ABSTRACT The object of this study is to investigate the unloading failure mechanism of hard rocks in the unloading process. A commercial finite element program LS-DYNA was employed to simulate the rock unloading process. The implicit and explicit methods were performed in sequence to simulate the static initialization-dynamic unloading process of rocks. The numerical results indicated that the rock failure can be induced by releasing of the initial stress, and the previous result of the equivalent initial stress release rate (EISRR) theory based on the 1D stress state is not suitable for 3D stress state. In 3D stress state, a new definition of equivalent strain energy release rate (ESERR) was introduced. The further study indicated that the ESERR can characterize the effect of different confining stresses and different unloading path on rock unloading. A significant finding is that the ESERR can quantitatively describe the characteristics of the unloading process under 3D stress state. This finding indicated that in practical underground excavation engineering, dynamically controlling the ESERR can be used to increase excavation potential of rocks and minimize the needed external excavation energy by using the initial energy.’
A number of standard methods have been proposed to determine the Mode I fracture toughness of rock. They include those based on short rod specimen, chevron bend specimen and cracked chevon-notched Brazilian disk specimen. The semi-circular bend (SCB) specimen shown in Figure1 has been widely used for fracture toughness determination of geomaterials owing to inherent favourable properties such as its simplicity, minimal requirement for machining and the convenience of testing that can be accomplished by applying 3-point compressive loading using a laboratory load frame. It is made from typical rock cores. Despite the application of compressive loading the stresses at the crack tip are tensile and it causes tensile failure due to the opening mode (i.e. Mode I) of the crack propagation. The critical crack initiation parameter in linear elastic fracture mechanics is defined as the fracture toughness Kic and is considered as a material property. However unless the crack tip process zone where the micro cracks that generate due to the tensile stresses coalesces and form a macro crack is contained within a small volume of area in comparison to the specimen size, the resulting toughness value may not be equal to the fracture toughness. This paper discusses the minimum size requirements as appalicable to the determination of plane strain fracture toughness of rock materials using the SCB specimen.
This paper presents a review of a systematic research program for understanding scale and stress effects on transport behaviours of fractured crystalline rocks, using a hybrid discrete element and particle tracking approach. The motivation is the importance of understanding stress effects on behaviours of contaminant transport in fractured crystalline rocks, an important issue of rock mechanics for environmental safety assessments of many rock engineering projects. The study is divided into three steps. The first step is a basic study that established the mathematical platform for deriving the conditions, criteria, basic approaches and test case results for investigating stress and scale effects on hydraulic behavior of the fractured rock concerned. At the second step, based on outstanding issues drawn from the first step, the study was extended to consider effects of the correlation between the fracture aperture and size (represented by trace length) on the permeability of the fractured rock, and uncertainties in deriving equivalent continuum properties of fractured rocks. The third step added the particle/solute transport processes to the mathematical platform, including different retardation mechanisms, so that impact of stress on safety can be directly evaluated, even it can only be done conceptually. The obtained results show that stress, scale and inter-parameter correlations of the fracture system geometry are dominant issues for understanding and characterization of coupled hydro-mechanical processes of fractured rocks and play a significant role for understanding the mass transport behaviour in them, with direct impact on geo-environmental safety.
This study presents an analysis of the changes that could be expected in geomechanics, fluid flow and seismic during steam injection in unconsolidated sandstone reservoirs. Variations of the rock, fluid properties and velocities due to steam injection in a heavy oil unconsolidated formation were predicted using laboratory testing, field data and logs. A 3D thermal, compositional and geomechanical reservoir model was built based on these data and a 2D synthetic seismogram was calculated for time lapse modeling. The synthetic seismogram results were used to calculate the variation in acoustic impedance and fluid substitution was evaluated using Gassmann’s equation. Thermal-Fluid-Geomechanics simulations indicated a 6% increase of the pore pressure inside the chamber during the injection and a 45% increase of the water saturation. This resulted in rock dilation and shear strain around the steam chamber, thus reducing the overburden and acoustic impedance during the steam injection process used to increase heavy oil recovery.
Failure in rocks is attributed to crack initiation and coalescence processes upon loading. Crack behavior study, which has been extensively conducted in various rock types containing artificially created flaws under a quasi-static loading condition over the past decades, is extended to dynamic loading conditions by using the Split Hopkinson Pressure Bar (SHPB) technique.
The loading tests are conducted on specimens containing a single pre-existing open flaw (60mm x 30mm x 20mm) in Carrara marble. Similar to those loaded quasi-statically, white patches are observed to develop in the specimens loaded dynamically at the early stage of the loading prior to the development of observable cracks. The respective failure modes are however significantly different. For quasi-static compression, normally two macro-cracks linking the two opposite corners of the specimen and the flaw tips cause the failure of the marble specimens, producing two main fragments. In contrast, under high strain rate, four fragments associated with “X” shape deformation bands are produced irrespective of the inclination angle of the flaws. The failure is attributed to the more or less simultaneous propagation of the horsetail and anti-wing cracks. In this paper, the detailed cracking processes will be compared and discussed.
Controlled blasting for loosening coal seam is one of the most important measures to enhance the gas drainage and thus to prevent coal and gas outburst. The formation and development of blasting-induced damage zone, as well as the gas flow in damaged coal seam, can be considered as a coupled process among gas flow, solid deformation and damage. In this work, a coupled multiphysical model for the interaction of blasting damage of coal seam and gas flow is proposed, based on which, the effect of loosening blasting on draining gas in coalbed is numerically simulated and the associated mechanisms for enhanced gas drainage induced by blasting damage is clarified. In this model the loosening of coal seam induced by blasting is considered as damage process that is dominated by the combined contribution of blasting stress wave and blasting-induced gas pressure. Then, by considering the effect of coal seam damage on the gas permeability, the gas drainage enhanced by blasting damage is quantified based on the numerical simulations. It is demonstrated that, the blasting damage around the borehole can not only alleviate the stress concentration at the perimeter of borehole, but also enhance the gas drainage due to the increase of the gas permeability in the damaged coal seam.