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• <br />CTL/THOMPSON, INC. <br />CONSULTING GEOTECHNICAL AND MATERIALS ENGINEERS <br />July 10, 1980 <br />Mr. Gary Meyers <br />Kerr Coal Company <br />Box G <br />Steamboat Springs, Colorado 80477 <br />Subject: Physical Properties of Overburden <br />Kerr Coal Mine <br />Walden, Colorado <br />Job No. 6396 <br />Dear Mr. Meyers: <br />As requested, we have reviewed data in our files regarding the Kerr Coal <br />Mine. The purpose of the review was to provide a concise summary of <br />the typical physical properties of overburden materials typically encoun- <br />tered in the existing mine. <br />Over the past several years, we have been at the site to analyze the sta- <br />bility of the pit walls for the existing mine, to investigate feasibility <br />of paving the County road extending from the mine to Walden, and at several <br />times have made trips to the site to make density tests of the material <br />backfilled into the pit to help evaluate the relationship between in-place <br />density and density after the material has been excavated, and replaced <br />as backfill in the pit. During the course of these various investigations <br />we have made standard and modified Proctor compaction tests to determine <br />density obtained for a given laboratory effort, performed field density <br />tests to evaluate the field placement procedures, and performed typical pro- <br />perty tests such as determination of in-place density, specific gravity, <br />Atterberg limits, and performed several types of strength tests on the ma- <br />terials both intact and disturbed. This report presents, in summary form, <br />data developed in the previous studies. <br />When the overburden is excavated three general types of materials can be <br />identified in the field on the basis of color and texture. A significant <br />portion of the materials excavated consists of gray sandstone. The sand- <br />stone in some areas is highly cemented and some samples obtained at depth <br />have unconfined strengths in the range of 5,000 psi. The second common ma- <br />terial is a gray to blue-gray shale. Intact pieces of this material have <br />an unconfined compressive strength ranging from 500 to 1,000 psi. There <br /> <br />1701 SOUTH FEDERAL BLVD. SUITE D DENVER, COLORADO 80219 (303) 934-5473 <br />