REV89474 - Laserfiche WebLink

Home Browse Search

.. Memo to Berhan Reffelew Carlton Tailings Lift Thickness - Supplemental page 2 More normally, tailings are assumed to be cohesionless when evaluating the conceptual stability of tailings structures. While I believe the evaluation performed by Golder models one possible scenario of concern, the "fast loading", I do not believe it is sufficient to alleviate my concern. I suggest it would be appropriate to also analyze the short term fill configuration assuming a more traditional tailings strength assumption of zero cohesion and a sensitivity range of conservative p values, such as a range from 20° through the original laboratory value of 38°. This sensitivity style analysis will provide additional insight into the probable short term stability of tailings placed at lower densities in unsaturated/drained conditions. Had undrained shear tests been performed on the materials we would have specific strength parameters for the evaluation. Long Term Scenario Apparently, I did not adequately communicate my longer term stability concern to CC&VG and Golder. I continue to be concerned that placing a low strength foundational layer of relocated Carlton Mill Tailings beneath the Arequa Gulch excess overburden fill and adjoining structure could facilitate a large scale rotational or glide failure, even though the relocated highway embankment will buttress the toe. Such a rotational failure could override the highway embankment with a large scale compression ridge or material flow, obliterating the surface and underdrain hydrology controls and conceivably subjecting the public to a health and safety risk within the relocated right-of-way. In order to evaluate this scenario, Golder should repeat the original permit application stability analysis for the gross cross-section transecting the thalweg of Arequa Gulch and the adjacent and superimposed excess overburden structure. Lacking specific Carlton Tailings material properties for lower densities, I believe a similar sensitivity style evaluation to be appropriate. Summary Comments If this more thorough short term and long term sensitivity style stability analysis demonstrates the relocated Carlton Mill Tailings structure to be stable, I will then recommend approval of CC&VG's revision application. As a word of caution I must note, since CC&VG continues to place the material in noncompliant lift thickness, they are risking a costly reexcavation, recompaction and replacement remedy. Finally, I also suspect that the placement of material in low density condition, even if intrisically stable, may result in operational problems when CC&VG attempts to place the superimposed excess overburden material later.