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STATE OF COLORADO <br />COLORADO GEOLOGICAL SURVEY- serving the people of Colorado C~L~~.AD~ <br />Department of Natural Resources <br />1313 Sherman Street, Room 715 ~~~~'~ E~ <br />Denver, CO 80203 <br />Phone: (303) 866-2611 <br />Fax: (303) 866-2461 <br />pUG ~ g ZOO1Dn, DEPARTMENT OF <br />Memorandum Division of ~ecSa ety NATURAL <br />Nlinin9 and ~S~URCES <br />To: Ms. Janet Binns, DRMS Bill Rider, Jr. <br />Governor <br />From: TC Wait, CGS -x3518 <br />Subject: Trapper Mine: G-Pit Landslide Stability Report- Task 2, March 2007 Execut°a D'Qeclor <br />Date: 15 August, 2007 vlncent Matthews <br />Division Director and <br />State Geologist <br />Thank you for forwarding a copy of the report to CGS, which I have reviewed. Per our conversation earlier <br />this week, this memo is intended to provide comments on the March 2007 landslide stability report by <br />Agapito Associates that pertain to the geology and geotechnical aspects of the investigation. <br />1) As stated in the report, the report contains preliminary conclusions based on geologic and <br />geotechnical information that was available prior to the landslide and contains no new data that has <br />been gathered since the event. <br />2) CGS finds the report to be thorough and accurate regarding the geologic conditions leading to the <br />landslide and agrees with preliminary conclusions presented, based on the limited data available at <br />the time of the report. <br />3) The modeling method used, FLAC3D, is a method that is widely used in rock mechanics and <br />mining applications. As with any modeling program, errors do occur and analysis depends on the <br />accuracy of data that is available. Agapito identified these occurrences in their report. <br />4) There is apparently little data on the properties, density, etc of the spoils pile that was used to infill <br />the strike line pit along the toe of the slide. It may be prudent in the future for spoils to be placed in <br />amore controlled, structural manner, particularly in regards to slopes and potential failures. This <br />may also help with any future land use in that there will be documentation on these fills and it will <br />be less likely for them to be considered "uncontrolled fill" and have to be removed and replaced. <br />5) The modeling sequences that were evaluated in the report include the assumption that <br />precipitation (or increased pore water pressure) was the trigger for the Gs-6 cut failure. While this <br />is a reasonable assumption, data should be provided as to whether this assumption is valid or if <br />other factors may have contributed to the failure fie blasting). If precipitation was the trigger for the <br />slide, a discussion of how that precipitation affected the down-dip portions of the identified slip <br />plane is necessary. Did the precipitation get down into the clay in the middle and lower portions of <br />the slide, and if so, how? <br />6) The scanned figures were difficult to read, so I could not determine where specific borings, <br />inclinometers, or survey markers were located. <br />7) Agapito offers some good questions and discussion points in the report. Further investigation into <br />the slide and the geologic conditions should address these topics. <br />Please don't hesitate to contact me with any questions or concerns you may have. I am looking forward to <br />additional information and analysis on this landslide and will be happy to offer comments as you deem <br />appropriate. <br />