Experimental study on failure characteristics of jointed rock mass under staged unloading conditions

Liwen He,Ping Cao,Guodong Tang

doi:10.21595/jve.2018.19970

Abstract

Strength and deformability characteristics of rock with pre-existing fissure are governed by cracking behavior. To further research the effects of pre-existing fissures on the mechanical properties and crack coalescence process under staged unloading conditions, a series of uniaxial compression tests were carried out. In this study, cement, fine sand, and water were used to fabricate a kind of brittle rock-like material rectangular model specimen. The mechanical properties of rock-like material specimen used in this research were all in good agreement with the brittle rock materials. The pre-existing fissure was created by mica sheets and the unloading zone was created by pre-filling Polylactic acid (PLA) resin material during the molding processes the model specimen. And the staged unloading test was conducted based on polylactic acid’s hot-melt characteristics. In order to investigate the crack initiation, propagation and coalescence process, inclination angle of pre-existing fissure was set to change from 0° to 90°. The mechanism of crack propagation and coalescence under photographic monitoring was discussed. And these experimental results indicate that the failure modes in the zone above unloading zone mainly consist of four modes: separation failure, shear-slip failure, mixed shear-separation failure and compression-shear failure. The experimental results are also expected to increase the understanding of the strength failure behavior and the cracking mechanism of rock under staged unloading.

Highlights

Study on mechanical behaviors of rock material containing different pre-existing flaws has been a significant project in current research [1,2,3,4]. It is of great theoretical value and practical significance to study the mechanical behavior of cracked rock mass and patterns of crack initiation, propagation and coalescence
After crack 5) stopped propagating, crack 6) in the middle of pre-existing fissure propagated to the staged unloading zone, and the secondary co-planar crack 7) which initiated at the right tip propagated rapidly to cause the overall failure of specimen
Overall separation failure mode in the zone above unloading zone mostly occurred when specimens were destroyed as a result of staged unloading to the place where near to right tip of pre-existing fissure

Summary

Introduction

Study on mechanical behaviors of rock material containing different pre-existing flaws (such as fissures, holes) has been a significant project in current research [1,2,3,4]. Yang Shengqi et al [9, 10] conducted uniaxial or triaxial tests on intact rocks and rocks containing single crack, double cracks, and multiple cracks respectively They investigated the failure characteristics and mechanical characteristics of brittle specimens containing pre-existing fracture. To further research the effects of unloading deformation failure, strength and fracture evolution of fractured rock masses, Huang Da et al [15] studied the physics model tests of double pre-existing. On the basis of these test data, we analyze the strength and deformation behavior, micro-cracks propagation process at the tip of fissure and investigate the relationship between inclination angle and crack coalescence process of rock-like material under staged unloading conditions

Specimen preparation

Excavation zone filling materials

Testing equipment and procedure

The unloading failure process of horizontal and vertical fissure specimens

The unloading failure process of inclined fissure specimens

Analysis of stress-strain curve

Inclined fissure specimen coalescence model and stress analysis

Analysis of failure modes above the staged unloading zone

Conclusions