For surface loading and when / is assumed to be zero 



qo= <= Nc (12) 



where N is (iT + 2) or 5.14 according to Prandtl , For loading in which the length 

 to width ratio is less than 2.0 (square or circular loads) equation 12 usually Is modified 

 to 



qo=1.3cN^ (13) 



Other Investigators believe that N^ constants 11 to 12 percent higher should be 

 used for strip, square, and circular loads at the surface. Different Nj- values must be 

 used below the surface (Fig. 24, after Skempton, 1951, p. 181). 



An example using data from core A 31 will Illustrate the use of one of these 

 formulas. A load with a buoyed mass of 5000 g and a square surface area of 100 cm 

 Is placed on the bottom in the immediate vicinity of core A 31 without Impact veloc- 

 ity. The resulting pressure or stress on the sediment Is 5000/100 or 50 g/cm . An 

 ultimate bearing capacity of at least the same amount Is required for support. Assuming 

 surface loading, qo = 6.7 c (equation 13), the cohesion necessary for support is 7 .5 

 g/cm (0.11 psi). in this core (Fig. 11), the sample from to 5 cm (0 to 2 in) was 

 too soft to test. The sample from 5 to 10 cm has a measured cohesion of 9.1 g/cm 

 (0,13 psi). Interpolation of the strength profile Indicates shear failure of the sediment 

 at a depth of somewhat less than 8 cm (3 in). Allowing a safety factor of 1 .5 (the 

 more usual value of 3, according to Meyerhof, 1951, p. 301, is more applicable to 

 sediment with a higher sensitivity), a cohesion of 10 g/cm (about 0.14 psi) Is required 

 for adequate support and Is found at an Interpolated depth of about 8 cm . 



It Is important to realize that the strength profile-depth relation determined In a 

 single core may not be valid a short distance away from where the core was collected 

 unless supplemented with tests of other nearby samples. Almost no Information exists 

 on the areal variability of strength in marine sediments. 



C. CONSOLIDATION AND SETTLEMENT 



The gradual reduction in volume of a sediment mass resulting from an increase In 

 compressive stress is termed consolidation (ASCE). If an object placed on the bottom 

 produces this stress due to its unit load, consolidation will occur and the object will 



4 . . 



This, and the following examples, were selected to show methods of calculation; 



they are not necessarily to be considered practical engineering problems. 



49 



