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_= iii <br />CTLITHOMPSON, INC. <br />CONSULTING GEOTECHNICAL AND MATERIALS ENGINEERS <br />iuiuiiiiiu iii <br />April 27, 1992 <br />Colowyo Coal Company <br />5731 State Highway 13 <br />Meeker, Colorado 81641 <br /> <br />tAY l~ 1 199 <br />Attention: Mr. Rich Atkinson <br />Mr. James Kiger <br />Subject: Underdrain Filter Requirements <br />West Pit and Section 16 <br />Pit Excess Spoil Fills <br />Job No. 18,638 <br />Gentlemen: <br />Mined Land <br />tie~l~.~..,,~.,., .,:..• <br />CTUThompson, Inc. was requested to assist Colowyo Coal Company with <br />development of additional evidence to support the field observations that gravity <br />segregation of spoil will result in an acceptable filter system sufficient to protect proposed <br />underdrains below the West Pit and Section 16 spoil fills. The work was performed in <br />response to concerns raised by MLRD (References 1 and 2). To produce the additional <br />evidence, Colowyo constructed a test section using representative materials for both the <br />drain rock and spoil. We observed the test section on March 20 and 31, 1992 to <br />document conditions and obtain samples. This letter presents our findings and opinions <br />of need for a filter layer to protect the underdrain. <br />To summarize, MLRD has decided "sufficient evidence" has not been provided to <br />satisfy the following: <br />Filter requirements -- Rule 4.09.2 (2)(c) <br />2. Avoiding underdrain contamination -Rule 4.09.2 (3)(c) <br />3. Inspection requirements for items 1 and 2 above -Rule 4.09.1(11) <br />INVESTIGATION <br />Our investigation included detailed observation of the test section and various end <br />dumped slopes to document the effects of large lift thickness spoil pile construction. The <br />intent of our investigation was to develop our opinions and sufficient evidence to <br />determine if the test section and large lift thickness spoil piles meet the requirements <br />regarding filters and contamination (MLRD items 1 to 3 above). The items which we <br />addressed include: <br />:9-: .:25T i_--... c.._' -ENVER. CC;~JR.;D:; 3:20= 130319-o~J:7: <br />