Heterogeneity is an inherent component of rock and may be present in different forms including mineralheterogeneity, geometrical heterogeneity, weak grain boundaries and micro-defects. Microcracks areusually observed ...Heterogeneity is an inherent component of rock and may be present in different forms including mineralheterogeneity, geometrical heterogeneity, weak grain boundaries and micro-defects. Microcracks areusually observed in crystalline rocks in two forms： natural and stress-induced; the amount of stressinducedmicrocracking increases with depth and in-situ stress. Laboratory results indicate that thephysical properties of rocks such as strength, deformability, P-wave velocity and permeability areinfluenced by increase in microcrack intensity. In this study, the finite-discrete element method （FDEM）is used to model microcrack heterogeneity by introducing into a model sample sets of microcracks usingthe proposed micro discrete fracture network （mDFN） approach. The characteristics of the microcracksrequired to create mDFN models are obtained through image analyses of thin sections of Lac du Bonnetgranite adopted from published literature. A suite of two-dimensional laboratory tests including uniaxial,triaxial compression and Brazilian tests is simulated and the results are compared with laboratory data.The FDEM-mDFN models indicate that micro-heterogeneity has a profound influence on both the mechanicalbehavior and resultant fracture pattern. An increase in the microcrack intensity leads to areduction in the strength of the sample and changes the character of the rock strength envelope. Spallingand axial splitting dominate the failure mode at low confinement while shear failure is the dominantfailure mode at high confinement. Numerical results from simulated compression tests show thatmicrocracking reduces the cohesive component of strength alone, and the frictional strength componentremains unaffected. Results from simulated Brazilian tests show that the tensile strength is influenced bythe presence of microcracks, with a reduction in tensile strength as microcrack intensity increases. Theimportance of microcrack heterogeneity in reproducing a bi-linear or S-shape failure envelope and itseffects on the mechanisms leading to spalling damage near an underground opening are also discussed.展开更多
Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographicall...Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographically by real-time holographic interferometry and shadow optical method of caustics, as soon as the loading process started. In the meantime, the AE (acoustic emission) signals were recorded by a micro crack information storage-analysis sys-tem. According to damage theory and location of micro fracture, we have studied the stain gap and gained: a) It is necessary that strain gap appears under the condition of linear elasticity theory, and its situation is relatively stable, corresponding to stress concentration. b) Micro fractures, which appear initially at area of high stress, occur rarely at the strain gap, and their locations are finally in the zone between the stress concentration area and the strain gap, which indicate the clusters or groups. However, the major macro fracture (final rupture) started from the shadow areas, and then grew quickly towards the strain gaps, which resulted in failure of sample.展开更多
Free granite blocks with size of 50 × 50 × 50 mm cubic form which were uniaxial compressed and pre treated as dry, water and Na 2SO 4 solution soaked, were experienced three freeze thaw stages of differe...Free granite blocks with size of 50 × 50 × 50 mm cubic form which were uniaxial compressed and pre treated as dry, water and Na 2SO 4 solution soaked, were experienced three freeze thaw stages of different temperature ranges. The temperature cycles were given and carried out in an environmental cabinet while the temperatures on the samples surface and inside 10 mm and 25 mm depth were recorded respectively. Samples' weight and ultrasonic transfer velocity were also measured before and after experiment. The results showed that, to these small free samples, there was no apparent temperature difference between those on the surface and inside the blocks. Rock temperatures varied with those of freeze thaw cycles but appeared 'relative stable' when temperatures within the total range of the cycles were below 0 ℃. The weight losses of samples were very small, but still suggested that the biggest change occurred in the group of the water soaked samples. Ultrasonic transfer velocity, to most samples, turned to be slow, specially those cross the microfractures of the samples had more change than those 'average ones'. These suggested that the internal pore volume of the samples probably enlarged and microfractures had apparent influence during the freeze thaw processes.展开更多
文摘Heterogeneity is an inherent component of rock and may be present in different forms including mineralheterogeneity, geometrical heterogeneity, weak grain boundaries and micro-defects. Microcracks areusually observed in crystalline rocks in two forms： natural and stress-induced; the amount of stressinducedmicrocracking increases with depth and in-situ stress. Laboratory results indicate that thephysical properties of rocks such as strength, deformability, P-wave velocity and permeability areinfluenced by increase in microcrack intensity. In this study, the finite-discrete element method （FDEM）is used to model microcrack heterogeneity by introducing into a model sample sets of microcracks usingthe proposed micro discrete fracture network （mDFN） approach. The characteristics of the microcracksrequired to create mDFN models are obtained through image analyses of thin sections of Lac du Bonnetgranite adopted from published literature. A suite of two-dimensional laboratory tests including uniaxial,triaxial compression and Brazilian tests is simulated and the results are compared with laboratory data.The FDEM-mDFN models indicate that micro-heterogeneity has a profound influence on both the mechanicalbehavior and resultant fracture pattern. An increase in the microcrack intensity leads to areduction in the strength of the sample and changes the character of the rock strength envelope. Spallingand axial splitting dominate the failure mode at low confinement while shear failure is the dominantfailure mode at high confinement. Numerical results from simulated compression tests show thatmicrocracking reduces the cohesive component of strength alone, and the frictional strength componentremains unaffected. Results from simulated Brazilian tests show that the tensile strength is influenced bythe presence of microcracks, with a reduction in tensile strength as microcrack intensity increases. Theimportance of microcrack heterogeneity in reproducing a bi-linear or S-shape failure envelope and itseffects on the mechanisms leading to spalling damage near an underground opening are also discussed.
基金The Dual Project of China Seismological Bureau (9691309020301) the Specialized Funds for National Key Basic Study (G1998040704) the project for the MOST under contract (2001BA601B02) and Youth Funds for applied basic study of the Science and Technolo
文摘Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographically by real-time holographic interferometry and shadow optical method of caustics, as soon as the loading process started. In the meantime, the AE (acoustic emission) signals were recorded by a micro crack information storage-analysis sys-tem. According to damage theory and location of micro fracture, we have studied the stain gap and gained: a) It is necessary that strain gap appears under the condition of linear elasticity theory, and its situation is relatively stable, corresponding to stress concentration. b) Micro fractures, which appear initially at area of high stress, occur rarely at the strain gap, and their locations are finally in the zone between the stress concentration area and the strain gap, which indicate the clusters or groups. However, the major macro fracture (final rupture) started from the shadow areas, and then grew quickly towards the strain gaps, which resulted in failure of sample.
文摘Free granite blocks with size of 50 × 50 × 50 mm cubic form which were uniaxial compressed and pre treated as dry, water and Na 2SO 4 solution soaked, were experienced three freeze thaw stages of different temperature ranges. The temperature cycles were given and carried out in an environmental cabinet while the temperatures on the samples surface and inside 10 mm and 25 mm depth were recorded respectively. Samples' weight and ultrasonic transfer velocity were also measured before and after experiment. The results showed that, to these small free samples, there was no apparent temperature difference between those on the surface and inside the blocks. Rock temperatures varied with those of freeze thaw cycles but appeared 'relative stable' when temperatures within the total range of the cycles were below 0 ℃. The weight losses of samples were very small, but still suggested that the biggest change occurred in the group of the water soaked samples. Ultrasonic transfer velocity, to most samples, turned to be slow, specially those cross the microfractures of the samples had more change than those 'average ones'. These suggested that the internal pore volume of the samples probably enlarged and microfractures had apparent influence during the freeze thaw processes.
