Experimental Study On Pore Characterization And Seepage Characteristics Of Plastic Blind Drain

As a new type of composite geotextile drainage material,the plastic blind drain has advantages in high surface opening rate,good water collecting,high porosity,strong compressive strength,good flexibility,convenient construction and so on,which can be applied and popularized in the underground water collection and drainage project of civil engineering,water conservancy,especially farmland drainage and other construction projects.However,at present the performance index of blind drain in using and construction process is still not clear.Due to the failure of measuring performance and clogging damage,the supervision suffers a setback.Therefore,it is urgent to conduct a systematic and comprehensive analysis to the pore characterization and seepage characteristics of plastic blind drain.Based on the above background,the pore characteristics of five commonly used plastic blind drains were measured and qualitatively described,and the permeability of plastic blind drain with different equivalent pore size and porosity was quantitatively analyzed.The research results have positive significance to the selection criteria and development application of plastic blind drain in agricultural water conservancy projects,and laid the foundation for developing theoretical research of new blind drains in agricultural irrigation engineering and ecological drainage engineering.In this paper,through the combination of theory and experiment,the following work has been carried out and conclusions are drawn:(1)Through the determintion and correlation analysis to equivalent pore size,porosity and surface porosity of five types of plastic blind drain,it is found that the porosity of plastic blind drain is between 65% and 90%.When the porosity of the blind drain is between 78.65%and 87.03%,porosity is positively associated with surface porosity,while the surface porosity tends to be closer to 71% after the porosity is more than 87.54%.When the porosity is between 65% and 84%,porosity is directly proportional to the equivalent pore size,and the equivalent diameter is closed to 11 mm after the porosity is more than 84.5%.(2)The seepage characteristics of plastic blind drain under constant pressure and free ouflow,as well as variable pressure and submerged outflow are quantitatively analyzed.By fitting the test data of hydraulic gradient,it is found that when the equivalent diameter is more than 8.03 mm and the porosity is greater than 65.45%,the relationship between hydraulic gradient and velocity is not linear anymore which means the flow pattern is between laminar flow and turbulent flow.And the smaller the equivalent pore size and porosity are,the closer the flow pattern is to Darcy's law.Compared with the constant pressure and free outflow condition,the seepage effect of plastic blind drain is enhanced by the inertia force of flow under the condition of variable pressure and submerged outflow,while the law of quadratic or the power curve is still consistent,but the curvature is obviously reduced.(3)According to the correlation curve fitted by Reynolds number and friction factor data,the non-Darcy percolation equation of plastic blind drain is derived.(4)Based on the NOF standard,the most suitable empirical formula for Non-Darcy seepage equation of plastic blind drain is selected.In the constant pressure free outflowcondition,second-order coefficient A and first-order coefficient B in the Forchheimer formula is the most consistent with the Sidiropoulou empirical formula.While in variable pressure and submerged outflow,the second-order coefficient A is the most consistent with the Ergun empirical formula,and the first-order coefficient B is the most consistent with the Ergun-Reichlet empirical formula.(5)Based on the NOF standard,the most suitable empirical formula for the coefficients of Non-Darcy seepage equation of plastic blind drain is selected.When the equivalent diameter of plastic blind drain is equal to 8.03 mm,the Ergun formula is the most suitable seepage equation for describing the seepage characteristics of plastic blind drain under two kinds of outflow condition.While the equivalent diameter of plastic blind drain is greater than8.03 mm,the Sidiropoulou formula is the most suitable seepage equation.