When the direct shear test is performed in accordance with ASTM guidelines, the measured shear stresses at failure estimate drained strength parameters. We investigate the possibility of estimating undrained strength using direct shear testing at variable shear displacement rates on specimens composed of various combinations of kaolinite and bentonite. Even at fast displacement rates, constant volume conditions are not achieved in the direct shear device because of changes in specimen height that are large relative to allowable ASTM thresholds for constant volume simple shear testing. However, undrained strengths established by constant volume simple shear testing at slow strain rates are well approximated by direct shear tests conducted at fast shear displacement rates (time to failure < t50/8, where t50=time to 50% consolidation in a conventional oedometer test). Because of the simplicity of direct shear testing, such estimates of undrained strength may be useful in engineering practice when access to a simple shear device is limited. Nevertheless, fast direct shear tests have shortcomings, including lack of control of rate effects, and constant volume testing is recommended for critical projects.
Estimating Undrained Strength of Clays from Direct Shear Testing at Fast Displacement Rates
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Geo-Congress 2013
Estimating Undrained Strength of Clays from Direct Shear Testing at Fast Displacement Rates
Abstract
Authors
Fugro Consultants, Inc., 1000 Broadway, Suite 440, Oakland, CA 94607. E-mail: abro@fugro.com
Dept. of Civil & Environmental Engineering, University of California, Los Angeles, 5731 Boelter Hall, Los Angeles, CA 90095. E-mail: jstewart@seas.ucla.edu
Group Delta Consultants, 370 Amapola Ave., Suite 212, Torrance, CA 90501. E-mail: dpradel@ucla.edu
Published online: March 28, 2013
Geo-Congress 2013
March 3-7, 2013 | San Diego, California, United States
© 2013 American Society of Civil Engineers