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<br />.... <br /> <br />('!j <br />.....t <br />o <br /> <br />'::.-) <br /> <br />than a membrane-lined canal. Earth moving costs for the 10-foot <br />shift for concrete are significantly less than for a 20-foot shift. <br />In Alternative M4; however, other factors cause construction costs <br />for the concrete lining alternatives to be more expensive than <br />Alternative M4. The final EA has been revised to clarify these <br />factors. <br /> <br />i.-I <br /> <br />Comment 6: For Alternative Ml, the canal centerline should be <br />shifted about 10 feet to the south as it is in the concrete lining <br />alternatives to make better use of the current fee land boundaries <br />of the canal. This would reduce additional land acquisition <br />requirements for the north O&M road. <br /> <br />ResDonse 6: This suggestion has been adopted and Alternative Ml <br />and M3 have been modified accordingly. <br /> <br />Comment 7: Concrete is favored over membrane, especially for the <br />first 3.5 miles (Reach lA), because concrete appears less <br />disruptive, seems more dependable, and is easier to repair and <br />maintain. Damage to membrane is difficult to find and repair. <br /> <br />ResDonse 7: The EA has been revised to address the merits of <br />concrete vs. membrane lining of Reaches lA, lB, and 2. The choice <br />between concrete and membrane requires weighing the pros and cons <br />of each lining type and then selecting the method which best <br />accomplishes the goal. what complicates this decision is that we <br />are comparing a technology (membrane lining) which is only about 35 <br />years old to one which is over a century old. Consequently, the <br />amount of information on both technologies is not equal. with this <br />in mind, Reclamation's natural tendency is to use the more proven <br />technology. However, experience in the Grand valley indicates that <br />membrane lining is a better choice. One of the more significant <br />factors in choosing membrane lining is the ability, or inability, <br />of the substratum in the Grand Valley to provide a stable <br />foundation for concrete lining. <br /> <br />As recently as one year ago Reclamation was considering the use of <br />concrete lining. But geological investigations for the canal <br />(Reclamation, 1991) found that there were not stable foundation <br />materials and a preponderance of silt sized particles which are <br />susceptible to frost were present. unstable foundation material <br />predominate beyond the first 1.9 miles of Reach 1. unstable <br />foundations cause significant cracking of a rigid concrete lining; <br />a flexible membrane lining can move with the ground. Problems with <br />the foundation along the canal is further aggravated by the <br />presence of groundwater. For Reach 1, the irrigation of lands both <br />above and below the canal amplifies the foundation problems more <br />than they would in Reach 2. <br /> <br />42 <br />