Study On Characteristics And Effect Of Sand Fixation Of HDPE Board Grid At Bridge And Subgrade Sand Disaster Section In Geku Railway

In order to further improve the railway network in northwest China,in 2014 China began to build the Geku Railway(Geermu to Kuerle).The Geku Railway crosses the Qaidam Basin and the Tarim Basin.These two regions have dry climate,large and high frequency wind,wide open terrain and abundant sand sources.The full-line sand accumulation and Gobi sandstorm disasters are more serious.In order to prevent the railway subgrade and bridge from being affected by sand damage,a test section was set up in the study area.Different sand control measures were set up in the test section.Through observation,it is found that the HDPE board grid has the best sand prevention effect and long service life.In this paper,the causes of sand damage near railway subgrade and bridge are analyzed by finite element software.Then the sand-preventing mechanism of HDPE board grid is studied by field test,wind tunnel test and numerical simulation,and the optimal porosity of the HDPE board grid is optimized by wind tunnel experiment and numerical simulation.The main contents of this paper are:(1)Through investigations on site,it is found that there are five sand damage forms mainly: sand-buried roadbeds,sand-eroded roadbeds,sand accumulation near bridges,sand accumulation in culverts and sand into the turnout.Through on-site monitoring,the sand movement speed of >6m/s in the year has reached 3417 times,accounting for 39% of the total statistics of the maximum wind speed in the whole year.The first dominant wind direction of the regional location is WNW.Through test,it was found that the grain size of sand was mainly composed of medium sand,fine sand and very fine sand(500-50?m),accounting for 80.51% of the experimental samples;and it is found that the roundness of sand in the study area was distributed between the minor circle and the polar circle.(2)Through test of HDPE board grid on site,it is found that,along from the direction away from the subgrade to the direction close to the subgrade,the percentage of coarse sand and medium sand in the whole gradually decreased,the content of sand,fine sand and silt increased gradually.(3)Through ANSYS FLUENT,the numerical simulation of the flow field structure and sand distribution in the wind-sand flow of railway subgrade and bridge is carried out.It is found that the flow field structure around the railway subgrade and the bridge has obvious zoning characteristics.According to the amount of sand accumulation,the sand accumulation is mainly distributed in the leeward side of the railway subgrade,the leeward side of the bridge deck,the windward side of the railway subgrade,upper surface of railway subgradeand and the surrounding of the pier.(4)Through wind tunnel experiments,the flow fields structure of HDPE board grid with different porosity(30%,40%,50%)at different wind speeds(8 m/s,12 m/s,18 m/s,24 m/s)is measured,it is found that the airflow field structure of the HDPE board grid with different porosity are similar,and can be divided into five parts: 1.windward side low speed zone;2.airflow uplift zone;3.airflow high speed zone(above the board);4.airflow deceleration area(in the middle and leeward side of the grid);5.airflow speed recovery area.Through the protection area of HDPE board grid,effective protection height and distance and windproof performance,the windproof effect of HDPE board grid is comprehensively evaluated.It is found that when the wind speed is 8m/s and 12m/s,the protective effect of 40% and 30% porosity HDPE board grid is the same.When the wind speed is 18m/s and 24m/s,for 30% porosity HDPE board grid,the protective effect began to decline,and the protection effect of the 40% porosity HDPE board grid is better,but regardless of the wind speed,the protection effect of the 50% porosity HDPE board grid is relatively poor.(5)The numerical simulation of the sand accumulation and velocity flow field of the HDPE board grid is carried out by ANSYS FLUENT.It is found that the simulation results of the flow field structure of the HDPE board grid are consistent with the wind tunnel experiment results,and the trend of the speed around the HDPE board grid is analyzed.Through the data sorting,the protection distance and protection height of the grids of different porosity HDPE board grid were compared and analyzed,it is concluded that the 40% porosity HDPE board grid has the best windproof effect.By analyzing the amount of sand accumulation of different,it is concluded that the 40% porosity HDPE board grid has the best windproof effect.At the same time 40% porosity HDPE board grid has the best sand fixation effect.(6)The results of wind tunnel experiments show that the protection effect of the 30% and 40% porosity HDPE board grid has little difference,but the choice of the optimal porosity has a certain correlation with the wind speed,and the numerical simulation shows that when the wind speed is greater than 12m/s,the efficiency of the sand-fixing is relatively small.The frequency of occurrence of wind speed greater than 12m/s in the study area is 11.5% in one year.To ensure the sand control effect,it is recommended to use 40% porosity HDPE board grid,and a high vertical sand barrier on the front edge of the HDPE board grid strip to reduce the wind speed,thus making the HDPE board grid efficiency of the sand fixation is maximized.In order to fully maintain the sand-fixing effect of the HDPE board grid,when the sand in the HDPE board grid reaches a height of 0.2 m,the HDPE board should be raised to a net height of 0.3 m.