| Alluvial-proluvial substances were distributed in the wide valley of Tibet plateau, whichwere thick and loose packed gravel layer as reservoir dam foundation. The permeability of layerwas critical for storage capacity and leakage of the reservoir, and also could affect the damnormal operation. In recent years, some small reservoirs in Shannan region of Tibet wereinvestigated. It found that the leakage was a more serious problem in early reservoir operation,but the amount of leakage was gradually reduced and the storage capacity was progressivelyrestored with the operation of reservoir storage. Therefore, mechanism of infiltration-cloggingand test for plateau loose dam foundation were carried out. It would be provide a theoreticalbasis for solving leakage problems of alpine gully river dam and artificial clogging infiltrationreduction. The research has important theoretical and practical value.In this paper, we focused on the thick and loose gravel layer of dam foundation in Zhuo Yureservoir. According to field investigation and field pit bicyclical water injection test,infiltration-clogging tests in the condition of different size range of coarse grained soil, naturalsoil and compacted soil were carried out. The infiltration-clogging process was researched frommacroscopic to microcosmic and also simulated by three-dimensional particle flow software.The primary results and conclusions as following:(1) Taking the alluvial fan edge along the upstream rivers of Zhuo Yu ditch as the researchobject, six locations were selected for sampling. And the dry density and soil particle sizedistribution curve are obtained. The local soil situ permeability was1.98×10-2cm/s through fieldpit bicyclical water injection test.(2) A series of indoor clogging tests was conducted on seven kinds of samples. The grainsize range of samples was from32mm to64mm,16mm to32mm,8mm to16mm,4mm to8mm,2mm to4mm,1mm to2mm and0.5mm to1mm separately. According to the monitoredfluid velocity and hydraulic gradient, and the result of sieving analysis at the end of test, theclogging can be divided into three types: surface clogging, internal blockage and unclogging.Then the internal blockage also can be divided into two types: internal partial pore blockage andsurface-internal clogging. The discrimination criterion of clogging types for gravel group soilwas obtained by “equivalent grain size criterion”. The effective porosity model of skeleton grainswas established. The clogging criterion based on “effective aperture of grain criterion” was that:when De/det(ratio of equivalent grain size of skeleton grains and equivalent grain size of addinggrains) less than1, clogging type was surface clogging; when De/detless than2and greater than1, clogging type was surface-internal clogging; when De/detless than4and greater than2, clogging type was internal partial pore blockage; when De/detgreater than4, clogging type wasunclogging. Characteristics aperture of six groups samples(16mm to32mm,8mm to16mm,4mm to8mm,2mm to4mm,1mm to2mm and0.5mm to1mm) were obtained by effectiveaperture of grain criterion, which is3.987mm、1.986mm、1.013mm、0.528mm、0.247mm、0.128mm separately. At the same time, the optimal range of clogging grain size are got, that is2.819-5.638mm,0.993-1.986mm,0.506-1.013mm,0.264-0.528mm,0.124-0.247mm,0.064-0.128mm separately. Compared with the range of clogging grain size corresponding to thephenomenon of obvious clogging in indoor test, the optimal range of clogging grain size wascalculated by effective aperture of grain criterion in the condition of the surface-internal cloggingtype was more consistent. It shows that using effective aperture of grain criterion can determinethe clogging types.(3) Three different gradations of natural soils and four same gradations but differentcompaction degree soils were tested. The optimal range of clogging grain size was calculated byeffective aperture of grain criterion. The tests of different compacted soil exhibited less seepageflow and lower permeability than tests of nature soil. And to reduce the seepage flow anddecrease permeability need by add clogging material.(4) Phenomenon of surface clogging, surface-internal clogging and internal partial poreblockage was analyzed on infiltration-clogging tests in the condition of different size range ofcoarse grained soil, natural soil and compacted soil. After the test, the natural soil and compactedsoil were sieved. Particle size range of three clogging phenomenon was statistics. The statisticalresults were consistent with the conclusions by effective aperture of grain criterion. Thegranularity fractal dimension value of skeleton grains and soil samples after tests were calculatedby the method of granularity fractal dimension. The value of soil samples after tests was muchmore than Skeleton grains’. It mean particle volume was increased and the effective pore volumewas decreased, namely, the clogging phenomenon. The value of first layer in natural soil washighest in four layers. It mean the mainly clogging type was surface-internal clogging. Forcompacted soil, the granularity fractal dimension values of first layer soil and entire soil wereincreasing with compaction times. The more times, the better clogging phenomenon appears.The most obvious clogging phenomenon appeared in the soil sample of105compaction times.(5) According to the field investigation and indoor physical simulation test, theinfiltration-clogging process of the loose dam foundation was split into three stages: particlesadjustment stage, seepage stabilization stage and seepage variation stage. And the seepagedeformation was divided into three types: seepage compaction, seepage failure and infiltrationclogging. Based on the mechanical characteristics of seepage deformation, the mechanism of thethree types of seepage deformation was analyzed. Particles movement in the pores was analyzed. When the particles along the throat were subject to a downward force, moving characteristic ofgrains appeared migrating, piping or suffosion erosion. When the particles along the throat weresubject to upward force, moving characteristic of grains appeared suspending. When the particlesalong the throat were subject to balance force，moving characteristic of grains appeared clogging.(6) Using three-dimensional particle flow code software, the whole process ofinfiltration-clogging was simulated. In numerical simulation, the velocity, position of singleparticle, porosity of specimens and permeability coefficient was monitored in the infiltrationclogging process. The results revealed that: clogging particle motion had randomness anduncertainty in infiltration clogging process; clogging particles changed the partial structures ofthe samples, and the porosity and permeability coefficients were also changed. When addingclogging materials, the porosity and permeability coefficient of specimens were graduallydecreases with time step for both natural soil and compacted soil. The numerical simulationresulted more consistent with the results of laboratory tests. |
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