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22 ,%.. — ton weight with a 30 to 60 ft drop. Two to four repetitions at about 10 to 15 ft spacing plus a final ironing pass are anticipated. Each drop of the DDC weight will produce a crater. Between coverages, it is customary to level the surface with a dozer. After the last "ironing" coverage, the surface materials should be excavated, moisture conditioned, and recompacted in accordance with criteria for structural fill to a depth of 12 inches or the crater depth from the last drop, whichever is greater. For the project site, we anticipate some craters potentially as deep as li to 2 feet. The average ground surface elevation in the treated areas is antici- pated to be over one foot below the existing ground surface. Care and appropriate safety precautions will be required because of the potential for flying glass when the DDC weight is dropped. Field Trials. The final combination of treatment parameters (weight, drop height, repetitions, and spacings) should be established based on field trials. Customarily, the rate of crater deepening and/or the change in SPT N value (sampler blow count) or cone petration test data is used to evaluate the trial results with different weights and drop heights. However, at the San Joaquin site the presence of glass prohibits cone penetration testing and produces questionable blow counts. Thus, the rate of crater deepening caused by repeated drops of different weight and height combinations will be the primary performance criteria. How the rate of crater deepening can be used to choose the optimum combination of weight, drop height, and number of drops is shown on Plate 41. The area to be treated is relatively small and the cost of mobilizing equipment will be a significant percentage of the-total costs. Thus, the field trials should be conducted at the beginning of the production DDC operation. We suggest the initial trials be conducted at: 1) a location known to be underlain by a significant thickness of glass (such as B-1 and B-2) ; 2) a location underlain by silty sand with no glass (such as B-5) ; and 3) a location underlain by a mixture of sand and glass (such as B-4) . Partial Removal and Replacement For many structures and equipment, it may be feasible to partially remove the surface materials, replace these excavated materials with compacted struc- tural fill, and found the structures on shallow foundations bearing in the fill. Since the mat of compacted structural fill will be underlain by glass and uncon- trolled fill, differential settlements will be larger than if the questionable 1144A/CC-22 :: McClelland