construction and refurbishment of earthen irrigation channel banks

10.2.3.3 Irregular compaction – bridging of layerIrregular compaction is where adjacent layers are compacted to significantlydifferent densities. Figure 10.14 shows the case where the lower layers arecompacted to a normal density, eg 95%. The upper layer is over-compacted, eg105%. This means that the swelling and shrinkage characteristics of each layerdiffer. Over time the over-compacted layer, because it has a greater density andstrength, may begin bridging when changing from a wetted state to a drier statewhile the lower layers remain in a similar position to when they were constructed.Therefore a gap will result between the layers which will cause a piping failure.Over-compacted layerNormally compacted layerFigure 10.14Piping failure due to irregular compactionThis failure mode can be prevented by ensuring compaction density atconstruction remains within construction tolerances.10.2.3.4 Lamination of the layersLamination of layers is caused by two smooth layers being in contact with eachother but not bonded. With wetting up and drying of the bank each layer willmove independently of the other. Therefore over time, a piping pathway betweenthe layers may form. This is prevented by good construction practice duringcompaction where the surface of each layer is tined or roughened before the nextlayer of material is placed on top for compaction.Unbonded laminated layerFigure 10.15 Piping failure caused by lamination of layers10.2.3.5 Case Study - PipingPiping failure occurred on the CG Channel 5/6 during 1995/96 season. A largeleak was discovered and the channel was drained to inspect the bank. A largepipe was discovered in the base of the bank.Piping failurePhoto 10-19 Entrance to pipe near bed of channelpipePhoto 10-20 View through pipe after channel was drainedConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 10-23