Shear Strength Testing

[Dirtguy4587]

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.

 

[rockiologist]

Direct Simple Shear (DSS) combines the direct shear test with the pore pressure measurments of the triaxial and allows the soil to shear along the weakest plane. It is not forced like the direct shear. The DSS sample is placed in a membrane (like and triaxial) but is shear like a direct shear. You run the test like a triaxial, confining pressure, strain rate, noraml load. Just make sure the test is run slowly if you are looking for a drained condition. The machine to run the test is very expensive so only a handful of labs have it. Hope this helps.

 

[BigH]

I've seen a simple graphic indicating the kind of test to run and "where" - I thought it was in Lambe and Whitman but couldn't find it. Basically, when the shearing plane is "near" horizontal, the strength parameters would be chosen using the direct shear test (or simple direct shear). When the shearing plane is "sloped" one would use the triaxial (compression or extension) where the failure plane in the specimen would 'closely' follow that in nature. In your case, it sounds like you are dealing with a fissured/slickensided material which has distinct geologic features. I would posit that you would want to determine the strengths that would be aligned "along" the geological feature and one that "cross" the feature. small specimens are likely to miss such features and one could argue that they are not realistic - which is why your area seems to use, in my view, the residual (or remoulded) strengths as being inherently the "worse-case" scenario. I'll try to find that graphic. Just some thoughts . . .

 

[GeoPaveTraffic]

I to am not familure with the direct simple shear test. Given that the project is large dam, thereby justifiying significant testing; at a minimum I would run both direct shear along bedding planes to determine peak strength along the plane and residual strenght and traixial shear testing to determine cross bed strength. Make sure to run a fair number of both kinds of tests so realistic design values can be determined. As for the DSS, maybe worthwile maybe not. If everyone is so unsure about it, not sure that anyone will believe the results and if you will not believe the results, then no point in runing the test.

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.

 

[Dirtguy4587]

Thanks for the comments. Mostly in line with my design philosphy. I am developing (and have) a large database of peak and residual direct shear results, for the various soil units. Certainly, using residual strengths (from direct shear) would be the lower-bound design case. I justify using slighly higher than residual values by looking at geologic undulation and considering asparaties, etc. Call it a "resonably conservative" design.

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.

 

[dgillette]

If the clay is fissured, seems to me that its strength is governed by the discontinuities, rather than intact material. As the DSS sample is ~3 cm thick, it may not capture the gross-scale strength. A wire cut or repeatedly sheared direct shear test could be a little conservative, but I don't have any better tricks up my sleeve. DSS machines are not real common. In fact, last week I traveled two time zones away to a lab doing some DSS tests for my project.

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.