Experimental study on the strength, deformation and crack evolution behaviour of red sandstone samples containing two ice-filled fissures under triaxial compression

Highlights

• Triaxial tests are conducted under different confining pressure and temperature.

• The strength of pre-flawed specimens were much lower than those of intact specimens.

• The relationships between the strength parameters and flaw combinations were studied.

• The crack growth is affected by the combination of pre-existing flaws.

Abstract

The artificial freezing method is an effective method of shaft construction in water-rich soft rock beds. Engineering frozen walls involves the use of intact rock material, flaws and ice. To better understand the mechanical properties of frozen walls, explore the failure laws of frozen fractured rock, and solve the problem of complex ice-rock coupling, it is necessary to carry out laboratory triaxial tests on frozen fractured rock. Herein, X-ray diffraction and mesostructure observations are used to analyse the mineral composition and microstructural characteristics of red sandstone specimens taken from the Shilawusu coal mine. Then, the strength, deformation and crack evolution behaviour of red sandstone containing two pre-existing ice-filled fissures are obtained under triaxial compression using the self-developed DRTS−500 subzero rock triaxial testing system. The experimental results show that the peak strength and elastic modulus of the specimens with pre-existing flaws are all much lower than those of the intact specimens, and the strength parameters are distinctly related to the flaw combinations. For the frozen red sandstone containing two intermittent ice-filled fissures, the peak strength, elastic modulus and shear strength parameters all have a linear relationship with the confining pressure and temperature, whereas the shear strength (σ₁ − σ₃) has a nonlinear relationship with the confining pressure. The deviatoric stress-strain curve of the samples is divided by four characteristic stresses (compacting stress, crack initiation stress, dilatancy stress and peak stress) into five stages from loading to failure. The lower the confining pressure or temperature is, the faster the curve falls after reaching the stress-strain peak value, indicating that the sample exhibits increasingly brittle characteristics. The ultimate failure modes of the specimens containing two pre-existing ice-filled flaws include mixtures of several types of cracks (shear cracks, main cracks, secondary cracks, tensile cracks, coplanar cracks, slip cracks and wing cracks), which depend on the flaw combinations. An obvious crack coalescence failure mode can be observed in the GG and GS specimens, but no obvious crack coalescence phenomenon can be observed in the SS specimens.

Introduction

There exists a great quantity of coal resources in the Ordos Basin of Northwest China (Bai et al., 2020). The topsoil in this area is shallow, the precipitation is low and the evaporation is high, as these characteristics are affected by the arid and semi-arid climate. This leads to desertification in some areas, especially in the northern region. However, data analysis shows that there are a large number of groundwater enrichment areas in these regions near the Yellow River. The geological conditions in this area are complex, and fracture structures generally exist in the engineering rock mass. The banded aquifer system, the water storage space in the fracture zone and the water collection gallery in the fracture zone are rich in groundwater. In the process of mine construction, there are always potential safety hazards, such as water inrushing, water gushing and water permeability. Fig. 1 shows the specific geographical location of the Shilawusu mine in Ordos Basin (Bai, 2019). Taking the Shilawusu mine as the engineering background, the artificial freezing method can effectively eliminate the threat of groundwater, and it is an effective method by which to realize the safe and efficient excavation of shafts in such water-rich bedrock. Fig. 2 shows the artificial freezing construction of the Shilawusu mine shaft.

After excavation, the mechanical properties of a rock mass, such as its strength, deformation and stability, are no longer primarily controlled by the characteristics of the rock itself but by the weak fracture structure, and research on the coupling mechanism of temperature and stress in fractured rock is one of the advanced fundamental studies in the field of rock mechanics. A frozen rock wall of this stratum contains many primary fractures, and its mechanical properties are different from those of an ordinary rock mass. If engineers do not have a clear understanding of these mechanical properties during design and construction, the corresponding designs of freezing parameters will be unreasonable. One option dictates that the overall strength of the frozen wall will be relatively low, and the frozen wall will be prone to deformation and damage after shaft excavation, leading to serious water leakage accidents in the shaft. The other option is that the designed strength of the frozen wall will be relatively high, which means that the wall strength will not be fully utilized, thereby increasing construction costs. In freezing engineering, for such ice-filled fractured rock, the frost heaving force produced by the phase transformation of water in internal pores and fractures induced by low temperatures is the source of the initial damage in the rock mass. However, the cementation of ice and mineral particles enhances the integrity of a rock mass. Under a constant temperature and static load, the expansion and transfixion of ice-filled fissures and the development of microcracks are the reasons for the damage and failure of frozen fractured rock. Therefore, it is very important to discuss the mechanical properties and damage degradation behavior of frozen rock wall containing ice-filled fractures.

Fractured rock is different from intact rock, and its strength is greatly affected by the occurrence of the fracture surface. Studies on the characteristics of fractured rock began in the 1960s (Brace and Bombolakis, 1963; Bieniawski, 1967). After half a century of development, remarkable results have been achieved. To study the crack initiation, propagation and coalescence behaviours of rock-containing flaws, many laboratory tests and theoretical analyses have been carried out on rock-like materials with pre-existing flaws (Bobet and Einstein, 1998a; Singh and Rao, 2002; Sagong and Bobet, 2002; Wong et al., 2004; Prudencio and Jan, 2007; Park and Bobet, 2009, Park and Bobet, 2010; Haeri et al., 2014; Gratchev et al., 2016; Le et al., 2019) and some real rock materials (Niandou et al., 1997; Li et al., 2005; Wong and Einstein, 2009a; Wong and Einstein, 2009b; Li et al., 2010; Lee and Jeon, 2011; Yang, 2011; Yin et al., 2016; Wu et al., 2019). One of the main objectives of these tests was to observe and characterize the fundamental cracking processes involved. Additionally, the influences of the flaw geometry and dip angle on the strength, deformation and crack propagation mechanism of rock materials containing two flaws have been obtained via these tests (Huang et al., 2016; Yang et al., 2013, Yang et al., 2016, Yang et al., 2017a). Moreover, some scholars have studied the effects of the strain rate (Feng et al., 2017), unloading condition (Yang et al., 2017b), confining pressure (Yang and Huang, 2017) and temperature (Wang et al., 2019) on the mechanical parameters and failure modes of fissured rock masses. To better understand the crack initiation at the tip of a flaw and the damage evolution process of fractured rock, computed tomography (CT) test technology, three-dimensional printing (3DP) technology and acoustic emission (AE) technology were used for specimen preparation and real-time observations of crack expansion in these tests (Feng et al., 2004; Zhou et al., 2008; Tian and Han, 2017; Zhang et al., 2017; Zhu et al., 2018). The crack initiation and propagation in specimens containing one or more flaws have been extensively studied by different numerical techniques, such as the boundary element method (BEM) (Eberhardt et al., 1998; Hosseini-Tehrani et al., 2005), the finite element method (FEM) (Liang et al., 2012; Xie et al., 2016; Lee et al., 2017), the displacement discontinuity method (DDM) (Bobet and Einstein, 1998b), and the discrete element method (DEM) (Lisjak and Grasselli, 2014; Gao and Kang, 2016). Recently, to simplify and visualize the generation of cracks, some scholars have modelled rock materials using the bonded particle model (BPM) incorporating particle flow code (PFC) (Lee and Jeon, 2011; Yang et al., 2018) to investigate the influences of pre-existing flaws on the stress-strain curves and strength and deformation parameters of rock material and discuss the initiation, propagation, and coalescence mechanisms of new cracks. These numerical simulation methods simplify the solving of complex theoretical equations, facilitating the research of the mechanical properties of multi-fissure rock that cannot be completed in laboratory tests, and allow the visualization of crack propagation in real time.

In addition, the influence of the environment (temperature and moisture content) on the mechanical properties of rock mass is complex. To study the strength, deformation and crack evolution behaviour of frozen rock and soil under different temperature conditions, relevant laboratory triaxial compression tests have been carried out, such as in the works of Lai et al. (2013), Nakamura et al. (2012), Kodama et al. (2013), Zakharov and Kurilko (2014), Liu et al. (2018) and Huang et al. (2019). Although they reported some valuable results, to date, no laboratory experiments have been carried out on frozen red sandstone specimens containing two intermittent fissures with different dip angle combinations.

In summary, previous research on the mechanical properties of fractured rock mass mainly focused on the following three methods: 1) mechanical property tests of rock-like materials with pre-existing flaws, 2) numerical simulation tests, and 3) mechanical property tests of natural rock (i.e., shale, sandstone, limestone, granite, etc.) specimens with single or intermittent pre-existing flaws. Although these three methods can solve some problems, rock-like materials are different from real rock materials, and none of these methods account for situations in which there is a filler in the flaw. However, engineering rock fissures contain certain fillings, which have a great influence on the overall strength of the rock mass and the corresponding law of fracture expansion. In particular, ice is a common filler in rock fissures in artificial freezing engineering. Hence, previous research results are substantially different from the results observed in engineering applications. Fissures in a rock mass are often discontinuous or discontinuously distributed, which is a characteristic closely related to the strength and deformation failure characteristics of the rock mass. The two-intermittent flaw mode is a common fissure combination form in rock mass and is the basic mode for solving complex rock mechanics problems.

Therefore, on the basis of previous research results, a series of triaxial compression tests was performed on frozen red sandstone with two ice-filled flaws under different temperatures and confining pressures. The strength, deformation and crack evolution characteristics of frozen rock specimens under different temperatures, confining pressures and flaw combination modes were explored. The research results are of great significance to the parameter design and safety evaluation of freezing shaft sinking in the Ordos Basin of Northwest China.