4 Runit (Yvonne) Cleanup and Crater Containment 409 POD was provided geological data from the Pacific Cratering Experiment (PACE) and the Exploratory Project on Enewetak (EXPOE) projects: The geology of the northern tip of Runit is a complex mosaic with great variability in both horizontal and vertical composition, cementation, and structure. Addedto its natural geologic complexity are the blast and shock effects of three lowyield nuclear detonations near the Cactus Crater and of the Lacrosse detonation. A review of Lacrosse Crater's geology revealed that it is surrounded by a well-cemented reef plate, which contains some large radial and tangential fractures. The crater is rimmed py an 8- to [0-foot thick rim of well-cemented back-reef sand and gravel whose physical properties arelike that of beach rock. !6 Cactus Crater’s geology is more complicated than that of Lacrosse Crater. It is located between the backreef and lagoon environments in what was a man-made extension of the island on the lagoonside of the reef. Muchof the underlying rock was severely fractured by three nuclear detonations (Zebra, Dog, Cactus). The beachrock on the northwestern portion clockwise to the southeastern portion of the crater is 3 to 6 feet thick, providing a satisfactory base for construction. Beachrockis limited on the lagoon side of the crater and what there is on the island side is fractured. The northwestern tip of Runit may be only a very recent transient sandbar and is undergoing rapid erosion back to the original island shape. Theoriginal island shape can be defined by the beachrock as it is elsewhere on the atoll. This geology caused some doubt as to the survivability of a containment structure placed in Cactus Crater. Would its contents be eroded away and undercut bytidal action on the ground water from the lagoon side? Could this be prevented by creating artificial beachrock or by grouting the existing, highly fractured rock? Could it exceed 10 feet in height (the maximum height—equilibrium point—to which typhoon waves will pile up sand and gravel) and yet survive the waves of severe typhoons (1 every 50 to 100 years frequency) ?!7 Cactus Crater is on the lagoon edge ofthe reef plate which servesas the foundation for all Enewetak islands, more on a sandbarthan onsolid rock, while Lacrosse Crater is centered within the reef plate. Yet, it was clear that Cactus was moreaccessible and could be used for containment much more economically and efficiently. There was a question of cost versus benefit, as well as of the real necessity for absolute integrity of the structure over millenia. Considering all factors, including permanent versus interim storage. Cactus Crater was selected. !8.19 To provide storage options up to 200,000 cubic yards ofsoil plus over 7,000 cubic yards of debris, Field Command proposed a cylindrical structure with walls not to exceed 9 feet in height.29 POD’s proposal was a domedstructure, not over 30 feet high, to be elongated as necessary based on the total volume of material to be contained. The 30-foot limit was eee