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.STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver, Colorado 80203 <br />Phone: (303) 866-}567 <br />FAX: (303) 8328106 <br />DATE: November 21, 2003 <br />TO: Tom Schreiner <br />,FROM: Allen Sorenson .~,~ <br />RE: Excavation Slope Stability and Protection of Yaluab[e Structures, <br />Varra Companies, Inc., Durham Pit, Permit No. M-1978-056 <br />:OLORADO <br />~ I V I s I O N O F <br />AIN SRALS <br />:EOLOGY <br />ECLA MATION•HIrvING <br />EA/ETY•ECIENOE <br />Bill Owens <br />Governor . <br />Greg E, Watcher <br />Executive Din2tor <br />Ronaid W. Cattany <br />Division Director <br />Natual Resource Trustee <br />The structure protection measures proposed for the Durham Pit aze to maintainan excavation slope of <br />no steeper than 1.25H:1 V and to maintain a minimum 10 foot setback from the crest of the excavation <br />slope and any potentially affected structure. The pit will be dewatered and mined dry with the <br />exception of potential pockets of gravel resource that maybe deeper than 40 feet below ground surface, <br />which maybe dredged. Whenever an Operator proposes to mine at a specific excavation slope, the <br />Division requires a description of earth moving methods, survey controls, and management measures <br />that will be applied to assure compliance with the excavation slope requirements. Maintenance of a <br />maximum 1.25H:1 V excavation slope at a working gravel pit is difficult for the following reasons: <br />• Mining from the toe of the slope using a front end loader tends to cause caving of the deposit <br />resulting in excavation slopes much steeper than 1.25H:1 V. <br />• Pushing the deposit down into the pit using a dozer is not safe on slopes steeper than 2H:1 V, so <br />this mining method could not be used to leave excavation slopes of 1.25H:1 V. <br />• Raking the deposit using an excavator or backhoe can be used fo maintain 1.25:1 excavation <br />slopes, but balancing the need to maximize productivity through heaped bucket bulk excavation <br />while at the same time operating the heavy machinery to maintain a required slope angle greater <br />than the angle of repose is a conflict for the equipment operator and would require frequent <br />survey control. <br />• Excavation to a 1.25H:1 V slope can also be accomplished using scrapers operating parallel to <br />the slope and leaving a benched pit wall with an overall slope no greater than 1.25H:1 V. <br />However, the same conflicts beh~een maximizing production and maintaining a designated <br />excavation slope angle exist, as does the need for frequent survey control. You reported that <br />during your recent inspection the deposit was being mined using scrapers, and that although no <br />slope angle measurements were taken during the inspection, it was your observation that the <br />excavation slope was being mined at angles substantially steeper than 1.25H:1 V. <br />• Management must instruct heavy equipment operators in the pit that the designated required <br />excavation slope angle cannot be exceeded. Management must provide the survey controls <br />necessary to allow the equipment operators to easily visualize and excavate to the required <br />angle, and management must emphasize the importance of the maximum excavation angle and <br />enforce the requirement with the front line employees or excavation contractors in the pit. <br />Office of Office of Colorado <br />Mined Land Reclamation Active and Inactive Mines Geological Survey <br />