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..i 21 larger equipment associated with the cogeneration facility are relatively settle- ment sensitive, the large and probably variable thickness of fill beneath this equipment may not be compatible with the need to minimize total and differential settlement. Therefore, even with the removal and replacement option, it is possible - if not probable - that some of the more critical equipment should be pile supported. However, after removal and replacement of the glass, a larger variety of driven and drilled deep foundation options may be possible. Considerations relative to reuse or disposal of the excavated materials and requirements for structural fill are addressed under the option for Partial Removal and Replacement. In considering the removal and replacement option, it should be realized that glass extends below groundwater in some areas. Thus, removal of the glass as well as replacement with engineered fill will require dewatering or water control measures. Because of the above factors, we do not consider this option to be a gener- ally cost-effective option for this site. Material Treated By Dynamic Deep Compaction Concepts. As an alternative to removal and replacement, the site appears to be suitable for in-place improvement by dynamic deep compaction (DDC) . DDC involves the repeated dropping of a heavy weight onto the ground surface. After DDC treatment, the potential subsurface variability will be reduced and the site - should be more uniform. To improve a large area, the weight is dropped several times at each point on a grid pattern. Areas with heavier loads can- then be further treated with additional repetitions at the grid points or by dropping the weight at locations between grid points. Drops also can be made at the actual footing locations. The DDC treatment is often completed with a final "ironing" pass where the weight is dropped from a relatively shallow height over a closely spaced grid as shown on Plate 41. San Joaquin Site. DDC will help to compress the medium dense silty sand or sand/glass mixture. In areas where glass is the predominant material, DDC potentially will break more unstable points of glass to glass contacts and produce a more stable matrix of glass. Depth of treatment is related to the size of the weight and fall height. The relatively shallow depths to be treated (15 ft) will require only a nominal size weight and a minimal fall height. Exper- ience and empirical relationship for these three variables suggests a 10 to 20 1144A/CC-21 McClelland