4. NMCC’s analysis of stability and water movement in the stacks appears in two appendices to their general description of the proposed the filtered tailings process: Appendix F on stack stability and Appendix E on seepage.
The studies are both labeled “pre-feasibility,” meaning that they are preliminary studies showing general conceptualizations where the most complex and difficult problems are avoided by making certain assumptions that will prevent the analyses from getting bogged down. Separating seepage from stability is one of the signs of this simplification, since we now understand that stability in filtered stacks depends, among other things, on water movement or seepage in the stacks.
The stability study does recognize the problem of tailings sliding along the liner. After analysis of failures in the filtered tailings and along the liner interface – it found that “[i]n all cases, the critically low failure surface was a block sliding failure along the liner interface.” Yet, in spite of that finding (that the risk of sliding was greater than the risk of localized collapse), the stability analysis seems to avoid the interface problems (the block sliding problems along the liner) that we would like to understand. See Appendix F, p. 24.
The tailings at the interface is assumed to be drained, that is, unsaturated. Ibid, Section 3.4.3, p. 20. Of course, this assumption results from the decision to separate stability issues from water movement issues. Saturation is completely assumed out of the question.
In addition, the friction at the liner interface is set at infinity, defining an “exclusion zone” which binds the tailings above the liner with the ground beneath it, excluding the possibility of slippage in this model of slippage failure. Ibid, Section 3.4.2, p. 20.
The study, then, does not answer the question I was asked: what keeps the whole thing from sliding downhill? I can’t answer that question and, it seems, NMCC also cannot.
Appendix F, however, does offer useful suggestions. First, it questions whether the expertise is available at Copper Flat Mine to manage the water flows in the tailings. This question recognizes the delicacy and importance of water management which we hypothesized in our imaginings in the first part of this article. Further, WSP, — NMCC’s consultants and authors of this study — recommend a full study of seismic safety for these tailing stacks. It found the data from the subsoil analysis partly lacking, so that its own seismic analysis was incomplete. Also lacking were proper data on the tailings that the mine would be generating. WSP had to hypothesize the physical characteristics of those tailings based on data from the 1980s. It warned that decisions on tailings had to depend directly on the deposited tailings and, therefore, had to be managed on the spot, bringing up again, the issue of competent expert management during operation.
Useful as this study is, it fails to assure us that the mine knows how to prevent a slide in the tailings, how probable a slide might be, and what the consequences of a slide might be. The application for mining does not contain any consideration of a catastrophic event. The reason for these omissions is simply because Appendix F is a preliminary, pre-feasibility study, not intended to address the serious and difficult questions of the project.
5. The same can be said of Appendix E, WSP’s seepage analysis. I mentioned how difficult it is to imagine water flow in unsaturated medium. In saturated medium, such as below the water table in the aquifer, hydrologists use the Darcy’s law, which is just a version of Newton’s laws of motion. It works in a saturated situation because water fills all the voids and water, not being compressible, transmits the pressure head throughout, the water moving through the medium as if through tiny ducts, resisted only by the medium’s overall resistance to flow (its permeability or conductivity).
But water flow in unsaturated medium, which is full of air pockets, does not follow Darcy’s Law. The water movement is not proportional to the pressure head because the air pockets compress and expand. Conductivity is not constant. The pressure inside those air pockets becomes a factor countering or aiding the applied pressure head. Because there are air pockets, the attraction of water to the surfaces of the medium becomes significant (I called this “surface tension” earlier; it’s also referred to as capillary forces; soil hydrologists call it “suction”).
Appendix E, dealing with water movement in the filtered tailings stacks, ignores all this: “The flow has been estimated according to Darcy’s law, which states that the seepage flow in a porous medium is proportional to the permeability of the medium (K), the hydraulic gradient (I) [what I called the pressure head], and the cross-sectional area of the analysis section (A).” (§4.5.1). The authors of this analysis are perfectly aware that Darcy’s law does not apply to the tailings stacks: “The CSTF [Cake Stack Tailings Facility] seepage analysis models considered flow established under saturated conditions (steady state). Seepage analysis under unsaturated flow conditions have not been considered at this stage of design.” (Id.)
The results of this analysis, therefore, are irrelevant to the actual situation being modeled. They give no clue as to how water moves in the tailings stacks and whether there will be saturated conditions within the stack that can cause collapse on the stack face or slippage of the stacks along the inclined liner. In fact, it can not even provide a certainty that there will be any internal water movement at all since it is just all saturated. Seepage turns out simply to mean precipitation percolating through saturated medium and draining out the bottom: “Flow boundary conditions were set at the CSTF surface to represent the net flow percolating through the structure: precipitation minus evaporation .” (Id.)
By focusing exclusively on seepage water as external water that flows through the tailings rather than the movement of water contain in the tailings, the analysis ignores the real movement of water in the stacks. If the tailings were entirely saturated, it probably would not stand, but the assumption of total saturation is required by the preliminary purpose of this study. As a result, it does not help us understand how movement of water in the unsaturated tailings can cause structural failures or help the miners prevent such failures which threaten the lives of workers even in small collapses during operations.
6. If we suppose that these two analyses represent NMCC’s understanding of the structural problems of stability in the tailings stacks, then clearly NMCC, like me, is unable to answer the question of what keeps the whole thing from sliding across Hwy 152, entering the various gulches and creek beds that lead to the Rio Grande valley, especially after mine closure when no one from the mine is around anymore, or when we get a big storm (getting bigger every year, they say) or a seismic event. The stacks are going to be there forever waiting to move.
7. After thinking this problem through, it seems important for me to remind the reader that we are trying to understand the most important aspect of the updated application: its risk to human life. Workers have died from filtered tailings failures. This is not a question of property, money, jobs, but primary safety considerations. The miners must demonstrate to the state (to regulators and to the public) that this operation will be safe. They haven’t done that. Safety depends not just on the miners themselves understanding the dangers but on the miners being able to find experienced, knowledgeable workers that we can trust. The experience and knowledge of filtered tailings processing is hard to come by in a country which has never done this kind of processing before on such a large scale.