2017-02-21_REVISION - M1988112 (3)

Home Browse Search

The tailing dam does not have a spillway or outlet works. Storm water is diverted around the perimeter of the impoundment via ditches and berms. The storm water run-on diversion system was originally designed to handle a 100 -year storm event. Most of the watershed is captured and diverted from the south side of the facility along a primary ditch referred to as the South Diversion Ditch. Flows in the South Diversion Ditch are routed over the left (south) abutment of the dam via a concrete and 48 -inch diameter corrugated metal pipe (CMP) structure that is referred to as the Drop Structure. For storms larger (more rare) than a 100 -year event, up to and including a probable maximum flood (PMF) event on the contributing watershed, the operational design envisioned that the perimeter diversion ditches and berms would spill over and allow excess water to enter the impoundment. There is sufficient volume available behind the dam and on top of the tailing deposits to completely contain the entire PMF storm runoff without overtopping the dam. In 2013 at the request of Battle Mountain Resources, Inc. (BMRI), Miller Geotechnical Consultants (MGC) conducted a comprehensive dam safety inspection and review of the tailing dam. The primary goals of the dam safety study were to evaluate the following: 1. Structural stability of the dam. 2. Current condition of the dam. 3. Anticipated hydraulic performance of the storm -water diversion systems during major hydrologic events. Based on site inspections conducted in 2013, review of available historic construction and design records for the facility, and additional engineering analyses performed as part of the comprehensive dam safety evaluation, the San Luis Tailing Facility dam was considered to be in good condition and the embankment was determined to be structurally stable in accordance with accepted engineering standards (factors of safety) under static and earthquake loading. The principal dam safety concern identified by the 2013 evaluation was related to uncertainties about the performance of the facility's storm water diversion system under rare major to extreme flood events, in excess of a 100 -year recurrence interval event. In the event of major floods approaching an extreme probable maximum flood (PMF), analyses presented in the Detailed Inspection Report (MGC, 2014) indicated that there is some risk of damage, uncontrolled erosion and scour along the South Diversion Ditch system, specifically due to predicted overtopping at the existing Drop Structure. The risk posed to the dam during extreme flood events is associated with the potential for deep scour erosion of the left dam abutment at the Drop Structure, which could potentially result in loss of the Drop Structure, head -cutting and erosion of the channel alongside the dam embankment. This specific condition was evaluated as a potential failure mode and is described in the 2014 report. The estimated range of probabilities and risk associated with this potential failure mode extends slightly above tolerable risk guidelines, but only for extreme flood events on the order of an extremely rare PMF. For lesser (more frequent) flood events, risks are below tolerable risk guidelines. In light of these findings, BMRI directed MGC to carry out additional hydrologic and engineering studies and to recommend appropriate upgrades to the South Diversion Ditch and Drop Structure to reduce risk to the dam under extreme flood events. -- - ----- ------------------------------------ - ----- -- - --- - - - - San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 2