construction and refurbishment of earthen irrigation channel banks

Coarse grained sand is recommended as finer sands may be washed away byrainfall. Sand should be of a homogeneous composition. Sands with no fines(material less than 75um) should also be avoided as the particles lack the cohesionto adequately bond to the embankment and each other.ApplicationSand, in a moist state, is transported to the site, spread evenly across theembankment surface area via a grader or similar machinery. The thickness of thelayer shall not be less than 75 millimetre. While in its moist state the material canbe lightly compacted using a flat drum roller.17.5.3 TopsoilCharacteristically, clay channel bank materials lack fertility. Their capacity forholding moisture is very low, and the available minerals are usually inadequate forthe healthy growth of most vegetation. In order to create favourable conditions forestablishment and subsequent growth, the faces of the channel bank need to becovered with topsoil.Material requirement• contains enough organic content to support vegetation establishment• pH between 6 and 8• erosion resistant material• free from sticks, excessive amount of stones and other deleterious material• must not be susceptible to excessive shrinkage when drying, otherwise theresulting cracks will extend through the topsoil cover into the compacted bank.• Liquid limit < 35 (AS1289.3.1.1). Refer to Section 15.4.2, Determination of theplasticity of fine-grained soils.Application• A minimum 150mm thickness of topsoil is required to establish a grass root mass.• Topsoil shall be evenly spread, in a moist state, over the finished crest atminimum 150mm thickness by conventional construction equipment, and lightlycompacted to hold the material in place.17.5.3.1 Chemical Treatment of Material available on SiteWhere topsoil as described in Section 17.5.3 is not available, the addition of lime,gypsum, and/or fertiliser may be applied to improve the soil structure andchemical/mineralogical composition of the available material. The application ofone or more of these additives should improve the insitu soil’s ability to grow asuitable protective cover.ApplicationThe application rates for each of these additives vary greatly depending on themineralogical and chemical make-up of the insitu soil. An assessment of thesoil’s requirements should be made by suitably qualified personnel andconsideration of the additives manufacturer’s instructions.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 17-6