A Study On Impact Force And Dynamic Response Of Debris Flow Retaining Structure Based On SPH-DEM-FEM Coupling

Debris flow is a special substance containing a lot of sediment fluids and solid particles.It is one of the most destructive and unpredictable geological disasters,constituting a great threat to people's lives and properties.The high-speed moving solidliquid debris flow mixture with huge mass will produce significant erosion on the flow path,and the erosion will significantly increase the kinetic energy and impact force of the debris flow,and have a significant impact on the dynamic response of the retaining structure on the flow path.Therefore,a research on the impact force of debris flow and the dynamic response of the retaining structure is of great significance to the prevention and control of debris flow.Taking Nanjiaogou in the western mountainous area of Fangshan District in Beijing as the background,this dissertation studies the dynamic response of the impact force of the solid-liquid debris flow mixture and the retaining structure.Based on the coupled SPH-DEM-FEM,this dissertation adopts the complex dynamic model of particle-fluidblocking structure.Firstly,the impact force and dynamic response of solid-liquid debris flow mixture impacting rigid retaining structure are studied.The process of debris flow impacting the retaining structure,the impact force of debris flow and the dynamic performance of retaining structure are studied respectively.Secondly,the impact force and dynamic response of the solid-liquid debris flow mixture impacting the flexible retaining structure are studied.Then,the influence of source erosion on the movement process and impact force of solid-liquid debris flow mixture is studied,a complex dynamic model of particle-fluid-erosion-retaining structure is established,and the impact force of debris flow with erosion and the dynamic response of retaining structure are analyzed.The secondary developed strain softening model is used to simulate the process of the mud-rock fluid transforming from a solid state to a transition state and finally to a liquid state.The impact process of debris flow,impact height behind the dam,deposition thickness,dynamic response of debris flow and impact force of debris flow are systematically studied,with and without erosion respectively.Finally,according to the research results,relevant suggestions for debris flow control in the study area are provided.The research results provide guidance for the debris flow control and the engineering design of the retaining structure in the study area,and provide the basis for the calculation of the impact force of the solid-liquid debris flow mixture in other areas,and provide a reference for the engineering design of the retaining structure.The main work and achievements of the paper are as follows:1.According to the material composition of debris flow,the SPH-DEM-FEM coupled numerical model is constructed.DEM is used to calculate solid debris flow particles,and elastic model is used for the contact between solid debris flow particles.The model simplifies the complexity of debris flow particles,but can simulate the interaction between debris flow,channels and retaining structures to the greatest extent.Particles will collide and dissipate energy during the interaction process.The particle force equation,the resultant external force and moment equation,and the interaction force equation between particles and the calculation principle are introduced.SPH is used to simulate debris flow fluid and eroded soil layers at the bottom and sides of the channel.The kernel approximation of the SPH method mainly includes the use of smooth functions and their derivatives,and the particle approximation function equation,kernel function equation,Cubic B-Spline smooth kernel function,particle approximation function equation and calculation principles are introduced.The realization principles of numerical simulation of DEM-FEM coupled governing equations,SPH-FEM coupled governing equations and SPH-DEM-FEM coupled governing equations are introduced respectively.2.The verification study of the coupled numerical model is carried out.First,the SPH-FEM coupling numerical model verification is carried out.By comparing with the indoor debris flow impact force and debris flow impact retaining structure test,the SPHFEM coupling numerical simulation verification is carried out,and the entire process of debris flow impacting the retaining structure and debris flow impact force are checked.The impact force time-history curve of debris flow basically fits the impact force timehistory curve of the indoor physical model test.The DEM-FEM coupling numerical model is verified,and the DEM-FEM coupling numerical simulation verification is carried out by comparing with the indoor debris flow impact test.The chip flow impact test fits well with the impact force time-history curve of the numerical simulation.The DEM-FEM coupling numerical model is verified,and the DEM-FEM coupling numerical simulation is verified by comparing with the indoor debris flow impact force and the debris flow impacting the retaining structure test.The impact force of the chip flow is checked and calculated,and the time-history curve of the impact force of the chip flow impact test and the numerical simulation fit well.The DEM-SPH coupling numerical analysis method is used to reproduce the whole process of the sphere's vertical impact on the water surface,and the velocity and displacement of the sphere's impact on the water surface are analyzed.The analysis shows that the curve of the velocity of the sphere with time is basically consistent with the test,the curve of the depth of the sphere with time,and the curve of the numerical simulation results and the test results are basically fit.The DEM-SPH-FEM coupling erosion numerical simulation method is used to analyze the erosion physical model test.The DEM-SPH-FEM coupling erosion numerical simulation is used to simulate the shape and thickness of debris flow at time t=0s,t=0.32 s and t=0.64 s,respectively.The profile results are basically consistent with the erosion test results.The coupled DEM-FEM numerical analysis method is used to verify the mechanical parameters of the large-scale direct shear test.The trial-and-error method is used to check the DEM parameters and the results of the large-scale direct shear test.The numerical simulation and the large-scale direct shear test shear stress-shear displacement curve fit well.It is proved that the coupled numerical model can be used for the later calculation of the impact force of debris flow and the study of the dynamic performance of the retaining structure.3.The dynamic response of debris flow impact force and rigid retaining structure is studied.Taking Nanjiaogou in the western mountainous area of Fangshan District in Beijing as the background,since the model mainly studies the impact force of debris flow and the dynamic response of retaining structures,the terrain is simplified accordingly,and the geometric model,material model and boundary model are constructed.The findings and model validation identified model parameters.Based on the coupled SPHDEM-FEM numerical method,the complex dynamic interaction of particle-fluidstructure is considered,and the whole process of impact,climbing and silting of debris flow impacting the retaining structure is reproduced.The coupled numerical analysis method reproduces the whole process of debris flow from formation,to blocking,and finally to silting.Based on the coupled numerical analysis method,the dynamic response analyses of the debris flow impacting the retaining structure under different slopes are compared.It can be seen from the research results that the larger the slope is,the earlier the dynamic response of the retaining structure and the displacement peak appear,and the larger the displacement peak is.The dynamic response law can provide a certain reference for the design of debris flow retaining structures.The variations of the impact force of debris flow are compared and compared with the empirical formula.The results of the two are relatively close.The numerical calculation method can provide a certain reference for the calculation of impact force of debris flow and the design of further retaining structures.The impact force distribution in the whole process of the impact of the solidliquid debris flow mixture on the retaining structure is studied.The distribution of the impact force has a certain reference value for the engineering design of the debris flow retaining structure.4.The dynamic response of debris flow impact force and flexible retaining structure is studied.Based on the coupled SPH-DEM-FEM numerical method,the complex dynamic interaction of particle-fluid-structure is considered,and the entire process of impact,blocking,water-rock separation,climbing,back-silting and deposition of debris flow impacting flexible retaining structures is reproduced.The dynamic response analysis of the debris flow impacting the flexible retaining structure under different slopes is compared,and the displacement time history of the middle position of the four supporting cables(NO.1,NO.2,NO.3,NO.4)of the flexible retaining structure is monitored and simulated.The results are consistent with the law and existing research results.The variation law of the impact force of debris flow under different slopes is compared,but the empirical formula does not take into account the complex dynamic interaction of water and stone separation and particle-fluid-structure,and the numerical simulation results will have errors between 12.1% and 27.4%.Therefore,the numerical analysis The method is more reasonable than the empirical method.The maximum deformation of the flexible blocking structure is analyzed,and the deformation has an obvious positive correlation with the slope of the channel.The maximum deformation position is located in the middle of the flexible blocking structure,and the support cable plays a significant role in restricting the flexible blocking net.The mechanical and dynamic characteristics of the flexible blocking structure and the rigid blocking structure under the impact of the solid-liquid debris flow mixture are compared.Since the rigidity of the flexible blocking structure is much smaller than that of the rigid blocking structure,the dynamic characteristics are significantly different,and the displacement time history of the two is significantly different.The difference is two orders of magnitude,and the recovery ability after deformation is much smaller than that of the rigid blocking structure.5.The impact force of debris flow with erosion and the dynamic response of retaining structure are studied.Based on the coupled SPH-DEM-FEM numerical method,the secondary development of the erodible soil strain softening model is carried out,and a complex dynamic interaction model of particle-fluid-erosion-structure is established,which reproduces the whole process of erosion,blocking and deposition.The source soil at the bottom and on both sides of the channel adopts the strain softening model of secondary development.When the shear stress exceeds the material strength of the soil,the soil will be transformed from a solid state to a fluid state.The effects of debris flow impact process,impact height behind the dam,sediment thickness,debris flow dynamic response and debris flow impact force with and without erosion are systematically studied.At t=6s,the solid-liquid debris flow mixture backs up and finally deposits at the bottom of the retaining structure to form a static load.The deposition volume of the erosion model is significantly larger than that of the model without erosion,and some debris flows flushed out of the retaining structure.All debris flows without the erosion model are blocked.The maximum impact height of debris flow with erosion is 5.13 m,while the maximum impact height without erosion is only 3.58 m.The sedimentary thickness of debris flow is 3.12 m considering erosion,but only 2.47 m without erosion.The maximum displacement of the midpoint of the retaining structure without considering erosion is 1.16 mm,while the maximum displacement of the retaining structure considering erosion is 2.08 mm.The peak value of the impact force of the debris flow without considering the erosion is 1315 k N,while the peak value of the impact force of the debris flow considering the erosion is 2339 k N.The results are basically consistent with the existing literature and data conclusions,further confirming that erosion has a significant impact on the impact force of debris flow and the dynamic response of the retaining structure.6.Combined with the research results,suggestions are provided for debris flow control in the study area.According to the calculation results of numerical simulation,and compared with the calculation results of the existing semi-empirical debris flow impact force peak calculation formula based on the hydrodynamic theory,the rigid retaining structure,flexible retaining structure and retaining structure considering the source erosion under debris flow impact are compared.The peak value of debris flow impact force is corrected by empirical parameters.It also provides suggestions on the impact force and engineering prevention of debris flow in the main ditch of Nanjiaogou,and calculates the velocity of debris flow,impact pressure,maximum flushing height and climbing height.The results show that under the consideration of erosion,the velocity of debris flow is greater than that without erosion,and the impact pressure,surge height and climbing height are also significantly larger than those without erosion.According to the previous parameter calculation,the calculated dam height is only 5.8m,and there is a significant difference.The research results provide guidance for the debris flow control and the engineering design of the retaining structure in the study area,and provide a basis for the calculation of the impact force of the solid-liquid debris flow mixture in other areas,and provide a reference for the engineering design of the retaining structure.