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Geotechnical Problem Solving by John C. Lommler

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2.4

Shear Strength Testing – What is Wrong with the Direct Shear Test?

2.4.1 Introduction to Direct Shear Testing

Geotechnical texts often state that you should use some form of the triaxial test to determine the drained or undrained shear strength of soil. The direct shear test is dismissed as having an uncontrolled stress state due to rotation of principle stresses.

The direct shear test is simple, inexpensive, and doesn’t require an 18-inch thick uniform, high-quality sample (Lambe and Whitman, 1969, see page 120). If you are testing remolded samples, such as sands for compliance with Federal Highway guidance on MSE (mechanically stabilized earth) reinforced backfill, it’s easier to compact three similar samples of sand for a direct shear test than prepare three comparable samples for triaxial testing. Besides, FHWA publication FHWA-NHI-10-024, November 2009, recommends use of the direct shear or triaxial tests to evaluate the friction angle of reinforced fill material (specifying that it be at least 34 degrees, as determined by the direct shear test). Earlier versions of the FHWA MSE design guide required use of the direct shear test only, and didn’t even mention use of the triaxial shear test.

So the question is: What is wrong with the direct shear test and why worry about rotation of principle stresses? You might not recall from college days what rotation of principle stresses is, so I’ll refresh your memory.

2.4.2 Direct Shear Rotation of Principle Stresses

Principle stresses ...

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