Research On The Mechanism And Regulation Technology Of Unsaturated Soil Water Flowing Around The Subsurface Drainage Pipe

Based on Engineering practice and research,a phenomenon was found that subsurface drainage pipe was not always below the design critical groundwater level for a long time after drip irrigation in some areas.The soil water around the subsurface drainage pipe is constantly switched between saturated and unsaturated areas with the rise and fall of groundwater level.The drainage capacity of the subsurface drainage pipe will decreased or even not draine,which will affect the treatment of saline alkali land seriously in arid areas when the residual head at the top of the subsurface drainage pipe is insufficient.Focusing on the critical problem of the action mechanism of unsaturated soil environment on subsurface drainage pipe,the characteristics of flow around the subsruface drainage pipe buried by traditional mode are observed by the infiltration test,which also are used to analyzed the influence factors.The envelope structure for regulating the bypass flow is constructed based on the theory of infiltration refraction and preferential flow,which is based on the characteristic problem of hydraulic function failure of envelope under unsaturated conditions,which provides a macro experimental basis for the reserch of the micro mechanism of bypass flow;Based on the continuous heterogeneous boundary characteristics of soil-envelope-drainage pipe,a transparent and visible microfluidic model is constructed which can show the seepage paths at different scales by camera and electron microscope.The seepage path can be used to analyze the flow behavior and distribution characteristics of fluid in the pore channel,clarify the flow mechanism and the working mechanism of the newly designed filter structure,provide technical support for the treatment of saline alkali land in arid areas.The research conclusions are as follows:(1)Hysteresis effect and capillary barrier are the important influence factors for the formation of bypass flow and the reasons for unsaturated soil water discharging from the subsurface drainage pipe difficultly.The subsurface drainage pipe buried in the sandy soil with large hysteresis effect can not drain,where deformation amplitude of soil water flow is large.The subsurface drainage pipe buried in the loamy clay with small hysteresis effect,which deformation amplitude of soil water flow is small and can drain at0.5-1.2l/h Meter drainage flux.(2)According to the streamline refraction theory and the preferential flow theory,the new type of envelope structure for drainage pipe can play a better drainage function than the traditional fully wrapped envelope;In this test,the capillary barrier formed by 0.125-0.2mm and 1-2mm envelope material is better than that formed by 0.125-0.2mm and 2-4mm envelope,and the funnel flow transverse diversion function formed at the capillary barrier interface is better.Even if soil water breaks through the capillary barrier and forms subsurface flow under the capillary barrier,which can better play the role of transverse diversion and promote the drainage function of drainage pipe.From the perspective of optimizing the drainage efficiency of the envelope,the envelope structure designed according to the refraction theory and priority flow theory is better than the traditional envelope.(3)The retention of gas at the interface of layered structure composed of soil,envelope and drainage pipe is another important factor for the generation of capillary barrier.Mastering the migration of gas in layered structural planes with different apertures plays an important guiding role in controlling the diversion effect of preferential flow.