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z-1 2.0 DAM INVESTIGATIONS AND RESULTS The existing condition of the lower tailings dam was defined by informa- tion obtained from (a) construction and post construction records, (b) investigations conducted by others, (c) previous investigations concern- ing seepage conducted by D'Appolonia and (d) additional investigations conducted by D'Appolonia specifically for the purpose of embankment evaluation. A brief discussion of the available information concerning the dam history and previous investigations is presented in this chapter. The need for and results of additional investigations are also discussed. The description of the existing dam conditions based on the investigations and observations made, is evaluated throughout the discussions. 2.1 DAM HISTORY AND PREVIOUS INVESTIGATIONS The lower tailings dam is located in a natural drainage as shown on Figure 1. The dam is an earthfill embankment, approximately 1,200 feet long with a maximum height of about 100 feet and crest elevation of about 8962 feet (MSL). The upstream slope is 3H:1V, the downstream slope is 2H:IV, and the crest width is 28 feet. IThe major historical aspects of the dam are as follows: o Construction of the dam in 1974 based on plans prepared by Davis Engineering Service, Inc., with geotechnical investigations and design recommen- dations by Lincoln-Devore Testing Laboratories. o Investigations and slope stability analyses in 1975 by Francis-Meador-Gellhaus, Inc. and Dames T and Moore in response to concerns about the stability of the embankment. o Construction of a seepage collection and pump - back system in 1978 by Homestake. o Investigation of the seepage problem by Enviro- logic Systems, Inc., in 1979 resulting in the installation of eight relief/collection wells in February 1980 to collect seepage in the toe area.