Yujing Jiang

Experimental Study on Mechanical and Acoustic Emission Characteristics of Rock-Like Material Under Non-uniformly Distributed Loads

The mechanical and acoustic emission characteristics of rock-like materials under non-uniform loads were investigated by means of a self-developed mining-induced stress testing system and acoustic emission monitoring system. In the experiments, the specimens were divided into three regions and different initial vertical stresses and stress loading rates were used to simulate different mining conditions. The mechanical and acoustic emission characteristics between regions were compared, and the effects of different initial vertical stresses and different stress loading rates were analysed. The results showed that the mechanical properties and acoustic emission characteristics of rock-like materials can be notably localized. When the initial vertical stress and stress loading rate are fixed, the peak strength of region B is approximately two times that of region A, and the maximum acoustic emission hit value of region A is approximately 1–2 times that of region B. The effects of the initial vertical stress and stress loading rate on the peck strain, maximum hit value, and occurrence time of the maximum hit are similar in that when either of the former increase, the latter all decrease. However, peck strength will increase with the increase in loading rate and decrease with the increase in initial vertical stress. The acoustic emission hits can be used to analyse the damage in rock material, but the number of acoustic emission hits cannot be used alone to determine the degree of rock damage directly.

Rheological Model of DMFC Rockbolt and Rockmass in a Circular Tunnel

In this paper, the time evolutions of the mechanical characteristics of a discretely mechanically or frictionally coupled (DMFC) rockbolt and surrounding rockmass are studied. A conceptual model is developed to analyze the interaction between the DMFC rockbolt and rockmass, and analytical solutions are presented for a tunnel supported with DMFC rockbolts. The formulation is based on the following assumptions: (1) a deep tunnel with a circular cross-section; (2) an axisymmetric problem, i.e., having a lateral pressure coefficient Kaxa0=xa01; (3) homogeneous and isotropic ground; (4) small deformations; (5) rockbolts simulated as two cylindrical surfaces and oppositely distributed forces, i.e., one at the head and the other at the bolt bond; and (6) viscoelastic material assumptions, i.e., a one-dimensional viscoelastic model for the rockbolt and a three-dimensional viscoelastic model for the rockmass. The analytical solutions of the rheological model are obtained by assuming the constitutive models of the rockmass and rockbolt in terms of a Maxwell model and Maxwell model (M–M), Kelvin model and Kelvin model (K–K), and Kelvin model and Maxwell model (K–M). The analytical solutions of the M–M model are compared with the results obtained from numerical simulations with FLAC3D. The analytical solutions are in good agreement with the numerical results in the case of small rheological rock deformation and close bolt arrangement. Furthermore, the variation of the axial force along the rockbolt and the evolution of stress and displacement in the rockmass are closely related to the rheological parameters and support parameters.

Shear Behaviour and Acoustic Emission Characteristics of Bolted Rock Joints with Different Roughnesses

To study shear failure, acoustic emission counts and characteristics of bolted jointed rock-like specimens are evaluated under compressive shear loading. Model joint surfaces with different roughnesses are made of rock-like material (i.e. cement). The jointed rock masses are anchored with bolts with different elongation rates. The characteristics of the shear mechanical properties, the failure mechanism, and the acoustic emission parameters of the anchored joints are studied under different surface roughnesses and anchorage conditions. The shear strength and residual strength increase with the roughness of the anchored joint surface. With an increase in bolt elongation, the shear strength of the anchored joint surface gradually decreases. When the anchored structural plane is sheared, the ideal cumulative impact curve can be divided into four stages: initial emission, critical instability, cumulative energy, and failure. With an increase in the roughness of the anchored joint surface, the peak energy rate and the cumulative number of events will also increase during macro-scale shear failure. With an increase in the bolt elongation, the energy rate and the event number increase during the shearing process. Furthermore, the peak energy rate, peak number of events and cumulative energy will all increase with the bolt elongation. The results of this study can provide guidance for the use of the acoustic emission technique in monitoring and predicting the static shear failure of anchored rock masses.

Macro-Micro Failure Mechanisms and Damage Modeling of a Bolted Rock Joint

The anchoring mechanism of a bolted joint subjected to a shear load was investigated using a bilinear constitutive model via the inner-embedded FISH language of particle flow code based on the discrete element method. The influences of the anchoring system on the macro-/micromechanical response were studied by varying the inclination angle of the bolt. The results indicate a clear relationship between the mechanical response of a bolted rock joint and the mechanical properties of the anchoring angle. By optimizing the anchorage angle, the peak strength can be increased by nearly 50% relative to that at an anchorage angle of 90°. The optimal anchorage angle ranges from 45° to 75°. The damage mechanism at the optimal anchorage angle joint is revealed from a macroscopic mechanical perspective. The concentration of the contact force between disks will appear in the joint and around the bolt, resulting in crack initiation. These cracks are mainly tensile cracks, which are consistent with the formation mechanism for compression-induced tensile cracks. Therefore, the macroscopic peak shear stress in the joint and the microscopic damage to the anchoring system should be considered when determining the optimal anchoring angle to reinforce a jointed rock mass.

Acoustic Emission Characteristics and Failure Mechanism of Fractured Rock under Different Loading Rates

To study the loading rate dependence of acoustic emissions and the failure mechanism of fractured rock, biaxial compression tests performed on granite were numerically simulated using the bonded particle model in Particle Flow Code (PFC). Uniaxial tests on a sample containing a single open fracture were simulated under different loading rates ranging from 0.005 to 0.5u2009m/s. Our results demonstrate the following. (1) The overall trends of stress and strain changes are not affected by the loading rate; the loading rate only affects the strain required to reach each stage. (2) The strain energy rate and acoustic emission (AE) events are affected by the loading rate in fractured rock. With an increase in the loading rate, AE events and the strain energy rate initially increase and then decrease, forming a fluctuating trend. (3) Under an external load, the particles within a specimen are constantly squeezed, rotated, and displaced. This process is accompanied by energy dissipation via the production of internal tensile and shear cracks; their propagation and coalescence result in the formation of a macroscopic rupture zone.