410 Y RADIOLOGICAL CLEANUP OF ENEWETAK ATOLL based on construction considerations and not on environmental geological, or radiological considerations. Up to that height, the cap structure would remain basically a series of slabs with no vertical walls. i.e., a paving project which could be accomplished with a minimum of design, equipment, and skill. POD estimated the volume of Cactus Crater up to a height of 3 feet above the reef, where tremie operations would no 5 e longer be required, as 51,917 cubic yards.2! POD estimated thatthis would contain 29,870 cubic yards of loose contaminated soil mixed in a Slurry, Additional soil would be placed on top ofthe slurry to which cement woulg be mixed by a disc-harrow, water would be applied, and the mixture compacted. Depending on the amount of material added in this sojj. cement operation, the volume of the dome would be increased,first by increasing the diameter of the keywall up to 430 feet and the domeheight up to 30 feet, then by extending the domeinlandas far as necessary. The POD design would accommodate up to the 200,000-cubic-yard worst case identified by Field Command and could accommodate more by simply increasing the extension (Figures 8-6, 8-7).22 The POD designers assumedthat sufficient contaminated soil would be stockpiled before the tremie operation began so that it could proceed efficiently and that, by the time the tremie operation was completed, an estimate of the contaminated soil remaining could be made accurately enoughthat the shape andsize of the domecould be determined.?3 [ To prevent scouring and undermining of the container by waveaction, POD designed a keywall to be constructed completely around the | ‘ contaminated material. The keywall was to be keyed | foot into the reef ‘ bedrock, where firm reef existed, and embedded to a depth of 8 feet below the top of the adjacent surface in areas where the reef was fractured or where no reef existed (Figure 8-8).24 The keywall would not prevent migration of fine material from the crater bottom through ground water action in the fractured rock and areas where no beach rock existed. Surveys in 1974 and 1975 indicated that a thick layer of material on the crater bottoms and in the fractures was more contaminated than the sediment covering it. Furthermore, this contamination was leaking into the lagoon from Cactus Crater.25.26 There was no EIS requirement to clean out the crater and fractures nor did there appear to be a practical means of accomplishing the task. DNA had advised POD that leakproof containment wasnotrequired or intended.27 POD believed that the tremie method would fix the material added during the cleanup project in place and prevent washouts through the Cactus Crater bottom if the slurry was ; \ placed properly. However, POD was concerned that the troop engineers had little experience with tremie placement of slurry and recommended that Field Command engagea qualified contractor to oversee and provide