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~. • iii iiiiiiiiiiiiiiii L7~ 999 CTL/THOMPSON, INC. CONSULTING GEOTECHNICAL AND MATERIALS ENGINEERS • • • April 27, 1992 Colowyo Coal Company 5731 State Highway 13 Meeker, Colorado 81641 ~..T~~~~~C~ 1AY d 1 199i Attention: Mr. Rich Atkinson Mr. James Kiger Subject: Underdrain Filter Requirements West Pit and Section 16 Pit Excess Spoil Fills Job No. 18,638 Gentlemen: Mined Land Re~l~m~t~~„ ~~..• ~ .. CTUThompson, Inc. was requested to assist Colowyo Coal Company with development of additional evidence to support the field observations that gravity ~ segregation of spoil will result in an acceptable filter system sufficient to protect proposed underdrains below the West Pit and Section 16 spoil fills. The work was performed in response to concerns raised by MLRD (References 1 and 2). To produce the additional evidence, Colowyo constructed a test section using representative materials for both the drain rock and spoil. We observed the test section on March 20 and 31, 1992 to document conditions and obtain samples. This letter presents our findings and opinions ~ of need for a filter layer to protect the underdrain. To summarize, MLRD has decided "sufficient evidence" has not been provided to satisfy the following: ~ 1. Filter requirements -- Rule 4.09.2 (2)(c) 2. Avoiding underdrain contamination -Rule 4.09.2 (3)(c) 3. Inspection requirements for items 1 and 2 above -Rule 4.09.1(11) • INVESTIGATION Our investigation included detailed observation of the test section and various end dumped slopes to document the effects of large lift thickness spoil pile construction. The ~ intent of our investigation was to develop our opinions and sufficient evidence to determine if the test section and large lift thickness spoil piles meet the requirements regarding filters and contamination (MLRD items 1 to 3 above). The items which we addressed include: • 1971 WEST 12TH AVENUE DENVER, COLORADO 80204 (303)825-0777