Angle of friction for backfill soils
Angle of friction for backfill soils
(OP)
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?
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?






RE: Angle of friction for backfill soils
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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RE: Angle of friction for backfill soils
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
https://res.cloudinary.com/engineering-com/image/upload/v1514553996/tips/Retaining_Wall_Details_2_puseid.pdf
RE: Angle of friction for backfill soils
Sorry, you are right 45o is not conservative. A better minimum value may be 25o. 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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RE: Angle of friction for backfill soils
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?
RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils
Something like "soft packed mud" or "plastic clay" may have a friction angle of 20o+... 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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RE: Angle of friction for backfill soils
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.
RE: Angle of friction for backfill soils
do you guys think that this is too conservative? shall i increase to 30 degrees? i will try both and see
RE: Angle of friction for backfill soils
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".
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RE: Angle of friction for backfill soils
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.
RE: Angle of friction for backfill soils
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RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils
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
RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils
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.
RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils
http://www.eng-tips.com/viewthread.cfm?qid=433845&...
RE: Angle of friction for backfill soils
RE: Angle of friction for backfill soils