| One of the main characteristics of soils in the loess area is that they often have layers and macro-pores (including fissures), and such soils possess different infiltration characteristics from the homogeneous soils. This paper presents a study on infiltration pattern of soils with layered structure and fissures in the loess area of Northwestern China, it is part of a research project funded by the National Natural Science Foundation of China. The goal of this study was to disclose and learn the infiltration features of such soils, and enrich the theory of unsaturated soil research and provide practical guidance. This study was based on extensive review of unsaturated soil water research achievements from domestic and abroad. Through systematic experiment, model study and analysis on mechanisms, the major achievements and findings are as follows:1. Developed a serial Marriot bottle device, which can maintain water level within ±0.5mm of preset values, and this new device made it possible to provide high-precision water supply source for field infiltration experiment;2. Studied infiltration characteristics of layered soils with different structures, major findings are:(1)The existence of a lower layer soil reduced the infiltration process and changed the upper layer soil's nonlinear cumulative infiltration process into a linear process when the wetting front reached the lower layer. An infiltration reduction ratio was proposed to assess layered soil infiltration reduction process and it has been found reasonable. Infiltration reduction and resistance of layered soils with a 'heavy top and light bottom' textures were caused by different water potentials of the two layers. Infiltration reduction and resistance of layered soils with a 'light top and heavy bottom' textures were caused by the different hydraulic conductivities of the two layers, temporary water table would form above the interface.(2)The thickness and initial water content of the upper layer soil had clear impact on infiltration process and wetting front. The influence of the lower layer soil texture on infiltration process was not obvious.3. Infiltration process of layered soils was expressed as different stages using common infiltration models and linear models, and the methods about how to obtain major parameters were also derived.4. A continuous functional model was proposed to simulate infiltration in layered soils, and it has been tested effective with experimental data. On the basis of the continuous model, a further model to consider layered soils with same texture but different bulk density was established and tested with experiment data, and good agreement was observed.5. Under certain 'heavy top and light bottom' layered soil structure, wetting front may induce finger flow. The reasons for this are: (1) top soil has lower water conductivity; and (2) the lower sandy soil is not distributed uniformly. A quantitative index Df, as the ratio of steady infiltration rate fp and lower layer conductivity Ks, for finger flow development was proposed, and its critical value was found to be 0.02.6. A length-to-width ratio (Cp=Lp/Bp) was used to express development of finger flow. Cp increased with infiltration time, and it was affected by median particle size, but not the finger flow width (Bp).7. The ratio of finger front movement to the finger root movement (Cv=Vp/V) was used to express the velocity of finger flow movement. When the top layer soil thickness is a constant, Cv decreased with increment of the lower layer soil median particle size. When the lower layer soil particle size was certain, Cv increased with steady state infiltration rate.8. Laboratory study on infiltration in fissured soils found that existence of fissures in soil increased soil infiltration capacity. The increment in infiltration concentrated at the beginning of the process, and it directly correlated with depth of the fissure. For certain soil texture, fissure infiltration was affected by fissure characteristics, or fissure... |
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