construction and refurbishment of earthen irrigation channel banks

lose some moisture by evaporation. The specimen is then rolled out by hand on asmooth surface or between the palms into a thread about 3mm in diameter. Thethread is then folded and re-rolled repeatedly. During this manipulation themoisture content is gradually reduced and specimen stiffens, finally loses itsplasticity, and crumbles when the plastic limit is reached.After the thread crumbles, the pieces should be lumped together with a slightkneading action continued until the lump crumbles.The tougher the thread near the plastic limit and stiffer the lump when it finallycrumbles, the more potent is the colloidal clay fraction in the soil. Weakness ofthe thread at the plastic limit and quick loss of coherence of the lump below theplastic limit indicates either inorganic clay of low plasticity, or materials such askaolin-type clays and organic clays. Highly organic clays have a very weak andspongy feel at the plastic limit.15.11.4 Boundary ClassificationProper boundary classification of a soil near the borderline between coarse-grainedand fine-grained soils is accomplished by classifying it first as a coarse-grained soiland then as a fine-grained soil. This type of dual classification is done for all soilswhich are visually judged to have between 40% and 60% of fine (or coarse)material.15.11.5 PlasticityThe hand classification method in the Description & Field Identification column inTable 15-10, can be used to determine whether the sample is of low, medium orhigh plasticity.Descriptive SymbolDescription & Field IdentificationTermLowPlasticityL May appear cloddy when dry, but dry lumps canreadily be broken by and powdered by pressure of thefingers and thumb. Will roll into a thick soft threadthat is easily broken. When dry will brush from thehands.IntermediatePlasticityI Fine textured soil smooth to the touch. May bemoulded into various shapes, will roll into a thread butbreaks on bending slightly.HighPlasticityH Very fine textured soil, smooth and very greasy. Rollsinto a very fine thread without breaking. Thread willbend to about 45° without breaking.Fat Clay H Threads will bend almost in two without breaking.Table 15-10Determination of plasticity by Field Identification15.11.6 Rapid Dispersion Test.Fill a clear glass jar half full (100ml) of distilled water or rainwater, gently place apea-size crumb (5 to 10 grams) of dry soil sample into the jar and leave undisturbedfor at least an hour, without shaking. At the end of the waiting period, the interfaceConstruction and Refurbishment of Earthen Channel Banks