Experimental investigation of cyclic wetting-drying effect on mechanical behavior of a medium-grained sandstone

Highlights

• Cyclic wetting-drying weathering effect on rock behavior is experimentally studied.

• Water absorption increase with increasing wetting-drying cycle in the treatment.

• Strength and deformation parameters decrease with the increase of wetting-drying cycle.

• Rock deterioration is mainly associated with micro-cracks generated inside the rock.

Abstract

As a typical form of weathering, cyclic wetting-drying treatment on host rock is commonly encountered in many applications of geotechnical engineering. How cyclic wetting-drying weathering affects the strength and deformation behavior of host rock is a topic worthy of comprehensively investigating. In this study, the influence of cyclic wetting-drying weathering on mechanical behavior of a medium-grained sandstone is experimentally investigated. The sandstone specimens are first subjected to different wetting-drying weathering cycles, and uniaxial compression tests and direct shear tests are then conducted on these prepared specimens. The results show that the water absorption of the rock monotonically increases with the increase of the number of wetting-drying cycle in the treatment. The strength and deformation properties (i.e., uniaxial compressive strength, Young's modulus, cohesion, and friction angle) are found to decrease with the increase of wetting-drying cycle in the treatment and the decrease gradually diminishes as the number of wetting-drying cycle becomes large. Upon failure of the rock specimen under both uniaxial compression and direct shear, the integrity is progressively weakened and more fragments can be observed as the number of wetting-drying cycle in the treatment gradually increases. The deterioration of rock strength and deformation properties is found to be mainly associated with the micro-cracks generated inside the rock in response to cyclic wetting-drying weathering. The data in this study provide useful guidance for long-term stability evaluation of engineering structures constructed under cyclic wetting-drying weathering condition.

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.