ABSTRACT

A cooperative investigation was made at a site in 95 m of water off South Pass by Texas A & M University, University of Illinois, Urbana, and ESSA, Miami, aboard the anchored R/V ALAMINOS in March-April, 1969. In place shear strength was measured in 30 cm increments, to a distance of 2.5 m below the bottom, by a 7.6 x 15.2 cm vane attached to telemetering equipment. High quality gravity cores up to 1.8 m long were raised from the same site. Shear strength was measured in three of the cores on board the ship using a laboratory vane shear machine. Three other cores were similarly investigated at Texas A & M and the University of Illinois seven weeks later. Laboratory-type vane shear strength data from short gravity cores collected during a commercial boring investigation and measured aboard the drilling ship, were also compared.

At a depth of about 10 cm, the in-place, shipboard, ashore, and boring strengths were in the range of 0.05 to 0.1 metric tons/m². At one meter, values were 0.4, 0.4, and 0.25 and 0.2 tons/m², respectively. At 2.2 m the in-place shear strength was 0.55 tons/m², and the boring value was 0.2 tons/m². Remolded shear strengths obtained by all methods were nearly zero near the surface. At 1 and 2.2 m the in-place values were greater by factors of 2 to 2.5. The significance of this study is that in-place shear at the location investigated are higher by a factor of 2 to 3 at depths below the bottom of about 1 m. Our results suggest that the foundations of engineering structures designed on the basis of shear strength data obtained from cores may be conservative and have an unnecessarily large factor of safety.

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