Internal angle of friction for crushed limestone 2

[DrMud]

I read with interest a thread now closed that dates back to July 2010 on a similar subject. I am trying to evaluate the internal angle of friction of crushed limestone as a backfill material (max. particle size of 25mm). I had 30cm x 30cm (1ft x 1ft) direct shear box tests on the material. The angle of internal friction came out pretty high (65 to 80 degrees). We are testing at very low normal stress ( 10 to 50kPa). Due to the Mohr envelope curvature, it is not unexpected to have high angles at such low pressures, but these values seem quite high. Anyone has some experience/feedback? Thanks.

 

[Ron]

I would expect a high phi angle in this material unless it is severely gap graded. When crushed, this material tends to autogenously cement itself together due to the high calcium carbonate content. When compacted it makes a very stable soil structure. It is commonly used as base material for roadways.

 

[DrMud]

Thanks, Ron, for your quick feedback. That was my reaction too, but some of my office colleagues felt there must be something wrong with the test results as the angles were "so high". I thought tehey were probably correct as I tried to envision those crushed rock particles trying to shera across past each other. That got to result in a fairly high angle. The one thing that might influence the test results is the size of the particles relative to the size of the box (even though it is large) and thus, at full scale, the effective friction angle might be smaller.

 

[AussieGeoEng]

What did the plot of vertical deformation v horizontal deformation look like?

Was there dilation? You may be measuring the dilation angle and friction angle.

 

[DrMud]

Yes, good point, I know what you mean. However, vertical displacements were negative with negative volumetric strain, which I inferred to mean contractive behavior, rather than dilative, unless the lab has a very unusual sign convention. I am in the process of getting them to clarify/confirm my interpretation that negative vertical displacements (with attendant negative volume changes) mean contractive behavior.

 

[AussieGeoEng]

Compression is commonly adopted as positive.

I had to think about it while running some basic shear box tests the other day.

 

[Ron]

If your lab started with a dry sample and then added water, it is likely that hydration occurred after the sample was compacted into the test mold. That would result in shrinkage.

 

[oldestguy]

That one foot square "cell" is good. I have used this system out on jobs with significant normal pressure, as that available from a back-hoe bucket and arm weight available. Results for soil with many gravel sizes have looked reasonable. My tests did not measure any volume changes, but application of the results has worked out fine, generally used for slope stability calcs. Then, no failures, so I like the method.

I've used it on various waste products also, with success.

The suggestion then, if results are critical, use more normal force.