Stress Distribution and Mechanical Behaviour of Rock Mass Containing Two Openings Underground: Analytical and Numerical Studies

In this study, stress solution for rock mass containing two rectangular openings was calculated based on the Schwarz alternating method to investigate the stress distribution in rock mass around openings with different layouts. In addition, large-scale numerical models were further established for the two-opening system by means of the PFC-FLAC coupling method, in which the stress evolution, failure patterns, and acoustic emission (AE) events were presented. With the combination of analytical and numerical solutions, the interaction mechanism between two openings under different layouts was discussed from the perspective of stress and failure. The result shows that the confining stress within a certain range contributes to relieving tensile stress concentration around openings. The stress condition within the connecting area and coalescence pattern between two adjacent openings is dominated by their layout. Compared with small-size rock specimens in laboratory tests, the failure patterns around openings show a better agreement with the stress concentration characteristics determined by analytical stress solutions.

Crack Coalescence and Failure Patterns in Brittle Rock-Like Specimens with Pre-Existing Fissures under Uniaxial Loading: Experimental Studies

In practical engineering, the joints or fissures with similar characteristics are commonly found in natural rocks, they decrease the mechanics properties of the rock mass and also can be seen as a source of initiation of new discontinuities. The previous studies (experimental or numerical) has promoted the understanding of coalescence patterns or failure process obtained from brittle specimens which contain single, two or three fissures. However, the failure characteristics of multi-fissure specimens has not been studied comprehensively. In this paper, the crack coalescence pattern and failure mode of the rock-like materials with multi-fissures have been explored. Based on the experiment results, under unaxial compression, we found that the coalescence modes between fissures can be generally classified into three categories: T/S/M pattern. Moreover, due to the difference of inclination angle and fissure numbers, the specimens present different failure mode. Careful examination all of specimens, the failure mode can be classified into 4 categories: Mix failure model(shear+splitting), Shear failure, Stepped path, Intact failure.

Lattice Boltzmann Simulation of Multiple Bubbles Motion under Gravity

The motion of multiple bubbles under gravity in two dimensions is numerically studied through the lattice Boltzmann method for the Eotvos number ranging from 1 to 12. Two kinds of initial arrangement are taken into account: vertical and horizontal arrangement. In both cases the effects of Eotvos number on the bubble coalescence and rising velocity are investigated. For the vertical arrangement, it has been found that the coalescence pattern is similar. The first coalescence always takes place between the two uppermost bubbles. And the last coalescence always takes place between the coalesced bubble and the bottommost bubble. For four bubbles in a horizontal arrangement, the outermost bubbles travel into the wake of the middle bubbles in all cases, which allows the bubbles to coalesce. The coalescence pattern is more complex for the case of eight bubbles, which strongly depends on the Eotvos number.

Experimental Study on Mechanism of Crack Coalescence between Two Pre-Existing Flaws under Dynamic Loading

More and more engineering practice indicates rock mass is prone to lose its stability through crack coalescence under dynamic loading, such as blasting and earthquake. However, the crack coalescence pattern of rock specimens containing two or more flaws has not been studied comprehensively under dynamic loading. In this paper, the mechanism of the crack coalescence and peak strength of sandstone-like materials containing two parallel flaws are studied under uniaxial static and dynamic loading with strain rates 1.7×10-5 s-1 and 1.7×10-1 s-1. Through the comparisons of the propagation length, coalescence pattern of the cracks and strength increase of the pre-cracked specimens under static and dynamic loading, the dynamic response of the crack coalescence is found different from static loading under different geometric setting of the flaws. The inertia effect of the crack propagation is revealed under dynamic loading, that is to say, the growth of the secondary cracks tends to the original propagation direction, and the direct and immediate coalescence is taken place easily between two pre-existing flaws, which is different from the kinking coalescence under static loading. So, the inertia effect of the crack propagation is regarded as the main cause of the strength increase of the brittle material under dynamic loading for medium strain rates. In virtue of the explanation, another cause of the mode II shear fracture occurred under earthquake is opened out.

The Mechanism Investigation on Propagation and Coalescence Pattern of Three Internal Plane Flaws in Brittle Materials

Under the action of compressive load, the growth and coalescence containing flaws in brittle materials (rock and rocklike materials e.g.) will result in the local buckling and global fracture of rockmass. But, the mechanisms on propagation and coalescence of 3-dimensional internal flaws are not clear till now. We examine brittle fractures of manmade specimens using frozen casting resin and rocklike material to observe 3D internal flaws growth process at about -30° C. A team of specimens containing three internal flaws is measured; flaws are made of three parallel oblong aluminum films. The propagation and coalescence pattern of three internal flaws is observed under compressive stress. An interesting phenomenon is that the crack initiated from the second flaw quickly turns to the one induced from the third flaw and forms a bigger fracture plane, then splits the specimen. It shows that the flaw distribution pattern will greatly affect the flaws growth and coalescence process. The mechanisms that lead to the wing and anti-wing crack initiation and coalescence are described.