a 6-inch diameter model which has ceased measurable settlement due to 

 its own dead load of 1.8-pounds submerged weight. Such settlement 

 averaged 0.3 inch. Since the thickness of the footing was 0.63 inch, 

 the top of the footing remained above the surrounding sediment surface 

 as illustrated. Figure 5 illustrates typical performance of a 6-inch 

 preconsolidated footing which has ceased measurable settlement due to 

 its 1.8-pound buoyant dead load and to the applied negative pressure 

 of 10 psi. Such settlement typically totaled 1.4 inch; thus the foot- 

 ing stabilized with its top surface 0.8 inch below the surrounding sed- 

 iment surface. Circumferential soil cracking outlines soil "slump 

 blocks" above the settlement bowl resulting from preconsolidation. 



As shown in Figure 6, preconsolidation significantly reduces 

 foundation settlement under subsequent gravity design load. Precon- 

 solidated 6-inch diameter models on the Seal Beach silt typically 

 settled less than 0.05 inch under a 3 1/3-pound design loading (the 

 accuracy of the measuring system is estimated to be 0.02 inch) . The 

 same load applied to a non-preconsolidated 6-inch footing typically 

 led to long-term settlements of 0.25 to 0.65 inch, or settlements 5 to 

 20 times greater. Figure 7 illustrates that the ratio of settlements 

 for non-preconsolidated and preconsolidated 2-inch and 12-inch diameter 

 footings were about the same as the 5 to 20 times found for the 6-inch 

 models. The wide variation in the settlement performance of non- 

 preconsolidated models, for example, curves one and two of Figure 6, is 

 not adequately understood. The initial condition of the supporting 

 soil at each of the model locations did not vary significantly, and 

 model placement and loading procedures were carried out similarly. 



The total settlement of non-preconsolidated 6-inch footings in- 

 cluded typically 0.30 inch due to the footing weight and 0.65 inch due 

 to the design loading, for a total on the order of 0.95 inch. The 

 total settlement of a preconsolidated 6- inch footing was typically 1.40 

 inch during preconsolidation and negligible under the design loading. 



The key depth variation from 1/8 and 1/4 diameter had no signifi- 

 cant effect on settlement resulting from preconsolidation or design 

 loading. 



Ultimate Vertical Load 



Preconsolidation of model footings on the Seal Beach silt reduces 

 the initial slope of the load-displacement curve and nearly doubles the 

 bearing capacity. (See Figure 8) 



Figures 8 and 9 indicate that the bearing capacity Increases as a 

 function of footing diameter. The Seal Beach silt bins were prepared 

 in such a way that the density and strength increased rapidly with 

 depth. The larger footings, which derive their support from a greater 

 depth of soil, should therefore be expected to exhibit a greater unit 

 bearing capacity. 



In general, footings failing in bearing capacity remained nearly 

 horizontal, and the displaced soil exhibited itself as a uniform 



