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STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />1313 Sherman SL, Room 215 <br />Denver, Colorado 80203 <br />Phone: (303) 866-3567 <br />FAX: (303)832-8106 <br />July 9, 2002 <br />DIVISION OF <br />MINERALS <br />GEOLOGY <br />RECLAMATION <br />MINING•SAFETY <br />Les Ewegen Bill Owens <br />124'h Estates Partners cnvernnr <br />th Greg E Wal<her <br />4545 East 60 AVe, Sulte 202 Executive Direcor <br />Commerce City, CO 80022 Ronald W. Cottony <br />Acting Division Director <br />RE: Clay Liner Design and Relationship to the Amount of Reclamation Bond, <br />Setbacks to Offsite Structures, l24` Avenue Estates, File No. M-2001-085 <br />The Division of Minerals and Geology (DMG) has completed a review of the Ground Engineering <br />Consultants report dated May 31, 2002 (Ground report). With submittal of the report, 124th Avenue Estates <br />Partners requests a reduction in the amount of reclamation bond required for permit issuance and requests a <br />reduction in the excavation setbacks from valuable manmade structures on surrounding properties. <br />Item No. 1-Setbacks to Valuable Structures <br />The Applicant has proposed a 10-foot setback from the Fulton Ditch easement line, and a 25-foot setback <br />from the U.S. Highway 85 right-of-way. The setbacks proposed for the north and south pit perimeters are 25 <br />feet from the property lines and existing fences. At the point of intersection with cross section A-A <br />"Proposed Water Storage Plan" map dated June 18, 2002 and prepared by Jehn and Associates, the edge of <br />the Fulton Ditch is between 40 and 50 feet from the proposed edge of the excavation. The depth of <br />excavation along the west side of the pit parallel to the Fulton Ditch is shown on the "Proposed Water <br />Storage Plan" map to be 46 feet. <br />Historically, the DMG has rarely approved setbacks as tight as those proposed for this project. In support of <br />the proposed setbacks, the Applicant has provided quantitative slope stability analyses included in the <br />Ground report. The clay stone bedrock component of the slope cross sections that were modeled in the <br />stability analyses is assumed to be a high shear strength material with cohesion of 1,000 pounds per square <br />foot. By setting the strength of the bedrock so high, only very high safety factors will result for any modeled <br />failure surfaces that would pass, even slightly, through the bedrock zone in the cross sections analyzed. The <br />Applicant recognizes this affect on page 5 of the Ground report where it is stated that "bedrock parameters <br />do not affect the results strongly." Given that the upper few feet of the clay stone bedrock is "highly <br />weathered locally" (page 4 of the Ground report), the DMG does not agree that the strength of the bedrock is <br />not going to be a factor in the stability of the excavation back cut prior to placement of the clay liner. In fact. <br />the geotechnical literature describes cases where the residual strength of weathered clay stone can exhibit <br />zero cohesive strength. However, the DMG is in agreement that the slopes proposed in the Ground report <br />will be sufficiently stable to protect stmctures, if in addition to the 10 foot setback to the Fulton Ditch <br />easement line, the Applicant will further commit to a minimum 40 foot setback from the top edge of the <br />excavation back cut to the east edge of the top of the bank of the Fulton Ditch. This requirement should not <br />impose much, if any hardship on the Applicant depending on how close the Fulton Ditch encroaches on the <br />easement line over the reach in question. <br />