where 



c = cohesion, 



w = buoyant unit weight of the sediment, 



d = depth to center of sample increment tested, 



B = width of structure footing, and 



N(-, Nq, and Ny are bearing capacity factors. 



The formula is applicable to structures where the length to width ratio of the base 

 is less than two (square or circular loads) and is usually reduced to q^ = 1.3 c N^. 

 Bearing capacity factors are a function of the angle of internal friction. Where the 

 angle Is zero (as is assumed for cohesive soils) N^ = 5.7, Nq = 1 .0, and Ny = as 

 determined by Terzaghi and Peck (1948). 



Results from tests of core number 62-22 will illustrate the application of this 

 formula. If a mass of 35 tons (buoyant weight) with dimensions 12' x 12' x 6' is 

 placed on the bottom at the location of core 62-22 and without impact velocity, the 

 resultant pressure or stress on the sediment would be 486 lbs/ft'^, and an ultimate 

 bearing capacity of at least the same amount is required for support of the mass. 

 Assuming a surface load of q,j = 7.4 c, the cohesion necessary for support is 0.46 psi. 

 In core 62-22 the core interval to 7 centimeters has a tested cohesion of 0.53 psi, 

 which (neglecting time) is sufficient for support of the mass. 



The majority of cores tested show a large increase In cohesion with depth in the 

 sediment, and inversions, when present, are small in magnitude. Figure 12, which 

 delineates areas in the TOTO of high and low cohesion values, is based on the aver- 

 age cohesion throughout the individual core. From this figure the cul-de-sac sediments, 

 except In two instances, have an average cohesion of less than 1 .0 psi, which is the 

 lowest in the channel. Near-flank sediments show a slightly higher cohesion, and 

 axial sediments, except for a zone of less cohesive sediments southeast of Middle Bight, 

 greatly exceed both areas. Although the differences in cohesion values throughout the 

 channel are slight, it might be pointed out that an increase of one unit in the measured 

 cohesion value presented in the example used in core 62-22 above would Increase the 

 ultimate bearing strength from 565 to 2,062 lbs/ft^. 



It Is noteworthy that cohesion values follow a trend corresponding to the 3 sedi- 

 mentary environments delineated In the TOTO, The near-flank and cul-de-sac areas 

 (low cohesion) represent environments of high water content, high organics, low den- 

 sity, and high rates of sediment accumulation, whereas, the axial area (high cohesion) 

 is characterized by relatively low water content, low organics, high density, and low 

 rates of sediment accumulation. Figure 1 3 delineates values of surface organic carbon 

 content and demonstrates the relationship between organic content and cohesion when 

 compared with Figure 12. 



49 



