construction and refurbishment of earthen irrigation channel banks

16.11.4 Moisture Content ControlBefore material is placed on the bank the moisture variation from OptimumMoisture Content should be within the specified limits. For channel bank projectsthis is typically ± 2% of optimum.Unless the moisture content of the borrow material is fairly uniform and within thespecified range, the possibility of achieving the density requirement is remote. Ifthe moisture content in a compacted layer is outside the limits and the fill should beloosened by ripping to full depth and moistened (too dry) or aerated (too wet) asnecessary.As far as practicable, the material should be brought to a uniform optimummoisture content in the borrow pit, prior to delivery to the bank.Bank construction is best undertaken at the time of year when the natural soilmoisture of the available bank material is close as possible to the optimum. Latespring, autumn or early winter are usually the best times. Construction is oftendifficult in mid-winter because sites are too wet, and the soil can be too dry duringmid-summer.If the soil is too dry, it can be wetted and conditioned by spraying with water at theborrow pit or by supplementary wetting during placement on the bank, asappropriate. Improved mixing of water will result if applied at the borrow site afew days before the soil is to be used.Photos\borrow pits\shep5 excavator mixing borrow wet&dry.jpgPhoto 16-6Excavator mixing wet and dry borrow materialIf the soil is too wet, it must be allowed to dry out. If wet conditions areexperienced during construction, this may mean that works are halted for a periodof time to allow the material to dry before continuing with compaction. If soil isplaced in too wet condition, it will be difficult to achieve the required density andfailure of the bank may result.Any major adjustments (greater than 1%) to water content should be carried out atthe borrow area, at least 24 hours and up to one week before hauling of the adjustedsoil to the bank. Minor adjustments of water content can be made on the bank priorto compaction. No standing water should be permitted on the bank.For most soils used in channel banks the chances of achieving the required densityare considerably enhanced if the fill is placed on the wet side of, rather than the dryside of optimum.Care should be taken to ensure that the material has an even moisture distributionthroughout each layer. Difficulty will be experienced in achieving goodcompaction if the moisture content varies more than 2% within a layer.Material having a placement moisture content of more than 2% dry of optimumcondition or more than 2% wet of the optimum condition should be rejected andremoved or reworked until the moisture content is within these limits.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 16-19