III. COHESION IN CORES 



Very Ill-tie Is known about the distribution of shear strength in sediments of the 

 sea floor. While a few studies have been mode on sediments in harbors and on the 

 continental shelf, the only investigations made In deeper water appear to be confined 

 to the Pacific Ocean, Arrhenlus (1952) made the first study of the distribution of 

 shear strength in a number of cores collected on the Swedish Deep-Sea Expedition 

 using the Swedish State Railways fall -cone pentrometer. Unfortunately, his measure- 

 ments are expressed as relative strength values and as such are of little use except to 

 show the relationship of strength profiles to depth , (To make these measurements of 

 more universal use, Moore and Richards (submitted) prepared a graph to convert the 

 relative strength values to conventional units In the English and metric systems). 

 More recently, Moore (1959, and In press) published shear strengths of a few selected 

 deep-sea samples in an analysis of bottom -slope stability using results from vane and 

 direct shear tests. 



Data are presented (Figs. 11 through 21) relating shear strength, expressed as 

 cohesion, to distance below the sea floor In cohesive sediments that were collected 

 in cores from water depths of 400 to 5,120 m (1,310 to 16,800 feet) In eight areas 

 of the North Atlantic Ocean, West Mediterranean Sea, and Central Pacific Ocean. 



Strength measurements were made In pounds per square Inch, psi, and converted 

 to g/cm^. Each compression test value Is plotted as If it were a point midway between 

 the ends of the sample. The compression test sample length for all cores Is 5 cm, ex- 

 cept as follows: Area D, (piston) core 1, 10.5 cm; Area F, core 6, 10.5 cm, core 11, 

 10 cm; and Area G, all cores, about 10 cm. Values obtained from vane shear tests 

 are representative of only about a 1 .9 cm vertical section in any given sample; these 

 values likewise are plotted as If they were taken at a point midway between the ends 

 of the sample . 



Vane and compression tests made alternately on succeeding core sections gener- 

 ally show fairly good agreement of values (Fig. 15, cores 47 and 48, and Fig. 18, 

 core 16), although there are exceptions (Fig. 21). In two examples, vane values for 

 an unknown reason are about 15 g/cm'^ less than compression values (Fig. 15, core 

 46, and Fig. 20). In another instance, both vane and compression values show an 

 apparently erratic distribution (Fig. 21), and it is uncertain which, if either, is 

 correct , 



The apparent inconsistency in curve fitting in the different figures Is intentional . 

 The degree of fit of a curve to the values shown, insofar as possible, is directly pro- 

 portional to their accuracy. Deviation of points less than 7 g/cm (0.1 psl) from a 

 curve are Insignificant because test precision is about 7 g/cm for compression tests 

 and 0.7 g/cm^ for vane tests. An example of lack of fit is shown for core D Ig 



24 



