instrumentation 



LOBSTER footing 



Concrete blocks, 48 inches in diameter, which apply 1 00 psf to the 

 sediment surface, are employed as the standard footings during the LOBSTER 

 tests. Settlement is measured relative to a stationary reference rod extending 

 1 feet into the sediment. The reference rod is protected from intermediate 

 settlement by a 9-foot-long outer casing or isolating tube. In one test (Figure 

 16) multiple reference rods were utilized to monitor soil compressions at var- 

 ious depths. Movement is recorded by either SCUBA divers (Figures 1 and 2) 

 or an automated electronics package (Figure 3). Nine long-term tests (duration 

 of observation up to 2 years) have been performed at sites near Port Hueneme, 

 California, in water depths varying from 4 to 1 ,200 feet. The soils in all cases 

 were soft cohesive materials with grain sizes predominantly in the silt and clay 

 ranges. The sediments' vane shear strengths at a depth of 12 inches varied 

 from 0.2 to 0.5 psi. 



In general, the settlement 

 data generated by these deployments 

 (Figure 17, typical example) have 

 confirmed analytical predictions 

 based upon the magnitude of pri- 

 mary consolidation measured during 

 laboratory consolidation tests. Settle- 

 ment predicted in this manner typically 

 varied from 1 to 3 inches. However, 

 the in-situ results also indicated a sur- 

 prisingly large amount of continuing 

 long-term settlement, apparently 

 caused by secondary compression 

 (about 4 inches in one case). Results 

 of the in-situ deployments also indi- 

 cated that undermining of the 

 foundation by burrowing animals 

 and hydraulic scour can cause large 

 settlement and, in some instances, 

 render the foundation useless. These 

 two effects have caused up to 7 and 

 8 inches of foundation settlement at 

 one particular site. In the latter case, 

 7 inches of settlement caused by scour was accompanied by a tilting of about 

 20 degrees. This tilting is equivalent to a differential settlement of about 18 

 inches. A more typical value of differential settlement, resulting from primary 

 and secondary compression only, was 1 inch. 



Figure 16. Early, shallow-water 

 LOBSTER structure 

 and reference systems 

 for investigation of long- 

 term foundation behavior 

 on the seafloor. 



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