construction and refurbishment of earthen irrigation channel banks

For economic construction, borrow material should be sought as close to thechannel site as possible. Material for bank re-construction is normally obtained byopening up borrow pits on properties in the immediate vicinity of the works.The borrow pits are generally used by landowners as farm re-cycling dams, and alllandowners within economic distance of the work site should be approached eitherpersonally, by letter or through advertisement in a local paper to determine if theyare interested in the construction of a farm dam. This could include an offer to payroyalties for material excavated from dams, as an added incentive to landowners inclose proximity to the work site. For economical construction, the minimum size ofa dam is typically 40m x 40m.A typical earthworks equipment cycle for re-construction works is shown in Figure20.6.In some situations, it may be acceptable to over-excavate the channel waterway anduse this material to re-construct the outside of the channel banks or top the crest ofthe bank to increase freeboard.20.4.4 Fill against structuresFill, adjacent to structures, such as pipes, culverts, abutments, retaining walls orother structural components, may need to take into account the following:a) The strength or age of cast in-situ concrete before filling can commence.b) The filter zone to be provided adjacent to weep holes or other subsoil drainagesystemsc) The quality of fill. Sand, natural gravel or quarry products may be specified tofacilitate compaction in confined areas, to minimise differential settlement whichmight otherwise overload the structure or to divert seepage to subsoil or otherfoundation drainage systems.d) The type and method of compaction compared with normal fill construction. Fillshould be brought up equally on each side of pipes and culverts, to avoidunbalanced loading. The first layers of fill over the top of structures will requirecareful placement. The design should specify the depth of fill to be placed overpipes or culverts, or special conditions which might apply to other structures. Insome cases, internal propping may be required if normal compaction is to beused immediately above or adjacent to structures.20.5 Re-forming Channel Banks with Waterway MaterialThe material that erodes from the inside batter of a channel bank and deposits at the toe ofthe bank and on the bed of the channel is often referred to as silt. Silt is a scientific termused to describe a soil particle size and type, (refer Section 15.4). In this manual thiswaterway material is referred to as eroded bank material (EBM). This is a moreappropriate term to use, as this material often contains clay from the original bank. It mayalso contain materials which have been carried by the channel water. Refer Figure 20.9.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 20-15