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A wireline adaptation of the borehole shear test (BST) with high-pressure shear plates was one of several in-situ test methods selected to measure strength of over-consolidated phosphatic clays and lime rock at proposed pier locations for the replacement Sunshine Skyway Bridge, Tampa Bay, FL. Although the site's soils ordinarily would be judged too hard for stage BSTs, an on-site decision was made to try it in lieu of single-point testing to save time. Of 18 stage tests attempted in very hard clay, 10 gave satisfactory failure envelopes, 5 gave envelopes indicative of residual shear strength, and 3 gave invalid envelopes attributed to progressive seating of the shear plates. Stage tests in the harder rocks and shell were less successful. The average cohesion from the 10 satisfactory stage tests in the overconsolidated clays was 91 kPa (1900 psf) and the average friction angle φ = 25.7°. The BST data proved useful for converting a large number of undrained penetration tests into the effective stress strength parameters needed for design. The additional use of special BSTs using smooth plates also permitted site-specific shear strength corrections for soil against steel piles and casings.
field test, shear strength, boreholes, angle of friction, cohesion, borehole shear test, underwater, coefficient of sliding friction
Professor of civil engineering and consultant, Iowa State University, Geotechnical Research Lab, Ames, IA
Principal, Schmertmann and Crapps, Inc., Gainsville, FL
Associate professor of civil engineering and consultant, Clarkson University, Potsdam, NY