Appendix B 



ANALYTICAL INVESTIGATION 



An analysis using soils data from two thoroughly investigated 

 NCEL test sites was made to determine the benefits of using full-scale 

 preconsolidating footings. Both sites are located in the Santa Barbara 

 Channel. The first is in 120 feet of water off Pitas Point; the soil 

 classified as ML by the Unified System and as clayey-silt by the Tri- 

 lineal System. It has a liquid limit, LL, of about 40; plasticity 

 index, PI, about 15; compression index, C =0.30; and coefficient of 

 consolidation, c^^ = 0.007 - 0.012 cm^/sec. The second site is in 

 1200 feet of water near the center of the Channel; the soil here is 

 classified as MH by the Unified System and clayey-silt to silty-clay by 

 the Trilineal System. It has a LL of about 92, PI about 49, C^ about 

 0,77, and c^ = 0.00075 - 0.0012 cm^/sec. 



The additional effective stresses in the soil resulting from an 

 applied negative pore water pressure were evaluated from a two- 

 dimensional flow net (see Figure 1) . This flow net assumed a homogene- 

 ous isotropic section. The vertical effective stress component from 

 the two-dimensional analysis was then applied directly to the three- 

 dimensional problem; the resulting errors were neglected. Void ratio 

 change estimates are therefore based on one-dimensional consolidation. 

 Shear strengths for the new void ratios were projected from the in-situ 

 e-log p relationship. Predictions were made of the effects of a 10-psi 

 preconsolidation pressure applied to a 10-foot diameter weightless 

 footing (see Table B-1) . 



The settlement reductions and bearing capacity increases predicted 

 for the real soil profiles were of the same relative magnitude as those 

 from the laboratory model tests. For instance, at Pitas Point, the 

 analytically determined settlement of a preconsolidated footing due to 

 design load is shown to be zero compared to 4.5 inches for the non- 

 preconsolidated footing design load settlement (see Table B-1). Allow- 

 ing for the inaccuracies of the laboratory measuring system, this 

 settlement reduction compares fairly well with the minimum 80 percent 

 reduction in laboratory model settlement. Preconsolidation increased 

 the analytically determined bearing capacity 2,8 times, from 380 psf 

 to 1060 psf, which compares favorably with the minimum two-fold 

 increase found in the laboratory. 



Lateral load capacity increases predicted for the real soil pro- 

 files differed in relative magnitude from the laboratory model test 

 results. For the Pitas Point profile, analytical lateral load pre- 

 dictions indicated a 6.6 times increase, from 5,600 pounds to 37,000 

 pounds; whereas model results in the bins showed only a two-fold 

 increase. The discrepancy probably largely arises because (1) the 

 assumed analytic failure plane was in the horizontal plane of the cut- 

 ting edge, which was not the case in the laboratory tests, and (2) the 

 test footings were subjected to an eccentric lateral load (above the 

 cutting edge plane) which resulted in tension stresses at the footing 

 heel. 



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