| The dynamic responses of flexible protective structures affected by geological hazards occurring on shallow slope are complex nonlinear multi-body dynamic problems.These bring great challenges to the prevention and control of the aforementioned hazards.An experiment,field survey,and theoretical analysis and numerical simulation based on impact mechanics were carried out to analyze the discretization theory and method that explains the effects of shallow slope geological hazards on flexible protective structures.These examinations included the discretization of flexible structures and geological hazard on shallow slope as well as the dynamic calculation theory and method for coupled flexible structure-geological hazards on shallow slope.Finally,along with identifying the failure modes of flexible protective structures exposed to the effects of actual disaster,this research formulated protection design suggestions and conducted back analysis.The main research contents and results are as follows:1.A discrete model of a wire-ring net was established(Chapter 2).The study performed quasi-static tests,including tensile tests on steel wires and one-ring specimens,puncture tests on net specimens,and impact tests on the wire-ring net.The test results revealed the nonlinear mechanical properties of the wire-ring net.A discrete model of a wire-ring net was established.The wire rings were modeled via an equivalent structure with a single winding and a circular cross-section.The equivalence between numerical and actual wire rings in terms of bending and tensile strength,contact with sliding friction,the flattening effect,and rupture behavior were also determined and presented.Compared with a traditional wire-ring net model,the discrete model can be simulated more accurately because it fully considers discretization characteristics,and the accuracy of P-(35)(tensile force-deformation curve)simulation under large deformation is visibly improved.2.A model of the multi-flexible discretization of flexible protective structures was established(Chapter 3).A dynamic model of a flexible protection system was constructed with consideration for large deformation in a net(has been studied in Chapter 2),considerable plastic deformation in an energy dissipation device,large slipping in a cable,and the mechanism motion effect in a support system.The model can simulate a series of strong nonlinear effects under large deformation of the flexible protective system,such as material,boundary and geometry nonlinear.A full scale impact test of three-span passive protective net system was performed,and a comprehensive comparative analysis of the impact mechanical behavior of the system was carried out on the basis of the analysis directed toward the multi-flexible discretization model.This model can support the accurate design and parametric expansion analysis of flexible protective structures.3.Three impact load models of typical geological hazards on shallow slope were established: granular flow on shallow slope,rockfall,and debris flow on shallow slope(Chapter 4).The discrete element method(DEM)was used to accurately examine friction-collision in granular flow on shallow slope during movement.With the failure of the discrete element parallel bond as basis,the research explored the fragmentation of rockfall.The coupled DEM-smoothed particle hydrodynamics method(DEM-SPH)was adopted to take into account the interaction between particle and particle friction-collision as well as the interaction between debris flow fluid and particles.The problem of large mesh distortion in debris flow simulation was solved.The accuracy of the three load models was verified via experiments.Three disasters cases were simulated,and the comprehensive characteristics of the impact evolution of geological disasters on shallow slope was analyzed.The results showed that DEM can completely reproduce the movement and accumulation characteristics of granular flow on shallow slope and the fragmentation characteristics of rockfall.The DEM-SPH method can reproduce the propagation,impact damage,and deposition of debris flow on shallow slope.4.Coupled models of flexible protective structures–geological hazards on shallow slope were established,and coupled impact dynamic behavior was quantitatively studied(Chapter 5).In the coupled model,the discrete mechanical behavior of the wire-ring net(has been studied in Chapter 2),the strong nonlinear effect of the structural model(has been studied in Chapter 3),and the characteristics of load(friction and collision of granular flow on shallow slope,fragmentation of rockfall,solid-liquid phase of debris flow on shallow slope)(has been studied in Chapter 4)were comprehensively considered and verified via in-situ tests.The coupled model was employed in examining the dynamics of granular flow-barrier interaction,such as the interaction process,impact force,energy distribution,and evolution of the internal forces of components.The analyses of impact force showed that rise and accumulation of incoming flow particles caused the maximum impact force in the run-up stage.Thus,when the force approach design neglects the effects of the run-up stage and dead zone formation,conservative estimates of impact force may be derived in the interaction process.The kinetic energy of a flow front dissipated mainly in the frontal impact stage.In the run-up stage,the kinetic energy of the subsequent incoming granular flow was dissipated mainly by friction.In the benchmark model,the sensitivity of the bulk density of granular flow on shallow slope,the impact height of such flow,the slope angle of a channel,and the active force of energy dissipators to the maximum impact force were 1,0.186,2.084,and 0.035,respectively.The high-sensitivity parameters were the main influencing parameters and should be given priority in the design of flexible protective structures.The effect of the fracture strength of rockfall on the coupled mechanical behavior of flexible protective structure was quantitatively studied.The maximum impact force,the internal force of a steel post,and the internal force of cable increased with the rising fracture strength of rockfall,but the rate of increase gradually diminished.In contrast,under no consideration of fragmentation in rockfall,the dispersion and divergence of the broken track of rockfall expanded,leading to a larger threat area, which tended to be unsafe from the perspective of protection scope.5.The failure mechanism of a flexible protective structure was analyzed,and corresponding design countermeasures were proposed(Chapter 6).On the basis of an investigation and analysis of 11 specific worksites in the mountains of southwest China,the failure modes of a flexible protective structure were analyzed,and the main failure mode was divided into six categories: net destruction,steel post buckling,support rope fracture,anchor end failure,overall overturning,and component rusting.The most prevalent failure mode was steel post buckling failure.The failure mechanism of the corresponding failure mode was analyzed,and the results showed that the slip lock in the boundary was the main reason for the failure of the flexible protective structure.Corresponding design countermeasures were put forward on the grounds of the failure mechanism.Finally,with the actual worksite where the flexible protective structure is located as reference,the failure mode and mechanism of the structure affected by granular flow on shallow slope were examined via inversion by comprehensively applying the coupled model in Chapter 5.Additionally,the failure mode and mechanism were verified.Quantitative research on the design countermeasures was carried out,and corresponding effective measures,such as increasing the flange width of steel posts and changing the structural system,were verified. |
PDF Full Text Request