Debris Flow Impact Forces On Bridge Piers

Mountainous areas account for 2/3 of China's land area.With the development of China's economy,more and more bridges will be built in mountainous areas.Natural hazards occur frequently in Mountain areas and the debris flow is one of the main geological hazards.Debris flow contains a large amount of sand and stones and can be treated as a solid-fluid-air three-phase flow.It has tremendous energy which can destroy bridges seriously.Therefore,it is urgent to understand the failure mechanism of debris flow impacting bridges,and one premise is to quantify the debris flow impact forces on bridge piers.The debris flow is separated by a pier and moves around the pier,which is quite different from the debris flow impacting on a barrier.This study starts from a small-scale experiment of debris flow impacting on piers.The experiment results are studied from macro and micro two aspects,i.e.,rheological model and stress components,respectively.Then the experiment results are used to calibrate and validate a numerical model.The numerical study based on the expanded small-scale experiment finds the debris flow impact three-layer model.The following full-scale numerical simulations improves the three-layer model.This thesis can be summaried as follows:(1)This thesis introduces the study background and significance and points out to quantify the debris flow impact forces on piers is the premise of exploring the failure mechanism of debris flow impacting bridges.Focusing on the experiment and numerical simulation,the overseas and domestic researches are summarized.Then,the exsited deficiency in other studies and the direction in this study are pointed out.Finally,the objectives and contens in this study are determained.(2)The experiment platform and setup are introduced.The experiment contains debris flow flume,a round and a square pier models,impact pressure and force sensors,and 10 debris flow materials.36 experiment sets are tested in the experiment.The original impact signals are denoised,post-processed,and then used in the following analysis.(3)The rheological parameters are tested.This study finds that the relation between debris flow rheological parameters and velocities can be fitted with the Manning formula,and the relation between debris flow rheological parameters and impact force coefficients can be fitted with the non-Newtonian Reynolds number.Furthermore,the complex fitting formula can be simplified as the impac force coefficients of dilute or viscous debris flow impacting on round or square piers.The simplified coefficients are compared with those in current code.(4)Experiment results show that the flow regimes of debris flow can be classified into Type A,Type B,and Type C,and two dimensionless numbers can be used as the classification creteria.The flow regime greatly influences the impact pressure and impact forces.The current popular empirical formula of impact pressure coefficient is physically deficient,which can be fixed based on the pressure components in debris flows.(5)The debris flow impacting on piers can be simulated based on Bingham model.The thesis introduces the debris flow movement governing equtions and the numerical simulation platform,i.e.,OpenFOAM.A moving belt flume experiment is used to validate the flow field of debris flow in the simulation,and the above small-scale impact experiment is used to validate the impact forces of debris flow on piers in the simulation.(6)To overcome the shortcommings of the flume experiments,a small scale numerical experiment with a larger tank and three more section shapes,i.e.,round-end,rectangle,and diamond,is simulated.The flow charasteristics,magnitude and distribution of impact pressure and impact forces of viscous and dilute debris flow impacting on piers with five different section shapes are studied.A new debris flow impact pier force model,i.e.three-layer(shear layer,plug layer,and runup layer)model,is presented.(7)The dimensional analysis shows that the scale law is inaccuracy in the small-scale experiment or simulation,so the full-scale debris flow impacting round pier is simulated.The general flow characteristics,heights and impact pressures of each layer in the three-layer model,and impact force coefficients are studied with the combination of different slopes and rheological parameters.The shear layer height and plug layer height can be obtained through theoretical analysis based on visco-plastic lubrication model,while other results are obtained throught fitting with parameters.