Differnetial settlement

[LonnieP]

Our structural office had started getting Soils Reports that indicate we should expect settlement in a thrity foot diameter circle, located anywhere across the building site. They give an expected range of settlement, just no location. Corner of the building gets real interesting.

This sounds like a CYA item their insurance company insisted they put in.

We're in So. Cal. Has anyone else out there had soils reports like that and if so, how did you handle it. Seems to me the soils guys should at least supply a spring constant for an elastic foundation analysis. The plan checkers are at a loss about what to do too.

Lonnie P.

 

[oldestguy]

As a writer of these reports, a spring constant has never been given. I use a different method for calculating settlement. There are different schools of thought on what is best. You give the geotech the individual load data and they can then estimate the settlement of each footing using their method, not necessarily as a spring. Usually some conservancy is required for a typical site, depending on variations found.

If you want a detailed location for a specific settlement, then you had better pay for borings about 10 feet on center through the area and beyond. In any case, a phone call usually will clear up the question.

 

[FixedEarth]

On a circular mat foundation, the center settles more than the edges. You can ask for the expected settlement values at both locations as well as the vertical subgrade modulus. If you have two tanks, then it becomes more interesting due to possible overlapping stresses.

http://www.soilstructure.com/

 

[LonnieP]

Ok, some more information to illustrate my problem. Hopefully I'm overthinking this issue.

I have a two story classroom building 150' long. Live load is 1200#/ lin. ft. Dead load is 1400#/ lin. ft. The soils engineer has given me approximate settlement numbers for both live and dead AND differential settlement numbers for both.

My question - in the thirty foot long section of footing that's 'settled', how much upward soil pressure is still present to offset the applied downward loads. Or do I assume total loss of soil pressure in that area?

Next question - If I have reduced pressure in the thirty foot area, I have to transfer the unresolved downward loading to the 'unsettled' footing on either side of the 'thirty foot settled area'. How long of a redistribution length should be used and wouldn't this area then also settle even more which would slightly reduce the differential settlement.

Obviously this a problem in indeterminate statics - hence the need for spring constants (K values?) or even two spring constants, one for the 'unsettled' footing and another for the area subject to greater settlement.

I called soils engineer with the above discussion and he said "He'd have to think about it." Still waiting.

On a similar soils report I went through the same thing and finally suggested a soils pressure profile I'd like to have and the Geotech wrote a letter suggesting that would be adequate. Not really what I was hoping for but at least we could move forward. The footing got huge! and the cantilevered condition at the end of the building got interesting.

The issue here is what to do with a long footing, not several isolated ones.

Lonnie P.

 

[oldestguy]

If you are looking for some advice, I think the readers need to know a lot more. To make any comments, I's want to know more about the site soil conditions. The little I can glean from your descriptions, something sounds really unusual. That's not an unusual building, so why all this fuss? At this stage, I think another getech should be consulted. I doubt that this site members can do more than general advice. Mine is to go elsewhere.

 

[LonnieP]

oldstguy, Yeah, I agree about all the fuss. We've been doing buildings for fifty years without getting this kind of issue. Unfortunately this is the second Soils guy (two different projects)to make this thirty foot differential settlement issue without giving any guidance about what to do about it. (Remember when liquifaction was new? Maybe it's back with a different name.) I'm thinking it's some kind of 'Cover your tail' wording required by their insurance company.

I was hoping someone who has also delt with this issue would see my post and let me know how they resolved it. On the previous project with this comment in the soils report (different Geotech), the plan checker suggested using piles. Then we thought the pilings would still be in the same 'sagging' soil, so that wouldn't work either. If fact, the sagging pile could make thing worse buy pulling downward.

I agree, I haven't supplied near enough info for a real solution. Just a general suggestion of which direction to look for a solution. I'll probably see if the Geotech more guidance than the last one did.

LonnieP

 

[cvg]

liquefaction is not the same as settlement and your report should be very clear about that.
settlement is not new. differential settlement is not new. it has always been a possibility in California because of the higher potential for earthquakes and the possibility of settlement caused by them. However to expect that your building and foundation does not settle right along with the ground is not a reasonable assumption. So, not sure how this really should change your design.

 

[BigH]

Why would you assume a "loss" of soil bearing pressure with settlement? All you are doing by putting on a load and "causing" settlement is rearranging the soil fabric - in granular soils it would be pretty quick as the soils will realign in a more dense packed fashion - the particles are still there working merrily away, just a little more cramped. In clayey soils it would be the same although the period that it takes for the settlement to occur would be longer while waiting for expulsion of pore-pressure. The soils engineer in giving you a settlement estimate is only saying that under the applied load, the foundation will, due to the rearrangement of material fabric, compress. The soil is still acting in accepting the loads. You might want to read up on differential settlement (see Skempton, Tomlinson's book on Foundation Design and Construction)

 

[LonnieP]

BigH, Good point. I'll check to see if that's what he's trying to tell me. If so, it's an easy deflection calc to find moments and shears in my grade beams. Hope it's that easy.

LonnieP