1998-01-10_REVISION - M1988112 - Laserfiche WebLink

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1 BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT CMLRD ADEQUACY LETTER RESPONSES Page 16 of 28 In terms of the limit equilibrium principles employed in pseudostatic analyses, if a seismic event of sufficient magnitude to cause the factor of safety to drop below unity were to occur, this signifies that some failure will occur. No quantification of the degree of failure is produced, under actual seismic excitation, the direction of the induced forces vary rapidly creating short periods when the structure is unstable followed by periods when the forces reverse, thereby increasing stability. Depending upon the duration of the unstable condition a section of the embankment will slide along the failure surface and will come to rest when the new mean stresses along the failure surface do not exceed the available strength. The amount of relative displacement of the failed mass wi'Il depend on the duration for which the stresses exceed the available strength. Under static conditions the stresses which cause failure will exist until large displacements change the geometry of the structure thereby changing the stress conditions. Under earthquake conditions the stresses causing displacement will exist only for a short time and the factor of safety after the earthquake will be the same as that prior to the earthquake provided that the displacements are not sufficiently large to alter the configuration of the embankment considerably. Considering the slopes and materials of the San Luis tailings embankment, a seismic event of sufficient magnitude to result in a displacement large enough to cause loss of the operational freeboard ' (5 feet minimum) and subsequent escape of tailings or solution is not predicted to occur. As indicated by the sensitivity analyses, a pseudostatic coefficient of 0.259 does not cause the factor of ' safety to drop below unity when utilizing conservative embankment shear strength parameters. A pseudostatic coefficient of 0.259 roughly corresponds to a peak ground acceleration of Oi.6g or that equivalent to the MCE for the project area. f i Should seismic induced displacements and slope adjustments cause the pipe network beneath the embankment to become inoperable, the drainage blanket has sufficient capacity to pass all drainage from the tailings facility. The embankment will remain unsaturated. Should minor displacement of the embankment occur during operation, the failure is indicated to occur along the synthetic liner to embankment interface. Damage to the synthetic liner may result, however, the compacted subsoil can be anticipated to retain its integrity. Repair of the liner and rehabilitation of the embankment slopes could be performed without significantly impacting continued operation of the facility. The rate of seepage and its predicted quality are such that little impact on groundwater would exist in the unlikely event that seismic induced failure occurred (see response to Question 11). IJ