Angle of friction for backfill soils

[kellez]

Hello everyone, I am designing a cantilever reinforced concrete retaining wall and I need to determine the angle of friction of backfill soil.
Since this is not an insitu soil how am I supposed to determine the angle of friction of the backfilled soil since i am not able to get samples to carry tests on?

 

[SlideRuleEra]

One way is to select a conservative friction angle, say 45^o. Edit: 45^o is wrong - see correction below.
If the wall height is "low", the value of the fiction angle won't make much difference anyway. Practical minimum wall/foundation thicknesses/reinforcement will govern.

Of course I don't know if the wall is "low"... you have got to tell us.

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[kellez]

I dont think the wall is low, the wall is 4.20m high with backfilled soil at 2.70m, the top 1.5m is for the perimeter wall of the building.
Please check link below

are you sure 45degrees is conservative? I am using 30 degrees and when i change it to 45 degrees my design has a higher factor if safety therefor its a safer design

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1514553996/tips/Retaining_Wall_Details_2_puseid.pdf[/url]

 

[SlideRuleEra]

I agree, 2.70m of backfill does not meet my definition of "low".

Sorry, you are right 45^o is not conservative. A better minimum value may be 25^o. See how that checks in your design.

On the drawing, I see the wall is 200mm thick and has 2 rebar mats. IMHO, that thickness is not practical. To meet minimum rebar cover requirements (on both sides of the wall) AND get meaningfull separation of the rebar mats thickness has to be at least 250mm... 300mm is better.

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[kellez]

ok thanks for the advice, i will be changing the thickness to 250mm.

Ok so your suggestion in regards to the angle of friction for the backfill is to take a conservative value of 25degrees? there is no other way to determine this right?

 

[robyengIT]

practical calc for wall thickness : 100 mm each meter of elevation (if the wall is 4 m high, then the wall at the base should be 400 mm thick, decreasing while going up)

 

[kellez]

what about the fact that the soil is only at 2.70m? there is no soil at the full height of the wall?

 

[SlideRuleEra]

kellez - Yes, 25^o should be conservative for any soil that could reasonable be considered backfill.
Something like "soft packed mud" or "plastic clay" may have a friction angle of 20^o+... but doubt anyone would use that for backfill.

Since force on the wall depends on the backfill's unit weight (kg/m³), what are you assuming?

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[Okiryu]

It will depend on the type of backfill:

Good Granular Fill: between 30 to 35 degrees. 20 kN/m3

Fine-grained fill: between 15 to 25 degrees. 18 kN/m3

You can look at foundation engineering books. In their section for retaining structures there should be some presumptive values for different type of backfill soils.

 

[kellez]

Backfill will be good granular fill and for the unit weight i am using a value of 20kN/m3 together with a conservative value for the angle of friction at 25 degrees
do you guys think that this is too conservative? shall i increase to 30 degrees? i will try both and see

 

[SlideRuleEra]

...how am I supposed to determine the angle of friction of the backfilled soil...

Backfill will be good granular fill...

Look up a reasonable range of values for friction angles and unit weights for "good granular fill", as Okiryu suggested. I would choose the most conservative values for each.
Be sure the backfill has good drainage... otherwise there may be hydrostatic pressure, too.

BTW, You know a lot more than you told us. Makes me feel kind of foolish trying to help you "guess". But... that's ok.

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[kellez]

BTW, You know a lot more than you told us. Makes me feel kind of foolish trying to help you "guess". ponder But... that's ok.

Why would you say that? Probably you misunderstood my question.
My main question was how can someone determine the angle of friction of a backfilled soil other than using a conservative value or picking up a number from a text book.

for example, for an insitu soil you can take samples and test the soil inside a lab to get the angle of friction, however this cannot be done with a backfill since the soil is not in an insitu state.

I do know the type of soil to be used as backfill but i dont really know how engineers choose their angle of friction for that material...obviously you helped me understand that there are standard range of values from which engineers choose from.

 

[PEinc]

Check of the attached file from NAVFAC DM-7.2 for various types of compacted soils.

www.PeirceEngineering.com

 

[kellez]

Thanks thats great

 

[BAretired]

When specifying a retaining wall, the type of backfill should also be specified. Ordinarily, a granular backfill is used against the back of wall. Some engineers specify drain rock. If the wall is long, you may want to specify drainage tile along the back of the wall and drainage holes through the wall every two or three meters which empty out a short distance above the lower grade.

It appears that the cross section indicates 100mm insulation under the foundation. If that is intended to prevent frost heave, it won't work because frost will penetrate the soil well beyond the edge of the insulation. For a wall as high as this one, a shear key will be needed below the footing to prevent lateral movement. This is usually a continuation of the wall below grade. The bottom of footing should extend below the lower grade by a distance equal to the frost depth unless frost heave is deemed acceptable.

The proportions of the footing appear wrong for the height shown. The overall width of base is usually at least 2/3 of the height and the toe of the footing usually extends beyond the face of wall.

Continuing the concrete wall above the upper grade will raise the possibility of raising grade higher than was anticipated in design.

I recommend that you examine standards such as the CRSI Design Handbook for further guidance in designing retaining walls.

BA

 

[oldestguy]

Your drawing does not show "the permiter wall of a building" clearly. If that building sits on that wall compressive stresses will come into play, possibly showing that little or less reinforcing is needed. You say nothing about what sits on that backfill. If it is a roadway where vehicle traffic may occur, you may get significantly higher wall stresses than from that fill alone. You say nothing abut how that backfill is placed. If it is compacted by compactors closer than one meter from the wall there likely will be built up horizontal pressures far in excess of natural gravity caused stresses. So getting angle of friction depends on a lot more than material type. Show that upper building wall and give what the downward load is on the top of that wall as a starter. Describe the nearby traffic and what compaction is done and where during placement. As one poster noted in order to get useful answers you have to give every possible factor that may affect the wall and much is needed or the answers from here may be useless or totally wrong.

 

[kellez]

Thank you everyone for commenting on my post, I know i did not post all the details regarding the design of the retaining wall,
however at first i just wanted to ask a simple question regarding the angle of friction of the backfill and not get into details of the design
I saved that for later.

Indeed there is a problem with the current wall design, and it shown in my calculations as well.
I will post a new thread with the actual design problem and include all my data and calculations as well.
Currently i am also missing some info, once i have that i will post the whole design in a new thread.

 

[robyengIT]

some info other than what suggested by PEinc (USSteel)

 

[kellez]

I have posted the complete design problem in another thread, please check the link

http://www.eng-tips.com/viewthread.cfm?qid=433845&action=updated

 

[JStructsteel]

Can you not define the soil parameters you want it to be backfilled with?