Study On Critical Tension Depth And Stability In Rockslides That Conform To The “Creep-Tension-Shear” Mechanism

The stability in large-scale rockslides located on the alp-gorge zone are closely linked to the locked section along a potential slip surface and are sudden to occur.For rockslides with a locked section,the sliding-tension cracking-shearing mechanism is the underlying mechanism of a typical geo-mechanical model.In this model,the sliding surface is directly determined by three sections:(a)the creep section along the weaker strata,(b)the steep tension section along the tension crack at the rear,and(c)the curved locked section in between.During the evolution of such rockslides,the sliding at the slope toe driven by continuous creep deformation causes the initiation of a tension crack from the head of the slope that then propagates to the critical tension depth resulting in the brittle shear failure of the locked section,as a result,rockslides with a locked section are usually large and suddenly become unstable,resulting in highspeed and long-distance runout.For solving the difficulty caused by the sudden instability of rockslides that conform to the “sliding-tension cracking-shearing” mechanism,this thesis,based on the empirical summary from previous researches,firstly derived the critical tension crack depth from the theoretical angle.And then,a stability assessment method was created to consider the “progressive evolution-sudden instability” mechanism.The research results can not only further reveal the inherent mechanism of the creep section,tension section and the locked section at the temporal and spatial evolution aspects,but also have the application value for the stability evaluation and instability early warning of large-scale rockslides.The main contributions are shown as follows:(1)Based on the summary analysis of rockslides that conform to the “slidingtension cracking-shearing” mechanism,the geological environment conditions and deformation-failure mechanism of such landslides were concisely summarized.Generally,rockslides that conform to the “sliding-tension cracking-shearing”mechanism,belong to gravity landslides.The sudden instability is controlled by the brittle shear rupture along a nearly arc failure path in the locked section.The tension crack at the rear propagating to the critical depth is the prerequisite and criterion for the occurrence of rockslides that conform to the “sliding-tension cracking-shearing”mechanism.(2)The floor-friction physical simulation experiment was carried out based on the typical slope prototype to explore the evolutionary mechanism of the tension crack at the rear under the action of front weak interlayer.The tension crack extends vertically to half the critical depth and then decelerates,and finally the critical tension crack depth is obtained.And then,there is a closed trend in the tension crack with the shear action along the arc-shaped path in the locked section.(3)The locked section must be shortened with the propagation of the tension crack at rear.Based on the compression-shear test results of rock bridge with different length,the evolutionary mechanism of rockslides that conform to the “sliding-tension cracking-shearing” mechanism and the rock bridge length effect,were further revealed by the PFC2 D numerical simulation and theoretical analysis of fracture mechanics.The tension crack at rear to improve the suddenly brittle failure of rockslides that conform to the “sliding-tension cracking-shearing” mechanism.(4)Controlled by the personality characteristics of the rockslide,different rockslides that conform to the sliding-tension cracking-shearing mechanism,ought to have different critical tension crack depth.The nearly arc-shaped failure path of the locked section was verified and analyzed by the numerical simulation using PFC2 D.On this basis,the critical tension crack depth threshold was theoretically derived based on the limit equilibrium state determined via the vector sum method,and then a new methodology was proposed for calculating the critical tension crack depth.The critical tension crack depth calculated via the abovementioned theory is more reliable and accurate than that calculated via the empirical equation.(5)For the evolutionary processes of landslide before and after reaching the critical tension crack depth,the strength evolution constitutive models of the locked section were established respectively:(1)the FWCS model considering slope time-dependent deformation that can effectively reflect the rock bridge length effect and the damage accumulation of locked section under the propagation of the tension crack,(2)the sharp deterioration model of the locked section strength parameters that can effectively reflect the suddenly brittle failure of slope.(6)Employing the FWCS model considering slope time-dependent deformation and the sharp deterioration model of the locked section strength parameters,the evolutionary processes of landslide before and after reaching the critical tension crack depth can be joined by the critical tension crack depth.And a numerical simulation method was further established to consider the progressive evolution-sudden instability mechanism.On this basis,a quantitative stability assessment method was established to realize the targeted evaluation of the stability of rockslides that conform to the“sliding-tension cracking-shearing” mechanism.Finally,based on the evolution of the tension crack at rear,an early identification method for such rockslides was established by the critical tension depth,which can help solve the difficulty of early warning for the rockslides that conform to the “sliding-tension cracking-shearing” mechanism.