sample (if NCEL possessed the capability to place the structure in the exact 

 location from which the core was taken). On the other hand, a 30 x 30-foot 

 manned structure may require a minimum of three undisturbed cores: one 

 at two diagonal corners and one in the center. The sample from each location 

 should represent the sediment column to a depth approximately equal to the 

 minimum overall lateral dimension of the foundation system. 



The core samples obtained in accordance with the above techniques 

 are then subjected to laboratory tests as prescribed by ASTM procedures for 

 testing soils. 22 Some tests are modified to accommodate seafloor sediments. 

 A few of these modified tests, examples of engineering properties data, and 

 results of the site survey for the STU 1 1 series are presented by Hironaka. 23 

 The type of tests to be conducted depends on the requirements of the struc- 

 ture and on the consistency of the samples. The desired data are strength, 

 consolidation, and other index properties that will be used in design and to 

 evaluate the foundation-supporting capability of the sediment. 



This first method for assessing the foundation engineering parameters 

 also includes investigation of bedrock by core drilling for samples. Obtaining 

 rock core samples from deep water is within sampling capabilities as has been 

 demonstrated by the Project MOHOLE and JOIDES operations. Samples 

 obtained in this manner are then subjected to laboratory tests as prescribed 

 by ASTM procedures. 24,25 In addition to evaluating the strength character- 

 istics and other properties of the samples, the location of bedrock below the 

 sediment surface can be defined in the core drilling process. This investigation 

 would be required in the event that piles or other foundation systems are to 

 be placed on bedrock. 



The second method for assessing the foundation engineering parameters 

 that is being developed consists of in-situ measurement techniques. Although 

 presently limited to sediment thicknesses of about 10 feet, some of these 

 techniques may soon be capable of evaluating the sediment column to a depth 

 of 1 00 feet. NCEL now has a vane shear device, a cone penetrometer device, 

 a plate bearing device, and long-term settlement devices (LOBSTERs). (These 

 devices will be discussed in detail in a later section.) Richards has also developed 

 in-situ test devices, 55 including a vane shear probe, a gamma ray density probe, 

 and a pore pressure probe. The Naval Oceanographic Office has also used a 

 density probe. NCEL is currently conducting research to provide design infor- 

 mation for a probe to measure sediment water content by nuclear methods. 



The geological characteristics can be determined by the combination 

 of core samples, seismic and magnetic surveys, accurate bathymetric charts, 

 bottom photographs, and visual observation from submersibles. The require- 

 ments of the structure will govern the thoroughness required for the survey. 

 In such a survey, it is desired to locate weak sublayers, indications of mass 

 movements (creep, slides, turbidity currents), tension cracks on slopes, faults, 

 and other features which may influence the stability of the structure. 



24 



