construction and refurbishment of earthen irrigation channel banks

The base coat and seal coat rates may be varied in order to achieve adhesion tothe surface and impregnation of the geotextile. The key factor is that thegeotextile will absorb about 1.0 L/m2. This is essentially an adaptation of atechnique being used to control reflective cracking and extend the life of roadseals.There is discussion about the suitability of geotextile materials based on eitherpolyester or polypropylene fibres. Polyester has a high melting point and isunaffected by hot bitumen whereas polypropylene begins to melt at 140 o C andbitumen from a tank at 150 o C can effect the geotextile. There is a converseargument that the polypropylene achieves a better bond because of thetemperature relationship although it would obviously be prudent to avoidbitumen spray temperatures that cause distortion or shrinkage.Failure of bitumen membranes is typically caused by too thin a membranecoating, penetration by weeds or rupture due to movement of the cover material.In hot climates, uncovered bitumen membranes absorb heat and soften,encouraging weed growth that can penetrate through the lining. Winterconditions can also present difficulties and this technique may not be well suitedto winter applications where cold, moist and wet conditions prevail.Australian and overseas experience with spray applied membranes has generallynot been good, with problems of poor quality control in the field. Use of sprayedapplied membranes should therefore be looked at with some degree of caution.22.8.3 Performance RequirementsThe most important performance parameters of geomembrane liners arePermeability, Durability and Mechanical Performance. These are outlined below.22.8.3.1 PermeabilityCommon to all channel liner applications is the requirement to provide arelatively impermeable barrier and to maintain this over a reasonably long servicelife appropriate to the individual application. Whilst polymer basedgeomembrane materials themselves can be some 10,000 times less permeablethan a compacted-earth liner, the vagaries of their installation and field seamingmeans that some leakage will occur.22.8.3.2 DurabilityCritical to the success is not only the material selection, but the accompanyingpreparation, installation and quality control program. The geomembraneconstruction methods and seaming practices should follow a QC/QA programthat allows meaningful inspection and testing both in the field and from thelaboratory.The durability of a geomembrane to survive as long as required by the life of theproject, in the environment in which it is used, is of paramount importance. Animportant consideration is also the survivability of the geomembrane duringplacement so that the lining is not damaged by construction procedures. Thegeomembranes are often exposed to the direct sunlight both permanently orConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-49