construction and refurbishment of earthen irrigation channel banks

composition of fly ash varies considerably with the type of coal burned to produceit. Fly ash does not materially affect the plasticity of the treated soil.Tests have not shown gypsum to be very effective in treating dispersive clays.Other additives, such as magnesium chloride, have been considered. Tests ondispersive clays treated with this chemical have shown it to be effective. Theprimary questions concerning this and other chemicals that have few field trials iswhether they have adequate longevity and how they may affect subsequentestablishment of vegetation.Material of Emerson Class No. 1 can only be used if there is substantial blanketof higher quality material over both the crest and batters or it is chemically treatedas described above.In existing channels, the addition of chemical additives, well-mixed in andcompacted with the upper 200 to 300mm of the outer surface layer of the bank,may be beneficial in stabilising the bank material. Refer to Section 15.9.2,Shrinkage, Chemical Treatment.15.9.3.4 Sand Filter ZoneThe internal erosion or piping of channel banks constructed from dispersive clayscan be prevented by the placement of a vertical sand filter zone in the bank.Fine clean sand is placed in a 0.5 to 1 metre wide vertical trench excavated in theconstructed bank, extending from the bank subgrade up to the height of themaximum water level.Eroding water flowing through the dispersive clay carries particles of the erodedsoil to the sand filter interface. Plugging of the flow occurs as particles build upon the face of the filter. For it to be effective, the sand filter must not have thepotential to crack itself.Crest and batter protection from external erosion would also be required. Referto Sections 17 and 18.15.10 Tests for Small ProjectsThere is a view that the level of investigation, design, material testing and constructioncontrol used on large channel projects cannot be justified on small low cost projects.While that may be the case in some situations, it is not just the size or the cost of thechannel works that should be the deciding factor in determining the amount of care that istaken in design and construction, but rather the consequences of a channel failure. Thetotal potential damage and economic loss that may result from a failure of the channelbank also needs to be considered.The scope of investigation and testing carried out is usually proportional to the size of theproject. A large channel project would justify the expense of detailed investigation andlaboratory testing, but on a small project laboratory testing facilities may not be available.Construction and Refurbishment of Earthen Channel Banks