Influence of Cyclic Wetting–Drying on the Shear Strength of Limestone with a Soft Interlayer

Abstract

Cyclic wetting–drying can greatly influence rock properties such that a significant decrease in mechanical strength can be expected for a rock undergoing wetting–drying cycles (Zhou et al. 2017; Xie et al. 2018; Gu et al. 2020). The water-weakening effect has been widely recognized to be relevant to many rock failures, typically represented by rockslides in reservoir areas, where rock masses within the riparian zone undergo periodic water-level fluctuations (Huang et al. 2018; Wang et al. 2020). In terms of the long-term instability of rock masses induced by cyclic wetting–drying, the strength degradation of rocks is of major concern (Castellanza et al. 2008). Over the past several decades, a large number of experimental studies have been carried out to investigate the influence of cyclic wetting–drying on the mechanical properties of sandstone (Zhang et al. 2014; Zhao et al. 2017; Yao et al., 2020), shale (Huang et al. 1995), limestone (Beck and Al-Mukhtar 2014), and other types of rock (Mitaritonna et al. 2009; Asahina et al. 2014). A detailed review of previous studies has revealed that most of the works on this topic focused on exploring the water-weakening behaviors of intact rocks. However, rock masses are typically characterized by joints, soft interlayers, bedding planes and other weak planes (Meng et al. 2017). With their low shear strength and sensitivity to water, weak planes usually play an adverse role on rock stability (Huang et al. 2017). Typical examples of such instabilities include the shear failure of rockslides along natural weak interlayers in reservoir areas (Gu et al. 2017) and fault-related sloughing at underground openings in water-enriched regions (Bruneau et al. 2003; Li et al. 2021). Therefore, it is crucial to evaluate the mechanical properties of weak planes in rock masses under the influence of water. However, due to the difficulties that lie in the preparation of rock specimens with natural discontinuities (specimens are particularly prone to break into two pieces during drilling or polishing procedures), the current experimental research on the water-weakening features of rock discontinuities mainly involves testing on samples with artificial joints (a bare discontinuity or a discontinuity filled with clay/cement mortar to simulate a soft interlayer). For example, Ma et al. (2019) carried out a series of ring shear tests on reconstituted soft interlayers to investigate the influences of the remolded water content on its shear behaviors; Kim and Jeon (2019) investigated the shear characteristics of rock discontinuities under various thermal–hydromechanical conditions by conducting shear tests on rocks with artificial saw-cut surfaces. However, the effect of cyclic wetting–drying on the mechanical behavior of natural rock discontinuities has rarely been reported in the literature. The main objective of this paper is to investigate the degradation characteristics of the shear strength of rock interlayers due to cyclic wetting and drying. A series of direct shear tests were conducted on rock specimens with a natural soft interlayer. The shear strengths were measured under different normal stresses and different numbers of wetting–drying cycles. The data were then compared to find the trend of the change in shear strength and to establish a predictive relationship between the strength parameters with the number of wetting–drying cycles. Finally, a damage evolution model * Wengang Zhang [email protected]