Laserfiche WebLink
<br />M-2000-158 TR Memo 4 May 17, 2001 <br />acceptable. Figure D-5 of the Division's guidance suggests that the base of the pit slope riprap should be keyed in <br />to at least three feet below the river invert elevation. The Applicant will need to either commit to following the <br />design specifications provided in the Division's attached guidance document or provide a technicaljustification <br />for their proposal. The Division's specification for the key depth results in little practical difference in the base <br />elevation of the pit slope riprap in this instance. This is because the invert of the river appears to be about 12 feet <br />below the original ground surface in the area to be mined. The riverbank armor installation plan depicted in the <br />TR shows riprap ending at two feet below the rivers normal water level. The Division's guidance recommends <br />that riverbank armor be keyed five feet below the river bed or, alternatively, extension of a thicker (doubled) layer <br />of riprap into the channel bed (DMG, 1998, Figures D-5 and D-8, copies attached). The Applicant will need to <br />commit to one of these toe protection options or provide technical justification for their proposed design. <br />Another pertinent concern relative to the riprap designs is the bedding for riprap installation. The bedding must <br />be filter compatible with the riprap. This bedding is typically a filter blanket of gravel, sand and gravel, or <br />synthetic material to prevent soil movement into or through the riprap. It is possible that the in situ materials will <br />create suitable bedding, particularly on the pit slopes since pit run sand and gravel usually makes a good filter <br />blanket. However, for reclamation cost estimating and bonding, the Division must assume that a layer of filter <br />material must be imported to the site and installed prior to riprapping. The Applicant must commit to preparation <br />of a filter compatible bedding prior to placement of any riprap, and must commit to conducting at least one <br />gradation test for each 500 linear feet of riprap installation to demonstrate and document filter compatibility of the <br />bedding. This is necessary 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. Physical <br />testing of the riprap to assure and document that it meets the gradation specification should be conducted, and <br />must be conducted if riprap will be buried such that inspection would be precluded. The Division further suggests <br />that slake testing and abrasion testing be conducted prior to importation of riprap to the site to ensure suitability. <br />The Division also encourages the applicant to consider incorporating soil and vegetation or biotechnical slope <br />protection measures into the mitigation designs. Section 2.3.2 of the Urban Drainage and Flood Control <br />guideline, copy attached, provides a discussion and specification for one such method. Also attached to this <br />memo are guidelines for riprap design and installation from the Soil Conservation Service that may be <br />incorporated into the applicant's Flood Con[rol/Mitigation 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 ton of <br />ordinary riprap at quarries in Jefferson County (considered the nearest source of suitable material), plus the cost of <br />delivery, the cost of bedding, and the cost of installation, [he Division has determined that the Applicant's <br />proposed cost is too low for bonding purposes. In addition, given the discussion of the keyways and thickness of <br />riprap layers in the previous section of this memo, the number of tons per linear foot of armoring required must be <br />increased to complete a reliable cost estimate. As will be discussed below, the Division need not bond for <br />riverbank armoring, but must bond for lakeshore armoring. In the TR, the Applicant commits to concurrent <br />armoring of the pit slopes such that no more than 500 linear feet of finished pit perimeter will require armoring at <br />any time during the life of the mine. However, in a bond forfeiture situation, the Division may have to armor an <br />entire working face that has not ye[ been mined to the ultimate pit perimeter at the time mining operations cease. <br />For example, the mining plan map (exhibit C-I) included with the application depicts a 3000 foot-plus working <br />face in Sharkey's Lake that may require riprapping if [he operation were to cease and default at [he point in time <br />illustrated on the map. The Applicant should provide an enforceable permit commitment specifying the <br />