Chapter 4 DESCRIPTION OF THE CONTAINMENT STROCTURE 4.1 General Observations The key-wall and concrete dome appear to be well finished and to consist of very good quality concrete. Some slightly open joints are visible between the slabs that form the dome, and a small number of very narrow cracks can be seen in a few of the slabs. Field observations by the committee revealed that fine cracking extends through the midpoint of 6 of the 358 panels. Most of these cracks appear to have resulted from normal shrinkage of the concrete. The existing cracks may become larger, and similar cracks may develop in other panels. The principal effect of this cracking is to reduce the effective size of the panel, functioning as riprap, to approximately 15 tons each half. Neither moisture movement nor future possible fissure to the underlying membrane is an issue with respect to durability of the dome. 4.2 The Drilling Program At the outset of the study, it was recognized that information concerning the quality of the concrete, the effectiveness of construction of the dome, and the condition of the material within the struc- ture could be obtained only by means of a drilling program. The Test Directorate, Pield Command, DNA, agreed to undertake the drilling program for the committee. The report on the results of the drilling (Ristvet 1980) is a comprehensive document that includes historical, geological, and seismic data. The drilling started on March 11 and ended on March 28, 1980, and much of it took place while committee members were on the site (March 21-28). Selection of the positions of many of the drill holes was made ‘dn cooperation with the committee, whose members were able to witness the drilling and recovery of cores and to examine the cores as they were extracted. Detailed descriptions of the cores are supplied by Ristvet (1980) but a general summary will be given here. . Twelve sections of the concrete cap were cored with a.4<inch e diamond bit. Thicknesses varied from 12.5 inches to 24 inches with a mean of 17.3 + 3.1 inches. The concrete was of high quality with some minor voids or air bubbles. One cap section showed a l= to 2-inch honeycombed zone with interconnections of voids. All concrete-cores tort S004: Ly on 19