1997-07-28_GENERAL DOCUMENTS - M1973021 (9)

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' feature could exist which, if mobilized, could allow for a relatively large-scale, albeit localized, slope failure. Review of the rock exposures prior to 1996 did not reveal any such features of concern. The anticipated most likely problem feature was a shallow dipping joint or shear which if daylighted and could form an unstable zone in the East Highwall slope. Such a feature was encountered during excavation of a part of the fourth bench of the East Highwall during 1996 resulting in the occurrence of a localized slope failure that i encompassed the second and third benches and part of the fourth ■ highwall near the south end of the East Highwall. An almost instantaneous slope failure occurred during excavation of the fourth bench of the East Highwall during late January 1996. It was initially apparent that failure was occurring along a clay shear feature that appeared to occur along dipping foliation and which daylighted about mid-highwall level in the fourth highwall. Creep slope movement was evident along this discontinuity for a period of 1 time thereafter until the repair was made. The failure was sudden in that creep movement was evident in the field within hours of having removed material that had before been supporting the rock mass ' above the shear feature, but the movements were never sudden or catastrophic. The clay shear feature is probably tectonic in origin and represents conditions not seen to this extent or with the resulting impact before. No such features were assumed in the working ' geologic model used for previous stability analyses and design. The failure was investigated by using new aerial photography, topographic base maps and geologic mapping; drilling rock core borings and assigning laboratory testing of rock core and clay samples ' from the discontinuity discussed above, and monitoring continued slope movements via the temporary inclinometer and by optical survey. The results of these investigations are summarized in the following paragraphs. ' 2. Geologic Mapping ' Geologic mapping was performed for the four uppermost highwalls in the East Highwall of the Central Quarry to document conditions for these four permanent highwalls at the final cut slope and to collect ' data around the 1996 slide area near the south end of the East Highwall. Data collected are shown on the geologic plan map in Appendix A. A summary of prevailing bedrock structure and character was provided above for the East Highwall in general. This section is intended to emphasize mapped features that are considered to be significant relative to the slide failure area. ' 10 G:\PROJECIS\20200\002\96ANNRPr.WPD •® a