Triaxial Shear Test on Retaining Wall Backfill Soil

[learning2geotech]

Hi,

If a triaxial shear test is required to obtain friction angle of retaining wall backfill soil, what confining pressures would you apply? Does it matter if I apply anything other than the typical 5, 10, 15 psi? Can I have just 2 circles instead of the typical 3? Is the test result representative of the actual soil condition (my thought is that the actual backfill soil has air in it while the testing soil does not have air since it's been saturated; and the actual backfill is not consolidated while the testing soils is consolidated (CU test))

 

[TigerGuy]

You want to model the actual circumstances in the field, in your lab testing. What confining pressure with the in-situ soil be subjected to in service? The densification of the fill is your consolidation. Why are you testing undrained, when it will be drained in service?

 

[learning2geotech]

Since the confining pressure of the backfill soil varies with depth and the objective of the test is to obtain the strength parameters of the backfill soils, does it matter what confining pressures to apply? If I run it at 5, 10, 15 psi versus, 2, 4, 8 psi, wouldn't I get the same strength parameters (phi and cohesion) at the end?

Densification is not the same as consolidation.

I haven't seen anyone run CD test because it is very time consuming. CU + pore pressure can get you the same results in much less time. I can see justification to run a CD test on an important project like dam, slope failure with very serious consequences. I just don't see justification running a CD test to get strength parameters of a ret wall backfill soil

 

[TigerGuy]

In theory, you should get similar results for phi and cohesion. They can vary slightly with confining pressure. I always like to try and model the test to reflect the actual field conditions. I've often had to throw out one of my circles, because something went wrong and it's nowhere near what it should be.

Densification can be viewed as a rapid consolidation. You are correct that it is not actual consolidation, but the material will consolidate much less if properly densified.

If the CD or CU are being tested the way I learned, the time is similar. You shear at a rate that permits uniform internal pore pressure throughout the sample. If you have high enough permeability, the test should go relatively fast.

 

[GeoPaveTraffic]

If the C-Phi line is linear; then the confining pressures do not matter. However, the line is not linear for all soils across all confining pressures. That said, unless your wall is very tall and your laboratory is VERY good, I would not bother trying to match the confining pressures to the expected field conditions.

As for CU versus CD, I've only seen a hand full of CD tests run in the 35+ years that I've been practicing.

 

[morecoffeeplz]

Run a direct shear test instead.