Earth pressure to resist overturning in shallow footings

[BW_Engineer]

thread507-367772
I have a client wanting a design for a shallow precast sign foundation. A prior engineer on the project utilized soil pressure (from the backfill) to resist sliding and overturning from wind, seismic, and impact. This is contrary to what I know is correct (for overturning at least). In my search for answers to justify this approach I came across the above referenced thread which bolstered my position. I also came across the white paper from Bently (link below) that details how to use their software to utilize the soil pressure for just this purpose.

https://docs.bentley.com/liveconten...UID-043EA13F-6AE5-452B-BC42-3CE3658DBA53.html

Granted with the software you can opt to use or not use the soil pressure but why even have it as an option for spread footings? Is there any code I can reference for my client that expressly prohibits or discourages the use of soil pressure to resist overturning?

Thanks in advance for your thoughts/comments....first time commenting on the site but have followed many conversations and gleaned a lot of information over the years.

 

[phamENG]

Depends on how deep it is. If it's only a few inches (okay where I am - frost depth is 8"), then I ignore it. If I have at least a foot of soil over the footing, I'll use it in my calculations as long as it's not subject to flooding and scour. If so, I use the soil depth after subtracting scour. But the soils is there, it has mass, and it exerts a downward force on the top of the footing.

For sliding, I only use it if a) the footing was cast against undisturbed earth on the sides of the excavation or b) the compaction around the footing is inspected.

If I'd do it for a retaining wall, I'll do it on one of these. The results are obviously less significant, but as I see it the principle is the same.

 

[bridgebuster]

Some years ago I was checking the design of a contractor designed temporary utility support structure. They did the same thing with the footings. The chief engineer was walking by, looked at the calculations and said “ I see he’s using passive pressure. Some guy comes along and takes a leak and there goes his passive pressure.”

I don't necessarily disagree with pham but is there a possibility that the effect of passive pressure could be lost to an unanticipated event?

 

[JAE]

bridgebuster...That's a lot of beer...

 

[BW_Engineer]

Thanks for the replies. I think there's always a chance of loss of passive pressure with shallow spread type footings...flood, scour, landscaper moving dirt, critters, kegger down the street For that reason I typically discount it. The client being a precaster though wants the foundation to use as little concrete as possible (lighter and less $$). I am using ASCE7 and AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and traffic signals for the basis of design. There is a bit of information that could be referenced from the IBC as well....but nothing in the code specifically prohibits use of passive.

 

[Smoulder]

Check deflection. Pasive pressure means the foundation has rotated non negligibly. What is the deflection at top of post? Also look at reduced passive from rainwater draining down. It's not as bad as flooding but it's not nothing.

 

[Lomarandil]

Came here to make the same point as Smoulder. If you choose to use passive, make sure the system can handle the required rotation (~0.5% for cohesionless, 2-4% for cohesive)

 

[HTURKAK]

My points based on my past experience,

- The contribution of full passive thrust to overturning resistance is not reasonable . The full passive thrust would develop after almost 50 mm horizontal displacement literally means some permanent deflection at top of post.

- Consider the wt of overburden soil above the spread ftg. to resist overturning .

- If there is pavement around precast sign post, essentially the sliding and overturning will be resisted by the pavement .

- The following picture is useful to see the concept of active, passive and at rest pressure .

Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[phamENG]

Right...I wouldn't use passive pressure for a shallow footing's overturning resistance (it would be tiny, anyway), but I do account for the weight of the soil in sliding and overturning. I thought that was what the OP was talking about as they didn't mention passive pressure.

 

[ZiggyKS]

If you use passive soil pressure to resist the applied load sliding force, then according to ASCE 7, §2.4.1 you can use (up to) 0.6H for sliding resistance where the soil is permanent or 0.0H where the soil resistance is temporary (i.e. neglect passive pressure). That is on top of the FoS already established by the Geotech Engr for the allowable passive pressure. I added "up to" implying that passive resistance stops once it matches the applied load. Using 0.6H gives you the maximum resistance allowed to reduce the applied load to a net zero value. In a spreadsheet or MathCad template you would need to use a Min or Max function such that F(passive) ≤ F(applied)

You can use the weight of the soil above the top of the footing to develop the passive wedge in front of the footing because the overburden soil weight is still present. I like to subtract 1-ft from the bury depth to be a little conservative accounting for soil settlement, grading tolerances, etc. This also assumes a level soil profile in front of the wall in question. If the soil slopes downhill away from the wall then you need to account for the difference in height to where the passive wedge intersects the slope line. Without getting into a doctoral thesis, consider neglecting the soil above the level plane from that point to simplify analysis. The passive resistance comes from the trapezoidal shape against the face of the footing thereby neglecting the passive pressure in the upper triangular section that acts against the stem wall. The soil above the footing has to compress in order to ramp up to full passive resistance and that soil, even if compacted, has been disturbed and will not compress at the same rate as the soil beyond the face of the footing so I never use the passive wedge above the top of footing unless it is really deep, like a basement wall or buried tank. Neglecting the full depth of soil down to the top of footing leaves you with a very small triangular stress block if taken from the top of footing which is the most conservative approach without neglecting the entire passive wedge altogether. This can occur at a treatment plant where the possibility exists for future piping to be laid in front of a retaining wall. For those cases it is better to increase the heel to increase frictional resistance of the vertical mass above the footprint.

For overturning you should have zero passive soil resistance in your analysis of a shallow footing. Engaging the passive wedge should only be used when designing a sheet pile wall, "flagpole" or a very deep footing block where rotation engages the soil wedge. Including the weight of soil above the footing as a gravity load is your choice and may need to be neglected in your analysis if construction phasing includes backfilling behind the wall prior to backfilling in front of the wall.

 

[milkshakelake]

Assuming 2' footing, Kp = 3.33, soil density 120 pcf: The pressure at the bottom, if activated at all before a catastrophic failure, would be 2x3.33x120=800 psf. (800/2 [force]) * (2/1 [resisting moment arm]) = 800 lb-ft of resistance.

I agree with the comments above about activating the pressure in the first place. But given that the resistance will be only about 800 lb-ft, I'd think that it wouldn't make much of a difference anyway. Such a margin could be achieved by changing other things, like width, by a few inches.

 

[azcats]

Whenever I played with this tactic, it was to really sharpen the pencil to make soil pressures work on OT calcs. Seemed to have a minimal impact.

 

[BW_Engineer]

thanks everyone....to milkshakes point....
Wind Pressure Pz = 0.00256KzKdGV^2Cd
With a wind velocity of 150mph and all other factors per AASHTO LTS the wind pressure on an 8ft tall by 3ft wide sign is approx 125psf. This creates an Mo of about 18000lb-ft. A Pp of 800lb-ft is supplying 4.5% of the overturning moment....which is sizeable in this context. A concrete footing that is 2ft deep with dimensions of 5ft X 5ft will weigh in at 7500lbs and provide Mr of approx 18750lb-ft. Resists but little to no FoS. The Pp provides some level of FoS or does it if it's not actually activated and may not always be present (at that level).

 

[milkshakelake]

If you increase that to 5'-3"x5'-3", resisting moment is 21,705lb-ft, which is 120% of overturning moment, as opposed to 5'x5' being 104%. What I generally meant is that adding a few inches is a much bigger effect, rather than rely on something sketchy like passive pressure for overturning moment. The soil above the footing will also help. I guess 800lb-ft isn't negligible, but personally I'd keep it in my back pocket as an extra cushion instead of designing using that.

 

[BW_Engineer]

in agreement with you milkshake....have a client though that is wanting to make this footing as small as possible and is using the design of a prior engineer as justification to use the Pp in order to do that. I'd rather not use it but am having a hard time arguing against it....was looking for something in one of the codes that explicitly says "can't use this dummy".

 

[WinelandV]

Well, if you're trying to not use Pp, many geotechs that I've worked with have stated that it's good practice to ignore passive pressure until you get below the local frost line. Sometimes that even gets clearly stated within the site geotech report.

Shoot, if the contractor bank forms the footing, you may end up with 5.25' square as a footing anyway.

Please note that is a "v" (as in Violin) not a "y".

 

[BridgeSmith]

150 mph wind speed seems high for sign that's only 10' tall to the top, unless this sign is going up in south Florida or the Aleutian islands(?).

 

[BW_Engineer]

yeah....AASHTO LTS and ASCE 7 have all of the coastal regions pretty much at that level or higher for the 300yr MRI which is the lowest risk category.

 

[BridgeSmith]

Btw, an 8'x4'x1' footing will provide 19,200 lb-ft of overturning resistance (in the directions it's needed), and only weights 4800 lbs.

 

[BridgeSmith]

Your profile says you're in California, so I thought you were designing it for there.