Shear Strength Testing
Shear Strength Testing
(OP)
I'm currently developing a design for a large dam founded on thick, heavily overconsolidated, pre-sheared marine clays. The residual strength of the foundation material is very low - experience in the area uses friction angles of 8 to 12 degrees for design.
Our standard practice is to use direct shear (ASTM D3080) to determine the residual angles (test results usually 6 - 10 degrees), and then add 2 degrees for design to account for large scale geologic features (i.e. undulation, etc.). This has been the practice in the area for 30-some years.
A collegue that is not familiar with the standard practice in the area has suggested that the direct shear tests are not accurate, and likely too conservative to use for design. The recommendation provided to me is to consider alternate testing, such as Direct Simple Shear (which I have never actually used - I don't think there is a DSS machine anywhere nearby), or triaxial testing.
My first reaction to triaxial testing is that the results would more likely give me a cross-bedded strength, rather than a residual angle (keep in mind that these are marine deposits, and the shearing planes are almost horizontal along the bedding).
Can anyone help me in explaining how a DSS or triaxial testing may be more 'accurate' than using the direct shear and local experience.
Our standard practice is to use direct shear (ASTM D3080) to determine the residual angles (test results usually 6 - 10 degrees), and then add 2 degrees for design to account for large scale geologic features (i.e. undulation, etc.). This has been the practice in the area for 30-some years.
A collegue that is not familiar with the standard practice in the area has suggested that the direct shear tests are not accurate, and likely too conservative to use for design. The recommendation provided to me is to consider alternate testing, such as Direct Simple Shear (which I have never actually used - I don't think there is a DSS machine anywhere nearby), or triaxial testing.
My first reaction to triaxial testing is that the results would more likely give me a cross-bedded strength, rather than a residual angle (keep in mind that these are marine deposits, and the shearing planes are almost horizontal along the bedding).
Can anyone help me in explaining how a DSS or triaxial testing may be more 'accurate' than using the direct shear and local experience.





RE: Shear Strength Testing
RE: Shear Strength Testing
RE: Shear Strength Testing
By the way, for stability analysis I would use software that allows different strengths for different orientations of failure plane. That way you can model the bedding and cross bedding shear strenghts.
RE: Shear Strength Testing
BigH is correct - this is slickensided material, with failure planes near horizontal. As such, I question the use of triaxial tests to determine shear strengths, aside from giving some insight into the cross bedded strength. Based on the comments with respect to DSS, I would think that the results from this type of testing would be similar to the results from triaxial testing (or perhaps a hybrid of the direct shear/triaxial results)- am I correct?
My level of comfort remains with basing my design on direct shear testing, supported with a large amount of data. Furthermore, I plan to run an instrumented load trial prior to construction to confirm actual in-situ conditions.
RE: Shear Strength Testing
Dirtguy is correct about the triaxial tests not capturing the strength of the horizontal shears.
A drawing like Big H is referring to is in Ladd's 1989 Terzaghi lecture, which I think was in the ASCE JGE in 1991.