construction and refurbishment of earthen irrigation channel banks

16.15 Acceptance of Test ResultsThe following acceptance criteria can be used as a general guide in interpreting channelbank test results:1. If the moisture content is within permitted range and the density requirement is equalor exceeded – Accept.2. If the moisture content is up to 1% outside the permitted range and the densityrequirement is equalled or exceeded – Accept only after consideration of the materialcharacteristics and risks involved.3. If the moisture content is within the permitted range and the density result is up to 2%below requirements – Order additional compaction of the placed layer, and re-testbefore acceptance.4. If the density is low – Reject.To achieve the required density, some or all of the following actions will be necessary:• loosen the placed layer to full depth, moisten or aerate as necessary, reduce layerthickness and undertake complete re-compaction.• remove the placed layer, stock pile the material, re-condition and repeat the laydown and compaction processes.If the moisture content is drier than the lower limit by more than 1%, additional passesare unlikely to achieve required density. It will only compact the upper portion of thelayer, without correcting the lower portion.If the moisture content is wetter than the upper limit by more than 1%, additional passesmay achieve the required density, provided the re-working can reduce the moisturecontent sufficiently to raise the density to the required level. Re-test before acceptance.The compaction of material in too dry a condition has been found to be one of the mainreasons for poor channel bank performance. This is attributed to the ease of handlingmaterials on the dry side of optimum, and the tendency for the moisture content to driftoutside the lower limit during the drier parts of the year when earth works are mostfrequently undertaken.No amount of compaction will produce the required density if the material is too dry. Thecompacted material may be very hard, but hard fill does not necessarily mean that therequired density has been achieved. A sample of clay which may have a strengthapproaching that of a weak concrete when it has been dried, can become mud whenimmersed in water.For channel banks, where construction pore pressures are normally not a concern, thechances of achieving the required density are increased if the material is compacted onthe wet side rather than on the dry side of optimum. By adopting this approach, a factorof safety is introduced against placing large quantities of dry material. Sufficient timebecomes available to detect the drying out, which can be offset by pre-wetting in theborrow area, rather than attempting to make moisture adjustments on the bank.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 16-32