Research On Brittle Failure Mechanism Of "Locking Section" Rock Bridge For Rock Slope

Due to the extremely complicated geological conditions in the mountainous areas of southwest China,the stability and disaster prevention of high rock slope are particularly serious.The locking section rock slope is the most common failure mode of large-scale rock slope.The locking section of rock slide refers to the rock bridge with relatively higher strength in the sliding zone.And the locking section of rock slide plays a key role in controlling the slope stability.Usually the occurrence of large-scale rock slide is accompanied with the stress concentration and sudden brittle failure of the locking section on the sliding surface.The locking section has an important position in deformation control and stability mechanism of rock slide,which is also the key to the geological hazard assessment and control of rock slide.The shortage of research on failure mechanism of locking section rock bridge and its importance has become a prominent contradiction in the control of large rock slope stability.In this paper,the uniaxial compression test and a series of triaxial loading and unloading tests are performed on rock-bridge samples.The mechanical characteristics and failure modes of the "locking section" type rock bridge under different stress states are deeply analyzed,and the failure mechanism of the "locking section" type rock bridge is explored.On this basis,a new method for evaluating the brittle failure of slope based on the strip-softening method is proposed,and the research results are verified with the backdrop of the Danba County back mountain landslide.The main achievements are as follows:(1)The stress strain curves of rock bridge samples under different stress states can be roughly divided into 5 stages according to the crack propagation process: compaction stage,linear elastic deformation stage,plastic deformation stage,failure stage,and residual strength stage.(2)The peak strength and strain of rock sample increase with the increase of the length of rock bridge and increases with the increase of confining pressure.A typical brittle failure of rock sample with the rock bridge is exhibited.The internal friction angle of the rock sample increases with the length of the rock bridge,and the length of rock bridge will affect the failure surface of rock sample.The strongest and most deformation characteristics of the rock bridge rock samples are triaxial loading and unloading tests,followed by the triaxial loading tests and the triaxial unloading tests.(3)The stress-strain curves of rock and Bridge samples will have the phenomenon of "pre-peak fluctuations" and "post-peak recovery".The phenomenon of “pre-peak fluctuation” is caused by partial overlap and penetration of cracks in the rock samples.The phenomenon of "post peak recovery" is due to the fact that residual rock mass still has some residual bearing capacity after the rock sample breaks through.(4)The failure modes of rock samples are summarized as five types: 1 Cracks are propagated upward from the joints of the lower part,and perforation failure occurs in the rock bridge area;2 Cracks are propagated upward from the joints of the lower part,and cracks penetrate the upper face of the rock sample;3 Cracks are propagated inwardly from the tip of the lower joint,and fracture of rock specimen by circumferential extrusion;4 Cracks are propagated outwardly from the tip of the lower joint,and rock samples are torsional to the tensile failure;5 Cracks are propagated downward from the tip of the lower joint,passing through the bottom face of rock samples.The destruction of each rock sample is either a single failure mode or a combination of multiple models.The extending direction of the main crack is basically parallel to the direction of the axial load or at a small angle to the axial load direction.The axial load is the main control factor for the crack propagation of the rock sample.The local cracks in rock samples basically correspond to the sudden drop of stress.(5)The AE characteristics of the rock samples with rock bridge at different stages are extremely significant.The change trend can provide an early warning of the overall destruction of rock samples.With the appearance of a large drop in stress,the count rate and energy of AE events will reach the peak point.The count rate and accumulated energy of AE events during the “quiet period” or “post-peak recovery” period are significantly greater than that in the plastic deformation stage,which indicates that rock samples with rock bridge are still expanding steadily over the “quiet period” or “post-peak recovery” stage,resulting in the overall penetration of rock samples.(6)A new stability evaluation method for dynamic progressive failure of slope is proposed,which combined the Sweden slice method and the strain-softening theory.For different brittle-plastic geomaterials,the difference in softening modulus will also lead to changes in the slope safety factor.The faster the post-peak strength index falls,the smaller the slope safety factor.There is also a double reduction factor in the drop of the strength index in the evaluation of slope dynamic stability based on the slice-softening method,ie,the reduction factor of ? is smaller than the reduction factor of c.Furthermore,the evaluation method of the safety factor is proposed by considering the brittle failure of the locking section,which provides a new way for the stability evaluation for rock slide.