Influence of water on mechanical behavior of surrounding rock in hard-rock tunnels: An experimental simulation

doi:10.1016/j.enggeo.2020.105816

Engineering Geology

Volume 277, November 2020, 105816

https://doi.org/10.1016/j.enggeo.2020.105816 Get rights and content

Highlights

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The effect of water on hard-rock tunnel was studied by true-triaxial test.
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The mechanism of water on the rockburst prevention was revealed
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Water changes the mesoscopic mechanism of rock spalling.
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Water exhibits a double effect on hard-rock tunnel.

Abstract

To investigate the influence of water on the mechanical behavior of rock surrounding hard-rock tunnels, a series of uniaxial and true-triaxial compression tests were performed on red sandstone samples with two different water contents (natural water content (NWC) and saturated water content (SWC)). The samples taken were cubic samples containing a circular hole and cylindrical samples. During the true-triaxial tests, hole failure was monitored and recorded in real-time with in-house developed monitoring equipment. The effects of water on the stress, energy, and fracture characteristics of rock failure in hard-rock tunnel were determined. The results indicate that, after reaching the SWC state, the strength and elastic modulus of red sandstone were reduced, and the shear characteristics became more obvious. The failure mode of the NWC holes was primarily slab ejection, while the failure mode of the SWC holes was primarily slab flaking. Water changes the mesoscopic mechanism of spalling and exhibits a double effect on hard-rock tunnels. The mechanisms of water on rockburst prevention are to reduce residual elastic strain energy, avoid excessive concentration of strain energy, and increase rockburst resistance. The ratio of the far-field maximum principal stress to the uniaxial compressive strength can be used as an index to evaluate the stability of hard-rock tunnels. The results help to rethink the influence of water on underground hard rock engineering, such as the failure mechanism of surrounding rock and the analysis of tunnel (or caverns) stability in water-rich stratum, and the mechanism of water on the rockburst prevention.

Introduction

At depths, rocks can contain a variety of interstitial fluids, and in most cases that fluid is water (Hermann et al., 2013). It is well known that water is an important factor affecting rock deformation and failure (Tang, 2018). Generally, water has a negative impact on the mechanical properties, long-term behavior, and stability of rocks (Wang et al., 2020; Yu et al., 2019). Even a small increase in water content may cause a significant decrease in the strength of rock (Erguler and Ulusay, 2009). Therefore, in the water-rich stratum, the impact is critical to the stability and safety of underground tunnels and caverns.

To understand the influence of water on rock strength and deformation, scholars have conducted a large number of uniaxial (Hawkins and McConnell, 1992; Shakoor and Barefield, 2009; Török and Vásárhelyi, 2010; Yu et al., 2019) and triaxial (Baud et al., 2000; Taibi et al., 2009; Li et al., 2012) compression tests on rock samples with different water contents. For example, Hawkins and McConnell (1992) performed a study on the uniaxial compressive strength of dry and saturated sandstone. They pointed out that the percentage of sandstone strength reduction with a uniaxial compressive strength greater than 200 MPa was significantly greater than that of sandstone with a uniaxial compressive strength less than 60 MPa. Shakoor and Barefield (2009) reported that sandstone has a significant decrease in strength over a range of water content below 20% saturation, while the reduction in strength was not significant when the water content exceeded 20% saturation. To quantify the weakening effect of water on sandstone, Baud et al. (2000) conducted a series of triaxial compression tests on four different sandstones (Gosford, Darley Dale, Berea, and Boise) under nominally dry and saturated conditions. Li et al. (2012) studied the effects of water content on the strength and deformation of two sedimentary rocks by conventional triaxial compression tests. Although the water content was small, its influence on the strength and deformation of the sedimentary rocks was obvious. In addition, the crack velocity increases dramatically with increasing water content (Nara et al., 2010, Nara et al., 2014). Increasing the relative humidity by three to four times resulted in an increase in crack velocity of one to four orders of magnitude (Nara et al., 2010). The above research primarily studied the influence of water on the mechanical properties of rock from the perspective of materials, but rarely studied the influence of water on the mechanical behavior and stability of rock surrounding tunnels and caverns.

Rocks exhibit significant differences in mechanical behavior under saturated water and dry conditions (Wasantha and Ranjith, 2014). In deep hard-rock tunnels, there is generally no rockburst when surrounding rock is rich in water; however, the relatively dry rock mass is prone to rockburst (Ortlepp, 1997; Mureithi and Fowkes, 2008; Fowkes, 2011). For example, Ortlepp (1997) analyzed rockbursts in many hard-rock mines and found that rock masses with high water content had fewer rockbursts. Fowkes (2011) noted that the possibility of rockburst activity in tunnels with water and mud is much less. Therefore, water content exhibits a significant effect on the mechanical behavior of rock surrounding tunnels or caverns. Scholars have conducted extensive experimental studies on the spalling and rockburst of hard-rock tunnels and caverns (Gong et al., 2012; Jiang et al., 2013; He et al., 2015; Du et al., 2016, Du et al., 2020; Kusui et al., 2016; Li et al., 2017; Liu et al., 2017; Su et al., 2017; Gong et al., 2019a, Gong et al., 2020; Luo et al., 2020; Qiu et al., 2020; Si and Gong, 2020), but these studies did not consider the effects of water. It is necessary to study the effects of water on the mechanical behavior and stability of rock surrounding hard-rock tunnels.

In this study, red sandstone was processed into cylindrical samples and cubic samples containing a circular hole (called a cubic hole sample) and divided into two groups. One group was in a natural water content (NWC) state, and the other group was soaked in water to achieve a saturated water content (SWC) state. First, uniaxial compression tests were performed on the cylindrical samples with two different states to study the influence of water on the physical and mechanical properties of the sandstone. Then, considering the initial in situ stress at a depth of 500 m, a series of true-triaxial tests were performed on cubic hole samples in each state to study the influence of water on the mechanical behavior of rock surrounding hard-rock tunnels. For these true-triaxial tests, specially developed in-house equipment was used to monitor and record the sidewall failure of the cubic hole samples in real-time. By comparing the test results of different water content samples, the effects of water on the stress, energy, and fracture characteristics of rock surrounding hard-rock tunnels were obtained, and a new understanding of the role of water on hard-rock tunnels was found. The results offer significant guidance for understanding the influence of water on the failure and stability of hard-rock tunnels, and the mechanism of water on the rockburst prevention.

Section snippets

Description of rock sample

Red sandstone is widely distributed in East, South, and Southwest China and was formed in the Cretaceous-Tertiary extinction event. In recent years, with the rapid development of underground infrastructure in these areas, mechanical problems associated with the nearby red sandstone have been increasing, such as weakening of rock strength in water-rich stratum, spalling, and rockburst, particularly during the excavation process of underground tunnels or hydropower caverns. Therefore, the rock

Water content and P-wave velocity

The steps for determining the NWC and SWC were as follows. First, three cylindrical samples were selected, and their NWC masses (m₁) were determined by electron weighing. Then, the three cylindrical samples were placed in a drying oven at 105 °C for 24 h to evaporate the internal water to obtain their dry masses (m₂). Finally, the three dried samples were immersed in water for 48 h to achieve a SWC state, and the SWC masses (m₃) were obtained. The NWC (w₁) and SWC (w₂) of the red sandstone were

True-triaxial test results

According to the test plan described in Section 2.2, the true-triaxial compression tests were performed on the cubic hole samples, and the loading stress paths were obtained as shown in Fig. 5. To prevent an overall failure of the cubic hole samples during the unloading process, the NWC cubic hole samples first unloaded the stress in the Z direction to the stress in the Y (when σ_y > σ_x, or X when σ_x > σ_y) direction. Then, the stresses in the Z and Y (when σ_y > σ_x, or X when σ_x > σ_y) directions

Discussion

To correctly understand the influence of water on the mechanical behavior of rock surrounding hard-rock tunnels, the stress, energy, and fracture characteristics of sidewall failures are analyzed and discussed in-depth below based on the results of uniaxial and true-triaxial tests.

Conclusions

In this study, a series of true-triaxial compression tests were performed on red sandstone cubic hole samples with two different water contents, and the failure process of the hole sidewalls were monitored and recorded in real-time. Combined with the results of uniaxial compression tests, the effect of water on the mechanical behavior of rock surrounding a hard-rock tunnel was revealed. The main conclusions of this study are as follows:

(1)
After red sandstone reaches the SWC state, its strength and

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 by the Projection of the National Natural Science Foundation of China (Grant No. 41472269, 51974163), the Research Project of Education Department of Hunan Province (Grant No. 18C0439), and the University of South China Research Fund (Grant No. 190XQD091).

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