Granular backfill slope, reducing active pressure
Granular backfill slope, reducing active pressure
(OP)
Sorry for the long post with my question. I've got some background in my question so basic soil mechanics won't need repeating.
It is common to use granular backfill behind retaining walls instead of cohesive backfill to reduce wall pressures. Reasoning being that a higher friction angle results in lower lateral coefficients and cohesive strength is typically ignored in design (at least in local firms to Iowa). My question concerns the geometry of the granular backfill behind a wall that is required to reduce the lateral pressures to those of granular material (when granular backfill is placed between the wall and existing cohesive soils). The two commonly reported slopes found in local geotechnical firms' standard sections are either 1:1 and 1:2 (Hor:Vert)extending up and away from the wall base. These slopes are imaginary lines where granular backfill is between the line and the wall and cohesive soil is allowed on the other side. I was wondering if anyone knew the theory behind a 1:2 slope, if anyone had seen any alternative recommendations, or if anyone had any interesting ideas/background on this topic. Again, we are looking at standard section verbiage here where no triax or plane strain testing has been completed to determine phi.
My thoughts are the 1:1 slope appears to be conservatively derived from a friction angle of 0 for cohesive soils and a failure wedge inclination of 45 + phi/2. But then by that math, a 1:2 would need a phi = 37 degrees, not conservative and not typically applicable to a cohesive soil wedge...
Thanks in advance for any help.
It is common to use granular backfill behind retaining walls instead of cohesive backfill to reduce wall pressures. Reasoning being that a higher friction angle results in lower lateral coefficients and cohesive strength is typically ignored in design (at least in local firms to Iowa). My question concerns the geometry of the granular backfill behind a wall that is required to reduce the lateral pressures to those of granular material (when granular backfill is placed between the wall and existing cohesive soils). The two commonly reported slopes found in local geotechnical firms' standard sections are either 1:1 and 1:2 (Hor:Vert)extending up and away from the wall base. These slopes are imaginary lines where granular backfill is between the line and the wall and cohesive soil is allowed on the other side. I was wondering if anyone knew the theory behind a 1:2 slope, if anyone had seen any alternative recommendations, or if anyone had any interesting ideas/background on this topic. Again, we are looking at standard section verbiage here where no triax or plane strain testing has been completed to determine phi.
My thoughts are the 1:1 slope appears to be conservatively derived from a friction angle of 0 for cohesive soils and a failure wedge inclination of 45 + phi/2. But then by that math, a 1:2 would need a phi = 37 degrees, not conservative and not typically applicable to a cohesive soil wedge...
Thanks in advance for any help.





RE: Granular backfill slope, reducing active pressure
Then there is also the issue of constructability. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Granular backfill slope, reducing active pressure
RE: Granular backfill slope, reducing active pressure
Still, this would require about the same amout of sand, maybe more, as just placing on a 45 degree angle. But it would allow space for a drain behind the wall, which would make cvg happy.
As for constructability, that's between the installer, OSHA, and the "not in my back yard" neighbor to discuss between themselves :)
RE: Granular backfill slope, reducing active pressure
It is of interest to check Terzaghi and Peck's empirical relationship for walls 20 ft in height or less. Of course the lateral pressures are higher for cohesive backfill materials, but is there a message for these size of walls. What are the savings.
RE: Granular backfill slope, reducing active pressure
Your vertical cut gets to use cohesive strength and can stay vertical for some time, although OSHA will have problems with you if you have a high vertical cut. (if you give your excavation wall enough time, the vertical face will begin to develop tension cracks which cause slabs of clay to fall away from the cut)
Here we are assuming c=0 as a conservative premise in computing lateral pressures,. i.e. the clay is being modeled just as a granular material with a very low friction angle. The geometry of this low friction angle material is therefore very important due to it increased later stress on the wall, per Rankine formula. So I would say you should be concerned. If, however, you choose to incorporate cohesion into your lateral pressures, you may find that the clay performs better than the granular and you may not be concerned about lateral pressures from the clay after all and just specify granular to allow wall drainage. Does this make sense?
I'm interested in the empirical relationship you mentioned. Can you point me towards a paper or an on-line source?
RE: Granular backfill slope, reducing active pressure
f-d
¡papá gordo ain't no madre flaca!
RE: Granular backfill slope, reducing active pressure
Right, there is merit to not considering cohesion. More than just conservatism I mean. You bring up a point I would like to explore.
The removal of cohesive strength above a "tensile crack" and application of cohesive strength below appears to the the approach suggested by soils mechanics class texts. Then adding hydraulic pressure or a reduced earth pressure to the tension crack zone. Do you think the cohesion would degrade over the time if the soil isn't in tension, i.e. below the tensile crack (where: overburden pressure x K > 2c)?
Seems to me like the answer to my own question would be "yes" in an active pressure situation where the wall moves enough to reduce confinement and allow tensile forces on the soil but "no" in an at-rest situation where lateral deflections should be minimal.
Thoughts anyone?
RE: Granular backfill slope, reducing active pressure
I don't trust cohesion on clay soils over time. I'd not trust cohesion whether I was modeling active or at-rest earth pressures.
This is what my professor (J. M. Duncan) told me and I took good notes.
f-d
¡papá gordo ain't no madre flaca!
RE: Granular backfill slope, reducing active pressure
There is some "real" cohesion in granular soils due to dilative behavior (interlocking) or locked in stresses however this might be in the range of 50 to 200 psf in addition to the friction angle. Someone might argue that if an active condition develops, one might have developed a slightly deformed failure zone which may disrupt this interlock. Unless you have plenty of data or a lot of confidence this cohesion is generally ignored. (This is also probably there in clays, but is probably even more controversial).
Even clays in undrained failure typically fail at 45 + phi/2 (looking at an unconfined compression test will confirm that) however if the friction angle is 20 degrees, 1:1 or 45 degrees is conservative.
RE: Granular backfill slope, reducing active pressure