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Mr. Gregg Squire May 24, 2000 Page 2 oj3 • Maximum loose lift thickness will be 9 inches for placement of Zone 1 and Zone 2 materials. • Zone 1 materials shall have a minimum percent of clay fines passing the United States Standard No. 200 sieve of no less than 50% by dry weight and shall have a plasticity index of 10 percent or greater for the portion of material passing a United States Standard no. 40 sieve. Comment 2 • The South Platte River bank adjacent to the proposed Lafarge Riverbend Operation features significant existing concrete rubble armoring to stabilize the main channel alignment. The Urban Drainage and Flood Control District historically has recognized the stabilization value of concrete rubble armoring. Recently, District staff viewed another South Platte mining site proposed by the operator within their jurisdiction, and recognized the existing concrete rubble armoring as partially meeting their requirements for riverside channel stabilization. Specifically, they are allowing the operator to reduce the mining setback from the river bank from the 400' `no stabilization' limit based on the existing concrete rubble armoring on the site within their jurisdiction. As [here may be a time delay between commencement of mining, and encroachment within the 400' (or other agreed distance) `no improvement' setback, the operator proposes to use that time to measure, monitor and evaluate the effectiveness of the existing concrete rubble for South Platte River channel stabilization. In addition, the operator proposes to use the time lag to mining encroachment to utilize other `soft' stabilization techniques, which may include: • minor bank regrading, • burial of concrete rubble armoring, and • planting and seeding of appropriate stabilizing bank vegetation. The time delay again would be used to evaluate, construct or install, and monitor the effectiveness of these soft improvements. Since monitoring and evaluation of the bank stability, improvements and techniques will be a major component of the operator's approach, reporting of interim findings and updates will be provided at least on an annual basis. Comment 3 • To permit mining and backfilling within the river setback area, the low flow season will be defined as the period from October 1~` to April 1~' of each year as recommended by the Division. • During each year that the Riverbend Operation is active, the applicant will provide to the Division a signed affidavit stating that the required 200-foot setback is established