Shear Strength Testing for Gravel Materials
Shear Strength Testing for Gravel Materials
(OP)
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.





RE: Shear Strength Testing for Gravel Materials
If it wasn't for the problem of soil moisture, you could also just make a trial excavation and back-calculate a friction angle.
f-d
¡papá gordo ain’t no madre flaca!
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
There are very few labs that can actually test material that coarse for strength, and it's likely to cost a pile of money. We once hired U of California at Berkeley to do some 12-inch (30 cm) diameter CIUC triaxial tests for us (long story), but we had to sieve out the larger pieces to be able to test in that size cell (which of course complicates the calculation of relative density and other things). A long time ago, both Berkeley and Raul Marsal at I forget which university in Mexico were doing larger triaxial tests that that, even on rock fill.
To get around the effects of moisture and capillarity, and any cementing agents in a test cut, you could try a test FILL (instead of exc) to see what you get for angle of repose, recognizing that it might be misleading if the stuff is very rounded. A test fill is likely cheaper than a lab program for material that coarse. A test fill can also tell you about compaction procedures, but with material that coarse, it will be tough even to measure density on the fill, which would likely require a 48-inch or larger ring test (water replacement), let alone min and max.
Or, unless this is a very large project with a very large testing budget, you could just do like fattdad says, and assume a conservative strength value (34-36, or more), and see if your design gives a decent factor of safety. Unless you are really tight on borrow quantity or have some unusual material, or there is some other condition that prohibits building flatter slopes or whatever it would take, you probably can't get in too much trouble that way. In 95 cases out of 100, that's probably what I would do (and have done).
RE: Shear Strength Testing for Gravel Materials
I then design my slope based on a factor of safety of 1.5. So when the materials are compacted at 95% Proctor during the actual construction, I will certainly have a higher friction angle and as a result a factor of safety of greater than 3. Do you have any comment on this approach? Thank you again.
RE: Shear Strength Testing for Gravel Materials
"A properly executed test fill program should determine the
most effective type of compaction equipment, the lift
thickness, and the number of passes; maximum rock sizes;
amount of degradation or segregation occurring during
rolling; and physical properties of the in-place fill, such as
density and grain-size distribution."
RE: Shear Strength Testing for Gravel Materials
Last things first:
Having a calculated FS of 1.5, it's rather unlikely that the actual FS could be 3, unless your stability analysis was not only conservative, but ridiculously conservative.
You will not be able to use Proctor tests on this material, and it is no small task to measure either the vibrated maximum density or the fill density with cobbles present, or even just large gravel. Think about whether you really need to quantify the density accurately, or if you can simply use a procedure spec, something along the lines of 50 cm lifts, each rolled by 4 passes of a ten-tonne vibratory smooth drum roller, with water applied on the fill as needed. Here is where a test fill is helpful by telling you whether the compaction has reached its limit with 4 passes or it needs 6(for example), whether additional passes would just break up the particles without accomplishing anything, whether a water hose helps, and things like that. Plus, it gives you an excuse to go out to the site and play with soil and machines. The test fill will need to be several lifts high, preferably two lanes wide, and long enough to replicate the actual placement techniques that will be used.
I don't think letting the stock pile age for a week is going to affect things much, unless there is a heavy rain that makes the slopes slough.
Depending on the shape of the particles, the angle of repose in stockpile may or may not give you a very good estimate of the angle of internal friction. If you put marbles in a drained triaxial test, they will give a pretty good φ, maybe 30 degrees or so. If you pour them out on the table, they roll away, angle of repose ~0. Rounded cobbles may segregate and roll when they are dumped onto a stock pile, and they can roll down fairly flat slopes if dislodged. On the other hand, if there is not a problem with rolling, and there is no capillarity or other "binder" present, the stock pile could give you a pretty good estimate of friction angle.
Bon chance!
RE: Shear Strength Testing for Gravel Materials
There are some articles by Breitenbach on placement of rockfill, materials, etc. that would prove useful. The articles I have seen seem to point towards using smaller rockfill than I am used to placing. Typically, I would not use more than 40 degrees in design and perhaps less depending on type of rock, potential for breakdown, etc.
RE: Shear Strength Testing for Gravel Materials
I then design my slope based on a factor of safety of 1.5. So when the materials are compacted at during the actual construction, I will certainly have a higher friction angle and as a result a factor of safety of greater than 1.5. Do you have any comment on this approach? Thank you again.
Thank you for your attention.
RE: Shear Strength Testing for Gravel Materials
It would really help this discussion if we knew what you were building, the slope heights and such.
Read the Duncan paper too! I'm sure you can get it through CGPR (Center for Geotechnical Practice and Research) at Virginia Tech.
f-d
¡papá gordo ain’t no madre flaca!
RE: Shear Strength Testing for Gravel Materials
Interesting that you Berkley run 12-inch-diameter triax tests. My thiess, also a very long time ago, included triax testing on course coal refuse. I compared test results on 1.5-inch, 3-inch, and 12-inch-diameter remolded samples.
Prevailing thought, at least at that time, was that when you tested smaller material you got a lower friction angle due to removal of larger (presumably stronger) particles. My testing indicated that was not always the case; however, the differences were generally small +/- 1.5 degrees.
ONENGINEER
As has been pointed out/asked; we really need to know more about what you are building to be able to provide useful information.
Mike Lambert
RE: Shear Strength Testing for Gravel Materials
Thanks everyone for the comments.
PS: Have some questions about GeoPaveTraffic seemeingly complicated research but that distances me from my main question here.
RE: Shear Strength Testing for Gravel Materials
Mike Lambert
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
However, how can one estimate the friction angle during a test fill, as indicated by dgillette. The side slopes of the fill may not stand under the angle of repose for a few lifts because the side angles are affected by the method of compaction, the machinery used, labour interference etc. Or does dgillette mean that I should excavate within the compacted fill and measure the angle of repose from the excavation side slopes
RE: Shear Strength Testing for Gravel Materials
http://www.ausenco.com/uploads/papers/64068_Rockfi...
Google " Breitenbach rockfill ".
I would make the rockfill test pad, say 8 m wide at the bottom and use a 1H:1V slope as you come up - you won't be putting on any more than 3 layers anyway (don't fret too much about "edges" - your compactor operator will not go to the edge anyway. You will want to try lifts of, say, 400 mm, 600 mm and perhaps even 800 mm thick and see what kind of compactive response you get - assuming you will be using a large 15 tonne or more vibratory roller. As noted by the paper above, paint some marks on the test pad layer and then take elevations of the mark after 2, 4, 6 and 8 single passes of the compactor; develop a pass vs elevation drop curve. Typically, we haven't gone more than 8 passes - so particles don't break down.
I am not sure what materials your embankment will retain (copper tailings?, uranium tailings?, etc) but you need to be cognizant, too, of other and likely more important aspects than friction angle of rock fill. You will need to address filters to the rockfill - presuming this might be a rockfill toe of a, say, till embankment.
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
I've used this method many times to find on-site properties of waste piles, natural earth as well as compacted fill. Once you run a few, it is a simple operation. My shear box was one foot square, but you can make it any size to suite your material, as long as your loading gear can do the job.
For weak stuff, such as fly ash or waste bark, all you need is a 5 gallon bucket for the load and a spring loaded scale for the lateral loading. Weigh the bucket with the scale.
RE: Shear Strength Testing for Gravel Materials
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!
RE: Shear Strength Testing for Gravel Materials
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.
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
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).
RE: Shear Strength Testing for Gravel Materials
RE: Shear Strength Testing for Gravel Materials
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?
RE: Shear Strength Testing for Gravel Materials
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
RE: Shear Strength Testing for Gravel Materials
Any references, links?
RE: Shear Strength Testing for Gravel Materials
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
RE: Shear Strength Testing for Gravel Materials
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.
RE: Shear Strength Testing for Gravel Materials
http://ascelibrary.org/doi/abs/10.1061/(ASCE)GT.19...
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