Remolded shear
Remolded shear
(OP)
Hello,
I have a simple question about the remolded shear testing, all the materials are not meeting the specs required (C of 590, Phi of 28 degrees) the materials are silty sands most of the time. i am performing the remolded shear on the material passing sieve #4, the ASTM don't mention anything about remolded shear passing a certain sieve.
your comments are appreciated
Sincerely,
I have a simple question about the remolded shear testing, all the materials are not meeting the specs required (C of 590, Phi of 28 degrees) the materials are silty sands most of the time. i am performing the remolded shear on the material passing sieve #4, the ASTM don't mention anything about remolded shear passing a certain sieve.
your comments are appreciated
Sincerely,





RE: Remolded shear
Are you using a direct shear box? Or triaxial setup?
How long are you allowing the samples to sit ("cure") before you are testing them?
RE: Remolded shear
all results are not meeting the specs, i think that is because of the materials, not enough cohesion, to meet their standards i know the materials should be clayey sands (Standards call for c:23.9 KN/m2 , Phi 28 degrees and 25% passing #200). some of the materials meets the passing 200 and the phi is close to 28, but i am getting low cohesion, the samples are being consolidated a minimum of half hour to 45 minutes in the direct shear box and at least 24 hours consoled in water. my wonders are maybe i shouldn't use the -#4 materials for remolded shear.
Sincerely,
RE: Remolded shear
RE: Remolded shear
Is there any way you could modify the specs and use effective stress parameters instead of parameters involving "cohesion?" The effective stress approach would yield a phi angle (without a cohesion intercept) that represents the conditions you are modeling (drained or undrained shear). This approach is fundamentally more correct in terms of soil behavior. The triaxial test would probably be a better set-up to monitor pore pressures during testing to allow you to use effective stress parameters. The cohesion intercept is, in many ways, one of the great "untruths" of the soil mechanics world. In my opinion, solid rock is really the only geo-material that has cohesion. Any cohesion you're observing in the test as you have it now is probably the result of pore pressure build-up because I don't believe that silty sand, by the description alone, would have any "cohesion", as anyone knows it, anyway. How much silt is present in the specimen? How does the remainder of the grain size distribution look?
On a side note, you could probably meet the "cohesion" specs by over consolidating the silty sand like crazy and running a very quick test (saturated, undrained). The pore pressure created in the highly oc silty specimen would cause a larger "cohesion" intercept. Of course I'm only joking now...this test would be a trick. It would not represent the actual loading conditions. Also, the "cohesion" intercept would dissipate with time.
RE: Remolded shear
And that brings me to an important point for your testing program: you could probably meet the test result requirements by increasing the normal (vertical) force. This will have the effect of reducing the phi value somewhat, but will increase the apparent value of cohesion.
But that isn't a good strategy, after all - it isn't ethical to choose only those test results that allow the material to pass the criteria. You would be better served to perform enough tests to define the curved failure envelope, then present the results to the engineer(s) who specified the criteria. Plot the criteria on the same graph, and show them that the soil meets their criteria under certain circumstances. This should help you resolve your problem -
RE: Remolded shear
RE: Remolded shear
I know that you understood the implications of 95% compaction behind your hypothetical retaining wall, but for the novice:
Specifying a high degree of compaction behind a retaining wall will cause the soils to be more dense, giving a larger effective value of phi. But it will also induce lateral stresses within the soil mass that can exceed the capacity of the wall, causing the wall to crack. The high stresses will also require more horizontal movement to occur before the design active pressures are achieved -
Just a little clarification.
RE: Remolded shear
The point I was making about the backfill parameters is that many times, the designer just puts this in as he doesn't know what the "fuzziness" of his design can take. Then, he can always blame the constructors.
Still, too, it is hard to fathom how experienced contractors keep on piling on the big rollers in the abutment backfill area!!
Best to you all