This report describes a laboratory evaluation of the benefits that 

 may be derived from the preconsolidation of seafloor footings. A 

 supporting analytical evaluation is described in Appendix B. 



LABORATORY INVESTIGATION 



The major purpose of the model testing program was to verify the 

 improvement in sediment behavior due to preconsolidation by the induced 

 negative pore pressure technique. Of particular interest was the degree 

 of reduction of long-term settlement under design load. The increases 

 in bearing capacity and resistance to lateral loads were also of major 

 interest. A secondary purpose of the model testing was to provide data 

 on the influence of the skirt-depth/footing-diameter ratio on precon- 

 solidation settlement and lateral load capacity. The latter informa- 

 tion will be useful for the economical design of prototype footings. 



Test Program 



The model tests were conducted in two bins, each 4-feet on a side 

 by 1^-te.et deep, containing artificially prepared soil simulating sea- 

 floor sediments. Two soils were used: a low-plasticity silt from Seal 

 Beach, California; and a low-plasticity clay from Rogers Lake, 

 California. (See Appendix A for detailed properties.) The soils were 

 placed in the bins as slurries and were consolidated by drainage to a 

 thickness of approximately 13^2 inches. The majority of tests were 

 conducted on the Seal Beach silt. Tests were conducted on the Rogers 

 Lake clay to confirm that the preconsolidation concept worked similarly 

 in a very fine-grained sediment. 



A typical model preconsolidating footing (see Figure 2) consisted 

 of a filter stone with its top face and sides enclosed by 1/8- inch 

 thick sheet steel. This sheet extended some distance below the filter 

 stone to lengthen the pore water drainage path. A vacuum hose connec- 

 tion on the footing top penetrated the sheet steel to drain the filter 

 stone. 



Three sizes of model footings were used to investigate the scale 

 effect: two, six, and 12 inches in diameter. Two ratios of keying 

 edge depth to diameter, 1:8 and 1:4, were employed to evaluate the 

 effects on settlement and lateral load resistance. The preconsolidation 

 pressure applied to the footings was usually 10 psi, with 5 psi used in 

 a few tests for comparative purposes. These negative pressures were 

 applied until measurable vertical movements ceased (assumed completion 

 of primary consolidation). Similarly, most non-preconsolidated footings 

 were not tested until they had ceased measurable movement under the 

 applied loadings, although a few tests at partial consolidation were 

 conducted to determine initial bearing capacity and lateral load 

 resistance. 



