Study On The Failure Mechanism Of Rock Bridges In A Jointed Rock Slope And Evaluation On Its Stability

There are large numbers of multi-scale discontinuities in the rock mass of a slope,such as joints,bedding surfaces and faults,etc.Due to their specific locations,occurrences and mechanical properties,as well as the complex stress distribution of the slope,which makes the study of the failure mechanism of rock bridges and the stability of the slope very complicated.For the rock slopes containing intermittent discontinuities,the stability is controlled by the rock bridge,so to reveal the failure patterns and failure evolutions of rock bridge is a key role to the control and prevention of this kind of rock slope disaster.Therefore,relied on the National Natural Science Foundation of China(41672300),the tests and numrrical simulations on fractured rock blocks or rock slope models which contain two kinds of common cracks(parallel and steep-gentle cracks)were carried out.The main work and results are as follows:(1)Since the shallow rock mass and deep rock mass of the slope are respectively in the state of uniaxial compression and three-dimensional compression stress,uniaxial compression and conventional triaxial compression tests were carried out on cylindrical specimens containing a parallel and steep-gente fissure pair.The failure patterns of the rock bridges and the crack growth processes of the specimens were analyzed.The influence of fissure inclination angle,rock bridge inclination angle,rock bridge length and confining pressure(only for triaxial compression tests)on the failure mechanism of specimens were studied.The characteristics of stress-strain curves of rock specimens with a parallel/steep-gentle fissure pair under uniaxial/triaxial compression was presented.The effects of geometries of pre-existing fissures and rock bridges on the mechanical parameters were explored.The failure patterns of rock specimens under uniaxial and triaxial are similar.Four coalescence modes of rock bridges' cracks for parallel-fissure rock mass are: tensile coalescence of wing cracks,tension-shear coalescence,shear coalescence and coalescence of multiple cracks.(2)In view of the stress unloading on the vertical slope induced by large-scale excavation,the mechanical tests of rectangular sandstone blocks containing a parallel and steep-gentle fissure pair under normal unloading-shear condition were carried out.The coalescence modes of cracks in rock bridges were studied.The test results were compared with that of direct shear tests,which is concerning to the failure patterns and the shear strength.And the failure rules of rock bridge are summarized.The characteristics of inelastic deformation components that cause rock bridge failure are analyzed.There are four failure modes for the parallel-fissure rock specimens: shear failure,tension failure,tension-shear failure and two-stage failure.The failure patterns of rock specimens with a steep-gentle fissure pair are: tension-shear failure showing a concave curve,tension-shear failure showing a closed straight line,failure with two coalesced cracks and shear failure showing a reversed “S” shape.(3)Aim at the fact that there are several groups of discontinuties with different orientations in the rock mass.In order to explore the failure patterns of the rock bridges between discontinuities with different orientations,the sample with non-uniform strike intermittent cracks(parallel and steep-slow in the central section)was produced by 3D printing technology,and uniaxial compression test was carried out.Combined with CT scanning technology,the crack growth pattern and rock bridge failure mode of the samples were described.The damage characteristics of the sample were evaluated.And the relationship between the strength and the fissure deflection angle was studied.With the variation of fissure deflection angle and fissure layout,the failure of the rock bridge presents three modes: spalling failure,coalescence of cracks at the short end and infrangibleness at the far end,and no coalescence of cracks.With the increase of fissure deflection angle,the crack initiation stress tends to increase.For the samples with a steep-gentle fissure pair,the deflection angle of the gentle-dip fissure has little effect on the failure morphology and mechanical response.(4)As the three-section landslide is a typical slope failure mode controlled by the rock bridge between steep and gentle discontinuities,the experimental study on the slope model with steep-gentle discontinuity pair was carried out.The instability mode,evolution process and bearing capacity of the three-section slope were studied.The slope stability calculation model and limit equilibrium calculation theory considering failure mode were established.The slope failure mode is mainly controlled by the inclination angle of the rock bridge,and there are three failure modes: tensile failure(Type 1),tension-shear failure(Type 2)and two-stage failure of slope body(Type 3).Type 3 has the largest bearing capacity,followed by Type 2 and the lowest by Type 1.(5)Large-scale excavation induced slope rock mass unloading is one of the main reasons for the engineering slope failure,therefore rock slopes with a parallel and steep-gentle fissure pair is simulated under the action of excavation,and the sliding failure modes of the slopes were analyzed.Based on the displacement vector diagrams and the displacements/deformations of the monitoring points,the failure evolution processes of the slopes were analyzed.The relationship between the safety factors and the geometry structures of the slope discontinuities were studied.For the parallel-discontinuity slopes,the upper parts are often tensile failure,the rock bridges in the middle parts are often characterized by tension-shear failure,and the shear failure appears at the slope toe.For the slopes containing a steep-gentle discontinuity pair,the failure pattern is mainly affected by the inclination angle and length of rock bridge,and the inclination angle of steep discontinuity,while the inclination angle of the gentle discontinuity has little influence.With the increase of the inclination angle of rock bridge or with the decrease of the rock bridge length,the tensile fracture property of rock bridge is stronger,and the shear failure property is weaker.The steep discontinuity mainly affects the failure of the rock bridge between the upper steep discontinuity and the lower gentle discontinuity.