submerged weight, the bearing pads apply a stress of 85 psf . Short-term 

 settlements from 0.5 to 1 .5 inches have been observed in soft cohesive sedi- 

 ments. No foundation performance difficulties have been experienced during 

 more than 30 deployments on the seaf loor in water depths to 5,600 feet. 



ESSA Bottom-Sitting Observation Stand. The Seattle, Washington, 

 Division of the Environmental Science Services Administration (ESSA) used 

 an observation stand equipped with a camera and current meter array to 

 observe ocean-bottom currents in the Tasman Sea (Ryan, 1969). The device, 

 which weighs 200 to 300 pounds in water, is pyramidal with a 12- by 12-foot 

 base fabricated from 1 -inch-diameter pipe. Water depths at test locations varied 

 from 2,600 to 1 5,000 feet. Sediments were predominantly calcareous oozes. 

 Although bottom penetration and settlement varied from site to site, no foun- 

 dation performance difficulties were experienced. Performance data are being 

 assembled by ESSA for future publication. 



ESSA Plate Load Device. Two series of plate bearing tests were 

 performed by Harrison and Richardson on sandy marine sediments in the 

 shallow waters of lower Chesapeake Bay (Figure 14) (Harrison and Richardson, 

 1967; Harrison, 1969). The behavior of the sediments was compared to the 

 theoretical behavior as predicted by the Terzaghi and Taylor equations for 

 terrestrial soil. 



A load frame (Figure 15), which weighed 82,000 pounds in air and 

 was estimated to weigh 48,000 pounds in water, supplied the reaction for each 

 of the in-situ load tests. The frame had a bearing area of approximately 48 

 square feet (giving an applied stress of 1 ,000 psf). A 20,000-pound calibrated 

 hydraulic jack on the frame was used to apply loads to the 1 2-, 1 9-, and 24- 

 inch-diameter plates. 



Before tests were performed, soil at the site was evaluated for grain 

 size, void ratio, density, and wet unit weight. A series of triaxial tests, con- 

 ducted in the laboratory, established the sediment's angle of internal friction. 



When the load frame was slowly placed on the seaf loor, the frame 

 settled 1 -1/2 to 3 inches into the sediment at Site A and 1 inch at Site B. 

 Once SCUBA divers had instrumented the frame, the plate bearing tests were 

 performed. Values of ultimate in-situ bearing capacity as determined by this 

 procedure were found to be generally higher (by factors of 2 to 3) than pre- 

 dicted by theory. The amount of settlement under a given stress increased as 

 the plate diameter increased, as predicted by existing terrestrial theory. 



NCEL LOBSTER. The NCEL LOBSTER (Long-Term Ocean Bottom 

 Settlement Test for Engineering Research) was designed to measure the in-situ 

 long-term compression of soft sediment under typical foundation loads. 



25 



