Presumptive Value of Soil for Retaining Wall Design without Geotech Report

[snipit]

Per IBC 1806, we're allowed to presume the soil bearing capacity without a geotech report. However, Table 1806.2 only have value for bearing capacity and friction/cohesion for the 4 classes of soil. However, friction angle φ wasn't given, neither the soil unit weight. How do you guys get these values? Would like to be able to refer to a doc/code or something in the general notes.

Thank you!

 

[phamENG]

For low consequence retaining walls (4, maybe 5 feet tall site walls on private property), I'll use conservative fluid pressure values. Anything more - roadway, public access, holding up a building, etc., I'll demand a geotech report.

 

[snipit]

For low consequence retaining walls (4, maybe 5 feet tall site walls on private property), I'll use conservative fluid pressure values.

Mine is 10' retaining 8' soil on private property. What do you mean by fluid pressure?

 

[phamENG]

'equivalent fluid pressure' is a simplification allowed in some building codes...so a certain soil type can be simplified as 30psf/ft, another as 45psf/ft, etc. Some geotechs will even put that in their report to make things 'easier' for us small minded structural engineers.

I probably wouldn't go for a wall that high...I'd want more information and support from a geotech. At the very least, I'd list my assumptions and put in the drawings that the contractor must hire a geotech to confirm them prior to construction.

 

[ClearCalcs]

If you look at Section 1610.1 in the IBC, they provide some minimum lateral soil pressure values for a wide range of soil types. I think that's what you're looking for.

What do you mean by fluid pressure?

Instead of calculating Ka using the friction angle, the IBC gives out the pressures in pcf - they're basically just doing the work of finding Ka * your soil unit weight for you for simple cases. The term used in literature is typically "Equivalent Fluid Pressure method" (EFP)

-Laurent

http://www.ClearCalcs.com

 

[JoelTXCive]

Pham is correct....equivalent fluid pressure is just a simplification.... see below

 

[milkshakelake]

OP also mentioned bearing pressure. For bearing pressure on the horizontal bottom of the footing, I have a quick way of doing it which is advocated by a transportation agency in my area. You add the two 6" blow counts at the desired bottom of footing and multiply by 0.1 TSF. For example, if your 6" blow counts are 11 and 14, your bearing pressure is approximately 2.5 TSF, and then you can make it like 2.0 TSF to be conservative. It also shouldn't exceed code prescriptive maximum bearing pressures (my building code has a limit for each type of soil). Your mileage may vary, because soils differ from region to region and there are different methods of blow counts.

As for active pressure, there are rough charts online for correlating angle of friction with different types of soil. Just match it up to a chart and be conservative by reducing the angle by a few degrees. I'd only do this for a "low consequence wall", as phamEng put it.

 

[BigH]

Milkshakelake - just wondering - are you talking the 2nd and 3rd 6 inch blow counts of an SPT test? If so, the two added together are actually the N value so why not just 0.1xN??

 

[271828]

At firms I used to work for, the standard retaining wall details would have granular backfill at some angle (45 deg.?) behind the wall. If that's used, then could one set the equivalent fluid pressure for that and not be concerned about the native soil?

There's still the question about the bearing pressure under the footing.

 

[milkshakelake]

@BigH Yeah I meant the 2nd and 3rd blow counts. Honestly I don't know much about it, I just blindly use the method since a city agency recommended it. I'm not sure what N value means, but if it's the same thing as 2nd + 3rd, then that's what I meant. Some borings don't specify the N-value.

 

[JLNJ]

If it's a restrained basement wall you will want to use the higher, "at rest", pressures.

Don't assume fully drained if it might not be. No point in under-designing an important retaining wall. I saw first hand a project where the equivalent fluid pressure was grossly underestimated and a 200' long 12' high cantilever retaining wall laid right over.