construction and refurbishment of earthen irrigation channel banks

damage in a channel situation and a cover of stable earth or crushed rock atleast 0.3 metres thick is usually spread over the thin lining for protection. Alower compaction standard than for earth lining is acceptable as the layer is forprotection and stabilisation only. Spreading a uniform thickness of bentonitecan be difficult, adequate mixing and compacting the material on the batterslopes can also be hard to achieve and for moist fine-grained soils, the thoroughmixing of the bentonite can be virtually impractical because of the stickyconditions. Under these difficult mixing conditions a pure buried liner ofbentonite may be a more suitable method of treatment.Bentonite channel linings in Australia and overseas have been found to frequentlyloose their effectiveness after only a few years of service, due to the decomposingeffect of wetting and drying, hard water and other contaminant and weatheringfactors. As well, bentonite may not be effective where the porosity of the subgradematerial is high, such as in the case of a prior stream location.Given their relatively high installation cost compared with other seepage controlmethods, the use of bentonite type linings should be looked at with some degree ofcaution.22.7.4 Compacted Soil Cement LiningSoil mixed with an optimum percentage of cement and water can be compacted intoa hard and stable concrete-like material. Stabilisation of soil with cement to improveits engineering and stability properties has been used for many years. Overseas it hasbeen used in the lining of channels and water reservoirs, however no sites wereidentified in Australia where compacted soil-cement had been used for irrigationchannel linings. No information was therefore available on the effectiveness, costand service live of this type of lining.A compacted soil-cement lining is commonly 100mm to 150mm thick and a typicalinstallation sequence would consist of:1. Portland cement is uniformly spread over the area to be treated.2. The cement is mixed into the soil using a rotary tiller, while water is addedsimultaneously to the mixture.3. When the cement and soil are thoroughly mixed and the moisture content is atoptimum, the mixture is compacted by flat drum or rubber-tyred compactors.4. After the mixture has been compacted to the specified density, the lining is curedfor 7 days, using the same methods employed for thin concrete sections.Laboratory tests are required for each different soil type to determine:• how much Portland cement is needed to adequately harden the soil?• how much water should be added to achieve optimum moisture content?• to what density should the soil-cement mixture be compacted?The amount of cement, which is related to the particular soil type, is likely to be inthe range from 5% to 15% by weight, high enough to produce a durable watertightlining, but not too high to cause serious cracking. In general, a well graded gravel orConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-34