2014-03-03_REVISION - M1988112 - Laserfiche WebLink

Home Browse Search

5.0 POTENTIAL FAILURE MODES This section documents the findings of a Potential Failure Mode Analysis (PFMA) that was conducted to support an evaluation of the current condition of San Luis Tailing Dam. PFMA is a systematic, proactive method for evaluating a dam to identify where and how it might fail (potential failure mode), and assessing the likelihood of the failure mode occurring and the effects (consequences) of such failure. The goal is to identify the parts of the dam that are most at risk of failure and may be in need of upgrade, modification, or monitoring. The PFMA considered a range of credible potential failure modes associated with normal operating (static) loading conditions, extreme flooding and earthquake load conditions. For purposes of this analysis, it is assumed that the structural modifications recommended in Section 6.2 of this report will be implemented. These modifications include raising the berm on the South Diversion Ditch and placing additional erosion protection adjacent to the drop structure in the anticipated overflow area. These improvements are intended to insure that all storm water collected by the South Diversion Ditch system up to a 100 -year event is diverted and will not enter the impoundment area, consistent with the original operational concept of the permitted facility. 5.1 GENERAL DESCRIPTION DAM AND LOADING CONDITIONS 5.1.1 Dam Data (elevations in NAVD 88) • Dam purpose: permanent mine tailing storage, and storage and evaporation of water (pumped groundwater and brine) that is delivered via pipeline to the impoundment from the nearby inactive mine site • Dam type: compacted granular (sand, silt and gravel) earth embankment; • upstream zone (Type 1 in design documentation) is described as a pervious granular zone designed to be relatively pervious to facilitate drainage of impounded tailing; • downstream zone (Type 4) is described as general fill up to 6 -inch maximum particle size derived from on -site materials; • an upstream "filter blanket" zone (Type 2) may have up to 35% passing No. 200 sieve (Phase II, Raise 1 Expansion, Final Construction Report, 1994, Section 5.4.1) — the design intent of this zone is "to prevent movement of tailing material into the granular embankment fill" (112 Permit Application Amendment, 1989, Section D.6.5.4.1). • Dam height: 155 feet • Dam length: 1,900 feet • Dam crest width: 25 feet • Dam crest elevation: 6,821.8 feet San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 34 February 2014