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Granular backfill slope, reducing active pressureHelpful Member! 

glacialclint (Geotechnical) (OP)
26 Feb 10 13:32
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.
 
Helpful Member!  fattdad (Geotechnical)
26 Feb 10 14:13
Well your Rankine geometries are correct. For a 10 to 12 ft tall wall, I'd likley step back three to four feet and then use a 1/2H:1V slope.  I'd also check the active wedge using the friction angle of the clay (measured from the toe) to see just how much gravel and clay is in the intercept.  Maybe I'd use a weighted value to account for the presence of this clay in the wedge.

Then there is also the issue of constructability. . .

f-d

¡papá gordo ain't no madre flaca!

cvg (Civil/Environmental)
26 Feb 10 14:51
the other reason for using granular backfill is to reduce hydrostatic pressure that could develop behind a wall. Granular backfill with foundation drain or weepholes will intercept the phreatic surface and drop it down to the level of the weepholes near the footing.  
glacialclint (Geotechnical) (OP)
26 Feb 10 15:53
Thanks for the input fattdad. By stepping back 4 feet on a 12' wall before stepping up 1H:2V; a conservative 45 degree failure wedge would intersect the granular/cohesive boundary 8 feet above the wall base. Given a triangular stress distribution with depth, almost 90% of the pressures being exerted by the cohesive lateral coefficient would be reduced to the lower granular coefficient. If you give the clay much of a friction angle at all (about 12 degrees), the failure wedge would be completely within the granular backfill, by rankine theory.

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 :)
VAD (Geotechnical)
21 Mar 10 3:12
Suppose I had to excavate a stiff clay soil. build a retaining wall and then backfill between the wall and the excavated soil face would I need to be concerned about the dimensions of the wedge of granular material. Assume that the clay soil would be stable during the course of excavation and building of the wall and the clay stands almost vertical.

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.

    
glacialclint (Geotechnical) (OP)
22 Mar 10 13:59
VAD, I think I understand your scenario.

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?  
fattdad (Geotechnical)
22 Mar 10 14:12
the cohesion intercept in clay will fatigue over time.  This will result in a stress increase that can rotate or fail the wall if cohesion is included in design.

f-d

¡papá gordo ain't no madre flaca!

glacialclint (Geotechnical) (OP)
22 Mar 10 14:30
f-d,

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?
 
fattdad (Geotechnical)
22 Mar 10 15:33
Let's not get into the modeling of tension cracks with fluid pressure on slope stability assessments in this discussion.  It's a different topic (well, at least to me).

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!

Geostructsparks (Geotechnical)
25 Mar 10 9:58
Some but not all cohesion is caused by negative pore pressures in the soil, therefore over time as the pore pressures due to initial construction dissipate, the strength will drop back to low cohesion and an effective stress friction angle will control again.  If you designed for significant cohesion, it still may not fail immediately, since capillary rise/suction may affect your surface soils, plus any significant overdesign (e.g. that 1.5 factor of safety) may also hold it up longer.  Since retaining walls are designed for the long term condition, and also to withstand extreme events (in this case, fully saturated clay such as due to prolonged wet weather or spring melt), one designs for the long-term condition, i.e. the effective stress strength, even in clays.

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.
Jalthi (Geotechnical)
26 Mar 10 14:46
Culmann theory can be used to determine the backslope behind the granuar fill.

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