Core Shortening 



With almost all of the standard oceanographic coring devices, it 

 is often difficult to determine the depth below the seafloor from which 

 a particular sample is obtained. A plug may form in a gravity corer 

 and cause weak strata to be pushed away and not sampled. A piston 

 corer in which the piston is not fixed relative to the seafloor may 

 suck in quantities of soft material. In both cases the depth of a 

 sample within the core will not correspond to its original depth below 

 the seafloor. Richards and Parker (1967) present a discussion of the 

 problem and offer some tentative techniques for evaluating its effects. 



TEST PROGRAM 



A test plan was developed in which the undrained shear strength of 

 soil from different seafloor sites would be measured under a variety of 

 situations ranging from in-situ to a moderately disturbed sample. The 

 samples were subjected to specific forms of disturbance so that the 

 influence of each, in terms of changes in strength, could be determined. 

 More importantly, the data could be used to develop procedures for 

 estimating the in-situ strength when given the strength of a partially 

 disturbed sample. 



To develop a procedure for correcting strengths for disturbance, 

 it was necessary to have a measure of the amount of disturbance to which 

 the sample had been subjected. From a review of the literature (Ladd 

 and Lambe, 1963; Noorany and Seed, 1965) , it was determined that the 

 most commonly used disturbance parameter is the residual pressure re- 

 tained by the pore water within each sample. These pressures are nega- 

 tive relative to atmospheric, and, in general, the more negative the 

 pressure, the smaller the amount of disturbance. These pressures 

 develop because of the natural tendency of soil to expand during 

 sampling as a result of the release of in-situ stress. In relatively 

 fine-grained soils this expansion is retarded by the development of 

 menisci in the pores near the sample surface, with a general state of 

 tension being set up throughout the samples pores. As the sample is 

 disturbed, however, these tensile stresses are gradually broken down. 



The general procedure followed in the experimental program was to 

 measure in-situ strength, the strength of samples with varying degrees 

 of disturbance, and the residual negative pore water pressure. The 

 data were then analyzed to determine how the in-situ strengths could be 

 estimated given the sample strengths and the residual pore pressures. 

 This involved an empirical correlation of the measured parameters. 



Coring Program 



At least five 10-foot, fixed-piston cores were taken at each of 

 three sites in the Santa Barbara Channel area off Southern California. 

 The coring system used operates in conjunction with the DOTIPOS plat- 

 form (Padilla, 1971) and has been described previously (Demars and 



