. The lining structure stresses, which arise from the rheological and elastic-plastic deformations of the surrounding rock mass, are the loading conditions that require consideration during the analysis of lining design and safety. The combined action of these factors is quite complicated because these are interconnected with each other. Therefore, research conducted on only a single factor cannot possibly meet the necessary requirements.

A chlorite schist formation was found during the excavation of a headrace tunnel at the Jinping II Hydropower Station. The formation had a total length of approximately 400 m, a burial depth of about 1550–1850 m, and gravity stress of approximately 42–50 MPa (Liu et al., 2013). After the tunnel excavation and initial support, the extrusion deformation became serious. The rheological effect was prominent, the wetting-induced softening effect was strong, and inside-to-outside seepage problem was existed. The aims of this study were to analyze and evaluate the lining structure safety during the operational period, based on a detailed inspection of the geological conditions, excavation method, initial support, and support reinforcement for the chlorite schist section. The processes of actual tunnel construction and tunnel operational conditions were analyzed using visco-elastic rheological model. The rheology mechanical behavior of the rock mass and the wetting-induced softening effect were also considered in this analysis. The fluid-solid coupling method was applied in this analysis. Then, several indexes, including the rheological deformation, stress, FAI (Failure Approach Index (Zhang et al., 2011), which is an index for the strain assessment) and plastic zone, were introduced to evaluate the safety of the surrounding rock mass and lining structure. Fig. 1 illustrates the flow chart of the research work in this paper.

Section snippets

Engineering geological characteristics of the chlorite schist section

The headrace tunnel of the Jinping II Hydropower Station consisted of four parallel headrace tunnels, with a total length between 16.658 and 16.675 km, maximum burial depth 2525 m, and average burial depth approximately 1610 m. The chlorite schist of the Jinping II Hydropower Station was mainly distributed in K1 + 537 to K1 + 800 of the #1 headrace tunnel, and K1 + 613 to K1 + 755 of the #2 headrace tunnel. It had a burial depth ranging from 1550 m to 1850 m and gravity stress value approximately 41–50 MPa.

Effects of the wetting-induced softening and the rheological characteristics of the chlorite schist

In this study, the mechanical engineering characteristics of the chlorite schist, the deformation strength characteristics, effects of the wetting-induced softening, and the rheological mechanical characteristics were examined through uniaxial, tri-axial, and rheological testing.

Selection of the rheological mechanics model

As previously detailed, the chlorite schist had an obvious rheological property, and in the later rheological stage, specimens develop great plastic deformation when subjected to high-stress conditions, which finally leads to the failure of the specimens. In other words, the plastic and viscous flows coexisted. Therefore, future modeling of the rheological mechanical behavior of the chlorite schist must consider the plastic deformation behavior of rock masses.

As can be seen in Fig. 9, Fig. 10,

Safety analysis and assessment of the surrounding rock and lining structure

The deformation rate of the rocks surrounding the highly squeezed and deformed chlorite schist section of the tunnel could be well controlled after its excavation, support installation, and reinforcement. The rate could be lowered to less than 0.2 mm/d, which met the pouring condition of the lining. Following the lining’s completion, water’s temporal connection through tunnel was lost and the lining was only subjected to deformation pressure from the surrounding rocks. Water penetrated into

Concluding remarks

During the construction of deep-buried long tunnels under the conditions of high-stress and soft rock environments, the stability of the tunnel construction is a formidable issue that needs to be highlighted. The chlorite schist tunnel section in the headrace tunnel of the Jinping II Hydropower Station had a buried depth of 1550–1850 m and a gravity stress between 42 and 50 MPa. During the construction, the tunnel encountered serious large deformations, and even collapsing problems, which posed

Acknowledgements

This research was supported by The National Program on Key Basic Research Project of China [Grant No. 2014CB046902], the National Science Foundation of China [Grant No. 41172288], [Grant No. 51427803], [Grant No. 51279201] and [Grant No. 51404240]. The work in this paper was also supported by funding from the Youth Innovation Promotion Association CAS.

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