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<br />Memo to Erica Crosby 5 May 15, 2001 <br />Another pertinent concern relative to the riprap designs is the bedding for all riprap installations. The <br />bedding must be filter compatible with the riprap. This bedding is typically a filter blanket of gravel, <br />sand and gravel, or synthetic material to prevent soil movement into or through the riprap. It is possible <br />that the in situ materials will create suitable bedding, particularly on the pit slopes since pit run sand and <br />gravel usually makes a good filter blanket. However, for reclamation cost estimating and bonding, the <br />Division must assume that a layer of filter material must be imported to the site and installed prior to <br />riprapping. The Applicant must commit to preparation of a filter compatible bedding prior to placement <br />of any riprap, and must commit to conducting at least one gradation test for each 500 linear feet of <br />riprap installation to detnonstrate and document filter compatibility of the bedding. This is necessary <br />for riprap installations along the riverbank as well as sensitive lake slope areas. <br />Finally, the applicant must commit to placement of riprap that is angular, durable, and non-slaking. <br />Physical testing of the riprap to assure and document that i[ meets the gradation specification should be <br />conducted, and must be conducted if riprap will be buried such that inspection would be precluded. The <br />Division further suggests that slake testing and abrasion testing be conducted prior to importation of <br />riprap to the site to ensure suitability. The Division also encourages the applicant to consider <br />incorporating soil and vegetation or biotechnical slope protection measures into the mitigation designs. <br />Section 2.3.2 of the Urban Drainage and Flood Control guideline, copy attached, provides a discussion <br />and specification for one such method. Also attached to this memo are guidelines for riprap design and <br />installation from the Soil Conservation Service that may be incorporated into [he applicant's Flood <br />Control/Mitiga[ion Plan for the S&H Mine. <br />Riprap Cost Estimate <br />The Applicant's estimated cost for riprap installed at the site is $3.00 per ton. Considering the cost per <br />ton of ordinary riprap at quarries in Jefferson County (considered the nearest source of suitable <br />material), plus the cost of delivery, the cost of bedding, and the cost of installation, the Division has <br />determined that the Applicant's proposed cost is too low for bonding purposes. In addition, given the <br />discussion of the keyways and thickness of riprap layers in the previous section of this memo, the <br />number of tons per linear foot of armoring required must be increased to complete a reliable cost <br />estimate. As will be discussed below, the Division need not bond for riverbank armoring, but must <br />bond for Lakeshore armoring. In the TR, the Applicant commits to concurrent armoring of the pit slopes <br />such that no more than 500 linear feet of finished pit perimeter will require armoring at any time during <br />the life of the mine. However, in a bond forfeiture situation, the Division may have to armor an entire <br />working face that has not yet been mined to the ultimate pit perimeter at the time mining operations <br />cease. For example, the mining plan map (exhibit C-1) included with the application depicts a 3000 <br />foot-plus working face in Sharkey's Lake that may require riprapping if the operation were to cease and <br />default at the point in time illustrated on the map. The Applicant should provide an enforceable permit <br />commitment specifying the maximum length for the working face, over and above the 500 foot <br />commitment for final pit perimeters, and this length plus the 500 feet should be used to determine the <br />bond to be required for pit slope armoring. <br />