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Daniel Hernandez April 21, 2006 Page 4 The GEI report recognizes the value of installing drains in the landslide and proposes a shallow interceptor drain just uphill of the left abutment on the landslide. As discussed in the submittal document and extensively in the file history, landslide movement has historically occurred during unusually wet periods and in response to ground saturation. While the stability berm and buttress were sized assuming high pore water pressures, we feel the GEI report underestimates the importance of drainage. At a minimum, the final design should incorporate the proposed interceptor drain and blanket drain as featwes for drainage of the left abutment. MCC is strongly encouraged to explore the use of deeper drainage features such as horizontal or directionally drilled drains back into the landslide. The GEI report, as does our office, recognizes the current configwation of the outlet works through Monument Dam is a dam safety concern. Of primary concern is the downstream location of the outlet pipe control valve causing pressurized conditions in the embankment and the potential for leakage and piping azound the conduit. Potential impacts to the outlet pipe and valve due to mine-induced seismic events and/or surface subsidence are not specifically addressed in the GEI report. The proposed preventive measures significantly reduce, but do not eliminate, the potential for differential settlement ofthe embankment dam and separation or cracking of the outlet pipe. MCC should include design of an upstream guard gate at the reservoir outlet structure as a part of the preventive measwes in order to remove the pressurized pipe condition and reduce the risk of catastrophic failure of the dam. Construction ofthe stability berm and buttress will require extension of the outlet pipe in the downstream direction. MCC notes the possibility of sliplining the pipe which would reduce the effective inside diameter of the outlet pipe. If this solution is contemplated, further analysis will need to be conducted to show that the capacity of the pipe can meet historic irrigation delivery needs and that the reservoir can be drawn down at an acceptable rate during emergency situations. Conclusions and Recommendations Conceptually, and with the current estimated soil strength parameters, the proposed preventive measures satisfactorily improve the factors of safety of the dam and landslide to withstand anticipated mine-induced seismic events. Ow office recognizes the efforts of MCC and feels that the proposed long-wall coal mining could occur as long as the above comments are sufficiently addressed and the proposed preventive measures are constructed. It is the opinion of this office that any form of mining with the potential to produce seismic activity in the proposed area should not occur until construction and implementation ofthese mitigating structures and activities has occurred. Construction of the stabilizing berm and buttress constitutes a modification to the dam and falls under Rule 6 of the State of Colorado's Rules and Regulations for Dam Safety and Dam Construction. Consequently, an application package with the required design reports and plans and specifications prepazed by a Professional Engineer registered and licensed in Colorado must be submitted to the State Engineer's Office for approval. While we recognize the design is in a conceptual phase, several items should be considered in the final design, including but not limited to the following: