| The subgrade along the river in the mountainous area is scoured by river water,leading to washout,which frequently triggers traffic interruption.Relying on the highway project along the river in the mountainous area of southern Shaanxi,combined with the main types and characteristics of highway washout,the flow field characteristics of the river flow in the form of washout such as outer bank erosion in bends and straight river course jet flow,and compression scouring are taken as the research object in this paper.By means of field investigation,model test and numerical simulation,the features of the flow field and its changing law from different hydraulic conditions,curve geometry,the angle of jet flow in straight channel,compression ratio and compression length are systematically studied in this paper by using the basic parameters of river hydraulic and geometric prototypes.On this basis,the calculation model for the maximum variables related to the flow field is established,and the determination of subgrade protection scope against washout is proposed to provide decisions and support for the management,maintenance and prevention of washout of the highway in the mountainous area.The main research contents and achievements are as follows:(1)Based on many years of field investigation,the characteristics of the natural environment along the highway in the mountainous area of southern Shaanxi are analysed,and the main types and influencing factors of washout are summarized.The results show that according to the characteristics of river flow and subgrade,the washout of the highway mainly includes outer bank erosion in channel bends,straight section compression and jet flow scouring,and the longitudinal scouring of flooded pavement.(2)The model tests of water flow in bends under different flow conditions are carried out to analyse the changing characteristics of flow depth and velocity distribution.Based on theory of hydrodynamics,The basic parameters of the hydraulic and geometric prototypes of the bends are adopted to simulate the flow field under the conditions of five rates of flow from 127.91m~3/s to 471.44 m~3/s,five central angles in bends from 50°to 170°and five aspect ratios from1.5 to 9.0.The influence laws of flow,central angle in bends,the ratio of curvature to width on flow field are analysed.Considering the influence of various factors,the formula for the amplitude of the maximum water depth along the outer bank subgrade relative to the normal water depth,the maximum bed shear stress and the maximum transverse water superelevation are established.(3)According to the change characteristics of the location of the maximum circulation intensity in the bend,the main flow and wall shear stress distribution,the relationship between the outer bank subgrade protection scope against washout and the hydraulic and geometric elements of the river course is obtained,and the formula for the outer bank subgrade protection scope against washout is established.The results show the following:The starting point for subgrade protection should be near the maximum circulating current intensity in the bend.The cross-section of the starting point for protection moves upstream with the increase of the central angle of the bend and the ratio of curvature to width,whereas it does not change with the increase of flow.The ending point should be at 2.5 B downstream of going off curve,where the0.625–1.25 B flow path going off curve is the key protection section of the subgrade.(4)The basic parameters of the hydraulic and geometric prototypes of the straight section compression and jet flow field are adopted to simulate under the conditions of five volumes of flow from 127.91 m~3/s to 471.44 m~3/s,five river width compression ratios from 0.2 to 0.6,five lengths of compression section from 37.5 m to 137.5 m,five angles of jet flow from 15°to 75°,and analyse the influencing laws of flow,compression ratio,compression length and angle of jet flow on flow field.The formulas for the maximum water depth along the bank,the maximum bed and wall shear stress are established.(5)Based on the characteristics of the bed and wall shear stress,the scope of protection against washout is obtained as follows:When a compression section is in the subgrade,the scope of protection against washout is the compression section flow path.The foots of the wall on the inlet and outlet of the compression section is especially protected.When a compression section is on the bank of the subgrade,the scope of protection should also include the influencing range of the downstream flow field on the outlet of the compression section.The protection length in the downstream of the outlet of the compression section is proportional to the flow and river width compression ratio.With the increase of the length of the compression section,the protection length in the downstream remains unchanged.The scope of protection for the jet flow scouring is the flow range of the continuous reduction of the water depth along the subgrade in the downstream of the outlet in the jet flow section.The protection length increases with the increase of the flow,compression ratio and angle of jet flow.(6)The influencing laws of the flow and geometric factors on the characteristics of the flow field,and the formulas for the maximum water depth along the subgrade,the maximum bed and wall shear stress,and the scope of protection for the subgrade against washout can provide a reference basis for selecting the scope of protection for the subgrade against washout,designing the route and elevation of the highway along the river,widening the straight section and designing the protection structure. |
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