C. LABORATORY PROCEDURE 



General Stat-ement -- Briefly summarized, the laboratory test program was to: 

 (1) carefully cut the plastic liner, or barrels of metal or plastic, into lengths of about 

 15 cm (6 in), (2) push 5.1 cm (2 in) long, thin-wall brass tubes of 4.9 cm (1 .94 in) 

 diameter into each length for measurement of mass and volume, (3) gently extrude 

 sediment from each tube for the compression test, and (4) quarter each sample length- 

 wise following the strength test; the first and third quarters were combined and used 

 to determine water content and the specific gravity of solids, while the second and 

 fourth quarters were combined and used for Atterberg limit tests and mechanical 

 analysis. Vane-shear tests were made in sediment contained In the brass tube. Sep- 

 arate samples were used for consolidation tests. Unless specifically noted later, the 

 5-cm sample length was standard. 



Shear Strength — A simple compression test device with plastic platens at either 

 end of the axial loading rod (Fig. 6) was used for measurement of unconfined compres- 

 sive strength. A brief resume of the test steps and ensuing computations used in the 

 BUDOCKS laboratory is given In the appendix. Test data result in a stress-strain re- 

 lationship (Fig. 7a). Failure of the sample was taken at the point of greatest curva- 

 ture of the stress-strain line or. If this point was undeterminable, arbitrarily at 20 

 percent axial strain. Compression strength, p^, is related to shear strength, s, by 



s = 0.5pc (5) 



A discussion of the theoretical reasons for this relationship is given by Skempton 

 (1942, Appendix 2). 



By mid-1959, commercial laboratory vane-shear equipment (Fig. 8) was obtained 

 (from Wykeham Farrance Engineering Ltd.) by the Hydrographic Office and used at 

 BUDOCKS solely, on some cores, or alternatively with the compression test equip- 

 ment. This machine uses a vane 1 ,3 cm (0.5 in) in diameter and 1 .9 cm (0.75 in) 

 long rotated by a constant-speed motor at a rate of 6 degrees per minute (0.1° per 

 second). The vane was buried in sediment prior to measurement by a distance not 

 less than Its length. Test procedure Is summarized in the appendix. Results from the 

 test are expressed In a stress -vane rotation relationship (Fig. 7b). Sample failure Is 

 determined at the major Inflection of the stress -rotation curve. When water-saturated, 

 cohesive sediments are tested at an unaltered water content, the vane test directly 

 measures shear strength. Osterberg (1957) has reviewed the history of the vane test. 



Certain samples were completely remolded to destroy all natural structure and 

 again tested to determine the remolding sensitivity (Terzaghi, 1944, p. 613), S|, at 

 an unaltered water content: 



-. "undisturbed" strength , , 



^t = rrz r^— (6 



remolded strength 



17 



