Study On Clogging Microcosmic Mechanism Of The Filtration System Of Xinjiang Underground Water Harvesting Engineering

Based on three-dimensional particle flow code theory, the numerical model of seepage erosion which takes into account the coupling of solid and fluid has been established using the built-in fish programming language of PFC. The numerical percolation models of uniform gradation foundation soil, good gradation foundation soil, gap graded foundation soil are simulated under various coefficients between layers. The variations of porosity, permeability, loss fraction of gains and the detention amount of base soil in filter are traced and recorded. And the characteristics of the base soil-filter system such as conserving soil,permeability, and prevention of silt plug have been evaluated. In a word, we hope that the coefficient between layers which is applicable to the design of the filter of the underground water engineering can be found, and the meso mechanism of infiltration clogging can be explained. The results show that :(1) Two kinds of infiltration clogging in underground water engineering may be happened,one is the "cake" clogging caused by fine-grained aggregates gathered at the surface, another is catchment channel siltation which could happen when a group of fine-grained soil go through the filter layer. The possibility and the kinds of infiltration clogging depend on the characteristics of gradation of base soil and the values of coefficient between layers.(2) For uniformly graded foundation soil, most of the foundation soil particles participated in the constitution of the skeleton structure, and under the effect of seepage force, there were no fine particles migrating along the skeleton particles clearance. After the foundation soil particles in interface run off, all the foundation soil particles begin to slowly move downstream. When the coefficient value between layers is not more than 5, the filtration system can not only maintain stability, but also have good permeability.(3) For continuous graded foundation soil,the smaller foundation soil particles are difficult to move in the foundation soil. And when coefficient is appropriate, the foundation soil has better self-filter-ability.The coarse and slightly fine particles detained around the interface, not only can prevent the fine losing from upstream to downstream, but also can form a overhead structure with larger permeability. In this case,if the value of D15/d85* is smaller than 2.3, the clogging occurs at the interface, and if the value of D15/d85* is between 2.3 and 4.3, the foundation soil has better self-filter-ability, and the filtration system will be with good permeability.(4) For gap graded foundation soil, the parts above horizontal segment are coarse particles, and below it are minute particles. The coarse particles constitute the skeleton and the fine particles fill the porosity between coarse particles. The deficiency of the intermediate grain diameter lead the fine particles to freely shuttle inter particles. From these characteristics we can conclude that, when the D15/d85* is less than5, the clogging will take place, and when the D15/d85* is more than 6, the fine particles will easily silting in the infiltration galleries, and when the D15/d85* is between 4 and 5, the filtration system can keep stable and be with good permeability after part of the foundation soil run off.