Typical EFP? 3

[SKIAK]

What are some "typical" values that get used for EFP?

I'm asking because I've been doing some work on a few reinforced concrete retaining walls recently. My supervisors pass off the important information and I go to work. With geotech and loading info in hand I try to keep sizes for my walls in general dimensions (all references I have say pretty much the same but looking specifically from Bowles "Foundation Analysis and Design" chapter 12-2 page 433... just so you know where I'm coming from). I have a hard time coming up with these dimensions and my supervisors are always suprised at how thick my walls are, how long my footings are for sliding (they would prefer that I didn't use keys), and how much rebar I have. After doing a little searching around it seems to me that the EFP that I get off the geotech report (60pcf for clay/silty) is a fairly high number in the spectrum of soils, which might be why (I'm hoping) that my layouts end up so large. Any thoughts?

 

[SKIAK]

OK, at second glance I could have asked this question differently. I'm not confused about EFP values, but the common proportions of retaining walls. Odviously the proportions will be different under different loading, I guess I am wondering what conditions these kind of proportions can be expected. Is it reasonable that 60pcf EFP (with level fill, no surcharge, say 10' retained height) would exceed these values? Just looking for a little dialogue on the subject.

 

[RWF7437]

For a very rough check on the resonablesness of your wall proportions you might check State Highway Standard drawings. Thousands of walls of many types have been built successfully using such standards. Still, be cautious in any generalizations you draw from these.

 

[fattdad]

For retaining walls that are allowed to rotate at the top (i.e., mobilize active earth pressure, 60 pcf seems a little high. It implies an at-rest earth pressure for a friction angle of 30 degrees and a moist unit weight of 120 pcf. For at-rest conditions (and also a 30 degree friction angle), you'd end up closer to 40 pcf.

Now the question is whether you want to design your wall to rotate 1 inch in 10 ft or not. Also, what backfill can you truely depend on, and will the slopes at the top and the toe be horizontal.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[fattdad]

Correction to third sentence: For ACTIVE conditions (and also a 30 degree friction angle), you'd end up closer to 40 pcf.

Sorry for any confusion.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Why don't you use the Coloumb or Rankine approch to see what they get vs your EFP comps? I have never used the EFP approach . . .

 

[SKIAK]

RWF7437,
Thats a good idea, I checked the state's typical drawings and they are for 27pcf EFP, but are fairly smaller in size.

fattdad,
I'm making the assumption that at-rest vs active conditions means the same as non-yielding vs yielding walls. The "Conclustions and Recommendations" in the geotech report are 90pcf and 60pcf for non-yielding and yielding walls respectively. I am not given a friction angle so I was back calculating to obtain that of 19.48 deg (I think thats OK, I've seen it done before). The deflection of the wall will not be a problem.

BigH,
I don't have enough info from the geotech report to be able to do Coloumb or Rankine. Otherwise I agree and would feel a little more confident where I have been taught them in school and have a better understanding of them.

 

[PEinc]

Check with AASHTO. I think there is a minimum active EFP of 35 PSF per VF. If this is not a highway project, you probably do not need to follow AASHTO.

 

[civilperson]

Try "CRSI Design Handbook" for tabular solutions to thousands of retaining wall designs. Chapter 14 has 10' high wall with level backfill using 12" thick stem wall with 3'-11" heel and 6'-5" width and no key, (130 pcf soil, friction mu=0.45 and phi of 30 degrees).

 

[DRC1]

90 psf for non yielding and 60 psf for yielding seem very high, esp. for a 10 ft wall. Because the soil is sity clay, the geotech may be concerned abut drainage and is putting a fudge factor (a big one) for water build up behind the wall. If you talk to the geotech, you may be able to specify drainage details he would be happy with. He probably does not realize the impact of his numbers.

 

[SKIAK]

PEinc,
Not AASHTO, just some wall out in amongst the grass.

civilperson,
Bingo. Good reference, I had no idea that was there. We only had the 1992 edition but our numbers were pretty close. Thanks.

DRC1,
The numbers seemed high to me too after doing a little digging around. I will suggest that it might be worth to do a little poking around and see if we can use any smaller number with proper detailing. I would imagine that it would help significantly if we could.

 

[fattdad]

Just to confirm, yes, active is yielding and at-rest is non-yielding. Regarding wall drainage: Given a frictin angle of 30 degrees and a moist unit weight of 120, the EFP for drained backfill would be 0.33*120=40 pcf. The submerged EFP would be 0.33*(120-62.4)+62.4=81.4 pcf. Approximating at-rest conditions you'd then have 120*(1-sin30)=60 for drained conditions and for submerged conditions you'd have (120-62.4)(1-sin30)+62.4=91.2 pcf.

Then again, the friction angle may not be 30 degrees.

f-d

¡papá gordo ain’t no madre flaca!

 

[SKIAK]

The friction angle could be part of my confusion. The geotech report did not give a value for it. I came up with a little spreadsheet to find both Coulomb and Rankine values for phi based on the EFP and density. You have to use goal seek (by changing phi in the yellow to get whatever the estimated Ka is from the top to be the same as the Ka below) to find phi. Using this I get around 15-20 degrees for phi. Is this a valid procedure? This seems very low. The spreadsheet is very crude, sorry.

 

[BigH]

SKIAK - most of the time, the phi angle is a "guesstimate" or based on "experience" anyway by the geotechnical engineer. The actual phi in practice would depend on the nature of the backfill and the level of compaction. You might specify 95% MDD (Std) but if, in fact, the contractor gets an average of 98% MDD, then obviously the phi would be higher (and ka would be lower). Using an experienced based phi - likely on the low side for upper limit ka - would be a safe bet for formulating your wall dimensions. There is also a big Retaining Wall Handbook - can't remember the name - it isn't as mentioned above . . .

 

[SKIAK]

Thats true BigH. I just don't have a good perception on the line between conservative and overly conservative. I am only finding phi to guesstimate a vertical load on the wall from the soil (I am given the EFP for horizontal and gamma for weight). The vertical load would only increase the bearing pressure, but could help in sliding and overturning although I wasn't considering that. With the dimensions I have to resist sliding my bearing pressures aren't really a concern anyways. If you can think of another reference other than the Bowles book I'd be interested to look into it. Thanks.

 

[fattdad]

What is the soil type anticpated for the wall backfill and will this backfill represent the soil material throughout the retained "active" wedge? There is no doubt it'll have to be compacted, but if you are designing a wall and all there is to use is lean clay with sand (or such other fine-grained, low-sand content material), then a low phi angle may be appropriate. If you are designing a wall and you are going to specify the wall backfill, you have control to make the specification requirement some sort of "select backfill" (i.e., clayey sand, poorly-graded sand with silt or such).

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[theCorkster]

Another thought;

Are the clays expansive?

If so, the efp will be increased more toward ko to reflect the creep of the clay.

 

[SKIAK]

Behind the wall will be a minimum of 30" of NFS soil, the top few inches will have an "impermeable" layer, but the rest will be filled with the native silt that was excavated to construct the wall. I don't expect any clays.

 

[lovethecold]

What is a NSF soil?

 

[hokie66]

I love it. One guy says NFS, the next one wants to know what is NSF.