IV. VALIDITY OF STRENGTH DATA 



Disturbance to natural, in place, sediment may result in micro-structural damage 

 with consequent loss of in-place strength. The amount of damage generally is propor- 

 tional to the grain size. Compared to sediments composed of sand- and silty sand- 

 size particles that possess little or no cohesion, cohesive, fine-grained sediments are 

 relatively immune to damage If they possess a low sensitivity or a low water content; 

 however, sediments of high sensitivity or high water content are very susceptable to 

 disturbance , 



A difficult question to answer is how valid are shear-strength measurements con- 

 sidering the various sources of sedimentary macro- and micro -structure I disturbance 

 that occur between the time sediment was in place on the sea floor and the time of 

 the test? These sources of disturbance Include: (1) collection of the sediment core 

 by the sampler, particularly gravity-type corers, (2) reduction of hydrostatic pressure 

 from in place to atmospheric pressure and its effect on the differential expansion of 

 interstitial water and mineral grains, (3) shipboard handling and storage of the core 

 in liner or barrel, including bacterial and chemical changes occurring during the 

 storage period, (4) transportation to the laboratory, and (5) test procedure in the 

 laboratory. Hvorslev (1949, pp. 182-206) presents an excellent detailed discussion 

 or many factors contributing to disturbance. 



An unknown but presumably small amount of structural disturbance results from 

 the slight expansion of water and grains with a reduction of confining pressure (about 

 one decibar or 1 ,47 psi per meter of sea-water depth). With care, given to most of 

 the cores described, it is believed that disturbance caused by handling and shipment 

 was reduced to a small or negligible amount. In the laboratory, a sample can be 

 tested within the core liner without extrusion by using a vane shear machine, thus 

 nearly eliminating disturbance from shear-test procedure. Loss of strength resulting 

 from extrusion for the compression and vane shear tests or cores described in this report 

 is presumed to be present, but of small quantity. Some of the strength lost by the 

 various processes may be regained by thlxotropy^ following core collection and prior 

 to testing. Studies by Skempton and Northey (1952, pp. 33-39) indicate that because 

 of the short time involved, the amount of strength regain probably would be small , 

 Without doubt. If due care has been exercised elsewhere, the major source of disturb- 

 ance and loss of strength is in the sampling process, Jakobson (1954, p, 56) con- 

 cluded from a series of tests, designed to provide Information on the Influence of 

 sampler type on shear strength of clay-size samples, that even the best type of sampler 



3 

 Thixotropy Is the property of a material that enables it to stiffen In a relatively 



short time on standing, but upon agitation or manipulation to change to a very soft 



consistency or to a fluid of high viscosity, the process being completely reversible 



(ASCE) , 



41 



