construction and refurbishment of earthen irrigation channel banks

The study identified some confusion in the irrigation industry over the term silt.Silt is a soil classification, but it is also the term widely used to describe the layer ofinorganic and organic sediment that forms on the bed of channels. To draw adistinction, these Guidelines refer to sediment deposits as eroded channel material.15.4.1 Determination of Particle Size DistributionThe classification of fine-grained soils is on the basis of Particle Size Distribution,Liquid Limit and Plasticity Index. While the classification of coarse-grained soils ison the basis of Particle Size Distribution only.A representative sample of soil (excluding particles larger than 63mm) is firstclassified as coarse grained or fine grained by determining whether 50% by weightof particles are less than 0.075mm in diameter. The laboratory method for gradingis described in AS 1289.3.6.1 Determination of the particle size distribution of asoil - Standard method of analysis by sieving. A summary of soil classificationgrading is shown in Table 15-3.Soils containing more than 50 percent of particles that are greater than 0.075mm indiameter are coarse-grained soils; soils containing more than 50% of particles lessthan 0.075mm in diameter are fine-grained soils.15.4.1.1 Coarse-grained soilsIf the soil is predominantly coarse grained, it is then identified as being a gravelor sand by determining whether 50% or more by weight, of the coarse grains arelarger or smaller than 2.36mm.If the soil is a gravel, it is next identified as being clean (containing little or nofines) or dirty (containing an appreciable amount of fines). For clean gravels,final classification is made by determining the gradation. The well-gradedgravels belong to the GW group, and poorly graded gavels belong to the GPgroup. Dirty gravels are of two types: those with non-plastic (silty) fines (GM),and those with plastic (clayey) fines (GC).If a soil is a sand, the same steps and criteria are used as for the gravels in orderto determine whether the soil is a well-graded clean sand (SW), poorly gradedclean sand (SP), sand with silty fines (SM), or sand with clayey fines (SC).15.4.1.2 Fine-grained soilsSilts and clays are collectively called fines and when dry and powdered areindistinguishable by eye. If a material is predominantly (more than 50 % byweight) fine-grained, it is classified into one of six groups (ML, CL, OL, MH,CH, OH) by determining its liquid limit, plasticity index and by identifying it asbeing organic or inorganic. Refer Table A1 from AS1726.Before discussing these three properties of fine material, it is necessary tocomment on the change of consistency with water content for fine-grained soils,particularly clays. Fine-grained soil may be liquid, plastic or solid. These stagesare related to the water content and although the transitions are gradual, testconditions have been established arbitrarily to delineate the water content as adefined point in the three stages of consistency:Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 15-9