IBC presumptive soil properties

[RotmTranspo]

I am designing a small retaining wall and using the presumptive soil properties given in the IBC (2021). The wall will retain 5' and assumed 2' embedment to frost line, for total H=7'. Per the NRCS, the soils on the site are inorganic clays and silts (ML).

I interpret the lateral pressures in Tables 1610.1 and 1806.2 to be a substitute for gamma*K in the earth pressure calculation. For instance: Force from active earth pressure of clay = 45 psf x 0.5 x h^2. Is that the correct way to use the table values?

When calculating the resistance to sliding, I used the cohesion value of 130 psf from Table 1806.2, and FS=1.5 I back-solve to find the footing width B=10'. I included the passive pressure at the toe. I know the IBC values are considered conservative, but does a 10' footing width for a 7' wall seem far-fetched to anyone else?

 

[JoelTXCive]

That does not seem too crazy.

For preliminary dimensioning, I always assume that the footing will be roughly the same size as the stem. So, if you stem is 7'ft tall, then the overall footing will be in the neighborhood of 7' to 8' ft. usually.

TxDOT (and I think FDOT) both have stock retaining walls that you can compare your dimensions and reinforcing too. Just google "TxDOT standard retaining wall"

Also, several software companies have 30 day trials if you want to download something. Software definitely helps when you want to change properties & dimensions quick.

My current favorite is 'QuickRWall' by IESWeb.

 

[EcoGen]

That didn't seem right to me at first so I ran it in ENERCALC and got about the same width (9 feet) to meet 1.5 F.S. for sliding. Another option would be to use a key. Even then to get to 1.5 F.S. you would need about a 24 inch deep x 12 inch wide key which gets you to about a 5 foot wide footing. Saving about 30% in concrete volume but now you need extra excavation for the key. I would double check your cohesion assumption to see if you're being too conservative. Also, if you have a slab on grade up against your retaining wall, you can eliminate the sliding check all together.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[Settingsun]

That cohesion value seems like a residual cohesion in the drained state judging from its magnitude, otherwise it's about the worst ground this side of quicksand and your wall will be monumemtal.

If it is drained, you can take friction as well - not clear whether you're doing that. If this is one of those walls with the base under the low side, giving little dead weight for ​friction, then again that's going to push the base width up.

Are you backfilling with the clay/silt?

 

[Celt83]

Also, if you have a slab on grade up against your retaining wall, you can eliminate the sliding check all together.

Eh be careful with this assumption it may check out in a closed basement but not an open basement setup.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[BigH]

I do not know what is "in front" of your retaining wall - but be careful with the passive pressure - you don't want someone coming later and installing a sewer line parallel to your wall opening up a "long line" in front . . .

 

[Guest090822]

I second "BigH" - passive pressure at the toe is usually ignored. You never know when someone may need to excavate in that area.

 

[dik]

I don't know, and may not be understanding the problem, but a footing width of 10' seems excessive for a 7' wall.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[le99]

10' footing length for 7' high wall does seem excessive unless the wall is placed on the saturated clay with the friction angle equal to zero. I agree that in no case that the passive toe pressure is to be included.

 

[dauwerda]

Sounds to me like getting a soils report would more than pay for itself.