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Memo to Larry Oehler August 6, 1993 convey the concerns of the Division regarding the potential for deformation o£ the embankment under strong shaking. The displacement analyses presented in the SRK design report are acceptable for the failure surfaces depicted in Figure 4.1 of the report. The concern I was attempting to convey, and which still needs to be addressed, is for failure that may result due to the inability of liquified tailing material to support a load. The proposed upstream raise is founded largely on tailing. Cone penetrometer testing of tailing has indicated that saturated zones may exist. Liquefaction of these saturated zones would result in a loss of supporting capacity and may result in settle~ent of that portion of the dam founded on tailing material. the type of failure that may result, and which has not been a dressed in operator submittals to date, is depicted in the purely iagrammatic illustration attached. Finite element analyses have been used to calculate embankment deformations on the basis of gravity loads and softened material properties (Lee and Roth, 1977), andl to predict the affect of slimes liquefaction on embankment stability (Lo et al., 1982). Liquefaction of tailing sands as it relate8 to dynamic response analysis is discussed in a paper presented by Sinn (1980). In the absence of finite element analyses to properly assess the failure mechanism that may be caused by the inability of liquified tailing to bear the load of the upstream raise, the operator may commit to providing for additional free-board which will be maintained, over and above that which is provided for storm water inflows and wave run-up, to prevent loss of containment in the case of lowered crest elevation resulting from settlement. Four feet of additional free-board would appear to be appropriate. AMPLIFIED GROUND MOTIONS The SRK memorandum dated July 28, 1993, concludes that amplified ground motions observed at Mexico City resulting front the 1985 Michoacan Earthquake have no correlation to the setting created at the San Luis tailing pond. This conclusion is drawn £rom the fact that, in the vicinity of the dam, the height of the tailing column at San Luis is 25 feet (as compared to columns of soiY at Mexico City that were 75 to 130 feet thick), and that the tailing materials are non-saturated and cohesionless. The conclusion is also predicated on the notion that amplification recor ed at some Mexico City sites involved low strain, low frequency'gro~nd motions resulting from a great earthquake (magnitude 8.1) occyrrinq some distance from Mexico City (approximately 190 miles), whereas, the design earthquake selected for the San Luis facility is for a nearby seismic source with a Maximum Credible Earthquake (MCE) of less than magnitude 8. The assertion is also made that liquefaction within the tailing would limit propagation of bedrock ground motions upward through the tails. The Division feels that the lessons learned from recording o£