Study On Drainage And Salt-removal Effect Of Reticulated Wall Dark Pipe In Unsaturated Soil

This study is to realize the sustainable utilization of saline-alkali farmland and perfect the theory of farmland drainage,by combining with the advantages of drip irrigation technology and the function of water discharge and salt drainage of the concealed pipe in unsaturated soil.Based on laboratory tests(geotextiles structure observation and hydraulic performance test,soil column test and soil tank test),this study researched the structural characteristics and hydraulic performance of geotextiles as an outsourcing filter layer for drainage concealed pipe in farmland.The effects of the porosity of the reticulated wall and the geotextiles on the drainage and salt discharge of the reticulated wall in unsaturated soil was observed.And the internal mechanism of these influences was analysis.The geotextiles tested in this study included three types(10 specifications)of needled short-fiber geotextiles(300g/m~2,400g/m~2 and 500g/m~2),polyester-filament geotextiles(300g/m~2 and 500g/m~2),and super-thin geotextiles(30 g/m~2,60 g/m~2,100 g/m~2,150 g/m~2 and 260 g/m~2),labeled as D300,D400,D500,C300,C500,B30,B60,B100,B150 and B260,respectively.Different laboratory experiments including the microscopic observation technique,permeability test,water drainage in changed placement angle of geotextiles,and the improved funnel-tension plate assembly,were used to test geotextiles’hydraulic elements including the pore-structure,saturated permeability,unsaturated water-holding capacity.The relationship between the hydraulic elements of geotextiles and their microstructure was determined.In this study,the porosity of the tube wall was treated at four levels,labeled as P1(3.02%),P2(31.6%),P3(71.3%)and P4(88.9%),respectively.The dark tubes used in the soil column test were composed of 4 orthogonal combinations of wall porosity(P1,P2,P3 and P4)and 5 kinds of geotextiles(D300,D500,C300,B30 and B100),a total of 20 combinations.The dark tubes used in the soil tank test were composed of 4 orthogonal combinations of wall porosity(P1,P2,P3 and P4)and 2 kinds of geotextiles(D300 and B30),with a total of eight combinations.In the soil column test or soil tank test corresponding to different dark pipes,the indexes of water displacement,salt discharge concentration,salt discharge,soil moisture content,soil matrix absorption,and soil salt content were observed,and the influences of the porosity of the wall of the dark pipe and the geotechnical cloth on the drainage and salt discharge effect of the dark pipe in unsaturated soil were analyzed.The main research conclusions of this paper are as follows:(1)With the increased volume density of geotextiles,the plane porosity and spatial porosity of geotextiles gradually decreases.At the same time,the thickness-directional porosity of short-fiber geotextiles and polyester-filament geotextiles increases,and that of super-thin geotextiles first increases and then decreases.The greater thickness-directional porosity of geotextiles,the better permeability of geotextiles in the vertical direction.The saturation of short silk and filament geotextiles decreases as the placement angle of geotextiles increases from small(0)to large(90).And with the increase of placement angle,the geotextiles with larger volume density has less water displacement.That is,the water holding capacity is stronger.Under the same saturation,the suction of these two types of geotextiles is positively correlated with their thickness.(2)Taking geotextiles as the outer filter layer of the concealed pipe can improve the pipe drainage of the net wall,the efficiency of salt drainage and slow down the flow of soil moisture around the concealed pipe.Geotextiles with a certain thickness(short-fiber geotextiles(D type)and filament geotextiles(C type))have higher water holding capacity than ultra-thin geotextiles(B type),which is beneficial to hold soil moisture around the pipe wall and slow down the flow velocity around the soil wetting front.The water conductivity is stronger down the arc working surface of geotextiles,and it is easier to form local saturated permeable zone at the side bottom of geotextiles,which provides more favorable conditions for water to enter the concealed pipe.The test results show that,under the condition of the same pipe wall opening rate,the drainage and salt discharge of the polyester-filament geotextiles(type C)and short-fiber geotextiles(type D)are generally larger than those of the super-thin geotextiles(type B).In the soil column test,the average displacement of concealed pipes corresponding to C300 geotextiles is the largest,and the average displacement of concealed pipes corresponding to D500,D300,B30 and B100 geotextiles is 55.92%,52.43%,30.68%and 17.09%of that of C300 geotextiles respectively.The average salt discharge of C300geotextiles is the largest,and the average salt discharge of B30,D500,D300 and B100 geotextiles is82.53%,72.05%,66.32%and 28.30%of that of C300 geotextiles.In the soil tank test,the concealed pipes corresponding to D300 geotextiles have a good effect on and salt discharge,while the concealed pipes corresponding to B30 geotextiles all have neither drainage nor salt discharge.(3)With the increase of the porosity of the pipe wall(mesh rate)(30%-90%),the permeable area of the pipe wall increases,and the seepage resistance of soil moisture entering the pipe wall decreases.Under unsaturated conditions,the flow around the soil wetting front along the pipe wall is slowed down,and a local high water content(low suction)area is formed around the pipe wall,which makes the drainage and salt discharge of the concealed pipe with the net wall gradually increase or have the best quality value.Compared with the traditional concealed pipe(the porosity of the pipe wall is about 3%),it has better drainage and salt discharge.In the soil column test,the average drainage amount and salt discharge amount of P4(88.9%)mesh rate are the largest,while P1(3.02%),P2(31.6%)and P3(71.3%)mesh rates are35.16%,69.41%and 96.58%of the average drainage amount of P4(88.9%)mesh rate.P1(3.02%),P2(31.6%)and P3(71.3%)mesh ratios correspond to the average salt discharge of concealed pipes,which are41.15%,67.63%and 93.22%of the average salt discharge of concealed pipes corresponding to P4(88.9%)mesh ratio,respectively.In the soil tank test,the drainage and total salt discharge of concealed pipes composed of D300 geotextiles and P3(71.3%)mesh rate are the largest.The drainage and total salt discharge of concealed pipes corresponding to P1(3.02%),P2(31.6%)and P4(88.9%)mesh rates are50.93%,88.20%and 71.43%of those of P3(71.3%)mesh rate,respectively.