ENGINEERING PROPERTIES 



Shear Strength 



A test for shearing strength (cohesion) was performed at the top, middle, and 

 bottom of all cores considered to be undisturbed and to have an unaltered water 

 content. Where obvious disturbance of the core had taken place during collection, 

 no shear strength tests were performed; however, in some instances, water content 

 measurements were feasible although the exterior of the core was disturbed. 



Shear strength measurements were performed on the same core increments used 

 in the sediment density determinations by carefully extruding the sample from with- 

 in the cylinder and into the testing device. 



The testing procedure is decribed in detail by Richards (1961) and is only briefly 

 discussed herein. An unconfined compression testing device with plastic platens 

 at either end of an axial rod was used to measure the compressive strength (2 times 

 the shear strength) on sediments of moderate firmness. A stress-strain relationship is 

 obtained by placing an ever increasing load on the upper end of the axial rod with 

 the sediment increment standing upright beneath. Failure of the sample was taken 

 by subsequently plotting the stress-strain data and taking failure at the point of 

 greatest curvature in the plotted line, or arbitrarily, at 20 percent axial strain if 

 the point of greatest curvature was undeterminable. 



When the sediment was soupy or not very cohesive, a vane-shear apparatus was 

 used In which a vane was inserted into the sediment and rotated by a constant-speed 

 motor. Degree of vane rotation and degree of applied torque was recorded at the be- 

 ginning and during the test. Sample failure was determined at the major inflection 

 point of the stress-rotation curve. 



The results of the shear strength tests are presented in Table VII and are thought 

 to be sufficiently accurate for most engineering work. Richards ( Ibid .) discussed the 

 sources of disturbance to the sediment during the sampling, transporting, and labora- 

 tory analysis. Because no Information was available to estimate quantitatively the 

 reduction of In-place strength, he concluded that values of shear strength obtained 

 through the method outlined in his report are conservative by an unknown amount 

 compared to In-place strength. 



Shear strength or cohesion values were determined in order to calculate the 

 ultimate bearing capacity of the sediment. The ultimate bearing capacity (q^) is 

 defined as the average load per unit area required to produce failure by rupture of a 

 supporting sediment mass, excluding any factor of safety, and is based on the formula: 



q^ = 1.3 cNc + w d Nq + 0.4 B w Ny 



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