| In recent years,many expressways have been built in western mountainous areas of China.In order to reduce the impact of highway infrastructure construction on regional ecological environment,longitudinal bridge is often used to replace the traditional roadbed.However,the large number of longitudinal piers has a serious impact on the river flow pattern,leading to the prominent local scour problem of piers,and the scour mechanism is complex.It is of great theoretical and practical significance to reveal the local scour mechanism and the determination of scour depth of longitudinal bridge piers in straight channel in mountainous area.On the basis of summarizing and analyzing the influencing factors of local scour of longitudinal piers in straight channel in mountainous areas,this paper studies the influence of different influencing factors on local scour depth of longitudinal piers by means of multi-factor and multi-level indoor model tests,aiming at the influence of such factors as the position,span of longitudinal bridge and the angle between the axis of longitudinal bridge and water flow,and puts forward the corresponding method to determine the local scour depth of longitudinal piers.The main research contents and conclusions of this paper are as follows:(1)First of all,this paper analyzes the influence factors of the maximum local scour depth of the longitudinal pier in mountainous channel from two aspects of natural factors and longitudinal bridge factors.The main factors selected for the study are: the span of the longitudinal bridge,the location of the longitudinal bridge and the angle between the longitudinal bridge axis and the flow direction.(2)On the basis of selecting the influencing factors,combined with the test conditions and the characteristics of the channel,the scale was selected,and six spans of 52 cm,64cm,80 cm,120cm,160 cm and 200 cm were selected for model test.The results show that the local scour depth of each pier is closely related to L/D(L is the pier span,D is the pier diameter).When L/D≤20,the local scour depth decreases with the increase of pier span;When L/D>20,the local scour depth increases with the increase of pier span;When L/D≥30,the span has no obvious effect on the local scour depth,and the local scour depth of the pier can be treated as a single pier.(3)When the longitudinal bridge span is the same,the maximum local scour depth of the pier group in the middle and one side of the river is compared.The results show that: when the longitudinal bridge is located on one side of the river,the change trend of the maximum local scour depth of each column of piers in the longitudinal direction is consistent with that when the longitudinal bridge is located in the middle of the river,and the change is obvious in the transverse direction.Affected by the river wall,the maximum local scour depth of each column of piers increases.(4)The maximum local scour depth of the first row pier is not affected by the back row pier,and its maximum value is close to the calculation result of 65-2 formula.The maximum scour depth of the rear piers decreases,and the reduction range is related to the longitudinal bridge span.Generally,it decreases firstly,then increases,and finally tends to be stable.The specific scour depth can be determined by reducing the calculation results of formula 65-2 with reference to the coefficients in table 4.1-4.3 and table 5.3-5.4 according to the layout of piers.(5)When there is an angle between the longitudinal bridge axis and the flow direction,the span is 52 cm and 64 cm,and the angle is 5°and 10°,the variation of local scour depth of longitudinal bridge is analyzed.The results show that when L/D≤16 and the angle between the longitudinal bridge axis and the flow direction is 10°,The change of the bridge pier span and the angle between the longitudinal bridge axis and the flow direction has little effect on the local scour depth around the bridge pier. |
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