Experimental and numerical investigation on the effects of bedding plane properties on the mechanical and acoustic emission characteristics of sandy mudstone

https://doi.org/10.1016/j.engfracmech.2021.107582Get rights and content

Highlights

  • Failure strength and failure pattern showed strong sensitivity to the bedding plane-loading angle.

  • The characteristics of AE counts and cracks were affected predominantly by the failure pattern.

  • The failure patterns of the specimens were influenced by the bedding plane strength and bedding plane number.

Abstract

Bedding planes have a strong influence on the mechanical characteristics of layered rocks. To study the effects of bedding plane properties on the strength, fracture and acoustic emission (AE) characteristics of rock, Brazilian tests of sandy mudstone with different angles between the bedding plane inclination and loading direction (the bedding plane-loading angle) were first conducted. Based on the cohesive zone model in ABAQUS, the effects of the number and strength of bedding plane on the fracture and crack distributions of specimens were then investigated through numerical simulation. The experimental results showed that with increasing bedding plane-loading angle, the failure strength of the disk specimens continuously increased. The characteristics of the AE count and crack distributions (RA-AF distribution) were predominantly affected by the failure pattern. The AE count distributions were divided into two types: unimodal distribution and multimodal distribution. The cracks that formed in the specimens were divided into three forms: tensile cracks, mixed cracks and shear cracks. The numerical simulation results showed that both the number and strength of the bedding planes strongly influence the fracture and crack distributions of specimens. With the increase in bedding plane strength, the failure patterns of the specimens under each bedding plane-loading angle transformed from layer activation failure to central failure under each bedding plane-loading angle tested. Furthermore, under each bedding plane-loading angle, the failure patterns of specimens with different numbers of bedding plane were the same, and the percentage of cracks on the bedding planes increased while the percentage of cracks within the rock matrix decreased.

Introduction

In recent years, many geotechnical engineering works such as underground construction [1], [2], [3], [4], [5], [6], slope support [7], [8], [9], petroleum drilling [10], [11], [12], and radioactive substance storage [13], [14], [15], [16] have been carried out in rock masses with bedding planes. The strength and deformation properties of rock with bedding planes usually show obvious anisotropy and the stability of these actual engineering projects will be strongly influenced by the bedding plane properties. Therefore, studying the influence of bedding planes on the mechanical characteristics of rock is of great significance.

In engineering fields that involve rock masses, rock materials under tensile stress are vulnerable to damage. The tensile strength of rock is much lower than the compressive strength. Because of the difficulties of experimentation, limited investigations have attempted to study the direct tensile properties of layered rocks [17], [18]. This has prompted the development of the Brazilian test method to measure the tensile strength of layered rock. Numerous research results on the anisotropy of tensile strength have been obtained by Brazilian testing [19], [20], [21], [22]. For instance, Tavallali and Vervoort [23] conducted the Brazilian test for sandstone to study the effect of the angle between the bedding plane inclination and loading direction (the bedding plane-loading angle) on the fracture pattern of disk specimens. The results showed that the switch angle of the failure pattern (from central failure to layer activation) was in the range of 45° and 60°. Dan et al. [24] conducted Brazilian tests on three different kinds of rocks (slate, gneiss and sandstone) with bedding planes. The results showed that the presence of bedding planes had different effects on the strength properties of slate, gneiss and sandstone. Compared with those of isotropic materials, the failure patterns of anisotropic materials are more complicated and mainly composed of mixed-mode cracks along the rock matrix and bedding plane.

As a supplement to laboratory tests, numerical simulation studies based on rock mechanical test results can be used to not only improve the understanding of the test conclusions but also determine more mechanisms and laws, which cannot be acquired by laboratory testing. Tan et al. [25] studied the effect of the bedding plane-loading angle on the failure strength of slate by experimental and numerical simulation methods. The experimental results showed that the failure strength of slate increased with increasing bedding plane-loading angle and that the failure pattern of slate transformed from layer activation to central failure. The numerical simulation results showed good agreement with the experimental results. Xu et al. [26] investigated the tensile failure behavior of phyllite with bedding planes with Brazilian tests and numerical simulations. The test results showed that the failure strength and acoustic emission (AE) characteristics of specimens were closely related to the failure patterns of the specimens under different bedding plane-loading angles. The numerical simulation results showed that the failure pattern and failure strength are strongly influenced by the bedding plane strength.

From the above analysis, it can be concluded that the study of the tensile properties of layered rock has mainly concentrated on the effect of the bedding plane-loading angle on the tensile strength and failure pattern of layered rock. However, the influence of bedding plane properties (strength and number of bedding planes) on the tensile properties of layered rock needs further study. In this study, Brazilian tests of sandy mudstone under different bedding plane-loading angles were first conducted to study the failure strength, fracture propagation and acoustic emission characteristics of disk specimens. In the finite element software ABAQUS, the failure of cohesive elements can be used to simulate the fracture and failure of rock specimens, thus, ABAQUS has been applied to research rock fractures [27], [28]. In this paper, based on the secondary development of ABAQUS through the Python programming language, cohesive elements were inserted into solid elements to establish a disk specimen model. Thus, the strength and failure characteristics of layered sandy mudstone were also studied via numerical simulation. By comparing the simulation and test results, the rationality of the established numerical model was verified. Finally, the effects of the strength and number of bedding planes on the fracture and crack distribution of specimens were studied through numerical simulation. The relevant findings could provide a theoretical basis and technical support for destruction evaluation of engineering works that involve layered rock masses.

Section snippets

Preparation of sandy mudstone

Sandy mudstone specimens were collected from Liuzhou city, in the Guangxi Zhuang Autonomous Region of China. This sandy mudstone has visible bedding planes in the natural state, and its average density and P-wave velocity were 2.33 g/cm3 and 1.169*103 m/s respectively. Based on the XRD results, the content of each mineral was calculated, as shown in Fig. 1(a). The sandy mudstone specimens were mainly composed of clay minerals (illite and kaolinite), quartz and feldspar. The mass percentages of

Failure strength

The equation for calculating the Brazilian tensile strength is expressed as follows [29]:σt=2FπDtwhere σt is the Brazilian tensile strength, F is the maximum vertical load, D is the diameter of the disk specimen, and t is the thickness of the disk specimen.

In the Brazilian test, two assumptions must be satisfied [30]: (1) The rock specimen is homogeneous and isotropic. (2) The main fracture formed in the disk specimen should initiate from the center of the disk specimen and then propagate along

Numerical simulation

When bedding planes are present in sandy mudstone, the failure strength and fracture propagation during the Brazilian test are affected not only by tensile stress but also the bedding plane properties. Therefore, the failure strength and failure pattern change with variations in the bedding plane properties. Indoor experimentation has been proven to be a straightforward methodology to measure the strength and failure characteristics of layered rocks [37]. However, limitations still exist in the

Conclusions:

In this study, a series of Brazilian tests of sandy mudstone with bedding planes were performed to investigate the fracture mechanism of disk specimens with different bedding plane-loading angles. High-speed cameras and acoustic emission technology were used to study the crack initiation and propagation characteristics. The secondary development of ABAQUS through the Python programming language was used to systematically study the effect of bedding plane properties on the mechanical behavior of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by National Natural Science Foundation of China (Nos. 51734009, 51904290) and the Natural Science Foundation of Jiangsu Province, China (BK20180663).

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