Shear Strength Testing for Gravel Materials 6

[ONENGINEER]

To find out the shear strength parameters of a fill (pit run) materials consisting of gravel and cobbles, what sort of laboratory tesing are recommended. Thank you.

 

[fattdad]

to the OP: If you are striving to place gravel and cobbles as an earth fill, you are stating that the maximum partical dimension will exceed 3 inches (the break between gravel and cobbles). Let's say your maximum size is 5 inches. That'd mean the minimum lift thickness would be about 15 inches. A test fill will also allow you to gauge the means and methods for placing these very materials in that very lift thickness. In goes the fill and then on goes the compactor (10T vibratory?).

After 2 passes, what's the density? How about after 5 passes? Did it increase? What was the moisture content for that trial? Change the moisture content and redo. Did you get greater density? Can you find the correct set of conditions that allow for the desired results?

This may have nothing to do (directly) with the friction angle. I'm just saying if you state 95 percent compaction and 8 in thick lifts, it's not correct. 8 in lifts for material with 5 in size is not good form.

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

Oldestguy: Is the a picture for the shear box method you suggested.

Fattdad: When we measure the density of the fill, what would be the target/maximum density to compare with. Really with cobble size materials, conducting a proctor test will not be practical.
I cannot do CPT, DCPT, SPT. Lets say how could I verify that the fill is at dense condition. One may say to use BPT but I doubt it because of calibration issues.

 

[cvg]

run a fully loaded water truck or scraper over the fill, visually observe and make your determination of how compact it is

 

[aeoliantexan]

In "Earth and Rockfill Dam Engineering", George Sowers provides a sketch of a direct shear box 6 feet square by 3 feet high used for L.M. Smith Dam in Alabama. He cites a publication by himself and Gore in the Fifth International Conference on Soil Mechanics and Foundation Engineering, Paris, 1961. It looks pretty simple if you don't need to test at high normal stresses. He also suggests measuring the angle of repose in a conical pile at least 25 feet high, then adding 2 to 4 degrees for compaction. I have measured the angle of such piles by shooting a photo with a plumb line in the foreground, then applying a protractor to the printed photo.

 

[oldestguy]

Not sure of your question as to picture of shear box. Unfortunately, having slowed down now (age 85), all that gear is gone. I am sure the general scheme of things can be worked up by a mechanic and /or welder. How to apply the normal load and how to apply the shearing force can be done by many means. For the normal load, it is a real bear to jack up against anything on rubber tires, since you have that vehicle on "springs". Better to jack against a fixed object unless you have lots of "throw" in your jack. The back-hoe is ideal, since it can get down in test pits and move about the job easily. A boring job for the operator however.

 

[ONENGINEER]

Thank you for the valuable answers by all. I have the answers to my questions at this stage.

 

[Mccoy]

Guys, isn't the angle of repose of a loose stock pile, rigorously speaking, closer to phi_cv rather than phi_peak? If so, that's inherently conservative, probably too much if any kind of compaction is carried out (I read about the suggestion of increasing a few degrees in such an instance and that sounds good although pretty guessestimatic).

Also: why not have a dynamic penetrometry done? Sort of like the DPSH super-heavy rig we use in Italy, it correlates to SPT and getting a phi_peak value is straightforward (pls note I do not sponsor the portrayed brand),

unless it's a cobble-fill, then something like the BPT or LPT would be in order, but I never saw them performed (again, given a certain budget I would use them to build a specific database and then proceed with the data collected).

 

[BigH]

@Mccoy - Nice response but, "Why not?" Cost, I would presume.

 

[dgillette]

The OP has probably gotten way more advice from us than he actually needed or wanted, on a fairly straight-forward issue.

For penetration testing in gravel, the Chinese (mainland) have a large diameter dynamic cone that's getting some attention in the US (notably from Les Youd and Kyle Rollins at Brigham Young U). It can be built at a tiny fraction of the cost of a Becker (BPT) rig, in any machine shop in any country. It's very simple, just a 120 kg donut hammer and a 75 mm conical tip that is larger in diameter than the rods, so the rod friction that complicates BPT analysis pretty much goes away. In China, they are using it for liquefaction potential in gravelly material. Very low tech, but has a lot of promise. Even if it doesn't ever replace the BPT in the US and Canada, it could be a real good tool for the rest of the world, and maybe here also, especially for small jobs.

McCoy - How did you put that picture in?

 

[Mccoy]

McCoy - How did you put that picture in?

Hi dgillette, that's called an 'hotlink' in the internet jargon, since you link up a picture on another website. Once a frowned-upon practice, now it's accepted because of the wide band. A curiosity: last time I posted an hotlink which was rejected happened with a Chinese website.
The procedure to post it is the following:
1)find a suitable picture in any website (watch out the Chinese ones, lol) then left click on it to enlarge
2) right click on it
3)click onto the 'properties' voice in the opened menu
3)copy the URL (be careful, all of it until all the extension)
4)while the mouse is on the wanted position while writing the post, click Ctrl+P, or click on the 'picture' image of the eng-tips menu
5)paste the previously copied URL into the prompt which opens up in the NW corner of the screen
6)click OK

 

[Mccoy]

For penetration testing in gravel, the Chinese (mainland) have a large diameter dynamic cone that's getting some attention in the US (notably from Les Youd and Kyle Rollins at Brigham Young U). It can be built at a tiny fraction of the cost of a Becker (BPT) rig, in any machine shop in any country. It's very simple, just a 120 kg donut hammer and a 75 mm conical tip that is larger in diameter than the rods, so the rod friction that complicates BPT analysis pretty much goes away. In China, they are using it for liquefaction potential in gravelly material. Very low tech, but has a lot of promise. Even if it doesn't ever replace the BPT in the US and Canada, it could be a real good tool for the rest of the world, and maybe here also, especially for small jobs.

Any references, links?

 

[Mccoy]

The OP has probably gotten way more advice from us than he actually needed or wanted, on a fairly straight-forward issue. yawn

Right, then we should grade them!
My grades:
1st: oldestguy method, nothing is best than a direct measurement according to the definition. But, the device should be pretty large since the OP specified gravel with cobbles
2nd: In situ testing by large penetrometers since the OP specified gravel with cobbles, can be costly
3rd: the cited Duncan method in Duncan, 2004 (Friction Angles for Sand, Gravel and Rockfill, Notes of a lecture presented at the Kenneth L. Lee Memorial Seminar Long Beach, California). remains a literature method, not a direct or indirect measurement

 

[Mccoy]

@Mccoy - Nice response but, "Why not?" Cost, I would presume.

Right, then the issue becomes an optimization problem.

If the original poster is working in an area with wide land stretches, cheap material and operators costs, then he can very well use the lower bound 36° value, which will entail a pretty flat (little inclined) angle for the embankment sides but who minds.

If the OP is working in Europe, where estate is very costly, rig costs are high, gravel and cobble is costly, the Eurocodes provisions take down the friction angle to a ghost of itself, then he'll pretty much gain a lot of money by some specific testing. Besides, if the place in Europe is a sesmic one like the place where I live, then we need every last residue of strenght we can collect because a stable slope will be pretty hard to achieve using the pseudostatic stability algorithms.

 

[dgillette]

Cao, Z, T.L. Youd, and X. Yuan "Chinese Dynamic Penetration Test (DPT) for Liquefaction Evaluation in Gravelly Soils" in the ASCE JGGE. It was published on line in October 2012, but I'm not sure whether it's in the print version yet. I can't seem to find it in the heap on my desk, so probably not. Should be soon.

http://ascelibrary.org/doi/abs/10.1061/(ASCE)GT.1943-5606.0000857

Note that it's only about half the diameter of the BPT (170 mm), so maybe not as good in really coarse stuff?

Best regards,
DRG