Modeling Of Step-path Failure Mechanism In Rock Slopes Using PFC2D And Evaluating On Its Stability

Step-path failure in rock slopes is very common such as the failure in strongunloading zones in the surface of high slopes in Xiaowan hydroelectric station reservoirbank which is controlled by steep dip structural surface and low-angle dip structuralsurface. Traditional research methods such as limit equilibrium method of slices andfinite element method based on medium continuity hypothesis are insufficient fordeformation and failure analysis and stability evaluation of this kind of slopes whichhave complex structure.This paper relied on the Youth Science Fund(No.40902078) andthe National Natural Science Foundation(No.41172243) of National Natural ScienceFoundation of my tutor, based on the theory of particle flow, focused on the coalescencemechanism of rock bridge, simulated the step-path failure mechanism in rock slopesusing PFC2D from the perspective of mesoscopic and macroscopic, and built the slopestability evaluation models using related mechanics theory. The main work and researchconclusions are as follow:①Based on the numerical modeling of uniaxial compression test, shear test andunilateral unloading test, the coalescence mechanism of rock bridge in specimen withtwo flaws from the perspective of mesoscopic and macroscopic is analyzed. Crackcoalescence of rock bridge is easy to happen when the dip angle of flaw is low underuniaxial compression and there are three modes of coalescence which are tensilecoalescence, shear coalescence and tensile-shear coalescence. Tensile coalescence iseasy to happen when the dip angle of flaw is lower than45°and the dip angle of rockbridge is bigger than90°, shear coalescence is easy to happen when the dip angle offlaw is60°and tensile-shear coalescence happens sometimes when the length of rockbridge is long. Tensile coalescence and shear coalescence happen too under unilateralunloading. There are tensile coalescence, nucleation failure and the combination of thetwo situations under shearing.②Based on the numerical modeling of uniaxial compression test, the deformationand peak stress character of specimen with two flaws are revealed, and strengthcriterions when crack initiation and coalescence(tensile coalescence and shearcoalescence) are given under compress-shearing stress. The deformation process ofspecimen can be distributed into four stages: elastic deformation, crack propagation ofrock bridge steadily, crack propagation of rock bridge unsteadily, and failure of specimen after peak strength. Elasticity modulus of specimen decreases and thenincreases with the increase of the dip angle of flaw, and is the lowest when the dip angleof flaw is30°; elasticity modulus decreases with the increase of the dip angle of rockbridge when the dip angle of flaw is lower than45°, decreases and then increases withthe increase of the dip angle of rock bridge when the dip angle of flaw is≥45°and isthe lowest when the dip angle of bridge is90°, elasticity modulus increases with theincrease of the length of rock bridge; peak strength of specimen decreases and thenincreases with the increase of the dip angle of flaw, and is the lowest when the dip angleof flaw is30°; peak strength decreases and then increases with the increase of the dipangle of rock bridge and is the lowest when the dip angle of rock bridge is90°, peakstrength is the highest when the length of rock bridge is12mm, reduces slightly whenthe length of rock bridge is24mm, and is the lowest when the length of rock bridge is6mm.③Step-path failure in slope numerical models with different geometry of fracturesand rock bridges based on the gravity increase method are analyzed. Slip surface isformed mainly by the shear coalescence of rock bridges when the dip angle of rockbridge is60°or40°, slip surface is formed mainly by the tensile coalescence of rockbridges when the dip angle of rock bridge is≥90°, and slip surface is formed sometimesby the tensile-shear coalescence of rock bridges when the length of rock bridge is long.Generally tensile fissures initiate from the slip surface and especially the tensile fissurein the trailing edge of the slope always extends to slpoe surface. Slope slids along theflat fractures by step-path firstly, slope displacement roughly has the characteristic ofregionalization because of the fissures, and finally the failure of slope is monolithic butwith local failure.④Based on the mesoscopic and macroscopic mechanical process analysis, thestep-path failure mechanism in rock slopes is expounded. It is a gradual process thatrock bridge fractures one by one from the bottom up of step-path failure in rock slopeunder the action of gravity. There is stress concentration at fracture tips and rock bridgesbefore slope failure and stress relief after slope failure. Tensile fissure development inthe trailing edge of the slope is due to the traction of lower part of slope and it is general.The step-path failure process can be distributed into four stages: steady deformation,coalescence for lower rock bridges, coalescence for upper rock bridges and tensilefissure initiates in the trailing edge of the slope, and slipping of slope after failure. It isthe final failure of slope in stage three and the start point is the critical point of instability.⑤Based on the analysis of the step-path failure process mechanism in rock slopes,three slope stability evaluation models of step-path failure by shear coalescence, tensilecoalescence, and tensile-shear coalescence of rock bridges are built using mechanicstheory(limit equilibrium theory and Mohr-Coulomb criterion). The shear strengthcriteria of macro slipping and the equivalent shear strength parameters calculation areexplored.