Investigating the Macro-Micromechanical Properties and Failure Law of Granite under Loading and Unloading Conditions

Chao Peng,Guilin Li,Jianbo Wang,Wei Zhao,Huanxin Liu

doi:10.1155/2021/9983427

Abstract

The excavation of rock significantly changes the initial stress state of rock slopes, which makes rock in complex loading and unloading conditions. However, the failure mechanisms and macro-micromechanical properties of rock under loading and unloading conditions are not very clear. This study investigates the macro-micromechanical properties and failure law of granite under loading and unloading conditions through traditional laboratory tests and particle flow simulations. Granite specimens are taken from Shuichang iron mine, and stress unloading experiments are designed based on the engineering practices. The stress-strain curves and failure modes under different confining pressures and unloading paths are obtained to analyze the granite properties from the viewpoint of the macroscopic mechanism. Moreover, numerical models are established in PFC software. The microcracks developments, failure characteristics, and energy evolutions under loading and unloading conditions are obtained and discussed. Results show that compared with the loading tests, the brittle failure characteristics of specimens under unloading tests are more obvious. When the confining pressure reduces to about 66% of the initial confining pressure, the specimen loses its load-bearing capacity and destroyed due to the lateral expansion. For loading tests, an inclined plane can be produced as the main failure surface. While for confining pressure unloading tests, there are many damage zones parallel to or intersecting with the main failure surface.

Highlights

Rock excavation will significantly change the stress state of rock mass, which affects the stability and performance of rock engineering [1]. e redistributed stress of rock after excavation can be in loading or unloading states
Based on the concept of Griffith microcrack in rock fracture mechanics, Lajta et al [5] proved that unloading could lead to the formation of cracks, and the tensile failure was related to residual stress
Loading Test Scheme. ere are two purposes of loading test: one is to analyze the mechanical characteristics of rock under loading conditions which will be used for comparisons with the subsequent unloading test; the other is to determine the peak stress of the specimen under different confining pressures which will provide the basis for the design of unloading test scheme

Summary

Introduction

Rock excavation will significantly change the stress state of rock mass, which affects the stability and performance of rock engineering (e.g., open-pit slope, shaft, tunnels, wells, and stopes) [1]. e redistributed stress of rock after excavation can be in loading or unloading states. Due to the research of rock mechanics has yielded fruitful results in loading conditions, more and more scholars begin to explore the unloading rock mechanics from various aspects such as the failure mechanism, stressstrain curve, and strength criterion. Huang et al [18, 19] studied the mechanical characteristics of granite unloading failure under different initial confining pressures from various aspects such as deformation, parameters, and fracture characteristics, obtaining the evolution of the deformation, brittle failure, and energy under unloading conditions. Is study attempts to discuss the macro-micromechanical properties and failure mechanism of granite under loading and unloading conditions by laboratory tests and numerical tests. Rough triaxial compression tests with constant axial stress-unloading confining pressures, the stress-strain curves of granite under different stress paths are obtained, and its macromechanical properties are studied. Using PFC3D software as the numerical analysis platform, triaxial loading and unloading simulations are carried out to study the micromechanical behavior of rock. e failure mechanism is revealed by analyzing the evolution law of microcrack and failure mode of microcrack under loading and unloading conditions

Experimental Procedures

Experimental Results

Analysis of Micromechanical Properties Based on Numerical Tests

Conclusion