Research On Damage Mechanism And Protection Measures Of Circular Pile Pier Impacted By Boulder In Mountainous River

From the end of last century,a large number of high-grade highways and highways extended to the western mountainous areas of Sichuan.From the aspects of topography,geology,engineering scale,construction difficulty,engineering cost and population distribution,a large number of highway corridors are arranged along the river.Due to the requirements of highway grade and linear index,a large number of river bridges or small angle bridges have appeared in the construction of high-grade highways in western mountainous areas,which have a certain impact on the flow pattern and flood discharge of the river.Natural disasters such as mountain floods,debris flows and landslides occur frequently in western Sichuan.Floods with sediment boulders directly impact piers,resulting in serious damage to the surface of piers and seriously endangering the safety of bridges.Due to the limited research on the protection of boulder impact on bridge piers,the research and countermeasures of boulder impact are imminent.By referring to the literature and combining with the geological characteristics and actual investigation in the mountainous area of western Sichuan,the causes,particle size and motion trajectory of boulders in the river channel under the water flow are analyzed to study the velocity and energy of boulders impacting the pier.The three-dimensional finite element analysis software was used to study the damage mechanism of pier column under the impact of boulder and the effectiveness and necessity of protection,and the protective structure with external rigidity and internal flexibility was proposed,which laid the foundation for the solid application.Based on the overall protection idea of using softness to overcome hardness,the reasonable protection structure form is explored through indoor entity test verification,and the suitable protection structure is selected according to the actual situation,and applied to entity engineering.The conclusions of this paper are as follows: Under the action of flood,the boulders sticks to the bottom of the river bed alternately to do the ’ bounce ’ movement.The boulder impact area is generally within the scope of 2m upward at the bottom of the river bed,and the protection is carried out according to 10 kJ impact energy in engineering.Combined with numerical simulation and buffer cushion indoor test,the buffer cushion can effectively increase the energy consumption time,reduce the impact force to less than 20 % of the unprotected state,greatly reduce the impact stress,and the impact reaction time history curve is stable and constant,so the buffer protection is effective and necessary.Steel casing + cushion structure after impact,will produce irreversible plastic deformation,even squeeze and cut the middle cushion,hurt the internal concrete layer.Compared with the two-layer structure,the protection performance of the improved high toughness outer shell + cushion layer + high toughness inner shell three-layer protection structure is greatly improved.