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• <br />APPENDIX D <br />March 2000 -2- 993-2099.150 <br />Based on field investigations conducted within the Phase IV area and observations made <br />during the remediation of the Phase III VLF, the mined excavations on the working levels <br />generally followed the mineralized veins, and were typically less than 12 feet wide. Mined <br />stopes were typically less than 10 to 12 feet wide, although the mining plans also indicate <br />locally wider stopes in some areas. The ore appears to have been extracted by various <br />forms of open stoping or shrinkage mining methods. Mining techniques employed at that <br />time rarely incorporated backfilling of the stopes and there is no evidence that any of the <br />excavations in the Phase IV area were ever backfilled with development rock or other <br />backfill material. The historical mine plan maps do indicate that some of the excavations <br />had collapsed and caved. <br />2.2 Cent Conditions <br />Surface disturbances from mining are present throughout the Phase IV area. These <br />disturbances generally consist of open shafts or mines. In addition, some of the features in <br />the Phase IV area are tunnel portals that provided access to deeper underground workings. <br />Most of the disturbances in the Phase IV area do not show any evidence of subsidence, <br />caving, or collapse from the impact of underground mining. Northeast of Phase IV, the <br />area was mined or collapsed through to the surface over a long narrow area. These <br />workings are related to the Thompson, Elkton, Katherine, and Tornado shafts and are <br />located sufficiently outside the Phase IV area as to not to have any impact on the stability <br />of the foundation. The only area beneath the Phase IV facility where the collapse of <br />underground workings is evident is immediately adjacent to the Ophir Incline shown in <br />Drawing No. 1. <br />The surficial soils within the Phase IV area range up to approximately 15-feet-thick over <br />most of the area, although locally the thickness has been shown to be up to 50 feet in some <br />C7 <br />of the drainages. As discussed in more detail later, this additional overburden thickness in <br />IA991 MFINAL REP0171 MAPP MM Golder Assoclates