Study On Anti-clogging Of Composite Subsurface Drainpipe Envelopes Based On Reuse Of Returned Water

In the planning,design,construction,and operation management of agricultural drainage and irrigation projects,it is necessary to grasp the movement patterns of groundwater and soil water under drainage and irrigation conditions,as well as the engineering control effects.Subsurface drainage in farmland has been widely used around the world,and has become an important component of modern irrigated agriculture.In the past,research on the envelope materials used in subsurface drainage projects often focused on improving drainage flow or water quality.For the research on the reuse of recycled water in subsurface drainage projects,the key lies in whether the envelope material can meet both of the above requirements.Related studies have shown that using absorbent materials(such as slag)as filter materials has a good desalination rate and a high removal rate of suspended solids.In addition,using geotextile+sand filter material(composite subsurface drainpipe envelopes)has a better anti clogging effect than laying geotextile only.Considering the need for recycled water reuse,the use of adsorbent materials instead of sand filter materials in composite subsurface drainpipe envelopes has become one of the research focuses on whether it can meet the requirements of anti-clogging.This thesis uses a one-dimensional constant head permeability testing device,jointly developed by the research group and Shanghai Dayou Instrument Equipment Company,to conduct anticlogging performance testing tests.Based on the experimental results,an improved gradient ratio method was proposed and applied to evaluate the clogging behavior of composite subsurface drainpipe envelopes.On this basis,numerical simulation was conducted using PFC-3D software to study the migration of soil particles in the filter material,and the anti-clogging mechanism of composite subsurface drainpipe envelopes was discussed.The main research content and results are as follows:(1)The preliminary experimental results of the reuse of recovered water from subsurface drainage indicate that the anti-clogging effect of geotextile+sand is the best.The research team contracted to study the salt discharge effect of Geotextile+adsorption filter material,and the salt discharge effect of Geotextile+slag is better.Based on the preliminary type selection test results of Geotextile+sand,zeolite and slag,two kinds of Geotextile and three grain sizes of sand and slag were selected to further carry out the anti-clogging test of improved composite subsurface drainpipe envelopes.The results show that the flow attenuation of SF27 Geotextile+slag with grain size of 0.9(±0.2)mm is low,and the anti-clogging effect is better than other schemes,which is the optimal solution that can meet the requirements of silt prevention and purification of water quality.At the same time,the gradient ratio characteristics in the test results show that the structure of composite subsurface drainpipe envelopes can protect Geotextile from clogging or reduce clogging to a certain extent.(2)The gradient ratio characteristic data of composite subsurface drainpipe envelopes in the test results are summarized and analyzed,and the GR123 and GR134 of filter material and Geotextile in each scheme respectively account for the GR124 weight of composite subsurface drainpipe envelopes through linear regression fitting.On the basis of gradient ratio criterion,a new improved gradient ratio method is proposed by combining the structural form of composite subsurface drainpipe envelopes,and its application is used to analyze and judge the siltation prevention of each test scheme.The results show that in the preliminary selection test,the GRcomposite the Geotextile+sand scheme is less than 1 and remains stable,and the siltation prevention performance is good;In the test of improving the anti-clogging of composite outer filter material,the GRcomposite of SF27 Geotextile+slag with particle size of 0.9(±0.2)mm is less than 1,and the value is the lowest and most stable among several groups,and the anti-clogging performance is better than other groups.The conclusion drawn from analyzing the anti-clogging situation of each group of schemes using the improved gradient ratio method is consistent with the conclusion drawn from the experimental results.(3)For the system of "composite subsurface drainpipe envelopes+soil",a fluid-structure interaction model was established by using PFC-3D software.Combining the soil particle size changes before and after the test,and the soil particle size changes before and after the numerical simulation results,the movement,loss and invasion of soil particles during the external filter material anti-clogging were analyzed from a microscopic perspective.The results show that the simulation scheme of Geotextile with particle size of 0.5(±0.2)mm is superior to other simulation schemes in terms of siltation prevention.The optimal solution for numerical simulation is different from the optimal solution obtained from actual experimental results.Firstly,the PFC-3D software still lacks smaller particles when generating particles within a given range.Secondly,the 0.9(±0.2)mm slag not only intercepts fine particles based on its own size,but also adsorbs fine particles through its surface porous structure,but its porous structure is difficult to display through PFC-3D software.There are numerical differences in soil particle size changes between the simulation and experimental plans for the same particle size filter material,but their overall trend of change is consistent.(4)The clogging mechanism and calculation method of geotextile and sand were summarized respectively.Based on this,the anti-clogging mechanism of composite subsurface drainpipe envelopes was elaborated.And based on the previous analysis and discussion,the differences in anti-clogging performance and working mechanism between sand composite subsurface drainpipe envelopes and adsorption composite subsurface drainpipe envelopes were analyzed and discussed.Considering the reuse of recycled water,measures and suggestions for anti-clogging by subsurface drainpipe engineering are proposed.The above research results can provide a reference basis for the setting of subsurface drainpipe envelopes in practical engineering.