construction and refurbishment of earthen irrigation channel banks

Unless the bank is very high, the seepage gradient is rarely critical to design. Inpractice, construction and vehicle access requirements generally dictate the width ofthe channel bank.A soil having a seepage gradient of more than 1:6 would generally be unsuitable forbank construction.12.16.4 Batter Slopes12.16.4.1 Inner SlopesEarthen channel batter slopes need to be stable under all anticipated conditions,and the angle of slopes should be carefully considered as part of design.Batter slopes ranging from 1 (vertical):1.5 (horizontal) to 1 (vertical) :2.0(horizontal) are practically standard for earthen trapezoidal cross-sections inAustralia, frequently without due regard to the soil types present.Long term channel bank performance indicates that some of the design batterslopes used in the past may have been too steep, and flatter batter slopes couldincrease bank life and reduce maintenance costs considerably.Experience has shown that where the batter slopes are too steep, channel waterlevel fluctuations can cause bank slumping and wave action can cause erosion,increasing rates of deterioration and gradually eroding the bank away.While the standard cross-section approach may be adequate for a range of sites,more careful consideration is recommended.Each channel should be evaluated separately, taking into consideration theoperating conditions, effects of water, the type of soil, shear strength, soilshrinkage conditions, depth of cutting or height of bank, surcharge loading,ground water, and climatic conditions. A study of the soils in which the channelis to be excavated will assist in determining batter stability and this should beestablished by drilling bores along the line on which the channel is to beconstructed.The batter slopes of earthen channels are principally determined by the stabilityrequirements of the material forming the channel. For some channels, themaximum batter slopes may be determined by economic methods ofconstruction.The batter slopes on earthen channels need to be flatter than the angle of reposeof the saturated bank material at least as far up the slope as the maximum waterlevel in the channel. Otherwise, under the less stable saturated soil conditions,the high pore water pressures in the bank can cause a shear failure and induceslumping of the batters into the channel. A similar failure can occur if the bank ismade of expansive material that swells in contact with water and loses all or partof its shearing strength.Experience has shown that these conditions speed up the bank deteriorationprocesses considerably, with the steeper the batter slope, the greater the impact.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-40