FIGURE 6-15. AOMON CRYPT JUST BEFORE BACKFILLING. All excavation completed, the pool is ready for backfill. (May 1979) 6.8.3 Excision When all the necessary equipment and materials were assembled, Holmes & Narver, Inc., the base support contractor, drove the sheetpile and excavated the soil and debris from the enclosed area. Approximately 10,600 cubie yards of soil and debris were removed from the Crypt area between 22 January 1979 and 30 April 1979. Excavation was halted when the sheetpile started to cave in along one side. The average depth of excavation was about 20 feet. During the course of excavation, it was observed that a fine grey-black, rubbery material would drain with the water from a pile of freshly excavated soil. Samples of the rubbery material were found to contain higher levels of TRU activity than the soil from which it drained. When all cells within the sheetpile area showed, by bottom sediment sampling, TRU concentrations less than 400 pCi/g, the pond (Figure 6-15) was allowed to stand undisturbed for several days. Then a "blanket" of cement mixed with soil was carefully placed on the bottom in an attempt to lock in any of the rubbery material which might have settled there. 182