Knowing the acoustic velocities of the material also provides the 

 marine construction engineer with a preliminary estimate of the relative 

 bearing capacity of the material. An increase in compressional velocity 

 generally implies an increase in the firmness of the material and, hence, 

 an increase in its relative bearing capacity (Patterson and Meidav, 1965). 

 Seismic information plus soil mechanics tests can be of immense value in 

 the evaluation of slope stability and subgrade behavior. 



There are, of course, limitations and dangers in the engineering 

 interpretation of seismic velocity data. Foremost, perhaps, is that in 

 the highly saturated sediments immediately underlying the water column, 

 acoustic velocities may be equal to or less than the acoustic velocity in 

 water (Jones, 1962; Morgan, 1969). The explanation for an anonymously low 

 acoustic velocity in the sediment is the presence of gases derived primarily 

 from the decay of organic matter (Jones, 1962) or the following explanation 

 quoted from Morgan (1969) : "Adding solid particles to water (decreasing 

 the porosity) increases the bulk density without an appreciable change in 

 compressibility (see equation (13), i.e., V = (l/fBp)-'-'^ Wood's eouation 

 given earlier); hence, decreasing the velocity. However, upon a further 

 decrease of porosity, the system assumes a grain-to-grain contact* and, 

 consequently, the compressibility decreases. The compressibility change 

 seems to dominate the bulk density change, causing the seismic velocity 

 to increase. The data show such behavior down to a porosity of about 

 0.75." 



A small amount of gas greatly increases the average compliance of the 

 material without significantly affecting the average density of the sedi- 

 ment (Wood, 1941). A velocity in the mixture less than that of either 

 constituent is possible. 



Shear Wave Velocity . Since the shear wave velocity is relatable to 

 shear strength parameters of a material, the measurement of its velocity 

 would be of immense value to the marine construction engineer. 



Shear waves are propagated through marine sediments; but, there are 

 considerable practical dif faculties in the measurement of its velocity. 

 Most seismic equipment, for example, is sensitive to the energy of a wave 

 and does not discriminate between the arrival of a shear or a compressional 

 wave. Further, since shear waves do not generally propagate through a 

 liquid to any marked degree, the transducer recording shear wave behavior 

 needs to be near to or on the seafloor (Auld et al . , 1969). Also, normally 

 the coefficient of rigidity of the sediments is so small that measurements 

 by direct timing of recovered sanroles have been accomplished only under 

 high pressure. 



Laughton (1957) measured shear wave velocities in a sample of globi- 

 gerina ooze compressed between porous disks. Shear waves were observed at 

 pressures of 500 kg/cm^ on the initial compression. On decompression, 

 however, shear waves could be identified at pressures as low as 64 kg/cm . 



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