Study On Visual Model Test Of Bridge Foundation Scour Erosion And Flexible Protection

Scour is the main cause of bridge flooding,which seriously threatens the safety of people’s lives and property.The scour of bridge pile foundation is a process in which water digs out and removes sand and other materials from around the pile foundation.It is a very complicated interaction problem between water flow,sand and structure.In response to this type of problem,model tests have always been the main research method.At present,domestic and foreign scholars have carried out a lot of research on the problem of bridge pile foundation scour.However,the existing scour tests mainly focus on the scour depth and the shape of the scour pit after the scour is completed,and the single pile.Analysis of changes in flow patterns around foundations,but few studies have been conducted on the morphological evolution of scouring pits during scouring and changes in flow patterns around complex pile foundation structures.In this paper,the visual model test technology combining artificially synthesized transparent soil material and particle image processing technology is applied to the pile foundation scour model test.First,the hydraulic characteristics of the transparent model sand are initially explored,and then the self-developed visual water tank test device is used.The focus is on the problems of pile foundation scour and flexible protection.The main work and conclusions are as follows:(1)A set of visual water tank test equipment was independently developed.The test equipment is composed of a circulating water tank system and a particle image measurement system,and the equipment has been debugged and calibrated.(2)Based on the transparent soil prepared from the mixture of fused silica sand,ndodecane and 15# white oil,the hydraulic properties of transparent model sand were studied through laboratory experiments,including basic physical properties,angle of repose and starting flow rate.The test results show that the bulk density of the model sands of each particle size group is very small,and the dry bulk density first increases and then decreases with the increase of the particle size;the model fluid is a typical Newtonian fluid;the submerged angle of repose increases with the increase of the median particle size.It is increased and compared with the natural angle of repose;the initial flow velocity of model sand with a particle size group of 0.1-0.2 mm increases with the increase of liquid depth,and it is fitted by Shamov’s formula,and the effect is better.(3)Two flow control factors,liquid depth and flow velocity,were selected to carry out 9 groups of visual model tests of erosion and erosion of bridge pile foundation.The influence of flow conditions on the flow structure and the evolution process of erosion pits around the pile foundation was analyzed.The experimental results show that the flow velocity distribution of the incoming flow satisfies the logarithmic distribution law,and the downstream flow field has a backflow area and gradually recovers with the downstream flow field.The scour starts from the middle of the upper end of pile foundation,develops to the corner points on both sides of the upper end of pile cap,and then gradually expands to both sides of the pile cap,but it will reach equilibrium in different stages due to the difference of flow conditions.The range of vertical section and horizontal plane of scour pit increases with the increase of flow velocity,but decreases slightly with the increase of liquid depth.(4)Three control factors of flexible protective body,depth,width and area,were selected to carry out seven groups of visual model tests of flexible protective body for bridge pile foundation scouring,and the influence of protective body parameters on the protection effect was analyzed.The test results show that the protection depth and the protection condition of the protection area show good protection effect.In the four working conditions in which the factor of protection width was explored,only 1/4 of the reference width of the protection working condition had obvious scour pits around the protection body,and the rest of the protection effect was good.