Research On The Influence Of Envelope Material On The Drainage Flow And Surrounding Hydraulic Characteristics Of Field Subsurface Pipe

The subsurface pipe drainage technology has been widely used in agricultural drainage projects both domestically and internationally due to its ability to effectively discharge excess water from fields,quickly reduce groundwater levels,and prevent water logging disasters in farmland.With the gradual advancement of modern agricultural ecological irrigation area construction,the reuse of irrigation return water in arid and semi-arid areas has become a research focus.In subsurface pipe drainage technology,the envelope material wrapped around the subsurface pipe is mainly used to improve the drainage effect of the subsurface pipe filter soil,which directly affects the hydraulic conditions around the subsurface pipe,and can also purify the water quality discharged into the subsurface pipe,Therefore,the setting of envelope materials and their hydraulic calculation have become crucial technical links in underground drainage systems.The thesis focuses on the quantitative study of the impact of envelop materials on the drainage flow of subsurface pipe.Taking the winter irrigation and salt washing in the Yinbei irrigation district of Ningxia as the research background,the influence of envelop materials on the drainage flow of subsurface pipe and the water head at different media interfaces is revealed through theoretical derivation,physical model tests of seepage tanks,and numerical simulation.Formulas for calculating the flow rate and effective radius of subsurface pipe wrapped with geotextile and envelop material under surface water logging conditions have been proposed.The main content and conclusions of the thesis are as follows:(1)In view of the situation of surface water accumulation(Winter irrigation scene in Yinbei Irrigation District,Ningxia),based on the "image method" and the potential flow superposition theory,the seepage problem of a single subsurface pipe in the semi-infinite seepage field is transformed into the seepage superposition problem of multiple subsurface pipe in the infinite seepage field,and a calculation formula considering the influence of the envelope material on the drainage flow of the subsurface pipe is proposed.The relative errors between the calculation results of this formula and the physical model test results of the seepage tank and the simulation results of the HYDRUS-2D software are within 7.00%and 2.05%,indicating the reliability of the formula.(2)Based on the Laplace equation and Darcy’s law,and based on the principle of flow continuity,formulas for calculating the water head at the interface of different media and for calculating the effective radius of subsurface pipe with geotextile and envelope material are derived,which overcomes the drawbacks of traditional effective radius calculation formulas that are not applicable to the calculation of synthetic materials and multi-layer envelope material.the calculation results of the formula are consistent with the experimental and simulated values,which proves that the effective radius calculation formula based on Laplace equation and Darcy’s law is reasonable and feasible.(3)Through comparison,it was found that the verification results of the formula for calculating the drainage flow of subsurface pipe with envelope material under the condition of surface water accumulation show that when the thickness of envelope material is less than 15 cm,the Numerov formula and the formula in this thesis are all consistent with the simulation results of the drainage flow of subsurface pipe;When the thickness of the envelope material is greater than 15cm,the calculated value of the formula in this thesis is more in line with the simulated value,and the formula in this thesis can more closely reflect the influence of the envelope material thickness and permeability coefficient on the drainage flow.(4)When surface ponding,with the gradual increase of the envelope material thickness,the unit drainage flow of the subsurface pipe is also increasing,but the growth rate gradually decrease.The water head at the interface between the envelope material and the soil decreases with the increase of the permeability coefficient of the envelope material,and the decrease amplitude decreases gradually.The water head at the interface of different media increases with the increase of the thickness of the envelop material,and growth rate gradually decreases.(5)In the calculation of unstable drainage,the distance between subsurface pipes considering the influence of envelop material is 27.239%larger than that without considering envelop material.In the calculation of stable drainage,the distance between subsurface pipes with the inclusion of envelop material is 9.978%larger than that without the inclusion of envelop material.For areas with strong evaporation in Yinbei,it is necessary to consider the issue of evaporation,otherwise it may lead to a larger spacing of the designed subsurface pipe,which cannot meet the drainage requirements.(6)The grey correlation ranking and correlation coefficient of the influence of subsurface pipe engineering factors on drainage flow are:buried depth of subsurface pipe(0.896)>radius of subsurface pipe(0.886)>thickness of envelop material(0.741)>spacing between subsurface pipes(0.739)>permeability coefficient of envelop material(0.557).The research results of the thesis have expanded the calculation method of subsurface pipe drainage,providing a reference basis for the design of subsurface pipe drainage schemes for soil salt leaching during winter irrigation in arid and semi-arid irrigation areas.