Key Engineering Materials Vols. 297-300

Abstract: Six types of numerical specimens containing two notches are set up to numerically investigate the effect of element size on rock shear strength and failure pattern using RFPA2D (rock failure process analysis) code. These specimens are of the same geometrical dimension 180 mm×180 mm and have been discretized into 61×61, 122×122, 183×183, 244×244, 305×305, and 366×366 elements.The width of notches is about 2.95 (180/61) mm and the length is 45mm. The specimens are placed in a direct shear box. A lateral confining pressure with a value of 0.15MPa is invariably loaded in the vertical direction and an increasing horizontal displacement with 0.002mm/step is applied in the horizontal direction. The whole shear failure progress and associated stress field for the specimens are visually represented. Results show that the crack propagation is mostly influenced by the stress field in the vicinity of the notch tip, the required element size would be necessary in order to obtain good results. In general, for a coarse mesh, the stress field close to the notch tip can’t be represented accurately and shear strength obtained by such discretisation is slightly higher than the accurate value. For a fine mesh, the notch tip spreads through a relatively large number of elements and the stress field in vicinity of notch tip is well represented by the finite element approximation, therefore the failure pattern is consistent with real physical fracture mode.

Abstract: Disastrous rock slope failures have been posing a hazard to people’s lives and causing enormous economic losses worldwide. Numerical simulation of rock slope failure can lead to improve the degree of understand of such phenomenon so as to predict and avoid the occurrence of these disastrous events. In order to simulate the global behaviors of rock slope failure under the high seepage pressure and the local behaviors of the occurrence of hydraulic fracture in the pre-existing rock joints effectively, a powerful finite element tools F-RFPA2D, is adopted. The simulation takes into account of the growth of existing fractures and the initiation of new fractures under various of hydraulic pressure in different heterogeneities medium. The behavior of fluid flow and damage evolution, and their coupling action are studied in small specimens that are subjected to both hydraulic and biaxial compressive loadings. The influence of the ratio (the initial horizontal stress to the initial vertical stress) and the distance between the two existing cracks on the fracture propagation behaviors are investigated. Moreover, based on the fundamental study of hydraulic fracture, the progressive failure of rock slope under the influence of the increase in hydraulic pressure was also studied in the paper.

Abstract: In this paper, the process of failure and falling of the rock masses in the underground mining is studied; and the relationship between the processes of failure and falling of rock masses is proposed based on the rockmass strength theory. Research indicates that the failures of the rock and rock masses are probably not at all in common. Even though the rock masses go into the plastic condition, even to the extent that come into being in the failure, they do not surely fall. Hence, the mechanical properties of rock mass, especially after the failure, still need to be further researched. The laneway and the stope in the mining engineering are the temporary engineering, the load- carrying ability of the rock masses should be bought into play as far as possible, and therefore the stabilization during failure process is a significant for the supporting in the underground mine under safety prerequisite. Since 1 million m3 of the mined-out area is formed by mine for many year at Xishimen iron mine in China, and the gob roof is only 54m from the ground, the mined-out area is potentially dangerous to the underground mine. This paper analyzes the pattern of the stability and falling process in the mined-out area based on the theoretical analysis and numerical calculation, in order to obtains the falling criterion of the roof rock masses and stabilize the mined-out area. The predicted falling shape and range of the mined-out area is compared well with the in-situ observations. This contribution has also been popularized in many mines of China.

Abstract: In this paper, nonlinear vibration techniques were applied to investigate stages of progressive damage in three vibration isolation systems induced by dynamic loadings. Analytical models for reinforced concrete structures of three isolation systems were developed based on FEM with discrete crack concept. Vibration response spectra and the spectra of forces transmitted through the isolators were computed with respect to stationary dynamic loads. In addition, fatigue properties of concrete structures were examined for given materials’ properties and given geometries. The results indicated that the proposed isolation system 3 can improve fatigue resistance by extending fatigue life and changing the failure mode from shear to flexure.

Abstract: By using numerical code RFPA2D (Rock Failure Process Analysis), the evolution of fracture around cavities subjected to uniaxial and polyaxial compression is examined through a series of model simulation. It is shown from the numerical results that the chain of events leading to the collapse of the cavity may involve all or some of the fractures designated as primary tensile, shear and remote fracture. Numerical simulated results reproduce the evolution of three types of fractures. Under the condition of no confining pressure, the tensile mode dominates with collapse coinciding with the sudden and explosive appearance of the secondary tensile fracture; at moderate higher confining pressure, the tensile mode is depressed, comparatively, the shear effect is strengthened. Nevertheless, tensile fractures especially in remote fractures stage still play a role; at higher pressure, the shear fracture dominates the remote fractures. In addition, the evolution and interact of fractures between multiple cavities is investigated, considering the stress redistribution and transference in compressive and tensile stress field.

Abstract: Borehole breakout is the process by which portions of borehole or tunnel wall fracture or spall when subjected to compressive stresses. The stress-strain characteristics of rock during loading and unloading confining pressure are studied firstly. To overcome the difficulties in analytical model studies, a numerical code, RFPA2D (Rock Failure Process Analysis), developed by CRISR, Northeastern University, China, is used to investigate the progressive failure of breakout around tunnel. The heterogeneity of rock was also taken into account in the software. The numerical simulation reproduces the formation notch in rocks by the growth, interaction and coalescence of randomly distributed macrocracks. It is illustrated from the numerical simulated results that breakout direction of tunnel is parallel with the minor stress tensor in the plane perpendicular to the borehole axis. Specifically due to the inclusion of heterogeneity, some peculiarities are studied both in the evolution of fracture and the influence of borehole on the peak intensity of specimen as well as the AE event patterns.

Abstract: Rock failure analysis is an important research in investigating the behavior of rocks, especially its failure process. And a rock sample, which contains pre-existing cracks, is a typical sample to investigate the rock failure behavior which under tension or compression, because almost all the natural materials exist micro-flaws more or less. This mode is closed to the natural material character. By using Rock Failure Process Analysis code, RFPA2D, we present a numerical simulation and similar materials experiment on rock samples with two pre-existing cracks in uniaxial compression were conducted to investigate the initiation, propagation, coalescence of cracks and failure mechanism of rock. Numerical simulations visually reproduce the process of crack initiation, propagation and coalescence in rock, which are well tallied with experiments in laboratory.

Abstract: The purpose of this paper is to investigate shear strength and failure pattern of rock containing two parallel open joints with different horizontal separations using RFPA2D (rock failure process analysis) code. Specimens are placed in a direct shear box. The upper is invariably loaded with normal stress 0.15MPa, the left is controlled by a constant increasing horizontal displacement 0.002mm/step. The whole shear failure process is visually represented and the failure pattern in reasonable accordance with previous experimental results is obtained. In general, only mixed mode (tensile and shear) is observed for the failure pattern in the numerical tests. Tensile cracks initiate from the tips of pre-existing joints respectively with an initiation angle of about 45°, then propagate towards another joint in a single stria; Shear cracks occur in the further process and the main direction of shear failure surface is roughly parallel to shear loading. The failure pattern of bridged rock is mainly controlled by the joint separation and the roughness of wavy shear failure surface is different, which is mostly influenced by the joint separation in the same way. The peak shear load, related to the failure patterns, decreases with the increase of joint separation, but the shear strength of intact rock is invariable.

Abstract: When the coal (rock) specimens are tested with infrared, one can observe that the specimens’ temperature field is changing. Commonly, causes of the changes are due to specimens’ plastic deformation and crack extension. But many coherent documents indicate that most specimens even has high temperature field after the peak strength. This paper put a forward that the influence of friction between the fracture planes on the specimens’ temperature field could not be ignored. Meanwhile, the paper gives a simple friction model of two blocks,. In the model, the friction surface plays the different roles to the two blocks as moving heat source and fixed heat source respectively. Those different heat sources have different effect on the blocks’ temperature field. In the end, analytic solutions of temperature change at any point of two blocks that caused by friction are given.

Abstract: Based on cusp-type catastrophe theory, a sample rock-rock (hypocenter surrounding the rock) model for studying the pillar rockburst mechanism is presented in this paper. It is expounded theoretically that the stiffness ratio, K, of the roof and floor to the pillar plays an important role in the outbreak of instability. Using a newly developed numerical code, RFPA2D, the progressive failure process and associated microseismic behavior of the twin rock samples are simulated. The numerically simulated results also confirm that a soft roof and floor promotes an unstable failure or collapse of pillars. Additionally, the simulated results reproduced the deformation jump and the energy release that occur during a pillar rockburst. It is demonstrated that the proposed model properly simulates the pillar failure process.