Direct Shear Test - Establishing Shear Rate

[GeoWade]

AASHTO T236 and ASTM D 3080 indicate that the specimen should be sheared slowly to ensure complete dissipation of excess pore pressure. They suggest that the rate of loading should be such that the time to failure will be at least 50t50 (t50 based on data from the consolidation phase of the test). However, to estimate the displacement rate, one must then estimate the displacement at failure. However, the specifications provide no commentary on how to do this. I was simply wondering how others have addressed this issue to determine the proper displacement rate. Is there any published discussion or commentary that provides any guidance regarding this issue? What is the practice of others to establish appropriate shear rates for the direct shear test.

 

[harney]

GeoWade--

The displacement at which peak strength is mobilized is usually based on experience.

K.H. Head has a discussion on this subject in Volume 2 of his "Manual of Soil Laboratory Testing" (Pentech Press, 1994). In the Second Edition, this is in section 12.7 on pp 247. He includes a table of recommended values, which depend on the type and density (or stiffness for cohesive) of soil. The range is from about 5 to 8 mm for loose sands (2 to 5 for dense) to about 1 to 2 mm for hard clay (2 to 5 for stiff). He recommends 8 mm for plastic clay.

Bill Kovacs (University of Rhode Island) notes that about 0.2 inches is adequate for most specimens (around 5 mm).

If inexperienced with this soil type, it may be wise to err on the side of caution and use a lower estimate of displacement at peak strength (giving a slower displacement rate).

Hope this helps.

M. Harney
University of Washington