Design of footing for settlement 1

[TiffanyEIT]

Hello structural pals-

So I have this horizontal vessel foundation I'm tasked to design. It will be sitting on a poorly graded loose sand, and per the soil report there 's potential for liquefaction in depths of about 15 to 20 ft. Due to some incidents of sinking foundation in similar conditions, I don't want to take the chance of not designing the foundation for settlement. Report gives around 2" in 40 feet long. I did some research on design for settlement and came across articles talking about use of slope deflection method (?). Can anyone help me with a reference or write equations on here please?

Thanks.

Tiffany-

 

[EireChch]

Hi Tiffany,

Can you give us some more information, what are the ground conditions i.e. soil strength (N values), where is the groundwater table. What is the size of the size of the foundation and the loading (applied bearing pressure). Is the load applied eccentrically?

The slope deflection method sounds like a method for determining the allowable settlement or tilt in a structure. Generally, as far as i know, geotechs dont provide an assessment of the expected tilt of a structure (if there is going to be differential settlement). If it is provided it would be a big range.

You also have two settlement issues:

1) settlement from static loading. In this case it will just be elastic settlement.
2) Liquefaction induced settlement from Seismic loading. You will need to undertake a liquefaction assessment to determine liquefaction induced settlement.

 

[oldestguy]

The slope deflection method is used for evaluating a structure where there are not sufficient knowns to analyze it statically. It is a method for analyzing an indeterminate structure. Probably does not apply to foundations.

 

[MotorCity]

Agree w/ OG, slope deflection is a method of solving indeterminate structures. Won't help your settling issues. So the concern with settling in your case is not so much the foundation, but more the integrity and operability of the tank. Your foundation will just go along for the ride w/ the soil whether it settles 3" or 3'. Realize there is a difference between settling and liquefaction. Settling usually is on the order of inches and is not usually a concern if the settlement is uniform or the differential settlement is small. Liquefaction is usually more severe than settlement, as it can result in a total washout of the soil below a foundation. I would look into a deep foundation system that extends a sufficient distance below the level of anticipated liquefaction. Then design your tank foundation to span between your deep foundation elements. Don't try to design a foundation that accommodates displacement due to liquefaction.

 

[TiffanyEIT]

Hi fellas-
Thanks for replying to my post and sorry for being late to reply back

Eire-
Yes it is liquefaction induced settlement, but the liquefaction per the soil report occurs below 20 ft which I think is not a concern, but the soil is bad.
Size of foundation is 15 ft x 20 ft with a concentric dead load of 23 kips and a base shear of V = 0.30*SDS*Woper*I/1.4 = (0.3)(1.13)(23)(1.5)/1.4 = 8.3 kips
Ground water is encountered at 18 feet below grade. See attached for N.

MotoCity-
I don't think I can tolerate even uniform settlement because it will impact pipe stresses.

Thank you all.

 

[TiffanyEIT]

not sure why attachment didn't work. anyway, Blow counts N=6 at 5ft depth and N=18 at 10ft

 

[WARose]

Liquefaction is the collapse of the structure of saturated non-plastic soils (typically sandy) due a loading event that occurs faster than excess pore water pressures can dissipate, causing effective stress to approach zero. It typically occurs in a seismic (or other short-term dynamic loading) event. It shouldn't be an issue from static loading.

If the goetech thinks liquifcation will be a issue in a seismic event.....typically they will recommend remedial measures for the site (i.e. compaction, soil replacement, piles, etc). You shouldn't be trying to design for some crazy settlement from it.

Since it's not clear to me what the goetech report says on this......I'd ask the geotech 2 point-blank questions:

1. If you think liquifcation will be an issue during a seismic event.....what are your recommendations? (You may want to be sure their suspicions are confirmed by something like the Seed-Idriss method.)

2. What is your recommended maximum bearing pressure and what will the settlement be at that pressure (after proper fill and/or compaction is in place)?

 

[TiffanyEIT]

Thank you very much WARose. The Geotech did actually recommend overexcavating for the foundation an extra 3 feet to replace soil with a certified fill.

 

[WARose]

[blue](TiffanyEIT)[/blue]

The Geotech did actually recommend overexcavating for the foundation an extra 3 feet to replace soil with a certified fill.

I would be 100% sure that is all they recommend to address the potential liquefaction issue. (Would definitely be worth a e-mail to them if the verbiage in the report isn't crystal clear.) That sounds more like immediate bearing surface prep than anything else. (I.e. such a shallow excavation.)

Also, in your OP you said that the report stated the "potential for liquefaction [is] in depths of about 15 to 20 ft". So I have my doubts that the 3' excavation will cover it.

 

[TiffanyEIT]

WARose,

Not even. The overexcavated bottom level is at about 7 feet below grade so we're above the liquefaction zone. It's just that a recent incident of a foundation that sunk left my boss traumatized so we're going all out conservative..