construction and refurbishment of earthen irrigation channel banks

Figure 10.10 shows longitudinal cracking. This may cause slumping of banks if thecrack is near the top of the inside bank and contribute to inside bank erosion.Figure 10.10Longitudinal cracks10.2.2.1 Case Study - CrackingGoulburn-Murray Water has suffered several bank failures caused by cracking,particularly on new banks during dry summers.10.2.3 PipingA 400 metre length of the CG channel 2/4/8 was remodelled during the winter of1995. In January 1997 both channel banks suffered numerous leaks caused bysevere cracking.A visual inspection was carried out and the following observations were made:• deep cracking up to 900 mm in depth• longitudinal and transverse cracking• sparse vegetation cover on banksAn investigation of the top cover material was then carried out. It was found tohave a high clay content with minimal organic content. It was discovered to beold bank material rather than topsoil.After consolidation due to rainfall and vehicular traffic, no significant vegetationcould establish on the bank. Two very dry Spring/Summer seasons followedconstruction which promoted cracking through the top cover which then extendedinto the compacted banks.The problem was rectified by scarifying the top 200mm of the bank to loosen theclay clods and then applying a new top cover of crushed rock. No significantcracking has occurred since this treatment was applied.Piping as illustrated in Figure 10.11 is a common failure mode of channel banks.Piping is generally caused by dispersion of the bank material. Dispersion is thetendency of a clay soil fraction to go into colloidal suspension in (or when exposedto) water. Thus the clay or colloidal fraction of the soil fails to cohere closely to orbond with other soil particles.The tendency for dispersive erosion in a given soil depends upon such variables asthe mineralogy and chemistry of the clay and the dissolved salts in the soil porewater and in the eroding water.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 10-21