Experimental investigation of cyclic wetting-drying effect on mechanical behavior of a medium-grained sandstone
Introduction
The strength and deformation characteristic of rock is a fundamental topic of great interest to engineers in rock mechanics. Proper estimation of rock strength and deformation parameters is important for stability evaluation of structures in many rock engineering applications, such as slopes, deep tunnels, boreholes, underground caverns, dams, wells, and foundations. A good understanding of the strength and deformation behavior of rock will facilitate proper design of engineering structures in these applications (Cai et al., 2004; Peng et al., 2014).
Due to long-term exposure to a water-and-temperature changing environment, the rock is generally suffering various kinds of weathering (Sumner and Loubser, 2008). As a typical type of weathering process, the cyclic wetting-drying phenomenon plays a vital important role in influencing the strength and deformation properties of rock (Li et al., 2021). For example, in the construction and operation of hydropower stations, the slope in the dam usually experiences periodic rainfall or cyclic storage and drainage of water (Cascini et al., 2014; Yang et al., 2018). The exercise is generally associated with cyclic wetting-drying weathering of rock in the slope, which significantly affects the overall stability of the structures in the dam site. Therefore, increasing attention has been paid to the physical and mechanical properties of rock subjected to cyclic wetting-drying weathering.
In recent years, the influence of cyclic wetting-drying treatment on the physical and mechanical properties of different rock types has been extensively studied in laboratory tests. Table 1 summarizes the recent experimental studies in terms of rock type, source, specimen size, treatment method, and test result. It is seen that various treatment methods have been used to prepare rock specimens experiencing different wetting-drying cycles.
Two methods are generally used in the wetting process, namely free immersion method and pressure immersion method. In the free immersion method, the rock specimens are immerged in water under atmospheric pressure condition (Cai et al., 2020; Dehestani et al., 2020; Gratchev et al., 2019; Hua et al., 2015; Khanlari and Abdilor, 2015; Liu et al., 2018; Momeni et al., 2017; O¨zbek, 2014; Qin et al., 2018; Sun and Zhang, 2019; Topal and Sozmen, 2003; Zhou et al., 2017). The time for wetting rock specimens is usually different in various studies. In order to ensure a saturated state of the rock after wetting treatment, gradual immersion of specimens in water is sometimes used in this method. For example, the lower one-quarter, half, lower three-quarters, and whole height of specimen are gradually immersed in water for 2 h, respectively (An et al., 2020; Yang et al., 2018; Zhao et al., 2017). In the pressure immersion method, a vacuum pressure condition is adopted when immersing the rock specimens in water to ensure a quick achievement of the saturated state of rock specimens (Liang and Fu, 2020; Liu et al., 2016; Xie et al., 2018; Zhang et al., 2014; Zhao et al., 2018). The water pressure used in this method is also different in various studies. In the drying process, air dry and oven (or heater) dry are usually used in recent studies (Chen et al., 2019a; Xie et al., 2018; Zhang et al., 2014). In the air dry, due to difference in rock type, the time used for drying is generally different in various studies. Similarly, the time and temperature used in the oven dry is also different in these experimental studies. In general, the time for air dry is longer than that for oven dry.
The results from these previous laboratory tests (Table 1) have revealed that the physical and mechanical properties of various rocks are significantly affected by the wetting-drying cycle in the treatment. The investigated physical and mechanical properties include water absorption (O¨zbek, 2014), porosity (Topal and Sozmen, 2003; Zhao et al., 2018), slake durability index (Liu and Zhang, 2020; Momeni et al., 2017; Zhou et al., 2017, Zhou et al., 2018), wave velocity (An et al., 2020; Yang et al., 2018), thermal conductivity (Sun and Zhang, 2019), surface condition (Zhao et al., 2020), point load index (Gratchev et al., 2019), fracture toughness (Cai et al., 2020; Dehestani et al., 2020; Hua et al., 2015, Hua et al., 2016, Hua et al., 2017; Liang and Fu, 2020), compressive strength (Khanlari and Abdilor, 2015; Xie et al., 2018), tensile strength (Liu et al., 2016; Yao et al., 2020; Zhao et al., 2017), shear strength (He et al., 2020; Liu et al., 2018; Qin et al., 2018; Yuan et al., 2018; Zhang et al., 2014), and Young's modulus (Chen et al., 2019a, Chen et al., 2019b; Huang et al., 2018, Huang et al., 2020). Although the wetting and drying methods are different among these studies, a consensus can be obtained that the physical parameters such as water absorption and porosity are found to monotonically increase with the increase of wetting-drying cycle in the treatment while other physical and mechanical parameters generally decrease. In other words, the cyclic wetting-drying treatment greatly deteriorate the performance of rock.
Apart from these physical and mechanical testing in laboratory, many microscopic testing methods are also used to examine the micro-structure evolution inside the rock experiencing cyclic wetting and drying treatment. These microscopic testing technologies include scanning electron microscopy (SEM) (Du et al., 2019; Liu et al., 2018; Yang et al., 2018; Zhang et al., 2014; Zhou et al., 2018), polarizing microscopy (PM) (Dehestani et al., 2020), laser scanning confocal microscopy (LSCM) (An et al., 2020), nuclear magnetic resonance (NMR) (Zhao et al., 2018; Xie et al., 2018), and computed tomography (CT) (Liu et al., 2016; Yao et al., 2020). The results from these microscopic studies have shown that the deterioration of rock performance is closely related to the micro-cracks or micro-pores generated inside the rock in response to the cyclic wetting and drying treatment. As the wetting and drying cycle in the treatment increases, the micro-cracks (micro-pores) increase rapidly, leading to a quick increase in porosity of the rock specimen.
Although a large number of experimental studies have been conducted recently, the underlying mechanism of how cyclic wetting and drying treatment affects the physical and mechanical properties of rock is still incompletely understood. In addition, previous studies have mainly focused on the mechanical behavior of rock under compressive and tensile loading, seldom investigation has been conducted to examine the direct shear behavior of cyclic wetting and drying weathered rock, which is also important in understanding the strength and deformation characteristic of rock. In this study, the influence of cyclic wetting and drying treatment on the mechanical behavior of a sandstone is experimentally investigated. Uniaxial compression test and direct shear test on sandstone specimens experiencing different numbers of wetting-drying cycles are conducted and the variations of strength and deformation properties of the rock are examined and discussed.
Section snippets
Specimen preparation
The rock sample tested in this study was collected from a slope in Xianfeng open pit mine in Yunnan province, China. To ensure high uniformity and repeatability of specimens used for laboratory testing, all specimens were prepared from a single large sandstone block. The dimensions of the collected rock block were 1000 mm in length, 600 mm in width, and 300 mm in height. All rock specimens were drilled out with the drilling proceeding along the height direction of the block. A diamond coring
Water absorption
In the water absorption test, the specimens' dry mass and saturated mass under different wetting-drying cycles are measured. Table 3 presents the variation of measured saturated mass in response to the change of wetting-drying cycle for the six specimens. The water absorption (wa) of each specimen is calculated according to the weight variation at different wetting-drying cycles aswhere md is the dry mass of the specimen without wetting and drying treatment, and ms is the
Discussion
Cyclic wetting and drying process is a common type of weathering of rock, which is widely encountered in rock engineering. The results in this study reveal that the cyclic wetting and drying treatment significantly deteriorates the strength parameters (i.e., UCS, cohesion, and friction angle) of rock. In general, the UCS, cohesion, and friction angle of the examined rock experiencing 30 wetting-drying cycles in the treatment are decreased by 62%, 50%, and 30% respectively when compared with the
Conclusions
In this study, the influence of cyclic wetting-drying weathering on mechanical behavior of a medium-grained sandstone is experimentally investigated. Uniaxial compression tests and direct shear tests are conducted on sandstone specimens subjected to different numbers of wetting-drying cycles. The results show that the water absorption and peak strain of the rock generally increase with increasing wetting-drying cycle in the treatment. On the other hand, the strength and deformation parameters
Declaration of Competing Interest
The authors declare that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
The research work presented in this paper is in part supported by the National Natural Science Foundation of China (Grant nos. 41702327, 41867033, and 51609178), the China Postdoctoral Science Foundation (Grant nos. 2019M650144 and 2018T110800), the Natural Science Foundation of Hubei Province (Grant no. 2018CFB593), open fund of State Key Laboratory of Safety and Health of Metal Mines (Grant no. zdsys2019-005), and the Open-end Research Fund of the State Key Laboratory for GeoMechanics and
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