Water-Weakening Effects on the Strength of Hard Rocks at Different Loading Rates: An Experimental Study

Water is one of the most important environmental factors affecting rock mechanical properties, because most rock masses in engineering practice are in the moist or saturated state (Fig. 1). Previous studies have concluded that the uniaxial compressive strength (UCS) of rock is generally reduced to various degrees in the presence of water (Shakoor and Barefield 2009; Eunhye et al. 2017). Furthermore, several mechanisms related to water-weakening effects have been revealed through numerous experiments, including (a) clay mineral softening (Van Eeckhout 1976; Erguler and Ulusay 2009), (b) stress corrosion (Brantut et al. 2014), (c) pore water pressure (Atkinson 1984; Zhou et al. 2016; Huang et al. 2019), and (d) friction-weakening effects (Baud et al. 2000). Among them, clay mineral softening arises in rocks containing clay mineral (as its name implies), and stress corrosion often occurs in siliceous rocks (Atkinson et al. 1981; Masuda 2001). It should be noted that, the effects mentioned above are usually combined, and there is no generally accepted explanation due to the great differences in rock types as well as in loading conditions. Therefore, it is challenging to detangle the contribution of a specific mechanism and describe the extent of its effect. The fracturing of engineering rock masses often occurs under varying loading rates. To date, many researchers have found increases in strength to various degrees as loading rate increases (rate-dependence), e.g., Lajtai et al. (1991), Li et al. (2017a) and Hou et al. (2019). Nevertheless, it is generally recognized that, pore water pressure becomes obvious for saturated rock from drained to undrained conditions with an increasing loading rate, implying great strength reduction (Obert and Duvall 1967; Zhou et al. 2018). Interestingly, there is a contradiction regarding the loading rate effects on strength for saturated rock. Hence, a comprehensive study is necessary to fill in the knowledge gaps concerning which effect is primary. The acoustic emission (AE) technique and wave propagation can be used to analyze the real-time activities of microcracks and has, therefore, become an important analytical tool in rock mechanics (Ohnaka and Mogi 1982; Fan et al. 2013; Li et al. 2015a, b). Based on the first motion polarity method and the moment tensor method proposed by Zang et al. (1998) and Ohtsu et al. (1998), Li et al. (2017b) concluded by statistical analysis of AE dominant frequencies that the waveforms of low dominant frequency are produced by micro-tensile failure and waveforms of high dominant frequency are caused by micro-shear failure. Afterwards, the micro-failure mechanisms of marble under uniaxial compression, direct tension and flat loading Brazilian tension based on statistical analysis of AE dominant frequencies were studied by Zhang et al. (2018) and Wang et al. (2019). The abovementioned findings concerning the statistical analysis of AE dominant frequencies provide a new approach to investigate the rock failure process, which will be used in this study. To eliminate the effects of clay mineral softening and stress corrosion, two rocks containing a single mineral and free of clay and siliceous minerals were selected for uniaxial * Jianhui Deng jhdeng@scu.edu.cn