construction and refurbishment of earthen irrigation channel banks

Low permeability and high stability and erosion resistance are important criteria forchannel banks, and this leads to a reasonably high target compaction of 95% to98% of maximum dry density being specified for channel bank projects.This percentage should be established for each project.A field density of 95% of maximum dry density should provide adequate stability,erosion resistance and impermeability for most channel bank projects. Somedifficult soils may require a field density of 98% of maximum dry density toovercome stability problems. Refer to Section 15, Material Selection and Testing.In most soils, a much greater degree of compactive effort is required to achieve98% as against 95% density, and the decision to specify higher levels ofcompaction needs to be soundly based.In levees used for flood control, low permeability is important, but generally not assignificant as in channel banks, and target compactions in the range of 90-95% ofstandard maximum dry density are often specified.Soils that have pronounced expansive qualities sometimes tend to swell or heave ifthey are over-compacted. Therefore for certain soils, problems can be caused byover-compaction.16.8.3 Quality AssuranceThe application of quality assurance in civil earthworks is not straightforward, andthere is a need for caution when applying quality assurance principles to channelbank earthworks. A technical overview of the work is required, withresponsibilities and authorities clearly defined, and compliance auditing and checktesting.There is however no substitute for the use of proper construction equipment andexperienced machine operators.16.9 Compaction SpecificationsA typical specification for channel bank compaction would be:1. Material placed within the moisture range from 2% dry to 2% wet of the optimummoisture content obtained on the same material using AS1289.5.1.1-1993.2. Field compaction to achieve a dry density not less than 95% of the maximum drydensity as obtained on the same material using AS1289.5.1.1-1993.A weakness identified in a number of irrigation authorities was the inappropriate use ofspecifications developed for other projects. Specifications have at times been taken froma previous project with different soil conditions and applied where the techniques do notadequately cover the type of material being used. Specifications need to be appropriate toeach job and prepared with a full understanding of the soil conditions.Refer to Section 15, Material Selection and Testing.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 16-11