Rock bridge fracturing characteristics in granite induced by freeze-thaw and uniaxial deformation revealed by AE monitoring and post-test CT scanning

https://doi.org/10.1016/j.coldregions.2020.103115Get rights and content

Highlights

  • AE parameter curves present skip and are in consistent with the stress drop points on the stress strain curves.

  • AE spectral frequency analysis reveals six kinds of cracks corresponding to different fracturing events.

  • Post-test CT scanning visualization of the internal crack coalescence pattern at the rock bridge segment.

  • Rock bridge structure is easy to be deteriorated for rock subjected to high F-T damage.

Abstract

This work presents the impacts of freeze-thaw (F-T) fatigue damage on the crack coalescence behaviors at the rock bridge segment for granite samples containing two unparallel flaws. The pre-existing flaw geometry in the rock was a combination of a horizontal flaw and an upper inclined flaw forming a rock bridge structure. In-situ acoustic emission (AE) detection technique and post-test X-ray computed tomography (CT) scanning were used to reveal the rock bridge fracturing characteristics during the whole deformation. Results show that both the F-T cycle and flaw inclined angle influence the crack propagation path, sample strength, deformation and AE pattern and the rock bridge fracture behavior. The AE counts and energy curves present skip phenomenon and are in consistent with the stress drop points on the macroscopic stress-strain curves. The accumulative AE energy and count decrease with increasing F-T cycles. In addition, the cracks during sample failure can be classified into six types by AE spectral frequency analysis, and the high amplitude - low frequency signal was good evident to predict the brittle fracture of the rock bridge. By the post-test CT scanning visualization of the internal crack coalescence pattern at the rock bridge segment, the most striking finding is that the rock bridge structure easily deteriorates for rock subjected to high F-T damage, which suggests that repeated F-T damage drives the crack propagation at the flaw tip and can accelerate the communication of the pre-existing flaws. The testing results are expected to enhance the understanding of the F-T fatigue damage effect on the rock bridge fracturing and can be favorable for predicting the stability of rock structures and rock mass in cold regions.

Introduction

The integrity of rock mass is greatly influenced by the freeze-thaw (F-T) fatigue damage that it sustains. The F-T fatigue damage process involves the repeated phase transformation of water and ice. It first thaws, then freezes, and the thaws again. Laboratory freeze-thaw testing is usually used to mimic the natural weathering of rock under low temperature at a faster rate. The deterioration of rock mechanical behaviors is primarily attributed to the repeated formation and release of frost heaving pressure. When water changes into ice, it expands in volume. In fact, it expands by about 9%, and this volumetric expansion increases the pressure in pores and micro-cracks (Halsey et al., 1998; Zhou et al., 2015; Yamabe and Neaupane, 2001; Tan et al., 2011). This increase in pressure pushes the growth of micro-cracks, creating more new micro-cracks, while the existing ones get wider and deeper. These changes to the rock meso-structures also result in changes to the corresponding mechanical properties as well. Natural rock generally has many discontinuities, such as fissures, weak surfaces, joints, flaws, cracks, faults, and any pre-existing crack or discontinuities greatly affect the general strength of the rock as well as crack propagation when the rock undergoes F-T damage fatigue (Bieniawski, 1967; Bobet and Einstein, 1998a, Bobet and Einstein, 1998b; Ko et al., 2006; Wong and Einstein, 2009). When the crack is filled with water, 9% volume expansion will accelerate the deterioration of F-T fatigue damage of the rock due to the tensile stress generated in the crack, especially when the water turns into ice.

The geomechanical properties of rock subjected to F-T conditions are impacted by many factors, such as the F-T temperature, F-T cycle number, rock porosity, rock mineral composition and the applied external stress, etc., (Takarli et al., 2008; Matsuoka, 2001; Bayram, 2012; Martínez-Martínez et al., 2013; Ghobadi and Babazadeh, 2015b; Mu et al., 2017; Fang et al., 2019). The deterioration of the rock physical and mechanical properties have been systematically studied in previous literatures (e.g., Yamabe and Neaupane, 2001; Liu et al., 2015; Ghobadi and Babazadeh, 2015a, Ghobadi and Babazadeh, 2015b; Wang et al., 2016; Huang et al., 2018a, Huang et al., 2018b), and corresponding damage models have been proposed to describe the F-T fatigue damage degree, mainly including the uniaxial compressive strength (UCS), elastic modulus, porosity, P- or S- velocities and mass loss rate (Hori and Morihiro, 1998; Takarli et al., 2008; Tan et al., 2011; Nicholson and Nicholson, 2015). Due to the existence of discontinuities in rock, they greatly decrease the strength of the rock mass when suffering the influence of F-T cycles. Arosio et al. (2013) found that the frost heaving pressure in the flaws is large enough to drive the crack propagation and the failure of rock. Apart from the intact rock subjected to F-T cycles, nowadays, more and more scholars pay attention to the effects of the pre-existing flaws on the F-T fatigue flaw of rock. The crack geometric shape on the evolution of frost heaving pressure was studied by Huang et al. (2018a) and Bost and Pouya (2017) and Liu et al. (2015). The formation and expansion of new frost heaving cracks caused by frost heaving pressure at the crack tip when the maximum tensile strength is exceeded were also investigated (Girard et al., 2013; Al-Omari et al., 2015; Huang et al., 2019). The cracking process and the law of cracking and coalescence of brittle/semi-brittle materials and natural rocks were studied experimentally: gypsum (Lajtai, 1974), molded gypsum (Bobet and Einstein, 1998a, Bobet and Einstein, 1998b; Wong and Einstein, 2009), glass (Hoek and Bieniawski, 1965), sandstone materials (Wong et al., 2001), granite (Miller and Einstein, 2008), marble (Li et al., 2005), sandstone (Yang et al., 2013), etc. In addition, a number of numerical models have been developed to simulate crack initiation and propagation, including finite element method (FEM), displacement discontinuity method (DDM), discrete element method (DEM), and real fracture process analysis (RFPA). However, the effect of water-ice phase transitions on rock cracking and freezing cracking have not been studied in depth. In fact, damage and frost heaving cracks caused by the repeated freeze-thaw cycle can even affect the crack growth paths, failure modes, and rock strength. Therefore, rock samples with flawed structures have gained attention from many scholars so that they can be examined during F-T cycling and under compression. There have been several experiments that have examined such cases. For example, Huang et al., 2018a, Huang et al., 2018b used a coupled thermo-hydro-mechanical model to study the frost heaving strain in a pre-existing imperfection, and they also used this model to study the influences of temperature and pore water/ice pressure on frost heaving strain. Later, Huang et al. (2019) completed laboratory testing on rock-like material that had only a single flaw and subjected it to F-T and uniaxial compression. They studied the crack propagation characteristics and the frost-heaving cracking behavior under these conditions. Lu et al. (2019) also explored rocks with only a single imperfection. They completed triaxial compression testing on sandstone with only a single flaw and the sandstone underwent F-T treatment. However, Lu et al. also considered several other influencing factors including: the confining pressure, the F-T action, and the loading on the deterioration of the rock mechanical properties. This information allowed them to establish a damage constitutive model to describe the damage of the rock. Other authors also studied the effects of F-T cycling on structures with imperfections on other types of rocks. Zhou et al. (2019) studied the effect of the freeze-thaw fatigue damage on the cracking behavior of sandstone containing two non-parallel cracks under uniaxial compression. In the process of specimen deformation, they recorded the crack formation process with a high-speed digital camera and were able to observe the coalescence shape of the crack in the field. Furthermore, Wang and Li (2020) studied the crack coalescing behavior of cracked granite under different freeze-thaw treatments, and revealed the macroscopic crack morphology of rock bridge. The above study confirmed the influence of freeze-thaw damage on the rock failure characteristics, indicating that it is different from the rock crack coalescence morphology at room temperature.

After literature review, the investigations of crack propagation and rock bridge fracture are almost all concentrated on the intact rock at room temperature. It is clear that few reports about the rock containing pre-existing flaws subjected to the F-T cycle and compression have been published. It has been proven that the discontinuities in rock affect the crack coalescence pattern and alter rock deformation and strength properties; in addition, the huge frost heaving stress acted on the discontinuities accelerates the damage and failure of rock. In addition, the current studies on flaw-contained rock are mainly focus on the macroscopic stress strain responses, where mapping of the internal strain field is typically only observed after failure, for example macroscopic crack pattern descriptions. What is more, the crack pattern of rock is observed at the rock surface, the rock bridge crack inside the sample is difficult to be detected. Therefore, the basic objective of the present work is to investigate the rock crack coalescence in the rock bridge area using real-time acoustic emission (AE) monitoring and post-test CT scanning technique. The AE parameter and spectral analysis is used to reveal the crack initiation, propagation and coalescence in the rock bridge segment, and CT imaging is used to visualize the crack coalescence pattern. This work is focused on revealing the effects of F-T fatigue damage on the cracking behavior of rock containing two unparallel flaws, and the results are helpful to understanding the deterioration mechanism of fractured rock mass in cold regions.

Section snippets

Test rock material descriptions

In this study, the rock material was from northwest China in the Xinjiang province. Specifically, the rock was obtained from the open pit slope at Hejing Beizhan open pit slope. The lithology of the source rock is granite. As shown in Fig. 1a, there is granite that is scattered across the western boundary of the open pit slope. After filed investigations, three sets of joints were measured on the rock outcrop of the western slope. A large number of joints are intermittent, forming a locking

Typical stress–strain responses

The impacts of the previous freeze-thaw fatigue damage on the axial stress strain curves for the granite samples with different inclined flaw angle are plotted in Fig. 8a–c. Different from the typical stress strain curves reported by the authors of previously (Liu et al., 2015), no obvious compaction deformation can be observed, which reflects the hard-brittle characteristics of the tested rock. The rock samples experience a linear elastic stage, crack initiation and stable propagation stage,

Discussions

The effects of F-T fatigue damage on the crack coalescence and the associated rock bridge fracturing has been investigated experimentally. Generally, a cyclic freeze-thaw process on the rock includes the shrink stress difference among the rock minerals under variable temperature, frozen heave force by the water phase change, change of mineral crystal phase and water dissolution of minerals during the freeze-thaw cycle. For the granite containing two pre-existing flaws in this work, the frozen

Conclusions

Here, we focused on studying the effects of the accumulative freeze-thaw fatigue damage on the crack coalescences pattern and rock bridge fracturing in granite samples containing two flaws. The samples have pre-existing flaws with different approach angles prepared by water-jet cutting. The testing results reveal the impacts of freeze-thaw damage on the strength, deformation, AE characteristics and rock bridge cracking for the tested samples. Based on the work presented, main conclusions can be

Author statement

We would like to submit the revised manuscript entitled “Rock bridge fracturing characteristics in granite induced by freeze-thaw and uniaxial deformation revealed by AE monitoring and post-test CT scanning” to “COLD REGIONS SCIENCE AND TECHNOLOGY”. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to thank the editors and the anonymous reviewers for their helpful and constructive comments. This study was supported by National Key Technologies Research & Development Program (2018YFC0808402), the Beijing Natural Science Foundation of China (8202033), the Key Laboratory of Geo-hazards Prevention and Geoenvironment Protection (Chengdu University of Technology)) (SKLGP2019K017), National Natural Science Foundation of China (51774021), and the Fundamental Research Funds

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