V. PRACTICAL APPLICATION OF DATA TO STRUCTURAL LOADS PLACED 

 ON THE SEA FLOOR 



A. GENERAL STATEMENT AND SIMPLE STRENGTH THEORY 



If an object is placed on sediments of the sea floor without Impact velocity, one 

 of two events will Immediately occur: (1) either the sediment will have Insufficient 

 strength and will fail In shear (rupture) or, (2) the sediment will have sufficient 

 strength to support the object, which will remain on the surface. Following either 

 of these events, a relatively slow and gradual settlement of the object may occur 

 because of sedimentary consolidation. 



Whether or not the object initially sinks, or penetrates. Is directly related to 

 the structural strength of the sediment, the amount of load, and can be determined 

 from a knowledge of the sediment bearing capacity. The assumption that an object 

 will penetrate until It is supported by sediment with a wet density equal to or greater 

 than that of the object is invalid (see Jurgenson, 1934, p. 199). Archimedes' prin- 

 ciple, which states that a body immersed In a fluid is buoyed up with a force equal 

 to the weight of the displaced fluid, does not apply because sediment has strength 

 and is not a fluid. This strength, however, may be very small If the material is fine- 

 grained and has a high water content. (The fact that an object placed In water is 

 buoyed up by the unit weight of water should not be confused with these statements. 

 The effective mass of the load on the sediment Is its buoyed weight). 



A simplified expression of shear strength In sediments has been developed from 

 a theory presented by Coulomb (1776) 



s = c + p tan / (8) 



where s Is the shear strength, c is the cohesion, p is the total Intergranular pressure 

 normal to the shear plane, and /^ Is the angle of internal friction. Results of Investi- 

 gations conducted by Hvorslev (1936, 1937) showed that cohesion Is a function of 

 water content, and Coulomb's equation should be modified to include an expression 

 of the effective normal stress on the shear plane. The earlier discovery of effective 

 stress by Terzaghl, and later experimental confirmation by Rendulic and others. Is 

 discussed in detail by Skempton (1960) and need not be considered here. 



Equation 8 is usually expressed in the form 



s = c + (cr - u) tan / (9) 



where s Is the shear strength, c Is the apparent cohesion, a Is the total stress normal 

 to the shear plane, u is the neutral stress (pore pressure) at the point on this surface. 



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