their response to footing loads as measured by the plate bearing device is 

 being studied to enable the prediction of the short-term bearing pressure- 

 settlement response for a proposed foundation on the basis of laboratory 

 tests on core samples. 



Long-Term Settlement Device 



Penetration Resistance (psi) Structures founded On Soft 



20 40 60 80 , .• , r *u 



cohesive sediments typical of the 

 deep-ocean areas will be subject 

 to large long-term settlements 

 resulting from consolidation of 

 the underlying sediment layers. 

 Such foundations can be designed 

 for these large total and differen- 

 tial settlements or they can be 

 designed to minimize these settle- 

 ments by proper load distribution. 

 Such designs require a knowledge 

 of the in-situ sediment properties 

 affecting long-term foundation 

 settlement. These properties may 

 be obtained from laboratory tests 

 on core samples; however, the 

 effects of core disturbance make 

 it preferable to measure some of 

 the more critical properties in situ. 

 The LOBSTER 34 (Long-Term Ocean Bottom Settlement Test for 

 Engineering Research), a schematic of which is shown in Figure 22, is an in-situ 

 test system designed to evaluate those sediment properties which control long- 

 term foundation settlement. The overall diameter of the device is 6 feet. It 

 is designed to be a large model footing capable of applying a low bearing 

 pressure (approximately 1 00 psf over an area which has a diameter of 4 feet) 

 to the seafloor. The resulting settlement is measured relative to an isolated 

 reference system founded deep in the sediment beneath the footing. The 

 settlement data including total settlement and differential settlement (foun- 

 dation tilting) are monitored and recorded hourly on highly sensitive and 

 accurate digital tape systems housed within instrument capsules attached to 

 the footing as shown in the figure. The LOBSTER has the capability of 

 measuring total settlements as great as 36 inches and foundation tilting as 

 great as 1 5 degrees. 



Figure 18. Static cone penetration resistance 

 versus depth for cohesive sedi- 

 ments located off Pitas Point, 

 California. 



33 



